Fluctuations in the CXB

Fluctuations in the CXB, which can be described as a power law of index _E = 0.8 and normalisation of 5.0 × 10^-4 photons cm^-2 sec^-1 keV^-1 at 1 keV giving ~0.7 counts sec^-1 r.m.s. over the full energy range (Hayashida et al. 1989; Butcher et al. 1997), can cause significant uncertainties in the spectra of weak sources. These uncertainties due to the CXB fluctuations can be estimeted by addding and subtracting the 1 r.m.s. CXB fluctuation spectrum to the data.

The fluctuation spectra are located on the directory $XACAL. The files diffuse_5per.phad and diffuse_8per.phad should be used with spectral files that have been background subtracted using the universal and local methods respectively. Both files have been compressed so that they contain top- and mid-layer spectra for the full complement of LAC detectors (they cannot be used when detectors have been ignored or the layers compress further, i.e. MPC2 and MPC3 data). Note that these files are HDS and should be added or subtracted to a background subtracted spectral file prior to spectral fitting with spf or conversion into a FITS file using the program ginga2xsp. Alternatively, for each 1  CXB fluctuation spectrum, there are two FITS format files (diffuse_xper_top.cor and diffuse_xper_mid.cor; x is 5 or 8) which xspec allows the user to include as a correction file during the spectral fitting process.