We present the analysis of the extraordinarily bright gamma-ray burst (GRB) 130427A under the hypothesis that the GRB central engine is an accretion-powered magnetar. In this framework, initially ...proposed to explain GRBs with precursor activity, the prompt emission is produced by accretion of matter on to a newly born magnetar, and the observed power is related to the accretion rate. The emission is eventually halted if the centrifugal forces are able to pause accretion. We show that the X-ray and optical afterglow is well explained as the forward shock emission with a jet break plus a contribution from the spin-down of the magnetar. Our modelling does not require any contribution from the reverse shock, that may still influence the afterglow light curve at radio and mm frequencies, or in the optical at early times. We derive the magnetic field (B ∼ 1016 G) and the spin period (P ∼ 20 ms) of the magnetar and obtain an independent estimate of the minimum luminosity for accretion. This minimum luminosity results well below the prompt emission luminosity of GRB 130427A, providing a strong consistency check for the scenario where the entire prompt emission is the result of continuous accretion on to the magnetar. This is in agreement with the relatively long spin period of the magnetar. GRB 130427A was a well-monitored GRB showing a very standard behaviour and, thus, is a well-suited benchmark to show that an accretion-powered magnetar gives a unique view of the properties of long GRBs.
X-ray flare candidates in short gamma-ray bursts Margutti, R.; Chincarini, G.; Granot, J. ...
Monthly Notices of the Royal Astronomical Society,
November 2011, Letnik:
417, Številka:
3
Journal Article
Recenzirano
Odprti dostop
We present the first systematic study of X-ray flare candidates in short gamma-ray bursts (SGRBs) exploiting the large 6-year Swift data base with the aim to constrain the physical nature of such ...fluctuations. We find that flare candidates appear in different types of SGRB host galaxy environments and show no clear correlation with the X-ray afterglow lifetime; flare candidates are detected both in SGRBs with a bright extended emission in the soft γ-rays and in SGRBs which do not show such component. We furthermore show that SGRB X-ray flare candidates only partially share the set of observational properties of long GRB (LGRB) flares. In particular, the main parameter driving the duration evolution of X-ray variability episodes in both classes is found to be the elapsed time from the explosion, with very limited dependence on the different progenitors, environments, central engine lifetimes, prompt variability time-scales and energy budgets. On the contrary, SGRB flare candidates significantly differ from LGRB flares in terms of peak luminosity, isotropic energy, flare-to-prompt luminosity ratio and relative variability flux. However, these differences disappear when the central engine time-scales and energy budget are accounted for, suggesting that (i) flare candidates and prompt pulses in SGRBs likely have a common origin; (ii) similar dissipation and/or emission mechanisms are responsible for the prompt and flare emission in LGRBs and SGRBs, with SGRBs being less energetic albeit faster evolving versions of the long class. Finally, we show that in strict analogy to the SGRB prompt emission, flares candidates fall off the lag-luminosity relation defined by LGRBs, thus strengthening the SGRB flare-prompt pulse connection.
Science with the Cherenkov Telescope Array The Cta Consortium, The Cta Consortium
The Astrophysical journal. Supplement series,
01/2019, Letnik:
240, Številka:
2
eBook, Journal Article, Book
Recenzirano
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This book summarizes the science to be carried out by the upcoming Cherenkov Telescope Array, a major ground-based gamma-ray observatory that will be constructed over the next six to eight years. The ...major scientific themes, as well as core program of key science projects, have been developed by the CTA Consortium, a collaboration of scientists from many institutions worldwide. CTA will be the major facility in high-energy and very high-energy photon astronomy over the next decade and beyond. CTA will have capabilities well beyond past and present observatories. Thus, CTA's science program is expected to be rich and broad and will complement other major multiwavelength and multimessenger facilities. This book is intended to be the primary resource for the science case for CTA and it thus will be of great interest to the broader physics and astronomy communities. The electronic version (e-book) is available in open access.
On the environment of short gamma-ray bursts Kopač, D; D'Avanzo, P; Melandri, A ...
Monthly Notices of the Royal Astronomical Society,
11 August 2012, Letnik:
424, Številka:
3
Journal Article
Recenzirano
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Abstract
In this paper, we present a sample of ten short gamma-ray bursts (GRBs) with a robust redshift determination, discovered by the Swift satellite up to 2011 January. We measure their X-ray ...absorbing column densities and collect data on the host galaxy offsets. We find evidence for intrinsic absorption and no correlation between the intrinsic absorbing column density and the projected offset of the GRB from its host galaxy centre. We find that the properties in the gamma regime (T
90, fluence and 1-s peak photon flux) of short GRBs with 'bright' and 'faint' X-ray afterglow likely disfavour different prompt emission mechanisms. The host galaxy offset and GRB duration (T
90) do not correlate. Instead, there is a hint of anticorrelation between the effective radius normalized host galaxy offset and T
90. Finally, we examine the properties of short GRBs with short-lived and long-lived X-ray afterglows, finding that some short GRBs with short-lived X-ray afterglows have their optical afterglow detected. In light of this, the X-ray afterglow duration does not seem to be a unique indicator of a specific progenitor and/or environment for short GRBs.
The coincidental detection of the gravitational wave event GW 170817 and the gamma-ray burst GRB 170817A marked the advent of multi-messenger astronomy and represented a milestone in the study of ...GRBs. Significant progress in this field is expected in the coming years with the increased sensitivity of gravitational waves detectors and the launch of new facilities for the high-energy survey of the sky. In this context, the launch of
SVOM
in mid-2022, with its two wide-field high-energy instruments ECLAIRs and GRM, will foster the possibilities of coincidental transient detection with gravitational waves and gamma-rays events. The purpose of this paper is to assess the ability of
SVOM
/ECLAIRs to detect and quickly characterize high-energy transients in the local Universe (z ≤ 0.3), and to discuss the contribution of this instrument to multi-messenger astronomy and to gamma-ray burst (GRB) astrophysics in the 2020’s. A list of local HE transients, along with their main characteristics, is constructed through an extensive literature survey. This list includes 41 transients: 24 long GRBs, 10 short GRBs and 7 SGR Giant Flares. The detectability of these transients with ECLAIRs is assessed with detailed simulations using tools developed for the
SVOM
mission, including a GEANT4 simulation of the energy response and a simulated trigger algorithm representative of the onboard trigger algorithm.
SVOM
/ECLAIRs would have been able to detect 88% of the short high-energy transients in our list: 22 out of 24 long GRBs, 8 out of 10 short GRBs and 6 out of 7 SGR Giant Flares. The SNR for almost all detections will be sufficiently high to allow the on-board ECLAIRs trigger algorithm to derive the localisation of the transient, transmitting it to the
SVOM
satellite and ground-based instruments. Coupled with the anti-solar pointing strategy of
SVOM
, this will enable an optimal follow-up of the events, allowing the observation of their afterglows, supernovae/kilonovae counterparts, and host galaxies. We conclude the paper with a discussion of the unique contribution expected from
SVOM
and of the possibility of simultaneous GW detection for each type of transient in our sample.
Context. One of the most intriguing features revealed by the Swift satellite are flares that are superimposed on the gamma-ray burst (GRB) X-ray light curves. The vast majority of flares occurs ...before 1000 s, but some of them can be found up to 106 s after the main event. Aims. We shed light on late-time (i.e. with peak time tpk ≳ 1000 s) flaring activity. We address the morphology and energetic of flares in the window ~103−106 s to put constraints on the temporal evolution of the flare properties and to identify possible differences in the mechanism producing the early and late-time flaring emission, if any. This requires the complete understanding of the observational biases affecting the detection of X-ray flares superimposed on a fading continuum at t > 1000 s. Methods. We consider all Swift GRBs that exhibit late-time flares. Our sample consists of 36 flares, 14 with redshift measurements. We inherit the strategy of data analysis from Chincarini et al. (2010) in order to make a direct comparison with the early-time flare properties. Results. The morphology of the flare light curve is the same for both early-time and late-time flares, but they differ energetically. The width of late-time flares increases with time similarly to the early-time flares. Simulations confirmed that the increase of the width with time is not owing to the decaying statistics, at least up to 104 s. The energy output of late-time flares is one order of magnitude lower than the early-time flare one,and is ~1%Eprompt. The evolution of the peak luminosity as well as the distribution of the peak-flux-to-continuum ratio for late-time flares indicate that the flaring emission is decoupled from the underlying continuum, differently from early-time flares/steep decay. A sizable fraction of late-time flares are compatible with afterglow variability. Conclusions. The internal shock origin seems the most promising explanation for flares. However, some differences that emerge between late- and early-time flares suggest that there could be no unique explanation about the nature of late-time flares.
Context. The cosmological origin of gamma-ray bursts (GRBs) has been firmly established, with redshifts up to $z = 6.29$. They are possible candidates for use as “distance indicators” for testing ...cosmological models in a redshift range hardly achievable by other cosmological probes. Asserting the validity of the empirical relations among GRB observables is now crucial for their calibration. Aims. Motivated by the relation proposed by Amati and collaborators, we look within the “fireshell” model for a relation between the peak energy Ep of the $\nu F_\nu$ total time-integrated spectrum of the afterglow and the total energy of the afterglow Eaft, which in our model encompasses and extends the prompt emission. Methods. The fit within the fireshell model, as for the “canonical” GRB 050315, uses the complete arrival time coverage given by the Swift satellite. It is performed simultaneously, self-consistently, and recursively in the four BAT energy bands (15–25 keV, 25–50 keV, 50–100 keV, and 100–150 keV), as well as in the XRT one (0.2–10 keV). It uniquely determines the two free parameters characterizing the GRB source, the total energy $E^{{\rm e}^\pm}_{\rm tot}$ of the e$^{\pm}$ plasma and its baryon loading B, as well as the effective CircumBurst Medium (CBM) distribution. We can then build two sets of “gedanken” GRBs varying the total energy of the electron-positron plasma $E^{{\rm e}^\pm}_{\rm tot}$ and keeping the same baryon loading B of GRB 050315. The first set assumes the one obtained in the fit of GRB 050315 for the effective CBM density. The second set assumes instead a constant CBM density equal to the average value of the GRB 050315 prompt phase. Results. For the first set of “gedanken” GRBs we find a relation $E_{\rm p} \propto (E_{\rm aft})^a$, with $a = 0.45 \pm 0.01$, whose slope strictly agrees with the Amati one. Such a relation, in the limit $B \to 10^{-2}$, coincides with the Amati one. Instead, no correlation is found in the second set of “gedanken” GRBs. Conclusions. Our analysis excludes the proper GRB (P-GRB) from the prompt emission, extends all the way to the latest afterglow phases, and is independent of the assumed cosmological model, since all “gedanken” GRBs are at the same redshift. The Amati relation, on the other hand, includes the P-GRB, focuses only on the prompt emission, being therefore influenced by the instrumental threshold that fixes the end of the prompt emission, and depends on the assumed cosmology. This might explain the intrinsic scatter observed in the Amati relation.
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