X-ray broad-band study of the symbiotic X-ray binary 4U 1954+31^*

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 Bologna, via Irnerio 48, 40126 Bologna, Italy
³ Dipartimento di Fisica, Università di Ferrara, via Saragat 1, 44100 Ferrara, Italy
⁴ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, Via Ugo La Malfa 153, 90146 Palermo, Italy (formerly IASF/CNR, Palermo)

Received: 6 October 2006
Accepted: 14 November 2006

Abstract

We present results of several pointed X-ray broad band observations of the “symbiotic X-ray binary” 4U 1954+31 performed with the satellites BeppoSAX, EXOSAT, ROSAT, RXTE and Swift between October 1983 and April 2006. We also studied the RXTE ASM data over a period of more than 10 years, from January 1996 to October 2006. Light curves of all pointed observations show an erratic behaviour with sudden increases in the source emission on timescales variable from hundreds to thousands of seconds. There are no indications of changes in the source spectral hardness, with the possible exception of the RXTE pointed observation. Timing analysis does not reveal the presence of coherent pulsations or periodicities either in the pointed observations in the range from 2 ms to 2000 s or in the long-term RXTE ASM light curve on timescales from days to years. The 0.2-150 keV spectrum, obtained with BeppoSAX, is the widest for this source available to date in terms of spectral coverage and is well described by a model consisting of a lower-energy thermal component (hot diffuse gas) plus a higher-energy (Comptonization) emission, with the latter modified by a partially-covering cold absorber plus a warm (ionized) absorber. A blackbody modelization of our BeppoSAX low-energy data is ruled out. The presence of a complex absorber local to the source is also supported by the 0.1-2 keV ROSAT spectrum. RXTE, EXOSAT and Swift X-ray spectroscopy is consistent with the above results, but indicates variations in the density and the ionization of the local absorber. A 6.5 keV iron emission line is possibly detected in the BeppoSAX and RXTE spectra. All this information suggests that the scenario that better describes 4U 1954+31 consists of a binary system in which a neutron star orbits in a highly inhomogeneus medium, accreting matter from a stellar wind coming from its optical companion, an M-type giant star.