Abstract
The analysis of the third INTEGRAL/Imager on Board Integral Satellite (IBIS) survey has revealed several new cataclysmic variables, most of which turned out to be intermediate polars, thus confirming that these objects are strong emitters in hard X-rays. Here, we present high-energy spectra of all 22 cataclysmic variables detected in the third IBIS survey and provide the first average spectrum over the 20–100 keV band for this class. Our analysis indicates that the best-fitting model is a thermal bremsstrahlung with an average temperature of ?kT?? 22 keV . Recently, 11 (10 intermediate polars and one polar) of these systems have been followed up by Swift/X-ray telescope (XRT) (operating in the 0.3–10 keV energy band), thus allowing us to investigate their spectral behaviour over the range ?0.3–100 keV. Thanks to this wide energy coverage, it was possible for these sources to simultaneously measure the soft and hard components and estimate their temperatures. The soft emission, thought to originate in the irradiated poles of the white dwarf atmosphere, is well described by a blackbody model with temperatures in the range ?60–120 eV. The hard emission, which is supposed to be originated from optically thin plasma in the post-shock region above the magnetic poles, is indeed well modelled with a bremsstrahlung model with temperatures in the range ?16–35 keV, similar to the values obtained from the INTEGRAL data alone. In several cases, we also find the presence of a complex absorber: one totally [with NH? (0.4– 28) × 1021 cm?2 ] and one partially [with NH? (0.7–9) × 1023 cm?2 ] covering the source. Only in four cases (V709 Cas, GK Per, IGR J06253+7334 and IGR J17303?0601), we find evidence for the presence of an iron line at 6.4 keV. We discuss our findings in the light of the systems parameters and cataclysmic variables/intermediate polars modelling scenario.
Highlights
Cataclysmic variables (CVs) are close binary systems, with orbital period typically less than one day, containing a white dwarf (WD) which is accreting material from a late-type main-sequency secondary star filling its Roche lobe
CVs can be broadly divided into two subclasses: nonmagnetic and magnetic objects depending on the strength of the WD magnetic field
Due to the strong magnetic field, cyclotron radiation cooling suppresses the high-temperature bremsstrahlung emission of a substantial fraction of the electrons in the shock region (Lamb & Masters 1979; Konig, Beuermann & Gansicke 2006). This may explain why most of hard X-ray detected CVs are intermediate polars (IPs), in which cyclotron emission is negligible
Summary
Cataclysmic variables (CVs) are close binary systems, with orbital period typically less than one day, containing a white dwarf (WD) which is accreting material from a late-type main-sequency secondary star filling its Roche lobe (for a review see Warner 1995). Due to the strong magnetic field, cyclotron radiation cooling suppresses the high-temperature bremsstrahlung emission of a substantial fraction of the electrons in the shock region (Lamb & Masters 1979; Konig, Beuermann & Gansicke 2006) This may explain why most of hard X-ray detected CVs are IPs, in which cyclotron emission is negligible. The soft X-ray component, originating by the reprocessing of the hard emission on the WD surface, is typically described by a blackbody emission with temperatures ranging from a few eV up to ∼100 eV (De Martino et al 2004a, 2006a,b) This component was mostly seen in polars but recently Evans & Hellier (2007), performing a systematic spectral analysis of XMM–Newton data of IPs, found this to be a common feature of IPs X-ray spectra.
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