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.
12 pages, including 4 figures. To appear in ApJ Letters