M-type giants as optical counterparts of X-ray sources 4U 1700+24 and 4U 1954+319

¹ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129 Bologna, Italy (formerly IASF/CNR, Bologna) e-mail: masetti@iasfbo.inaf.it
² Dipartimento di Fisica, Università di Ferrara, via Saragat 1, 44100 Ferrara, Italy

Received: 15 February 2006
Accepted: 9 March 2005

Abstract

We observed with Chandra two peculiar galactic X-ray sources, 4U 1700+24 and 4U 1954+319, which are suspected to have a M-type giant star as optical counterpart, in order to get a high-precision astrometric position for both of them. The peculiarity of these sources lies in the fact that these are the only two cases among low-mass X-ray binaries (LMXBs), besides the confirmed case of GX 1+4, for which the companion can possibly be a M-type giant. We found that in both cases the field M-type giant star is indeed the counterpart of these X-ray sources. We also determined the distance to 4U 1954+319 to be ~1.7 kpc. This result suggests that a number of faint ( erg s^-1) Galactic X-ray sources are “symbiotic X-ray binaries”, that is, wide-orbit LMXBs composed of a compact object, most likely a neutron star, accreting from the wind of a M-type giant.