Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002apj...574l.143c&link_type=abstract
The Astrophysical Journal, Volume 574, Issue 2, pp. L143-L146.
Astronomy and Astrophysics
Astronomy
8
Stars: Binaries: Close, Stars: Individual: Alphanumeric: Gx 17+2, X-Rays: Stars
Scientific paper
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.
Anderson Scott
Callanan Paul J.
Curran Peter
Deutsch Eric
Fender Rob P.
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