National Aeronautics and Space Administration

GRS 1915+105

C. R. Robinson, B. A. Harmon, W. S. Paciesas, K. J. Deal, S. N. Zhang, M. L. McCollough, and C. A. Wilson, for the Compton Observatory BATSE Team; E. B. Waltman, Naval Research Laboratory; and E. Morgan, Massachusetts Institute of Technology, report: The galactic superluminal jet source GRS 1915+105 has reached the highest flux levels detected in prior outbursts from BATSE, for data averaged over one day. Previously, three strong outbursts were observed, with peaks averaging around 350 mCrab (20-100 keV) and characterized by a power law with photon index between -3.0 and -2.5. This latest hard x-ray outburst has lasted 8 months, rising slower to its peak flux than the other outbursts and exhibiting, until recently, a spectrum steeper than -3.0. Starting around Nov. 24, the source has remained above 350 mCrab, reaching at times 500 mCrab. The spectrum is now harder, with an index of -2.82 +/- 0.04. During this same interval, the RXTE/ASM data (2-10 keV) show average countrates and variability at around their lowest levels thus far detected from this source. Public-domain data from the NSF-NRAO-NASA Green Bank Interferometer, however, reveal the source to be flaring during this interval of high, hard x-ray flux. The largest flare was detected on Dec. 6 at a flux level of nearly 150 mJy (2.25 GHz). Additional observations of this source are encouraged.

Scorpius X-1

A. G. Peele and N. E. White, Goddard Space Flight Center, NASA, write: The public RXTE ASM lightcurve of Sco X-1 covering the interval 1996 Feb. 22-Nov. 28 shows an intensity modulation with a period of about 37 days. The period is detected using a Lomb-Scargle periodigram, and it has a false-alarm probablility of < 10E-6; it is also detected using minimum entropy and chi**2- folding techniques. Detection of the period is complicated by the well-known x-ray flaring from Sco X-1. The modulation is most evident in the low-energy (1.3-3.0-keV) channel, where the flaring amplitude is a minimum, and it is a sinusoidal-like modulation of the quiescent flux. Fitting a sine wave to the data in this channel gives an ephemeris of JD 2450160.9 (+/- 1.0) + (36.8 +/- 0.4)E, with phase zero defined as the modulation minimum and a full amplitude of 13 +/- 2 percent. The modulation amplitude for the quiescent flux decreases by a factor of 4-5 between the soft- and hard-energy (5.0-12.2-keV) channels. The 37-day modulation does not appear to be related to the flaring episodes. The modulation is not detected in archival Vela 5B and Ariel 5 ASM x-ray data (but upper limits do not rule out its presence). We encourage a search for the 37-day period in the optical and radio emission from Sco X-1 (= V818 Sco).

2S 0114+650

R. Corbet, Goddard Space Flight Center (GSFC) and Universities Space Research Association, for the RXTE ASM team at GSFC and the Massachusetts Institute of Technology; and J. P. Finley, Purdue University, report: RXTE ASM observations of 2S 0114+650 obtained between 1996 Jan. 5 and Dec. 5 show the presence of a modulation at a period of 2.7388 +/- 0.0006 hr and a peak-to-peak amplitude of approximately 60 percent. We identify this with the 2.78-hr (+/- 0.01 hr) period reported by Finley et al. (1992, A.Ap. 262, L25), showing that this is a persistent coherent property of the source. In addition, the ASM data indicate a modulation at a period of 11.7 +/- 0.2 days (amplitude approximately 60 percent), consistent with the 11.6-day optical period (Crampton et al. 1985, Ap.J. 299, 839). These results can be interpreted in terms of a neutron star rotating with a 2.7-hr period in an 11.6-day eccentric orbit around its early-type companion.

GRO J2058+42

A. J. Castro-Tirado, Laboratorio de Astrofisica Espacial y Fisica Fundamental, Madrid, communicates: We have examined the 8' RXTE error box of the x-ray pulsar GRO J2058+42 (IAUC 6514) on a deep Schmidt plate taken by K. Birkle at the German-Spanish Calar Alto Observatory on 1995 Oct. 15 in response to the report of the outburst on IAUC 6238. On that date, the pulsed signal was about 70 mCrab (C. A. Wilson, private communication.). A new object of mag about 18.5 not present on the corresponding Palomar Sky Survey plate is seen at R.A. = 20h58m58s.0, Decl. = +41d43'06" (equinox 2000.0, uncertainty +/- 1"). A second object of mag about 19.5 that seems to be variable by about 0.5 mag is also seen at R.A. = 20h59m07s.7, Decl. = +41d44'46". A finding chart can be obtained via anonymous ftp at laeff.esa.es (under /pub/users/ajct).

GRO J2058+42

C. A. Wilson, NASA Marshall Space Flight Center; T. Strohmayer, NASA Goddard Space Flight Center and Universities Space Research Association; and D. Chakrabarty, Massachusetts Institute of Technology, report: The Compton Observatory/BATSE instrument has observed a series of five outbursts from the 195.6-s accreting x-ray pulsar GRO J2058+42, with a bright initial outburst in Sept. 1995 (IAUC 6238) followed by four weaker (15-20-mCrab pulsed flux in the 20-50-keV band) outbursts, all spaced by about 110 days. The weak outbursts have each lasted about two weeks, with a marginal decline in peak intensity over consecutive outbursts. The most recent outburst peaked on 1996 Nov. 25 +/- 2 with a 20-50-keV pulsed flux of 14 +/- 1 mCrab. The barycentric pulse frequency on Nov. 25.0 UT was 5.1125 +/- 0.0001 mHz. A target-of-opportunity scan of the Compton/OSSE/BATSE error box (IAUC 6239) on Nov. 28 with the Proportional Counter Array (PCA) on the Rossi X-ray Timing Explorer (RXTE) constrained the 90-percent-confidence region for the source position to a circle with 4' radius and centered at R.A. = 20h59m.0, Decl. = +41d43' (equinox 2000.0). RXTE/PCA observed a peak flux of 11 +/- 1 mCrab (2-30 keV) and a pulse frequency consistent with the BATSE measurement. Optical observations of the new error circle are strongly encouraged in order to identify the optical companion, most likely a Be star.

GX 5-1

M. van der Klis and R. Wijnands, Astronomical Institute 'Anton Pannekoek', University of Amsterdam; E. Kuulkers, Oxford University; F. K. Lamb and D. Psaltis, University of Illinois at Urbana; S. Dieters, University of Alabama at Huntsville (UAH); J. van Paradijs, UAH and University of Amsterdam; W. H. G. Lewin, Massachusetts Institute of Technology; and B. Vaughan, California Institute of Technology, report: RXTE observations of the low-mass x-ray binary (and Z source) GX 5-1 made on Nov. 6 and 16 show high-frequency quasiperiodic oscillations (QPO). Power-density spectra of the 10- 50-keV flux show two QPO peaks in the range 325-895 Hz. The centroid frequency of the stronger of the two peaks increases from 567 +/- 14 to 895 +/- 12 Hz, its width decreases from 230 +/- 40 to 50 +/- 40 Hz, and its rms amplitude decreases from 6.7 +/- 0.5 to 2.0 +/- 0.3 percent (10-50 keV) from the left end of the horizontal branch to the top of the normal branch in the x-ray color-color diagram (i.e., for increasing mass-accretion rate). The weaker of the two peaks is only detected in part of this range, at a frequency that increases with accretion rate from 325 +/- 9 to 448 +/- 25 Hz. The frequency separation of the two peaks is consistent with being constant near 325 Hz. The peaks are at least 1.3 to 2.4 times weaker in the band 1-10 keV than in the band 10-50 keV. GX 5-1 is the second Z source (after Sco X-1; IAUC 6319) to show kHz QPO.

Cygnus X-1

S. N. Zhang and C. R. Robinson, Universities Space Research Association; and W. Cui, Massachusetts Institute of Technology (MIT), report: We have detected the 5.6-day orbital period of the blackhole candidate Cyg X-1 in the ASM/RXTE data, confirming a similar detection with the Ginga ASM in its low-energy band during the x-ray hard state (Kitamoto et al., submitted to Ap.J.). The modulation is only detectable in the hard state (before around 1996 May 17 and after around Aug. 10) and is strongest in the lowest ASM energy band (1.3-3.0 keV), with a full amplitude of around 10 percent and x-ray flux minimum near superior conjunction of the compact object. This seems to indicate that the modulation is due to an asymmetry in the photoelectric absorption. However, the lack of any significant orbital modulation in the soft state suggests that another effect may be responsible for the detected modulation, unless the power-law component is much more heavily absorbed than the blackbody component, since the ASM 1.3-3.0-keV detector counting rate is dominated by the power-law component in the hard state and by the blackbody component in the soft state.

Cygnus X-3

W. Cui, Massachusetts Institute of Technology (MIT), reports for the RXTE ASM Team at MIT and the Goddard Space Flight Center: Cyg X-3 has just started another x-ray outburst, as seen by the RXTE All Sky Monitor. This is only about 3 weeks after the end of the previous one, which peaked at about 220 mCrab (1.3-12 keV) and lasted for about 50 days (see the ASM light curves at http://xte.mit.edu/XTE/ASM_lc.html). The daily averaged ASM measurements show that it has already reached a level of about 300 mCrab (299 +/- 7 mCrab on Nov. 21), which is about a factor of 3 higher than its 'normal' persistent flux level. During this active period, observations of this source at other wavelengths, especially radio and infrared, are strongly encouraged.

GX 13+1

R. Corbet, Goddard Space Flight Center (GSFC) and Universities Space Research Association, reports on behalf of the RXTE ASM team at GSFC and the Massachusetts Institute of Technology: Analysis of the RXTE ASM lightcurve of the low-mass x-ray binary GX 13+1 (= 4U 1811-17) obtained between 1996 Feb. 23 and Nov. 14 suggests the presence of a modulation with a period of 24.7 +/- 1 days. The folded lightcurve on this period shows a mean peak-to-peak modulation of approximately 15 percent. This periodicity is apparently confirmed by the Ariel 5 ASM lightcurve of GX 13+1, as a Fourier Transform of this shows that the largest peak, for periods < 125 days, occurs at 25.2 +/- 0.15 days, consistent with the RXTE result. In the RXTE data, the modulation is most pronounced at low energies (1.3-4.8 keV) and is reduced at higher energies (4.8-12.2 keV). In addition, there is an anticorrelation between hardness ratio and intensity in the RXTE data, which may suggest a similarity with Cyg X-2 (Wijnands et al. 1996, Ap.J. 473, L45). If this periodicity is a persistent property of this source, it may also be present in the variability of the infrared counterpart.

4U 1820-30

A. P. Smale, Goddard Space Flight Center (GSFC), NASA and Universities Space Research Association; and W. Zhang and N. E. White, GSFC, report: RXTE observations of the low-mass x-ray binary 4U 1820-30 performed on Oct. 15 and Oct. 26-30 reveal the presence of high-frequency quasiperiodic oscillations (QPOs) when the source is in its low state (160-200 mCrab). The centroid frequency of the main peak varies between 546 +/- 2 and 796 +/- 6 Hz, and is tightly correlated with source intensity. The QPO width is typically 20-30 Hz, with rms amplitude 5.0 +/- 0.5 percent. When the source is at the upper end of this range, a second significant QPO peak appears with frequency 1066 +/- 5 Hz, width 46 +/- 12 Hz, and rms amplitude 3.2 +/- 0.5 percent. When the source brightens above 200 mCrab, neither QPO is detected.

Rapid Burster (X1730-333)

W. H. G. Lewin, D. W. Fox, J. M. Kommers, and E. Morgan, Massachusetts Institute of Technology; R. E. Rutledge, Max-Planck- Institut fur Extraterrestrische Physik; L. Bildsten, University of California, Berkeley; T. Dotani, Institute of Space and Astronautical Science; L. Lubin, Carnegie Observatories; M. van der Klis, Astronomical Institute 'Anton Pannekoek', University of Amsterdam (UA); and J. van Paradijs, University of Alabama in Huntsville and UA, report: Following the recurrence of activity from the rapid burster MXB 1730-335 = X1730-333 (IAUC 6504), we have carried out a series of observations with the RXTE PCA. On Nov. 6.8 and 9.0 UT, we made 3000-s observations and detected the source at a persistent emission level of roughly 160 and 105 mCrab (2-12 keV), respectively, with no type II x-ray bursts seen. One burst with type-I characteristics was seen during each observation; these bursts exhibited a about 3-s rise time and a about 200-s exponential decay. No significant (3-sigma) coherent or quasiperiodic oscillations (QPOs) were detected in the persistent or burst emission between 10 and 2048 Hz; 2-sigma upper limits on the strength of any 2-10-Hz QPO in the burst decline on those days are 3 and 5 percent rms, respectively. On Nov. 10.7 (3000-s observation), 11.9 (2000-s), and 17.0 (3000-s), we detected the source at fluxes of roughly 90, 80, and 70 mCrab, respectively, with one small burst of low statistics (and unknown type) seen on Nov. 11. This behavior is reminiscent of that displayed by the rapid burster for about 10 days in 1983 Aug. (Kunieda et al. 1984, PASJ 36, 807), after which it began its usual type-II bursting behavior. We note that the current active phase of the rapid burster follows the last one by about 6 months (IAUC 6390, 6409), strengthening suspicions that this is the recurrence time for the rapid burster (Lewin and Joss 1983, in Accretion-Driven Stellar X- ray Sources, Cambridge University Press, p. 41, and references therein). Sun-angle constraints do not allow us to continue PCA observations.

Rapid Burster (X1730-333)

R. Remillard, Massachusetts Institute of Technology (MIT), reports for the RXTE ASM Team at MIT and the Goddard Space Flight Center: The rapid burster X1730-333 is again seen to be in x-ray outburst. Since the end of the previous outburst (1996 May 7), the daily average measurements with the RXTE All Sky Monitor were generally upper limits near 10 mCrab. On Oct. 30, the average flux suddenly increased to 178 +/- 8 mCrab (2-12 keV). During the interval Oct. 31-Nov. 7, the ASM coverage is very sparse, due to occasions of source confusion and also to the details of the RXTE pointing program. Yet there are additional detections, leading to a measurement of 115 +/- 10 mCrab on Nov. 7.

New Transient (XTE J1856+053)

SGR 1806-20

C. Kouveliotou, Universities Space Research Association; J. van Paradijs, University of Amsterdam, and University of Alabama in Huntsville (UAH); G. J. Fishman and C. A. Meegan, Marshall Space Flight Center, NASA; S. Dieters and M. S. Briggs, UAH; K. Hurley, University of California, Berkeley; T. Murakami, Institute of Space and Aeronautical Sciences, I. Smith, Rice University; and D. Frail, National Radio Astronomical Observatory, report: We observed SGR 1806-20 on Nov. 5 and 6 with the Rossi X-Ray Timing Explorer during an active episode detected by BATSE (IAUC 6501). In addition to the large outbursts, also seen with BATSE, we detected numerous small outbursts coming in groups from the source, with peak fluxes about 2 orders of magnitude fainter than those of bursts seen earlier with BATSE. Burst rates as high as one per minute were observed.

SGR 1806-20

C. Kouveliotou, Universities Space Research Association; G. J. Fishman, C. A. Meegan, Marshall Space Flight Center, NASA; J. van Paradijs, University of Amsterdam and University of Alabama in Huntsville (UAH); M. S. Briggs and G. Richardson, UAH; and K. Hurley, University of California, Berkeley, report: The BATSE has triggered on renewed activity from the soft gamma-ray repeater SGR 1806-20 on Oct. 30.95220 and on Oct. 31.54094 and 31.97024 UT. The fluence and peak flux of the first event are 10E-7 erg cmE-2 and 10E-6 erg cmE-2 sE-1, respectively. The reactivation of the source has initiated several Target of Opportunity observations with other satellites and groundbased observatories; currently RXTE has detected several bursts from the source and is observing during Nov. 6. We have changed the BATSE trigger criteria to trigger on (weaker) bursts during the RXTE observations. We strongly encourage follow-up observations in other wavelengths.

4U 1145-61

Galactic Center

T. Strohmayer, Laboratory for High Energy Astrophysics and Universities Space Research Association (LHEA/USRA); U. Lee, Tohoku University, Sendai; and K. Jahoda, LHEA, Goddard Space Flight Center, report: We have detected x-ray brightness oscillations with a frequency of 589 Hz during three type-I x-ray bursts from a burster in the galactic-center region. The bursts occurred on May 15.814, June 4.612, and June 19.414 UT during routine monitoring observations of GRO J1744-28 by the Rossi X-ray Timing Explorer (RXTE). The oscillations have amplitudes (rms) in the range 2-4 percent (above 8 keV) and are detected only in the few seconds after burst peak. Each burst shows evidence for modest photospheric radius expansion. The current data constrain the source position to a 0.15-degree-wide region whose shape is approximately that of one quarter of an annulus. The center line of the annular region extends from (R.A., Decl.) = (17h43m40s.8, -29o03'00", equinox 2000.0) to (17h45m00s.0, -29o16'48"), with midpoint at (17h44m07s.2, -29o09'00"), strongly excluding GRO J1744-28 as the burst source. The known burster nearest to this region is MXB 1743-29, but at present this association is only suggestive. This represents the third detection by RXTE of a near- millisecond oscillation from an x-ray bursting low-mass x-ray binary (see IAUC 6387, 6437).

GRO J1744-28

T. Augusteijn and C. Lidman, European Southern Observatory; and P. Blanco, University of California, San Diego; report: A re- analysis of the Feb. 8 K-band image taken of the field of the transient GRO J1744-28 (IAUC 6321) shows that the source that was identified as the infrared counterpart of the transient (IAUC 6369) may not be real. The image is the superposition of ten individual, mutually-shifted images. The source was in the field-of-view of seven of these; however, it only appears in two. The detections do not coincide with any recorded bursts in the Ulysses GRB data (K. Hurley, private communication), and we suspect that the detections may be artifacts of the detector. Further analysis of the Feb. 8 and Mar. 28 images do not show any source in the vicinity of the x-ray position down to K about 16 and varying by > 0.5 mag. We encourage continuous deep-infrared imaging to secure the identification of the counterpart of GRO J1744-28.

X 0115+634 and Cygnus X-3

R. Remillard, Massachusetts Institute of Technology (MIT); and B. Thomas and T. Takeshima, Goddard Space Flight Center (GSFC), report for the RXTE ASM Team at MIT and GSFC: The Be binary/pulsar X0115+634 is again increasing in x-ray brightness. Beginning on Sept. 26, the RXTE All Sky Monitor began detecting the source, with an average flux of 15 +/- 3 mCrab at 2-12 keV on that day. By Sept. 30, the average flux had increased to 40 +/- 3 mCrab. Cygnus X-3 has also displayed increasing brightness. During Mar.-Aug., the 2- to 12-keV flux usually varied between 70 and 140 mCrab. However, during Sept. most of the average daily measurements were between 140 and 230 mCrab, with increasing variability and spectral softening. On Sept. 28 and 29, the average flux was 275 +/- 5 mCrab -- while on Sept. 30, the flux dropped to 146 +/- 3 mCrab with continued variability on timescales of hours.

GX 1+4

D. Chakrabarty, Massachusetts Institute of Technology (MIT); M. H. Finger, Universities Space Research Association; and T. A. Prince, California Institute of Technology, report for the Compton Observatory BATSE team: The accreting x-ray pulsar GX 1+4 has been undetected by BATSE since Aug. 18, with a 90-percent-confidence upper limit of 15 mCrab (20-60 keV) for the pulsed intensity. This is the first time that GX 1+4 has remained below our 15-mCrab detection threshold for longer than a month, since we began continuous BATSE monitoring of its pulsed flux in 1991. When last detected on Aug. 18.5 UT, the phase-averaged pulsed intensity of GX 1+4 was 29 +/- 4 mCrab and the barycentric pulse frequency was (8.0381 +/- 0.0001) x 10E-3 Hz. The pulse frequency derivative changed from (-3.7 +/- 0.2) x 10E-12 Hz sE-1 during May 28-June 12, to (0.0 +/- 0.2) x 10E-12 Hz sE-1 during July 27-Aug. 6, suggesting that a torque reversal may have occurred in early Aug. (cf. IAUC 6105, 6153). The phase-averaged pulsed intensity flared from 59 +/- 5 mCrab on June 4.5 to a maximum of 204 +/- 5 mCrab on Aug. 5.5, before dropping below our detection threshold.

W. Cui and D. Chakrabarty, MIT, write: RXTE/PCA observations of GX 1+4 on Sept. 5.03 UT detect a phase-averaged pulsed intensity of 4.9 mCrab (2-60 keV) and a barycentric pulse frequency of (8.03156 +/- 0.00042) x 10E-3 Hz, implying that the pulsar continued to spin down after the last BATSE detection on Aug. 18, with a mean pulse frequency derivative of (-4.3 +/- 0.3) x 10E-12 Hz sE-1. A second RXTE/PCA observation on Sept. 25.55 did not detect any pulsations, with a 90-percent-confidence upper limit of 0.2 mCrab on the pulsed intensity. This is comparable to the low state established by EXOSAT non-detections of GX 1+4 in 1983 and 1984 (IAUC 3872). Deeper x-ray observations and optical/infrared observations of the red-giant companion are strongly encouraged.

SMC X-1

S. N. Zhang, C. R. Robinson, R. B. Wilson, B. A. Harmon, and D. M. Scott, on behalf of the BATSE/CGRO team; and R. Remillard, on behalf of the ASM/RXTE team, report: Quasiperiodic flux variations have been observed from the 0.7-s pulsar SMC X-1 with a period around 60 days, in the energy bands 1.3-12 keV (ASM/RXTE) and 20-100 keV (BATSE/CGRO), confirming the HEAO-1 results in 1977-1978 (Gruber and Rothschild 1984, Ap.J. 283, 546). A sequence of five outbursts between early Oct. 1995 and mid-Aug. 1996 is apparent in the BATSE/CGRO lightcurve, of which the last three coincide with those observed from ASM/RXTE (the first two of the three ASM outbursts have been reported by Levine et al. 1996, Ap.J. 469, L33). The peak flux is about 30 mCrab in both bands. No significant outbursts were observed in the BATSE data between Apr. 1991 and Oct. 1995, though a weak level of emission at about 10-20 mCrab was sometimes detected. Eclipses with a duration of about 20 percent of the orbital period of 3.9 days are observed in both data sets. The current outburst was detected about Aug. 27.

Cygnus X-1

S. N. Zhang, B. A. Harmon, W. S. Paciesas, C. R. Robinson, D. M. Scott, and W. Yu, on behalf of the BATSE/CGRO team; and R. Remillard and W. Cui, on behalf of the ASM/RXTE team, report: The blackhole candidate Cyg X-1 is returning to the hard state, following the recent hard x-ray flux increase (IAUC 6447). Hard x-ray (20-200 keV) and soft x-ray (1.3-12 keV) fluxes are very close to the values before the hard-to-soft-state transition in mid-May 1995 (IAUC 6405). Spectral hardening has also been observed in both bands. The ASM (1.3-12 keV) and BATSE (20-200 keV) long-term lightcurves are anticorrelated, with the total 1.3- to 200-keV luminosity remaining approximately unchanged within 15 percent during the entire state transition from mid-May to late August 1996. The BATSE and ASM soft-state power-density spectra (PDS) of Cyg X-1 (roughly 10E-7 to 10E-4 Hz), which can be approximately represented by a power law with an index of approximately -1.0, are apparently a low-frequency extension of the same soft-state PDS detected by PCA/RXTE above 10E-3 Hz (Cui et al., submitted to Ap.J. Lett.). Flaring episodes separated by approximately 45-50 days and of duration 5-30 days were observed superimposed on the spectral transition in both ASM and BATSE data. (The mid-1991 to early-1996 BATSE lightcurve for Cyg X-1 does not show evidence of any significant periodicity near 50 days.) A possible 15-day delay of the flares in the ASM band with respect to the flares in the BATSE band was also detected from the cross correlation between ASM and BATSE lightcurves.

Cygnus X-2

A. P. Smale, NASA/Goddard Space Flight Center and Universities Space Research Association; E. Kuulkers, Astrophysics Division, ESTEC, European Space Agency; and R. A. D. Wijnands, Astronomical Institute 'Anton Pannekoek', University of Amsterdam, report: The public RXTE ASM lightcurve (covering the interval 1996 Feb. 22-Aug. 8) of the 9.8-day low-mass x-ray-binary Cyg X-2 shows a pronounced intensity modulation with a period of 77.0 +/- 1.0 days and full amplitude of 40 percent. A shallower (20 percent) secondary minimum is also observed. The spectral hardness and source intensity are anticorrelated on this cycle. We find that this long-term periodicity can be detected independently in the HEASARC archival datasets from Vela 5B (see also Smale and Lochner 1992, Ap.J. 395, 582) and the Ariel V ASM. From these three datasets we derive a single overall ephemeris from 1969 to 1996 of HJD 2442202.5 (+/- 5.0) + 77.13 (+/- 0.05) E (where phase zero is the primary minimum). The occurrence of different intensity states in the Einstein, EXOSAT and Ginga data from Cyg X-2 (Kuulkers et al. 1996, A.Ap. 311, 197) is also consistent with this 77.1-day periodicity, which we tentatively associate with the precession of a tilted accretion disk in the system.

4U 1735-444

R. A. D. Wijnands and M. van der Klis, Astronomical Institute 'Anton Pannekoek', University of Amsterdam; J. van Paradijs, University of Amsterdam and University of Alabama in Huntsville; W. H. G. Lewin, Massachusetts Institute of Technology; F. K. Lamb, University of Illinois, Urbana/Champaign; B. Vaughan, California Institute of Technology; E. Kuulkers, Astrophysics Division, ESTEC, European Space Agency; and T. Augusteijn, European Southern Observatory, report: RXTE observations of 4U 1735-444 performed on Aug. 1.60-1.68 UT show a single kHz quasiperiodic oscillation at frequency 1149 +/- 3 Hz. The power spectral peak width is 30 +/- 10 Hz and the root-mean-square amplitude 3.1 +/- 0.3 percent (0.24- 18.37 keV). This is the seventh example of kHz oscillations in a low-mass x-ray binary.

Cygnus X-1

W. Cui, Center for Space Research, Massachusetts Institute of Technology; W. Focke, University of Maryland and Goddard Space Flight Center (GSFC); and J. Swank, GSFC, report: The ASM lightcurve indicates that Cyg X-1 is still in the high state, and its 1.3-12-keV flux has been varying between about 0.8 and 1.5 Crab since the transition (IAUC 6404). So far, twelve snapshots (public) of the source have been made with RXTE. In the PCA band (2-60 keV), the x-ray flux varied between about 0.4 and 1 Crab. After an apparent 'settling period' (covered by the first three observations), both the energy spectrum and power-density spectrum (PDS) seem to be stabilized. The energy spectrum can be described by a model consisting of a soft-blackbody component (kT about 0.3 keV) and a broken power law whose photon index breaks from about 2.9 to 2.0 at about 11 keV. The PDS can be characterized by a power law, with a slope of about -1 (i.e., 1/f), below about 16 Hz and a steeper one above, with a slope of about -2. The observation made on June 17.332-17.380 UT shows the presence of quasiperiodic oscillations (QPOs) centered at about 52 mHz (Lorentzian width about 19 mHz), only when the source reaches a flux peak at about 1 Crab. The integrated fractional rms of the QPO is about 8, 11, and 10 percent in the energy bands 2-6.5, 6.5-13.1, and 13.1-60 keV, respectively. It is not clear if this QPO is related to the 40-mHz QPO reported for the low state (IAUC 5576, 5580).

KS 1731-260

E. H. Morgan and D. A. Smith, Massachusetts Institute of Technology, report: RXTE observations of KS 1731-260 performed on July 14.10-14.29 UT show persistent emission of 250 mCrab and a single type-I burst with a peak emission of 1.5 Crab at July 14.18. The total burst duration is 30 s, with a FWHM of 6 s in the band 2- 60 keV. During the peak of the burst, a periodic signal at 524 Hz is detected. This signal is only seen for two seconds, starting 2 s after the rising edge of the burst. The pulse fraction is 12 +/- 2 percent. This 524-Hz signal is similar to the 363-Hz signal found only during bursts from 4U 1728-34 (IAUC 6387). No periodic or quasiperiodic signals are detected in the persistent emission or in the tail of the burst. We speculate that we have detected the rotation frequency of the neutron star, in which case the spin period is 1.91 ms.

GRS 1915+105

B. Thomas, Hughes STX and NASA Goddard Space Flight Center (GSFC); R. Corbet, Universities Space Research Association and GSFC; J. Swank, GSFC; W. Focke, University of Maryland and GSFC; and F. Marshall, GSFC, report: Observations made of GRS 1915+105 on July 11.357-11.397 UT with the RXTE PCA show the presence of strong x-ray (1-12 keV) quasi-periodic oscillations (QPOs) at 2.64 Hz (rms amplitude 9.2 +/- 0.3 percent, Lorentzian width 0.25 Hz) with a weaker feature at 5.27 Hz (3.3 +/- 0.4 percent, width 2 Hz). We note that the previously observed QPO between 0.5 and 100 mHz and at 67 Hz (IAUC 6392) were not apparent in this observation. This change in properties may be related to the recent radio outburst from this source (IAUC 6432). The flux during this observation was about 700 mCrab and the variability < 10 percent during the observation interval.

4U 1636-536 and 4U 0614+091

M. van der Klis, Astronomical Institute "Anton Pannekoek", University of Amsterdam; J. van Paradijs, University of Amsterdam and University of Alabama in Huntsville; W. H. G. Lewin, Massachusetts Institute of Technology; F. K. Lamb, University of Illinois, Urbana/Champaign; B. Vaughan, California Institute of Technology; E. Kuulkers, Astrophysics Division, ESTEC, European Space Agency; and T. Augusteijn, European Southern Observatory, report: RXTE observations of 4U 1636-536 performed on Feb. 28.24-28.35 UT show two simultaneous kHz quasiperiodic oscillations (QPOs) at frequencies of 898.7 +/- 1.4 and 1171 +/- 11 Hz. Power spectral peak widths are 28 +/- 5 and 88 +/- 26 Hz (FWHM), respectively, and root-mean-square amplitudes 6.6 +/- 0.2 and 6.1 +/- 0.7 percent (7-20 keV). The higher-frequency peak is the fastest x-ray QPO yet reported. Two RXTE observations of 4U 0614+091 (cf. IAUC 6426) that were performed on Mar. 16.35-16.46 and Apr. 13.41-13.54 each show a single kHz QPO peak, with frequencies, FWHMs, and rms amplitudes of 727 +/- 5 Hz, 137 +/- 18 Hz, and 15.8 +/- 0.8 percent; and 630 +/- 11 Hz, 126 +/- 27 Hz, and 16.5 +/- 1.5 percent (2-20 keV), respectively.

4U 0614+091

E. Ford, P. Kaaret, and M. Tavani, Columbia University; B. A. Harmon, Marshall Space Flight Center (MSFC), NASA; S. N. Zhang, MSFC and Universities Space Research Association; and D. Barret, P. Bloser, and J. Grindlay, Harvard-Smithsonian Center for Astrophysics, report the detection of high-frequency, quasiperiodic oscillations (QPOs) from the x-ray burster 4U 0614+091 with the Rossi X-ray Timing Explorer: Using 125- microsecond-resolution PCA data, two peaks are apparent in the power-density spectrum with frequencies near 900 and 570 Hz. The frequency of each peak is approximately linearly proportional to the total countrate, varying from 750 to 950 Hz (high-frequency peak) and 400 to 600 Hz (low-frequency peak) as the countrate varies from 750 to 850 counts/s (full PCA). The frequency difference between the two QPOs is constant at 327 +/- 4 Hz. The relative widths (FWHM/nu) are constant at 15 and 6 percent, respectively. From gaussian fits, the amplitude above the poisson noise appears constant. A feature at 326 +/- 8 Hz with marginal significance is present during one 30-min interval. This feature, if confirmed, may be direct evidence for a 3.0-ms spin period of the neutron star.

QPO in Sco X-1

M. van der Klis and R. Wijnands, Astronomical Institute "Anton Pannekoek", University of Amsterdam; W. Chen, University of Maryland at College Park and Goddard Space Flight Center, NASA; F. K. Lamb and D. Psaltis, University of Illinois at Urbana-Champaign; E. Kuulkers, Astrophysics Division, European Space Agency and ESTEC; W. H. G. Lewin, Massachusetts Institute of Technology; B. Vaughan, California Institute of Technology; J. van Paradijs and S. Dieters, University of Alabama at Huntsville and University of Amsterdam; and K. Horne, University of St. Andrews, report: Rossi X-ray Timing Explorer observations of Sco X-1 performed on May 25.0-25.2 UT show two kHz quasiperiodic oscillations (QPOs) similar to those observed in Feb. (IAUC 6319; van der Klis et al., Ap.J. Lett., submitted). Sco X-1 is at a lower accretion rate (as evidenced by the absence of 6-Hz QPO) than in the Feb. observations and the QPOs are at lower frequency (634 +/- 2 and 926 +/- 1 Hz). Their frequency difference is 292 +/- 2 Hz, significantly larger than that in February (247 +/- 3 Hz). This implies that the beat- frequency interpretation proposed for the two kHz QPOs in 4U 1728-34 (IAUC 6320, 6387; Strohmayer et al., Ap.J. Lett., submitted), where one QPO is due to the Keplerian rotation of the inner edge of the accretion disk and the other to the beat of this same Keplerian frequency with the neutron-star spin, can, for Sco X-1, be excluded.

New X-ray transient in the Large Magellanic Cloud

F. E. Marshall, Laboratory for High Energy Astrophysics, Goddard Space Flight Center (LHEA/GSFC); and R. Corbet and T. Strohmayer, Universities Space Research Association and LHEA/GSFC, report the discovery of a new hard-x-ray source in the direction of the LMC using the RXTE's Proportional Counter Array. The 2- to 10-keV flux on June 8.19 and 11.31 UT was about 18 mCrab (luminosity1.3 x 10E38 erg/s at the distance of the LMC). The best-fit position is R.A. = 5h22m.5 +/- 0m.2, Decl. = -69o37' +/- 9' (equinox 2000.0). An absorption event on June 11.3154 lasting about 100 s suggests a high-inclination binary system. No pulsations were seen, with an upper limit on the pulsed fraction of 1 percent for frequencies between 0.1 and 40 Hz. Additional x-ray observations are urged to establish an accurate position and possibly to determine a binary period.

GRO J1744-28

J. M. Kommers, R. E. Rutledge, D. W. Fox, W. H. G. Lewin, and E. H. Morgan, Massachusetts Institute of Technology; C. Kouveliotou, Universities Space Research Association; and J. van Paradijs, University of Alabama in Huntsville and University of Amsterdam, report: We found strong quasiperiodic oscillations (QPOs) with a frequency of about 0.4 Hz in a burst from GRO J1744-28 observed on about June 4.617 UT with the PCA instrument onboard the Rossi X-ray Timing Explorer. The burst was the sixth of seven seen on June 4. The oscillations are clearly visible in the lightcurve during the last 25 s of the 40-s burst. A power spectrum of these 25 s shows a peak near 0.4 Hz, with FWHM < 0.08 Hz (the upper limit being set by our frequency resolution) and a fractional rms amplitude of the mean burst flux during these 25 s (in excess of the quiescent flux) of 25 +/- 5 percent (not corrected for deadtime). We have also identified QPOs with frequencies in the range 0.3-0.5 Hz in the emission following at least 10 of 82 bursts observed between late January and late April. These QPOs typically lasted for about 30 s, although in one burst they lasted as long as 80 s; the fractional rms amplitudes exceeded 15 percent in some cases.

GRO J1744-28

K. Jahoda, Goddard Space Flight Center (GSFC); T. Strohmayer and R. Corbet, GSFC and Universities Space Research Association; and M. Stark, GSFC and University of Maryland, report: GRO J1744-28 resumed bursting activity between May 29 and June 1. The Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA) observed three bursts during a 151-min observation on June 1, and seven bursts during a 179-min observation on June 2. The average power spectrum of the June 2 bursts shows 6-percent rms variability at the 2.1-Hz pulse period of GRO J1744-28, indicating that these bursts come from this source, and the peak power at 2.1 Hz reaches 10 percent rms. The brightest burst was 9000 counts/s (0.7 Crab). All of these bursts have rise times less than a few seconds; while most of the bursts have durations of 20-30 s, some continue up to 100 s. The burst spectrum can be characterized by a hard power-law (photon index 1.3) cutoff at about 20 keV with a 10-keV e-folding energy, similar to the spectrum previously measured for GRO J1744-28. While a nearby source hampers measurements of the quiescent flux, the nonbursting pulsed flux is 13 +/- 0.5 counts/s, an increase of at least a factor of two since May 29. This suggests that the quiescent flux has also increased. RXTE will continue to observe GRO J1744-28 regularly.

May 27, 1996 - IAUC 6409

MXB 1730-335

W. H. G. Lewin, E. Morgan, D. W. Fox, and R. Rutledge, Massachusetts Institute of Technology; L. Bildsten, University of California, Berkeley; T. Dotani, Institute of Space and Astronautical Science; L. Lubin, Carnegie Observatories; M. van der Klis, Astronomical Institute 'Anton Pannekoek', University of Amsterdam (UA); and J. van Paradijs, University of Alabama in Huntsville and UA, report: Following detection of the rapid burster MXB 1730-335 with the RXTE satellite's All-Sky Monitor (IAUC 6390), we observed the source on May 3, 7, and 13 with its PCA (2-60 keV). On the first two occasions, it exhibited type-II bursts (spasmodic accretion) with durations of 10-15 s and recurrence times of roughly 100 s (May 3) and 400 s (May 7). These bursts exhibited the usual 'ringing' during their decay (cf. Tan et al. 1991, MNRAS 251, 1). Detailed inspection of the burst power spectra also shows 5- to 7-Hz quasiperiodic oscillations on May 3 but not on May 7. No coherent power is seen above the 99.9-percent confidence threshold between 10 and 2048 Hz on either date. For the first two observations, we can place an upper limit of 44 mCrab on any persistent emission from the rapid burster (assuming a Crab- like spectrum). On the third observation, no bursts were observed, and our upper limit for any persistent emission is 21 mCrab. Quick-look results from the ASM team indicate that the source is no longer active.

May 24, 1996 - IAUC 6406

GRO J1655-40

K. Horne, E. T. Harlaftis, and R. Baptista, University of St. Andrews; C. Hellier and A. Allan, Keele University; H. Johnston, Anglo-Australian Observatory; J. Patterson and J. Kemp, Columbia University; C. Haswell, University of Sussex and Columbia University; and W. Chen, Goddard Space Flight Center, report on the optical/ultraviolet brightening of GRO J1655-40 during the recent x-ray outburst (IAUC 6393): Cerro Tololo photometry on May 10.41 UT gave Johnson magnitudes U = 17.0, B = 16.8, V = 15.4, R = 14.4, I = 13.4, which fit a power-law spectrum with slope 1/3, reddened by E(B-V) = 1.5 mag. Anglo-Australian Telescope spectra obtained on May 10.48, 11.57, and 12.57 confirm the red continuum slope, with emission from He II at 468.6 nm and CIII/NIII at 464.0 nm (FWHM = 2 nm, EW = 0.2 nm), and broad absorption filled in by multi-component emission at H-beta and H-alpha. Simultaneous HST/XTE data on May 13.9-14.3 show 5- to 10-percent variations on timescales of 2 s-10 min. Deep 220-nm absorption in the HST spectrum suggests E(B-V) = 1.3 mag. HST/XTE observations were planned for May 20.59-20.80, 27.8-28.1, June 8.53, and two additional times in June.

May 21, 1996 - IAUC 6404

Cygnus X-1 AND GRS 1915+105

W. Cui, Center for Space Research, Massachusetts Institute of Technology, reports for the RXTE/ASM team: Observations with the RXTE All Sky Monitor (ASM) reveal that Cygnus X-1 is in a bright flaring state. It was measured at about 0.5 Crab (2-12 keV) before it started to brighten on May 10. The intensity reached about 1 Crab on May 17 and about 2 Crab on May 19. Another blackhole candidate, GRS 1915+105, has also become very bright. Early this year, this source was in a highly variable state (IAUC 6392). Its peak-to-peak amplitude became about a factor of 2 smaller between Apr. 7 and May 17, and its mean brightness decreased linearly. It was measured at about 0.8 Crab on May 17 and has since brightened, reaching about 3 Crab on May 19.

May 14, 1996 - IAUC 6399

GRO J1655-40

R. Corbet, Goddard Space Flight Center (GSFC) and Universities Space Research Association; and K. Jahoda, GSFC, report on behalf of the RXTE Science Operations Facility and PCA team: We continue to detect significant emission from GRO J1744-28, and the following approximate quiescent flux levels were measured in recent observations: Apr. 26, 100 mCrab, 116 min observation time; 28, 95 mCrab, 52 min; 30, 90, 100; May 3, 70, 105; 10, 60, 33; 12, 50, 58; 13, 50, 75. During these observations the following bursts were detected: Apr. 26.069 UT, 890 mCrab; 28.656, 120; May 3.785, 100; 12.688, 700. The May 12 burst had high absorption and timing and spectral characteristics typical of a type I burst. Owing to the 1-deg PCA field of view we cannot yet completely rule out the possibility that the lower-amplitude bursts might be due to another contaminating nearby source. Pulsations at 2.1 Hz were evident during all observations, although since May 12 the pulsed fraction has decreased to below 1 percent rms. Lightcurves are available under 'TOO News' at http://heasarc.gsfc.nasa.gov/docs/xte/xte_1st.html/.

May 8, 1996 - IAUC 6393

GRO J1655-40: recurrence of bright X-ray emission

R. Remillard, H. Bradt, W. Cui, A. Levine, E. Morgan, B. Shirey, and D. Smith, Massachusetts Institute of Technology, report: Observations with the RXTE All Sky Monitor (ASM) reveal a recent recurrence of bright x-ray emission from GRO J1655-40, a dynamical blackhole binary (Bailyn et al. 1995, Nature 378, 157) that began x-ray eruptions during 1994 and became a source of relativistic radio jets (Tingay et al. 1995, Nature 374, 141). Combining the ASM measurements at 2-12 keV in 1-day time bins (UT), we see an upper limit of 12 mCrab (3 sigma) on 1996 Apr. 23, followed by detections of 71 +/- 15 mCrab on Apr. 25, 420 +/- 30 mCrab on Apr. 26, 586 +/- 15 mCrab on Apr. 27, 802 +/- 9 mCrab on Apr. 28, and 1077 +/- 30 mCrab on Apr. 29. Between Apr. 30 and May 5, the x-ray flux ranges between 1.3 and 1.5 Crab. Observations at all wavelengths are strongly encouraged.

May 8, 1996 - IAUC 6393

Comet C/1996 B2 (Hyakutake)

F. E. Marshall, C. Lisse, M. Mumma, and R. Petre, NASA Goddard Space Flight Center; and K. Dennerl, J. Englhauser, and J. Trumper, Max-Planck-Institut fur Extraterrestrische Physik, Garching, report: The x-ray emission from comet C/1996 B2 is primarily at energies < 2 keV. No evidence for emission in the band 2-10 keV was seen during a fortuitous scan lasting 24 s across the comet on Mar. 31.2836 UT with the Proportional Counter Array onboard the Rossi X-ray Timing Explorer. The 90-percent-confidence upper limit for the flux in the band 2-10 keV, assuming a power-law spectrum with photon index 2.0, is 1.6 x 10E-11 erg sE-1 cmE-2. The limit is 1.7 x 10E-11 erg sE-1 cmE-2 for an index of 3.0. During this observation, the comet was 0.22 AU from the earth and 0.91 AU from the sun. Assuming the comet's luminosity in the band 0.1-2 keV was the same as during the non-flaring intervals of the ROSAT observation on Mar. 26-28 (IAUC 6373), we can rule out spectral shapes harder than a power law with photon index 2.0 or thermal bremsstrahlung emission with a temperature of 2.5 keV. For both these assumed spectral shapes, > 50 percent of the 0.1- to 10-keV luminosity is at energies < 1.25 keV.

May 2, 1996 - IAUC 6392

GRS 1915+105

E. Morgan and R. Remillard, Massachusetts Institute of Technology; and J. Greiner, Max-Planck-Institut fuer Extraterrestrische Physik, Garching, communicate: Recent RXTE observations have shown that GRS 1915+105, a galactic x-ray transient (IAUC 5590) with relativistic radio jets (Mirabel and Rodriguez 1994, Nature 371, 46), is bright and highly variable in the range 2-25 keV. More than 800 measurements from the All Sky Monitor (ASM) at 2-12 keV during 1996 Feb. 20-Apr. 26 show large intensity variation from 0.2 to 2.0 Crab. The ASM captured several episodes of quasiperiodic oscillations (QPOs) with timescales of 30-40 s and amplitudes as large as 1 Crab. Five PCA observations taken in April show many narrow QPO features between 0.5 and 100 mHz with complex spectral and dynamic signatures and large amplitudes. There is a trend toward smaller amplitude and higher frequency variability, although the average intensity has remained about 1.5 Crab. In the two most recent PCA observations, we find a 67-Hz QPO. If this feature is related to the frequency of the minimum stable Keplerian orbit around a Schwartzschild blackhole, then the implied blackhole mass is 33 M(sun). Radio Observations are urged to look for new episodes of superluminal mass ejection. RXTE will continue to monitor this source.

May 1, 1996 - IAUC 6390

GRS 1739-278

T. Takeshima, J. K. Cannizzo, and R. Corbet, Goddard Space Flight Center (GSFC) and Universities Space Research Association; and F. E. Marshall, GSFC, report: The Rossi X-ray Timing Explorer (RXTE) observed GRS 1739-278 during Mar. 31.754-31.862 UT. The pointing position was slightly offset (to R.A. = 17h42m05s, Decl. = -27o37'.2, equinox 2000.0) in order to reduce possible contamination from the nearby bright source GRO 1744-28. The source flux in the PCA band (2-60 keV) was 490 mCrab, corrected for the offset from the source, and the spectrum is very soft. There were no detectable oscillations with periods between about 0.002 and about 256 s. In addition, GRS 1739-278 was detected during several slews between other targets, and the following fluxes were found: Mar. 16, 810 mCrab; 22, 430; Apr. 21, 300. Errors on the fluxes from the slew data are conservatively estimated to be 10 percent. After standard processing, data from the pointed observation of GRS 1739-278, as well as other public domain XTE observations, are available over the Internet. Information on how to obtain these data can be found at http://heasarc.gsfc.nasa.gov/docs/xte/xte_1st.html/ (see 'TOO News' for a spectrum and lightcurve).

May 1, 1996 - IAUC 6390

X-ray sources

A. M. Levine, H. Bradt, D. Chakrabarty, W. Cui, J. G. Jernigan, E. Morgan, R. Remillard, R. Shirey, and D. A. Smith, Center for Space Research and Department of Physics, Massachusetts Institute of Technology, report that five galactic and Large Magellanic Cloud x-ray sources, in preliminary results from the RXTE All Sky Monitor (ASM) data, appear to have intensified in the band 2-10 keV since late March. The blackhole candidate LMC X-3 has increased from about 15 mCrab in late March to about 40 mCrab. The recurrent transient 4U 1630-47 intensified from < 20 mCrab to about 200 mCrab around Mar. 20 and remains steady at that level. The strength of burster and QPO source 4U 1705-44 has grown linearly with time from Mar. 20 to Apr. 28, at which time it had a flux of about 200 mCrab. The rapid burster MXB 1730-335 suddenly appeared in outburst around Apr. 18, when it appeared to peak at about 200 mCrab; it has weakened since the peak to a level of about 100 mCrab. Finally, the ASM also detects GRS 1739-278 (IAUC 6348) at a level of about 250 mCrab in early Mar., reaching a peak of about 800 mCrab around Mar. 20, and declining to an intensity of about 350 mCrab in late April.

April 30, 1996 - IAUC 6389

4U 1630-47: outburst detected

F. E. Marshall, Laboratory for High Energy Astrophysics, Goddard Space Flight Center, reports the detection of the recurrent ultrasoft x-ray transient 4U 1630-47 in outburst: 4U 1630-47 is thought to be a blackhole x-ray transient system because of its spectrum and the lack of detected pulsations or bursts. A flux in the band 2-10 keV of 0.26 times that of the Crab was observed on Apr. 23.794 UT during a brief scan with the Proportional Counter Array (PCA) onboard the Rossi X-ray Timing Explorer. The flux has increased from 0.004 Crab on Feb. 16.428 and 0.08 Crab on Mar. 12.885. The latest observation occurs at phase 0.29 of the 602-day period for outbursts from this source reported by Parmar et al. (1995, Ap.J. 452, L129). The current outburst is thus unusually delayed from the time expected from previous outbursts. No optical counterpart has been identified for this source, whose position was measured during an earlier observation with the HRI on the Einstein Observatory. A finding chart is given in Parmar et al. (1986, Ap.J. 304, 664). Other blackhole transients also flare at radio wavelengths during x-ray outbursts (e.g., GRS 1915+105, IAUC 6269).

April 26, 1996 - IAUC 6387

4U 1728-34: oscillations in bursts

T. Strohmayer, W. Zhang, A. Smale, and C. Day, Universities Space Research Association (USRA) and the Laboratory for High Energy Astrophysics, Goddard Space Flight Center (LHEA/GSFC); J. Swank, LHEA/GSFC; L. Titarchuk, George Mason University and LHEA/GSFC; and U. Lee, Tohoku University, report: Of the seven bursts that we have observed from 4U 1728-34 during a recent campaign with RXTE, five show oscillations with a frequency of 363 Hz. In the first few seconds of two of the seven bursts, the oscillations do not appear to be coherent. The oscillation frequency drifts by as much as 0.5 Hz. During the other three bursts, the coherence of the oscillations is only limited by the durations of the bursts. The fractional rms of the oscillations ranges from 1.5 to 5 percent. We have also found in the same data set two simultaneously-present kHz quasiperiodic oscillations (QPOs), one of which has been reported earlier (IAUC 6320). The centroid frequencies of the two QPOs change with intensity and time, but their difference appears to be always near 363 Hz. These observations are consistent with a neutron star spin period of 2.75 ms.

April 21, 1996 - IAUC 6385

GRO J1744-28: source continues to burst as flux declines

A. B. Giles, NASA Goddard Space Flight Center and Universities Space Research Association, and the PCA instrument team report: The PCA experiment on RXTE has now observed the galactic-center transient GRO J1744-28 (cf. IAUC 6291) on 29 occasions. The source behavior continues as reported before (IAUC 6338), with no sign of the large bursts ceasing. On Apr. 19, the nonburst flux was about 225 mCrab (2-60 keV) or about 11 percent of its late-Jan. level. The rate of decline of the nonburst flux has decreased a little, but the flux has followed a smooth decrease given by flux(mCrab) about 2592 - 22D, where D is the day number in 1996. If the present trend were to continue, the nonburst flux would be close to PCA background levels by early May. The decline in the integrated burst flux has continued to lag behind that for the nonburst flux, and a linear projection places it near zero in late May.

April 4, 1996 - IAUC 6371

GRO J1744-28: BATSE and XTE pulse fraction

J. Kommers and W. H. G. Lewin, Massachusetts Institute of Technology; C. Kouveliotou and M. H. Finger, Universities Space Research Association; G. J. Fishman, C. A. Wilson, and R. B. Wilson, Marshall Space Flight Center; M. S. Briggs, University of Alabama in Huntsville (UAH); and J. van Paradijs, University of Amsterdam and UAH, report on the 2.1-Hz pulsations: For a sample of 61 bursts from GRO J1744-28 observed by BATSE over a 3-month period (Jan.-Mar.), the average value of the pulsed fraction within bursts in the range 20-60 keV is 14 +/- 1 percent (rms variation). The average value for the persistent emission observed by BATSE (covering the period Jan. 10-16) is 16.8 +/- 0.3 percent (rms) in the energy range 20-40 keV. By comparison, the pulse fraction in the persistent emission observed by the PCA instrument (M. J. Stark, private communication) onboard XTE on Feb. 19 is 8 percent (rms) in the range 2-12.5 keV and 12 percent (rms) for E > 12.5 keV. No background subtraction or correction for the PCA response was performed. The pulse fraction in the upper energy range is therefore an underestimate. Dotani et al. (ASCA) report a 'peak- to-peak' pulsed fraction in the persistent emission of 10 percent in the band 2-10 keV (IAUC 6337, and T. Dotani, private communication). For comparison with the numbers given above, we note that this converts to a pulse fraction of roughly 3.5 percent (rms). [For an approximately sinusoidal pulse shape, as is the case here, the peak-to-peak pulse fraction is 2(2)E0.5 times the rms pulse fraction.] From the combined BATSE-XTE-ASCA observations, it is very clear that there is a strong energy dependence of the pulsed fraction from GRO J1744-28.

March 8, 1996 - IAUC 6338

GRO J1744-28: XTE sees decrease in persistent emission flux; micro- and spiky- bursts

B. Giles and T. Strohmayer, Goddard Space Flight Center and Universities Space Research Association, report: The PCA experiment onboard XTE has now observed the galactic-center transient GRO J1744-28 (IAUC 6291) fourteen times for 1698 min during the period Jan. 18-Mar. 5. The nonbursting flux below 15 keV shows a linear decline since late January, which if continued will reach zero about 1 week into April. This is a slower decrease than reported by BATSE for 20-100 keV (IAUC 6335). The flux in the main bursts is also decreasing, but at a slower rate, and will if continued reach zero about 40-50 days later. A total of 50 large bursts has been seen (6- to 8-fold increase over the persistent emission, 10-15 min exponential recovery). In addition, at least 5 'mini-bursts' have occurred, where the flux increases by a factor of 2 and the recovery takes 20 s or less. The mini-burst duration appears slightly less than the more frequent, larger bursts. Many even smaller and shorter burst events often seem to be clustered in the few minutes prior to a large burst and are good predictors of the main events; these 'micro-bursts' last 1-2 s and show a 20- to 50-percent increase in countrate over the mean persistent rate. Finally, there are occasions when the flux appears to be primarily comprised of numerous spiky bursts of duration about 1 s. This spiky variability is substantial compared to the statistical error over the 0.1- to 2-s timescale and to the about 10-percent modulation at 2.2 Hz. The appearance of the similar bursting behavior (burst followed by a recovery in the flux) over such a wide range of amplitudes argues that a single phenomenon, such as accretion of plasma blobs of varying mass, may account for essentially all the source emission.

March 6, 1996 - IAUC 6336

4U 1608-52: detection of high-frequency QPO

J. van Paradijs, University of Alabama in Huntsville and University of Amsterdam; W. Zhang, Universities Space Research Association (USRA) and Goddard Space Flight Center (GSFC); F. Marshall and J. H. Swank, GSFC; T. Augusteijn, European Southern Observatory; E. Kuulkers, European Space Agency; W. H. G. Lewin, Massachusetts Institute of Technology; F. Lamb, University of Illinois; I. Lapidus, University of Cambridge; J. Lochner and T. Strohmayer, USRA/GSFC; M. van der Klis, University of Amsterdam; B. Vaughan, California Institute of Technology; report: Following the recent outburst of 4U 1608-52 (IAUC 6331), we have observed this low-mass x-ray binary for 3 hr on Mar. 3 with the Rossi XTE (total exposure about 2 hr). The source showed strong quasi- periodic intensity oscillations (QPOs) throughout the observation. In the beginning, the QPOs were observed with a centroid frequency of 850 Hz, a FWHM of 6.8 Hz, and a fractional rms of 6.5 percent. Twenty minutes into the observation, the centroid frequency started increasing and 15 min later reached 890 Hz, with a fractional rms value of 5.2 percent and a FWHM of 15 Hz. Then the centroid frequency started decreasing through the end of the observation, when it reached 828 Hz with a fractional RMS of 6.8 percent and a FWHM of 6.7 Hz. In the course of the observation, the source intensity in the XTE/PCA band (2-60 keV) showed a slight overall decrease from 3200 counts/s in the beginning to 3100 counts/s in the end. The excursion to higher frequency did not correlate with any change in intensity. With earlier detections of kHz QPOs in the source 4U 1728-34 (IAUC 6320) and the Z source Sco X-1 (IAUC 6319), our results suggest that kHz QPO are a regular feature of low-mass x-ray binaries.

March 4, 1996 - IAUC 6334

GRO J1744-28: Correction and further report on pular phase lag observed by XTE

M. J. Stark, University of Maryland at College Park, and the Laboratory for High Energy Astrophysics, Goddard Space Flight Center (LHEA/GSFC); A. Baykal, National Research Council and LHEA/ GSFC; T. Strohmayer, Universities Space Research Association and LHEA/GSFC; and J. H. Swank, LHEA/GSFC, write: We report a correction to IAUC 6324 and confirmation with XTE of the result reported by Jung et al. on IAUC 6321 . On IAUC 6324 , we reported an incorrect direction of the phase shift in the pulsar arrival times of GRO J1744-28. Following each of the x-ray bursts, we reported as an advance in the pulse arrival time what is actually a lag. We reported the correct magnitude (25 ms) and duration (onset < 25 s; decay about 1500 s). Combining the data from seven bursts observed on Jan. 18, we have confirmed a lag during the bursts in pulse-arrival times of 15 percent. This lag persists for the duration of the burst (8-10 s) in the majority of pulses, which keep the same shape. For a few periods at high intensity, the pulse is not present or is doubly peaked. After the burst, the pulse-arrival time lag drops to 5 percent within a few seconds and decays further in about 1500 s, as we reported earlier.

March 1, 1996 - IAUC 6331

4U 1608-52: detection of outburst

F. E. Marshall, Laboratory for High Energy Astrophysics, Goddard Space Flight Center (LHEA/GSFC); L. Angelini, Universities Space Research Association and LHEA/GSFC; and the X-ray Timing Explorer (XTE) Science Operations Center Team report the detection of the recurrent soft-x-ray transient 4U 1608-52 (cf. IAUC 2859, 3090) in outburst: 4U 1608-52 is a low-mass x-ray binary that also exhibits type-I x-ray bursts. During a brief scan of the Galactic plane with the Proportional Counter Array (PCA) onboard XTE, 4U 1608-52 was seen on Feb. 23.883 UT with a countrate of about 5700 counts/s (about 44 percent of the rate for the Crab). The 3- to 20-keV spectrum of the source can be qualitatively described by a thermal bremsstrahlung model with kT = 5 keV and absorption with column density NH = 3 x 22 atoms cmE-2. About ten outbursts from this source have been observed since 1970 (Lochner and Roussel- Dupre 1994, Ap.J. 435, 840). Flaring from the proposed optical counterpart (QX Nor; V = 18.2) was seen during the 1977 outburst (Grindlay and Liller 1978, Ap.J. 220, L127). This neutron-star transient is of particular interest because it sometimes shows some characteristics similar to the blackhole candidate Cyg X-1. Observations at other wavelengths and by other x-ray observatories are encouraged.

February 23, 1996 - IAUC 6324

GRO J1744-28: XTE observes Pulsar Phase Jumps Associated with the Bursts

M. J. Stark, University of Maryland and the Laboratory for High Energy Astrophysics, Goddard Space Flight Center (LHEA/GSFC); A. Baykal, National Research Council and LHEA/GSFC; T. Strohmayer, Universities Space Research Association and LHEA/GSFC; and J. H. Swank, LHEA/GSFC, communicate: We have detected, with the Proportional Counter Array (PCA) onboard the X-ray Timing Explorer (XTE), glitches in the arrival times of x-ray pulses from GRO J1744-28 that occur during the x-ray/gamma-ray bursts seen from this source. Timing of the pulses before and after the burst events shows a clear advance in the pulse arrival times of about 25 ms (5 percent of the 0.467-s period). The advance occurs within a time interval no longer than 25 or 30 s, after which the advance decays. The functional form of this decay is not yet precisely known, but the best-fit of an exponential function to it has a time constant of about 500 s. This behavior is exhibited during all bursts so far observed from this source by XTE, and these pulse timing considerations have been used to identify additional bursts that occurred when the PCA was not operating. The flux from GRO J1744-28 is depressed slightly following a burst but recovers to its pre-burst level within a few hundred seconds, while the phase shift recovers in about 1500 s. The pulse shapes before and after the bursts are very similar. Interpreted in terms of a fractional change in the angular velocity, the short time during which the advance occurs indicates such a change of at least 10E-3, whereas the largest such change seen in radio-pulsar glitches is 10E-6. If 10E20 g of material with the angular momentum that is characteristic of the co-rotation radius (10^8 cm) are accreted, the fraction of the star's moment of inertia participating in the glitch is about 10E-6, equivalent to the moment of inertia of a thin shell containing 10E-6 of the neutron-star mass. This is much less than the fraction of the moment of inertia thought to participate in radio-pulsar glitches.

February 20, 1996 - IAUC 6320

4U 1728-34: detection of high-frequency QPO

T. Strohmayer and W. Zhang, Universities Space Research Association (USRA) and the Laboratory for High Energy Astrophysics, Goddard Space Flight Center (LHEA/GSFC); and J. Swank, LHEA/GSFC, report the detection with the proportional counter array (PCA) onboard the X-ray Timing Explorer (XTE) of very-high-frequency variability in the quiescent x-ray intensity from the low-mass x- ray binary 4U 1728-34: Two features, perhaps distinct, are seen in the power spectrum and appear to be correlated with the source intensity. When the PCA source countrate is 1500-1800 counts/s, a quasiperiodic oscillation (QPO) is seen with a centroid frequency of 800 Hz, FWHM about 94 Hz, and fractional rms variation in the x- ray intensity of 8.4 percent. This feature is accompanied by strong, high-frequency noise extending up to 200 Hz. When the PCA source countrate > 2100 counts/s, a much narrower power spectral feature is seen at 740-800 Hz, with a FWHM of only 8 Hz. The strength of this feature corresponds to a fractional rms variation of about 5 percent. The high-frequency noise seen in the low- countrate state is not present in the high-countrate state. Preliminary searches with data sampled at 16 microseconds do not reveal any additional significant features in the power spectrum above 1 kHz. Analysis of data segments separated in time suggests that the center frequencies of the features can vary by as much as 5-6 percent over several hours. To our knowledge, this represents the highest-frequency time variability yet seen from a neutron-star system. Explanations in terms of either keplerian frequencies or a beat-frequency model cannot yet be ruled out, although no evidence has yet been seen for a coherent pulsar frequency in the same data.In addition, we note that oscillation modes associated with the crust of a neutron star have frequencies in the range observed.

February 20, 1996 - IAUC 6319

Scorpius X-1: detection of high-frequency QPO

M. van der Klis, Astronomical Institute 'Anton Pannekoek', University of Amsterdam; J. Swank, W. Zhang, and K. Jahoda, Goddard Space Flight Center, NASA; E. Morgan and W. Lewin, Massachusetts Institute of Technology; B. Vaughan, California Institute of Technology; and J. van Paradijs, University of Alabama at Huntsville and University of Amsterdam, report: We have observed quasiperiodic oscillations (QPOs) with frequencies between 1060 and 1130 Hz in two different 2- to 60-keV observations of Sco X-1 with the X-ray Timing Explorer satellite. In the first observation, on Feb. 14.470-14.492 UT, Sco X-1 was mostly in the lower normal branch of the Z track in the x-ray color-color diagram, with clear 6- to 7-Hz, normal-branch QPOs. Power spectra of two simultaneous, statistically-independent time series (resolution 16 and 62.5 microseconds, respectively) show a narrow QPO peak at 1062 +/- 7 and 1077 +/- 8 Hz, respectively, with a FWHM of about 60 Hz and a deadtime-corrected, fractional, rms amplitude, r, of 0.74 +/- 0.10 percent. In the second observation, on Feb. 18.203-18.234, Sco X-1 changed state several times, moving along the Z track between the lower normal and flaring branches, showing 7- and 16-Hz QPOs, respectively. Using 250-microsecond data, a very narrow QPO peak is seen in the normal branch at 1089.1 +/- 0.5 Hz with FWHM 15 +/- 3 Hz and r = 0.72 +/- 0.06 percent. In the flaring branch, the peak is at 1133 +/- 7 Hz, FWHM is roughly 40 Hz, and r = 0.49 +/- 0.09 percent. In another segment of the same observation, 16- and 62-microsecond data taken in the normal branch again show similar high-frequency, comparatively-narrow peaks. All peak significances are between 3.5 and 4.5 sigma (single trial). In both observations, there is evidence that the QPOs increase in frequency along the normal branch and into the flaring branch (i.e., with mass-transfer rate), and that part of the observed peak widths is due to this motion. In a third observation, when Sco X-1 was in the flaring branch, no significant kHz peak was seen. This is the fastest QPO yet seen in an accreting compact object. The high-QPO frequency, and its increase with mass-transfer rate, suggest that we may be seeing the keplerian frequency at the inner edge of the disk near the magnetospheric boundary, or its beat frequency with a slower (about 100 Hz) pulsar. The fact that the high-frequency QPOs have a similar dependence of strength and frequency on mass-transfer rate as the well-known 6- to 20-Hz QPOs in Sco X-1 indicates that the two phenomena might be related.

February 9, 1996 - IAUC 6309

GRO J1744-28: XTE refined position

T. Strohmayer, Universities Space Research Association; and K. Jahoda and F. Marshall, Goddard Space Flight Center, NASA, communicate for the XTE science and instrument teams: Additional proportional counter array (PCA) scanning observations of GRO J1744-28, obtained since the first observations by XTE on Jan. 18- 19 (cf. IAUC 6291), have been analyzed the further to constrain the object's position. The resulting position obtained by averaging the results of seven independent determinations is R.A. = 17h44m34s.3 +/- 2s.8, Decl. = -28o45'22" +/- 47" (equinox 2000.0). The quoted uncertainties are the standard deviations among the positions determined from seven independent scanning observations. These uncertainties are consistent with our current understanding of the possible systematic errors.

February 2, 1996 - IAUC 6302

4U 1755-33: off state

M. S. E. Roberts and P. F. Michelson, Stanford University; L. R. Cominsky, Sonoma State University; and F. E. Marshall, R. H. D. Corbet, and E. A. Smith, Goddard Space Flight Center, NASA, report: On Jan. 30-31, the black-hole candidate 4U 1755-33 was observed by XTE to be in an off state for the first time. Preliminary analysis gives a conservative flux limit of < 1 mCrab, a factor of 100 below the source's typical intensity. This offers an unprecedented opportunity to perform optical spectroscopic measurements of the companion star. The optical counterpart normally appears as a faint (V about 18) blue object associated with reprocessed radiation from the accretion disk. Radial-velocity measurements over the 4.4-hr orbital period could lead to a determination of the mass function of the compact object. In addition, a detailed optical spectrum could determine the spectral type of the companion and resolve the issue of extreme underabundance of heavy metals in the accreted material.

February 1, 1996 - IAUC 6300

GRO J1744-28: QPO discovery

W. Zhang, Goddard Space Flight Center (GSFC) and Universities Space Research Association; E. Morgan, Massachusetts Institute of Technology; J. Swank and K. Jahoda, GSFC; and G. Jernigan and R. Klein, University of California at Berkeley, report: GRO J1744-28 showed quasi-periodic intensity oscillations (QPO) in the energy band 3-12 keV during an 11-hr XTE observation starting on Jan. 18.77 UT. The power spectrum in the frequency band 5-100 Hz consists of a red-noise component that can be characterized by a power law with an index of -1 and three QPO peaks centered, respectively, at 20, 40, and 60 Hz. The peaks at 20 and 60 Hz are clearly discernible, yet hard to quantify at this stage of the analysis. The peak at 40 Hz can be well characterized by a Lorentzian profile with a FWHM of 27 Hz. The total integrated power under the 40-Hz peak amounts to a fractional RMS variation of 5.6 percent (not corrected for the effect of detector deadtime). The effect of flares on the centroid frequencies and the strength of the oscillations is currently under study. It is not clear whether the standard beat frequency model can accommodate this QPO because of the high frequencies. The pulsation at 2 Hz (IAUC 6285) and the energy spectrum (IAUC 6291) imply a strong magnetic field, which probably prevents the accretion disk from extending so close to the neutron star as to have keplerian frequencies as high as the observed QPO frequencies, even if one takes into account the presumably very high luminosity.

January 21, 1996 - IAUC 6291

GRO J1744-28: X-ray detection and position

J. Swank, NASA Goddard Space Flight Center, reports, for the X-Ray Timing Explorer instrument and operations team: XTE observed the new galactic transient GRO J1744-28 (IAUC 6272 , 6275, 6276, 6284, 6285, 6286, 6290) for 11 hours on Jan. 18-19. A source pulsing at 2.1 Hz, as well as nine bursts 2-8 times brighter than the persistent emission, was observed from a position within the error box reported by Hurley et al. (IAUC 6286). The XTE best-fit position, from scans across the source, is RA: 266.13 +/- 0.06, DEC: -28.75 +/- 0.01; all in decimal degrees J2000 , where the errors are systematic. A sinusoidal pulse shape was observed, and the burst flux was sometimes strongly modulated. Following each burst, the flux is depressed and takes a few minutes to recover. The spectra of both the pulsed and the burst flux are characteristic of bright accreting pulsars; they are consistent with a power law with number index about 1.2 and a rollover above 13 keV with an e-folding energy of 14 keV. The absorbing column density is less than 10**22 cm**-2. The low column density implies that the source is probably closer than the galactic center and that at least the non-burst luminosity probably does not exceed the Eddington limit. The non-burst flux was about 2 times 10**-7 erg cm**-2 s**-1 (2-100 keV). These observations were taken during the period of initial instrument verification, so the calibrations are preliminary. See http://heasarc.gsfc.nasa.gov/docs/xte/SOF/TOONews.html for further information about results, data and future observations.