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 a = 2 + b, where b is 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 (t sub(0)) for each steep decay component by fitting the light curves with the constraint of a = 2 + b. Our results show that the t sub(0) values 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 61 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.
In 2006 March the soft gamma-ray repeater SGR 1900+14 resumed its bursting activity after image2 yr of quiescence. The Swift mission observed the source several times. We report on the intense burst ...'forest' recorded on March 29, which lasted for image30 s, when Swift was pointing at the source with the narrow field of view instruments. More than 40 bursts were detected by BAT and XRT, 7 of which were rare intermediate flares (IFs). The BAT data were used to carry out time-resolved spectroscopy in the 14-100 keV range down to 8 ms timescales. BAT and XRT simultaneous data were used to characterize the broadband energy spectra of IFs and verify the results obtained from the BAT-only spectral fits. This unique data set allowed us to test the magnetar model predictions, such as the magnetically trapped fireball and twisted magnetosphere, over an unprecedented range of fluxes and with large statistics. We confirmed that a two-blackbody component adequately fits the time-resolved and integrated spectra of IFs. However, Comptonization models give comparably good reduced image. Moreover, we found a change of behavior, around image10 super(41) erg s super(-1), above which the softer blackbody shows a sort of saturation, while the harder one still grows to a few times 10 super(41) erg s super(-1), and a rather sharp correlation between temperature and radii of the blackbodies, which holds for the most luminous parts of the flares (image erg s super(-1)). Within the magnetar model, the majority of these findings are accounted for in terms of thermalized emission from the E-mode and O-mode photospheres. Interestingly, the maximum observed luminosity coming from a region of image15 km matches the magnetic Eddington luminosity at the same radius, for a surface dipole field of image G (virtually equal to that deduced from the spin-down of SGR 1900+14).
Correlation studies of prompt and afterglow emission from gamma-ray bursts (GRBs) between different spectral bands have been difficult to do in the past because few bursts had comprehensive and ...comparable afterglow measurements. In this paper we present a large and uniform data set for correlation analysis based on bursts detected by the Swift mission. For the first time, short and long bursts can be analyzed and compared. It is found for both classes that the optical, X-ray, and gamma-ray emission are linearly correlated, but with a large spread about the correlation line; stronger bursts tend to have brighter afterglow, and bursts with brighter X- ray afterglow tend to have brighter optical afterglow. Short bursts are, on average, weaker in both prompt and afterglow emission. No short bursts are seen with extremely low optical-to-X-ray ratios, as occurs for "dark" long bursts. Although statistics are still poor for short bursts, there is no evidence yet for a subgroup of short bursts with high extinction, as there is for long bursts. Long bursts are detected in the dark category in the same fraction as pre-Swift bursts. Interesting cases of long bursts that are detected in the optical, and yet have a low enough optical-to-X-ray ratio to be classified as dark, are discovered. For the prompt emission, short and long bursts have different average tracks on flux versus fluence plots. In Swift, GRB detections tend to be fluence-limited for short bursts and flux-limited for long events.
Context.We present the results of a set of observations of nine TeV detected BL Lac objects performed by the XRT and UVOT detectors on board the Swift satellite between March and December 2005. ...Aims.Our main goal is the accurate measurement of the spectral shape of TeV detected BL Lacs. Particular attention was given to the presence of intrinsic spectral curvature in the X-ray band. Methods.To perform our X-ray spectral analysis we have assumed either a log-parabolic or a simple power-law model. Results.The X-ray data of many objects in our sample clearly show highly significant spectral curvature. However, in sources with spectral energy distribution peaked at energies lower than ~0.1 keV the X-ray spectrum is steep and generally consistent with a power law. In most of these cases poor statistics did not allow us to obtain tight constraints on the spectral curvature. We have used UVOT observations to verify if X-ray spectra can be extrapolated to lower frequencies and to search for multiple emission components. Conclusions.The results of our analysis are useful for the study of possible signatures of statistical acceleration processes predicting intrinsically curved spectra and for modelling the SED of BL Lacertae objects up to TeV energies where a corresponding curvature is likely to be present.
Two short (< 2 s) γ-ray bursts (GRBs) have recently been localized and fading afterglow counterparts detected. The combination of these two results left unclear the nature of the host galaxies of the ...bursts, because one was a star-forming dwarf, while the other was probably an elliptical galaxy. Here we report the X-ray localization of a short burst (GRB 050724) with unusual γ-ray and X-ray properties. The X-ray afterglow lies off the centre of an elliptical galaxy at a redshift of z = 0.258 (ref. 5), coincident with the position determined by ground-based optical and radio observations. The low level of star formation typical for elliptical galaxies makes it unlikely that the burst originated in a supernova explosion. A supernova origin was also ruled out for GRB 050709 (refs 3, 31), even though that burst took place in a galaxy with current star formation. The isotropic energy for the short bursts is 2-3 orders of magnitude lower than that for the long bursts. Our results therefore suggest that an alternative source of bursts-the coalescence of binary systems of neutron stars or a neutron star-black hole pair-are the progenitors of short bursts.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We present observations of GRB 060124, the first event for which both the prompt and the afterglow emission could be observed simultaneously and in their entirety by the three Swift instruments. ...Indeed, Swift-BAT triggered on a precursor similar to 570 s before the main burst peak, and this allowed Swift to repoint the narrow field instruments to the burst position similar to 350 s before the main burst occurred. GRB 060124 also triggered Konus-Wind, which observed the prompt emission in a harder gamma-ray band (up to 2 MeV). Thanks to these exceptional circumstances, the temporal and spectral properties of the prompt emission can be studied in the optical, X-ray and gamma-ray ranges. While the X-ray emission (0.2-10 keV) clearly tracks the gamma-ray burst, the optical component follows a different pattern, likely indicating a different origin, possibly the onset of external shocks. The prompt GRB spectrum shows significant spectral evolution, with both the peak energy and the spectral index varying. As observed in several long GRBs, significant lags are measured between the hard- and low-energy components, showing that this behaviour extends over 3 decades in energy. The GRB peaks are also much broader at soft energies. This is related to the temporal evolution of the spectrum, and can be accounted for by assuming that the electron spectral index softened with time. The burst energy ( E_{\rm iso} \sim 5 \times 10 super(53) erg) and average peak energy ( E_{\rm p} \sim 300 keV) make GRB 060124 consistent with the Amati relation. The X-ray afterglow is characterized by a decay which presents a break at t_{\rm b} \sim 10 one fourth s.
Short Gamma-ray bursts: a bimodal origin? Salvaterra, R.; Cerutti, A.; Chincarini, G. ...
Monthly notices of the Royal Astronomical Society. Letters,
July 2008, Letnik:
388, Številka:
1
Journal Article
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Short-hard Gamma-Ray Bursts (SGRBs) are currently thought to arise from gravitational wave driven coalescences of double neutron star systems forming either in the field or dynamically in globular ...clusters. For both channels, we fit the peak flux distribution of BATSE SGRBs to derive the local burst formation rate and luminosity function. We then compare the resulting redshift distribution with Swift 2-yr data, showing that both formation channels are needed in order to reproduce the observations. Double neutron stars forming in globular clusters are found to dominate the distribution at z≲ 0.3, whereas the field population from primordial binaries can account for the high-z SGRBs. This result is not in contradiction with the observed host galaxy type of SGRBs.
Aims. The association of Type Ic supernovae (SNe) with long-duration gamma-ray bursts (GRBs) is well established. We endeavor, through accurate ground-based observational campaigns, to characterize ...these SNe at increasingly high redshifts. Methods. We obtained a series of optical photometric and spectroscopic observations of the Type Ic SN 2012bz associated with the Swift long-duration GRB 120422A (redshift z = 0.283) using the 3.6-m TNG and the 8.2-m VLT telescopes during the time interval between 4 and 36 days after the burst. Results. The peak times of the light curves of SN 2012bz in various optical filters differ, with the B-band and i’-band light curves reaching maximum at 9 ± 4 and 23 ± 3 rest-frame days, respectively. The bolometric light curve has been derived from individual bands photometric measurements, but no correction for the unknown contribution in the near-infrared (probably around 10–15%) has been applied. Therefore, the present light curve should be considered as a lower limit to the actual UV-optical-IR bolometric light curve. This pseudo-bolometric curve reaches its maximum (Mbol = −18.56 ± 0.06) at 13 ± 1 rest-frame days; it is similar in shape and luminosity to the bolometric light curves of the SNe associated with z < 0.2 GRBs and more luminous than those of SNe associated with X-ray flashes (XRFs). A comparison with the model generated for the bolometric light curve of SN 2003dh suggests that SN 2012bz produced only about 15% less 56Ni than SN 2003dh, about 0.35 M⊙. Similarly the VLT spectra of SN 2012bz, after correction for Galactic extinction and for the contribution of the host galaxy, suggest comparable explosion parameters with those observed in SN 2003dh (EK ~ 3.5 × 1052 erg, Mej ~ 7 M⊙) and a similar progenitor mass (~25–40 M⊙). GRB 120422A is consistent with the Epeak − Eiso and the EX,iso − Eγ,iso − Epeak relations. GRB 120422A / SN 2012bz shows the GRB-SN connection at the highest redshift so far accurately monitored both photometrically and spectroscopically.
Short duration ($\la $2 s) Gamma-ray bursts (GRBs) have been a mystery since their discovery. Until May 2005 very little was known about short GRBs, but this situation has changed rapidly in the last ...few months since the Swift and HETE-2 satellites have made it possible to discover X-ray and optical counterparts to these sources. Positional associations indicate that short GRBs arise in close-by galaxies ($z<0.7$). Here we report on a detailed study of the short GRB 050724 X-ray afterglow. This burst shows strong flaring variability in the X-ray band. It clearly confirms early suggestions of X-ray activity in the 50–100 s time interval following the GRB onset seen with BATSE. Late flare activity is also observed. These observations support the idea that flares are related to the inner engine for short GRBs, as well as long GRBs.
Long gamma-ray bursts (GRBs) are important for the study of the Universe near and beyond the epoch of reionization. In this paper, we describe the characteristics of an ‘ideal’ instrument that can be ...used to search for GRBs at z≥ 6–10. We find that the detection of these objects requires soft-band detectors with high sensitivity and a moderately large field of view. In light of these results, we compare available and planned GRB missions, deriving conservative predictions of the number of high-redshift GRBs detectable by these instruments along with the maximum accessible redshift. We show that the Swift satellite will be able to detect various GRBs at z≥ 6, and likely at z≥ 10 if the trigger threshold is decreased by a factor of ∼2. Furthermore, we find that INTEGRAL and GLAST are not the best tools to detect bursts at z≥ 6, the former being limited by the small field of view, and the latter by its hard energy band and relatively low sensitivity. Finally, future missions (SVOM, EDGE and, in particular, EXIST) will provide a good sample of GRBs at z≥ 6 within a few years of operation.
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