|
Description
Apparently diffuse X-ray emission has been known to exist along the central quarter of the Galactic Plane since the beginning of X-ray astronomy; this is referred to as the Galactic Ridge X-ray emission (GRXE). Recent deep X-ray observations have shown that numerous X-ray point sources account for a large fraction of the GRXE in the hard band (2 - 8 keV). However, the nature of these sources is poorly understood. Using the deepest X-ray observations made in the Chandra bulge field, the authors present the result of a coherent photometric and spectroscopic analysis of individual X-ray point sources for the purpose of constraining their nature and deriving their fractional contributions to the hard-band continuum and Fe K line emission of the GRXE. Based on the X-ray color-color diagram, they divided the point sources into three groups: A (hard), B (soft and broad spectrum), and C (soft and peaked spectrum). The group A sources are further decomposed spectrally into thermal and non-thermal sources with different fractions in different flux ranges. From their X-ray properties, the authors speculate that the group A non-thermal sources are mostly active galactic nuclei and the thermal sources are mostly white dwarf (WD) binaries such as magnetic and non-magnetic cataclysmic variables (CVs), pre-CVs, and symbiotic stars, whereas the group B and C sources are X-ray active stars in flares and quiescence, respectively. In the log N - log S curve of the 2 - 8 keV band, the group A non-thermal sources are dominant above ~10<sup>-14</sup> erg/cm<sup>2</sup>/s, which is gradually taken over by Galactic sources in the fainter flux ranges. The Fe K-alpha emission is mostly from the group A thermal (WD binaries) and the group B (X-ray active stars) sources. The authors retrieved 10 archived data sets of the Chandra bulge field (CBF) taken with the Advanced CCD Imaging Spectrometer-I (ACIS-I; 0.5 - 8.0 keV energy band with a spectral resolution of ~280 eV for the full width at half-maximum at 5.9keV) array on board Chandra. The observations were carried out from 2008 May to August with a total exposure time of ~900 ks. The authors first extracted point-source candidates using the wavdetect algorithm in the CIAO package. They set the significance threshold at 2.5 x 10<sup>-5</sup>, implying that one false positive detection would be expected at every 4 x 10<sup>4</sup> trials. As a result, 2596 source candidates were found. The number of their source candidates is nearly the same as that found by Revnivtsev et al.(2009, A&A, 507, 1211) in the same region. To select significant point sources from the candidates, the authors examined their validity based on their photometric significance (PS) and the probability of no source (P<sub>B</sub>). The PS is defined as the background-subtracted source counts (C<sub>net</sub>) divided by its background counts normalized by the area. P<sub>B</sub> is the probability that the source is attributable to a background fluctuation, assuming Poisson statistics. The authors recognized a source to be valid if it satisfied both these criteria: PS >= 1.0 and P<sub>B</sub> <= 1.0 x 10<sup>-2</sup>. As a result, they obtained 2002 valid point sources. This table was created by the HEASARC in December 2014 based on <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/ApJ/766/14">CDS catalog J/ApJ/766/14</a> file table2.dat. This is a service provided by NASA HEASARC .
|
