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Register a new userSymbiotic stars in X-rays
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Added by
Gerardo Juan Manuel Luna
Publication date
2012
fields
Physics
and research's language is
English
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Until recently, symbiotic binary systems in which a white dwarf accretes from a red giant were thought to be mainly a soft X-ray population. Here we describe the detection with the X-ray Telescope (XRT) on the Swift satellite of nine white dwarf symbiotics that were not previously known to be X-ray sources and one that had previously been detected as a supersoft X-ray source. The nine new X-ray detections were the result of a survey of 41 symbiotic stars, and they increase the number of symbiotic stars known to be X-ray sources by approximately 30%. The Swift/XRT telescope detected all of the new X-ray sources at energies greater than 2 keV. Their X-ray spectra are consistent with thermal emission and fall naturally into three distinct groups. The first group contains those sources with a single, highly absorbed hard component that we identify as probably coming from an accretion-disk boundary layer. The second group is composed of those sources with a single, soft X-ray spectral component that probably originates in a region where low-velocity shocks produce X-ray emission, i.e., a colliding-wind region. The third group consists of those sources with both hard and soft X-ray spectral components. We also find that unlike in the optical, where rapid, stochastic brightness variations from the accretion disk typically are not seen, detectable UV flickering is a common property of symbiotic stars. Supporting our physical interpretation of the two X-ray spectral components, simultaneous Swift UV photometry shows that symbiotic stars with harder X-ray emission tend to have stronger UV flickering, which is usually associated with accretion through a disk. To place these new observations in the context of previous work on X-ray emission from symbiotic star.............
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We describe the X-ray emission as observed with Suzaku from five symbiotic stars that we selected for deep Suzaku observations after their initial detection with ROSAT, ASCA and Swift. We find that the X-ray spectra of all five sources can be adequately fit with absorbed, optically thin thermal plasma models, with either single- or multi-temperature plasmas. These models are compatible with the X-ray emission originating in the boundary layer between an accretion disk and a white dwarf. The high plasma temperatures of kT$~>3$ keV for all five targets were greater than expected for colliding winds. Based on these high temperatures, as well as previous measurements of UV variability and UV luminosity, and the large amplitude of X-ray flickering in 4 Dra, we conclude that all five sources are accretion-powered through predominantly optically thick boundary layers. Our X-ray data allow us to observe a small, optically thin portion of the emission from these boundary layers. Given the time between previous observations and these observations, we find that the intrinsic X-ray flux and the intervening absorbing column can vary by factors of three or more on a time scale of years. However, the location of the absorber and the relationship between changes in accretion rate and absorption are still elusive.
Any white dwarf or neutron star that accretes enough material from a red giant companion, such that this interaction can be detected at some wavelength, is currently termed Symbiotic Star (typical P(orb)=2-3 years). In the majority of ~400 known systems, the WD burns nuclearly at its surface the accreted material, and the resulting high temperature (T(eff)=10(^5)~K) and luminosity (L(hot)=10(^3)-10(^4) Lsun) allow ionization of a large fraction of the cool giants wind, making such symbiotic stars easily recognizable through the whole Galaxy and across the Local Group. X-ray observations are now revealing the existence of a parallel (and larger ?) population of optically-quiet, accreting-only symbiotic stars. Accretion flows and disks, ionization fronts and shock, complex 3D geometries and new evolution channels are gaining relevance and are reshaping our understanding of symbiotic stars. We review the different types of symbiotic stars currently in the family and their variegated outburst behaviors through an unified evolution scheme connecting them all.
Symbiotic stars show emission across the electromagnetic spectrum from a wide array of physical processes. At cm-waves both synchrotron and thermal emission is seen, often highly variable and associated with outbursts in the optical and X-rays. Most models of the radio emission include an ionized region within the dense wind of the red giant star, that is kept ionized by activity on the white dwarf companion or its accretion disk. In some cases there is on-going shell burning on the white dwarf due to its high mass accretion rate or a prior nova eruption, in other cases nuclear fusion occurs only occasionally as recurrent nova events. In this study we measure the spectral indices of a sample of symbiotic systems in the Southern Hemisphere using the Australia Telescope Compact Array. Putting our data together with results from other surveys, we derive the optical depths and brightness temperatures of some well-known symbiotic stars. Using parallax distances from Gaia Data Release 3, we determine the sizes and characteristic electron densities in the radio emission regions. The results show a range of a factor of 10^4 in radio luminosity, and a factor of 100 in linear size. These numbers are consistent with a picture where the rate of shell burning on the white dwarf determines the radio luminosity. Therefore, our findings also suggest that radio luminosity can be used to determine whether a symbiotic star is powered by accretion alone or also by shell burning.
Thanks to the high sensitivity of the instruments on board the XMM-Newton and Chandra satellites, it has become possible to explore the properties of the X-ray emission from hot subdwarfs. The small but growing sample of hot subdwarfs detected in X-rays includes binary systems, in which the X-rays result from wind accretion onto a compact companion (white dwarf or neutron star), as well as isolated sdO stars in which X-rays are probably due to shock instabilities in the wind. X-ray observations of these low mass stars provide information which can be useful also for our understanding of the winds of more luminous and massive early-type stars and can lead to the discovery of particularly interesting binary systems.
We report the detection of X-ray emission from the jet-driving symbiotic star MWC 560. We observed MWC 560 with XMM-Newton for 36 ks. We fitted the spectra from the EPIC pn, MOS1 and MOS2 instruments with XSPEC and examined the light curves with the package XRONOS. The spectrum can be fitted with a highly absorbed hard X-ray component from an optically-thin hot plasma, a Gaussian emission line with an energy of 6.1 keV and a less absorbed soft thermal component. The best fit is obtained with a model in which the hot component is produced by optically thin thermal emission from an isobaric cooling flow with a maximum temperature of 61 keV, which might be created inside an optically-thin boundary layer on the surface of the accreting with dwarf. The derived parameters of the hard component detected in MWC 560 are in good agreement with similar objects as CH Cyg, SS7317, RT Cru and T CrB, which all form a new sub-class of symbiotic stars emitting hard X-rays. Our previous numerical simulations of the jet in MWC 560 showed that it should produce detectable soft X-ray emission. We infer a temperature of 0.17 keV for the observed soft component, i.e. less than expected from our models. The total soft X-ray flux (i.e. at < 3 keV) is more than a factor 100 less than predicted for the propagating jet soon after its birth (<0.3 yr), but consistent with the value expected due its decrease with age. The ROSAT upper limit is also consistent with such a decrease. We find aperiodic or quasi-periodic variability on timescales of minutes and hours, but no periodic rapid variability. All results are consistent with an accreting white dwarf powering the X-ray emission and the existence of an optically-thin boundary layer around it.
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