Abstract
Aims. We study populations of soft and super-soft X-ray sources (SSSs) in nearby galaxies of various morphological types with particular emphasis on characterizing populations of stable nuclear burning, accreting white dwarfs (WDs). Methods. Analyzing the content of the Chandra archive, we assembled a sample of nearby galaxies suitable for studying populations of SSSs. Our sample includes four spiral galaxies, two lenticular galaxies, and three ellipticals with stellar mass exceeding 1010 M⊙ and X-ray sensitivity of the order of a few × 1036 erg s−1. We used a combination of hardness ratio and median energy to pre-select X-ray sources with soft spectra, and used the temperature–X-ray luminosity diagram to identify SSSs, likely nuclear-burning, accreting WDs. Results. For spiral galaxies, there is a distinct and rare population of super-soft sources that are largely detached from the rest of the sources on the kTbb − LX plane. The boundary between these sources and the much more numerous population of harder (but still soft) sources is consistent with the boundary of stable hydrogen burning on the surface of WDs. The combined spectrum of soft sources located outside this boundary shows clear emission lines of Mg and S, the equivalent width of which is similar to that in the combined spectrum of a large number of confirmed supernova remnants in M 83. This supports earlier suggestions that the vast majority of the so-called quasi-soft sources are supernova remnants. In early-type galaxies, populations of super-soft sources are about a factor of eight less abundant, in broad agreement with the population synthesis calculations. Specific frequencies of super-soft sources are (2.08 ± 0.46) × 10−10 M⊙−1 in spiral galaxies and (2.47 ± 1.34) × 10−11 M⊙−1 in lenticular and elliptical galaxies, with the ratio of the latter to the former being 0.12 ± 0.05.
Highlights
CAL 83 and CAL 87, the two prototypical super-soft X-ray sources (SSSs) were discovered by Einstein observatory in the course of a survey of the Large Magellanic Clouds (Long et al 1981)
The proposition that the main source of energy in SSSs is thermonuclear burning of the accreted hydrogen naturally explained the unusual softness of their X-ray spectra, the combination of the energy output expected in hydrogen fusion and the surface area of a typical white dwarfs (WDs) giving effective temperatures in the correct range, close to the observed values
This can be understood considering that populations of accreting nuclear burning WDs are determined by the long-term star-formation history of their host galaxy rather than by its mass, as discussed
Summary
CAL 83 and CAL 87, the two prototypical super-soft X-ray sources (SSSs) were discovered by Einstein observatory in the course of a survey of the Large Magellanic Clouds (Long et al 1981). With the advent of Chandra and XMM-Newton, similar sources were found in other, more distant galaxies The spectra of these sources are very soft and do not extend beyond ∼1−2 keV, and approximation with the blackbody model gives temperatures of the order of ∼10−100 eV and bolometric luminosities in the ∼1036−1038 erg s−1 range (Greiner et al 1991; Kahabka et al 1994). The proposition that the main source of energy in SSSs is thermonuclear burning of the accreted hydrogen naturally explained the unusual softness of their X-ray spectra, the combination of the energy output expected in hydrogen fusion and the surface area of a typical WD giving effective temperatures in the correct range, close to the observed values (van den Heuvel et al 1992). The nature of soft X-ray sources and SSSs is described in Sect. 6 where we estimate their specific frequencies and compare our results with those of previous studies, and in Sect. 7 we summarize our findings
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
