The optical counterpart to XTE J1855-026

ATel #102; Francesco Verrecchia (ASDC, Rome), Ignacio Negueruela (University of Alicante), Stefano Covino (INAF - Merate Observatory), GianLuca Israel (INAF - Rome Observatory)
on 14 Jul 2002; 19:29 UT
Credential Certification: Ignacio Negueruela (ignacio@astronomia.disc.ua.es)

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We report observations of the likely optical counterpart to the 361-s X-ray pulsar XTE J1855-026 (Corbet et al. 1999, ApJ 517, 956; Corbet & Mukai 2002, astroph/0207181), which is part of an eclipsing binary with a 6.01-d orbit.

Our candidate is a relatively bright star right on the edge of the new 12" ASCA error circle (Corbet & Mukai 2002, astroph/0207181), which doesn't contain any obvious candidate, and therefore the likely optical counterpart (coordinates from SIMBAD RA: 18:55:30.5, Dec: -02:36:21).

The object was identified in 1999 from observations taken at the Loiano Observatory (Bologna, Italy) as the only H-alpha emission line object inside the 2' XTE error circle of XTE J1855-026. Low-resolution spectroscopy (4800-7000A) taken on 15th September 1999 with the ESO 3.6-m telescope at La Silla shows weak H-alpha emission (EW=-6A) and HeI absorption lines on a very steep continuum (B=16.1, I=11.5), indicating a very reddened early-type star. Likely emission-infilling in H-beta and a shallow weak HeII4541A line suggested that the object was likely not a supergiant, as the orbital solution seemed to imply.

A red intermediate resolution spectrum taken on 17th July 2000 with the OHP 1.93-m telescope shows double-peaked (with much stronger red peak) H-alpha emission with an EW=-5.5A and apparent weak P-Cygni profiles in the HeI lines, which, if real, should indicate mass loss.

The observed asymmetric emission line profiles may be interpreted as those of a Be star with a Global One-Armed Oscillation in its circumstellar disk. However, as this object is likely to be the counterpart (and therefore have a neutron star orbiting in a 6-d orbit), we interpret them as P-Cygni profiles due to extended mass loss. Further observations are underway in order to confirm if the object is a supergiant. We note that systems containing a neutron star accreting from a non-supergiant early star via localized Roche-Lobe overflow are known (c.f. LMC X-4), however the position of the pulsar in the Corbet diagram favours wind accretion from a blue supergiant.