Infrared Candidates for the Intense Galactic X-ray Source GX 17+2

Abstract

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 sub-arcsecond 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 sub-arcsecond 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.

15 pages including 3 figures and 3 tables. Accepted for publication in The Astrophysical Journal

Cited by in corpus (13)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• Detecting gravitational wave emission from the known accreting neutron stars
• RXTE observations of the neutron star low-mass X-ray binary GX 17+2: correlated X-ray spectral and timing behavior
• Evidence for a jet contribution to the optical/infrared light of neutron star X-ray binaries
• Empirical Uncertainty Estimators for Astrometry from Digital Databases
• A Hard Look at the Neutron Stars and Accretion Disks in 4U 1636-53, GX 17+2, and 4U 1705-44 with $\text{NuSTAR}$
• Simultaneous radio and X-ray observations of Galactic Centre low mass X-ray binaries
• The infrared counterpart of the Z source GX5-1
• Infrared Photometric Variability of GX13+1 and GX17+2
• A study of accretion discs around rapidly rotating neutron stars in general relativity and its applications to four Low Mass X-ray Binaries
• Gaia EDR3 parallaxes of type I X-ray bursters and their implications on the models of type I X-ray bursts: a generic approach to the Gaia parallax zero-point and its uncertainty
• Spectroscopic Identification of the Infrared Counterpart to GX5-1
• A Possible Period for the K-band Brightening Episodes of GX 17+2