arXiv subjects

Biswajit Paul

Publications and source records attributed to Biswajit Paul.

At least 163 records · Page 9Linked to original sources

arXiv2011

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 cut-off 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 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 suggests the dipping activity in Cen X-3 is caused by obscuration of the neutron star by dense matter, probably structures in the outer region of the accretion disk, as in case of dipping low mass X-ray binaries.

astro-ph.SR↗

arXiv2011

Prospect of polarisation measurements from black hole binaries in their thermal state with a scattering polarimeter

X-ray polarisation measurement is a unique tool which may provide crucial information regarding the emission mechanism and the geometry of various astrophysical sources, like neutron stars, accreting black holes, pulsar wind nebulae, AGNs, Supernova Remnants etc. and can help us to probe matter under extreme magnetic fields and extreme gravitational fields. Although the three other domains of X-ray Astronomy i.e. timing, spectral and imaging are well developed, there has been very little progress in X-ray Polarimetry with only one definitive polarisation measurement and a few upper limits available so far. Radiation from accreting black holes in their thermal dominated (High Soft) state is expected to be polarised due to scattering in the plane parallel atmosphere of the disk. Also, special and general relativistic effects in the innermost parts of the disk predicts energy dependent rotation in the plane of polarisation and some distinct signatures which can be used as a probe for measuring the parameters of the black hole like its spin, emissivity profile and the angle of inclination of the system. We present the results from an analysis of expected minimum detectable polarisation from some of the galactic black hole binaries GRO J1655-40, GX339-4, H1743-322, Cyg X-1 and XTE J1817-330 in their thermal dominated state with a proposed Thomson X-ray Polarimeter. A proposal for a scattering polarimeter has been submitted to the Indian Space Research Organization (ISRO) for a dedicated small satellite mission and a laboratory unit has been built. Along with the measurement of the degree of polarisation, the polarisation angle measurement is also important, hence the error in the polarisation angle measurement for a range of detection significance is also obtained.

astro-ph.HE↗

arXiv2011

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.

astro-ph.HE↗

arXiv2011

Search for pulsations in the LMXB EXO 0748-676

We present here results from our search for X-ray pulsations of the neutron star in the low mass X-ray binary EXO 0748-676 at a frequency near a burst-oscillation frequency of 44.7 Hz. Using the observations made with the Proportional Counter Array on board the Rossi X-ray Timing Explorer we did not find any pulsations in the frequency band of 44.4 Hz to 45.0 Hz and obtained a 3σupper limit of 0.47 % on the pulsed fraction for any possible underlying pulsation in this frequency band.

astro-ph.HE↗

arXiv2010

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_αemission 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.

astro-ph.SR↗

arXiv2010

Eclipse Timings of the LMXB XTE J1710-281: Orbital Period Glitches

We present an X-ray eclipse timing analysis of the transient low mass X-ray binary XTE J1710-281. We report observations of 57 complete X-ray eclipses, made with the proportional counter array detectors aboard the RXTE satellite. Using the eclipse timing technique, we have derived a constant orbital period of 0.1367109674 (3) d, during the period from MJD 52132 upto MJD 54410; and 1σ limits of -1.6 $\times$ 10^ -12 d d-1 and 0.2 $\times$ 10^ -12 d d-1 , on the period derivative. This puts constraints on the minimum timescale of secular orbital period evolution of 2.34 $\times$ 10^8 yr for a period decay and 18.7 $\times$ 10^8 yr for a period increase. We also report detection of two instances of discontinuity in the mid-eclipse time, one before and one after the above MJD range. These results are interpreted as three distinct epochs of orbital period in XTE J1710-281. We have put lower limits of 1.4 ms and 0.9 ms on orbital period change at successive epochs.

astro-ph.HE↗

arXiv2010

Development of a Thomson X-ray Polarimeter

We describe the current status of the design and development of a Thomson X-ray polarimeter suitable for a small satellite mission. Currently we are considering two detector geometries, one using rectangular detectors placed on four sides of a scattering element and the other using a single cylindrical detector with the scattering element at the center. The rectangular detector configuration has been fabricated and tested. The cylindrical detector is currently under fabrication. In order to compensate any pointing offset of the satellite, a collimator with a flat topped response has been developed that provides a constant effective area over an angular range. We have also developed a double crystal monochromator/polariser for the purpose of test and calibration of the polarimeter. Preliminary test results from the developmental activities are presented here.

astro-ph.IM↗

arXiv2010

Flares, Broadening of the Pulse frequency peak, and Quasi Periodic Oscillations in the Transient X-ray Pulsar 4U 1901+03

After a long quiescence of three decades, the transient X-ray pulsar 4U 1901+03 became highly active in 2003 February. From the analysis of a large number of Rossi X-ray Timing Explorer/ Proportional Counter Array (RXTE/PCA) observations of this source, we report here the detection of X-ray flares, a broadening of the pulse frequency peak and Quasi Periodic Oscillations (QPOs). The X-ray flares showed spectral changes, had a duration of 100 s - 300 s, and were more frequent and stronger during the peak of the outburst. In most of the observations during the outburst we also detected a broadening of the pulse frequency peak. We have also found intensity dependent changes in the pulse profile at very short timescales. This reveals a coupling between the periodic and the low frequency aperiodic variabilities. In addition, near the end of the outburst we have detected a strong QPO feature centered at ~ 0.135 Hz. The QPO feature is broad with a quality factor of 3.3 and with an rms value of 18.5+/-3.1%. Using the QPO frequency and the X-ray luminosity during the QPO detection period we estimated the magnetic field strength of the neutron star as 0.31*10^12 G which is consistent with the value inferred earlier under the assumption of spin equilibrium.

astro-ph.HE↗

arXiv2010

New measurement of orbital and spin period evolution of the Accretion Disk Corona source 4U 1822-37

4U 1822-37 is a Low Mass X-ray Binary (LMXB) system with an Accretion Disk Corona. We have obtained 16 new mid-eclipse time measurements of this source during the last 13 years using X- ray observations made with the RXTE-PCA, RXTE-ASM, Swift-XRT, XMM-Newton and Chandra observatories. These, along with the earlier known mid-eclipse times have been used to accurately determine the timescale for a change in the orbital period of 4U 1822-37. We have derived an orbital period Porb = 0.23210887(15) d, which is changing at the rate of \cdot Porb = 1.3(3) x 10-10 d d-1 (at T0 = MJD 45614). The timescale for a change in the orbital period is Porb/ \cdot Porb of 4.9(1.1) x 106 yr. We also report the detection of 0.59290132(11) s (at T0 = MJD 51975) X-ray pulsations from the source with a long term average \cdot Pspin of -2.481(4) x 10-12 s s-1, i.e., a spin-up time scale (Pspin/ \cdot Pspin) of 7578(13) yr. In view of these results, we have discussed various mechanisms that could be responsible for the orbital evolution in this system. Assuming the extreme case of conservative mass transfer, we have found that the measured \cdot Porb requires a large mass transfer rate of (4.2 - 5.2) x 10-8 M\odot yr-1 which together with the spin up rate implies a magnetic field strength in the range of (1-3) x 108 G. Using the long term RXTE-ASM light curve, we have found that the X-ray intensity of the source has decreased over the last 13 years by ? 40% and there are long term fluctuations at time scales of about a year. In addition to the long term intensity variation, we have also observed significant variation in the intensity during the eclipse.

astro-ph.HE↗

arXiv2010

Variations in the X-ray eclipse transitions of Cen X-3

We report here an investigation of the X-ray eclipse transitions of the high mass X-ray binary pulsar Cen X-3 in different intensity states. Long term light curve of Cen X-3 obtained with RXTE-ASM spanning for more than 5000 days shows strong aperiodic flux variations with low and high states. We have investigated the eclipse transitions of Cen X-3 in different intensity states with data obtained from pointed observations with the more sensitive instruments on board ASCA, BeppoSAX, XMM-Newton, Chandra and RXTE. We found a very clear trend of sharp eclipse transitions in the high state and longer transitions in the low state. This is a confirmation of this feature first observed with the RXTE-ASM but now with much better clarity. From the light curves obtained from several missions, it is seen that the eclipse egress in the low state starts earlier by an orbital phase of 0.02 indicating that the observed X-rays originate from a much larger region. We have also performed spectral analysis of the post-eclipse part of each observations. From BeppoSAX observations, the out-of-eclipse X-ray fluxes is found to differ by a factor of ~ 26 during the high and low intensity states while the eclipse count rates differ by a factor of only ~ 4.7. This indicates that in the low state, there is an additional scattering medium which scatters some of the source photons towards the observer even when the neutron star is completely eclipsed. We could also resolve the three iron line components using XMM-Newton observation in the low state. By comparing the iron line equivalent width during the high and low states, it is seen that the width of iron line is relatively large during the low state which supports the fact that significant reprocessing and scattering of X-rays takes place in the low state.

astro-ph.HE↗

arXiv2010

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.

astro-ph.HE↗

arXiv2010

An X-ray Bright Nucleus in the Low Surface Brightness Galaxy UGC 6614

We report a study of the X-ray emission from the nuclear region of the low surface brightness (LSB) galaxy UGC 6614. Very little is known about the central objects in LSB galaxies especially their X-ray properties and X-ray spectra. In this study we have used XMM-Newton archival data to study the characteristics of the X-ray spectrum and the X-ray flux variability of the AGN in the LSB galaxy UGC 6614. The nucleus of UGC 6614 is very bright in X-ray emission with an absorption corrected 0.2-10.0 keV luminosity of ~1.1 x 10^{42} erg s^{-1}. The X-ray spectrum is found to be power-law type with a moderate column density. A short time scale of intensity variation and large X-ray flux is indicative of the presence of a black hole at the centre of this galaxy. Using the method of excess variance, we have determined the black hole mass to be ~0.12 x 10^{6} solar mass. The X-ray spectral properties are similar to that of the Seyfert I type AGNs. Our study thus demonstrates that although LSB galaxies are poor in star formation, they may harbour AGNs with X-ray properties comparable to that seen in more luminous spiral galaxies.

astro-ph.GA↗

arXiv2009

Spectral and temporal changes associated with flux enhancement in 4U 1626-67

4U 1626-67 is an accretion powered X-ray pulsar that shows remarkably stable X-ray luminosity above hours timescale and gradual intensity variation on a few years timescale. Here we report a significant increase in the X-ray intensity in the long term RXTE-ASM light curve of 4U 1626-67. Similar enhancement in the X-ray flux has also been detected in the Swift-BAT light curve. The increase in the X-ray flux took place over a long period of about 100 days and there appears to be two episodes of flux enhancement. We have investigated the spectral and timing features of 4U 1626-67 during its current state of enhanced flux emission with data obtained from the Proportional Counter Array and the High-Energy X-ray Timing Explorer on board the Rossi X-ray Timing Explorer. The source has entered a new spin-up phase with a spin-up rate of 4.02(5) $\times$ 10$^{-13}$ Hz s$^{-1}$. The present spin-up rate is almost half of the earlier spin-up and spin-down trends. A significant excess in soft X-ray photon emission is observed during the enhanced flux state, which is similar to the energy spectrum obtained during the spin-up era of the pulsar before 1990. We also report detection of a significant broadening in the wings of the 130 mHz peak and a change in the shape of the continuum of the power spectrum.

astro-ph.HE↗

arXiv2009

Effect of pulse profile variations on measurement of eccentricity in orbits of Cen X-3 and SMC X-1

It has long been argued that better timing precision allowed by satelites like Rossi X-ray Timing Experiments (RXTE) will allow us to measure the orbital eccentricity and the angle of periastron of some of the bright persistent high mass X-ray binaries (HMXBs) and hence a possible measurement of apsidal motion in these system. Measuring the rate of apsidal motion allows one to estimate the apsidal motion constant of the mass losing companion star and hence allows for the direct testing of the stellar structure models for these giant stars present in the HMXBs. In the present paper we use the archival RXTE data of two bright persistent sources, namely Cen X-3 and SMC X-1, to measure the very small orbital eccentricity and the angle of periastron. We find that the small variations in the pulse profiles of these sources rather than the intrinsic timing accuracy provided by RXTE, limit the accuracy with which we can measure arrival time of the pulses from these sources. This influences the accuracy with which one can measure the orbital parameters, especially the very small eccentricity and the angle of periastron in these sources. The observations of SMC X-1 in the year 2000 were taken during the high flux state of the source and we could determine the orbital eccentricity and $ω$ using this data set.

astro-ph.HE↗

arXiv2009

Orbital X-ray modulation study of three Supergiant HMXBs

We present the orbital X-ray modulation study of three high mass X- ray binary systems, IGR J18027-2016, IGR J18483-0311 and IGR J16318-4848 using data obtained with RXTE-ASM, Swift-BAT and INTEGRAL-ISGRI. Using the long term light curves of the eclipsing HMXB IGR J18027-2016, obtained with Swift-BAT in the energy range 15-50 keV and INTEGRAL-ISGRI in the energy range 22-40 keV, we have determined three new mid eclipse times. The newly determined mid eclipse times together with the known values were used to derive an accurate value of the orbital period of 4.5693(4) d at MJD 52168 and an upper limit of 3.9(1.2)x10^-7 d d^-1 on the period derivative. We have also accurately determined an orbital period of 18.5482(88) d for the intermediate system IGR J18483-0311, which displays an unusual behaviour and shares many properties with the known SFXTs and persistent supergiant systems. This is a transient source and the outbursts occur intermittently at intervals of 18.55 d. Similarly, in the third supergiant system, IGR J16318-4848, we have found that the outbursts are separated by intervals of 80 d or its multiples, suggesting a possible orbital period.

astro-ph.HE↗

arXiv2009

Near-infrared/optical identification of five low-luminosity X-ray pulsators

We present the identification of the most likely near-infrared/optical counterparts of five low-luminosity X-ray pulsators (AX J1700.1-4157, AX 1740.1-2847, AX J1749.2-2725, AX J1820.5-1434 and AX J1832.3-0840) which have long pulse periods (> 150 s). The X-ray properties of these systems suggest that they are likely members of persistent high mass X-ray binaries or intermediate polars. Using our Chandra observations, we detected the most likely counterparts of three sources (excluding AX J1820.5-1434 and AX J1832.3-0840) in their ESO - NTT near-infrared observations, and a possible counterpart for AX J1820.5-1434 and AX J1832.3-0840 in the 2MASS and DSS observations respectively. We also performed the X-ray timing and spectral analysis for all the sources using our XMM-Newton observations, which further helped us to constrain the nature of these systems. Our multiwavelength observations suggest that AX J1749.2-2725 and AX J1820.5-1434 most likely harbor accreting neutron stars while AX J1700.1-4157, AX J1740.1-2847 and AX J1832.3-0840 could be intermediate polars.

astro-ph.HE↗

arXiv2009

Discovery of a short orbital period in the Supergiant Fast X-ray Transient IGR J16479-4514

We report here discovery of a 3.32 day orbital period in the Supergiant Fast X-ray Transient (SFXT) source IGR J16479-4514. Using the long term light curve of this source obtained with Swift-BAT in the energy range of 15-50 keV, we have clearly detected an orbital modulation including a full eclipse of duration ~0.6 day. In the hard X-ray band of the BAT instrument, the eclipse ingress and egress are rapid. We have also used the long term light curve obtained with the RXTE -ASM in the energy range of 1.5-12 keV. Taken independently, the detection of orbital modulation in the RXTE -ASM light curve is not significant. However, considering a clear detection of orbital modulation in the BAT light curve, we have used the ASM light curve for a more precise determination of the orbital period. IGR J16479-4514 has the shortest orbital period among the three SFXTs with measured/known orbital period. We discuss the implication of a short orbital period with the various mechanisms proposed to explain the transient nature of this class of sources.

astro-ph.HE↗

arXiv2008

Orbital Phase Spectroscopy of four High Mass X-ray Binary Pulsars to Study the Stellar Wind of the Companion

Our work focuses on a comprehensive orbital phase dependent spectroscopy of the four High Mass X-ray Binary Pulsars (HMXBPs) 4U 1538-52, GX 301-2, OAO 1657-415 & Vela X-1. We hereby report the measurements of the variation of the absorption column density and iron-line flux along with other spectral parameters over the binary orbit for the above-mentioned HMXBPs in elliptical orbits, as observed with the Rossi X-ray Timing Explorer (RXTE) and the BeppoSAX satellites. A spherically symmetric wind profile was used as a model to compare the observed column density variations. Out of the four pulsars, only in 4U 1538-52, we find the model having a reasonable corroboration with the observations, whereas in the remaining three the stellar wind seems to be clumpy and a smooth symmetric stellar wind model appears to be quite inadequate in explaining the data. Moreover, in GX 301-2, neither the presence of a disk nor a gas stream from the companion was validated. Furthermore, the spectral results obtained in the case of OAO 1657-415 & Vela X-1 were more or less similar to that of GX 301-2.

astro-ph↗