Attenuation of super-soft X-ray sources by circumstellar material

Recent studies have suggested the possibility of significantly obscuring super-soft X-ray sources in relatively modest amounts of local matter lost from the binaries themselves. If correct, then this would have explained the paucity of observed super-soft X-ray sources and would have significance for the search for single-degenerate type Ia supernova progenitors. We point out that earlier studies of circumbinary obscuration ignored photo-ionisations of the gas by the emission from the super-soft X-ray source. We revisit the problem using a full, self-consistent calculation of the ionisation state of the circumbinary material photo-ionised by the radiation of the central source. Our results show that the circumstellar mass-loss rates required for obcuration of super-soft X-ray sources is about an order of magnitude larger than those reported in earlier studies, for comparable model parameters. While this does not entrirely rule out the possibility of circumstellar material obscuring super-soft X-ray sources, it makes it unlikely that this effect alone can account for the majority of the missing super-soft X-ray sources. We discuss the observational appearance of hypothetical obscured nuclear burning white dwarfs and show that they have signatures making them distinct from photo-ionised nebulae around super-soft X-ray sources imbedded in the low density ISM.Comment: MNRAS, accepted; 11 pages, 10 figures, 1 tabl

Progenitors of type Ia supernovae in elliptical galaxies

Although there is a nearly universal agreement that type Ia supernovae are associated with the thermonuclear disruption of a CO white dwarf, the exact nature of their progenitors is still unknown. The single degenerate scenario envisages a white dwarf accreting matter from a non-degenerate companion in a binary system. Nuclear energy of the accreted matter is released in the form of electromagnetic radiation or gives rise to numerous classical nova explosions prior to the supernova event. We show that combined X-ray output of supernova progenitors and statistics of classical novae predicted in the single degenerate scenario are inconsistent with X-ray and optical observations of nearby early type galaxies and galaxy bulges. White dwarfs accreting from a donor star in a binary system and detonating at the Chandrasekhar mass limit can account for no more than ~5% of type Ia supernovae observed in old stellar populations.Comment: To be published in Proceedings of "Astrophysics of neutron stars", Cesme, 201

He II recombination lines as a test of the nature of SN Ia progenitors in elliptical galaxies

To date, the question of which progenitor channel can reproduce the observed rate of Type Ia supernovae (SNe Ia) remains unresolved, with the single and double degenerate scenarios remaining the leading contenders. The former implies a large population of hot accreting white dwarfs with photospheric temperatures of T ~ 10^5-10^6 K during some part of their accretion history. We show that in early-type galaxies, a population of accreting white dwarfs large enough to reproduce the SN Ia rate would contribute significantly to the ionizing UV radiation expected from the stellar population. For mean stellar ages < ~5 Gyr, single degenerate progenitors would dominate the ionizing background produced by stars, increasing the continuum beyond the He II-ionizing limit more than ten-fold. This opens a new avenue for constraining the progenitors of SNe Ia, through consideration of the spatially extended low-ionization emission-line regions now found in many early-type galaxies. Modelling the expected emission, we show that one can constrain the contribution of the single degenerate channel to the SN Ia rate in E/S0 galaxies from upper limits on the luminosity of He II recombination lines in the optical and FUV. We discuss future directions, as well as possible implications for the evolution of SNe Ia in old stellar populations.Comment: 12 pages, 13 figures, MNRA

X-ray diagnostics of chemical composition of the accretion disk and donor star in ultra-compact X-ray binaries

Non-solar composition of the donor star in ultra-compact X-ray binaries may have a pronounced effect on the fluorescent lines appearing in their spectra due to reprocessing of primary radiation by the accretion disk and the white dwarf surface. We show that the most dramatic and easily observable consequence of the anomalous C/O abundance, is the significant, by more than an order of magnitude, attenuation of the Ka line of iron. It is caused by screening of the presence of iron by oxygen - in the C/O dominated material the main interaction process for a E ~ 7keV photon is absorption by oxygen rather than by iron, contrary to the solar composition case. Ionization of oxygen at high mass accretion rates adds a luminosity dependence to this behavior - the iron line is significantly suppressed only at low luminosity, log(LX) less than 37-37.5, and should recover its nominal strength at higher luminosity. The increase of the EW of the Ka lines of carbon and oxygen, on the other hand, saturates at rather moderate values. Screening by He is less important, due to its low ionization threshold and because in the accretion disk it is mostly ionized. Consequently, in the case of the He-rich donor, the iron line strength remains close to its nominal value, determined by the iron abundance in the accretion disk. This opens the possibility of constraining the nature of donor stars in UCXBs by means of X-ray spectroscopy with moderate energy resolution.Comment: Accepted by MNRA

Is the lack of pulsations in low mass X-Ray binaries due to comptonizing coronae?

The spin periods of the neutron stars in most Low Mass X-ray Binary (LMXB) systems still remain undetected. One of the models to explain the absence of coherent pulsations has been the suppression of the beamed signal by Compton scattering of X-ray photons by electrons in a surrounding corona. We point out that simultaneously with wiping out the pulsation signal, such a corona will upscatter (pulsating or not) X-ray emission originating at and/or near the surface of the neutron star leading to appearance of a hard tail of Comptonized radiation in the source spectrum. We analyze the hard X-ray spectra of a selected set of LMXBs and demonstrate that the optical depth of the corona is not likely to be large enough to cause the pulsations to disappear

Can AGN and galaxy clusters explain the surface brightness fluctuations of the cosmic X-ray background?

Fluctuations of the surface brightness of cosmic X-ray background (CXB) carry unique information about faint and low luminosity source populations, which is inaccessible for conventional large-scale structure (LSS) studies based on resolved sources. We used Chandra data of the XBOOTES field ($\sim9\,\mathrm{deg^2}$) to conduct the most accurate measurement to date of the power spectrum of fluctuations of the unresolved CXB on the angular scales of $\sim3\,$arcsec $-$ $\sim17\,$arcmin. We find that at sub-arcmin angular scales, the power spectrum is consistent with the AGN shot noise, without much need for any significant contribution from their one-halo term. This is consistent with the theoretical expectation that low-luminosity AGN reside alone in their dark matter halos. However, at larger angular scales we detect a significant LSS signal above the AGN shot noise. Its power spectrum, obtained after subtracting the AGN shot noise, follows a power law with the slope of $-0.8\pm0.1$ and its amplitude is much larger than what can be plausibly explained by the two-halo term of AGN. We demonstrate that the detected LSS signal is produced by unresolved clusters and groups of galaxies. For the flux limit of the XBOOTES survey, their flux-weighted mean redshift equals \left\sim0.3, and the mean temperature of their intracluster medium (ICM), \left\approx 1.4 keV, corresponds to the mass of $M_{500} \sim 10^{13.5}\,\mathrm{M}_\odot$. The power spectrum of CXB fluctuations carries information about the redshift distribution of these objects and the spatial structure of their ICM on the linear scales of up to $\sim$Mpc, i.e. of the order of the virial radius.Comment: 25 pages, 20 figures, submitted to MNRAS, comments welcom