Rxte Observations of a New X-ray Transient: Grs 1737-31

We present the results from RXTE observations of a new X-ray transient, GRS 1737-31, near the Galactic center during an outburst. Over a span of two weeks, the source was seen to vary significantly on time scales down to a few seconds. The rapid flares in the light curves bear resemblance to those observed for Cygnus X-1, a well-known black hole candidate (BHC). The power-density spectrum is also very typical of BHCs in the hard (or low) state: it is flat below a characteristic break frequency of ∼0.03 Hz, and becomes roughly a 1/f power law above. The observed X-ray spectrum can be characterized by a simple power law with a photon index of ∼1.7 over a broad energy range 2–200 keV, which is remarkably similar to that of Cyg X-1 in the hard state. The similarities to Cyg X-1 make GRS 1737-31 a likely BHC. However, unlike most transient BHCs, GRS 1737-31 does not seem to reach a “soft state” even during an outburst. We discuss this phenomenon in light of the comparison to a class of hard X–ray BHCs including transients GRO J0422+32, V404Cyg and GRO J1719-24, and persistent sources 1E1740.7-2942 and GRS 1758-258. Subject headings: binaries: general — stars: individual (GRS 1737-31) — X-rays: stars 1. Discovery and ASM Light Curve A new X-ray transient, GRS 1737-31, was discovered during an outburst by the SIGMA detector aboard the GRANAT satellite on March 14, 1997 (Sunyaev et al. 1997). The source position was later refined with follow-up observations with the Wide Field Camera on the BeppoSAX satellite (Heise 1997) and again with ASCA observations (Ueda et al. 1997). In Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139 Center for Astrophysics and Space Sciences, University of California, San Diego, La Jolla, CA 92093 NASA/Goddard Space Flight Center, Code 662, Greenbelt, MD 20771 Space Sciences Laboratory, University of California, Berkeley, Berkeley, CA 94720