National Aeronautics and Space Administration

4U 1915-05

D. Barret, J. F. Olive, and L. Boirin, Centre d'Etude Spatiale des Rayonnements, Toulouse; J. E. Grindlay, P. F. Bloser, and Y. Chou, Harvard-Smithsonian Center for Astrophysics; and J. H. Swank and A. P. Smale, Goddard Space Flight Center, NASA, report: "Observations of the x-ray burster 4U 1915-05, obtained with the RXTE PCA between 1996 Feb. and Oct., show clear evidence for high- frequency quasiperiodic oscillations (HFQPO) in its persistent x- ray emission. The strongest signals were detected as follows: May 16, 1016 +/- 10 Hz, 6-sigma detection, FWHM = 75 +/- 10 Hz, rms = 18 +/- 3 percent, countrate 35.6 counts/s/PCU unit (5-30 keV); May 23, 830 +/- 10 Hz, 7-sigma, 80 +/- 10 Hz, 15 +/- 2 percent, 34.8; June 1, 935 +/- 10 Hz, 7-sigma, 70 +/- 10 Hz, 15 +/- 2 percent, 31.2; Sept. 6, 860 +/- 20 Hz, 8-sigma, 200 +/- 20 Hz, 17 +/- 2 percent, 31.6. HFQPOs at comparable frequencies may also be present in the May 15 and May 17 data (890, 920 Hz), with lower significance. Furthermore, on May 16 we see evidence (3.3 sigma) for a second HFQPO peak at about 650 +/- 15 Hz (FWHM = 50 +/- 20 Hz, rms = 12 +/- 2 percent). A signal of 600 +/- 10 Hz at 5.3 sigma is also detected in the Oct. 29 data (FWHM = 50 +/- 10 Hz, rms = 11 +/- 2 percent, 33.4 counts/s/PCU units) with no accompanying HFQPO detection above about 900 Hz. For the two observations for which the countrates were significantly larger (March 13, 68.5 counts/s/PCU; May 5, 56.5), no HFQPOs were detected. Further analysis is continuing; nevertheless these results may be important for the planning of further RXTE observations of the source."

GS 1354-64

W. Heindl, P. Blanco, D. Gruber, M. Pelling, D. MacDonald, D. Marsden, and R. Rothschild, Center for Astrophysics and Space Sciences, University of California at San Diego, report: "Pointed observations of GS 1354-64 (IAUC 6772, 6774, 6775, 6779, 6781) with the RXTE/HEXTE on Nov. 22.40, Dec. 5.21, and 12.81 UT yield fluxes of 150, 150, and 130 mCrab (15-200 keV), respectively. In all cases, the spectrum can be described by an exponentially-cutoff power law with photon index 1.0 +/- 0.1 and an e-folding energy of about 60 keV. Although GS 1354-64 is in outburst, this spectrum is similar to blackhole candidates in the low (hard) state. The spectrum in the HEXTE band is not consistent with a single power law. However, fitting only in the band 30-100 keV gives photon indices of 2.0 +/- 0.1, somewhat harder than the value (2.7) reported with BATSE (IAUC 6774) for these times. Because the source remains active, additional observations at x-ray and other wavelengths continue to be desirable."

XTE J0053-724

R. Corbet, Goddard Space Flight Center (GSFC) and Universities Space Research Association (USRA); F. E. Marshall, GSFC; J. C. Lochner, GSFC and USRA; and M. Ozaki and Y. Ueda, Institute of Space and Astronautical Science, report: "After the apparent detection of SMC X-3 with RXTE (IAUC 6777), we performed ASCA observations on Dec. 12. While no source was found at the location of SMC X-3 itself, a source was detected about 8' away, at R.A. = 0h53m53s, Decl. = -72o26'42" (equinox 2000.0; +/- 1'.2, 90-percent confidence) that exhibited modulation at the 92-s period seen with RXTE, confirming that this is the same source. The mean countrate (per GIS detector) of 0.08 counts/s corresponds to a 2-10-keV flux of 1.45 x 10E-11 erg sE-1 cmE-2. A search of archival ROSAT images reveals a variable source in this error box located at position end figures 55s, 48" (+/- 5")."

Lochner and L. A. Whitlock, GSFC and USRA; and N. Brandt, Pennsylvania State Universtity, report: "In a preliminary analysis of the RXTE PCA observations of XTE J0053-724 from Nov. 25-Dec. 11, the spectra have been fit with a cutoff power law (with photon indices typically 0.5-0.7 and energy cutoffs in the range 11-15 kev) show no evidence for intrinsic x-ray absorption. Flux measurements in the range 2-10 keV are as follows (units of 10E-11 erg sE-1 cmE-2): Nov. 25.46 UT, 6.72; 27.45, 6.74; 29.25, 6.51; Dec. 2.18, 6.06; 2.35, 5.97; 5.18, 5.30; 8.06, 4.82; 11.85, 4.12. The average pulsation period measured from these observations is 91.13 s."

SMC X-3

F. E. Marshall, Goddard Space Flight Center (GSFC); and J. C. Lochner and T. Takeshima, GSFC and Universities Space Research Association, report "the detection with the RXTE PCA of an outburst from the x-ray transient SMC X-3 and the discovery of a period of 92 +/- 1.5 s with a complex pulse profile: 'First detected on Nov. 15.44 UT with a 2-10-keV flux of 1.5 mCrab, the source flux had increased to about 4 mCrab by Nov. 25.44. Further RXTE observations are planned, and observations of the suggested optical counterpart are encouraged.' "

X1354-644

R. Remillard, F. Marshall, and T. Takeshima report for the RXTE All Sky Monitor (ASM) Team at Massachusetts Institute of Technology and Goddard Space Flight Center: "The Rossi X-ray Timing Explorer's ASM has detected a modest outburst from the recurrent transient and blackhole-candidate binary, X1354-644. Renewed x-ray activity began around Nov. 1, and the average flux over the interval Nov. 1-9 was 16 mCrab at 2-12 keV, increasing to 26 mCrab during Nov. 11-16 and to 50 mCrab on Nov. 17-18. An RXTE PCA observation between Nov. 18.02 and 18.11 UT found an average flux of 50 mCrab (2-10 keV). There were aperiodic variations, with a range of time scales from subseconds to 1 min and flare amplitudes as large as a factor of 3. We encourage radio, optical, and infrared observations of the counterpart, BW Cir (cf. IAUC 4357)."

XTE J1739-302

D. M. Smith, Space Sciences Laboratory, University of California at Berkeley, communicates: "The position originally reported for XTE J1739-302 on IAUC 6748 was incorrect. The correct position is 7' away, at R.A. = 17h38m53s, Decl. = -30o15'.6 (equinox 2000.0). An error radius of 3' at 99-percent confidence is still appropriate, excluding the original field."

XTE J1739-302

D. M. Smith and D. Main, Space Sciences Laboratory, University of California at Berkeley; F. Marshall and J. Swank, Goddard Space Flight Center, NASA; W. Heindl, University of California at San Diego; and M. Leventhal, University of Maryland, College Park, report: "The RXTE Proportional Counter Array detected a new x-ray transient on Aug. 12.60 and 12.72 UT during slews across the galactic-center region. It was not seen on Aug. 3, 14, 20, or 28. The position is R.A. = 17h38m59s, Decl. = -30o08'.4 (equinox 2000.0; 3' error radius at 99-percent confidence). The 2-20-keV flux was 70 mCrab, and the spectrum could be fit with a hard (photon index -1.6) power law plus a 4-keV disk blackbody. The rms variability was 13 percent from 0.005 to 10 Hz. The power spectrum showed no flattening at low frequencies and no pulsations in this range. Although it is no longer bright in x-rays, prompt observations, particularly in the radio, are encouraged."

POSSIBLE NEW SOFT gamma-RAY REPEATER

K. Hurley, Space Sciences Laboratory, Berkeley, on behalf of the Ulysses Gamma-Ray Burst Team; C. Kouveliotou, Universities Space Research Association (USRA), on behalf of the Gamma-Ray Observatory BATSE team; and T. Cline and E. Mazets, on behalf of the KONUS-WIND team, report: "Ulysses has observed three similar short intense bursts whose triangulated positions are consistent with a single location. The first two occurred on June 29 (and were also observed by KONUS-WIND), and the third on Sept. 12; the second and third are the BATSE triggers described below. For each burst, we can derive a single annulus of possible arrival directions. They intersect over a region defined by the following coordinates (equinox 2000.0): R.A. = 18h14m50s, Decl. = -13o36'.6; 18h14m25s, -14o01'.7; 18h15m26s, -13o15'.0; 18h15m02s, -13o39'.8."

C. Kouveliotou, USRA and Marshall Space Flight Center (MSFC), NASA; G. J. Fishman and C. A. Meegan, MSFC; and P. Woods, University of Alabama in Huntsville, report on behalf of the BATSE Team: "BATSE triggered on two very intense, short, soft events on June 29.27189 and Sept. 12.25652 UT. Each lasted about 2 s, and their (preliminary) spectra can be fitted with a power-law with indices of about -3.8 and -4.2, respectively. Given the characteristics and common location noted above, we classify them as soft-gamma-ray-repeater events from a previously unknown source."

D. A. Smith, A. M. Levine, E. H. Morgan, and R. A. Remillard, Massachusetts Institute of Technology (MIT), on behalf of the RXTE/ASM team at MIT and Goddard Space Flight Center, NASA; and R. Rutledge, Max-Planck-Institut fur Extraterrestrische Physik, write: "The RXTE All Sky Monitor has detected the Sept. 12 event reported above; ASM camera 3 detected a bright x-ray flash that lasted about 2.8 s and yielded a peak countrate of about 270 counts/s. The inferred peak flux, including corrections for background and location in the field-of-view, is about 10 Crab (2-12 keV). We obtain a single long, narrow line of position. Our formal analysis gives an error box with corners at R.A. = 18h25m39s.0, Decl. = -14o51'48"; 18h25m42s.6, -14o48'50"; 18h34m24s.9, -15o28'10"; 18h34m04s.5, -15o23'31" (equinox 2000.0). However, the determination of the ends of the error box along the long direction is subject to systematic errors of about 2 deg. With this consideration, the extension of our error box by about 2.8 deg in the long direction is consistent with the intersection of the position annuli from the IPN reported above. The combined IPN/ASM error box is centered at R.A. = 18h14m.7, Decl. = -13o40' and has a radius (90-percent confidence) of 5'."

GRB 970828

T. Murakami and Y. Ueda, Institute of Space and Astronautical Science; A. Yoshida and N. Kawai, Institute of Physical and Chemical Research; and F. E. Marshall, R. H. D. Corbet, and T. Takeshima, Goddard Space Flight Center, report on behalf of the ASCA and RXTE teams: "We analyzed all of the ASCA observational data from Aug. 29.91 to 30.85 UT (net exposure 36 000 s). After the attitude determination, we refined the ASCA x-ray position (reported on IAUC 6729) to R.A. = 18h08m32s.2, Decl. = +59o18'54" (equinox 2000.0; error radius 0'.5). Its average flux in the band 2-10 keV is 4 x 10E-13 erg cmE-2 sE-1. The fluxes clearly show fading behavior. This strongly supports the conclusion that the ASCA x-ray source is a fading x-ray source associated with GRB 970828 (IAUC 6726, 6728). Considering the flux reported by the PCA/RXTE observations (IAUC 6727), together with the ASCA fluxes, the x-ray source appears to be fading consistent with a time power- law having an index of about -1.4."

GRB 970828

D. Smith, A. Levine, R. Remillard, and A. Wood, Massachusetts Institute of Technology, communicate: "A refined analysis of the RXTE ASM observations of GRB 970828 (described on IAUC 6726), using the two 90-s observations simultaneously, yields a best-fit location and 90-percent confidence ellipse. The center of this ellipse lies at the position reported on IAUC 6726; its axes lie along the local R.A. and Decl. lines, such that its major axis spans 5'.0 in R.A. and its minor axis 2'.0 in Decl. This 90- percent-confidence ellipse includes an estimate of the systematic errors added in quadrature, and it is smaller than the 1-sigma error region reported previously." K. Hurley, Space Sciences Laboratory, Berkeley, on behalf of the Ulysses Gamma-Ray Burst Team; C. Kouveliotou, G. Fishman, C. Meegan, and V. Connaughton, on behalf of the Gamma Ray Observatory BATSE Team; and J. van Paradijs, University of Amsterdam and University of Alabama in Huntsville, report: "A preliminary triangulation has been done using Ulysses and BATSE data. The resulting position is an annulus whose half-width is conservatively estimated to be 0.027 deg at 3-sigma confidence, centered at R.A. = 10h46m13s, Decl. = +19o10'.3 (equinox 2000.0), with central radius 83.518 deg. This annulus passes through the corners of the RXTE error box (IAUC 6726) but does not reduce its size substantially. The annulus is therefore also consistent with the RXTE PCA source reported on IAUC 6727. Further processing will reduce the annulus width."

GRB 970828

F. E. Marshall, Goddard Space Flight Center (GSFC); and J. K. Cannizzo and R. H. D. Corbet, GSFC and Universities Space Research Association, on behalf of the RXTE Science Operations Team, report the detection with the RXTE PCA of an x-ray source whose position appears consistent with that reported on IAUC 6726: "Multiple slews across this position, beginning at Aug. 28.890 UT and lasting a total of 40 min, revealed a source intensity in the band 2-10 keV of about 0.5 mCrab. Since we know of no catalogued x-ray sources at this position, we believe this is the detection of an x-ray afterglow from this burst. A refined error box should be possible after more detailed analysis."

GRB 970828

R. Remillard, A. Wood, D. Smith, and A. Levine, Massachusetts Institute of Technology, communicate: "The RXTE All Sky Monitor has detected the gamma-ray burst of Aug. 28.73931 UT that was logged as BATSE trigger no. 6350. The burst was detected by ASM camera 1 with a mean flux of 756 mCrab (2-12 keV) averaged over the 40 s in which the burst was seen before the camera was rotated. Ninety seconds later, ASM camera 2 scanned over the source and determined a crossing line of position and an average flux of 238 mCrab, declining in an exponential-like fashion with a 30-s time constant. The x-ray emission began roughly simultaneously with the gamma-ray trigger, but was detectable with the ASM over at least 160 s. The combined camera detections yield an x-ray position of R.A. = 18h08m29s, Decl. = +59o18' (equinox 2000.0; 1-sigma uncertainties of +/- 3'.2 in R.A. and +/- 1'.2 in Decl.). Optical observations are urgently needed."

XTE J1755-324

R. N. Ogley and T. D. C. Ash, Open Univeristy; and R. P. Fender, Sussex University, report that a search was made on Aug. 18.17-18.69 UT for a radio counterpart to the recently detected x- ray source XTE J1755-324 (IAUC 6710). Observations were made using the Australia Telescope Compact Array at 1384 and 2496 MHz, and no radio emission was detected down to a level of 0.3 mJy (1384 MHz) and 0.2 mJy (2496 MHz) within the 1' error box of XTE. The nearest source detected was 7' from the reported position of the x-ray source. At the time of these observations, XTE J1755-324 had an x- ray (2-12 keV) flux of about 7 counts/s, measured by the ASM on XTE.

GRB 970815

D. A. Smith, A. M. Levine, E. H. Morgan, and A. Wood report, on behalf of the RXTE/ASM team at the Massachusetts Institute of Technology and Goddard Space Flight Center, their independent detection with the ASM detector of an x-ray burst that they identify as a counterpart to the gamma-ray burst source detected by the Compton Gamma Ray Observatory's BATSE detector on Aug. 15.50491 UT (posted at http://asd.gsfc.nasa.gov/docs/gamcosray/legr/ bacodine/baco_notes.html): "The burst rise was detected in one camera on Aug. 15.50597. At Aug. 15.50623, the cameras were rotated through 6 deg, such that the burst came into the field-of- view of a second camera. Observations with two cameras at different orientations enabled us to estimate the position of the burst source to be within an error box having the following corners: R.A. = 16h06m36s, Decl. = +81o27'36"; 16h10m50s, +81o33'36"; 16h08m10s, +81o31'12"; 16h09m17s, +81o30'00" (equinox 2000.0). The burst lasted about 130 s and showed a double-peaked structure, reaching a maximum intensity of almost 2 Crab (2-12 keV) in the second peak."

V1333 AQUILAE

C. Chevalier and S. A. Ilovaisky, Observatoire de Haute- Provence, write: "CCD monitoring of the optical counterpart of the recurrent low-mass x-ray transient Aql X-1 with the OHP 1.2-m telescope on Aug. 6.9, 7.9, 8.9, 9.9, and 10.9 UT shows that the object is hovering around V = 17.5, indicating that the current outburst has probably reached a maximum level typical of previous such mild events (see IAUC 6638). Recent publicly available XTE data (up to Aug. 11.29) show that the x-ray flux reported on IAUC 6714 indeed appears to have leveled off since Aug. 8.5."

V1333 AQUILAE

XTE J1755-324

R. Remillard and A. Levine, Massachusetts Institute of Technology (MIT); and J. Swank and T. Strohmayer, Goddard Space Flight Center (GSFC), report for the RXTE ASM Team at MIT and GSFC: "The All Sky Monitor of the Rossi X-ray Timing Explorer detected a new x-ray source late on July 25 UT, whose position was determined with the PCA early on July 29 to be R.A. = 17h55m28s.6, Decl. = -32o28'39" (equinox 2000.0; uncertainty 1'). By July 28, the flux had increased to 170 mCrab (2-12 keV). The x-ray spectrum is substantially steeper than that of the Crab nebula. The spectrum appears complex, roughly consistent with a 0.7-keV disk blackbody combined with a hard component that extends above 10 keV. The flux is 2.9 x 10E-9 erg cmE-2 sE-1 at 2-20 keV. The interstellar column density appears to be substanitally less than 1.0 x 10E22 atoms cmE-2. Optical and radio observations are strongly encouraged."

BL LACERTAE

J. E. Grove and W. N. Johnson, Naval Research Laboratory, on behalf of the OSSE team, report: "We have detected emission from BL Lac between 50 and 300 keV with the OSSE instrument on the Compton Gamma Ray Observatory. For the period July 15.7-21.0 UT, the flux in this energy band is (9.0 +/- 1.4) x 10E-4 photons cmE-2 sE-1. There is no evidence for variability. The power-law spectrum is hard, with energy index 0.5 (+/- 0.7, 95-percent confidence). Although OSSE has not previously observed BL Lac, objects in this class are not generally detected in this energy range."

G. Madejski, Goddard Space Flight Center (GSFC) and University of Maryland; T. Jaffe, GSFC and Hughes/STX; and M. Sikora, Copernicus Center, Warsaw, report the detection of strong x-ray flux from BL Lac, which is undergoing an outburst in the radio, optical, and GeV bands: "The x-ray flux, measured with the Proportinal Counter Array aboard the Rossi X-ray Timing Explorer about every two days between July 16 and 20, increased from about 2 to about 3.6 x 10E-11 erg sE-1 cmE-2 in the band 2-10 keV, with a very hard energy spectrum, well described as a power law with an energy index of 0.4 +/- 0.1. This is an increase of flux by about a factor of three, and a very significant hardening (from an energy index of 0.9 +/- 0.1) of the x-ray spectrum as compared with the Nov. 1995 ASCA observation."

BL LACERTAE

MXB 1730-335

R. Guerriero, W. H. G. Lewin, and J. Kommers, Massachusetts Institute of Technology, report: "Following detection of the rapid burster MXB 1730-335 with the RXTE's All-Sky Monitor, we observed the source on June 26.19 and 26.33 UT for 1500 and 2300 s, respectively, with the RXTE PCA. A persistent emission level of 4100 +/- 300 counts/s (total PCA countrate, 2-60 keV; 1 Crab = 13 500 counts/s) was inferred for the rapid burster, after subtracting a contribution of 1200 +/- 200 counts/s from 4U 1728-34. Seven type-II bursts were detected, with peak countrates of 1100-2900 counts/s above the persistent emission. The bursts had a 10-20-s smooth rise and a 40-60-s decay, and occurred 450-650 s apart. Type-I bursts show a characteristic spectral softening during burst decay, which was not present in any of these bursts. The spectral hardness ratios during bursts showed little variation from those of the persistent emission. A preliminary analysis of fourier power spectra shows no significant coherent or quasiperiodic oscillations between 1 and 2048 Hz in the bursts or persistent emission. Upper limits (95-percent confidence) on the strength of any narrow features in the power spectra are 3.4- percent rms. This active phase followed the previous one by 7 months (IAUC 6506). Infrared and radio observations are encouraged, particularly if coordinated with the ongoing public RXTE PCA observations."

GRB 970616

V. Connaughton, National Academy of Sciences/National Research Council and Marshall Space Flight Center (MSFC); R. M. Kippen, R. Preece, and G. N. Pendleton, University of Alabama at Huntsville and MSFC; and S. D. Barthelmy, Universities Space Research Association (USRA) and Goddard Space Flight Center (GSFC), report for the BATSE team: "The transient x-ray XTE source reported below by Marshall et al. lies in the error box of a gamma-ray burst seen by BATSE on June 16.757 UT. The Rapid Burst Reponse location, which was distributed through the GRB Coordinates Network/BATSE Coordinates Distribution Network, 20 min after the initial trigger, was R.A. = 1h21m51s, Decl. = -7 09'.4 (equinox 2000.0), with a 1-sigma error radius of 0.3-degree statistical and about 2 deg systematic. The burst was multipeaked, with a total duration of about 200 s. A maximum flux of 23.8 +/- 0.5 photons cmE-2 sE-1 (above 20 keV) was measured 90 s after the trigger time, placing this event in the brightest 2 percent of all BATSE bursts. Its total fluence above 20 keV is estimated to be 4.01 +/- 0.14 x 10E-5 erg cmE-2."

F. E. Marshall, GSFC; T. Takeshima, and S. D. Barthelmy, USRA and GSFC; C. R. Robinson, USRA and MSFC; and K. Hurley, University of California at Berkeley, report the detection of an x-ray afterglow for the gamma-ray burst detected with BATSE on June 16.7568 UT. Scanning observations with the Proportional Counter Array on the Rossi X-ray Timing Explorer (RXTE) 4.0 hr after the burst revealed a source at R.A. = 1h18m.9 +/- 0m.7 (90-percent statistical confidence), Decl. = -5 30' +/- 18' (equinox 2000.0) with a countrate of 4.5 +/- 1.6 counts/s or 1.1 x 10E-11 erg sE-1 cmE-2 in the band 2-10 keV. It is the brightest source detected within the 2-deg BATSE error circle, and it also overlaps the preliminary inter-planetary network (IPN) annulus determined from the time difference in the BATSE and Ulysses detections. The IPN annulus has a radius of 49.808 deg, is centered at R.A. = 22h10m30s, Decl. = -27o24'.8, and has a full width of 4'.6. An annulus with a smaller width will be available after further processing. The intersection of the error regions has corners at 1h19m.6, -5 49'; 1h19m.6, -5 40'; 1h18m.4, -5 12'; 1h18m.2, -5 16'. A subsequent RXTE observation on June 18.08 detected no significant flux from this position, with a preliminary 90-percent confidence upper limit of 2.5 counts/s.

Eta Car

GRS 1915+105

GRS 1915+105

R. Remillard and E. Morgan, Massachusetts Institute of Technology (MIT); J. Swank, Goddard Space Flight Center (GSFC); and the RXTE ASM instrument team at MIT and GSFC report: The RXTE All Sky Monitor has detected bright flares from GRS 1915+105, ending a long interval of steady hard-x-ray emission at 200-300 mCrab (2-12 keV) that had been observed since early Dec. 1996. Recent flares reached 0.69 Crab on 1997 May 6.75 UT and 0.81 Crab on May 8.09. The flares are evident on timescales at least as short as hours, and there is a moderately softer x-ray spectrum at 2-12 keV. We encourage radio and infrared observations.

X 0726-260

R. Corbet, Goddard Space Flight Center (GSFC) and Universities Space Research Association, and A. Peele, GSFC and National Research Council report: A scan over the position of the transient Be star x-ray source X0726-260 on May 5 with the PCA on RXTE detected emission at a level of approximately 10 mCrab, showing the source to be active. An examination of the ASM light curve of X0726-260, obtained between 1996 Jan. 5 and 1997 May 1, shows a mean flux of approximately 2 mCrab, and the Fourier Transform of this curve has its strongest peak at a period of 34.5 days and an amplitude of approximately 1.6 mCrab. A sine-wave fit at this period yields maximum predicted flux on May 4, consistent with the scan detection. With this ephemeris the next predicted maximum is on June 7, and further observations are encouraged at that time.

V1333 AQUILAE

GRO J1655-40

E. Kuulkers, University of Oxford; T. Belloni, M. Mendez, M. van der Klis, and R. Wijnands, Astronomical Institute 'Anton Pannekoek', University of Amsterdam (UoA); and J. van Paradijs, University of Alabama at Huntsville and UoA, report: Using RXTE All Sky Monitor data, we discovered about twenty drops in the 2--12-keV intensity by about 25-95 percent during 90-s measurements of the relativistic jet blackhole binary GRO J1655-40 in its current outburst (IAUC 6393). During these drops, the spectral hardness increases. Public RXTE PCA observations on Feb. 26.8-27.0 UT show two additional drops by about 92 percent with durations of order 1 min. A period of 2.621 +/- 0.001 days (1-sigma) is best- fit with the occurrence of the ASM and PCA intensity drops. This is consistent with the optically determined period of the system (Orosz and Bailyn 1997, Ap.J. 477, 876). All drops occurred between orbital phases 0.72 and 0.86. This constitutes the first orbital evidence of GRO J1655-40 in x-rays. Similar orbital dips have previously been seen in low-mass x-ray binary 'dip' sources. Further monitoring at all wavelengths at these orbital phases is encouraged.

X 2127+119

R. Corbet, Goddard Space Flight Center (GSFC) and Universities Space Research Association; A. Peele, GSFC and National Research Council; and D. A. Smith, Massachusetts Institute of Technology (MIT), report on behalf of the RXTE ASM team at GSFC/MIT: Observations of this low-mass x-ray binary in M15, obtained between 1996 Jan. 1 and 1997 Apr. 10 with the ASM on RXTE, show (in addition to considerable non-periodic variability) the presence of a modulation with a period of 37 days about the average 2-12-keV flux of 15 mCrab. This modulation is most prominent in the lowest energy band (1.3-3.0 keV) at an average level of about 35 percent and a FWHM in the Fourier transform of about 3.5 days, but is undetectable in the highest energy band (4.8-12 keV). Maximum brightness occurs at an epoch of about 1996 May 25. Modulation at the 17.1123-hr orbital period is also seen. We encourage continued observations at x-ray and optical wavebands to search for other manifestations of the 37-day period and to determine whether this is a persistent property of X2127+119.

GRS 1737-31

W. Cui and E. H. Morgan, Massachusetts Institute of Technology; W. A. Heindl, University of California, San Diego; J. H. Swank, Goddard Space Flight Center; and D. M. Smith, University of California, Berkeley, report: Preliminary results of the RXTE observation noted on IAUC 6603 reveal remarkable similarities to the blackhole candidate Cyg X-1 in its hard (or low) state. The power-density spectrum is flat below about 0.03 Hz and roughly inversely proportional to the frequency above, with an integrated rms variability of 21.4 percent. The energy spectrum can be characterized by a power law with photon index about 1.6. A more detailed analysis is required to determine if an iron line at about 6.4 keV is accounted for by the diffuse x-ray emission in the central region of the galactic plane. The best-fit hydrogen column density is 4-6 x 10E22 cm^2, depending on the diffuse background, in any case consistent with a location near the galactic center. Fluxes in the bands 2-10, 10-35, 35-75, and 75-150 keV were 18, 37, 60, and 90 mCrab, respectively. Assuming a distance of 8.5 kpc, the 2-150-keV source luminosity was 1.9 x 10E37 erg/s. Similarities to Cyg X-1 make GRS 1737-31 a likely black hole candidate. Further observations at all wavelengths, in particular, infrared and radio, are encouraged.

GRS 1737-31

F. E. Marshall, Goddard Space Flight Center; and D. M. Smith, University of California, Berkeley, report for the RXTE team: Recent scans over the position of the new x-ray transient GRS 1737-31 (IAUC 6599) with the Proportional Counter Array on the Rossi X-ray Timing Explorer reveal that the outburst began between Feb. 17.0 and 20.4 UT and peaked about 2 weeks later. The following fluxes in milliCrabs (2-10 keV) were detected: Feb. 17.0, < 3; 20.4, 10; 27.1, 14; 27.9, 25; Mar. 6.1, 23; 9.9, 25; 11.0, 20; 19.9, 17. A pointed observation with the PCA and HEXTE was carried out during Mar. 21.69-21.73.

GX 17+2

M. van der Klis, J. Homan, and R. Wijnands, Astronomical Institute 'Anton Pannekoek', University of Amsterdam (UA); E. Kuulkers, Oxford University; F. K. Lamb and D. Psaltis, University of Illinois at Urbana/Champaign; S. Dieters, University of Alabama at Huntsville (UAH); J. van Paradijs, UAH and UA; W. H. G. Lewin, Massachusetts Institute of Technology; and B. Vaughan, California Institute of Technology, report: RXTE observations of the Z-source and low-mass-x-ray-binary GX 17+2 that we obtained during Feb. 6-9 show quasiperiodic oscillations (QPOs) at the kHz level. We find two peaks in the power spectra of the x-ray countrate variations. The frequency of the lower-frequency peak decreases from about 880 to 682 +/- 5 Hz when the countrate increases from 1600 to 1920 counts/s. This is consistent with a positive correlation between frequency and mass-accretion rate, as the source is on the normal branch in the x-ray color-color diagram during our observations, where countrate and accretion rate are thought to be anticorrelated. The higher-frequency peak is detected only at high countrate, at a frequency of 988 +/- 3 Hz. The peak separation is then 306 +/- 5 Hz. Peak widths range from 30 to 200 Hz, and QPO amplitudes are between 3 and 5 percent (rms), with no significant dependence on countrate. We also observe simultaneous horizontal-branch oscillations of about 60 Hz, but no 6-Hz QPOs, consistent with the source being in the upper-normal branch. The kHz QPOs were detected in the band 8-60 keV and are very difficult to see in the more usual 2-60-keV band. X-ray observers are advised to use high time-resolution modes, which allow isolating the high-energy photons, in ongoing observational programs of bright low-mass x-ray binaries in order to be able to detect the phenomenon.

V1333 Aquilae

A. M. Levine, Massachusetts Institute of Technology (MIT), and B. Thomas, Goddard Space Flight Center (GSFC), on behalf on the RXTE ASM team at GSFC and MIT; M. Garcia and P. Berlind, Smithsonian Astrophysical Observatory; and P. Callanan, University College Cork, report that the low-mass x-ray binary Aquila X-1 (= V1333 Aql) appears to be in a moderate-intensity outburst in quick- look results from the RXTE All Sky Monitor. Recent daily-average flux levels in the band 2-12 keV are as follows: Jan. 27, about 60 mCrab; 28, 120; 29, 190; 30, 260; Feb. 1, 310; 2-5, about 360. Observations of the optical counterpart V1333 Aql from the Whipple Observatory 1.2-m telescope on Feb. 6.543 UT yield V = 16.9. Observations at other wavelengths are encouraged.

GRO J2058+42

R. Corbet, Goddard Space Flight Center (GSFC) and Universities Space Research Association, on behalf on the RXTE ASM team at GSFC and the Massachusetts Institute of Technology (MIT); A. Peele, GSFC and National Research Council; and R. Remillard, MIT and RXTE ASM team, report: Analysis of the RXTE ASM light curve of the 195.6-s x-ray pulsar GRO J2058+42 obtained between 1996 Jan. 5 and 1997 Jan. 30 shows the presence of a modulation with a period of approximately 54 days. This period is half the 110-day period reported from BATSE observations (IAUC 6514), although the ASM is sensitive to lower-energy x-rays in the range 2-12 keV. A sine- wave fit to the light curve yields a period of 53.9 +/- 1.2 days with epoch of maximum flux at 1996 Oct. 2.4 UT +/- 2.0 days. The mean x-ray flux during this period was approximately 3 mCrab with outbursts reaching approximately 8 mCrab.

GRO J1744-28

M. J. Stark, University of Maryland and Goddard Space Flight Center (GSFC); K. Jahoda and J. H. Swank, GSFC; and T. Strohmayer, GSFC and Universities Space Research Association, report: We observed GRO J1744-28 on Jan. 18 with the Rossi X-ray Timing Explorer (RXTE) and confirm the earlier report (IAUC 6530) that it is again in outburst. The flux detected by the Proportional Counter Array on RXTE is roughly equivalent to the flux detected from the Crab nebula. The fluxes observed during our first eight days of monitoring are as follows: Jan. 18, 12 800 counts/s; 19, 12 700; 20, 13 100; 21, 13 100; 22, 12 800; 24, 13 500; 25, 13 400; 26, 13 200; 27, 13 500 (where 10 000 counts/s implies a luminosity of 3.5 x 10E38 erg/s in the range 2-60 keV for this source at 8 kpc). The luminosity during bursts reaches as high as 1.2 x 10E40 erg/s. The average interval between bursts is 38 min. The burst duration is 10-12 s, which is similar to the duration of bursts on 1996 Mar. 1, when the source was last at this intensity. The current x-ray flux is intermediate between the flux corresponding to the possible infrared detection of a counterpart (x-ray flux about 1.7 Crab) and the flux corresponding to several upper limits (x-ray flux about 0.5 Crab; IAUC 6369; Cole et al. 1997, Ap.J., in press, with preprint available at http://xxx.lanl.gov/e-print/ astro-ph/9610079). Despite the difficulty of observing near the sun we urge renewed searches for the infrared counterpart.

XTE J1709-267

R. M. Hjellming and M. P. Rupen, National Radio Astronomy Observatory, report the discovery of a candidate for the radio counterpart of the x-ray nova XTE J1709-267 (IAUC 6543). VLA observations on Jan. 25.62 UT were used to detect a radio source with flux densities of about 104 and about 111 mJy at 1.42 and 4.9 GHz, respectively, at a position of R.A. = 17h09m09s.23, Decl. = -26d42'19".0 (equinox 2000.0, estimated error < 1"). This was the only detected radio source in the RXTE error region, and the probability of a 4.9-GHz background source of this strength in the field is about 0.002. They add that further observations are underway to search for the variability typical of x-ray novae.

Cygnus X-1

R. P. Fender and C. Brocksopp, University of Sussex; and G. G. Pooley, University of Cambridge, report: We have detected the 5.6-day orbital modulation of the blackhole candidate Cyg X-1 in radio monitoring at 15 GHz with the Ryle Telescope (Cambridge), undertaken since 1996 Oct., during which time the source has been in a hard x-ray state. This confirms the apparent orbital modulation in previous Very Large Array data (Han and Hjellming, Ap.J., submitted). The modulation is approximately sinusoidal, with a semiamplitude of about 3 mJy superimposed on a mean flux density of about 14 mJy for the entire data set. Orbital flux minimum occurs at superior conjunction of the compact object, as observed also in ASM/XTE x-ray data (IAUC 6510). This implies that the radio-emitting region is associated with the compact object, and the orbital modulation may be due to varying line-of-sight optical depth through the stellar wind of the OB-type companion star. The orbital modulation is superimposed upon other trends that also appear to be correlated with the x-ray activity.

4U 1636-53

W. Zhang, Goddard Space Flight Center (GSFC); I. Lapidus, University of Cambridge; J. H. Swank and N. E. White, GSFC; and L. Titarchuk, GSFC and George Mason University, report: Four type-I x-ray bursts from 4U 1636-53 were detected during 1996 Dec. 25-1997 Jan. 10 with the RXTE. Each burst has nearly-coherent brightness oscillations at frequency 581 Hz, with fractional rms amplitudes ranging from 2.5 +/- 0.3 to 8.7 +/- 0.5 percent in the RXTE/PCA 2- 60-keV band (stronger bursts having lower fractional rms amplitudes). No similar oscillations were detected at any other frequencies during these bursts. Also detected are two QPOs with frequencies in the ranges 840-920 and 1150-1200 Hz in the persistent flux; when both are present at the same time, their frequency difference appears to be a constant of 255 +/- 25 Hz, similar to that reported on IAUC 6428. The 581 Hz differs from the frequency difference of the two simultaneous QPOs in the persistent flux. Considering its stability from burst to burst and its level of coherence, we think that 581 Hz, or possibly 290 Hz (although we have not observed any significant excess power for it), is most likely the neutron-star spin frequency. This is a second case, after Sco X-1 (IAUC 6424), where the frequency difference of the two QPOs in the persistent flux is not necessarily a valid measure of the neutron-star spin frequency, as a simple beat-frequency XTE model would require. This is in contrast to the case of 4U 1728-34 (IAUC 6387), where the frequency difference and the frequency found during bursts are the same (namely, 363 Hz).

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.