arXiv subjects

Manoneeta Chakraborty

Publications and source records attributed to Manoneeta Chakraborty.

34 records · Page 2Linked to original sources

arXiv2021

Broadband time-resolved spectroscopy of thermonuclear X-ray bursts from 4U 1636-536 using AstroSat

Broadband spectral studies of Type-I X-ray bursts can put strong constraints on the physics of burst spectra as well as their interaction with the environment. We present the results obtained from the broadband time-resolved spectroscopy of 15 thermonuclear bursts detected simultaneously from the neutron star atoll source 4U 1636-536 using LAXPC and SXT onboard AstroSat. During the observations with AstroSat, the Low mass X-ray binary (LMXB) 4U 1636-536 is observed to show a modest spectral evolution within the island state. The broadband burst spectra are observed to show an excess in addition to the thermal emission from the neutron star surface near the peak of the bursts. We investigate the interpretation of the excess observed near the peak of the burst as re-emission/reprocessing of the photons by the accretion disk/corona or scattering of the photons in the neutron star atmosphere or the enhanced persistent emission due to Poynting-Robertson drag. This is the first reported broadband simultaneous study of Type-I bursts using LAXPC and SXT onboard AstroSat. This kind of study may provide a better understanding of the burst-accretion interaction and how the bursts influence the overall accretion process contributed by the accretion disk as well as the corona.

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arXiv2021

Magnetar Giant Flare Originated GRB 200415A: Transient GeV emission, Time-Resolved $\rm E_p~ -~L_{iso}$ Correlation, and Implications

Giant flares (GFs) are unusual bursts from soft gamma-ray repeaters (SGRs) that release an enormous amount of energy in a fraction of a second. The afterglow emission of these SGR-GFs or GF candidates is a highly beneficial means of discerning their composition, relativistic speed, and emission mechanisms. GRB 200415A is a recent GF candidate observed in a direction coincident with the nearby Sculptor galaxy at 3.5 Mpc. In this work, we searched for transient gamma-ray emission in past observations by Fermi-LAT in the direction of GRB 200415A. These observations confirm that GRB 200415A is observed as a transient GeV source only once. A pure pair-plasma fireball cannot provide the required energy for the interpretation of GeV afterglow emission and a baryonic poor outflow is additionally needed to explain the afterglow emission. A baryonic rich outflow is also viable, as it can explain the variability and observed quasi-thermal spectrum of the prompt emission if dissipation is happening below the photosphere via internal shocks. Using the peak energy ($E_p$) of the time-resolved prompt emission spectra and their fluxes ($F_p$), we found correlation between $E_p$ and $F_p$ or $E_p$ and isotropic luminosity $L_{\rm iso}$ for GRB 200415A. This supports the intrinsic nature of $E_p$-$E_{\rm iso}$ correlation found in SGRs-GFs, hence favoring a baryonic poor outflow. Our results also indicate a different mechanism at work during the initial spike, and that the evolution of the prompt emission spectral properties in this outflow would be intrinsically due to the injection process.

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arXiv2021

Search for Intermittent X-ray Pulsations from Neutron Stars in Low Mass X-ray Binaries

We present the results of our extensive binary orbital motion corrected pulsation search for 13 low mass X-ray binaries (LMXBs). These selected sources exhibit burst oscillations in X-rays with frequencies ranging from 45 to 1122 Hz, and have a binary orbital period varying from 2.1 to 18.9 hours. We first determined episodes that contain weak pulsations around the burst oscillation frequency by searching all archival Rossi X-ray Timing Explorer (RXTE) data of these sources. Then, we applied Doppler corrections to these pulsation episodes to discard the smearing effect of the binary orbital motion and searched for recovered pulsations at the second stage. Here we report 75 pulsation episodes that contain weak but coherent pulsations around the burst oscillation frequency. Furthermore, we report eight new episodes that show relatively strong pulsations in the binary orbital motion corrected data.

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arXiv2021

Thermonuclear X-ray Bursts with late secondary peaks observed from 4U 1608-52

We report the temporal and spectral analysis of three thermonuclear X-ray bursts from 4U 1608-52, observed by the Neutron Star Interior Composition Explorer (NICER) during and just after the outburst observed from the source in 2020. In two of the X-ray bursts, we detect secondary peaks, 30 and 18 seconds after the initial peaks. The secondary peaks show a fast rise exponential decay-like shape resembling a thermonuclear X-ray burst. Time-resolved X-ray spectral analysis reveals that the peak flux, blackbody temperature, and apparent emitting radius values of the initial peaks are in agreement with X-ray bursts previously observed from 4U 1608-52, while the same values for the secondary peaks tend toward the lower end of the distribution of bursts observed from this source. The third X-ray burst, which happened during much lower accretion rates did not show any evidence for a deviation from an exponential decay and was significantly brighter than the previous bursts. We present the properties of the secondary peaks and discuss the events within the framework of short recurrence time bursts or bursts with secondary peaks. We find that the current observations do not fit in standard scenarios and challenge our understanding of flame spreading.

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arXiv2020

GRB 200415A: A Short Gamma-Ray Burst from a Magnetar Giant Flare?

The giant flares of soft gamma-ray repeaters (SGRs) have long been proposed to contribute to at least a subsample of the observed short gamma-ray bursts (GRBs). In this paper, we perform a comprehensive analysis of the high-energy data of the recent bright short GRB 200415A, which was located close to the Sculptor galaxy. Our results suggest that a magnetar giant flare provides the most natural explanation for most observational properties of GRB 200415A, including its location, temporal and spectral features, energy, statistical correlations, and high-energy emissions. On the other hand, the compact star merger GRB model is found to have difficulty reproducing such an event in a nearby distance. Future detections and follow-up observations of similar events are essential to firmly establish the connection between SGR giant flares and a subsample of nearby short GRBs.

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arXiv2018

Variable Absorption Line in XTE J1810-197

We report the results of a long-term spectral and timing study of the first transient magnetar, XTE J1810-197 which was discovered in 2003 when its X-ray luminosity increased 100 fold. We fit X-ray spectra of all archival X-ray observations using a two-component blackbody model, where the cool component is most likely originating from the whole surface of the neutron star and the hot component is from a much smaller hot spot. We investigate the long-term evolution of the surface emission characteristics via tracing its surface temperature, apparent emitting area and the pulsed fraction. We evaluate the pulsed fraction in two energy intervals (<1.5 keV and >1.5 keV) and show that the XTE J1810-197 exhibits slightly higher pulsed emission at energies above 1.5 keV. We explore the characteristics of an absorption line detected around 1.1 keV. We find that the absorption feature is highly variable and its profile is asymmetric. To accurately represent this feature, we introduced an asymmetric Gaussian profile and quantified the level of asymmetry of the absorption feature.

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arXiv2017

Time and energy dependent characteristics of Thermonuclear burst oscillations

We have investigated temporal and spectral properties of a large sample of thermonuclear bursts with oscillations from eight different sources with spin frequencies varying from 270 to 620 Hz. For our sample we chose those bursts, for which the oscillation is sufficiently strong and of relatively long duration. The emission from the hot-spot that is formed during a thermonuclear burst is modulated by several physical processes and the burst oscillation profiles unavoidably carry signatures of these. In order to probe these mechanisms, we examined the amplitude and phase lags of the burst oscillations with energy. We also studied the frequency variation of oscillations during these thermonuclear bursts. We observed that the frequency drifts are more frequent in the cases where the spin frequency is lower. We found that the phase lag of the burst oscillations shows no systematic evolution with energy between the bursts, and also in between different sources. In 7 cases, we do indeed observe lag of soft energy photons, while there are a significant number of cases for which hard lag or no lag is observed.

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arXiv2016

Burst and Outburst Characteristics of Magnetar 4U 0142+61

We have compiled the most comprehensive burst sample from magnetar 4U 0142+61, comprising 27 bursts from its three burst-active episodes in 2011, 2012 and the latest one in 2015 observed with Swift/BAT and Fermi/GBM. Bursts from 4U 0142+61 morphologically resemble typical short bursts from other magnetars. However, 4U 0142+61 bursts are less energetic compared to the bulk of magnetar bursts. We uncovered an extended tail emission following a burst on 2015 February 28, with a thermal nature, cooling over a time-scale of several minutes. During this tail emission, we also uncovered pulse peak phase aligned X-ray bursts , which could originate from the same underlying mechanism as that of the extended burst tail, or an associated and spatially coincident but different mechanism.

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arXiv2016

Magnetar-like X-ray Bursts from a Rotation Powered Pulsar, PSR J1119-6127

Two energetic hard X-ray bursts have recently triggered the Fermi and Swift space observatories from the rotation powered pulsar, PSR J1119-6127. We have performed in depth spectral and temporal analyses of these two events. Our extensive searches in both observatory data for lower luminosity bursts uncovered 10 additional events from the source. We report here on the timing and energetics of the 12 bursts from PSR J1119-6127 during its burst active phase of 2016 July 26 and 28. We also found a spectral softer X-ray flux enhancement in a post burst episode, which shows evidence of cooling. We discuss here the implications of these results on the nature of this unusual high-field radio pulsar, which firmly place it within the typical magnetar population.

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arXiv2016

Variation of Spectral and Timing Properties in the Extended Burst Tails from the Magnetar 4U 0142+61

Extended emission episodes with intensity above the pre-burst level are observed following magnetar bursts from a number of soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs). Such extended tail emission were observed subsequent to two events detected from AXP 4U 0142+61. We investigated in detail the evolution of spectral and temporal properties during these two tail segments using RXTE/PCA observations, and report distinct variations both in the spectral and temporal behavior throughout the tails. In particular, sudden enhancement of pulsation amplitude in conjunction with bursts, and smooth decline of X-ray emission (cooling) during the tail were observed in both cases. We suggest that an inefficiently radiating trapped fireball formed during the burst, which can heat up the stellar surface, is able to explain the tail properties and its energetics. We also present the episodic detection of absorption and emission features during tails. One possible mechanism that has been proposed to give rise to such spectral lines is the proton/ion cyclotron resonance process which has been suggested to offer a valuable tool in probing the complex magnetic field of magnetars.

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arXiv2015

An Extraordinary Outburst of the Magnetar Swift J1822.3-1606

The 2011 outburst of Swift J1822.3--1606 was extraordinary; periodic modulations at the spin period of the underlying neutron star were clearly visible, remarkably similar to what is observed during the decaying tail of magnetar giant flares. We investigated the temporal characteristics of X-ray emission during the early phases of the outburst. We performed a periodicity search with the spectral hardness ratio (HR), and found a coherent signal near the spin period of the neutron star, but with a lag of about 3 radians. Therefore, the HR is strongly anti-correlated with the X-ray intensity, which is also seen in the giant flares. We studied time evolution of the pulse profile and found that it evolves from a complex morphology to a much simpler shape within about a month. Pulse profile simplification also takes place during the giant flares, but on a much shorter timescale of about few minutes. We found that the amount of energy emitted during the first 25 days of the outburst is comparable to what was detected in minutes during the decaying tail of giant flares. Based on these similarities, we suggest that the triggering mechanisms of the giant flares and the magnetar outbursts are likely the same. We propose that the trapped fireball that develops in the magnetosphere at the onset of the outburst radiates away efficiently in minutes in magnetars exhibiting giant flares, while in other magnetars, such as Swift J1822.3--1606, the efficiency of radiation of the fireball is not as high and, therefore, lasts much longer.

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arXiv2015

Burst Tails from SGR J1550-5418 Observed with Rossi X-ray Timing Explorer

We present the results of our extensive search using the Bayesian block method for long tails following short bursts from a magnetar, SGR J1550-5418, over all RXTE observations of the source. We identified four bursts with extended tails, most of which occurred during its 2009 burst active episode. The durations of tails range between ~13 s and over 3 ks, which are much longer than the typical duration of bursts. We performed detailed spectral and temporal analysis of the burst tails. We find that the spectra of three tails show a thermal nature with a trend of cooling throughout the tail. We compare the results of our investigations with the properties of four other extended tails detected from SGR 1900+14 and SGR 1806-20 and suggest a scenario for the origin of the tail in the framework of the magnetar model.

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arXiv2014

Evidence of thermonuclear flame spreading on neutron stars from burst rise oscillations

Burst oscillations during the rising phases of thermonuclear X-ray bursts are usually believed to originate from flame spreading on the neutron star surface. However, the decrease of fractional oscillation amplitude with rise time, which provides a main observational support for the flame spreading model, have so far been reported from only a few bursts. Moreover, the non-detection and intermittent detections of rise oscillations from many bursts are not yet understood considering the flame spreading scenario. Here, we report the decreasing trend of fractional oscillation amplitude from an extensive analysis of a large sample of Rossi X-ray Timing Explorer Proportional Counter Array bursts from ten neutron star low-mass X-ray binaries. This trend is 99.99% significant for the best case, which provides, to the best of our knowledge, by far the strongest evidence of such trend. Moreover, it is important to note that an opposite trend is not found from any of the bursts. The concave shape of the fractional amplitude profiles for all the bursts suggests latitude-dependent flame speeds, possibly due to the effects of the Coriolis force. We also systematically study the roles of low fractional amplitude and low count rate for non-detection and intermittent detections of rise oscillations, and attempt to understand them within the flame spreading scenario. Our results support a weak turbulent viscosity for flame spreading, and imply that burst rise oscillations originate from an expanding hot spot, thus making these oscillations a more reliable tool to constrain the neutron star equations of state.

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arXiv2012

Thermonuclear X-ray bursts from the 401 Hertz accreting pulsar IGR J17498-2921: indication of burning in confined regions

We use the 2011 Rossi X-ray Timing Explorer (RXTE) proportional counter array (PCA) data of the 401 Hz accreting pulsar and burster IGR J17498-2921 to perform timing analysis and time-resolved spectroscopy of 12 thermonuclear X-ray bursts. We confirm previously reported burst oscillations from this source with a much higher significance (8.8σ). We notice that the bursts can be divided into three groups: big photospheric radius expansion (PRE) bursts are about ten times more luminous than medium bursts, while the latter are about ten times more luminous than small bursts. The PCA field-of-view of these observations contains several known bursters, and hence some of the observed bursts might not be from IGR J17498-2921. The oscillations during big bursts at the known pulsar frequency show that these bursts were definitely from IGR J17498-2921. We find that at least several of the other bursts were also likely originated from IGR J17498-2921. Spectral analysis reveals that the luminosity differences among various bursts are primarily due to differences in normalizations, and not temperatures, even when we consider the effects of colour factor. This shows burning on a fraction of the stellar surface for those small and medium bursts, which originated from IGR J17498-2921. The low values of the upper limits of burst oscillation amplitude for these bursts suggest a small angle between the spin axis and the magnetic axis. We find indications of the PRE nature of a medium burst, which likely originated from IGR J17498-2921. If true, then, to the best of our knowledge, this is the first time that two PRE bursts with a peak count rate ratio of as high as {\approx} 12 have been detected from the same source.

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arXiv2011

Terzan 5 transient IGR J17480-2446: variation of burst and spectral properties with spectral states

We study the spectral state evolution of the Terzan 5 transient neutron star low-mass X-ray binary IGR J17480-2446, and how the best-fit spectral parameters and burst properties evolved with these states, using the Rossi X-ray Timing Explorer data. As reported by other authors, this is the second source which showed transitions between atoll state and `Z' state. We find large scale hysteresis in the almost `C'-like hardness-intensity track of the source in the atoll state. This discovery is likely to provide a missing piece of the jigsaw puzzle involving various types of hardness-intensity tracks from `q'-shaped for Aquila X-1, 4U 1608-52, and many black holes to `C'-shaped for many atoll sources. Furthermore, the regular pulsations, a diagonal transition between soft and hard states, and the large scale hysteresis observed from IGR J17480-2446 argue against some of the previous suggestions involving magnetic field about atolls and millisecond pulsars. Our results also suggest that the nature of spectral evolution throughout an outburst does not, at least entirely, depend on the peak luminosity of the outburst. Besides, the source took at least a month to trace the softer banana state, as opposed to a few hours to a day, which is typical for an atoll source. In addition, while the soft colour usually increases with intensity in the softer portion of an atoll source, IGR J17480-2446 showed an opposite behaviour. From the detailed spectral fitting we conclude that a blackbody+powerlaw model is the simplest one, which describes the source continuum spectra well throughout the outburst. We find that these two spectral components were plausibly connected with each other, and they worked together to cause the source state evolution. (Truncated).

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arXiv2011

X-ray bursts from the Terzan 5 transient IGR J17480-2446: nuclear rather than gravitational

The 2010 outburst of the transient neutron star low-mass X-ray binary IGR J17480-2446 has exhibited a series of unique X-ray bursts, as well as millihertz (mHz) quasi-periodic oscillations (QPOs) related to these bursts. It has been recently proposed that these are type-II bursts, powered by the gravitational energy. This implies that the current nuclear-burning based model of mHz QPOs is not correct, and this timing feature cannot be used as a tool to measure the neutron star parameters. We report the analysis of the Rossi X-ray Timing Explorer data of IGR J17480-2446 to show that the burst properties of this source are quite different from the properties of the type-II bursts observed from the rapid burster and GRO J1744-28. For example, the inferred ratio (~ 50-90) of the non-burst fluence to burst fluence is consistent with the thermonuclear origin of IGR J17480-2446 bursts, and is significantly different from this ratio (< 4) for type-II bursts. Our results suggest that the bursts and the mHz QPOs from IGR J17480-2446 are powered by the nuclear energy.

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