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Astron. Astrophys. 334, L13-L16 (1998)

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1. Introduction

The global spectrum of the cosmic X-ray background is a primary piece of information for understanding its origin. The 3-50 keV CXRB spectrum observed with HEAO-1 A2 can be well described by a [FORMULA] keV thin thermal plasma-like spectral shape (Marshall et al. 1980; Boldt 1987), which can be approximated by a power-law with a photon index of [FORMULA] in [FORMULA] keV. BBXRT (Jahoda et al. 1992) and ASCA (Gendreau et al. 1995; Ishisaki et al. 1998) measurements show that this power-law component extends down to 1 keV, below which an excess is observed.

A number of authors report ROSAT measurements in the 0.5-2 keV band (e.g. Hasinger 1992; Georgantopoulos et al. 1996) and show about 30% larger flux than the Gendreau et al.'s (1995) ASCA SIS result at 1 keV, with different slopes (see Hasinger 1996 for review). The disagreement may be contributed by the differences in the position/solid angle of the measured sky, problems arising from incomplete modelings, and/or calibration problems.

In order to separate these effects, we have made a series of joint spectral fits of ROSAT PSPC, ASCA GIS, and ASCA SIS spectra from two fields of the sky, where sufficient amount of blank-sky data exist after thorough screening. Because of the limited data meeting the criteria, the work presented in this paper is not intended to determine the current best estimate of the global CXRB spectrum, but rather a comparison of measurements among ASCA and ROSAT instruments in the same parts of the sky with consistent modelings.

In Sect. 2, we describe the ASCA and ROSAT data used in the analysis. Joint spectral fits are described in Sect. 3. The results are discussed in Sect. 4.

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© European Southern Observatory (ESO) 1998

Online publication: May 12, 1998