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M. R. Goad

Bio: M. R. Goad is an academic researcher from University of Leicester. The author has contributed to research in topics: Gamma-ray burst & Afterglow. The author has an hindex of 21, co-authored 36 publications receiving 4515 citations. Previous affiliations of M. R. Goad include Space Telescope Science Institute.

Papers
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Journal ArticleDOI
TL;DR: A homogeneous X-rays analysis of all 318 gamma-ray bursts detected by the X-ray telescope (XRT) on the Swift satellite up to 2008 July 23 is presented; this represents the largest sample ofX-ray GRB data published to date.
Abstract: We present a homogeneous X-ray analysis of all 318 gamma-ray bursts detected by the X-ray telescope (XRT) on the Swift satellite up to 2008 July 23; this represents the largest sample of X-ray GRB data published to date. In Sections 2-3, we detail the methods which the Swift-XRT team has developed to produce the enhanced positions, light curves, hardness ratios and spectra presented in this paper. Software using these methods continues to create such products for all new GRBs observed by the Swift-XRT. We also detail web-based tools allowing users to create these products for any object observed by the XRT, not just GRBs. In Sections 4-6, we present the results of our analysis of GRBs, including probability distribution functions of the temporal and spectral properties of the sample. We demonstrate evidence for a consistent underlying behaviour which can produce a range of light-curve morphologies, and attempt to interpret this behaviour in the framework of external forward shock emission. We find several difficulties, in particular that reconciliation of our data with the forward shock model requires energy injection to continue for days to weeks.

1,613 citations

Journal ArticleDOI
TL;DR: In this article, the early X-ray afterglows of the first 27 gamma-ray bursts (GRBs) well observed by the Swift X-Ray Telescope (XRT).
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 � � with 3P � 1 P5), followed by (2) a very shallow decay (0:5P � 2 P1:0), and finally (3) a somewhat steeper decay (1P � 3 P1:5). These power-law segments are separated by two corresponding break times, tbreak;1 P500 s and 10 3 sPtbreak;2P 10 4 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 (� 2) 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 k4 and augments the already severe requirements for the efficiency of the prompt gamma-ray emission. Subject headingg gamma rays: bursts — radiation mechanisms: nonthermal

709 citations

Journal ArticleDOI
TL;DR: In this article, the early X-ray emission for a sample of 40 gamma-ray bursts (GRBs) obtained using the Swift satellite, for which the narrow-field instruments were pointed at the burst within 10 minutes of the trigger.
Abstract: We present observations of the early X-ray emission for a sample of 40 gamma-ray bursts (GRBs) obtained using the Swift satellite, for which the narrow-field instruments were pointed at the burst within 10 minutes of the trigger. Using data from the Burst Alert Telescope and the X-Ray Telescope, we show that the X-ray light curve can be well described by an exponential that relaxes into a power law, often with flares superimposed. The transition time between the exponential and the power law provides a physically defined timescale for the burst duration. In most bursts, the power law breaks to a shallower decay within the first hour, and a late emission hump is observed, which can last for many hours. In other GRBs the hump is weak or absent. The observed variety in the shape of the early X-ray light curve can be explained as a combination of three components: prompt emission from the central engine, afterglow, and the late hump. In this scenario, afterglow emission begins during or soon after the burst, and the observed shape of the X-ray light curve depends on the relative strengths of the emission due to the central engine and that of the afterglow. There is a strong correlation such that those GRBs with stronger afterglow components have brighter early optical emission. The late emission hump can have a total fluence equivalent to that of the prompt phase. GRBs with the strongest late humps have weak or no X-ray flares.

448 citations

Proceedings ArticleDOI
01 May 2006
TL;DR: In this paper, the early X-ray afterglows of the first 27 gamma-ray bursts (GRBs) well observed by the Swift X-Ray Telescope (XRT).
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 � � with 3P � 1 P5), followed by (2) a very shallow decay (0:5P � 2 P1:0), and finally (3) a somewhat steeper decay (1P � 3 P1:5). These power-law segments are separated by two corresponding break times, tbreak;1 P500 s and 10 3 sPtbreak;2P 10 4 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 (� 2) 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 k4 and augments the already severe requirements for the efficiency of the prompt gamma-ray emission. Subject headingg gamma rays: bursts — radiation mechanisms: nonthermal

307 citations

Journal ArticleDOI
20 Jul 2006
TL;DR: In this article, the authors presented a self-consistency test for this scenario with a sample of 36 prompt emission tails/flares in 22 light curves observed by the Swift XRT.
Abstract: The X-ray light curves of many GRBs have a steep tail following the gamma-rays and have some erratic flares. We assume that these tails and flares are of ''internal'' origin and that their decline behaviors are dominated by the curvature effect. This effect suggests that the decay slope of the late steep decay part of the light curves is � ¼ 2 þ � , whereis the X-ray spectral index. We present a self-consistency test for this scenario with a sample of 36 prompt emission tails/flares in 22 light curves observed by the Swift XRT. We derive the zero time (t0) for each steep decay component by fitting the light curves with the constraint of � ¼2 þ � . Our results show that the t0values of the prompt emission tails and the tails of well-separated flares are self-consistent with the expectation of the internal dissipation models, indicating that each X-ray flare forms a distinct episode of the central engine activity and the central engine remains active after the prompt emission is over, sometimes up to � 1 day after the GRB trigger. This challenges the conventional models and calls for new ideas to restart the central engine. We further show that the onset time of the late central engine activity does not depend on the GRB duration. We also identify a minority group of GRBs whose combined BAT-XRT light curves are smoothly connected, without an abrupt transition between the prompt emission and the afterglow. These GRBs may have an external origin for both the prompt emission and the afterglow. Subject headings: gamma rays: bursts — methods: statistical Online material: color figuresONLINE MATERIALCOLOR FIGURES

230 citations


Cited by
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Journal ArticleDOI
TL;DR: In this article, a reanalysis of broad emission-line reverberation-mapping data was carried out for 35 active galactic nuclei (AGNs) based on a complete and consistent reanalysis, and it was shown that the highest precision measure of the virial product cτΔV2/G is obtained by using the cross-correlation function centroid (cf.
Abstract: We present improved black hole masses for 35 active galactic nuclei (AGNs) based on a complete and consistent reanalysis of broad emission-line reverberation-mapping data From objects with multiple line measurements, we find that the highest precision measure of the virial product cτΔV2/G, where τ is the emission-line lag relative to continuum variations and ΔV is the emission-line width, is obtained by using the cross-correlation function centroid (as opposed to the cross-correlation function peak) for the time delay and the line dispersion (as opposed to FWHM) for the line width and by measuring the line width in the variable part of the spectrum Accurate line-width measurement depends critically on avoiding contaminating features, in particular the narrow components of the emission lines We find that the precision (or random component of the error) of reverberation-based black hole mass measurements is typically around 30%, comparable to the precision attained in measurement of black hole masses in quiescent galaxies by gas or stellar dynamical methods Based on results presented in a companion paper by Onken et al, we provide a zero-point calibration for the reverberation-based black hole mass scale by using the relationship between black hole mass and host-galaxy bulge velocity dispersion The scatter around this relationship implies that the typical systematic uncertainties in reverberation-based black hole masses are smaller than a factor of 3 We present a preliminary version of a mass-luminosity relationship that is much better defined than any previous attempt Scatter about the mass-luminosity relationship for these AGNs appears to be real and could be correlated with either Eddington ratio or object inclination

1,893 citations

15 Mar 1979
TL;DR: In this article, the experimental estimation of parameters for models can be solved through use of the likelihood ratio test, with particular attention to photon counting experiments, and procedures presented solve a greater range of problems than those currently in use, yet are no more difficult to apply.
Abstract: Many problems in the experimental estimation of parameters for models can be solved through use of the likelihood ratio test. Applications of the likelihood ratio, with particular attention to photon counting experiments, are discussed. The procedures presented solve a greater range of problems than those currently in use, yet are no more difficult to apply. The procedures are proved analytically, and examples from current problems in astronomy are discussed.

1,748 citations

Journal ArticleDOI
TL;DR: A homogeneous X-rays analysis of all 318 gamma-ray bursts detected by the X-ray telescope (XRT) on the Swift satellite up to 2008 July 23 is presented; this represents the largest sample ofX-ray GRB data published to date.
Abstract: We present a homogeneous X-ray analysis of all 318 gamma-ray bursts detected by the X-ray telescope (XRT) on the Swift satellite up to 2008 July 23; this represents the largest sample of X-ray GRB data published to date. In Sections 2-3, we detail the methods which the Swift-XRT team has developed to produce the enhanced positions, light curves, hardness ratios and spectra presented in this paper. Software using these methods continues to create such products for all new GRBs observed by the Swift-XRT. We also detail web-based tools allowing users to create these products for any object observed by the XRT, not just GRBs. In Sections 4-6, we present the results of our analysis of GRBs, including probability distribution functions of the temporal and spectral properties of the sample. We demonstrate evidence for a consistent underlying behaviour which can produce a range of light-curve morphologies, and attempt to interpret this behaviour in the framework of external forward shock emission. We find several difficulties, in particular that reconciliation of our data with the forward shock model requires energy injection to continue for days to weeks.

1,613 citations

Journal ArticleDOI
TL;DR: In this article, the authors reported that NGC 2617 went through a dramatic outburst, during which its X-ray flux increased by over an order of magnitude followed by an increase of its optical/ultraviolet (UV) continuum flux.
Abstract: After the All-Sky Automated Survey for SuperNovae discovered a significant brightening of the inner region of NGC 2617, we began a ∼70 day photometric and spectroscopic monitoring campaign from the X-ray through near-infrared (NIR) wavelengths. We report that NGC 2617 went through a dramatic outburst, during which its X-ray flux increased by over an order of magnitude followed by an increase of its optical/ultraviolet (UV) continuum flux by almost an order of magnitude. NGC 2617, classified as a Seyfert 1.8 galaxy in 2003, is now a Seyfert 1 due to the appearance of broad optical emission lines and a continuum blue bump. Such 'changing look active galactic nuclei (AGNs)' are rare and provide us with important insights about AGN physics. Based on the Hβ line width and the radius-luminosity relation, we estimate the mass of central black hole (BH) to be (4 ± 1) × 10{sup 7} M {sub ☉}. When we cross-correlate the light curves, we find that the disk emission lags the X-rays, with the lag becoming longer as we move from the UV (2-3 days) to the NIR (6-9 days). Also, the NIR is more heavily temporally smoothed than the UV. This can largely be explained bymore » a simple model of a thermally emitting thin disk around a BH of the estimated mass that is illuminated by the observed, variable X-ray fluxes.« less

1,473 citations

Journal ArticleDOI
TL;DR: In this article, the mass scaling relationship between line widths and luminosity is used for estimating the central black hole mass in nearby AGNs and distant luminous quasars alike using either optical or UV single-epoch spectroscopy.
Abstract: We present four improved empirical relationships useful for estimating the central black hole mass in nearby AGNs and distant luminous quasars alike using either optical or UV single-epoch spectroscopy. These mass scaling relationships between line widths and luminosity are based on recently improved empirical relationships between the broad-line region size and luminosities in various energy bands and are calibrated to the improved mass measurements of nearby AGNs based on emission-line reverberation mapping. The mass scaling relationship based on the Hβ line luminosity allows mass estimates for low-redshift sources with strong contamination of the optical continuum luminosity by stellar or nonthermal emission, while that based on the C IV λ1549 line dispersion allows mass estimates in cases where only the line dispersion (as opposed to the FWHM) can be reliably determined. We estimate that the absolute uncertainties in masses given by these mass scaling relationships are typically around a factor of 4. We include in an appendix mass estimates for all of the Bright Quasar Survey (PG) quasars for which direct reverberation-based mass measurements are not available.

1,289 citations