The Spectra and Variability of X‐Ray Sources in a DeepChandraObservation of the Galactic Center

Abstract

We examine the X-ray spectra and variability of the sample of X-ray sources with LX ≈ 1031-1033 ergs s-1 identified within the inner 9' of the Galaxy by Muno et al. Very few of the sources exhibit intraday or intermonth variations. We find that the spectra of the point sources near the Galactic center are very hard between 2-8 keV, even after accounting for absorption. When modeled as power laws, the median photon index is Γ = 0.7, while when modeled as thermal plasma we can only obtain lower limits to the temperature of kT > 8 keV. The combined spectra of the point sources is similarly hard, with a photon index of Γ = 0.8. Strong line emission is observed from low-ionization, He-like, and H-like Fe, both in the average spectra and in the brightest individual sources. The line ratios of the highly ionized Fe in the average spectra are consistent with emission from a plasma in thermal equilibrium. This line emission is observed whether average spectra are examined as a function of the count rate from the source or as a function of the hardness ratios of individual sources. This suggests that the hardness of the spectra may in fact be to due local absorption that partially covers the X-ray-emitting regions in the Galactic center systems. We suggest that most of these sources are intermediate polars, which (1) often exhibit hard spectra with prominent Fe lines, (2) rarely exhibit either flares on short timescales or changes in their mean X-ray flux on long timescales, and (3) are the most numerous hard X-ray sources with comparable luminosities in the Galaxy.