National Aeronautics and Space Administration

CYGNUS X-1

G. Pooley, Cavendish Laboratory, University of Cambridge, communicates: "Recent observations of Cygnus X-1 with the Ryle Telescope show unusually low radio emission, with a mean flux density of about 0.95 mJy at 15 GHz over the period Oct. 1-9. For the past five years, the source has usually been in a low/hard x-ray state, with radio emission always present at the level 5-40 mJy. The current lack of radio emission corresponds to a transition to the high/soft x-ray state, as seen in the publicly available RXTE monitoring data."

M. P. Rupen, A. J. Mioduszewski, and V. Dhawan, National Radio Astronomy Observatory, further report: "Observations with the Very Large Array on Oct. 3 confirm the Ryle results noted above, putting even lower limits on the radio flux density, with no detections to rms noise levels of 0.58, 0.17, 0.12, and 0.34 mJy/beam at 1.425, 4.860, 8.460, and 14.939 GHz, respectively. Further observations of this unusual radio/x-ray state at other wavelengths are strongly encouraged."

XTE J1650-500

M. Revnivtsev and R. Sunyaev, Max-Planck-Institut fuer Astrophysik and Russian Space Research Institute, write: "Analysis of public-domain data of the target-of-opportunity RXTE observations of XTE J1650-500 (cf. IAUC 7707, 7708, 7710) from Sept. 9 and 10 reveals the presence of two quasiperiodic- oscillation (QPO) peaks (likely harmonics). The centroid-peak requencies changed from 1.13 +/- 0.02 and 2.28 +/- 0.02 Hz on Sept. 9.1 UT to 1.33 +/- 0.02 and 2.73 +/- 0.03 Hz on Sept. 10.5. The amplitudes of the QPOs are about 5-7 percent. The preliminary spectral analysis of PCA (3-20 keV) and HEXTE (20-200 keV) data shows that the source has a spectrum similar to that of Cyg X-1 and other black-hole binaries in the low/hard spectral state. The spectrum has a power-law-like shape with photon index around 1.5-1.8 and exponential cutoff with e-folding value about 90 keV. The investigation of reported QPOs in the optical band could provide additional information about properties of the accretion disk and the binary."

R. Wijnands, J. M. Miller, and W. H. G. Lewin, Massachusetts Institute of Technology, report on behalf of a larger collaboration: "A timing analysis of the RXTE PCA data obtained for the new x-ray transient XTE J1650-500 on four occasions on Sept. 6.98 (0.37 Crab), 7.13 (0.39 Crab), 8.38 (0.44 Crab), and 9.11 UT (0.45 Crab) demonstrates that the rapid x-ray variability of this source can be characterized by a strong (about 22 percent rms amplitude in the energy band 2-60 keV), band-limited noise component, which follows a power law with index of 1-1.5 at high frequencies with a break at 0.2 Hz, below which the spectrum is relatively flat. Superimposed on this noise component, broad (width of 0.5-1.5 Hz) features or QPOs are present with a frequency of 1.6-2.3 Hz. Also a broad (width of 0.4 Hz) feature at half the QPO frequency is detected. An example of the power-density spectrum of the source can be found at http://space.mit.edu/~rudy/XTEJ1650-500PDS.ps (obtained for the observation on Sept. 9.11). The spectral shape (IAUC 7707) and the timing behavior indicate that this is a black-hole binary in the low (spectrally hard) state."

M. M. Buxton, Research School of Astronomy and Astrophysics, Australian National University; and D. Londish, University of Sydney, report that the optical counterpart of XTE J1650-500 was visible at B = 18.21 +/- 0.2 on Sept. 10.42 UT on two 300-s CCD exposures taken with the 1.88-m telescope (+ Monash Cassegrain Imager) at Mount Stromlo Observatory.

XTE J1650-500

R. Remillard, reporting for the RXTE All-Sky Monitor (ASM) team at the Massachusetts Institute of Technology and Goddard Space Flight Center (GSFC), NASA, writes: "The RXTE ASM has detected x-ray emission from a new source, XTE J1650-500. The ASM position is centered at R.A. = 16h50m00s, Decl. = -50o00' (equinox 2000.0; estimated uncertainty 6'). The first detection was made on Sept. 5.637 UT, with an x-ray intensity (2-12 keV) of 0.14(2) Crab, rising to 0.24(1) Crab on Sept. 6.432 and 0.27(1) Crab on Sept. 6.896. The ASM detections in the three energy bands suggest x-ray colors consistent with a `soft x-ray transient', rather than an x-ray pulsar. Observations at radio, optical, and infrared wavelengths are needed as soon as possible."

C. Markwardt, J. Swank, and E. Smith, GSFC, report on behalf of the RXTE team at GSFC: "The transient x-ray source XTE J1650-500 discovered with the RXTE ASM was confirmed with PCA measurements on Sept. 6.39 (0.33 Crab) and 6.62 UT (0.39 Crab). PCA scans in the second observation give a best fitting R.A. = 16h50m00s, Decl. = -49o59' (equinox 2000.0; estimated uncertainty about 2'). The continuum spectrum was a power law with a photon index of 1.5, plus a blackbody with temperature kT = 1.2 keV (about 10 percent of total), which is typical of hard states of black-hole candidates. No pulsations were seen."

A. J. Castro-Tirado, Instituto de Astrofisica de Andalucia, Granada; P. Kilmartin, A. Gilmore, O. Petterson, and I. Bond, Mt. John University Observatory; P. Yock, Auckland University; and C. Sanchez-Fernandez, Laboratorio de Astrofisica Espacial y Fisica Fundamental, Madrid, communicate: "We have observed the RXTE error box of the probable soft x-ray transient XTE J1650-500 with the Mt. John Observatory 0.6-m telescope at Lake Tekapo, New Zealand. A 180-s image obtained with a wide blue-band filter centered on the RXTE position reveals a possible candidate of mag about 17 (preliminary) that might be present as a faint object (near the detection limit) in the corresponding blue Palomar Sky Survey image. A preliminary position is R.A. = 16h50m01s.0, Decl. = -49o57'45" (equinox 2000.0; +/- 2"), which is nearly 1' south of the bright star HDE 151476 (V = 8.0). Spectroscopic observations are encouraged in order to confirm whether this is the optical counterpart to XTE J1650-500. An identification chart is posted at http://www.iaa.es/~ajct/XRBs/xtej1650.html."

GRS 1758-258

D. M. Smith, Space Sciences Laboratory, University of California at Berkeley; C. Markwardt, Goddard Space Flight Center (GSFC); W. A. Heindl, Center for Astrophysics and Space Science, University of California at San Diego; and J. H. Swank, GSFC, using data from the RXTE PCA, report: "The black-hole candidate and microquasar GRS 1758-258 has entered an extremely dim state. The power law (usually photon index -2), which normally dominates the spectrum, has dropped by an order of magnitude in luminosity (from 10**-9 to 10**-10 erg cm**-2 s**-1, 2-20 keV) and softened to index -3, leaving the spectrum dominated by the faint thermal component that is often present (kT = 0.4 keV). The change occurred abruptly: the power law was as bright as usual on Feb. 21, and on Feb. 27, Mar. 2, and 6 was in the dim state, with little apparent change among the latter three observations. We identify this with the 'off' state last reported by SIGMA in 1991-1992 (Gilfanov et al. 1993, Ap. J. 418, 844). The remaining soft emission may be the extended emission reported by Chandra from this source (Heindl and Smith 2000, Bull. A.A.S. 32, 1603). Observations are strongly encouraged at all wavelengths not excessively absorbed near the Galactic center, both to characterize this x-ray state and to search for changes both to characterize this x-ray state and to search for changes in the optical candidates for this source's companion, now that the accretion disk may have turned off."

XTE J1550-564

XTE J1550-564 J. A. Tomsick, University of California at San Diego (UCSD); E. Smith and J. Swank, Goddard Space Flight Center; R. Wijnands, Massachusetts Institute of Technology; and J. Homan, University of Amsterdam, report: "Recent observations of the blackhole x-ray transient XTE J1550-564 with the Rossi X-ray Timing Explorer (RXTE) indicate that the source is active again. The last outburst ended in 2000 August, and a Chandra observation made on 2000 Aug. 16 indicated a flux level of 1 x 10**-13 erg cm**-2 s**-1 (0.5-8 keV). A 2.5-20-keV source flux of 9 x 10**-10 erg cm**-2 s**-1 was measured by RXTE on 2001 Jan. 28.7 UT. The spectral and timing properties indicate that the source is in the low-hard state. The energy spectrum can be described by a power law with a photon index of 1.52 and a neutral iron line with an equivalent width of 160 eV. The level of timing noise is about 40-percent rms (0.01-100 Hz; 2-60 keV), with violent flaring activity having a repetition period of 100-160 s. Currently, it is not clear if the observed activity will lead to a full outburst. Additional RXTE observations are scheduled, and coordinated observations at other wavelengths are encouraged."

R. Jain and C. Bailyn, Yale University; and J. Tomsick, UCSD, report YALO consortium observations of the optical counterpart of XTE J1550-564 using the Yale 1-m telescope (+ ANDICAM instrument) at Cerro Tololo Interamerican Observatory: "XTE J1550-564 has entered a new optical outburst. By Jan. 28.34, 29.34, and 30.34 UT, the object was at V = 18.28 +/- 0.01, 18.27 +/- 0.01, and 18.25 +/- 0.01, respectively. The source is significantly brighter than the quiescent level of V about 21.6."

MARKARIAN 421

XTE J0052-723

XTE J0052-723 R. Corbet, Goddard Space Flight Center (GSFC) and Universities Space Research Association; F. E. Marshall, GSFC; and C. B. Markwardt, GSFC and University of Maryland, report: "We have discovered a new transient x-ray pulsar in the direction of the Small Magellanic Cloud from RXTE Proportional Counter Array observations made on 2000 Dec. 27 and 2001 Jan. 5. Pulsations were seen with a period of 4.782 +/- 0.001 s and a double-peaked pulse profile. Subsequent slew observations on Jan. 9 give a source location of R.A. = 0h52m11s, Decl. = -72o20'.3 (equinox 2000.0) with a 90-percent statistical uncertainty of about 2' in R.A. and 1' in Decl. Some additional uncertainty in the source position may be present, due to source variability. The mean source intensity on Jan. 9 was about 8 mCrab (2-10 keV). Earlier observations of this region on 2000 Dec. 13 and 20 did not detect this source."