arXiv subjects

B. Paul

Publications and source records attributed to B. Paul.

86 records · Page 5Linked to original sources

arXiv2001

Lock-in detection using a cryogenic low noise looped current preamplifier for the readout of resistive bolometers

We implemented a low noise current preamplifier for the readout of resistive bolometers. We tested the apparatus on thermometer resistances ranging from 10 Ohm to 500 Mohm. The use of current preamplifier overcomes constraints introduced by the readout time constant due to the thermometer resistance and the input capacitance. Using cold JFETs, this preamplifier board is shown to have very low noise: the Johnson noise of the source resistor (1 fA/Hz1/2) dominated in our noise measurements. We also implemented a lock-in chain using this preamplifier. Because of fast risetime, compensation of the phase shift may be unnecessary. If implemented, no tuning is necessary when the sensor impedance changes. Transients are very short, and thus low-passing or sampling of the signal is simplified. In case of spurious noise, the modulation frequency can be chosen in a much wider frequency range, without requiring a new calibration of the apparatus.

physics.ins-det↗

arXiv2001

Different Types of X-ray Variability Observed in Micro-Quasar GRS 1915+105 with the IXAE

The galactic micro-quasar source GRS 1915+105 has been observed with the Indian X-ray Astronomy Experiment (IXAE) several times over a period extending from 1996 to 2000. The source has been observed in different luminosity states and exhibits a rich variety of random and regular X-ray variability over a wide range of time scales. It also shows a unique variability characteristic namely the occurrence of quasi-regular X-ray bursts with period ranging from 20 s to about 100 s which have slow rise and fast decay. The X-ray spectrum of these bursts is hardest near the end of decay. In the low-hard state and bright-soft state with rapid variability, the source exhibits quasi-periodic oscillations (QPOs) with variable frequency which is luminosity dependent. Regular X-ray intensity dips have been detected coincident with the occurrence of a giant radio outburst. The source made a transition from a low-state to a high-state in 1999 in less than a day and from a soft-bright state to a hard-low state in 2000 in less than 100 minutes during the IXAE observations. Details of these and other variability characteristics of the source are presented in this paper.

astro-ph↗

arXiv2000

Detection of a Series of X-ray Dips Associated with a Radio Flare in GRS 1915+105

We report the detection of a series of X-ray dips in the Galactic black hole candidate GRS 1915+105 during 1999 June 6-17 from observations carried out with the Pointed Proportional Counters of the Indian X-ray Astronomy Experiment on board the Indian satellite IRS-P3. The observations were made after the source made a transition from a steady low-hard state to a chaotic state which occuered within a few hours. Dips of about 20-160 seconds duration are observed on most of the days. The X-ray emission outside the dips shows a QPO at ~ 4 Hz which has characteristics similar to the ubiquitous 0.5 - 10 Hz QPO seen during the low-hard state of the source. During the onset of dips this QPO is absent and also the energy spectrum is soft and the variability is low compared to the non-dip periods. These features gradually re-appear as the dip recovers. The onset of the occurrence of a large number of such dips followed the start of a huge radio flare of strength 0.48 Jy (at 2.25 GHz). We interpret these dips as the cause for mass ejection due to the evacuation of matter from an accretion disk around the black hole. We propose that a super-position of a large number of such dip events produces a huge radio jet in GRS 1915+105.

astro-ph↗

arXiv2000

Rapid State Transitions in the Galactic Black Hole Candidate Source GRS 1915+105

We examine the X-ray spectral and temporal behavior of the Galactic black hole candidate source GRS 1915+105 during its spectral state transition observed in 1997 March - August. The source was making a slow transition from a low-hard state to a high-soft state in about three months and during this transition it was exhibiting a series of fast variations which can be classified as bursts. During one type of bursts, called the irregular bursts, it was found that the source makes rapid transitions between two intensity states. We have analyzed the RXTE PCA data obtained on 1997 June 18 (when the source was making rapid state transitions) and compared the results with two sets of data each pertaining to the low hard-state and high-soft states. We find that the power density spectrum (PDS) during the burst shows remarkable similarity to that seen during the high-soft state of the source. The PDS during the quiescence, on the other hand, is quite flat and is very similar to the PDS commonly seen during the canonical low-hard state of black hole X-ray binaries. A low frequency QPO at $\sim$3 Hz is observed during the quiescence, a property seen during the low-hard state of the source. The X-ray spectrum of the source during quiescence is similar to the spectrum seen during the low-hard state of the source and the spectra during the burst and high-soft state are very similar to each other. These observations corroborate the suggestion made by Yadav et al. (1999) that during the irregular bursts the source shows spectral state variations at very fast time scales. We explain such fast state changes using the two component accretion flow model in which the accretion disk consists of a geometrically thin (and optically thick) standard disk co-existing with an advection dominated hot thick disk.

astro-ph↗

arXiv2000

Changes in the long term intensity variations in Cyg X-2 and LMC X-3

We report the detection of changes in the long-term intensity variations in two X-ray binaries, Cyg X-2 and LMC X-3. In this work, we have used the long-term light curves obtained with the All-Sky Monitors (ASMs) of the Rossi X-Ray Timing Explorer (RXTE), Ginga, Ariel 5, and Vela 5B and the scanning modulation collimator of HEAO 1. It is found that in the light curves of both the sources, obtained with these instruments at various times over the last 30 years, more than one periodic or quasi-periodic component is always present. The multiple prominent peaks in the periodograms have frequencies unrelated to each other. In Cyg X-2, RXTE-ASM data show strong peaks at 40.4 and 68.8 days, and Ginga-ASM data show strong peaks at 53.7 and 61.3 days. Multiple peaks are also observed in LMC X-3. The various strong peaks in the periodograms of LMC X-3 appear at 104, 169, and 216 days (observed with RXTE-ASM) and 105, 214, and 328 days (observed with Ginga-ASM). The present results, when compared with the earlier observations of periodicities in these two systems, demonstrate the absence of any stable long period. The 78 day periodicity detected earlier in Cyg X-2 was probably due to the short time base in the RXTE data that were used, and the periodicity of 198 days in LMC X-3 was due to a relatively short duration of observation with HEAO 1.

astro-ph↗

arXiv2000

Study of the Long Term Stability of two Anomalous X-ray Pulsars 4U 0142+61 and 1E 1048.1-5937 with ASCA

We present new observations of two anomalous X-ray pulsars (AXP) 4U 0142+61 and 1E 1048.1-5937 made in 1998 with the ASCA. The energy spectra of these two AXPs are found to consist of two components, a power-law and a blackbody emission from the neutron star surface. These observations, when compared to earlier ASCA observations in 1994 show remarkable stability in the intensity, spectral shape and pulse profile. However, we find that the spin-down rate in 1E 1048.1-5937 is not constant. In this source, we have clearly identified three epochs with spin-down rates different from each other and the average value. This has very strong implications for the magnetar hypothesis of AXPs. We also note that the spin-down rate and its variations in 1E 1048.1-5937 are much larger than what can normally be produced by an accretion disk with very low mass accretion rate corresponding to its low X-ray luminosity.

astro-ph↗

arXiv1999

Different types of X-ray bursts from GRS 1915+105 and their origin

We report the X-ray observations of the Galactic X-ray transient source GRS 1915+105 with the PPCs of the Indian X-ray Astronomy Experiment(IXAE) onboard the Indian satellite IRS-P3 during 1997 June - August, which have revealed the presence of four types of intense X-ray bursts. All the observed bursts have a slow exponential rise, a sharp linear decay, and they can broadly be put in two classes: irregular and quasi-regular bursts in one class, and regular bursts in another class. The regular bursts are found to have two distinct time scales and they persist over extended durations. There is a strong correlation between the preceding quiescent time and the burst duration for the quasi-regular and irregular bursts. No such correlation is found for the regular bursts. The ratio of average flux during the burst time to the average flux during the quiescent phase is high and variable for the quasi- regular and irregular bursts while it is low and constant for the regular bursts. We suggest that the peculiar bursts that we have seen are charact- eristic of the change of state of the source. The source can switch back and forth between the low-hard state and the high-soft state near critical accretion rates in a very short time scale. A test of the model is presented using the publicly available 13-60 keV RXTE/PCA data for irregular and regular bursts concurrent with our observations.

astro-ph↗

arXiv1998

Quasi-periodic oscillations discovered in the new X-ray pulsar XTE J1858+034

We report the discovery of low frequency quasi-periodic oscillations centered at 0.11 Hz in the newly discovered 221 s X-ray pulsar XTE J1858+034. Among about 30 known transient X-ray pulsars this is the sixth source in which QPOs have been observed. If the QPOs are produced because of inhomogeneities in the accretion disk at the magnetospheric boundary, the low frequency of the QPOs indicate a large magnetosphere for this pulsar. Both the Keplerian frequency model and the beat frequency model are applicable for production of QPOs in this source. The QPOs and regular pulsations are found to be stronger at higher energy which favours the beat frequency model. The magnetic field of the pulsar is calculated as a function of its distance. The energy spectrum is found to be very hard, consisting of two components, a cut-off power law and an iron fluorescence line.

astro-ph↗

arXiv1997

Observations of Cygnus X-1 during the two spectral states with the Indian X-ray Astronomy Experiment (IXAE)

We present the time variability characteristics of Cygnus X-1 in its two spectral states. The observations were carried out using the Pointed Proportional Counters (PPC) on-board the Indian X-ray Astronomy Experiment (IXAE). The details of the instrument characteristics, the observation strategy, and the background modeling methods are described. In the soft state of Cyg X-1, we confirm the general trend of the Power Density Spectrum (PDS) obtained using the Proportional Counter Array (PCA) on-board the RXTE satellite. The hard state of the source just prior to the spectral transition was not observed by the PCA and we present the PDS obtained in this state. We find that the low frequency end of the PDS is flatter than that observed during the spectral transition. Additionally, we find that there is one more component in the low frequency end of the PDS, which is independent of the spectral state of the source. The time variability is also examined by taking the statistics of the occurrence of shots and it is found that the shot duration and shot energy follow an exponential distribution, with time constants significantly different in the two spectral states. In the soft state of the source a shot with a very large strength has been identified and it has exponential rise and decay phases with time constants of 0.4 s. We examine these results in the light of the current models for accretion onto black holes.

astro-ph↗

arXiv1997

X-ray variability of GRS 1915+105 during the low-hard state observed with the Indian X-ray astronomy experiment (IXAE)

The galactic superluminal transient X-ray source GRS 1915+105 was observed with the pointed proportional counters (PPCs) onboard the Indian satellite IRS-P3 during 1996 July 23-27. We report here details of the behavior of this source during the relatively quiet and low luminosity state. Large intensity variations by a factor of 2 to 3, generally seen in black-hole candidates, are observed at a time scale of 100 ms to few seconds. No significant variation is detected over larger time scale of minute or more. The intensity variations are described as sum of shots in the light curve, and the number distribution of the shots are found to be exponential function of the fluence and duration of the shots. The cross correlation spectrum between 6-18 keV and 2-6 keV X-rays is found to have asymmetry signifying a delay of the hard X-rays by about 0.2 to 0.4 sec. This supports the idea of hard X-rays being generated by Compton up-scattering from high energy clouds near the source of soft X-rays. Very strong and narrow quasi periodic oscillations in the frequency range 0.62 to 0.82 Hz are observed. We discuss about a model which explains a gradual change in the QPO frequencies with corresponding changes in the mass accretion rate of the disk.

astro-ph↗

arXiv1997

Quasi-regular X-ray bursts from GRS 1915+105 observed with the IXAE: possible evidence for matter disappearing into the event horizon of the black hole

Three different types of very intense, quasi-regular X-ray bursts have been observed from the Galactic superluminal X-ray transient source GRS 1915+105 with the Pointed Proportional Counters of the Indian X-ray Astronomy Experiment onboard the Indian satellite IRS-P3. The observations were carried out from 1997 June 12 to June 29 in the energy range of 2-18 keV and revealed the presence of persistent quasi-regular bursts with different structures. Only one of the three types of bursts is regular in occurrence revealing a stable profile over extended durations. The regular bursts have an exponential rise with a time scale of about 7 s to 10 s and a sharp linear decay in 2 to 3 s. The X-ray spectrum becomes progressively harder as the burst evolves and it is the hardest near the end of the burst decay. The profile and energetics of the bursts in this black hole candidate source are distinct from both the type I and type II X-ray bursts observed in neutron star sources. We propose that the sharp decay in the observed burst pattern is a signature of the disappearance of matter through the black hole horizon. The regular pattern of the bursts can be produced by material influx into the inner disk due to oscillations in a shock front far away from the compact object.

astro-ph↗

arXiv1997

Torque-luminosity correlation and possible evidence for core-crust relaxation in the X-ray pulsar GX 1+4

We present the detection of a positive correlation between spin-down rate $\dot{P}$ and pulsed X-ray luminosity in the BATSE archival data of the bright hard X-ray pulsar GX 1+4. We have also seen a delay of 5.6 $\pm$ 1.2 days between the luminosity change and the corresponding change in the spin-down rate. The observed correlation between $\dot{P}$ and L_X is used to reproduce the period history of GX 1+4 based on the observed luminosity alone, and it is found that the spin period can be predicted correct to 0.026% when the luminosity is adequately sampled. The idea that at a higher luminosity more matter is accreted and the accretion disk extends closer to the neutron star thereby transferring more angular momentum to the system, seems not to be the case with GX 1+4. The observed lag between the spin-down rate and the luminosity is reported here for the first time in any such binary X-ray pulsar, and is found to be consistent with the time scale for the core-crust relaxation in a neutron star.

astro-ph↗

arXiv1997

Rapid X-ray variability of the superluminal source GRS1915+105

The superluminal X-ray transient source GRS 1915+105 was observed during July 20-29, 1996 with the Indian X-ray Astronomy Experiment (IXAE) on the Indian satellite IRS-P3 launched on March 21, 1996 from Shriharikota Range in India. During our observations covering the energy band 2-18 keV, we have seen strong erratic intensity variations on time scale of 0.1s - 10s. Quasi Periodic Oscillations (QPOs) in a frequency range of 0.62 to 0.82 Hz were detected with a rms fraction of about 9%. The rapid X-ray intensity variations in GRS1915+105 are similar to those observed in some other black hole binaries and thus provide further support for the hypothesis that this source is likely to be a black hole. We discuss the possible emission region and mechanism of the observed quasi-periodic oscillations. Comparing the observed QPOs with the ones observed in other neutron star and black-hole systems, we argue that GRS1915+105 is possibly a black-hole.

astro-ph↗

arXiv1997

A double peaked pulse profile observed in GX 1+4

The hard X-ray pulsar GX 1+4 was observed several times in the last few years with a pair of balloon-borne Xenon filled Multi-cell Proportional Counters (XMPC). In a balloon flight made on 22 March 1995, the source was detected in a bright state, the average observed source count rate being $8.0\pm0.2/s$ per detector. X-ray pulsations with a period of $121.9\pm0.1$ s were detected in the source with a broad double peak pulse feature. When observed in December 1993 with the same instrument, the pulse profile of GX 1+4 showed a single peak. This change in the pulse profile to a double pulse structure in about 15 months indicates either activation of the opposite pole of the neutron star if the magnetic field is asymmetric or possibly a change in the beam pattern, from a pencil beam to a fan beam. Assuming a fan beam configuration, the pulse profile is used to find the inclinations of the magnetic axis and the viewing axis with the spin axis. The derived angles support the GINGA observations of a dip in the pulse profile which was resolved to have a local maximum in one of the observations and was explained with resonance scattering of cyclotron line energy photons by the accretion column (Makishima et al., \markcite{maki1988}, Dotani et al., \markcite{dotani1989}.). Compared to our previous observation of the same source with the same telescope (Rao et al., \markcite{rao1994}) a period change rate of $0.72 \pm 0.40 s/yr$ is obtained which is the lowest rate of change of period for this source since its discovery. Average pulse fraction in the hard X-ray range is low (30%), consistent with its anti correlation with luminosity as reported by us earlier (Rao et al., \markcite{rao1994}) and the observed spectrum is very hard (power law photon index $1.67\pm0.12$).

astro-ph↗