Infrared candidates for the intense galactic X-ray source GX 17+2

We present new astrometric solutions and infrared Hubble Space Telescope ob servations of GX 17+2 (X1813 - 140), one of the brightest X-ray sources on the celestial sphere. Despite 30 years of intensive study and the existence of a strong radio counterpart with a subarcsecond position, the object rem ains optically unidentified. The observed X-ray characteristics strongly su ggest that it is a so-called Z source, the rare but important category that includes Sco X-1 and Cyg X-2. Use of the USNO-A2.0 catalog enables us to m easure the position of optical and infrared objects near the radio source t o subarcsecond precision within the International Celestial Reference Frame for direct comparison with the radio position, which we also recompute usi ng modern calibrators. With high confidence we eliminate the V similar to 1 7.5 star NP Ser, often listed as the probable optical counterpart of the X- ray source, as a candidate. Our HST NICMOS observations show two Taint obje cts within our 0 ".5 radius 90% confidence error circle. Even the brighter of the two, star A, is far fainter than expected (H approximate to 19.8), g iven multiple estimates of the extinction in this held and our previous und erstanding of Z sources, but it becomes the best candidate for the counterp art of GX 17+2. The probability of a chance coincidence of an unrelated fai nt object on the radio position is high. However, if the true counterpart i s not star A, it is fainter still, and our conclusion that the optical coun terpart is surprisingly underluminous is but strengthened.