TY - JOUR
T1 - Infrared candidates for the intense galactic X-ray source GX 17+2
AU - Deutsch, Eric W.
AU - Margon, Bruce
AU - Anderson, Scott F.
AU - Wachter, Stefanie
AU - Goss, W. M.
PY - 1999/10/10
Y1 - 1999/10/10
N2 - We present new astrometric solutions and infrared Hubble Space Telescope observations 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 remains optically unidentified. The observed X-ray characteristics strongly suggest 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 measure the position of optical and infrared objects near the radio source to subarcsecond precision within the International Celestial Reference Frame for direct comparison with the radio position, which we also recompute using modern calibrators. With high confidence we eliminate the V ∼ 17.5 star NP Ser, often listed as the probable optical counterpart of the X-ray source, as a candidate. Our HST NICMOS observations show two faint objects within our 0″.5 radius 90% confidence error circle. Even the brighter of the two, star A, is far fainter than expected (H ≈ 19.8), given multiple estimates of the extinction in this field and our previous understanding of Z sources, but it becomes the best candidate for the counterpart of GX 17+2. The probability of a chance coincidence of an unrelated faint object on the radio position is high. However, if the true counterpart is not star A, it is fainter still, and our conclusion that the optical counterpart is surprisingly underluminous is but strengthened.
AB - We present new astrometric solutions and infrared Hubble Space Telescope observations 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 remains optically unidentified. The observed X-ray characteristics strongly suggest 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 measure the position of optical and infrared objects near the radio source to subarcsecond precision within the International Celestial Reference Frame for direct comparison with the radio position, which we also recompute using modern calibrators. With high confidence we eliminate the V ∼ 17.5 star NP Ser, often listed as the probable optical counterpart of the X-ray source, as a candidate. Our HST NICMOS observations show two faint objects within our 0″.5 radius 90% confidence error circle. Even the brighter of the two, star A, is far fainter than expected (H ≈ 19.8), given multiple estimates of the extinction in this field and our previous understanding of Z sources, but it becomes the best candidate for the counterpart of GX 17+2. The probability of a chance coincidence of an unrelated faint object on the radio position is high. However, if the true counterpart is not star A, it is fainter still, and our conclusion that the optical counterpart is surprisingly underluminous is but strengthened.
KW - Infrared: stars
KW - X-rays: stars
UR - http://www.scopus.com/inward/record.url?scp=0033543892&partnerID=8YFLogxK
U2 - 10.1086/307803
DO - 10.1086/307803
M3 - Article
AN - SCOPUS:0033543892
SN - 0004-637X
VL - 524
SP - 406
EP - 413
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 1 PART 1
ER -