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

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.

IGR J16194–2810: a new symbiotic X-ray binary

“…This result is compatible with those from other SyXBs (Masetti et al 2002(Masetti et al , 2006b. Table 2 are also shown.…”

“…3) extracted in the 0.5−7 keV band, in which the two pointings allowed us to collect sufficient statistics from IGR J16194−2810, show erratic fluctuations of the source emission on variability timescales from hundreds to thousands of seconds. This is typical of accreting Galactic sources in general and of SyXBs in particular (see Masetti et al 2002Masetti et al , 2006b). The hardness ratio between the 3−7 keV and 0.5−3 keV energy bands did not change significantly in each of the two single XRT observations, as well as between them.…”

“…The PSD obtained from the XRT data has the characteristics of the 1/ f -type shot-noise variability often seen in this class of objects (e.g., Masetti et al 2002Masetti et al , 2006b) and likely due to random instabilities in the accretion process, or to inhomogeneities in the accreting stellar wind captured by a NS (see e.g. Kaper et al 1993).…”

“…Next, following our past experience on SyXBs (Masetti et al 2002(Masetti et al , 2006b), we fit the averaged spectrum of the source with a more physical model composed of a thermal Comptonization (Titarchuk 1994) attenuated by a neutral hydrogen column. With this model we obtained a satisfactory description of the spectrum, as reported in Table 2 and in Fig.…”

The Nature of the Hard X‐Ray–Emitting Symbiotic Star RT Cru

“…Although a few symbiotics contain neutron star accretors (e.g., GX 1+4, 4U 1700+24, 4U 1954+31, and IGR J16194À2810; Davidsen et al 1977;Masetti et al 2002Masetti et al , 2007aMasetti et al , 2007bGalloway et al 2002), the accreting compact object is usually a white dwarf (WD). Typical binary separations are on the order of AU, with orbital periods on the order of a few hundred days to a few decades (Kenyon 1986;Belczyński et al 2000).…”