2002ApJ...574L.143C

We discuss the nature of the infrared (IR) counterpart of GX 17+2, one of the most luminous of the persistently bright X-ray binaries. Chandra HRC-S astrometry is consistent with either NP Ser (the original counterpart of GX 17+2 proposed by M. Tarenghi & C. Reina in 1972) or star ``A'' of E. W. Deutsch et al. as the counterpart of the X-ray source. However, we present Keck K-band observations that reveal a bright counterpart in the radio error circle of Deutsch et al. 0".9 north of NP Ser itself. Furthermore, the position of this counterpart is consistent with that of star A to within 0".1, implying an amplitude of variation of ∼25-33 between the Keck observations and the Hubble Space Telescope (HST) measurements of Deutsch et al. Subsequent Keck imaging also reveals star A in an ``IR-faint'' state (K=18.3 mag, with a corresponding amplitude of variability of ∼22). In addition, archival Cerro Tololo Inter-American Observatory observations provide evidence for K-band variability, albeit of smaller amplitude. The HST and Keck K-band variations, however, do not appear to be accompanied by any changes in the overall X-ray luminosity of GX 17+2 as measured by contemporaneous (but not simultaneous) Rossi X-Ray Timing Explorer All-Sky Monitor observations. We propose instead that the large radio outbursts observed when the source is in the horizontal branch of its ``Z'' state are likely to give rise to synchrotron flares in the IR. The amplitude of the radio flares is in agreement with this scenario. Such IR variability, unrelated (directly) to X-ray reprocessing and the gross characteristics of the mass accretion rate, may be present in the IR flux of other low-mass X-ray binaries but harder to see owing to the intrinsically brighter IR fluxes of the longer period systems.