Sudip Bhattacharyya

Tata Institute of Fundamental Research, Mumbai, Mahārāshtra, India

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Publications (44)182.59 Total impact

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    Manoneeta Chakraborty, Sudip Bhattacharyya
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    ABSTRACT: 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.
    The Astrophysical Journal 07/2014; 792(1). · 6.73 Impact Factor
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    ABSTRACT: Simultaneous, precise measurements of the mass $M$ and radius $R$ of neutron stars can yield uniquely valuable information about the still uncertain properties of cold matter at several times the density of nuclear matter. One method that could be used to measure $M$ and $R$ is to analyze the energy-dependent waveforms of the X-ray flux oscillations seen during some thermonuclear bursts from some neutron stars. These oscillations are thought to be produced by X-ray emission from hotter regions on the surface of the star that are rotating at or near the spin frequency of the star. Here we explore how well $M$ and $R$ could be determined by generating, and analyzing using Bayesian techniques, synthetic energy-resolved X-ray data that we produce assuming a future space mission having 2--30 keV energy coverage and an effective area of 10 m$^2$, such as the proposed \textit{LOFT} or \textit{AXTAR} missions. We find that if the hot spot is within 10$^\circ$ of the rotation equator, both $M$ and $R$ can usually be determined with an uncertainty of about 10% if there are $10^6$ total counts from the spot, whereas waveforms from spots within 20$^\circ$ of the rotation pole provide no useful constraints. These constraints can usually be achieved even if the burst oscillations vary with time and data from multiple bursts must be used to obtain 10$^6$ counts from the hot spot. This is therefore a promising method to constrain $M$ and $R$ tightly enough to discriminate strongly between competing models of cold, high-density matter.
    The Astrophysical Journal 04/2013; · 6.73 Impact Factor
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    Arunava Mukherjee, Sudip Bhattacharyya
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    ABSTRACT: A kilohertz quasi-periodic oscillation (kHz QPO) is an observationally robust high-frequency timing feature detected from neutron star low-mass X-ray binaries (LMXBs). This feature can be very useful to probe the superdense core matter of neutron stars, and the strong gravity regime. However, although many models exist in the literature, the physical origin of kHz QPO is not known, and hence this feature cannot be used as a tool yet. The energy dependence of kHz QPO fractional rms amplitude is an important piece of the jigsaw puzzle to understand the physical origin of this timing feature. It is known that the fractional rms amplitude increases with energy at lower energies. At higher energies, the amplitude is usually believed to saturate, although this is not established. We combine tens of lower kHz QPOs from a neutron star LMXB 4U 1728-34 in order to improve the signal-to-noise-ratio. Consequently, we, for the first time to the best of our knowledge, find a significant and systematic decrease of the fractional rms amplitude with energy at higher photon energies. Assuming an energy spectrum model, blackbody+powerlaw, we explore if the sinusoidal variation of a single spectral parameter can reproduce the above mentioned fractional rms amplitude behavior. Our analysis suggests that the oscillation of any single blackbody parameter is favored over the oscillation of any single powerlaw parameter, in order to explain the measured amplitude behavior. We also find that the quality factor of a lower kHz QPO does not plausibly depend on photon energy.
    The Astrophysical Journal 09/2012; 756(1). · 6.73 Impact Factor
  • Arunava Mukherjee, Sudip Bhattacharyya
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    ABSTRACT: Kilohertz quasi-periodic oscillations (kHz QPOs) are the fastest variability features from the low mass X-ray binaries (LMXBs) originally discovered in 1996 with Rossi X-ray Timing Explorer (RXTE). Most of the Z sources as well as several atoll sources have shown this timing feature. The physical origin of the kHz QPOs is still unknown, although many theoretical models are available in the literature. The energy dependence of rms amplitude from the kHz QPOs could be a potentially important tool to understand the spectral origin of this timing feature. We have studied this energy dependence in details for a large number kHz QPOs from a neutron star LMXB. We have also done simulations to test basic models. Here we will present the observed results and the simulations as well as the basic implications of the results. We will briefly mention the role of ASTROSAT in this regard.
    07/2012;
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    Manoneeta Chakraborty, Sudip Bhattacharyya
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    ABSTRACT: 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 (8.8\sigma). 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.
    Monthly Notices of the Royal Astronomical Society 02/2012; 422(3). · 5.52 Impact Factor
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    Sudip Bhattacharyya
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    ABSTRACT: Relativistic spectral lines from the accretion disc of a neutron star low-mass X-ray binary can be modelled to infer the disc inner edge radius. A small value of this radius tentatively implies that the disc terminates either at the neutron star hard surface, or at the innermost stable circular orbit (ISCO). Therefore an inferred disc inner edge radius either provides the stellar radius, or can directly constrain stellar equation of state (EoS) models using the theoretically computed ISCO radius for the spacetime of a rapidly spinning neutron star. However, this procedure requires numerical computation of stellar and ISCO radii for various EoS models and neutron star configurations using an appropriate rapidly spinning stellar spacetime. We have fully general relativistically calculated about 16000 stable neutron star structures to explore and establish the above mentioned procedure, and to show that the Kerr spacetime is inadequate for this purpose. Our work systematically studies the methods to constrain EoS models using relativistic disc lines, and will motivate future X-ray astronomy instruments.
    Monthly Notices of the Royal Astronomical Society 09/2011; 415. · 5.52 Impact Factor
  • Manoneeta Chakraborty, Sudip Bhattacharyya
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    ABSTRACT: A thermonuclear burst has been reported from the RXTE data of the transient neutron star low-mass X-ray binary IGR J17498-2921 (Atel #3568), which is also a 401 Hz pulsar and went into an outburst in August 2011 (Atels #3551, #3555, #3556, #3558, #3559, #3560, #3561, #3562, #3563, #3601, #3606, #3622, #3638). We report the detection of nine additional bursts from the RXTE data up to September 7, 2011.
    The Astronomer's Telegram. 09/2011;
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    Arunava Mukherjee, Sudip Bhattacharyya
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    ABSTRACT: We have analyzed the new Rossi X-ray Timing Explorer Proportional Counter Array data of the atoll neutron star (NS) low-mass X-ray binary (LMXB) system XB 1254-690. The colour-colour diagram shows that the source was in the high-intensity banana state. We have found two low-frequency candidate peaks with single trial significances of ~ 2.65 X 10^{-8} and ~ 7.39 X 10^{-8} in the power spectra. After taking into account the number of trials, the joint probability of appearance of these two peaks in the data set only by chance is ~ 4.5 X 10^{-4}, and hence a low-frequency QPO can be considered to be detected with a significance of ~ 4.5 X 10^{-4}, or, ~ 3.5\sigma for the first time from this source. We have also done the first systematic X-ray spectral study of XB 1254-690, and found that, while one-component models are inadequate, three-component models are not required by the data. We have concluded that a combined broken-powerlaw and Comptonization model best describes the source continuum spectrum among 19 two-component models. The plasma temperature (~ 3 keV) and the optical depth (~ 7) of the Comptonization component are consistent with the previously reported values for other sources. However, the use of a broken-powerlaw component to describe NS LMXB spectra has recently been started, and we have used this component for XB 1254-690 for the first time. We have attempted to determine the relative energy budgets of the accretion disc and the boundary layer using the best-fit spectral model, and concluded that a reliable estimation of these budgets requires correlations among time variations of spectral properties in different wavelengths.
    Monthly Notices of the Royal Astronomical Society 07/2011; 411. · 5.52 Impact Factor
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    Arunava Mukherjee, Sudip Bhattacharyya
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    ABSTRACT: We report the discovery ($20\sigma$) of kilohertz quasi-periodic oscillations (kHz QPOs) at ~ 690 Hz from the transient neutron star low-mass X-ray binary EXO 1745-248. We find that this is a lower kHz QPO, and systematically study the time variation of its properties using smaller data segments with and without the shift-and-add technique. The quality (Q) factor occasionally significantly varies within short ranges of frequency and time. A high Q-factor (264.5 +- 38.5) of the QPO is found for a 200 s time segment, which might be the largest value reported in the literature. We argue that an effective way to rule out kHz QPO models is to observationally find such high Q-factors, even for a short duration, as many models cannot explain a high coherence. However, as we demonstrate, the shift-and-add technique cannot find a very high Q-factor which appears for a short period of time. This shows that the coherences of kHz QPOs can be higher than the already high values reported using this technique, implying further constraints on models. We also discuss the energy dependence of fractional rms amplitude and Q-factor of the kHz QPO.
    The Astrophysical Journal Letters 04/2011; 730(2). · 6.35 Impact Factor
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    ABSTRACT: 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).
    Monthly Notices of the Royal Astronomical Society 02/2011; 418. · 5.52 Impact Factor
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    Arunava Mukherjee, Sudip Bhattacharyya
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    ABSTRACT: We study the low-frequency timing properties and the spectral state evolution of the transient neutron star low-mass X-ray binary EXO 1745-248 using the entire Rossi X-ray Timing Explorer Proportional Counter Array data. We tentatively conclude that EXO 1745-248 is an atoll source, and report the discovery of a ~ 0.45 Hz low-frequency quasi-periodic oscillation and ~ 10 Hz peaked noises. If it is an atoll, this source is unusual because (1) instead of a `C'-like curve, it traced a clear overall clockwise hysteresis curve in each of the colour-colour diagram and the hardness-intensity diagram; and (2) the source took at least 2.5 months to trace the softer banana state, as opposed to a few hours to a day, which is typical for an atoll source. The shape of the hysteresis track was intermediate between the characteristic `q'-like curves of several black hole systems and `C'-like curves of atolls, implying that EXO 1745-248 is an important source for the unification of the black hole and neutron star accretion processes.
    01/2011;
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    ABSTRACT: Accretion disk winds are revealed in Chandra gratings spectra of black holes. The winds are hot and highly ionized (typically composed of He-like and H-like charge states), and show modest blue-shifts. Similar line spectra are sometimes seen in "dipping" low-mass X-ray binaries, which are likely viewed edge-on; however, that absorption is tied to structures in the outer disk, and blue-shifts are not typically observed. Here we report the detection of blue-shifted He-like Fe XXV (3100 +/- 400 km/s) and H-like Fe XXVI (1000 +/- 200 km/s) absorption lines in a Chandra/HETG spectrum of the transient pulsar and low-mass X-ray binary IGR J17480-2446 in Terzan 5. These features indicate a disk wind with at least superficial similarities to those observed in stellar-mass black holes. The wind does not vary strongly with numerous weak X-ray bursts or flares. A broad Fe K emission line is detected in the spectrum, and fits with different line models suggest that the inner accretion disk in this system may be truncated. If the stellar magnetic field truncates the disk, a field strength of B = 0.7-4.0 E+9 Gauss is implied, which is in line with estimates based on X-ray timing techniques. We discuss our findings in the context of accretion flows onto neutron stars and stellar-mass black holes.
    The Astrophysical Journal Letters 01/2011; 731(1). · 6.35 Impact Factor
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    Manoneeta Chakraborty, Sudip Bhattacharyya
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    ABSTRACT: 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.
    The Astrophysical Journal Letters 12/2010; 730(2). · 6.35 Impact Factor
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    ABSTRACT: X-ray charge-coupled devices (CCDs) are the workhorse detectors of modern X-ray astronomy. Typically covering the 0.3-10.0 keV energy range, CCDs are able to detect photoelectric absorption edges and K shell lines from most abundant metals. New CCDs also offer resolutions of 30-50 (E/ΔE), which is sufficient to detect lines in hot plasmas and to resolve many lines shaped by dynamical processes in accretion flows. The spectral capabilities of X-ray CCDs have been particularly important in detecting relativistic emission lines from the inner disks around accreting neutron stars and black holes. One drawback of X-ray CCDs is that spectra can be distorted by photon "pile-up," wherein two or more photons may be registered as a single event during one frame time. We have conducted a large number of simulations using a statistical model of photon pile-up to assess its impacts on relativistic disk line and continuum spectra from stellar-mass black holes and neutron stars. The simulations cover the range of current X-ray CCD spectrometers and operational modes typically used to observe neutron stars and black holes in X-ray binaries. Our results suggest that severe photon pile-up acts to falsely narrow emission lines, leading to falsely large disk radii and falsely low spin values. In contrast, our simulations suggest that disk continua affected by severe pile-up are measured to have falsely low flux values, leading to falsely small radii and falsely high spin values. The results of these simulations and existing data appear to suggest that relativistic disk spectroscopy is generally robust against pile-up when this effect is modest.
    The Astrophysical Journal 11/2010; 724(2):1441. · 6.73 Impact Factor
  • Manoneeta Chakraborty, Sudip Bhattacharyya
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    ABSTRACT: We report Rossi X-ray Timing Explorer data analysis of the Terzan 5 source IGR J17480-2446, which is currently in outburst (ATels #2919, #2920, #2922, #2924, #2929, #2932, #2933, #2935, #2937, #2939, #2946, #2952, #2958, #2974, #3000). Galloway and in 't Zand (ATel #3000) reported that after the 2010 October 13 thermonuclear burst, no other burst up to 2010 October 26 showed significant (>2 sigma confidence) cooling during burst decay.
    The Astronomer's Telegram. 11/2010;
  • Arunava Mukherjee, Sudip Bhattacharyya
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    ABSTRACT: The current outburst (ATEL #2919) of a transient X-ray source in Terzan 5, which is plausibly EXO 1745-248 (ATEL #2920; but see ATEL #2933), provides motivation for archival data analysis of EXO 1745-248. In the archival RXTE PCA data of 2000-09-30 (T15:29:24 to T16:24:09; ObsId 50054-06-11-00), an extremely strong kHz QPO at ~690+-23 Hz with a fractional rms amplitude of ~0.08 has been detected. To the best of our knowledge, this is the first high frequency feature detected from EXO 1745-248.
    The Astronomer's Telegram. 10/2010;
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    ABSTRACT: A number of neutron star low-mass X-ray binaries (LMXBs) have recently been discovered to show broad, asymmetric Fe K emission lines in their X-ray spectra. These lines are generally thought to be the most prominent part of a reflection spectrum, originating in the inner part of the accretion disk where strong relativistic effects can broaden emission lines. We present a comprehensive, systematic analysis of Suzaku and XMM-Newton spectra of 10 neutron star LMXBs, all of which display broad Fe K emission lines. Of the 10 sources, 4 are Z sources, 4 are atolls, and 2 are accreting millisecond X-ray pulsars (also atolls). The Fe K lines are fit well by a relativistic line model for a Schwarzschild metric, and imply a narrow range of inner disk radii (6-15 GM/c 2) in most cases. This implies that the accretion disk extends close to the neutron star surface over a range of luminosities. Continuum modeling shows that for the majority of observations, a blackbody component (plausibly associated with the boundary layer) dominates the X-ray emission from 8 to 20 keV. Thus it appears likely that this spectral component produces the majority of the ionizing flux that illuminates the accretion disk. Therefore, we also fit the spectra with a blurred reflection model, wherein a blackbody component illuminates the disk. This model fits well in most cases, supporting the idea that the boundary layer illuminates a geometrically thin disk.
    The Astrophysical Journal 08/2010; 720(1):205. · 6.73 Impact Factor
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    Sudip Bhattacharyya
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    ABSTRACT: A neutron star low-mass X-ray binary is a binary stellar system with a neutron star and a low-mass companion star rotating around each other. In this system the neutron star accretes mass from the companion, and as this matter falls into the deep potential well of the neutron star, the gravitational potential energy is released primarily in the X-ray wavelengths. Such a source was first discovered in X-rays in 1962, and this discovery formally gave birth to the "X-ray astronomy". In the subsequent decades, our knowledge of these sources has increased enormously by the observations with several X-ray space missions. Here we give a brief overview of our current understanding of the X-ray observational aspects of these systems. Comment: 21 pages, 13 figures, An Invited Review, published in Current Science
    02/2010;
  • 02/2010;
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    Sudip Bhattacharyya
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    ABSTRACT: Measurement of at least three independent parameters, for example, mass, radius and spin frequency, of a neutron star is probably the only way to understand the nature of its supranuclear core matter. Such a measurement is extremely difficult because of various systematic uncertainties. The lack of knowledge of several system parameter values gives rise to such systematics. Low mass X-ray binaries, which contain neutron stars, provide a number of methods to constrain the stellar parameters. Joint application of these methods has a great potential to significantly reduce the systematic uncertainties, and hence to measure three independent neutron star parameters accurately. Here, we review the methods based on: (1) thermonuclear X-ray bursts; (2) accretion-powered millisecond-period pulsations; (3) kilohertz quasi-periodic oscillations; (4) broad relativistic iron lines; (5) quiescent emissions; and (6) binary orbital motions.
    Advances in Space Research 01/2010; · 1.18 Impact Factor

Publication Stats

358 Citations
182.59 Total Impact Points

Institutions

  • 2008–2014
    • Tata Institute of Fundamental Research
      • Department of Astronomy and Astrophysics
      Mumbai, Mahārāshtra, India
  • 2006
    • University of Illinois, Urbana-Champaign
      Urbana, Illinois, United States
  • 2004–2006
    • University of Maryland, College Park
      • Department of Astronomy
      Maryland, United States