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Register a new userBowen Fluorescence from Companion Stars in X-ray Binaries
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J. Casares
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This paper will review a new technique of detecting companion stars in LMXBs and X-ray transients in outburst using the Bowen fluorescence lines at 4634-4640 Angs. These lines are very efficiently reprocessed in the atmospheres of the companion stars, and thereby provide estimates of the K2 velocities and mass functions. The method has been applied to Sco X-1, X1822-371 and GX339-4 which, in the latter case, provides the first dynamical evidence for the presence of an accreting black hole. Preliminary results from a VLT campaign on V801 Ara, V926 Sco and XTE J1814-338 are also presented.
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We present simultaneous high time resolution (1-10 Hz) X-ray and optical observations of the persistent LMXBs Sco X-1 and V801 Ara(=4U 1636-536). In the case of Sco X-1 we find that the Bowen/HeII emission lags the X-ray light-curves with a light travel time of ~11-16s which is consistent with reprocessing in the donor star. We also present the detection of three correlated X-ray/optical bursts in V801 ara. Although this latter project is still in progress our preliminary results obtained by subtracting the Continuum light-curve from the Bowen/HeII data provide evidence of orbital phase dependent echoes from the companion star.
We present preliminary results of a simultaneous X-ray/optical campaign of the prototypical LMXB Sco X-1 at 1-10 Hz time resolution. Lightcurves of the high excitation Bowen/HeII emission lines were obtained through narrow interference filters with ULTRACAM, and these were cross-correlated with X-ray lightcurves. We find evidence for correlated variability, in particular when Sco X-1 enters the Flaring Branch. The Bowen/HeII lightcurves lag the X-ray lightcurves with a light travel time which is consistent with reprocessing in the companion star.
In this paper we give a review of the Bowen fluorescence survey, showing that narrow emission lines (mainly NIII and CIII lines between 4630 and 4660 A) appear to be universally present in the Bowen blend of optically bright low mass X-ray binaries. These narrow lines are attributed to reprocessing in the companion star giving the first estimates of K_2, and thereby providing the first constraints on their system parameters. We will give an overview of the constraints on the masses of the compact objects and briefly highlight the most important results of the survey. Furthermore, we will point out the most promising systems for future follow-up studies and indicate how we think their estimates of the component masses can be improved.
Stellar evolutionary models predict that most of the early type subdwarf stars in close binary systems have white dwarf companions. More massive companions, such as neutron stars or black holes, are also expected in some cases. The presence of compact stars in these systems can be revealed by the detection of X-rays powered by accretion of the subdwarfs stellar wind or by surface thermal emission. Using the Swift satellite, we carried out a systematic search for X-ray emission from a sample of twelve subdwarf B stars which, based on optical studies, have been suggested to have degenerate companions. None of our targets was detected, but the derived upper limits provide one of the few observational constraints on the stellar winds of early type subdwarfs. If the presence of neutron star companions is confirmed, our results constrain the mass loss rates of some of these subdwarf B stars to values <10^{-13}-10^{-12} Msun/yr.
We discuss three classes of x-ray transients to highlight three new types of transients found with the Wide Field Cameras onboard BeppoSAX. First there are the transients related to Low Mass X-ray Binaries in outburst, typically lasting weeks to months and reaching luminosities of the Eddington limit for a few solar masses. Recently another subclass of outbursts in such binaries has been discovered, which are an order of magnitude fainter and last shorter than typical hours to days. We discuss whether they constitute a separate subset of x-ray binaries. A second class of x-ray transients are the x-ray bursts. Thermonuclear explosions on a neutron star (type I x-ray bursts) usually last of order minutes or less. We discovered a second type (called super x-ray bursts) with a duration of several hours. They relate to thermonuclear detonations much deeper in the neutron star atmosphere, possibly burning on the nuclear ashes of normal x-ray bursts. The third class are the enigmatic Fast X-ray Transients occurring at all galactic latitudes. We found that the bright ones are of two types only: either nearby coronal sources (lasting hours) or the socalled x-ray flashes (lasting minutes). The new class, the X-ray flashes, may be a new type of cosmic explosion, intermediate between supernovae and gamma ray bursts, or they may be highly redshifted gamma ray bursts. It thus appears that the three classes of x-ray transients each come in two flavors: long and short.
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