RXTE GOF: 2000 IAU Circulars

RXTE GOF	2000 RXTE IAU Circulars	RXTE FAQ

With the kind permission of Dr Brian Marsden of the International Astronomical Union's Central Bureau for Astronomical Telegrams, the full text of IAU circulars concerning RXTE is reprinted here - a week after their original promulgation by the IAU.

Please note that the text below includes only the parts of the IAUC relevant to RXTE, and that some IAUC contain more than one bulletin about RXTE.

7531, 7523, 7522, 7487, 7485, 7482, 7456, 7454, 7446, 7429, 7427, 7425, 7424, 7402, 7401, 7399, 7389, 7377, 7369, 7365, 7363, 7358, 7355

November 29, 2000 - 7531 KS 1947+300 = GRO J1948+32 KS 1947+300 = GRO J1948+32 J. Swank, Goddard Space Flight Center; and E. Morgan, Massachusetts Institute of Technology, report that the transient x-ray source KS 1947+300 (IAUC 7523) is a pulsar with a barycentric period on Nov. 21 of 18.7579(5) s. This period is consistent with the object's being identical with the pulsar GRO J1948+32 (Chakrabarty et al. 1995, Ap.J. 446, 826), having slowed down from the 18.70 s of an outburst in 1994 at an average rate of 8 ms/yr. The flux was 4.5 x 10^-10 erg cm-2 s-1 above 2 keV, with a cutoff power-law spectrum (photon index about 0.6, with a 10-keV e-folding energy above 6 keV) and a small column density (< 10^21 cm-2). A sequence of RXTE observations was undertaken, with the source last seen on Nov. 28 at a flux diminished by a factor of four. November 24, 2000 - 7523 KS 1947+300 KS 1947+300 A. Levine, Massachusetts Institute of Technology (MIT), and R. Corbet, Goddard Space Flight Center (GSFC) and Universities Space Research Association, report on behalf of the RXTE ASM team at MIT and GSFC: "RXTE All-Sky Monitor observations of the transient x-ray source KS 1947+300 (Borozdin et al. 1990, Sov. Astron. Lett. 16, 345) show that the source is currently exhibiting an outburst that began around Oct. 23 and reached a maximum flux of about 20 mCrab (1.5-12 keV) around Nov. 8. The source is at most weakly detected (flux < 6 mCrab) prior to the outburst. A search for periodic modulation in the pre-outburst light curve suggests a possible period of 41.7 +/- 0.1 days with a semi-amplitude of about 1 mCrab and an average maximum flux of about 4 mCrab. The epoch of maximum flux is 1998 July 24.5 UT +/- 4 days. These parameters would predict a maximum near Nov. 4. The optical counterpart of KS 1947+300 may be an early-type star (e.g., Goranskij et al. 1991, Sov. Astron. Lett. 17, 399). Further observations are encouraged." November 20, 2000 - 7522 HERCULES X-1 HERCULES X-1 B. Boroson, College of Wooster; S. D. Vrtilek, Smithsonian Astrophysical Observatory; P. Boyd, NASA Goddard Space Flight Center; and A. Levine, Center for Space Research, Massachusetts Institute of Technology, write: ``Data from the All Sky Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE) suggest that the x-ray binary pulsar Hercules X-1 has emerged from the x-ray low state it entered in Mar. 1999 (IAUC 7139). In the period from Nov. 13.0 to 17.2 UT, four out of five averages of 25 ASM observations each detect Hercules X-1 with at least 15-sigma significance. The count rate peaks near 5 counts/s, corresponding to a flux of 60 mCrab. The average of 25 observations that was not 15-sigma- significant overlapped the expected eclipse of Hercules X-1. Hercules X-1 was also detected by the ASM over an about 5-day period with a peak of 6 counts/s (80 mCrab) on Oct. 12 (35 days prior to the current detection). Hercules X-1 historically has shown a 35-day x-ray cycle peaking near 6 counts/s in the RXTE ASM. Pointed x-ray observations to determine the spectrum and spin evolution are encouraged, as are observations at other wavelengths. Real-time quick-look data can be obtained at http://heasarc.gsfc.nasa.gov/cgi-bin/xte_weather/xte_lc?source=HERX1, and daily averages of quick-look data over the entire mission can be obtained at http://xte.mit.edu/." September 5, 2000 - 7482 4U 0115+63 4U 0115+63 W. Coburn, R. Rothschild, and W. A. Heindl, University of California at San Diego, report: "An outburst of the Be-transient x-ray pulsar 4U 0115+63 has been detected by the RXTE/ASM. The daily averaged ASM countrate began to increase on Aug. 27, which coincided with periastron of the source. In the first four days, the ASM countrate rose to > 7 counts/s. This is a faster rise than the 1999 Mar. outburst (IAUC 7116), which began several days after periastron, and a similar countrate was achieved 9 days after that. On 2000 Sept. 1.06 UT, the RXTE made a 1000-s pointed observation. Strong pulsations were detected at the 3.61-s pulse period, confirming that it is the pulsar currently in outburst. The 2-100- keV flux was (6.5 +/- 0.3) x 10**-9 erg cm**-2 s**-1, and at a distance of 3500 pc had an x-ray luminosity of 9.6 x 10**36 erg/s. Preliminary fitting of the joint PCA/HEXTE spectrum reveals two cyclotron absorption features at about 15 and 23 keV, similar to what was observed the previous outburst (IAUC 7126). Because phase-averaged spectra are complex due to pulse-phase variability, these line energies are not well determined. Additional observations are encouraged. The RXTE/ASM real-time light curve is available at http://heasarc.gsfc.nasa.gov/FTP/xte_weather/." September 1, 2000 - 7485 MXB 1659-29 MXB 1659-29 R. Wijnands, Massachusetts Institute of Technology; T. Strohmayer, Goddard Space Flight Center; and L. M. Franco, University of Chicago, report: "Using archival data from the PCA instrument on the Rossi X-ray Timing Explorer, we have detected x-ray brightness oscillations with a frequency of about 567 Hz in five type-I x-ray bursts observed from the x-ray transient and eclipsing binary MXB 1659-29. This source is still active more than a year after its reappearance in Apr. 1999 (IAUC 7138, 7139). The oscillations have fractional rms amplitudes of 10-20 percent and are most likely related to the spin frequency of the neutron star in MXB 1659-29. The oscillations are preferentially detected between orbital phase 0.05 and 0.75 (using the orbital ephemeris provided by Wachter et al. 2000, Ap.J. 534, 367) and only marginally detectable during intervals of x-ray dipping activity. This strongly suggests that the mechanism behind the x-ray dips inhibits the detectability of the oscillations. More observations are needed of MXB 1659-29 outside the x-ray dips and x-ray eclipses (i.e., between orbital phase 0.05 and 0.75) in order to study these oscillations before the source returns to quiescence." August 25, 2000 - 7482 EXO 1745-248 EXO 1745-248 C. B. Markwardt, University of Maryland and Goddard Space Flight Center (GSFC), NASA; and T. E. Strohmayer, J. H. Swank, and W. Zhang, GSFC, write: "The transient EXO 1745-248 in the globular cluster Terzan 5 (cf. IAUC 7454) has recently increased in intensity. RXTE PCA pointed and scanning observations from July 13-Aug. 15 show that the persistent intensity has varied erratically, increasing to 600 mCrab (2-10 keV) on Aug. 21.8 UT, which is near the Eddington limit for a source at 7600 pc. Dipping activity is also present, but no eclipse events have been seen. Fifteen x-ray bursts were detected, with peak fluxes between 200 and 350 mCrab and an average separation of 25 min. The x-ray spectrum of the bursts at the peak was consistent with blackbody emission with kT = 2.4-2.8 keV and radii of 2-4 km. Only modest cooling is suggested in the tails of the bursts. The short burst recurrence time, the range of peak luminosities, and the weak cooling suggest that the bursts may be of type II (accretion instability), but we cannot exclude a thermonuclear origin. The persistent emission of Aug. 13 is best fitted by a blackbody and power law with exponential cut-off plus an iron emission feature near 6.6 keV, typical of bright, low-mass x-ray binaries. Two quasiperiodic features were present on Aug. 13, with centroids of 65 and 134 mHz, the fundamental having a fractional r.m.s. amplitude of 10 percent. Optical observations are encouraged." July 16, 2000 - 7456 XTE J1859+226 XTE J1859+226 J. A. Tomsick and W. A. Heindl, Center for Astrophysics and Space Sciences, University of California at San Diego, write: "An RXTE observation of the x-ray transient XTE J1859+226 made on July 8.5 UT indicates a 2.5-20-keV flux level of 3 x 10**-11 erg cm**-2 s**-1, which is well above the expected quiescent x-ray flux level. The higher-than-expected x-ray flux may be related to the optical minioutburst reported on IAUC 7451. The energy spectrum is well- described by a power law with a photon index of 2.03 +/- 0.14. There is evidence for two short (< 16 s) x-ray dips during the 3000-s RXTE observation, possibly indicating a moderately high binary inclination. During the deepest dip, the flux drops to 33 +/- 19 percent of the mean flux level in the energy band 2.9-10.5 keV. Further x-ray observations are required to determine if the dips are related to the binary orbit." July 14, 2000 - 7454 EXO 1745-248 = XB 1745-25 EXO 1745-248 = XB 1745-25 C. B. Markwardt, University of Maryland and Goddard Space Flight Center (GSFC); and J. H. Swank, GSFC, report:"RXTE PCA scans of the galactic bulge on July 2.73 and 6.69 UT have detected a transient source. The position is consistent with a reappearance of the source EXO 1745-248 = XB 1745-25, an x-ray burster in the globular cluster Terzan 5 (cf., e.g., IAUC 3506). The best position as observed by ROSAT was R.A. = 17h48m05s.0, Decl. = -24o46'47" (equinox 2000.0; Johnston et al. 1995, A.Ap. 298, L21). The RXTE observations are inconsistent with the position of Johnston's nearby source S2 (= 1RXS 174746.0-24451; 6'.9 away) at a high confidence level. The 2-10-keV x-ray fluxes in the PCA observations were 54 and 94 mCrab, indicating that the intensity is rising. Pointed PCA observations are being scheduled; follow-up observations by imaging instruments are encouraged." July 1, 2000 - 7446 4U 2206+54 4U 2206+54 R. Corbet, Goddard Space Flight Center (GSFC) and Universities Space Research Association; R. Remillard, Massachusetts Institute of Technology (MIT); and A. Peele, University of Melbourne, report on behalf of the RXTE ASM team at GSFC and MIT: "We have found that the RXTE ASM lightcurve of the Be star and x-ray source 4U 2206+54 had previously been made using a position differing by 0.5 deg from the correct location of R.A. = 22h07m57s.0, Decl. = +54o31'06" (equinox 2000.0; Steiner et al. 1984, Ap.J. 280, 688). We have now produced and analyzed a revised lightcurve. The mean source flux is about 5 mCrab, and we find quasisinusoidal modulation at a period of 9.570 +/- 0.004 days with a semiamplitude of about 1.5 mCrab. There was a maximum on 1998 July 12.1 +/- 0.2 UT. If this periodicity is due to orbital motion, then it would be one of the shortest known for a Be-star x-ray source. However, the relatively low luminosity (< 10**35 ergs/s at 2.5 kpc) would run counter to the general trend for short-orbital-period Be/neutron-star systems to have higher luminosities. Observations at other wavelengths, including optical spectroscopy and photometry, are encouraged." May 24, 2000 - 7429 V1333 AQUILAE R. Jain, C. Bailyn, and P. Coppi, Yale University; M. Garcia, Harvard-Smithsonian Center for Astrophysics; A. Levine, Massachusetts Institute of Technology; J. Orosz, Utrecht University; P. Lu, Western Connecticut State University; and J. Espinoza and D. Gonzalez, Cerro Tololo Interamerican Observatory (CTIO), report YALO consortium observations: "The optical outburst of V1333 Aql = Aquila X-1, which began on May 11.23 UT (IAUC 7423), did not develop into a full outburst. By May 17.25, the object was back to quiescence. The peak amplitude of the outburst was 0.34 +/- 0.02 mag brighter than the quiescent mean in R and occurred on May 12.37. Two observations with the PCA instrument on RXTE lasting 1500 and 2500 s were taken on May 14.38 and 16.91, respectively. There is an indication that the source was detected with the PCA, but the count rates were heavily background-dominated and low enough that preliminary analysis does not allow a definitive determination of the flux. We therefore quote an upper limit of 4 x 10**-11 erg cm**-2 s**-1 (2.5-20 keV). A similar truncated 'mini-outburst' also occurred in Sept. 1998 (IAUC 7017, 7025)." May 20, 2000 - 7427 XTE J1118+480 XTE J1118+480 C. A. Haswell, Open University; D. Skillman, Laurel, MD; J. Patterson, Columbia University; R. I. Hynes, Southampton University; and W. Cui, Massachusetts Institute of Technology, on behalf of a larger collaboration, report: "Hubble Space Telescope observed XTE J1118+480 for a total integration time of 2.54 hr (range 115-170 nm) on Apr. 8. The power density spectrum (PDS) is roughly flat below about 0.02 Hz and breaks into a power law (roughly 1/f) above. Such a PDS shape is characteristic of blackhole candidates or 'atoll-type' low-mass x-ray binaries in the low state. In addition, the ultraviolet PDS reveals a broad quasiperiodic oscillation (QPO) at 0.08 Hz with FWHM 0.02 Hz. Unfiltered CCD observations (0.68-m f/2.9 telescope) during Apr. 30-May 3 with 0.32-Hz sampling show a QPO at 0.101 Hz (FWHM 0.008 Hz). Both the low-state PDS shape and the QPO are confirmed by preliminary results from RXTE, which indicate that the source has remained in such a low state throughout the outburst. Comparison of simultaneous HST near- ultraviolet (160-235 nm) and RXTE lightcurves reveals a correlation on timescales of seconds. A preliminary analysis indicates that ultraviolet variability lags behind x-rays by 1-2 s. This is likely due to light-echoes caused by reprocessing of the x-ray flares within the binary system; the delay is consistent with expected light-travel-time delays within a 4-hr x-ray binary (IAUC 7397). Further high-time-resolution observations at all wavelengths are urged to investigate light-echoes further (see http://phys-ftp.open.ac.uk/pub/1118/satobs)." K. Yamaoka, Y. Ueda, and T. Dotani, Institute of Space and Astronautical Science; and P. Durouchoux and J. Rodriguez, Service d'Astrophysique, Saclay, report observations of XTE J1118+480 with the ASCA satellite during from May 11.527-12.031 UT: "The x-ray intensity showed rapid variability over an averaged flux level of 8 x 10**-10 erg cm**-2 s**-1 (2-10 keV). A QPO feature was found at 0.115 +/- 0.002 Hz with an amplitude of about 6 percent rms in the averaged power spectrum; this QPO was confirmed by simultaneous RXTE data, which covered two epochs during May 11.729-11.748 and 11.781-11.931. The QPO frequency has shifted from the value reported before May 4 (0.085 +/- 0.002 Hz) by Revnivtsev et al. (http://xxx.lanl.gov/abs/astro-ph/?0005212). The ASCA GIS spectrum in the range 0.7-10 keV is about represented by a power law, over which a slight soft excess is seen below 2 keV. Applying a disk blackbody and power-law model yields an acceptable fit with a temperature of the innermost disk of 0.2 +/- 0.1 keV, and a power- law photon index of 1.76 +/- 0.02, with an upper limit to an absorption column density of 10**21 cm**-2." May 17, 2000 - 7425 LMC X-3 LMC X-3 J. Homan, P. G. Jonker, and M. van der Klis, Astronomical Institute 'Anton Pannekoek', University of Amsterdam; and E. Kuulkers, Space Research Organization Netherlands, Utrecht, and Astronomical Institute, Utrecht University, report: "The blackhole candidate LMC X-3 was observed with the RXTE/PCA during May 13.894- 13.995 UT. The 2-30-keV power spectrum of that observation reveals strong variability, with a fractional rms amplitude of about 40 percent (0.01-100 Hz). The shape of the power spectrum can be characterized by a broken power law, with a break at 1.1 +/- 0.4 Hz. In addition a quasiperiodic oscillation was found at 0.42 +/- 0.02 Hz, with a strength of 12 +/- 2 percent (3.7-sigma, single trial), and a width of 0.10 +/- 0.04 Hz. Spectral fits yield values similar to those reported by Boyd and Smale (IAUC 7424). This is the first time that LMC X-3 has been found to show rapid time variability similar to that of blackhole candidates in their low/hard state. Together with the spectral measurements, this strongly suggests that LMC X-3 has entered such a low/hard state." May 13, 2000 - 7424 LMC X-3 LMC X-3 P. T. Boyd and A. P. Smale, Universities Space Research Association and Goddard Space Flight Center, report: "RXTE/ASM observations of LMC X-3, a bright, persistent blackhole binary usually observed in the high/soft state, indicate that the source has entered a rare extended low/hard state, beginning about Apr. 10. RXTE/PCA observations on May 5.76 and 10.01 UT reveal a pure power-law spectrum with photon index 1.64 +/- 0.11 and 2-10-keV flux of 8.55 x 10**36 erg/s at 50 kpc. An additional PCA observation is planned. Radio and optical observations are encouraged so that LMC X-3 can be compared to persistent Galactic blackhole binaries that are most commonly observed in the low/hard state." April 14, 2000 - 7402 SMC X-2 SMC X-2 R. Corbet, Goddard Space Flight Center (GSFC) and Universities Space Research Association; and F. E. Marshall, GSFC, report on behalf of the RXTE teams at GSFC and the Massachusetts Institute of Technology: "We have discovered pulsed x-ray emission from a position consistent with the location of SMC X-2. RXTE ASM observations indicate activity since the middle of January with a mean flux of about 10 mCrab. Slews with the PCA over SMC X-2 on Apr. 9 and 12 show a source at R.A. = 0h54m53s, Decl. = -73o38', with a 90-percent-confidence error circle of 3' radius and a 2-10- keV flux of about 3 mCrab. A pointed PCA observation on Apr. 12 shows pulsations at a period of 2.374 +/- 0.007 s with a pulsed fraction of about 30 percent. Imaging x-ray observations, and observations of the suggested optical counterpart (Murdin et al. 1979, MNRAS 186, 43P), are encouraged." April 13, 2000 - 7401 XTE J1118+480 C. Mauche, Lawrence Livermore National Laboratory; R. Hynes and P. Charles, University of Southampton; and C. Haswell, Open University, on behalf of the EUVE project at the University of California at Berkeley, report: "EUVE obtained a 20 500-s exposure of XTE J1118+480 coincident with RXTE and Hubble Space Telescope (HST) observations during Apr. 8.10-8.71 UT. The mean count rate in the DS photometer was about 1 count/s and the extreme- ultraviolet (EUV) spectrum is apparent in the SW spectrometer from the left edge of the detector at about 7.0 nm to about 12.0 nm. A crude estimate of the flux density at 9 nm is 4 x 10**-13 erg cm**-2 s**-1 A**-1. This is the first EUV spectrum of a blackhole- candidate x-ray binary. Additional EUVE observations are planned during observations by RXTE on Apr. 13 (20 000 s) and by RXTE, Chandra, and the HST on Apr. 18 (100 000 s)." April 10, 2000 - 7399 XTE J1550-564 XTE J1550-564 D. A. Smith, A. M. Levine, R. Remillard, and D. Fox, Massachusetts Institute of Technology; and R. Schaefer, Goddard Space Flight Center, NASA, on behalf of the RXTE/ASM Team, report: "Observations over Apr. 6-10 with the RXTE All-Sky Monitor show a clear detection of a source with maximum likelihood position 1' +/- 3' from the position of XTE J1550-564 (IAUC 7009, 7197, etc.). It is therefore highly likely that we have detected renewed activity of XTE J1550-564. A 2-12-keV light curve from this location shows an approximately linear flux increase at a rate of about 15 +/- 2 mCrab/day. Extrapolation of this rate indicates zero flux on Apr. 2. The energy spectrum is slightly harder than that of the Crab, and there is as yet no evidence for spectral evolution." N. Masetti, Istituto Tecnologie e Studio Radiazioni Extraterrestri, CNR, Bologna; and R. Soria, Mullard Space Science Laboratory, University College, London, communicate: "We observed the optical counterpart of the soft-x-ray transient XTE J1550-564 on Apr. 10.58 UT, using the CCD imager on the 1.02-m telescope at Siding Spring Observatory. From a preliminary analysis of the data, we find B = 19.73 +/- 0.05, V = 18.14 +/- 0.03, R = 17.16 +/- 0.05. This shows that XTE J1550-564 is again in an outburst phase. Further multiwavelength observations of the source are encouraged." Soria and Masetti add that further CCD images of the optical counterpart, centered on Apr. 10.79 UT, show that the source is brightening at a rate of 0.30 +/- 0.05 mag/day in the B band, 0.25 +/- 0.05 mag/day in the V band, and 0.20 +/- 0.03 mag/day in the R band. March 30, 2000 - 7389 XTE J1118+480 XTE J1118+480 R. Remillard, E. Morgan, D. Smith, and E. Smith report for the RXTE ASM team at the Massachusetts Institute of Technology and at Goddard Space Flight Center, NASA: "The RXTE All-Sky Monitor has detected x-ray emission from a new source, XTE J1118+480. The ASM position is centered at R.A. = 11h18m17s, Decl. = +48o03'.0 (equinox 2000.0; 90-percent-confidence radius 6'). The average x-ray intensity (2-12 keV) was 39(8) mCrab on Mar. 29. Retrospective ASM analysis indicates that the x-ray flux has been slowly rising since Mar. 5. Another modest outburst had previously occurred during Jan. 2-29, peaking at 37(3) mCrab on Jan. 6. The SIMBAD database lists only an unremarkable guide star (V about 11) near the x-ray position. The source was confirmed in an 800-s RXTE pointed observation beginning on Mar. 29.968 UT. Rapid flares (e.g., 10 s) were seen, reaching a factor of 5 above the baseline flux near 25 mCrab and suggesting a Galactic x-ray source. No pulsations were detected. The x-ray spectrum does not vary appreciably during intense flares. Preliminary spectral analysis indicates a power law visible to 30 keV. The photon index is roughly 1.8, which is similar to Cyg X-1 in its hard state. The high galactic latitude (+62 deg) and x-ray properties present a puzzling combination, and observations at other wavelengths are urgently needed." March 09, 2000 - 7377 SAX J1747.0-2853 SAX J1747.0-2853 C. B. Markwardt, University of Maryland and Goddard Space Flight Center (GSFC); and F. E. Marshall and J. H. Swank, GSFC, report that RXTE PCA scans of the galactic bulge on Mar. 2.45 UT have detected a bright transient source near the galactic center. The position is consistent with a reappearance the bursting source SAX J1747.0-2853, discovered by the SAX Wide Field Camera in 1998 (IAUC 6846). The source was again detected in two follow-up pointed PCA observations and a subsequent galactic bulge scan. No significant pulsations or quasiperiodic oscillations were found. One x-ray burst was seen during the scan of Mar. 7.94. The approximate x-ray intensities in the band 2-10 keV were: Mar. 2.45, 42 mCrab; 6.37, 129; 7.94, 140; 8.38, 130. As the intensity of the source may still be increasing, further observations are encouraged. February 28, 2000 - 7369 XTE J1543-568 XTE J1543-568 T. Takeshima, Goddard Space Flight Center (GSFC) and Universities Space Research Association (USRA); F. E. Marshall, GSFC; R.~H.~D. Corbet, GSFC and USRA; and J. in 't Zand, Space Research Organization Netherlands, report an improved position for the 27.12-s pulsar XTE J1543-568 (IAUC 7363) from an RXTE/PCA observation on Feb. 14.04 UT: "The best-fit position is R.A. = 15h44m13s, Decl. = -56o44'.5 (equinox 2000.0; 90-percent confidence error radius 4'). The source flux and the pulsed fraction remain at about 6 mCrab and 60 percent (2-10 keV), respectively. Observations at other wavelengths are encouraged." February 14, 2000 - 7365 CYGNUS X-3 M. L. McCollough and G. J. Fishman, Marshall Space Flight Center, for the Compton Gamma Ray Observatory (CGRO) BATSE team; and E. B. Waltman, Naval Research Laboratory, report: "The galactic jet source Cygnus X-3 has been exhibiting very low flux levels in radio and hard x-rays (20-100 keV) since Feb. 4. Such behavior usually precedes major radio flaring episodes, with radio fluxes > 3 Jy (McCollough et al. 1999, Ap.J. 517, 951). Very low radio fluxes of < 30 mJy have been observed at 2 GHz with the Green Bank Interferometer between Feb. 4 and 9. Daily averages for Jan. 29- Feb. 11 of the hard-x-ray flux, measured using the BATSE/CGRO earth-occultation technique, have been below the 1-day detection limit of about 100 mCrab. Previous observations of Cyg X-3 with periods of very-low-radio and hard-x-ray flux, having durations from days to a few weeks, are followed by major radio flares. The last such episode occurred in 1997 (IAUC 6673). Target-of- opportunity observations have been initiated for both RXTE and CGRO. Observations at other wavelengths are encouraged during the next few weeks." February 11, 2000 - 7363 XTE J1543-568 XTE J1543-568 F. E. Marshall, Goddard Space Flight Center (GSFC); T. Takeshima, GSFC and Universities Space Research Association; and J. in 't Zand, Space Research Organization Netherlands, report the discovery of a transient, pulsating x-ray source designated XTE J1543-568: "The source was first seen in a slew of the Proportional Counter Array on the Rossi X-ray Timing Explorer at Feb. 6.84 UT with a 2-10-keV flux of 2 x 10**-10 erg cm**-2 s**-1. A subsequent observation with the PCA on Feb. 10.69 found a similar intensity and located the source in an error box that corresponds to galactic coordinates l = 324.88 +/- 0.2 deg, b = -1.45 +/- 0.1 deg, centered on R.A. = 15h43m33s, Decl. = -56o48'.0 (equinox 2000.0). The flux is strongly modulated with a period of 27.12 +/- 0.02 s. Observations at other wavelengths are encouraged." February 7, 2000 - 7358 SAX J1808.4-3658 SAX J1808.4-3658 M. van der Klis, University of Amsterdam; D. Chakrabarty, J. C. Lee, E. H. Morgan, and R. Wijnands, Massachusetts Institute of Technology; C. B. Markwardt, University of Maryland and Goddard Space Flight Center (GSFC); and J. H. Swank, GSFC, write: "The x-ray transient SAX J1808.4-3658, the only known accretion-powered millisecond pulsar (IAUC 6876, 6877), has been detected at a low x-ray-flux level of 3-12 mCrab (2-10 keV) since Jan. 21.1 UT with the PCA instrument on the Rossi X-Ray Timing Explorer (RXTE). The start time of this outburst is not known because the source was unobservable by RXTE from about 1999 Nov. 2 to 2000 Jan. 21, due to its proximity to the sun. On 1999 Nov. 2, the upper limit on the source flux was < 0.5 mCrab (2-10 keV). X-ray pulsations at the 401-Hz spin frequency were detected in several short observations between Jan. 21.1 and Feb. 6.2. Since Feb. 2, the source has exhibited violent quasi-periodic x-ray flaring with an rms amplitude varying between 40 and 100 percent of the average flux and a repetition frequency varying between 0.9 and 1.5 Hz. This is unlike any phenomenon previously observed in a neutron-star low-mass x-ray binary. The source is still close to the sun, but observations at other wavelengths (particularly radio and infrared) are strongly encouraged." February 1, 2000 - 7355 SAX J2103.5+4545 SAX J2103.5+4545 A. Baykal, Middle East Technical University; M. Stark, Marietta College; and J. H. Swank, Goddard Space Flight Center, report: "Timing analysis of RXTE PCA observations of the transient x-ray pulsar SAX J2103.5+4545 (Hulleman et al. 1998, A.Ap. 337, L25), in outburst since 1999 Oct. 25, shows an eccentric orbit with P = 12.68 +/- 0.25 days, e = 0.4 +/- 0.2, and projected a = 72 +/- 6 s. The pulse period of 358.62171(88) is consistent with the 1997 period, but the spin-up rate during 1999 Nov. 19-27 was 2.50(15) about 10**-13 Hz s**-1, at a flux of 15-20 mCrab, implying (for a 10**12 Gauss field) a luminosity of 8 x 10**35 erg s**-1 at a distance of 4 kpc -- probably beyond the star HD 200709, which was found just outside the BeppoSAX error circle. The RXTE ASM record shows that the x-ray flux is sometimes modulated on a time scale of 13 days, and as much as 20 percent during the 1997 outburst. Deeper searches for an optical counterpart are urged." If you have a question about RXTE, please send email to one of our help desks. This page is maintained by the RXTE GOF and was last modified on Wednesday, 24-Aug-2022 11:10:31 EDT.

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