Astron. Astrophys. 345, L1-L4 (1999)

1. Introduction

The X-ray source 1WGA J1958.2+3232 was serendipitously detected on May 1993 within the field of view of the Position Sensitive Proportional Counter (PSPC; 0.1-2.4 keV) in the focal plane of the ROSAT X-ray telescope. Highly significant pulsations at a period of 72114 s were discovered in the ROSAT data (Israel et al. 1998). An ASCA observation performed on May 1998 detected 1WGA J1958.2+3232 at the flux level expected from the ROSAT pointing and confirmed the presence of a strong periodic signal at 7341 s (Israel et al. 1999). A luminosity of 10³³(d/1 kpc)² erg s^{-|| 1} in the 2-10 keV energy band was obtained (assuming an absorbed power-law model). Due to the large uncertainty in the period determined by ROSAT, it was not possible to determine whether the system contains an accreting magnetic white dwarf or a neutron star, based on the period derivative. Even the spectral characteristics were consistent with both scenarios. Accreting neutron stars in binary systems are often associated with O-B stars, while cataclysmic variables with K-M main sequence companion stars; in both cases strong emission-lines are expected to be detected. So far no unambiguous association of an accreting white dwarf to an OB star has been found. Expected X-ray luminosities are in the 10³² erg s^-1 range for wind accretors . Identifying the optical counterpart of 1WGA J1958.2+3232 and studying its spectrum provides decisive clues on the nature of system.

We present here the results of an optical program aimed at studying the stars included in the X-ray 30" radius error circle of 1WGA J1958.2+3232. The observations were performed between May and September 1998 at the Loiano Astronomical Observatory. In order to select objects with peculiar emission-lines, as expected from the companion star of this kind of binary systems, slitless multiobject spectroscopy as described by Polcaro & Viotti (1998) was used. This method allows to obtain a good spectrum for a large number of stars and quickly select stars with strong emission-lines, down to magnitudes of m_V 16-18. Moreover the absence of a slit eliminates the light loss due to poor seeing, while sky and nebular lines are spread out over the whole image, resulting only in a small increase of the background level.

A Be spectral-type star was found well within the X-ray error circle. The probability of finding by chance a Be star with V16.0 mag within the small position uncertainty region is 10^-6 . Therefore the Be star represents a very likely optical counterpart of 1WGA J1958.2+3232, making this source one of the few accreting X-ray pulsars with a pulse period P 500 s in a Be/X-ray binary system.

© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999
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