National Aeronautics and Space Administration

XTE J1739-285

C. B. Markwardt, University of Maryland and Goddard Space Flight Center (GSFC); F. E. Marshall and J. H. Swank, GSFC; and Wei Cui, Massachusetts Institute of Technology, write: "RXTE PCA scans of the galactic-center region on Oct. 19.4 UT revealed a transient x-ray source, designated XTE J1739-285. The best-fitting position for the source is R.A. = 17h39m.9, Decl. = -28o29' (equinox 2000.0; estimated uncertainty about 5', due primarily to nearby contaminating sources in the collimator field-of-view). Approximate x-ray fluxes (with uncertainties of < 2 percent) derived from PCA scans, in units of mCrab in the band 2-10 keV: Oct. 19.4, 77; 21.2, 66; 24.4, 170; 27.1, 200; 30.2, 157. The source is distinct from known persistent x-ray sources SLX 1737-282 and GRS 1734-292. A pointed PCA exposure of 1400 s on Oct. 29.6 revealed no detectable quasiperiodic oscillations or high-frequency variability. Follow-up observations by imaging instruments are encouraged."

XTE J1859+226

R. I. Hynes, Southampton University; and C. A. Haswell, A. J. Norton, and S. Chaty, Open University, write: "Following optical and near-infrared monitoring of XTE J1859+226 (IAUC 7279, 7284), Hubble Space Telescope (HST) STIS observations were performed on Oct. 18.10-18.38 UT spanning 112-1026 nm. The ultraviolet spectrum shows broad (12 000 km/s FWZI) and deep Ly-alpha absorption, strong C IV 155-nm emission (EW = 1.1 nm; 4000 km/s FWZI) and weaker emission lines of C III, N V, O III, O IV, O V, Si IV, and He II. Sharp absorption lines of Si II, Si III, Mg II, and other species also appear with possible interstellar origin. Preliminary analysis of the 220-nm interstellar feature suggests E(B-V) = 0.58 +/- 0.07. The dereddened optical spectral energy distribution can be represented by a steep blue power law (spectral index +1.2), flattening in the ultraviolet. This resembles energy distributions of other short-period soft-x-ray transients, suggesting that XTE J1859+226 has a binary period < 1 day. A 20-percent decline in 110-170-nm fluxes during the observation coincides with an x-ray decline seen in RXTE/ASM lightcurves. Shorter timescale variations are present, but no persistent coherent modulation was detected. Preliminary analyses of 33 hr of optical time-series data collected between Oct. 13.86 and 20.24 at the Nordic Optical Telescope (J. E. Solheim, R. Ostensen, T. M. C. Abbott), Braeside Observatory (R. Fried, J. McFarland), the Jacobus Kapteyn Telescope (D. J. Rolfe, D. A. Lott), Keele (Z. Ioannou), Mt. Laguna (A. Shafter), St. Andrews (K. O'Brien, K. Horne), and Manastash Ridge Observatory (K. Krisciunas), and also 2.5 hr of infrared time-series data at the U.K. Infrared Telescope (R. J. Ivison) on Oct. 18.19-18.31, suggests a 1-percent modulation with period 22-23 min. Further HST observations are scheduled for Nov. 6.82-6.93 and 19.43-19.55; coordinated groundbased observations are encouraged."

HDE 245770

F. Giovannelli, Istituto di Astrofisica Spaziale, CNR, Rome; L. Sabau-Graziati, Division de Ciencias del Espacio, INTA, Madrid; S. Bernabei, Osservatorio Astronomico di Bologna; and S. Galleti, Dipartimento di Astronomia, Universita di Bologna, write: "Low- resolution spectrosopy [Delta(lambda) about 0.66 nm] of HDE 245770 (the optical counterpart of the x-ray pulsar A0535+26; e.g., IAUC 3167, 3655, 5746; Giovannelli and Sabau-Graziati 1992, Space Sci. Rev. 59, 1) made on Oct. 8 with the 1.5-m Loiano telescope reveals H-alpha in emission with EW about -0.5 nm. Also, He I lines (587.6, 667.8, and 706.5 nm) are seen in emission and show doublings separated by about 1.5 nm. H-beta is in weak absorption (EW about +0.017 nm). H-gamma and H-delta are seen in absorption (EW about +0.16 and +0.30 nm, respectively). This result clearly shows the resumption of optical activity after a period from early 1996 to early 1999 in which a depletion of the circumstellar disk was clearly seen (Piccioni et al. 1999, IAU Coll. No. 175, Alicante, Spain). For instance, H-alpha was in absorption on 1998 Nov. 11 and 12 with EW about +0.04 and +0.05 nm, respectively. Also H-beta, H-gamma, and H-delta were in absorption on 1998 Nov. 9 with EW = +0.19, +0.28, and +0.34 nm, respectively. Photometry of HDE 245770 performed on 1999 Oct. 9 and 11 gives B = 9.78 +/- 0.04 and 9.77 +/- 0.04, respectively. The historical B lightcurve has in the past 15 yr shown a continuous trend toward lower luminosity (starting from a stable mag of about 9.5 during the mid-1970s to the mid-1980s, and fading to mag about 10), but this trend is now reversing. As a consequence of this behavior, we expect the x-ray pulsar companion A0535+26 to have a normal x-ray outburst (following the classification of Giovannelli and Sabau-Graziati, ibid.) of intensity about 0.1 Crab on 1999 Dec. 27-31. Optical indicators (e.g., H-gamma, usually in absorption, will enhance to fill the continuum or even show emission; optical UBVRI flare of intensity about 0.1 mag) will precede the x-ray outburst by about 7-10 days. This prediction is based on RXTE data (http://space.mit.edu/XTE/asmlc/ASM.htlm) and on the orbital period of the system (110.8 days, from the best-fit multi-year optical data of Giovannelli's group and others; see Guarnieri et al. and de Martino et al. 1985, in Multifrequency Behaviour of Galactic Accreting Sources, pp. 310 and 326; and Priedhorsky and Terrell 1983, Nature 303, 681, who derived a period of 111.0 +/- 0.4 days from VELA-5B data, related to x-ray quiescent emission from

XTE J1859+226

A. Wood and D. A. Smith, Massachusetts Institute of Technology (MIT); and F. E. Marshall and J. Swank, Goddard Space Flight Center (GSFC), on behalf of the RXTE All-Sky Monitor (ASM) team at MIT and GSFC, report the emergence of a new x-ray transient: "The source's intensity was measured to rise slowly during approximately a 12-hr interval to 160 +/- 15 mCrab (2-12 keV) on Oct 9.658 UT and 250 mCrab by Oct. 10.523, confirmed by a PCA observation (with source position given by Markwardt et al., below). The rise was consistent with a linear increase at a rate of about 6 mCrab/hr. Extrapolation indicates that the outburst began around Oct. 8.55 UT. The ASM spectrum is roughly Crab-like and shows no indication of evolution. Neither the ASM catalogue nor SIMBAD lists any known x-ray sources in or near the positional error box. Observations at other wavelengths would be timely."

C. B. Markwardt, University of Maryland and GSFC; and F. E. Marshall and J. H. Swank, GSFC, write: "The RXTE PCA observed the transient source XTE J1859+226 on Oct. 10.56-10.60 UT for about 2000 s. The intensity of the source was 1.7 x 10**-8 erg s**-1 cm**-2 in the band 2-60 keV (= 250 mCrab in the band 2-10 keV), with fractional r.m.s. variability of about 32 percent. PCA scans during the observation determined a source position of R.A. = 18h58m35s, Decl. = +22o39'.3 (equinox 2000.0), with a 95-percent uncertainty radius of 1', which also accounts for the intrinsic variability of the source. A power spectrum shows a quasiperiodic oscillation and two harmonics, the fundamental being centered at 0.45 Hz (FWHM 0.11 Hz), with a fractional r.m.s. amplitude of about 17 percent. No high-frequency pulsations or variability were seen. The energy spectrum can be modeled as a power law with photon index 1.7 and neutral hydrogen absorption of about 3 x 10**21 cm**-2. These timing and spectral characteristics together are reminiscent of a galactic blackhole candidate. Additional PCA observations are planned."

GRS 1758-258

D. M. Smith, University of California, Berkeley; W. A. Heindl, University of California, San Diego; and J. H. Swank, Goddard Space Flight Center, write: "Weekly RXTE PCA monitoring of blackhole candidate GRS 1758-258 shows that this persistent source, which has been in a hard state since at least the beginning of 1996, has since 1999 Feb. 19 entered a period of oscillation between hard (power-law index 1.4-1.7, thermal disk component undetectable or < 10 percent of the 2-10-keV luminosity) and soft (index 2.0-2.3, thermal disk > 20 percent) states. The first cycles had only minor spectral changes and a 1-month timescale. The changes have become gradually more extreme, and, since Sept. 12, have begun to occur on scales < 1 week. The state changes may modulate mass loss to the radio jets. Observations in the x-ray and infrared-to-radio bands are strongly encouraged, frequent monitoring in particular."

GM SAGITTARII AND SAX J1819.3-2525 = XTE J1819-254

M. L. McCollough and M. H. Finger, Universities Space Research Association and Marshall Space Flight Center; and P. M. Woods, University of Alabama, Huntsville, report for the BATSE team: "GM Sgr appears as a strong and highly variable source in the BATSE energy range. The recent x-ray event (IAUC 7253) was first detected in the BATSE data on Sept. 14.88 UT with a 20-100-keV flux of about 5 Crab (9.0 x 10**-8 erg cm**-2 s**-1 with a photon index of -3.5). The occultation measurement prior to this showed no excess flux above background. From Sept. 15.36 to 15.68, the source was detected at fluxes reaching about 8 Crab (20-100 keV). Power spectra made from 20-50-keV rates during this interval were all consistent with a featureless 1/f continuum from 5 to 500 mHz. The fractional rms amplitudes for the range 10-500 mHz varied from 17 to 38 percent. For the remainder of the day, the source was undetected except for several short flares of about 10-500 s. The peak flux values of these events reach 1.2 x 10**-7 erg cm**-2 s**-1 (20-100 keV), with the source being clearly detected above 100 keV." C. B. Markwardt, University of Maryland and Goddard Space Flight Center (GSFC); J. H. Swank, GSFC; and E. H. Morgan, Massachusetts Institute of Technology, report that the RXTE PCA observed the transient source XTE J1819-254 starting on Sept. 15.89 UT for about 3000 s. The intensity of the source during the observation ranged from (1.8 to 860) x 10**-10 erg s**-1 cm**-2 in the band 2-60 keV. After a flaring period, characterized by rapid shots and lasting about 1000 s, it entered a quiescent phase at the minimum flux level. The x-ray spectrum during the flare was consistent with a power law having a photon index of 0.9, a rolloff above 20 keV, a soft excess, and neutral hydrogen absorption < 2 x 10**21 cm**-2. During the quiescent phase, the spectrum steepened to a photon index of 3. An iron emission line is present in both spectra, with a best-fit centroid of 6.5 keV and equivalent width of about 500 eV. No significant pulsations or quasiperiodic variability were detected, although red noise was present below 30 Hz. Subsequent PCA observations on Sept. 16.06, 16.12, and 16.18 also showed quiescent behavior at similar intensities. We have also undertaken regular monitoring observations of the galactic- center region. These show that the source has varied irregularly in the band 2-60 keV between (0.2 and 12) x 10**-10 erg s**-1 cm**-2 over the past 7 months, the most recent measurements being (in the same units) Sept. 9.30, 2.1; 12.15, 3.1; 15.10, 4.1. Additional public RXTE PCA observations are planned.

GM SAGITTARII AND SAX J1819.3-2525 = XTE J1819-254

R. Stubbings, Drouin, Vic., Australia, reports that GM Sgr is undergoing a very bright outburst after showing recent rapid variations, his visual magnitude estimates being: Aug. 11.488 UT, 12.3; 14.436, 12.5; 29.424, 12.7; 31.496, 13.0; Sept. 9.462, 11.2; 11.599, 11.9; 12.409, 11.1; 13.397, 11.9; 14.491, 11.4; 15.395, 8.8; 15.408, 8.8. Additional m_v estimates by A. Pearce, Nedlands, W. Australia: Sept. 15.550, 9.5; 15.571, 9.6.

D. A. Smith, A. M. Levine, and E. H. Morgan, Massachusetts Institute of Technology (MIT), on behalf of the RXTE All-Sky Monitor (ASM) team at MIT and Goddard Space Flight Center, report that, in response to the visual observations of GM Sgr, SAX J1819.3-2525 (an x-ray transient likely to be associated with GM Sgr; cf. IAUC 7119, 7120) was measured by the ASM on Sept. 15.420 UT to have an intensity of 1.55 +/- 0.04 Crab (2-12 keV; estimated error based solely on counting statistics). Subsequent measurements showed a rapid increase in x-ray intensity: Sept. 15.422, 1.32 +/- 0.04 Crab; 15.423, 1.43 +/- 0.04; 15.488, 2.41 +/- 0.04; 15.489, 3.74 +/- 0.05; 15.701, 10.0 +/- 0.2; 15.703, 12.2 +/- 0.2. The spectrum is harder than that of the Crab nebula.

The star attributed by Goranskij (1990, IBVS 3464) to GM Sgr has been measured on the Digital Sky Survey by G. V. Williams, Smithsonian Astrophysical Observatory, to be at R.A. = 18h19m21s.61, Decl. = -25o24'26".3 (equinox 2000.0; uncertainty +/- 1".0 in both coordinates); this is 1' north of the GCVS position from Luyten (1927, HCO Bull. No. 852).

V368 CEPHEI

D. A. Smith and R. Remillard, Massachusetts Institute of Technology (MIT), report on behalf of the All-Sky Monitor (ASM) team at MIT and NASA Goddard Space Flight Center: "During Aug. 28, the RXTE ASM reported three significant and consecutive detections of a bright x-ray flare, likely to originate in the variable star HD 220140 (= V368 Cep). This star is also the counterpart for the weak HEAO 1 x-ray source H2311+77. The net ASM error box is a parallelogram (7'.5 x 16'.2) with corners (90-percent confidence, including systematic effects; equinox J2000.0) at: R.A. = 23h19m58s.7, Decl. = +79o00'18"; 23h17m05s.2, +79o12'51"; 23h17m25s.9, +79o06'00"; 23h20m17s.5, +78o53'27". SIMBAD catalogues show only HD 220140 within the ASM error box. The average source intensity (2-12 keV) in 90-s exposures was 34 +/- 13 mCrab on Aug. 28.93237, 110 +/- 14 mCrab on Aug. 28.93348, and 150 +/- 13 mCrab on Aug. 28.93460. Observations of this location before and after the flare measured 2-sigma upper limits on the source intensity of 37 mCrab on Aug. 28.56274 and 20 mCrab on Aug. 29.06626. For a distance of 20 pc (Halliwell 1979, Ap.J. Suppl. 41, 173), the maximum luminosity observed with the ASM during this flare was 2 x 10**32 erg/s."

XTE J1859+083

F. E. Marshall, Goddard Space Flight Center (GSFC); J. J. M. in 't Zand, Space Research Organization of the Netherlands; T. Strohmayer, GSFC; and C. B. Markwardt, National Research Council and GSFC, report the discovery of a transient, pulsating x-ray source designated XTE J1859+083: "The source was first seen with the Proportional Counter Array on the Rossi X-ray Timing Explorer on Aug. 8.78 UT with a 2-10-keV flux of 2 x 10**-10 erg cm**-2 s**-1. Subsequent observations with the PCA located the source at R.A. = 18h59m.1, Decl. = +8o15' (equinox 2000.0), with an estimated 90-percent-confidence uncertainty of 2'. The flux is strongly modulated with a period of 9.801 +/- 0.002 s. The 2-10-keV flux on Aug. 16.95 was 1.4 x 10**-10 erg cm**-2 s**-1. Observations at other wavelengths are encouraged."

V382 VELORUM

K. Mukai and J. Swank, Goddard Space Flight Center, write: "The Rossi X-ray Timing Explorer performed a public TOO observation of V382 Vel during May 26.22-26.26 UT, about 3 days after optical maximum. In the band 2.5-10 keV, a nominal excess over the estimated background of 0.58 +/- 0.11 counts/s was obtained in four Proportional Counter Units (PCUs); however, given the fluctuations in the cosmic x-ray background, this should not be considered a detection. As an approximate upper limit, 0.8 counts/s in four PCUs would correspond to 2.5 x 10**-12 erg cm**-2 s**-1 in the band 2.5-10 keV, significantly below the BeppoSAX and ASCA detections."

XTE J1550-564

W. Cui and L. Wen, Massachusetts Institute of Technology; S. N. Zhang and X.-B. Wu, University of Alabama, Huntsville, report: "We have detected a periodic signal in the x-ray intensity of blackhole candidate XTE J1550-564, using public data from the All-Sky Monitor (ASM) aboard RXTE. A Lomb-Scargle periodogram is constructed from the ASM light curve of this transient source for the period when it was in a recent x-ray outburst (1998 Sept. 4- 1999 May 22). A peak is apparent at 1.398 +/- 0.003 days in the periodogram, with a power of about 40 (the Poisson noise power is normalized to 1 in the same units). Taking into account the intrinsic red noise of the source, we estimate that the detection bears a false-alarm probability of about 10E-4. We have also tried to break up the outburst light curve into segments and have performed the same analysis on each segment. We have found that the signal is always present at the same position in the periodogram (within measurement uncertainties). Similar results are obtained from Fourier analyses of the light curve. The epoch- folded light curve is roughly of sinusoidal form, with a mean peak- to-peak amplitude about 60 percent in each of the three ASM bands (1.3-3, 3-5, and 5-12 keV). The detected period is remarkably close to what was suggested for the binary orbital period of XTE J1550-564 (Soria and Wu, IAUC 7184), based on a theoretical relationship between the orbital period and the mean density of the secondary star, which is assumed to have filled its Roche lobe. Considering that few blackhole candidates have been observed to show any x-ray modulations at the binary orbital period, the detection of such modulation in XTE J1550-564 perhaps implies a special environment or geometry of the binary system, like in the well-known case of Cyg X-1 (Wen et al. 1999, Ap.J., in press). Future optical observations of the source in x-ray quiescence can hopefully shed more light on the issue."

eta CARINAE

M. F. Corcoran and S. A. Drake, Universities Space Research Association, Goddard Space Flight Center (GSFC); J. H. Swank and R. Petre, Laboratory for High Energy Astrophysics, GSFC; K. Ishibashi, K. Davidson and R. Humphreys, University of Minnesota; A. Damineli, Instituto Astronomico e Geofisico da University de Sao Paulo; K. Koyama and Y. Tsuboi, Kyoto University; and R. Viotti, Instituto Astrofisica Spaziale, CNR, Rome, report: "We compared two x-ray spectra of the peculiar variable star eta Car obtained by the ASCA observatory on 1993 Aug. 24 and 1999 Feb. 2. These spectra were both obtained near phase 0.20, where phase is calculated according to the epoch and period of Damineli et al. (1997, New Astron. 2, 107). We find that the absorbed 2-10-keV x-ray flux in 1999 is (6.4 +/- 0.05) x 10E-11 erg cmE-2 sE-1, while that in 1993 was (5.8 +/- 0.09) x 10E-11 erg cmE-2 sE-1. Our RXTE monitoring of the 2- 10-keV x-ray flux for +/- 20 days around the ASCA observation (1999 Jan. 20-Mar. 1) shows a steady decline in this energy band (from 23 to 18 PCU counts/s), with no sign of any x-ray flaring activity. Thus the observed change in the ASCA spectra suggests that the 2- 10-keV x-ray flux has undergone a long-term brightening of about 10 percent that is independent of the Damineli cycle. The x-ray variation may be related to a long-term increase in the optical brightness of the star (cf. van Genderen et al. 1999, A.Ap. 343, 847) and/or the recent brightening at optical and infrared wavelengths (IAUC 7146). The x-ray brightening we report is not the result of a change in circumstellar absorption, since circumstellar absorption is negligible at energies above 2 keV. Our preliminary analysis suggests that the flux increase is primarily due to an increase in the emission measure of the x-ray emitting region during the interval 1993-1999."

V1333 AQUILAE

C. Chevalier and S. A. Ilovaisky, Observatoire de Haute- Provence (OHP), write: "CCD images of V1333 Aql = Aquila X-1 taken with the 1.2-m OHP telescope on May 13.1, 14.1, and 15.1 UT show a rapid increase in mean brightness over this time interval, from V = 17.9 to 17.2. These levels are comparable to the maximum values that we observed during the mid-1996 mini-outburst and the mid-1997 moderate outburst. However, quick-look data from the XTE All-Sky Monitor for May 14-17 show low count rates (< 70 milliCrab). This low x-ray-to-optical luminosity ratio may perhaps signal a repeat of the 1996 mini-outburst."

4U 1630-47

M. L. McCollough, Universities Space Research Association and Marshall Space Flight Center (MSFC); B. A. Harmon, MSFC, NASA; S. Dieters, University of Alabama, Huntsville; and R. Wijnands, Astronomical Institute, University of Amsterdam, communicate: "The CGRO BATSE earth-occultation monitoring (20-100 keV) indicates that the recurrent blackhole x-ray transient 4U 1630-47 (cf. IAUC 6389, 6822) has shown increased activity. It has risen over the last 18 days from 70 mCrab (20-100 keV) on Apr. 20 to 140 mCrab on May 7. It has a hard spectrum (power-law index -2.6 +/- 0.2) with emission extending past 200 keV. A public target-of-opportunity RXTE PCA observations on May 8.05 UT showed the source to be at 75 mCrab (2-20 keV) with a spectrum described by a power law with an index of -1.6 +/- 0.06 and no evidence of a blackbody component. A strong quasiperiodic oscillation was found in the data (2-60 keV) with a centroid frequency of 0.855 +/- 0.005 Hz, FWHM of 0.13 +/- 0.01, and fractional rms amplitude of 16.3 +/- 0.5 percent. This renewed activity of 4U 1630-47 follows a significant outburst in Feb. 1998 (IAUC 6822) and does not fall within the expected about 690-day outburst cycle of this source. With an outburst, radio emission is expected, and observations at radio wavelengths is encouraged."

MXB 1659-29

C. B. Markwardt, National Research Council and Goddard Space Flight Center (GSFC); E. Smith and J. Swank, GSFC; and J. in 't Zand, Space Research Organization of the Netherlands, Utrecht, write: "Observations of the transient source MXB 1659-29 (IAUC 7138) were performed by RXTE on Apr. 5.83-6.05 UT. The source was detected at a level of about 38 mCrab (2-10 keV), but was punctuated by a series of x-ray dips that lasted 5-350 s. A single x-ray eclipse was observed between Apr. 5.97206 and 5.98257 (barycenter-corrected). Cominsky and Wood (1989, Ap.J. 337, 485) have published an orbital period of 7.1161138(17) hr, based on previous activity in 1976-1978, and we conclude that the observed eclipse is number 27708 according to their numbering convention. The observed center of the eclipse arrived 7.0 +/- 2.6 min before the predicted time, based on a constant period, implying an orbital dP/dt = (-4.8 +/- 1.8) x 10E-11. No x-ray bursts were seen, and no pulsations or QPOs were detected. Follow-up observations are being performed by RXTE."

HERCULES X-1

A. M. Levine, Center for Space Research, Massachusetts Institute of Technology (MIT); and R. Corbet, Goddard Space Flight Center (GSFC), NASA, and Universities Space Research Association, report on behalf of the RXTE/ASM team at MIT and NASA/GSFC: "The 35-day longterm cycle has been obvious in the RXTE All-Sky Monitor 1.5-12-keV lightcurve for Hercules X-1, with the 'main high state' (see, e.g., Scott and Leahy 1999, Ap.J. 510, 974) being clearly detected every 35 days for > 3 yr. The first exception to this regular appearance of the main high states has recently occurred. No main high state appeared as expected around Mar. 23, with an upper limit of about 15 mCrab. Earlier main high states typically reached peak intensities of 40-100 mCrab. This may represent an expansion of the accretion disk, such that the neutron star remains occulted through the entire 35-day cycle. Optical observations to determine whether the companion, HZ Her, is still being heated by x-rays are encouraged."

XTE J1723-376

F. E. Marshall, Goddard Space Flight Center (GSFC); Y. Ueda, Institute of Space and Astronautical Science; and C. B. Markwardt, National Academy of Sciences/National Research Council and GSFC, report an improved position for XTE J1723-376 (IAUC 7103): R.A. = 17h23m38s, Decl. = -37o39'.7 (equinox 2000.0; estimated uncertainty 0'.5, from an ASCA observation on Mar. 4 UT). The x-ray spectrum is typical of low-mass x-ray binaries, and the absorption is about 4 x 10E22 cmE-2. Type-1 x-ray bursts were observed with the RXTE PCA on Feb. 3.942 and 4.055, with peak intensities of 0.51 and 0.43 Crab. These results indicate that the source is at a distance comparable to that of the Galactic Center. The most recent RXTE observation shows a persistent flux of 0.05 Crab on Mar. 22.9.

4U 0115+63

W. A. Heindl and W. Coburn, University of California, San Diego, report: "The current outburst of the 3.6-s x-ray pulsar 4U 0115+63 (IAUC 7116) has been monitored with daily RXTE pointings since Mar. 3. The flux (2-10 keV) has increased from 5.6 x 10E-9 (Mar. 3) to 1.1 x 10E-8 erg cmE-2 sE-1 on Mar. 12. Fitting the Mar. 3 joint PCA/HEXTE spectrum reveals cyclotron absorption features at about 14 and 21 keV. Because the phase-averaged spectra are complex due to pulse-phase spectral variability, these line energies are not well-determined. A cyclotron absorption feature was first seen in this source with HEAO 1 (Wheaton et al. 1979, Nature 282, 240). In addition to the short monitoring observations, we have made two long pointings on Mar. 7.16-7.45 and 11.89-12.32 UT. The second of these spanned periastron passage (according to the ephemeris of Bildsten et al. 1997, Ap.J. Suppl. 113, 367) at Mar. 11.95. During this observation, the source exhibited large oscillations with a timescale of about 500 s. The oscillation rms deviations were about 30 percent of the mean flux. Such slow oscillations are not consistent with the beat-frequency QPO model whose expected frequencies are of order 5 Hz. The oscillations may have been present during 2500 s of the Mar. 7 observation, but at greatly reduced amplitude. Additional observations are encouraged. The RXTE/ASM real-time light curve is available at http://heasarc.gsfc.nasa.gov/FTP/xte_weather/."

XTE J1550-564

R. Remillard, E. Morgan, and A. Levine, Massachusetts Institute of Technology; J. McClintock and G. Sobczak, Center for Astrophysics; C. Bailyn and R. Jain, Yale University; and J. Orosz, Pennsylvania State University, write: "There is continued evolution of the high-frequency quasiperiodic oscillations (QPO) in the x-ray nova and blackhole candidate XTE J1550-564 (see IAUC 7121). An RXTE PCA observation on Mar. 5.52 UT reveals a QPO at 283 +/- 5 Hz with a FWHM of 39 +/- 9 Hz, and an rms amplitude of 4.0 +/- 0.7 percent at 6-30 keV, similar to that reported for Mar. 4.80. However, subsequent PCA observations show changes as follows: 235 +/- 6 Hz (FWHM 30 +/- 11; amplitude 2.8 +/- 0.8 percent at 6-30 keV) on Mar. 7.50; 182.0 +/- 2.5 Hz (FWHM 12 +/- 4; amplitude 0.91 +/- 0.23 percent at 2-30 keV) on Mar. 8.37. Thus the frequency has returned to the value at which the QPO was originally discovered (IAUC 7025). The steep increase in amplitude with photon energy (IAUC 7121) has lessened on Mar. 8, as the QPO is weakly visible in the band 2-6 keV, while the profile fits have poor quality when the energy range is restricted to 6-30 keV."

XTE J1550-564

J. Homan, R. Wijnands, and M. van der Klis, Astronomical Institute 'Anton Pannekoek', University of Amsterdam, report: "The blackhole candidate x-ray transient source XTE J1550-564 (IAUC 7008) seems to have undergone a sudden major state transition. The source was observed with the RXTE PCA instrument on Mar. 4.80 UT. The source flux (2-20.0 keV) was 6.0 x 10E-8 erg sE-1 cmE-2. A quasiperiodic oscillation (QPO) was detected at 284.2 +/- 1.3 Hz, with a width (FWHM) of 29 +/- 4 Hz and an rms amplitude of 3.9 +/- 0.2 percent (6.4-60 keV; 11.5 sigma). The QPO amplitudes in the energy ranges 2-60, 2-6.4, 6.4-13, and 13-60 keV, are 1.3 +/- 0.2, < 0.8, 3.2 +/- 0.2, and 8.4 +/- 0.9 percent, respectively. This high-frequency QPO is most likely related to the 184-Hz QPO previously observed in XTE J1550-564 (IAUC 7025; Remillard et al. 1999, Ap.J. Let., submitted). Simultaneously with the high- frequency QPO, QPOs at 5.92 +/- 0.02 Hz [FWHM 2.02 +/- 0.05 Hz; rms amplitude 2.98 +/- 0.03 percent (2-60 keV; 43.6 sigma)] and at 10.4 +/- 0.1 Hz [FWHM 5.8 +/- 0.3 Hz; rms amplitude 2.55 +/- 0.06 percent (2-60 keV; 23.7 sigma)] are observed. An RXTE observation taken the day before (on Mar. 3.06), shows a 2-20.0-keV x-ray flux about 20 percent lower than that of Mar. 4.80 (4.9 x 10E-8 erg sE-1 cmE-2), a softer x-ray spectrum, and a power spectrum with no QPO, dominated by a weak (2.7 +/- 0.2 percent; 2-60 keV) power-law component with index -1.23 +/- 0.04. These properties are typical for those of XTE J1550-564 during the last few months. This pronounced change in x-ray properties, which took place within 1.8 days, shows that XTE J1550-564 has undergone a major source state transition. Observations at all wavelengths are encouraged. More x-ray observations with the RXTE/PCA instrument are required to study the high-frequency QPO in more detail."

XTE J1819-254, XTE J1743-363, XTE J1710-281, XTE J1723-376

XTE J1550-564

R. Jain and C. Bailyn, Yale University; J. McClintock and G. Sobczak, Harvard-Smithsonian Center for Astrophysics; R. Remillard, Massachusetts Institute of Technology; and J. Orosz, Pennsylvania State University, report YALO consortium observations of XTE J1550-564: "We have obtained B-, V-, and I-band images virtually every night since Jan. 4.3 UT and most recently covering 3.5 hr of elapsed time per night. We find that, between Jan. 4.3 and 22.3, the source was steady at V = 16.04, B = 17.48, and I = 14.15. Then on Jan. 22.3, there was a 0.3-mag dip in the B, V, and I magnitudes. The source then remained steady, with a slight decline to a new level (V = 16.17, B = 17.62, I = 14.29) until Feb. 6.3, when another small dip (0.2 mag in I) occurred. This was followed by another period of steady slow decline. Most recently, a much larger decrease in optical brightness has occurred: since Feb. 21, the source has declined by > 0.5 magnitudes in B, V, and I. RXTE PCA observations show that the 2-20-keV x-ray flux increased from 6.5 x 10E-8 erg cmE-2 sE-1 on Feb. 21.5 to 7.4 x 10E-8 erg cmE-2 sE-1 on Feb. 23.9. The x-ray spectrum from 2 to 20 keV is dominated by the disk component, but the change in optical intensity has been accompanied by a factor of two increase in the power-law component of the x-ray flux from Feb. 21.5 to 23.9. We will continue to monitor this object on a daily basis for the foreseeable future, and we strongly encourage other observations, particularly of optical spectroscopy, to follow the current change in state of this source."

RX J0812.4-3114

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: "An analysis of the RXTE All Sky Monitor light curve of the transient x-ray source RX J0812.4-3114 (= LS 992, Motch et al. 1999, A.Ap. 323, 853) obtained between 1996 Jan. 5 and 1999 Jan. 28 reveals a modulation at a period of about 81 days. The source appears to have been in a lower flux state until early in 1998, when the 81-day outbursts became visible at levels of about 1 ASM count/s (10 mCrab). If these outbursts continue, then the next predicted maximum is at about 1999 Mar. 25, and observations are encouraged at that time to search for pulsations from this likely Be/neutron star binary."

XTE J1723-376

F. E. Marshall, Goddard Space Flight Center (GSFC); and C. B. Markwardt, National Academy of Sciences/National Research Council and GSFC, on behalf of the RXTE Science Operations Center, report the discovery of a transient x-ray source with quasiperiodic oscillations (QPO) at 816 Hz with an rms amplitude of 5 percent. The transient was first seen on Jan. 25.21 UT during a scan with the PCA on RXTE, with a 2-10-keV flux of 1.5 x 10E-9 erg cmE-2 sE-1. The QPO were detected during a pointed observation at Jan. 31.1, when the flux was 1.2 x 10E-9 erg cmE-2 sE-1. The best-fit source position is R.A. = 17h23m.6, Decl. = -37o39' (equinox 2000.0), with an estimated uncertainty of 2'. Additional RXTE observations are planned.

3C 279

M. Villata, C. M. Raiteri, G. Sobrito, G. De Francesco, and L. Lanteri, Osservatorio Astronomico di Torino; and R. C. Hartman, Goddard Space Flight Center, for the CGRO/EGRET Team, write: "3C 279 is optically bright and active; CCD data taken with the Torino 1.05-m telescope from Jan. 14 to 26 show fast variations in R in the range 13.78-14.50 (+/- 0.02; see http://www.to.astro.it/Groups/Extragal/blazars.htm). Preliminary results from EGRET observations show a flux (> 100 MeV) of about 2 x 10E-6 photon cmE-2 sE-1, with some evidence for time variation over the first 4.5 days (Jan. 20.5-24.0 UT). From preliminary analysis of data from RXTE, A. Lawson and I. McHardy report that the x-ray flux is at an intermediate historical level, consistent with the present EGRET flux. The RXTE PCA countrate (3-10 keV) is very roughly 3 counts/s for 3 PCUs."

MXB 1730-335

D. W. Fox and W. H. G. Lewin, Massachusetts Institute of Technology, report on behalf of a collaboration: "We have performed a timing analysis of the first second of data from each of 31 type-I x-ray bursts from the rapid burster observed from Nov. 1996 to Feb. 1998 with the Rossi X-ray Timing Explorer (energy range 5.5-16 keV only). We find a statistically significant pair of peaks in the summed power-density spectrum, with peak centroid frequencies of 154.9 +/- 0.1 and 306.6 +/- 0.1 Hz, respectively. We suggest that this finding indicates a neutron-star spin period of 6.5 ms for the rapid burster."