arXiv subjects

R. M. Bandyopadhyay

Publications and source records attributed to R. M. Bandyopadhyay.

25 records · Page 2Linked to original sources

arXiv2002

Eclipse Timings of the Low Mass X-ray Binary EXO0748-676 III. An Apparent Orbital Period Glitch Observed with USA and RXTE

We present 7 eclipse timings of the low mass X-ray binary EXO0748-676 obtained with the USA experiment during 1999-2000 as well as 122 eclipse timings obtained with RXTE during 1996-2000. According to our analysis, the mean orbital period has increased by ~8 ms between the pre-RXTE era (1985-1990) and the RXTE/USA era (1996-2000). This corresponds to an orbital period derivative of P(orb)/(dP(orb)/dt)~2x10^7 years. However, neither a constant orbital period derivative nor any other simple ephemeris provides an acceptable fit to the data: individual timings of eclipse centers have residuals of up to 15 or more seconds away from our derived smooth ephemerides. When we consider all published eclipse timing data including those presented here, a model that includes observational measurement error, cumulative period jitter, and underlying period evolution is found to be consistent with the timing data. We discuss several physical mechanisms for LMXB orbital evolution in an effort to account for the change in orbital period and the observed intrinsic jitter in the mid-eclipse times.

astro-ph↗

arXiv2001

USA Observation of Spectral and Timing Evolution During the 2000 Outburst of XTE J1550-564

We report on timing and spectral observations of the 2000 outburst of XTE J1550-564 made by the Unconventional Stellar Aspect (USA) Experiment on board the Advanced Research and Global Observation Satellite (ARGOS). We observe a low-frequency quasi-periodic oscillation (LFQPO) with a centroid frequency that tends to increase with increasing flux and a fractional rms amplitude which is correlated with the hardness ratio. The evolution of the hardness ratio (4--16 keV/1--4 keV) with time and source flux is examined. The hardness-intensity diagram (HID) shows a cyclical movement in the clockwise direction and possibly indicates the presence of two independent accretion flows. We observe a relationship between the USA 4--16 keV count rate and radio observations and discuss this in the context of previously observed correlations between X-ray, radio, optical and IR data. We examine our results in the context of models invoking two accretion flows: a thin disk and a hot sub-Keplerian flow.

astro-ph↗

arXiv2000

USA and RXTE Observations of a Variable Low-Frequency QPO in XTE J1118+480

The USA experiment on ARGOS and RXTE have exensively observed the X-ray transient XTE J1118+480 during its recent outburst in 2000 April--June. We present detailed monitoring of the evolution of a low frequency QPO which drifts from 0.07 Hz to 0.15 Hz during the outburst. We examine possible correlations of the QPO frequency with the flux and spectral characteristics of the source, and compare this QPO to low frequency QPOs observed in other black hole candidates.

astro-ph↗

arXiv1999

The USA X-ray Timing Experiment

The USA Experiment is a new X-ray timing experiment with large collecting area and microsecond time resolution capable of conducting a broad program of studies of galactic X-ray binaries. USA is one of nine experiments aboard the Advanced Research and Global Observation Satellite which was launched February 23, 1999. USA is a collimated proportional counter X-ray telescope with about 1000 cm$^2$ of effective area per detector with two detectors sensitive to photons in the energy range 1-15 keV. A unique feature of USA is that photon events are time tagged by reference to an onboard GPS receiver allowing precise absolute time and location determination. We will present an overview of the USA instrument, capabilities, and scientific observing plan as well as the current status of the instrument.

astro-ph↗

arXiv1999

Infrared spectroscopy of V616 Mon (=A0620--00): the accretion disc contamination

We have obtained for the first time $K$-band infrared spectra of the soft X-ray transient V616 Mon (=A0620--00). We determine the 2-sigma upper limit to the fraction of light arising from the accretion disc to be 27 percent. The effect this has on the binary inclination, determined from modelling the infrared ellipsoidal variations is to increase it by less than 7 degrees and decrease the mass of the black hole by less than 3.6 Msun.

astro-ph↗

arXiv1999

Infrared spectroscopy of low-mass X-ray binaries II

Using CGS4 on UKIRT, we have obtained 2.00-2.45 micron infrared spectra of a number of low-mass X-ray binaries including Sco X-1, Sco X-2, and GX13+1. Sco X-1 shows emission lines only, supporting our previous conclusion that the spectral type of the evolved secondary must be earlier than G5. Emission lines are also seen in the spectrum of Sco X-2, confirming the identity of the IR counterpart. We report the detection of CO bands in GX13+1 and estimate the most likely spectral type of the secondary to be K5 III. We also find P Cygni type profiles in the Brackett gamma lines of Sco X-1 and GX13+1, indicating the presence of high velocity outflows in these systems. We present spectra of candidate IR counterparts to several other elusive X-ray binaries. Finally, implications for the nature and classification of these systems are discussed.

astro-ph↗

arXiv1998

An ``outside-in'' outburst of Aql X--1

We present optical spectroscopy and optical and infrared photometry of the neutron star soft X-ray transient Aql X--1 during its X-ray outburst of August 1997. By modelling the X-ray, optical, and IR light curves, we find a 3 day delay between the IR and X-ray rise times, analogous to the UV-optical delay seen in dwarf novae outbursts and black hole X-ray transients. We interpret this delay as the signature of an ``outside-in'' outburst, in which a thermal instability in the outer disc propagates inward. This outburst is the first of this type definitively identified in a neutron star X-ray transient.

astro-ph↗