Astronomy and Astrophysics – Astrophysics
Scientific paper
2006-11-15
Astronomy and Astrophysics
Astrophysics
13 pages, 5 figures (2 in the Appendix), 4 tables (1 in the Appendix), accepted for publication on A&A, main journal. Revised
Scientific paper
10.1051/0004-6361:20066517
(Abridged) We present results of several X-ray observations of the X-ray binary 4U 1954+31 performed with the satellites BeppoSAX, EXOSAT, ROSAT, RXTE, and Swift. We also studied the RXTE ASM data over a period of more than 10 years. Light curves of all 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 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 from a stellar wind coming from its optical companion, an M-type giant star.
Amati Lorenzo
Cusumano Giancarlo
Frontera Filippo
Maiorano Elisabetta
Masetti Nicola
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