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Register a new userFaint X-ray sources resolved in the ASCA Galactic plane survey and their contribution to the Galactic ridge X-ray emission
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The X-ray emission from the central region of the Galactic plane, |l|<45 deg and |b|<0.4 deg, was studied in the 0.7-10 keV energy band with a spatial resolution of ~3 with the ASCA observatory. We developed a new analysis method for the ASCA data to resolve discrete sources from the extended Galactic ridge X-ray emission (GRXE). We resolved 163 discrete sources with a flux down to 10^-12.5 ergs cm^-2 s^-1 and determined the intensity variations of the GRXE as a function of the Galactic longitude with a spatial resolution of ~1 deg. The longitudinal variation of the GRXE in the energy band above 4 keV shows a large enhancement within |l|<30 deg. This suggests a strong enhancement of X-ray emissivity inside the 4-kpc arms. Searches for identifications of the resolved sources with cataloged X-ray sources and optical stars show that the 66% are unidentified. Spectral analysis of each source shows that a large number of the unidentified sources have hard X-ray spectra. We classified the sources into several groups according to the spectra and analyzed the spectra summed within each group. Possible X-ray origins of these sources are discussed based on the grouping analysis. Also, we derived the LogN-LogS relations of the resolved sources in the energy bands below and above 2 keV. The obtained LogN-LogS relation of the Galactic X-ray sources above 2 keV is represented by a power-law with an index of -0.79+/-0.07. This flat LogN-LogS relation suggests that the spatial distribution of the sources should have an arm-like structure in which the Solar system is included. The integrated surface brightness of the resolved sources is about 10% of the total GRXE in both energy bands. The approximately 90% of the emission remaining is still unresolved.
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The Galactic Ridge X-ray Emission (GRXE) is apparently extended X-ray emission along the Galactic Plane. The X-ray spectrum is characterized by hard continuum with a strong Fe K emission feature in the 6-7 keV band. A substantial fraction (~80%) of the GRXE in the Fe band was resolved into point sources by deep Chandra imaging observations, thus GRXE is mostly composed of dim Galactic X-ray point sources at least in this energy band. To investigate the populations of these dim X-ray point sources, we carried out Near-Infrared (NIR) follow-up spectroscopic observations in two deep Chandra fields located in the Galactic plane at (l,b)=(0.1{arcdeg}, -1.4{arcdeg}) and (28.5{arcdeg}, 0.0{arcdeg}) using NTT/SofI and Subaru/MOIRCS. We obtained well-exposed NIR spectra from 65 objects and found that there are three main classes of Galactic sources based on the X-ray color and NIR spectral features: those having (A) hard X-ray spectra and NIR emission features such as HI(Br{gamma}), HeI, and HeII (2 objects), (B) soft X-ray spectra and NIR absorption features such as HI, NaI, CaI, and CO (46 objects), and (C) hard X-ray spectra and NIR absorption features such as HI, NaI, CaI and CO (17 objects). From these features, we argue that class A sources are Cataclysmic Variables (CVs), and class B sources are late-type stars with enhanced coronal activity, which is in agreement with current knowledge. Class C sources possibly belong to a new group of objects, which has been poorly studied so far. We argue that the candidate sources for class C are the binary systems hosting white dwarfs and late-type companions with very low accretion rates. It is likely that this newly recognized class of the sources contribute to a non-negligible fraction of the GRXE, especially in the Fe K band.
We present the results of the identification of six objects from the ASCA Galactic center and Galactic plane surveys: AXJ173548-3207, AXJ173628-3141, AXJ1739.5-2910, AXJ1740.4-2856, AXJ1740.5-2937, AXJ1743.9-2846. Chandra, XMM-Newton, and XRT/Swift X-ray data have been used to improve the positions of the optical counterparts to these sources. Thereafter, we have carried out a series of spectroscopic observations of the established optical counterparts at the RTT-150 telescope. Analysis of X-ray and optical spectra as well as photometric measurements in a wide wavelength range based on optical and infrared catalogs has allowed the nature of the program sources to be determined. Two X-ray objects have been detected in the error circle of AXJ173628-3141: one is a coronally active G star and the other may be a symbiotic star, a red giant with an accreting white dwarf. Three sources (AXJ1739.5-2910, AXJ1740.5-2937, AXJ1743.9-2846) have turned out to be active G-K stars, presumably RS CVn objects, one (AXJ1740.4-2856) is an M dwarf, and another one (AXJ173548-3207) may be a low-mass X-ray binary in its low state. The distances and corresponding luminosities of the sources in the soft X-ray band have been estimated; analysis of deep INTEGRAL Galactic Center observations has not revealed a statistically significant flux at energies higher 20 keV from any of them.
We analyze a map of the Galactic ridge X-ray emission (GRXE) constructed in the 3-20 keV energy band from RXTE/PCA scan and slew observations. We show that the GRXE intensity closely follows the Galactic near-infrared surface brightness and thus traces the Galactic stellar mass distribution. The GRXE consists of two spatial components which can be identified with the bulge/bar and the disk of the Galaxy. The parameters of these components determined from X-ray data are compatible with those derived from near-infrared data. The inferred ratio of X-ray to near-infrared surface brightness I(3-20 keV) (1e-11 erg/s/cm2/deg2)/I_(3.5micron)(MJy/sr)=0.26+/-0.05, and the ratio of X-ray to near-infrared luminosity L_(3-20 keV)/L_(3-4 micron)=(4.1+/-0.3)e-5. The corresponding ratio of the 3-20 keV luminosity to the stellar mass is L_x/M_Sun= (3.5pm0.5) 10^{27} erg/s, which agrees within the uncertainties with the cumulative emissivity per unit stellar mass of point X-ray sources in the Solar neighborhood, determined in an accompanying paper (Sazonov et al.). This suggests that the bulk of the GRXE is composed of weak X-ray sources, mostly cataclysmic variables and coronally active binaries. The fractional contributions of these classes of sources to the total X-ray emissivity determined from the Solar neighborhood data can also explain the GRXE energy spectrum. Based on the luminosity function of local X-ray sources we predict that in order to resolve 90% of the GRXE into discrete sources a sensitivity limit of ~10^{-16} erg/s/cm2 (2--10 keV) will need to be reached in future observations.
An unresolved X-ray glow (at energies above a few kiloelectronvolts) was discovered about 25 years ago and found to be coincident with the Galactic disk -the Galactic ridge X-ray emission. This emission has a spectrum characteristic of a 1e8 K optically thin thermal plasma, with a prominent iron emission line at 6.7 keV. The gravitational well of the Galactic disk, however, is far too shallow to confine such a hot interstellar medium; instead, it would flow away at a velocity of a few thousand kilometres per second, exceeding the speed of sound in gas. To replenish the energy losses requires a source of 10^{43} erg/s, exceeding by orders of magnitude all plausible energy sources in the Milky Way. An alternative is that the hot plasma is bound to a multitude of faint sources, which is supported by the recently observed similarities in the X-ray and near-infrared surface brightness distributions (the latter traces the Galactic stellar distribution). Here we report that at energies of 6-7 keV, more than 80 per cent of the seemingly diffuse X-ray emission is resolved into discrete sources, probably accreting white dwarfs and coronally active stars.
We present the Chasing the Identification of ASCA Galactic Objects (ChIcAGO) survey, which is designed to identify the unknown X-ray sources discovered during the ASCA Galactic Plane Survey (AGPS). Little is known about most of the AGPS sources, especially those that emit primarily in hard X-rays (2-10 keV) within the F_x ~ 10^-13 to 10^-11 erg cm^-2 s^-1 X-ray flux range. In ChIcAGO, the subarcsecond localization capabilities of Chandra have been combined with a detailed multi-wavelength follow-up program, with the ultimate goal of classifying the >100 unidentified sources in the AGPS. Overall to date, 93 unidentified AGPS sources have been observed with Chandra as part of the ChIcAGO survey. A total of 253 X-ray point sources have been detected in these Chandra observations within 3 of the original ASCA positions. We have identified infrared and optical counterparts to the majority of these sources, using both new observations and catalogs from existing Galactic plane surveys. X-ray and infrared population statistics for the X-ray point sources detected in the Chandra observations reveal that the primary populations of Galactic plane X-ray sources that emit in the F_x ~ 10^-13 to 10^-11 erg cm^-2 s^-1 flux range are active stellar coronae, massive stars with strong stellar winds that are possibly in colliding-wind binaries, X-ray binaries, and magnetars. There is also a fifth population that is still unidentified but, based on its X-ray and infrared properties, likely comprise partly of Galactic sources and partly of active galactic nuclei.
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