Cosmic X-ray Background in HXD/PIN Data

Updated Dec 9, 2008

While the HXD team provides HXD/PIN background event files, they only account for the non X-ray background (NXB) and do not contain the CXB component. The CXB is approximately 5% of the NXB component, which must be estimated separately. One method is to estimate the CXB level by using the PIN response for the flat emission distribution. Such response files prepared by the HXD team is available from:

Epoch File

2005.8.17 -- 2006.5.13 ae_hxd_pinflate1_20080129.rsp

2006.5.13 -- 2006.10.2 ae_hxd_pinflate2_20080129.rsp

2006.10.2 -- 2007.7.28 ae_hxd_pinflate3_20080129.rsp

2007.7.28 -- 2008.8.31 ae_hxd_pinflate4_20080129.rsp

2008.9.1 -- ** ae_hxd_pinflate5_20080716.rsp

These files are available from the Suzaku CALDB (2008-12-04 release or later; epochs 1-3 responses were available since 2008 February), and assume that the uniform emission is from the region of 2 deg x 2 deg. As described above, there are 4 files, each appropriate for a range of observation dates. They differ in the bias voltages set on-board and in low energy threshold used in ground processing.

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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This file was last modified on Tuesday, 09-Dec-2008 18:43:58 EST