Low frequency flickering of TT arietis: Hard and soft X-ray emission region

Using archival ASCA observations of TT Arietis, X-ray energy spectra and power spectra of the intensity time series are presented for the first time. The energy spectra are well-fitted by a two continuum plasma emission model with temperatures 1 keV and 10 keV. A coherent feature at 0.643 mHz appeared in the power spectra during the observation.     

The Evolution of Massive Close Binaries

We present results of evolutionary computations for massive close binaries with the Brussels simultaneous evolution code for conservative and non-conservative Roche lobe overflow (RLOF). We discuss mass transfer in massive close binaries during phases of RLOF, common envelope, spiral-in and merging. We examine the effects of stellar wind during successive stellar evolution phases and the final fate of primaries. We show how our library can be used to explain well-known binaries such as the WR + OB system V444 Cyg, HMXBs Vela X-1 and Wray 977, LMXBs like Her X-1, and binary pulsars. More details on the evolution of massive close binaries can be found in “The Brightest Binaries” (Vanbeveren et al., 1998).     

Gamma-Ray Emission from Be/X-ray Binaries

Be/X-ray binaries are systems formed by a massive Be star and a magnetized neutron star, usually in an eccentric orbit. The Be star has strong equatorial winds occasionally forming a circumstellar disk. When the neutron star intersects the disk the accretion rate dramatically increases and a transient accretion disk can be formed around the compact object. This disk can last longer than a single orbit in the case of major outbursts. If the disk rotates faster than the neutron star, the Cheng-Ruderman mechanism can produce a current of relativistic protons that would impact onto the disk surface, producing gamma-rays from neutral pion decays and initiating electromagnetic cascades inside the disk. In this paper we present calculations of the evolution of the disk parameters during both major and minor X-ray events, and we discuss the generation of gamma-ray emission at different energies within a variety of models that include both screened and unscreened disks.     

The secondary stars in cataclysmic variables and low-mass X-ray binaries

We critically re-examine the available data on the spectral types, masses and radii of the secondary stars in cataclysmic variables (CVs) and low-mass X-ray binaries (LMXBs), using the new catalogue of Ritter &38; Kolb as a starting point. We find there are 55 reliable spectral type determinations and only 14 reliable mass determinations of CV secondary stars (10 and 5, respectively, in the case of LMXBs). We derive new spectral type–period, mass–radius, mass–period and radius–period relations, and compare them with theoretical predictions. We find that CV secondary stars with orbital periods shorter than 7–8 h are, as a group, indistinguishable from main-sequence stars in detached binaries. We find that it is not valid, however, to estimate the mass from the spectral type of the secondary star in CVs or LMXBs. We find that LMXB secondary stars show some evidence for evolution, with secondary stars which are slightly too large for their mass. We show how the masses and radii of the secondary stars in CVs can be used to test the validity of the disrupted magnetic braking model of CV evolution, but we find that the currently available data are not sufficiently accurate or numerous to allow such an analysis. As well as considering secondary star masses, we also discuss the masses of the white dwarfs in CVs, and find mean values of ⁻ M  = 0.69 ± 0.13 M_⊙ below the period gap, and ⁻ M  = 0.80 ± 0.22 M_⊙ above the period gap.     

Results of Optical Monitoring of Cataclysmic Variable GK Per in 2004

The preliminary results of ground-based UBV photometric and CCD observations of cataclysmic variable GK Per during the outburst in autumn of 2004 are presented.     

A preliminary version of a catalog of high-low-mass X-ray binaries

We introduce a catalogue of X-ray binaries. The full catalogue is available at http://astroa.physics.metu.edu.tr/XRBC/. The aim of this catalogue is to provide basic information about X-ray sources and their counterparts. The catalog contains positions, information about distances and counterparts, spectral, photometric and timing properties of the X-ray sources in the X-ray and optical bands, as well as references. The list of references, comments and the catalog as a whole will be periodically updated to include the most up-to-date information about all X-ray binaries and guide users to the recent literature on individual sources. In some cases, there is some doubt about the nature of an X-ray source, which has been noted. The sources are ordered according to right ascension. Here, we present a guide to the organization of the full catalogue. The full catalogue is represented here with an excerpt containing two HMXBs and two LMXBs as examples.     

The spectroscopic orbit of the K‐giant binary γ Canis Minoris

We have determined an improved orbit for the bright, evolved, double lined binary γ Canis Minoris. The system has an orbital period of 389.31 days and an eccentricity of 0.2586. We have revised the secondary to primary mass ratio to 0.987. The spectral types of the primary and secondary are K4 III and K1: III, respectively, and the components have a V magnitude difference of 2.2. Orbital fits to the Hipparcos astrometry are not definitive, but they suggest an orbital inclination of ∼ 66°, which produces masses of 1.88 and 1.85 M_⊙ for the components. A comparison with evolutionary tracks results in an age of 1.3 Gyr. STELLA very low amplitude radial velocity residuals of the secondary indicate a period of 278 days. We interpret this as the rotation period of the secondary, detectable because of star spots rotating in and out of view. This period is nearly identical to the pseudosynchronous rotation period of the star. The primary is rotating more slowly than its pseudosynchronous rate. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The changing X-ray light curves of the intermediate polar FO Aquarii

Comparison of five X-ray observations of the intermediate polar FO Aqr reveals that the morphology of the X-ray light curve changes considerably with time. In particular, power spectra of the 1988 Ginga   and 1993 ASCA   data reveal strong sideband and orbital variations, whereas the 1990 Ginga   observation does not. This suggests that the amount of stream-fed accretion varies with epoch, and the system was accreting predominantly from a disc in 1990.   In contrast to other intermediate polars, the X-ray spin-pulse profiles show significant variations between observations, ranging from relatively sinusoidal to sawtooth-shaped at medium energies. During the 1988 and 1990 observations a notch is visible at spin phase zero, due to the presence of an interpulse at phase 0.85, which is absent during the other observations. At lower energies a narrow pulse of emission is seen at spin phase 0.2.   We interpret the pulse profile from the 1990 Ginga   observation using a model for accretion from a disc on to a dipolar magnetic field, the axis of which is offset from the white dwarf centre by ∼ 0.15 white dwarf radii. In order to account for the later occurrence of the hardness-ratio maximum in 1988 and 1993, we suggest that the accretion-rate profile changes so that accretion is favoured along the field lines which trail the magnetic pole. This also accounts for the disappearance of the interpulse and notch in 1993.     

The X-ray eclipse of OY Car resolved with XMM–Newton: X-ray emission from the polar regions of the white dwarf

We present the XMM–Newton X-ray eclipse light curve of the dwarf nova OY Car. The eclipse ingress and egress are well resolved for the first time in any dwarf nova placing strong constraints on the size and the location of the X-ray emitting region. We find good fits to a simple linear eclipse model, giving ingress/egress durations of  30 ± 3 s (Δφ_orb= 0.0054 ± 0.0005)  . Remarkably, this is shorter than the ingress/egress duration of the sharp eclipse in the optical, as measured by Wood et al. (1989) and ascribed to the white dwarf  (43 ± 2 s)  . We also find that the X-ray eclipse is narrower than the optical eclipse by  14 ± 2 s  , which is precisely the difference required to align the second and third contact points of the X-ray and optical eclipses. We discuss these results and conclude that X-ray emission in OY Car arises most likely from the polar regions of the white dwarf.
Our data were originally reported by Ramsay et al. (2001b) , but they did not make a quantitative measurement of eclipse parameters. We have also corrected important timing anomalies present in the data available at that time.     

The multi-temperature X-ray spectrum of the intermediate polar V1223 Sagittarii

We describe fits to the Ginga LAC and ASCA GIS and SIS X-ray spectra of the intermediate polar V1223 Sgr using a multi-temperature emission model and including reflection from the white dwarf surface. Shock temperatures of and were obtained for the Ginga LAC, ASCA GIS and SIS instruments, respectively, giving a mean value of The data reveal significant amounts of reflection as well as a heavy metal underabundance by a factor of 2.
Multiple absorption components are required to successfully model the observed X-ray spectra at low energies, suggesting the presence of complex intrinsic absorption. All data sets reveal a low-energy spectral hardening at the minimum in the spin cycle caused by an increase in the amount of absorption at this phase. The requirement of such complex absorption is becoming increasingly common in the good quality X-ray spectra from magnetic cataclysmic variables.     

The X-ray and extreme-ultraviolet flux evolution of SS Cygni throughout outburst

We present the most complete multiwavelength coverage of any dwarf nova outburst: simultaneous optical, Extreme Ultraviolet Explorer and Rossi X-ray Timing Explorer observations of SS Cygni throughout a narrow asymmetric outburst. Our data show that the high-energy outburst begins in the X-ray waveband 0.9–1.4 d after the beginning of the optical rise and 0.6 d before the extreme-ultraviolet rise. The X-ray flux drops suddenly, immediately before the extreme-ultraviolet flux rise, supporting the view that both components arise in the boundary layer between the accretion disc and white dwarf surface. The early rise of the X-ray flux shows that the propagation time of the outburst heating wave may have been previously overestimated.
The transitions between X-ray and extreme-ultraviolet dominated emission are accompanied by intense variability in the X-ray flux, with time-scales of minutes. As detailed by Mauche & Robinson, dwarf nova oscillations are detected throughout the extreme-ultraviolet outburst, but we find they are absent from the X-ray light curve.
X-ray and extreme-ultraviolet luminosities imply accretion rates of  3 × 10¹⁵ g s⁻¹  in quiescence,  1 × 10¹⁶ g s⁻¹  when the boundary layer becomes optically thick, and  ∼10¹⁸ g s⁻¹  at the peak of the outburst. The quiescent accretion rate is two and a half orders of magnitude higher than predicted by the standard disc instability model, and we suggest this may be because the inner accretion disc in SS Cyg is in a permanent outburst state.     

ASCA X-ray observations of EX Hya: spin-resolved spectroscopy

We analyse the spectral changes over the spin modulation in the intermediate polar EX Hya using archival ASCA data. We find that the modulation can be modelled as either (1) the effect of occultation of the accretion poles by the limb of the white dwarf, or (2) the effect of phase-dependent photoelectric absorption. We argue, on the basis of the partial X-ray eclipse, that the accretion columns in the system are tall, with shock height ∼ R _wd, and hence that the spin modulation is caused mainly by occultation. We find that the temperature distribution along the accretion shocks is incompatible with the calculations of Aizu, except for a restricted parameter regime with a high M _wd. Hence the material in the shock must cool faster than predicted by theory.     

Superorbital period variations in the X-ray pulsar LMC X-4

We report the discovery of a decay in the superorbital period of the binary X-ray pulsar LMC X-4. Combining archival data and published long term X-ray light curves, we have found a decay in the third period in this system (P ∼ 30.3 day, P ∼ −2 × 10⁻⁵ s s⁻¹). Along with this result, a comparison of the superorbital intensity variations in LMC X-4, Her X-1 and SMC X-1 is also presented.     

A search for radio emission from Galactic supersoft X-ray sources

We have made a deep search for radio emission from all the northern hemisphere supersoft X-ray sources using the Very Large Array (VLA) and multi-element radio-linked interferometer network (MERLIN) telescopes, at 5 and 8.4 GHz. Three previously undetected sources, T Pyx, V1974 Cygni and RX J0019.8+2156, were imaged in quiescence using the VLA in order to search for any persistent emission. No radio emission was detected in any of the VLA fields down to a typical 1 σ rms noise of 20 μJy beam⁻¹, however, 17 new point sources were detected in the fields with 5-GHz fluxes between 100 and 1500 μJy, giving an average 100-μJy source density of ∼200 deg⁻², comparable to what was found in the MERLIN Hubble Deep Field survey. The persistent source AG Draconis was observed by MERLIN to provide a confirmation of previous VLA observations and to investigate the source at a higher resolution. The core is resolved at the milliarcsec scale into two components that have a combined flux of ∼1 mJy. It is possible that we are detecting nebulosity, which is becoming resolved out by the higher MERLIN resolution. We have investigated possible causes of radio emission from a wind environment, both directly from the secondary star, and also consequently, of the high X-ray luminosity from the white dwarf. There is an order of magnitude discrepancy between observed and modelled values that can be explained by the uncertainty in fundamental quantities within these systems.     

Magnetic cataclysmic variables: optical counterparts of X-ray and EUV sources

The magnetic cataclysmic variables (mCVs) are a small but astrophysically important class of X-ray source. This paper discusses some of the work carried out by the author and his collaborators on these objects, which were discovered as part of various optical identification programs of both X-ray and extreme ultraviolet sources observed by the HEAO-1 and ROSAT satellites. Definitions are given of the two classes of mCVs; the Polars and the Intermediate Polars. The reasons why mCVs are significant sources of X-ray and EUV radiation are also discussed. Individual systems which have been the subject of intensive follow-up programmes at SAAO are then described, and the implications arising from these observations are discussed.