Transient High Mass X-ray Binaries

High Mass X-ray Binaries (HMXBs) are interesting objects that provide a wide range of observational probes to the nature of the two stellar components, accretion process, stellar wind and orbital parameters of the systems. A large fraction of the transient HMXBs are found to be Be/X-ray binaries in which the companion Be star with its circumstellar disk governs the outburst. These outbursts are understood to be due to the sudden enhanced mass accretion to the neutron star and is likely to be associated with changes in the circumstellar disk of the companion. In the recent years, another class of transient HMXBs have been found which have supergiant companions and show shorter bursts. X-ray, infrared and optical observations of these objects provide vital information regarding these systems. Here we review some key observational properties of the transient HMXBs and also discuss some important recent developments from studies of this class of sources. The X-ray properties of these objects are discussed in some detail whereas the optical and infrared properties are briefly discussed.Comment: 21 Pages, 8 Figures, To appear in the special issue of the Bulletin of the Astronomical Society of India on Transients at different wavelengths, eds D.J. Saikia and D.A. Gree

Load flow analysis of unbalanced radial distribution systems

A thru tactic for unbalanced three-phase distribution load flow solutions is presented in this work. The distinctive topological characteristics of distribution networks have been fully exploited to make the direct solution possible. For unbalanced distribution system backward sweep is used for calculation of branch current and forward sweep is used for bus voltage. Due to the distinctive solution techniques of the approached method, the time-consuming Lower and Upper decomposition and forward or backward substitution of the Jacobian matrix or Y admittance matrix required in the traditional load flow methods are no longer necessary. Therefore, the approached method is robust and time efficient. Test results demonstrate the validity of the approached method. The approach shows excessive prospective to be used in distribution automation applications

Suzaku observation of the transient X-ray pulsar GRO J1008-57

We report the timing and broad-band spectral properties of the Be transient high mass X-ray binary pulsar GRO J1008-57 using a Suzaku observation in the declining phase of its 2007 November-December outburst. Pulsations with a period of 93.737 s were clearly detected in the light curves of the pulsar up to the 80-100 keV energy band. The pulse profile was found to be strongly energy dependent, a double peaked profile at soft X-ray energy bands (< 8 keV) and a single peaked smooth profile at hard X-rays. The broad-band energy spectrum of the pulsar, reported for the first instance in this paper, is well described with three different continuum models viz. (i) a high energy cut-off power-law, (ii) a Negative and Positive power-law with EXponential cut-off (NPEX), and (iii) a partial covering power-law with high energy cut-off. Inspite of large value of absorption column density in the direction of the pulsar, a blackbody component of temperature ~0.17 keV for the soft excess was required for the first two continuum models. A narrow iron K_\alpha emission line was detected in the pulsar spectrum. The partial covering model, however, is found to explain the phase averaged and phase resolved spectra well. The dip like feature in the pulse profile can be explained by the presence of an additional absorption component with high column density and covering fraction at the same pulse phase. The details of the results are described in the paper.Comment: 9 pages, 7 figures, Accepted for publication in MNRA

Discovery of a 0.02 Hz QPO feature in the Transient X-ray Pulsar KS 1947+300

We report the discovery of Quasi Periodic Oscillations (QPO) at 0.02 Hz in a transient high mass X-ray binary pulsar KS 1947+300 using {\em RXTE}-PCA. The QPOs were detected during May-June 2001, at the end of a long outburst. This is the 9th transient accretion powered high magnetic field X-ray pulsar in which QPOs have been detected and the QPO frequency of this source is lowest in this class of sources. The unusual feature of this source is that though the outburst lasted for more than 100 days, the QPOs were detected only during the last few days of the outburst when the X-ray intensity had decayed to 1.6% of the peak intensity. The rms value of the QPO is large, $\sim15.4\pm1.0$ with a slight positive correlation with energy. The detection of QPOs and strong pulsations at a low luminosity level suggests that the magnetic field strength of the neutron star is not as high as was predicted earlier on the basis of a correlation between the spin-up torque and the X-ray luminosity.Comment: Accepted in MNRA

Timing and broad band spectroscopy of 1A 1118-61 with Suzaku

We present a timing and broad-band pulse phase resolved spectral analysis of the transient Be X-ray binary pulsar 1A 1118-61 observed during its outburst in January 2009 using Suzaku observations. Suzaku observations were made twice, once at the peak of the outburst, and once 13 days later at its declining phase.Pulse profiles from both observations exhibit strong energy dependence with several peaks at low energies and a single peak above ~10 keV. A weak, narrow peak is detected at the main dip of the pulse profiles from both observations in the energy bands below 3 keV, indicating the presence of a phase dependent soft excess in the source continuum. The broad-band energy spectrum of the pulsar could be fitted well with a partial covering cutoff power-law model and a narrow iron fluorescence line. We also detect a broad cyclotron feature at ~50 keV from both observations which is a feature common for accretion powered pulsars with high magnetic field strengths. Pulse phase-resolved spectral analysis shows an increase in the absorption column density of the partial covering component, as well as variation in the covering fraction at the dips of the pulse profiles, that naturally explains the energy dependence of the same. The cyclotron line parameters also show significant variation with pulse phase with a ~ 10 keV variation in the cyclotron line energy and a variation in depth by a factor of three. This can be explained either as the effect of different viewing angles of the dipole field at different pulse phases, or a more complex underlying magnetic field geometryComment: 22 pages,9 figures and 1 table.Accepted for publication in MNRA

RXTE-PCA observations of 1A 1118--61: timing and spectral studies during an outburst

We report detailed timing and spectral analysis of RXTE-PCA data obtained from observations during the outburst of a transient X-ray pulsar 1A 1118--61 in January 2009. The pulse profile showed significant evolution during the outburst and also significant energy dependence - a double peaked profile upto 10 keV and a single peak at higher energy. We have also detected quasi-periodic oscillations (QPO) at 0.07--0.09 Hz. The rms value of the QPO is 5.2% and it shows a significant energy dependence with highest rms of 7% at 9 keV. The QPO frequency changed from 0.09 Hz to 0.07 Hz within 10 days. The magnetic field strength calculated using the QPO frequency and the X-ray luminosity is in agreement with the magnetic field strength measured from the energy of the cyclotron absorption feature detected in this source. The 3-30 keV energy spectrum over the 2009 outburst of 1A 1118--61 can be well fitted with a partial covering power-law model with a high energy cutoff and an iron fluorescence line emission. The pulse phase resolved spectral analysis shows that the partial covering and high energy cutoff model parameters have significant changes with the pulse phase.Comment: Accepted for publication in MNRA