The First Survey of X-ray Flares from Gamma Ray Bursts Observed by Swift: Spectral Properties and Energetics

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. The flares were sometimes intense, had rapid rise and decay phases, and occurred late relative to the ``prompt'' phase. Some remarkable flares are observed with fluence comparable to the prompt GRB fluence. Many GRBs have several flares, which are sometimes overlapping. Short, intense, repetitive, and late flaring can be most easily understood within the context of the standard fireball model with the internal engine that powers the prompt GRB emission in an active state at late times. However, other models for flares have been proposed. Flare origin can be investigated by comparing the flare spectra to that of the afterglow and the initial prompt emission. In this work, we have analyzed all significant X-ray flares from the first 110 GRBs observed by Swift. From this sample 33 GRBs were found to have significant X-ray flares, with 77 flares that were detected above the 3$\sigma$ level. In addition to temporal analysis presented in a companion paper, a variety of spectral models have been fit to each flare. In some cases, we find that the spectral fits favor a Band function model, which is more akin to the prompt emission than to that of an afterglow. We find that the average fluence of the flares is 2.4e-7 erg/cm^2/s in the 0.2-10 keV energy band, which is approximately a factor of ten below the average prompt GRB fluence. These results, when combined with those presented in the companion paper on temporal properties of flares, supports the hypothesis that most X-ray flares are late-time activity of the internal engine that spawned the initial GRB; not an afterglow related effect.Comment: accepted by ApJ; 39 pages with 14 figures and 7 table

High Resolution X-Ray Spectroscopy of SN 1987A: Monitoring with XMM-Newton

We report the results of our XMM-Newton monitoring of SN 1987A. The ongoing propagation of the supernova blast wave through the inner circumstellar ring caused a drastic increase in X-ray luminosity during the last years, enabling detailed high resolution X-ray spectroscopy with the Reflection Grating Spectrometer. The observations can be used to follow the detailed evolution of the arising supernova remnant. The fluxes and broadening of the numerous emission lines seen in the dispersed spectra provide information on the evolution of the X-ray emitting plasma and its dynamics. These were analyzed in combination with the EPIC-pn spectra, which allow a precise determination of the higher temperature plasma. We modeled individual emission lines and fitted plasma emission models. Especially from the observations between 2003 and 2007 we can see a significant evolution of the plasma parameters and a deceleration of the radial velocity of the lower temperature plasma regions. We found an indication (3-sigma-level) of an iron K feature in the co-added EPIC-pn spectra. The comparison with Chandra grating observations in 2004 yields a clear temporal coherence of the spectral evolution and the sudden deceleration of the expansion velocity seen in X-ray images ~6100 days after the explosion.Comment: 10 pages, 8 Figures; accepted by A&

The Swift X-Ray Telescope: Status and Performance

We present science highlights and performance from the Swift X-ray Telescope (XRT), which was launched on November 20, 2004. The XRT covers the 0.2-10 keV band, and spends most of its time observing gamma-ray burst (GRB)afterglows, though it has also performed observations of many other objects. By mid-August 2007, the XRT had observed over 220 GRB afterglows, detecting about 96% of them. The XRT positions enable followup ground-based optical observations, with roughly 60% of the afterglows detected at optical or near IR wavelengths. Redshifts are measured for 33% of X-ray afterglows. Science highlights include the discovery of flaring behavior at quite late times, with implications for GRB central engines; localization of short GRBs, leading to observational support for compact merger progenitors for this class of bursts; a mysterious plateau phase to GRB afterglows; as well as many other interesting observations such as X-ray emission from comets, novae, galactic transients, and other objects.Comment: 9 pages, 14 figure

GRB 081008: from burst to afterglow and the transition phase in between

We present a multi-wavelength study of GRB 081008, at redshift 1.967, by Swift, ROTSE-III and GROND. Compared to other Swift GRBs, GRB 081008 has a typical gamma-ray isotropic equivalent energy output (10^53 erg) during the prompt phase, and displayed two temporally separated clusters of pulses. The early X-ray emission seen by the Swift/XRT was dominated by the softening tail of the prompt emission, producing multiple flares during and after the Swift/BAT detections. Optical observations that started shortly after the first active phase of gamma-ray emission showed two consecutive peaks. We interpret the first optical peak as the onset of the afterglow associated with the early burst activities. A second optical peak, coincident with the later gamma-ray pulses, imposes a small modification to the otherwise smooth lightcurve and thus suggests a minimal contribution from a probable internal component. We suggest the early optical variability may be from continuous energy injection into the forward shock front by later shells producing the second epoch of burst activities. These early observations thus provide a potential probe for the transition from prompt to the afterglow phase. The later lightcurve of GRB 081008 displays a smooth steepening in all optical bands and X-ray. The temporal break is consistent with being achromatic at the observed wavelengths. Our broad energy coverage shortly after the break constrains a spectral break within optical. However, the evolution of the break frequency is not observed. We discuss the plausible interpretations of this behavior.Comment: 16 pages, 4 figures, accepted for publication in Ap

A refined position catalog of the Swift XRT afterglows

We present a catalogue of refined positions of 68 gamma ray burst (GRB) afterglows observed by the Swift X-ray Telescope (XRT) from the launch up to 2005 Oct 16. This is a result of the refinement of the XRT boresight calibration. We tested this correction by means of a systematic study of a large sample of X-ray sources observed by XRT with well established optical counterparts. We found that we can reduce the systematic error radius of the measurements by a factor of two, from 6.5" to 3.2" (90% of confidence). We corrected all the positions of the afterglows observed by XRT in the first 11 months of the Swift mission. This is particularly important for the 37 X-ray afterglows without optical counterpart. Optical follow-up of dark GRBs, in fact, will be more efficient with the use of the more accurate XRT positions.Comment: 4 pages, 4 figures, 1 table ; accepted for publication in A&A Letters. The revised version contains updated position

Observations of the Prompt Gamma-Ray Emission of GRB 070125

The long, bright gamma-ray burst GRB 070125 was localized by the Interplanetary Network. We present light curves of the prompt gamma-ray emission as observed by Konus-WIND, RHESSI, Suzaku-WAM, and \textit{Swift}-BAT. We detail the results of joint spectral fits with Konus and RHESSI data. The burst shows moderate hard-to-soft evolution in its multi-peaked emission over a period of about one minute. The total burst fluence as observed by Konus is $1.79 \times 10^{-4}$ erg/cm$^2$ (20 keV--10 MeV). Using the spectroscopic redshift $z=1.548$, we find that the burst is consistent with the ``Amati'' $E_{peak,i}-E_{iso}$ correlation. Assuming a jet opening angle derived from broadband modeling of the burst afterglow, GRB 070125 is a significant outlier to the ``Ghirlanda'' $E_{peak,i}-E_\gamma$ correlation. Its collimation-corrected energy release $E_\gamma = 2.5 \times 10^{52}$ ergs is the largest yet observed.Comment: 25 pages, 6 figures; accepted for publication in ApJ. Improved spectral fits and energetics estimate

On the spherical-axial transition in supernova remnants

A new law of motion for supernova remnant (SNR) which introduces the quantity of swept matter in the thin layer approximation is introduced. This new law of motion is tested on 10 years observations of SN1993J. The introduction of an exponential gradient in the surrounding medium allows to model an aspherical expansion. A weakly asymmetric SNR, SN1006, and a strongly asymmetric SNR, SN1987a, are modeled. In the case of SN1987a the three observed rings are simulated.Comment: 19 figures and 14 pages Accepted for publication in Astrophysics & Space Science in the year 201

The First Survey of X-ray Flares from Gamma Ray Bursts Observed by Swift: Temporal Properties and Morphology

We present the first systematic investigation of the morphological and timing properties of flares in GRBs observed by Swift/XRT. We consider a large sample drawn from all GRBs detected by Swift, INTEGRAL and HETE-2 prior to 2006 Jan 31, which had an XRT follow-up and which showed significant flaring. Our sample of 33 GRBs includes long and short, at low and high redshift, and a total of 69 flares. The strongest flares occur in the early phases, with a clear anti-correlation between the flare peak intensity and the flare time of occurrence. Fitting each X-ray flare with a Gaussian model, we find that the mean ratio of the width and peak time is = 0.13+/-0.10, albeit with a large scatter. Late flares at times > 2000 seconds have long durations, Delta t>300 s, and can be very energetic compared to the underlying continuum. We further investigated if there is a clear link between the number of pulses detected in the prompt phase by BAT and the number of X-ray flares detected by XRT, finding no correlation. However, we find that the distribution of intensity ratios between successive BAT prompt pulses and that between successive XRT flares is the same, an indication of a common origin for gamma-ray pulses and X-ray flares. All evidence indicates that flares are indeed related to the workings of the central engine and, within the standard fireball scenario, originate from internal shocks rather than external shocks. While all flares can be explained by long-lasting engine activity, 29/69 flares may also be explained by refreshed shocks. However, 10 can only be explained by prolonged activity of the central engine.Comment: submitted to Ap

Toward a standard Gamma Ray Burst: tight correlations between the prompt and the afterglow plateau phase emission

To reveal and understand astrophysical processes responsible for the Gamma Ray Burst (GRB) phenomenon, it is crucial to discover and understand relations between their observational properties. The presented study is performed in the GRB rest frames and it uses a sample of 62 long GRBs from our sample of 77 Swift GRBs with known redshifts. Following the earlier analysis of the afterglow {\it characteristic luminosity $L^*_a$ -- break time $T^*_a$} correlation for a sample of long GRBs \citep{Dainotti2010} we extend it to correlations between the afterglow and the prompt emission GRB physical parameters. We reveal a tight physical scaling between the mentioned afterglow luminosity $L^*_a$ and the prompt emission {\it mean} luminosity $_{45} \equiv E_{iso}/T^*_{45}$. The distribution, with the Spearman correlation coefficient reaching 0.95 for the data subsample with most regular light curves, can be fitted with approximately $L^*_a \propto {_{45}}^{0.7}$. We also analyzed correlations of $L^*_a$ with several other prompt emission parameters, including the isotropic energy $E_{iso}$, the peak energy in the $\nu F_{\nu}$ spectrum, $E_{peak}$, and the variability parameter, $V$, defined by \cite{N000}. As a result, we reveal significant correlations also between these quantities, with an exception of the variability parameter. The main result of the present study is the discovery that the highest correlated GRB subsample in the \citet{Dainotti2010} afterglow analysis, for the GRBs with canonical X\,-\,ray light curves, leads also to the highest {\it prompt-afterglow} correlations and such events can be considered to form a sample of standard GRBs for astrophysics and cosmology.Comment: The Data Table will appear after the paper will be accepte