The near-IR counterpart to the high-mass X-ray binary X1908+075

ATel #291; T. Morel (Palermo Observatory, Italy) and Y. Grosdidier (McGill University, Canada)
on 18 Jun 2004; 14:05 UT
Credential Certification: Thierry Morel (morel@astropa.unipa.it)

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We report the near-IR identification of the likely counterpart to X1908+075, a highly-absorbed Galactic X-ray source recently suspected to belong to the rare class of OB supergiant-neutron star binary systems (Wen et al. 2000, ApJ, 532, 1119; Levine et al. 2004, ApJ, submitted, astro-ph/0404428). The Einstein and HEAO 1 satellites have localized this X-ray source with an uncertainty of about 50''.

JHKs-band imaging of the field was carried out on service observing mode on August 9, 2001 (JD 2,452,131.50) with the near-IR camera CAIN-II mounted on the Carlo Sanchez Telescope (TCS) at Teide Observatory (Spain). The observations were carried out under photometric sky conditions and a seeing of about 1.6''. We identified a source localized at 19h 10m 48.20s and +07deg 35' 52.3'' (J2000) with: J=13.199+/-0.018, H=11.380+/-0.012 and Ks=10.457+/-0.018. These near-IR magnitudes are compatible with an early-type star lying at d~7 kpc and suffering A(V)~16.5 mag of extinction, the latter value being in good agreement with the hydrogen column density derived from the modelling of the X-ray spectrum.

Follow-up, medium resolution HK spectroscopic observations of this candidate was obtained on service observing mode with the near-IR spectrograph CGS4 at UKIRT (JD 2,452,419.04). The star presents He I features in absorption, but no He II or C IV lines. N III 2.1155 and Brackett gamma are in emission. These spectral diagnostics lead to a O7.5-O9.5 If classification, although a very early B supergiant cannot be ruled out (Hanson et al. 1996, ApJS, 107, 281; Hanson et al. 1998, AJ, 116, 1915). This classification is internally consistent with the near-IR photometric properties.

A paper reporting these results has been recently submitted to MNRAS.