P. J. Meintjes

Optical and X-ray properties of CAL 83 – I. Quasi-periodic optical and supersoft variability

We have studied the long-term (∼ years) temporal variability of the prototype supersoft X-ray source (SSS) CAL 83 in the LMC, using data from the MACHOand OGLE projects. The CAL 83 light curve exhibits dramatic brightness changes of ∼ 1 mag on timescales of ∼ 450 days, and spends typically ∼ 200 days in the optical low state. Combined with archival XMM-NewtonX-ray observations these represent the most extensive X-ra y/optical study to date of this system, and reveal in much greater detail that th e X-ray light curve is anticorrelated with the optical behaviour. This is remarkably s imilar to the behaviour of the “transient” SSS, RX J0513.9-6951, where the SSS outbursts recur on a timescale of ∼ 168 days, and also anti-correlate with the optical flux. We performed s imple blackbody fits to both high and low state X-ray spectra, and find that the blackbody tempe rature and luminosity decrease when the optical counterpart brightens. We interpret these long-term variations in terms of the limit-cycle model of Hachisu & Kato (2003a), which provides further support for these systems containing massive (∼ 1.3 M⊙) white dwarfs. In addition, we have refined their orbital periods in the MACHO and OGLE-III light curves to values of 1.047529(1) days and 0.762956(5) days for CAL 83 and RX J0513.9-6951, respectively.

Anisotropic inverse Compton scattering of photons from the circumstellar disc in PSR B1259–63

The gamma-ray binary system PSR B1259-63 consists of a 48 ms pulsar orbiting a Be star. The system is particularly interesting because it is the only gamma-ray binary system where the nature of the compact object is known. The non-thermal radiation from the system is powered by the spin-down luminosity of the pulsar and the unpulsed radiation originates from the stand-off shock front which forms between the pulsar and stellar wind. The Be star/optical companion in the system produces an excess infrared flux from the associated circumstellar disc. This infrared excess provides an additional photon source for inverse Compton scattering. We discuss the effects of the IR excess near periastron, for anisotropic inverse Compton scattering and associated gamma-ray production. We determine the infrared excess from the circumstellar disc using a modified version of a curve of growth method, which takes into account the changing optical depth through the circumstellar disc during the orbit. The model is constrained using archive data and additional mid-IR observations obtained with the VLT during January 2011. The inverse Compton scattering rate was calculated for three orientations of the circumstellar disc. The predicted gamma-ray light curves show that the disc contribution is a maximum around periastron and not around the disc crossing epoch. This is a result of the disc being brightest near the stellar surface. Additional spectroscopic and near-infrared observations were obtained of the system and these are discussed in relation to the possibility of shock heating during the disc crossing epoch.

The effect of the infrared excess from the Be star on inverse Compton gamma-ray production in PSR B1259−63/SS 2883

The binary system PSR B 1259-63 consists of a 48-ms pulsar in a 3.4-yr orbit around a Be star, and unpulsed TeV gamma-ray emission has been detected near previous periastron passages. A likely source of the gamma-rays is the inverse Compton upscattering of target photons from the Be star by the ultrarelativistic electron/positron pulsar wind in the region of the plerion shock front. In this study, the effect of the infrared emission from the Be stars circumstellar disc on inverse Compton gamma-ray production in PSR B1259―63 is investigated by considering an isotropic photon/electron approximation. The modified photon density distribution is determined by using a curve of growth method fitted to previous optical and infrared observations. The inverse Compton scattering rate is calculated using the modified photon distribution and the exact scattering equation. It is shown that including the infrared emission increases the GeV gamma-ray flux by a factor of ≳2.

The Photometry Pipeline of the Watcher Robotic Telescope

The Watcher robotic telescope was developed primarily to perform rapid optical follow-up observations of Gamma-Ray Bursts (GRBs). Secondary scientific goals include blazar monitoring and variable star studies. An automated photometry pipeline to rapidly analyse data from Watcher has been implemented. Details of the procedures to get image zero-point, source instrumental measurement, and limiting magnitude are presented. Sources of uncertainty are assessed and the performance of the pipeline is tested by comparison with a number of catalogue sources.

Watcher: A Telescope for Rapid Gamma-Ray Burst Follow-Up Observations

The Watcher telescope is planned to begin operation in Spring 2004 in South Africa.The system has been designed to respond primarily to very precise (arcminute) gamma‐ray burst locations distributed via the internet by the GCN. Watcher will be fully automatic and the planned response time for GRBs is ∼ 30 seconds or better. In addition, the telescope will be used for blazar monitoring and the photometric detection of extra‐solar planets when GRBs are not being observed.

Orbital and superorbital monitoring of the Be/X-ray binary A0538–66: constraints on the system parameters

We combine the decade long photometry of the Be/X-ray binary system A0538-66 provided by the MACHO and OGLE IV projects with high resolution SALT spectroscopy to provide detailed constraints on the orbital parameters and system properties. The ~420d superorbital modulation is present throughout, but has reduced in amplitude in recent years. The well-defined 16.6409d orbital outbursts, which were a strong function of superorbital phase in the MACHO data (not occurring at all at superorbital maximum), are present throughout the OGLE IV coverage. However, their amplitude reduces during superorbital maximum. We have refined the orbital period and ephemeris of the optical outburst based on ~25 yrs light curves to HJD = 2455674.48 +/- 0.03 + n*16.6409 +/- 0.0003d. Our SALT spectra reveal a B1 III star with vsini of 285 km/s from which we have derived an orbital radial velocity curve which confirms the high eccentricity of e = 0.72 +/- 0.14. Furthermore, the mass function indicates that, unless the neutron star far exceeds the canonical 1.44 Msun, the donor must be significantly undermassive for its spectral type. We discuss the implications of the geometry and our derived orbital solution on the observed behaviour of the system.

Supersoft X-Ray Source CAL 83: A Possible AE Aqr-like System

CAL83 is a close binary supersoft X-ray source in the Large Magellanic Cloud. A ~67 s periodicity detected in supersoft X-rays is most probably associated with the spin period of a highly spun-up white dwarf (WD). The variability in the period is ascribed to the obscuration of the WD by the hydrogen burning envelope surrounding it, rotating with a period that is close to, but not quite synchronized with, the WD rotation period. Optical spectra obtained with SALT exhibit accretion disc emission lines with broad wing structures and P Cyg profiles, indicating mass outflows. Timing analysis of photometrical observations performed at the South African Astronomical Observatory (SAAO) revealed variable signals at ≤1 mHz which are thought to be associated with quasi-periodic oscillations from an accretion disc. The short spin period inferred for CAL83 can be the result of spin-up by accretion disc torques during a long mass transfer history, placing this source on a similar evolutionary track as the cataclysmic variable AE Aqr.

Optical spectroscopic classification of a selection of Southern Hemisphere Fermi -LAT unclassified blazars

The Fermi-LAT has detected more than 3000 sources in the GeV

ANISOTROPIC SCATTERING FROM THE CIRCUMSTELLAR DISC IN PSR B1259-63

\gamma

Transient emission of selected CRTS Cataclysmic Variables

-ray regime. The majority are extra-galactic and these sources are dominated by blazars. However,