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The majority of short gamma-ray bursts (SGRBs) are thought to originate from the merger of compact binary systems collapsing directly to form a black hole (BH). However it has been proposed that both SGRBs and long gamma-ray bursts (LGRBs) may, on rare occasions, form an unstable millisecond pulsar (magnetar) prior to final collapse. GRB 090515, detected by the Swift Satellite, was extremely short (T90 = 0.036±0.016 s) and had a very low fluence (2×10-8 erg cm-2 s-1). Additionally, it had a faint optical afterglow of magnitude r ∼26.4 at 1.7 hours after the burst. Despite this, the 0.3-10 keV flux in the first 200 s was the highest observed for a SGRB by the Swift X-ray Telescope (XRT). The X-ray light curve showed an unusual plateau and steep decay, becoming undetectable after ∼500 s. This behaviour is similar to that observed in some LGRBs proposed to have magnetars contributing to their emission. [ABSTRACT FROM AUTHOR]

We have fitted the complete Swift BAT and XRT light curves of 88 GRBs for which we have a redshift with a total of 331 pulses. For each GRB we also include an afterglow component to fit the plateau phase and the late decay seen in the XRT data. The combination of pulses and afterglow model all the emission detected, prompt plus afterglow, including late X-ray flares detected only in the XRT. Each pulse is described by a simple physical model which includes the spectrum at peak and the temporal characteristics of the pulse. We find that the pulse peak luminosity is correlated with both the mean photon energy in the bolometric band of the pulse spectrum at the peak, referred to as Ezbol, and the temporal parameter Tzf which is a measure of the pulse width. An empirical bivariate luminosity model set up with these parameters provides a good fit to the pulse luminosity. The analysis indicates that prompt pulses and X-ray flares are one and the same and arise from the same physical process and this physical process is responsible for the bivariate nature of the luminosity. [ABSTRACT FROM AUTHOR]

The core-collapse of a massive star may lead either to a black hole or a, possibly unstable, rapidly-rotating magnetar. A magnetar provides an potential additional source of rotational energy which is available to power emission during the burst. If the energy reservoir is used up the magnetar may then suffer gravitational collapse ending the emission period. We have carried out a systematic search for the signature of a spinning-down magnetar in the long GRBs observed by the Swift satellite up to the end of 2008. We find 10 GRBs that have a feature-which we call an 'internal plateau'-in their light curves that can be associated with the injection of energy into the jet by a magnetar. The duration and intensity of the internal plateau provides constraints on the initial spin period and magentic field of the magnetar. The derived magnetic fields are large compared to those of long-lived Galactic magnetars, but are within the most extreme values predicted for magnetars. [ABSTRACT FROM AUTHOR]

Based on light curves from the Swift Burst Analyser, we investigate whether a 'dip' feature commonly seen in the early-time hardness ratios of Swift-XRT data could arise from the juxtaposition of the decaying prompt emission and rising afterglow. We are able to model the dip as such a feature, assuming the afterglow rises as predicted by Sari and Piran [1]. Using this model we measure the initial bulk Lorentz factor of the fireball. For a sample of 23 GRBs we find a median value of Γ0 = 225, assuming a constant-density circumburst medium; or Γ0 = 93 if we assume a wind-like medium. [ABSTRACT FROM AUTHOR]

Although many X-ray afterglows follow a “canonical” steep-shallow-standard decay pattern, some break the mould. The start of the Swift X-ray light-curve of GRB 080307 showed an unusual smooth rise, at the beginning of which the emission softened. After this brightening, the emission followed a simple power-law decay, with no requirement for breaks. It is conjectured that the early softening is related to the tail of the prompt emission, which then fades rapidly away, allowing the rise of the afterglow to be seen. The optical afterglow was briefly detected by Gemini, Faulkes Telescope South and UKIRT, and the host galaxy by WHT, though no redshift was determined. [ABSTRACT FROM AUTHOR]

We study the collisions of compact objects induced by neutron star (NS) birth kicks. If a binary system survives the first supernova (SN) explosion, in some cases the second SN may place the stellar remnants into bound highly eccentric orbits. The two compact objects will tidally interact and quickly spiral into each other, possibly originating a short GRB. We also discuss the presence of a debris disk around the newborn NS. [ABSTRACT FROM AUTHOR]

GRBs 050223 and 050911 are examples of Swift bursts with two of the faintest X-ray afterglows at (relatively) early times. While a faint, fading X-ray afterglow was located for GRB 050223, GRB 050911 was not detected, making any X-ray afterglow extremely faint. The faintness of the afterglow of GRB 050223 could be explained by a large opening or viewing angle, or by the burst being at high redshift. The non-detection of GRB 050911 may indicate the burst occurred in a low-density environment, or, alternatively, was due to a compact object merger, in spite of the apparent long duration of the burst. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

We have analysed BAT and XRT data for 40 gamma-ray bursts (GRBs) observed using the Swift satellite. The earliest X-ray light curve can be well described by an exponential which relaxes into a power law, often with flares superimposed. The transition time between the exponential and the power law gives a physically defined timescale for the burst duration. In most GRBs the power law decay changes to a shallower decay within the first hour. The resultant “late emission hump” can last for several tens of ks. In other bursts the late hump is weak or absent. The observed variety in light curve shape can be explained as a combination of three components: prompt emission from the central engine; afterglow; and the late hump, which may also be due to the central engine. GRBs with stronger afterglow components are more likely to have brighter early optical emission. The late emission hump can have a total fluence equivalent to that of the prompt phase. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

During the first year of operations of the Swift observatory, the X-ray Telescope has made a number of discoveries concerning the nature of X-ray afterglows of both long and short GRBs. We highlight the key findings, which include rapid declines at early times, a standard template of afterglow light curve shapes, common flaring, and the discovery of the first short GRB afterglow. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present Swift observations of the faint burst GRB 050421. The X-ray light-curve shows two flares: the first peaking at ∼ 110 s after the BAT trigger (T0) and the second one peaking at ∼ 154 s. We argue that the mechanism producing these flares is probably late internal shocks. The X-ray light-curve is consistent with a power-law with a temporal index α1 ∼ 3.1. The X-ray flux decays from ∼ 10-9 erg cm-2 s-1 at T0 + 100 s to < 7 × 10-13 erg cm-2 s-1 at T0 + 1000 s. An X-ray spectral softening is also observed with time from β ∼ 0.1 to ∼ 1.2, where β is the spectral index such as Fv ∝ v-βt-α. A good joint fit to the BAT and XRT spectra indicates that the early X-ray and Gamma-ray emissions are produced by the same mechanism. The X-ray spectral softening is likely due to a shift down to lower energies of the peak of the prompt emission, and the rapid decline of the X-ray emission is probably the tail of the prompt emission. This suggests that the X-ray emission is completely dominated by high latitude radiation and the external shock, if any, is extremely faint and below the detection threshold. GRB 050421 is likely the first “naked burst” detected by Swift. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

GRBs 050223 and 050911 were discovered by the Swift Burst Alert Telescope (BAT) on 23rd February and 11th September 2005 respectively. The observation of GRB 050223 showed a faint, fading X-ray source, which was identified as the afterglow; GRB 050911, however, was not detected, making any X-ray afterglow extremely faint. The faintness of the afterglow of GRB 050223 could be explained by a large opening or viewing angle, or by the burst being at high redshift. The non-detection of GRB 050911 may indicate the burst occurred in a low-density environment, or, alternatively, was due to a compact object merger, in spite of the apparent long duration of the burst. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Observations of gamma ray bursts (GRBs) with Swift produced the initially surprising result that many bursts have large X-ray flares superimposed on the underlying afterglow. These flares were sometimes intense, rapid, and late relative to the nominal prompt phase. The most intense of these flares was observed by XRT with a flux > 500× the afterglow. This burst then surprised observers by flaring again after > 10000 s. The intense flare can be most easily understood within the context of the standard fireball model, if the internal engine that powers the prompt GRB emission is still active at late times. Recent observations indicate that X-ray flares are detected in ∼1/3 of XRT detected afterglows. By studying the properties of the varieties of flares (such as rise/fall time, onset time, spectral variability, etc.) and relating them to overall burst properties, models of flare production and the GRB internal engine can be constrained. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We have measured coronal line emission for a sample of 19 Narrow-Line Seyfert 1 galaxies. Kinematic analysis shows that the Coronal Line Region (CLR) consists of gas in a decelerating outflow. We intend to use these data as a test of the EUV emission of NLSls. By comparison with photoionization modelling for a number of power-law + black body spectra, we plan to deduce the most likely EUV continua for these objects as well as explore physical conditions in the CLR. [ABSTRACT FROM AUTHOR]