S. K. Patel

Universities Space Research Association, Houston, Texas, United States

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Publications (24)79.87 Total impact

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    ABSTRACT: We present detailed spectral and timing analysis of the hard X-ray transient IGR J16358-4726 using multisatellite archival observations. A study of the source flux time history over 6 yr suggests that lower luminosity transient outbursts can be occurring in intervals of at most 1 yr. Joint spectral fits of the higher luminosity outburst using simultaneous Chandra ACIS and INTEGRAL ISGRI data reveal a spectrum well described by an absorbed power-law model with a high-energy cutoff plus an Fe line. We detected the 1.6 hr pulsations initially reported using Chandra ACIS also in the INTEGRAL ISGRI light curve and in subsequent XMM-Newton observations. Using the INTEGRAL data, we identified a spin-up of 94 s ( = 1.6 × 10-4), which strongly points to a neutron star nature for IGR J16358-4726. Assuming that the spin-up is due to disk accretion, we estimate that the source magnetic field ranges between 1013 and 1015 G, depending on its distance, possibly supporting a magnetar nature for IGR J16358-4726.
    The Astrophysical Journal 12/2008; 657(2):994. · 6.73 Impact Factor
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    ABSTRACT: We present new observations of the early X-ray afterglows of the first 27 gamma-ray bursts (GRBs) well observed by the Swift X-Ray Telescope (XRT). The early X-ray afterglows show a canonical behavior, where the light curve broadly consists of three distinct power-law segments: (1) an initial very steep decay (t-α with 3 α1 5), followed by (2) a very shallow decay (0.5 α2 1.0), and finally (3) a somewhat steeper decay (1 α3 1.5). These power-law segments are separated by two corresponding break times, tbreak,1 500 s and 103 s tbreak,2 104 s. On top of this canonical behavior, many events have superimposed X-ray flares, which are most likely caused by internal shocks due to long-lasting sporadic activity of the central engine, up to several hours after the GRB. We find that the initial steep decay is consistent with it being the tail of the prompt emission, from photons that are radiated at large angles relative to our line of sight. The first break in the light curve (tbreak,1) takes place when the forward shock emission becomes dominant, with the intermediate shallow flux decay (α2) likely caused by the continuous energy injection into the external shock. When this energy injection stops, a second break is then observed in the light curve (tbreak,2). This energy injection increases the energy of the afterglow shock by at least a factor of f 4 and augments the already severe requirements for the efficiency of the prompt gamma-ray emission.
    The Astrophysical Journal 12/2008; 642(1):389. · 6.73 Impact Factor
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    ABSTRACT: We report X-ray studies of the environs of SN 1998bw and GRB 980425 using the Chandra X-Ray Observatory 1281 days after the gamma-ray burst (GRB). Eight X-ray point sources were localized, three and five each in the original error boxes, S1 and S2, assigned for variable X-ray counterparts to the GRB by BeppoSAX. The sum of the discrete X-ray sources plus continuous emission in S2 observed by Chandra on day 1281 is within a factor of 1.5 of the maximum and the upper limits seen by BeppoSAX. We conclude that S2 is the sum of several variable sources that have not disappeared and therefore is not associated with the GRB. Within S1, clear evidence is seen for a decline of approximately a factor of 12 between day 200 and day 1281. One of the sources in S1, S1a, is coincident with the well-determined radio location of SN 1998bw and is certainly the remnant of that explosion. The nature of the other sources is also discussed. Combining our observation of the supernova with others of the GRB afterglow, a smooth X-ray light curve, spanning ~1400 days, is obtained by assuming that the burst and supernova were coincident at 35.6 Mpc. When this X-ray light curve is compared with those of the X-ray "afterglows" of ordinary GRBs, X-ray flashes, and ordinary supernovae, evidence emerges for at least two classes of light curves, perhaps bounding a continuum. By 3-10 yr, all these phenomena seem to converge on a common X-ray luminosity, possibly indicative of the supernova underlying them all. This convergence strengthens the conclusion that SN 1998bw and GRB 980425 took place in the same object. One possible explanation for the two classes is that a (nearly) standard GRB was observed at different angles, in which case X-ray afterglows with intermediate luminosities should eventually be discovered. Finally, we comment on the contribution of GRB afterglows to the ultraluminous X-ray source population.
    The Astrophysical Journal 12/2008; 608(2):872. · 6.73 Impact Factor
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    ABSTRACT: The new transient IGR J16358-4726 was discovered on 2003 March 19 with INTEGRAL. We detected the source serendipitously during our 2003 March 24 observation of SGR 1627-41 with the Chandra X-Ray Observatory at the 1.7 × 10-10 ergs s-1 cm-2 flux level (2-10 keV) with a very high absorption column (NH = 3.3 × 1023 cm-2) and a hard power-law spectrum of index 0.5(1). We discovered a very strong flux modulation with a period of 5880(50) s and peak-to-peak pulse fraction of 70(6)% (2-10 keV), clearly visible in the X-ray data. The nature of IGR J16358-4726 remains unresolved. The only neutron star systems known with similar spin periods are low-luminosity persistent wind-fed pulsars; if this is a spin period, this transient is a new kind of object. If this is an orbital period, then the system could be a compact low-mass X-ray binary.
    The Astrophysical Journal 12/2008; 602(1):L45. · 6.73 Impact Factor
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    ABSTRACT: SGR 1627-41 was discovered in 1998 after a single active episode that lasted ~6 weeks. We report here our monitoring results of the decay trend of the persistent X-ray luminosity of the source during the last 5 years. We find an initial temporal power-law decay with index 0.47, reaching a plateau that is followed by a sharp (factor of 10) flux decline ~800 days after the source activation. The source spectrum is best described during the entire period by a single power law with high absorption [NH = 9.0(7) × 1022 cm-2]; the spectral index, however, varies dramatically between 2.2 and 3.8 spanning the entire range for all known soft gamma repeater sources. We discuss the cooling behavior of the neutron star assuming a deep crustal heating initiated by the burst activity of the source during 1998.
    The Astrophysical Journal 12/2008; 596(1):L79. · 6.73 Impact Factor
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    ABSTRACT: Magnetars are a special class of slowly rotating (period approximately 5-12 s) neutron stars with extremely strong magnetic fields (>10(14 )G)--at least an order of magnitude larger than those of the 'normal' radio pulsars. The potential evolutionary links and differences between these two types of object are still unknown; recent studies, however, have provided circumstantial evidence connecting magnetars with very massive progenitor stars. Here we report the discovery of an infrared elliptical ring or shell surrounding the magnetar SGR 1900+14. The appearance and energetics of the ring are difficult to interpret within the framework of the progenitor's stellar mass loss or the subsequent evolution of the supernova remnant. We suggest instead that a dust-free cavity was produced in the magnetar environment by the giant flare emitted by the source in August 1998. Considering the total energy released in the flare, the theoretical dust-destruction radius matches well with the observed dimensions of the ring. We conclude that SGR 1900+14 is unambiguously associated with a cluster of massive stars, thereby solidifying the link between magnetars and massive stars.
    Nature 06/2008; 453(7195):626-8. · 38.60 Impact Factor
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    ABSTRACT: The anomalous X-ray pulsar 4U 0142+61 was observed with Swift XRT again on 10 Feb 2007 (MJD 54141) in Window Timing mode with 1.766 ms time resolution. At the observed XRT total count rate (5.0 c/s) pile-up was negligible. We find a good fit (chi^2/dof=291.2/272) to the data using an absorbed blackbody plus power law model with N_H=(0.90+/-0.08 ) x 1e22 cm^-2, PL index= 3.8 +/- 0.2 and kT_BB = 0.38 +/- 0.02 (errors are at 90% CL).
    The Astronomer's Telegram. 02/2007;
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    ABSTRACT: The spectra of short soft gamma repeater (SGR) bursts at photonenergies above ˜ 15 keV are often well described by an optically thin thermal bremsstrahlung model (i.e., F(E) ∝ E-1 e(-E/kT) ) with kT = 20-40 keV. However, the spectral shape burst continuum at lower photon energies (down to ˜ 2 keV) is not well established. It is important to better understand the SGR burst spectral properties at lower energies since inadequate description of the burst spectral continuum could lead to incorrect conclusions, such as existence of spectral lines. Here, we present detailed spectral investigations (in 2-200 keV) of 163 bursts from SGR 1806-20, all detected with Rossi X-ray Timing Explorer during the 2004 active episode that included the giant flare on 27 December 2004. We find that the great majority of burst spectra are well represented by the combination of a blackbody plus a OTTB models.
    Progress of Theoretical Physics Supplement - PROG THEOR PHYS SUPPL. 01/2007; 169:12-15.
  • The Astrophysical Journal. 01/2006; 642:389-400.
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    ABSTRACT: We present new observations of the early X-ray afterglows of the first 27 gamma-ray bursts (GRBs) detected with the Swift X-ray Telescope (XRT). The early X-ray afterglows show a canonical behavior, where the light curve broadly consists of three distinct power law segments. These power law segments are separated by two corresponding break times. On top of this canonical behavior of the early X-ray light curve, many events have superimposed X-ray flares, which are most likely caused by internal shocks due to long lasting sporadx activity of the central engine, up to several hours after the GRB. We find that the initial steep decay is consistent with it being the tail of the prompt emission: from photons that are radiated at large angles relative to our line of sight. The first break in the light curve takes place when the forward shock emission becomes dominant, with the intermediate shallow flux decay likely caused by the continuous energy injection into the external shock. When this energy injection stops, a second break is then observed in the light curve. This energy injection increases the energy of the afterglow shock by at least a factor of f greater than or approx. equal to 4, and augments the already severe requirements for the efficiency of the prompt gamma-ray emission.
    02/2005;
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    ABSTRACT: We report here the results of an analysis of the first photons detected with the Chandra X-ray Observatory. The observation took place immediately following the opening of the last door protecting the X-ray telescope. We discuss the unusual operational conditions as the first extra-terrestrial X-ray photons reflected from the telescope onto the ACIS camera. One bright source was apparent to the team at the control center and the small collection of photons that appeared on the monitor were sufficient to indicate that the telescope had survived the launch and was approximately in focus, even prior to any checks and subsequent adjustments. We present an analysis of the sources in the field and an identification of the first Chandra source which had been named Leon X-1 to honor the momentous contributions of the Chandra Telescope Scientist, Leon Van Speybroeck.
    08/2004;
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    ABSTRACT: The new transient IGR J16358-4726 was discovered on 2003 March 19 with INTEGRAL. We detected the source serendipitously during our 2003 March 24 observation of SGR 1627 – 41 with the CHANDRA X-ray Observatory at the 1.7 × 10−10 ergs s−1 cm−2 flux level (2–10 keV) with a very high absorption column (NH=3.3×1023 cm−2) and a hard power law spectrum of photon index 0.5(1). We discovered a very strong flux modulation with a period of 5880(50) s and peak-to-peak pulse fraction of 70(6) % (2–10 keV), clearly visible in the x-ray data. The nature, however, of IGR 16357-4726 remains unresolved. Most likely, we are looking at a galactic binary neutron star system as evidenced by the detection of rest frame fluorescence line emission from neutral Fe K (6.4 keV) in the Chandra spectrum. If the detected modulation is a spin period, this transient is a new kind of object, belonging to a class of very hard transients recently revealed with the unique INTEGRAL capabilities. The only neutron star systems known with similar spin periods are low luminosity persistent wind-fed pulsars, which exhibit very soft X-ray spectra. If this is an orbital period, then the system could be a compact Low Mass X-ray Binary.
    Nuclear Physics B - Proceedings Supplements 06/2004; 132:542-547. · 0.88 Impact Factor
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    ABSTRACT: One of the most fundamental characteristics of any explosion is its energy, yet despite lively debates, the gamma-ray burst (GRB) community has still to reach a consensus on whether GRBs are standard bombs with a total energy in their relativistic ejecta approximately 10(exp 51) ergs, or a broad range of phenomena with energies varying by many orders of magnitude. While low energy GRBs may well exist, we show here that observations of GRB 031203, do not require a sub-energetic nature for that event. In fact, contrary to previous claims, the data are more consistent with a typical, powerful GRB seen at an angle of about twice the opening angle of the central jet.
    02/2004;
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    ABSTRACT: We present X-ray imaging, timing, and phase resolved spectroscopy of the anomalous X-ray pulsars (AXPs) 1E 2259+586 and 4U 0142+61 using the Chandra X-ray Observatory (CXO). The pulsar spectra are well described by a power law plus blackbody model; we find no evidence for spectral features between 0.5-7.0 keV. We derive a new, precise X-ray position for the sources and determine their spin periods. Phase resolved X-ray spectra of 1E 2259+586 show no significant variation as a function of pulse phase. However, phase resolved X-ray spectroscopy of 4U 0142+61 shows evidence for evolution in phase in either Gamma, or kTBB or some combination thereof as a function of pulse phase. We have detected emission beyond 4 arcsec from the central source and extending beyond 100 arcsec, likely due to dust scattering in the interstellar medium and, for 1E 2259+586, possibly due to the supernova remnant G109.1-1.0.
    06/2003; -1:207-214.
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    ABSTRACT: SGR 1627-41 was discovered in 1998 after a single active episode which lasted approx. 6 weeks. We report here our monitoring results of the decay trend of the persistent X-ray luminosity of the source during the last 5 years. We find an initial temporal power law decay with index 0.47, reaching a piateau which is followed by a sharp (factor of ten) flux decline approx. 800 days after the source activation. The source spectrum is best described during the entire period by a single power law with high absorption (N(sub H) = 9.0(7) x 10(exp 22)/sq cm); the spectral index, however, varies dramatically between 2.2-3.8 spanning the entire range for all known SGR sources. We discuss the cooling behavior of the neutron star assuming a deep crustal heating initiated by the burst activity of the source during 1998.
    02/2003;
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    ABSTRACT: The X-ray transient XTE J1908+094 was serendipitously discovered during RXTE ToO observations of SGR 1900+14 in February 2002. Following the discovery, RXTE routinely monitored the region. At the onset, the source was found in a spectrally low/hard state lasting for approximately 40 days, followed by a quick transition to the highhoft state. At the highest X-ray intensity level (seen on 2002 April 6), the source flux (2-10 keV) reached approximately 105 mCrab, then decayed rapidly. Overall outburst characteristics resemble the transient behavior of galactic black hole candidates. Here, we present the long term light curves, and detailed spectral and timing investigations of XTE J1098+094 using the RXTE/PCA data. We also report the results of Chandra ACIS observations which were performed during the decay phase.
    02/2003;
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    ABSTRACT: We report the precise localization of the Soft Gamma Repeater SGR 1627-41 with the Chandra X-ray Observatory. The best position for SGR 1627-41 was determined to be RA=16:35:51.844, DEC=-47:35:23.31 (J2000) with an accuracy of 0.6 arcsec. We present the results of our search for an IR counterpart to SGR 1627-41 and compare our results to the existing detections and limits of other magnetar infrared and optical observations in the literature. We also present new observations of SGR 1806-20 obtained during the recent reactivation of the source. In addition, we have determined a precise location for archival Chandra observations and reanalyzed archival IR data in the search for a counterpart.
    02/2003;
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    ABSTRACT: We have instituted a program whose goal is to better communicate astronomy among students selected from an economically underprivileged area of Huntsville, Alabama. To achieve this goal, six local teachers were trained as astronomy teacher resource agents. These teachers were teamed with six scientists; the teams presented materials for 29 teachers, mostly from the Southeastern United States Topics included the electromagnetic spectrum, X-ray optics, gratings and spectroscopy, supernovae (and remnants), black holes, and the tools and techniques to analyze Chandra X-ray data. Beginning in September our teacher-scientist teams, in collaboration with the Huntsville Housting Authority, will present programs to specially selected students at the rate of one or two per month, until April 2004. These young people will be mentored and encouraged to develop a research project. Students, which complete a project by April 2, will be paid for their work.
    02/2003;
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    ABSTRACT: Soft gamma-ray repeaters (SGRs) are a small class of high-energy transients characterized by the emission of repetitive, brief bursts of hard X-rays and soft gamma-rays. The SGR sky distribution (four are located very close to the Galactic plane, one is in the Large Magellanic Cloud) and the discovery of X-ray pulsations indicates that they are young neutron stars, an inference currently supported by the potential association of two SGRs with massive star clusters. The currently favored model attributes the various SGR properties to neutron stars with superstrong (>1014-15 G) magnetic fields, so-called magnetars. Recently, however, a number of models involving accretion from a circumstellar disk were also proposed to explain the persistent behavior of the SGRs. Unraveling the nature of these objects has been hampered by the lack of optical/IR counterparts to SGRs which could potentially distinguish between the two models. We recently obtained Chandra observation of the field of SGR 1627-41, which provide the first precise localization of the source. We also present near-IR observations of the field acquired with the CTIO 4m telescope and will report on the search for the IR counterpart of SGR 1627-41.
    11/2002; 34:1313.
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    ABSTRACT: There exists a group of persistently faint galactic X-ray sources that based on their location in the galaxy, high Lx/Lopt, association with X-ray bursts, and absence of X-ray pulsations are thought to be low-mass X-ray binaries (LMXBs). We present results from Chandra observations for 7 of these systems: 1708-408, 1711-339, 1735-269, 1736-297, 1746-331, 1746.7-3224, and 1812-12. Improved locations for all sources, excluding 1736-297 and 1746-331 (which were not detected) are presented. Our observations are consistent with previously reported transient behavior of 1736-297, 1746-331, and 1711-339 (which we detect in one of two observations). Energy and power spectra are presented for 1735-269, 1711-339, and 1746.7-3224. The energy spectra are hard, consistent with typical faint LMXB spectra. Further, we present a newly discovered source, a very faint, soft, source, separated by 2.7' from 1746.7-3224.
    10/2002; -1:52.

Publication Stats

407 Citations
79.87 Total Impact Points

Institutions

  • 2008
    • Universities Space Research Association
      Houston, Texas, United States
  • 2001
    • The University of Manchester
      Manchester, England, United Kingdom