Sachindra Naik

ON THE ORIGIN OF THE VARIOUS TYPES OF RADIO EMISSION IN GRS 1915+105

We investigate the association between the radio plateau states and the large superluminal flares in GRS 1915+105 and propose a qualitative scenario to explain this association. To investigate the properties of the source during a superluminal flare, we present Giant Metrewave Radio Telescope observations during a radio flare that turned out to be a preplateau flare, as shown by the contemporaneous Ryle Telescope observations. A major superluminal ejection was observed at the end of this plateau state (as described by V. Dhawan et al.), associated with highly variable X-ray emission showing X-ray soft dips. This episode thus has all three types of radio emission: a preplateau flare, a plateau state, and superluminal jets. We analyze all the available Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array data during this episode and show that (1) the preflare plateau state consists of a three-component X-ray spectrum that includes a multicolor disk-blackbody, a Comptonized component, and a power law and (2) the Compton cloud, which is responsible for the Comptonizing component, is ejected away during the X-ray soft dips. We investigate all the available monitoring data on this source, identify several candidate superluminal flare events, and analyze the contemporaneous RXTE pointed observations. We detect a strong correlation between the average X-ray flux during the plateau state and the total energy emitted in radio during the subsequent radio flare. We find that the sequence of events is similar for all large radio flares, with a fast rise and exponential decay morphology. Based on these results, we propose a qualitative scenario in which the separating ejecta during the superluminal flares are the interaction of the matter blob ejected during the X-ray soft dips with the steady jet already established during the plateau state. This picture can explain all the types of radio emission observed from this source in terms of its X-ray-emission characteristics.

Cyclotron resonance energies at a low X-ray luminosity: A0535+262 observed with Suzaku

The binary X-ray pulsar A0535+262 was observed with the Suzaku X-ray observatory on 2005 September 14 for a net exposure of 22 ks. The source was in the declining phase of a minor outburst, exhibiting 3-50 keV luminosity of ~3.7 × 1035 ergs s-1 at an assumed distance of 2 kpc. In spite of the very low source intensity (about 30 mcrab at 20 keV), its electron cyclotron resonance was detected clearly with the Suzaku Hard X-Ray Detector, in absorption at about 45 keV. The resonance energy is found to be essentially the same as that measured when the source is almost 2 orders of magnitude more luminous. These results are compared with the luminosity-dependent changes in the cyclotron resonance energy, observed from 4U 0115+63 and X0331+53.

TIMING AND SPECTRAL STUDIES OF LMC X-4 IN HIGH AND LOW STATES WITH BeppoSAX: DETECTION OF PULSATIONS IN THE SOFT SPECTRAL COMPONENT

We report here detailed timing and spectral analysis of two BeppoSAX observations of the binary X-ray pulsar LMC X-4 carried out during the low and high states of its 30.5 day long superorbital period. Timing analysis clearly shows 13.5 s X-ray pulsations in the high state of the superorbital period, which allows us to measure the mideclipse time during this observation. Combining this with two other mideclipse times derived earlier with ASCA, we have derived a new estimate of the orbital period derivative. Pulse-phase-averaged spectroscopy in the high and low states shows that the energy spectrum in the 0.1-10 keV band comprises a hard power law, a soft excess, and a strong iron emission line. The continuum flux is found to decrease by a factor of ~60 in the low state, while the decrease in the iron-line flux is only by a factor of ~12, suggesting a different site for the production of the line emission. In the low state, we have not found any significant increase in the absorption column density. The X-ray emission is found to come from a very large region, comparable to the size of the companion star. Pulse-phase-resolved spectroscopy in the high state shows a pulsating nature of the soft spectral component with some phase offset compared to the hard X-rays, as is known in some other binary X-ray pulsars.

New measurements of orbital period change in Cygnus X-3

The nonlinear nature of the binary ephemeris of Cygnus X-3 indicates either a change in the orbital period or an apsidal motion of the orbit. We have made extended observations of Cygnus X-3 with the Pointed Proportional Counters (PPCs) of the Indian X-ray Astronomy Experiment (IXAE) during 1999 July 3 13 and October 11 14. Using the data from these observations and the archival data from ROSAT, ASCA, BeppoSAX and RXTE, we have extended the database for this source. Adding these new arrival time measurements to the published results, we make a comparison between the various possibilities, (a) orbital decay due to mass loss from the system, (b) mass transfer between the stars, and (c) apsidal motion of the orbit due to gravitational interaction between the two components. The orbital decay due to mass loss from the companion star seems to be the most probable scenario.

A Catalog of Transient X-Ray Sources in M31

From 1999 October to 2002 August, 45 transient X-ray sources were detected in M31 by Chandra and XMM-Newton. We have performed spectral analysis of all XMM-Newton and Chandra ACIS detections of these sources, as well as flux measurements of Chandra HRC detections. The result is absorption-corrected X-ray light curves for these sources covering this 2.8 yr period, along with spectral parameters for several epochs of the outbursts of most of the transient sources. We supply a catalog of the locations, outburst dates, peak observed luminosities, decay time estimates, and spectral properties of the transient sources, and we discuss similarities with Galactic X-ray novae. Duty cycle estimates are possible for eight of the transients and range from 34% to 2%; upper limits to the duty cycles are estimated for an additional 15 transients and cover a similar range. We find five transients that have rapid decay times and may be ultracompact X-ray binaries. Spectra of three of the transients suggest that they may be faint Galactic foreground sources. If even one is a foreground source, this suggests a surface density of faint transient X-ray sources of 1 deg-2.

Spectral variations of the X-ray binary pulsar LMC X-4 during its long period intensity variation and a comparison with Her X-1

We present spectral variations of the binary X-ray pulsar LMC X-4 using the RXTE/PCA observations at different phases of its 30.5 day long super-orbital period. Only out of eclipse data were used for this study. During the high state of the super-orbital period of LMC X-4, the spectrum is well described by a high energy cut-off power-law with a photon index in the range of 0.7-1.0 and an iron emission line. In the low state, the spectrum is found to be flatter with power-law photon index in the range 0.5-0.7. A direct correlation is detected between the continuum flux in 7-25 keV energy band and the iron emission line flux, The equivalent width of the iron emission line is found to be highly variable during the low intensity state, whereas it remains almost constant during the high intensity state of the super-orbital period. It is observed that the spectral variations in LMC X-4 are similar to those of Her X- I (using RXTE/PCA data). These results suggest that the geometry of the region where the iron line is produced and its visibility with respect to the phase of the super-orbital period is similar in LMC X-4 and Her X-1. A remarkable difference between these two systems is a highly variable absorption column density with phase of the super-orbital period that is observed in Her X-1 but not in LMC X-4.

Suzaku Discovery of Hard X-Ray Pulsations from a Rotating Magnetized White Dwarf, AEAquarii

The fast rotating magnetized white dwarf, AE Aquarii, was observed with Suzaku, in October 2005 and October 2006 with exposures of 53.1 and 42.4 ks, respectively. In addition to clear spin modulation in the 0.5–10 keV band of the XIS data at the barycentric period of 33.0769 ±0.0001 s, the 10–30 keV HXD data in the second half of the 2005 observation also showed statistically significant periodic signals at a consistent period. On that occasion, the spin-folded HXD light curve exhibited two sharp spikes separated by ∼ 0.2 cycles in phase, in contrast to approximately sinusoidal profiles observed in energies below ∼ 4 keV. The folded 4–10 keV XIS light curves are understood as a superposition of those two types of pulse profiles. The phase averaged 1.5–10 keV spectra can be reproduced by two thermal components with temperatures of 2.90 +0.20 −0.16 keV and 0.53 +0.14 −0.13 keV, but the 12-25 keV HXD data show a significant excess above the extrapolated model. This excess can be explained by either a power-law model with photon index of 1.12 +0.63 −0.62 or a third thermal component with a temperature of 54 +26 −47 keV. At a distance of 102 pc, the 4–30 keV luminosities of the thermal and the additional components become 1.7 +1.3 −0.6 and 5.3 +15.3 −0.3 ×10 29 erg s −1 , respectively. The latter corresponds to 0.09% of the spin down energy of the object. Possible emission mechanisms of the hard pulsations are discussed, including in particular non-thermal ones.

TIMING AND SPECTRAL PROPERTIES OF Be/X-RAY PULSAR EXO 2030+375 DURING A TYPE I OUTBURST

We present results from a study of broadband timing and spectral properties of EXO 2030+375 using a Suzaku observation. Pulsations with a period of 41.41?s and strong energy-dependent pulse profiles were clearly detected up to 100?keV. Narrow dips are seen in the profiles up to ~70?keV. The presence of prominent dips at several phases in the profiles up to such high energy ranges was not seen before. At higher energies, these dips gradually disappeared and the profile appeared to be single-peaked. The 1.0-200.0?keV broadband spectrum is found to be well described by a partial covering high-energy cutoff power-law model. Several low-energy emission lines are also detected in the pulsar spectrum. We fitted the spectrum using neutral as well as partially ionized absorbers along with the above continuum model yielding similar parameter values. The partial covering with a partially ionized absorber resulted in a marginally better fit. The spectral fitting did not require any cyclotron feature in the best-fit model. To investigate the changes in spectral parameters at dips, we carried out pulse-phase-resolved spectroscopy. During the dips, the value of the additional column density was estimated to be high compared to other pulse phases. While using a partially ionized absorber, the value of the ionization parameter is also higher at the dips. This may be the reason for the presence of dips up to higher energies. No other spectral parameters show any systematic variation with pulse phases of the pulsar.

X-RAY SPECTROSCOPY OF THE HIGH-MASS X-RAY BINARY PULSAR CENTAURUS X-3 OVER ITS BINARY ORBIT

We present a comprehensive spectral analysis of the high-mass X-ray binary (HMXB) pulsar Centaurus X-3 with the Suzaku observatory covering nearly one orbital period. The light curve shows the presence of extended dips which are rarely seen in HMXBs. These dips are seen up to as high as ~40 keV. The pulsar spectra during the eclipse, out-of-eclipse, and dips are found to be well described by a partial covering power-law model with high-energy cutoff and three Gaussian functions for 6.4 keV, 6.7 keV, and 6.97 keV iron emission lines. The dips in the light curve can be explained by the presence of an additional absorption component with high column density and covering fraction, the values of which are not significant during the rest of the orbital phases. The iron line parameters during the dips and eclipse are significantly different compared to those during the rest of the observation. During the dips, the iron line intensities are found to be lesser by a factor of 2-3 with a significant increase in the line equivalent widths. However, the continuum flux at the corresponding orbital phase is estimated to be lesser by more than an order of magnitude. Similarities in the changes in the iron line flux and equivalent widths during the dips and eclipse segments suggest that the dipping activity in Cen X-3 is caused by an obscuration of the neutron star by dense matter, probably structures in the outer region of the accretion disk, as in the case of dipping low-mass X-ray binaries.

Suzaku Observations of Hercules X-1: Measurements of the Two Cyclotron Harmonics

The accretion-powered pulsar Her X-1 was observed with Suzaku twice in its main-on state, on 2005 October 5-6 and 2006 March 29-30, for a net exposure of 30.5 ks and 34.4 ks, respectively. In the 2005 and 2006 observations, the source was detected at an average 10-30 keV intensity of 290 mCrab and 230 mCrab, respectively. The intrinsic pulse period was measured on both occasions at 1.23776 s by HXDPIN, after barycentric and binary corrections. The pulse phase-averaged spectra in the energy range above 10 keV are well fitted by “Negative and Positive power-law times EXponential (NPEX)” model, multiplied by a fundamental cyclotron resonance scattering feature at ∼36 keV which appears very significantly in the HXD-PIN data. The resonance profiles were reproduced successfully by the Lorentzian type scattering cross section, rather than by a Gaussian type alternative. The pulse phase-averaged HXD-GSO data, covering 50-120 keV, are featureless. However, in a differential spectrum between the pulse-decay phase and off-pulse phase, the second harmonic cyclotron resonance was detected in the GSO data at ∼73 keV, with a depth of 1.6 +0.9 −0.7 . This makes Her X-1 a 6th pulsar with established second harmonic resonance. Implications of these results are briefly discussed.