The low-energy cosmic X-ray background spectrum observed by the BeppoSAX LECS

Abstract

The spectrum of the 0.1-7.0 keV cosmic X-ray background (CXB) at galactic latitudes >|25| degrees has been measured using the BeppoSAX Low-Energy Spectrometer Concentrator (LECS). Above 1 keV the spectrum is consistent with a power-law of photon index 1.47 +/- 0.10 and normalization 11.0 +/- 0.8 photon cm^-2 s^-1 keV^-1 sr^-1 at 1keV. The overall spectrum can be modeled by a power-law with 2 thermal components, or by a broken power-law and a single thermal component. In both cases the softer thermal emission dominates at less than 0.3 keV and is seen through a column of a few 10^19 atom cm^-2. The other components have columns consistent with the mean line of sight value. The metal abundances for the thermal components are poorly constrained, but consistent with cosmic values. The power-law together with 2 thermal components model has been used to fit recent combined ASCA and ROSAT CXB measurements. Here, the soft thermal component is interpreted as emission from the local hot bubble and the hard thermal component as emission from a more distant absorbed region. While such a 2 component thermal model is consistent with the LECS spectrum, it is not required, and the hard thermal component may result from inadequate modeling of the extragalactic contribution. The observed low-energy spectral complexity may therefore originate primarily in the local hot bubble. There is no evidence for the presence of a very soft CXB component with a temperature <0.1 keV. The emission measure seen by ROSAT is rejected at 90% confidence.