Cosmic X-ray Background in HXD/PIN Data
Updated Dec 9, 2008
A "typical" CXB spectrum is reported as follows, based on the HEAO-1 results (Boldt 1987)
CXB(E) = 9.0x10-9 * (E/3keV)**(-0.29) * exp(-E/40keV) erg/cm2/s/str/keV
Since the flat PIN response files are for 4 square degree field of view, we need to multiply this result by the appropriate ratio (4 deg2/1 sr). Since 1 sr = 3283 deg2, this becomes
CXB(E) = 1.097x10-11 * (E/3keV)**(-0.29) * exp(-E/40keV) erg/cm2/s/FOV/keV
Converting this into values appropriate for XSPEC, which assumes the power-law is normalized at 1 keV and is in photons, not ergs, we obtain:
CXB(E) = 9.412x10-3 * (E/1keV)**(-1.29) * exp(-E/40keV) photons/cm2/s/FOV/keV
Then we can simulate the CXB contribution to the PIN background with XSPEC, using, for example
model pow*highecut 1.29 9.412e-3 1e-4 40.0 fakeit none ae_hxd_pinflate4_20070914.rsp
This particular combination of model and response (appropriate for the current epoch since 2007 July 28) predicts an expected count rate of 0.025 cts s-1, compared with the NXB contribution of ~0.5 cts s-1. It is important to note that the above model is valid only with the flat response file. The same spectrum, when fitted with the point source response observed at the HXD nominal position (e_hxd_pinxinome4_20070914.rsp), requires pow normalization of 8.125x10-4 instead.
Please note that the level of CXB and its point-to-point scatter are an active research topic. Other estimates of CXB spectrum can be converted to XSPEC models following the same steps as above.
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