The temporal and spectral analysis of nine bright X-ray flares out of a sample of 113 flares observed by Swift reveals that the flare phenomenology is strictly analogous to the prompt γ-ray emission: ...high-energy flare profiles rise faster, decay faster and peak before the low-energy emission. However, flares and prompt pulses differ in one crucial aspect: flares evolve with time. As time proceeds, flares become wider, with larger peak lag, lower luminosities and softer emission. The flare spectral peak energy Ep,i evolves to lower values following an exponential decay which tracks the decay of the flare flux. The two flares with best statistics show higher than expected isotropic energy Eiso and peak luminosity Lp,iso when compared to the Ep,i–Eiso and Ep,i–Liso prompt correlations. Ep,i is found to correlate with Liso within single flares, giving rise to a time-resolved Ep,i(t)–Liso(t). Like prompt pulses, flares define a lag–luminosity relation: L0.3–10 keVp,iso∝t−0.95±0.23lag. The lag–luminosity is proven to be a fundamental law extending ∼5 decades in time and ∼5 decades in energy. Moreover, this is direct evidence that γ-ray burst (GRB) X-ray flares and prompt γ-ray pulses are produced by the same mechanism. Finally we establish a flare–afterglow morphology connection: flares are preferentially detected superimposed to one-break or canonical X-ray afterglows.
GRB 090423 at a redshift of z 8.1 Racusin, J. L; Amati, L; Guidorzi, C ...
Nature (London),
10/2009, Letnik:
461, Številka:
7268
Journal Article
Recenzirano
Odprti dostop
Gamma-ray bursts (GRBs) are produced by rare types of massive stellar explosion. Their rapidly fading afterglows are often bright enough at optical wavelengths that they are detectable at ...cosmological distances. Hitherto, the highest known redshift for a GRB was z = 6.7 (ref. 1), for GRB 080913, and for a galaxy was z = 6.96 (ref. 2). Here we report observations of GRB 090423 and the near-infrared spectroscopic measurement of its redshift, z = . This burst happened when the Universe was only about 4 per cent of its current age. Its properties are similar to those of GRBs observed at low/intermediate redshifts, suggesting that the mechanisms and progenitors that gave rise to this burst about 600,000,000 years after the Big Bang are not markedly different from those producing GRBs about 10,000,000,000 years later.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Aims. With an observed and rest-frame duration of <2 s and <0.5 s, respectively, GRB 090426 could be classified as a short GRB. The prompt detection, both from space and ground-based telescopes, of a ...bright optical counterpart to this GRB offered a unique opportunity to complete a detailed study. Methods. Based on an extensive ground-based observational campaign, we obtained the spectrum of the optical afterglow of GRB 090426, measuring its redshift and obtaining information about the medium in which the event took place. We completed follow-up observations of the afterglow optical light curve down to the brightness level of the host galaxy that we firmly identified and studied. We also retrieved and analyzed all the available high-energy data of this event, and compared the results with our findings in the optical. This represents one of the most detailed studies of a short-duration event presented so far. Results. The time properties qualify GRB 090426 as a short burst. In this case, its redshift of $z = 2.61$ would be the highest yet found for a GRB of this class. On the other hand, the spectral and energy properties are more similar to those of long bursts. LBT late-time deep imaging identifies a star-forming galaxy at a redshift consistent with that of the GRB. The afterglow lies within the light of its host and shows evidence of local absorption.
Gamma-ray bursts (GRBs) are short, intense flashes of soft gamma-rays coming from the distant Universe. Long-duration GRBs (those lasting more than approximately 2 s) are believed to originate from ...the deaths of massive stars, mainly on the basis of a handful of solid associations between GRBs and supernovae. GRB 060614, one of the closest GRBs discovered, consisted of a 5-s hard spike followed by softer, brighter emission that lasted for approximately 100 s (refs 8, 9). Here we report deep optical observations of GRB 060614 showing no emerging supernova with absolute visual magnitude brighter than M(V) = -13.7. Any supernova associated with GRB 060614 was therefore at least 100 times fainter, at optical wavelengths, than the other supernovae associated with GRBs. This demonstrates that some long-lasting GRBs can either be associated with a very faint supernova or produced by different phenomena.
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Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We compute the luminosity function (LF) and the formation rate of long gamma-ray bursts (GRBs) by fitting the observed differential peak flux distribution obtained by the Burst and Transient Source ...Experiment (BATSE) in two different scenarios: (i) the GRB luminosity evolves with redshift and (ii) GRBs form preferentially in low-metallicity environments. In both cases, model predictions are consistent with the Swift number counts and with the number of detections at z > 2.5 and >3.5. To discriminate between the two evolutionary scenarios, we compare the model results with the number of luminous bursts (i.e. with isotropic peak luminosity in excess of 1053 erg s−1) detected by Swift in its first 3 yr of mission. Our sample conservatively contains only bursts with good redshift determination and measured peak energy. We find that pure luminosity evolution models can account for the number of sure identifications. In the case of a pure density evolution scenario, models with Zth > 0.3 Z⊙ are ruled out with high confidence. For lower metallicity thresholds, the model results are still statistically consistent with available lower limits. However, many factors can increase the discrepancy between model results and data, indicating that some luminosity evolution in the GRB LF may be needed also for such low values of Zth. Finally, using these new constraints, we derive robust upper limits on the bright end of the GRB LF, showing that this cannot be steeper than ∼2.6.
Although the link between long γ-ray bursts (GRBs) and supernovae has been established, hitherto there have been no observations of the beginning of a supernova explosion and its intimate link to a ...GRB. In particular, we do not know how the jet that defines a γ-ray burst emerges from the star's surface, nor how a GRB progenitor explodes. Here we report observations of the relatively nearby GRB 060218 (ref. 5) and its connection to supernova SN 2006aj (ref. 6). In addition to the classical non-thermal emission, GRB 060218 shows a thermal component in its X-ray spectrum, which cools and shifts into the optical/ultraviolet band as time passes. We interpret these features as arising from the break-out of a shock wave driven by a mildly relativistic shell into the dense wind surrounding the progenitor. We have caught a supernova in the act of exploding, directly observing the shock break-out, which indicates that the GRB progenitor was a Wolf-Rayet star.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The Swift X-ray telescope BURROWS, David N; HILL, J. E; CHINCARINI, G ...
Space science reviews,
10/2005, Letnik:
120, Številka:
3-4
Journal Article
Recenzirano
he Swift Gamma-Ray Explorer is designed to make prompt multiwavelength observations of gamma-ray bursts (GRBs) and GRB afterglows. The X-ray telescope (XRT) enables Swift to determine GRB positions ...with a few arcseconds accuracy within 100 s of the burst onset. The XRT utilizes a mirror set built for JET-X and an XMM-Newton/EPIC MOS CCD detector to provide a sensitive broad-band (0.2-10 keV) X-ray imager with effective area of > 120 cm^sup 2^ at 1.5 keV, field of view of 23.6 × 23.6 arcminutes, and angular resolution of 18 arcseconds (HPD). The detection sensitivity is 2×10^sup -14^ erg cm^sup -2^ s^sup -1^ in 10^sup 4^ s. The instrument is designed to provide automated source detection and position reporting within 5 s of target acquisition. It can also measure the redshifts of GRBs with Fe line emission or other spectral features. The XRT operates in an auto-exposure mode, adjusting the CCD readout mode automatically to optimize the science return for each frame as the source intensity fades. The XRT will measure spectra and lightcurves of the GRB afterglow beginning about a minute after the burst and will follow each burst for days or weeks.PUBLICATION ABSTRACT
We show that all X-ray decay curves of gamma -ray bursts (GRBs) measured by Swift can be fitted using one or two components, both of Which have exactly the same functional form comprised of an early ...falling exponential phase followed by a power-law decay. The first component contains the prompt gamma -ray emission and the initial X-ray decay. The second component appears later, has a much longer duration, and is present for approximately 80% of GRBs. It most likely arises from the external shock that eventually develops into the X-ray afterglow. In the remaining approximately 20% of GRBs the initial X-ray decay of the first component fades more slowly than the second and dominates at late times to form an afterglow. The temporal decay parameters and gamma /X-ray spectral indices derived for 107 GRBs are compared to the expectations of the standard fireball model including a search for possible "jet breaks." For similar to 50% of GRBs the observed afterglow is in accord with the model, but for the rest the temporal and spectral indices do not conform to the expected closure relations and are suggestive of continued, late, energy injection. We identify a few possible jet breaks, but there are many examples where such breaks are predicted but are absent. The time T alpha at which the exponential phase of the second component changes to a final power-law decay afterglow is correlated with the peak of the gamma -ray spectrum, E sub(peak). This is analogous to the Ghirlanda relation, indicating that this time is in some way related to optically observed break times measured for pre-Swift bursts.
The 22 Month Swift-BAT All-Sky Hard X-ray Survey Tueller, J; Baumgartner, W. H; Markwardt, C. B ...
The Astrophysical journal. Supplement series,
02/2010, Letnik:
186, Številka:
2
Journal Article
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 (8t super(-a) with 3 a sub(1) 5), followed by (2) a very shallow decay (0.5 a sub(2) 1.0), and finally (3) a somewhat steeper decay (1 a sub(3) 1.5). These power-law segments are separated by two corresponding break times, t sub(break,1) 500 s and 10 super(3) s t sub(break,2) 10 super(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 (t sub(break,1)) takes place when the forward shock emission becomes dominant, with the intermediate shallow flux decay (a sub(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 (t sub(break,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.
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