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Register a new userAn Overview of the Bowen Survey; detecting donor star signatures in Low Mass X-ray Binaries
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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.
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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 analyzed in a systematic way the public INTEGRAL observations spanning from December 2002 to September 2016, to investigate the hard X-ray properties of about 60 High Mass X-ray Binaries (HMXBs). We considered both persistent and transient sources, hosting either a Be star (Be/XRBs) or a blue supergiant companion (SgHMXBs, including Supergiant Fast X-ray Transients, SFXTs), a neutron star or a black hole. INTEGRAL X-ray light curves (18-50 keV), sampled at a bin time of about 2 ks, were extracted for all HMXBs to derive the cumulative distribution of their hard X-ray luminosity, their duty cycle, the range of variability of their hard X-ray luminosity. This allowed us to obtain an overall and quantitative characterization of the long-term hard X-ray activity of the HMXBs in our sample. Putting the phenomenology observed with INTEGRAL into context with other known source properties (e.g. orbital parameters, pulsar spin periods) together with observational constraints coming from softer X-rays (1-10 keV), enabled the investigation of the way the different HMXB sub-classes behave (and sometimes overlap). For given source properties, the different sub-classes of massive binaries seem to cluster in a suggestive way. However, for what concerns supergiant systems (SgHMXBs versus SFXTs), several sources with intermediate properties exist, suggesting a smooth transition between the two sub-classes.
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.
We report on unusually very hard spectral states in three confirmed neutron-star low-mass X-ray binaries (1RXS J180408.9-342058, EXO 1745-248, and IGR J18245-2452) at a luminosity between ~ 10^{36-37} erg s^{-1}. When fitting the Swift X-ray spectra (0.5 - 10 keV) in those states with an absorbed power-law model, we found photon indices of Gamma ~ 1, significantly lower than the Gamma = 1.5 - 2.0 typically seen when such systems are in their so called hard state. For individual sources very hard spectra were already previously identified but here we show for the first time that likely our sources were in a distinct spectral state (i.e., different from the hard state) when they exhibited such very hard spectra. It is unclear how such very hard spectra can be formed; if the emission mechanism is similar to that operating in their hard states (i.e., up-scattering of soft photons due to hot electrons) then the electrons should have higher temperatures or a higher optical depth in the very hard state compared to those observed in the hard state. By using our obtained Gamma as a tracer for the spectral evolution with luminosity, we have compared our results with those obtained by Wijnands et al. (2015). We confirm their general results in that also our sample of sources follow the same track as the other neutron star systems, although we do not find that the accreting millisecond pulsars are systematically harder than the non-pulsating systems.
Binary systems with a neutron-star primary accreting from a companion star display variability in the X-ray band on time scales ranging from years to milliseconds. With frequencies of up to ~1300 Hz, the kilohertz quasi-periodic oscillations (kHz QPOs) represent the fastest variability observed from any astronomical object. The sub-millisecond time scale of this variability implies that the kHz QPOs are produced in the accretion flow very close to the surface of the neutron star, providing a unique view of the dynamics of matter under the influence of some of the strongest gravitational fields in the Universe. This offers the possibility to probe some of the most extreme predictions of General Relativity, such as dragging of inertial frames and periastron precession at rates that are sixteen orders of magnitude faster than those observed in the solar system and, ultimately, the existence of a minimum distance at which a stable orbit around a compact object is possible. Here we review the last twenty years of research on kHz QPOs, and we discuss the prospects for future developments in this field.
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