Average spectral properties of galactic X-ray binaries with 3 years of MAXI data

Nazma Islam1,2*T. Mihara3 ,M. Sugizaki3,B. Paul1and Biman B. Nath1
1Raman Research Institute,Sadashivanagar, Bangalore-560080, India
2Joint Astronomy Programme, Indian Institute of Science, Bangalore-560012, India
3MAXI team, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

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The energy spectra of X-ray binaries (XRBs) have been investigated during the last few decades with many observatories in different energy band and with different energy resolutions. However, these studies are carried out in selected states of XRBs like during outbursts, transitions, quiescent states, and are always done in limited time windows of pointed observations. It is now possible to investigate the long term averaged spectra of a large number of X-ray binaries with the all sky monitor MAXI, which also has a broad energy band. Here we present the average spectral behaviour of a few representative XRBs. The long term averaged spectrum of Cyg X-1 is described by a sum of two power-laws having Γ1 ∼2.8 and Γ2 ∼1.2, along with a multi color disk blackbody having an inner disk temperature of 0.5 keV, GX 301-2 is described by a power-law with a high energy cut-off at Ec ∼ 15 keV and a blackbody component at 0.2 keV and that of Aql X-1 is described by a multi color disk blackbody at 2 keV and a power-law of Γ ∼ 2.2. We have also constructed the combined X-ray spectrum of the X-ray binaries in theMilkyWay, which can be compared to the XRBs spectra of other galaxies observed with Chandra and XMM-Newton. These measurements are also relevant to investigate the X-ray interaction with the ISM and its contribution to the ionizing X-ray background in the early universe.

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Keywords : X-rays: binaries – X-rays: galaxies – techniques: spectroscopic