A general scheme for modelling γ-ray burst prompt emission

Kumar, Pawan; McMahon, E.

doi:10.1111/j.1365-2966.2007.12621.x

Cited by 109 publications

(137 citation statements)

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“…Rees & Meszaros, 1994;Piran, 2005;Meszaros, 2006, and references therein) also encounters difficulties: Kumar & McMahon (2008) have shown that the traditional synchrotron model can be applied to the prompt emission only if it occurs at r > 10 17 cm. A proposed solution to this problem, via the inverse Compton process, suffers from an "energy crisis" (see e.g.…”

Section: Open Issues In Current Theoretical Modelsmentioning

confidence: 99%

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GRB060614: a “fake” short GRB from a merging binary system

Caito

Bernardini

Bianco

et al. 2009

A&A

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Context. GRB060614 observations by VLT and by Swift have infringed the traditionally accepted gamma-ray burst (GRB) collapsar scenario that purports the origin of all long duration GRBs from supernovae (SN). GRB060614 is the first nearby long duration GRB clearly not associated with a bright Ib/c SN. Moreover, its duration (T 90 ∼ 100 s) makes it hardly classifiable as a short GRB. It presents strong similarities with GRB970228, the prototype of a new class of "fake" short GRBs that appear to originate from the coalescence of binary neutron stars or white dwarfs spiraled out into the galactic halo. Aims. Within the "canonical" GRB scenario based on the "fireshell" model, we test if GRB060614 can be a "fake" or "disguised" short GRB. We model the traditionally termed "prompt emission" and discriminate the signal originating from the gravitational collapse leading to the GRB from the process occurring in the circumburst medium (CBM). Methods. We fit GRB060614 light curves in Swift's BAT (15 − 150 keV) and XRT (0.2 − 10 keV) energy bands. Within the fireshell model, light curves are formed by two well defined and different components: the proper-GRB (P-GRB), emitted when the fireshell becomes transparent, and the extended afterglow, due to the interaction between the leftover accelerated baryonic and leptonic shell and the CBM. Results. We determine the two free parameters describing the GRB source within the fireshell model: the total e ± plasma energy (E e ± tot = 2.94 × 10 51 erg) and baryon loading (B = 2.8 × 10 −3 ). A small average CBM density ∼ 10 −3 particles/cm 3 is inferred, typical of galactic halos. The first spikelike emission is identified with the P-GRB and the following prolonged emission with the extended afterglow peak. We obtain very good agreement in the BAT (15 − 150 keV) energy band, in what is traditionally called "prompt emission", and in the XRT (0.2 − 10 keV) one. Conclusions. The anomalous GRB060614 finds a natural interpretation within our canonical GRB scenario: it is a "disguised" short GRB. The total time-integrated extended afterglow luminosity is greater than the P-GRB one, but its peak luminosity is smaller since it is deflated by the peculiarly low average CBM density of galactic halos. This result points to an old binary system, likely formed by a white dwarf and a neutron star, as the progenitor of GRB060614 and well justifies the absence of an associated SN Ib/c. Particularly important for further studies of the final merging process are the temporal structures in the P-GRB down to 0.1 s.

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Section: Open Issues In Current Theoretical Modelsmentioning

confidence: 99%

“…The attempt to use the internal shock model for the prompt emission (see e.g. Rees & Meszaros, 1994;Piran, 2005;Meszaros, 2006, and references therein) only applies to regions where r > 10 17 cm (Kumar & McMahon, 2008).…”

Section: Introductionmentioning

confidence: 99%

GRB060614: a “fake” short GRB from a merging binary system

Caito

Bernardini

Bianco

et al. 2009

A&A

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“…It is an alternative to the SSC scenario (see e.g. Panaitescu & Mészáros 2000;Baring & Braby 2004;Kumar & McMahon 2008), to the comptonization scenario Ghisellini & Celotti 1999), or to other propositions to modify the standard synchrotron radiation, related to the timescale of the acceleration process (Stern & Poutanen 2004;Asano & Terasawa 2009), the pitch-angle distribution of electrons (Lloyd-Ronning & Petrosian 2002) or the small scale structure of the magnetic field (Medvedev 2000;Pe'er & Zhang 2006). A summary of the measurements of the low-energy photon index α and its distribution in GRBs is given in Sect.…”

Section: Introductionmentioning

confidence: 99%

Reconciling observed gamma-ray burst prompt spectra with synchrotron radiation?

Daigne

Bošnjak

Dubus

2011

A&A

207

253

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Context. Gamma-ray burst emission in the prompt phase is often interpreted as synchrotron radiation from high-energy electrons accelerated in internal shocks. Fast synchrotron cooling of a power-law distribution of electrons leads to the prediction that the slope below the spectral peak has a photon index α = −3/2 (N(E) ∝ E α ). However, this differs significantly from the observed median value α ≈ −1. This discrepancy has been used to argue against this scenario. Aims. We quantify the influence of inverse Compton (IC) and adiabatic cooling on the low energy slope to understand whether these processes can reconcile the observed slopes with a synchrotron origin. Methods. We use a time-dependent code developed to calculate the GRB prompt emission within the internal shock model. The code follows both the shock dynamics and electron energy losses and can be used to generate lightcurves and spectra. We investigate the dependence of the low-energy slope on the parameters of the model and identify parameter regions that lead to values α > −3/2. Results. Values of α between −3/2 and −1 are reached when electrons suffer IC losses in the Klein-Nishina regime. This does not necessarily imply a strong IC component in the Fermi/LAT range (GeV) because scatterings are only moderately efficient. Steep slopes require that a large fraction (10−30%) of the dissipated energy is given to a small fraction ( < ∼ 1%) of the electrons and that the magnetic field energy density fraction remains low ( < ∼ 0.1%). Values of α up to −2/3 can be obtained with relatively high radiative efficiencies (>50%) when adiabatic cooling is comparable with radiative cooling (marginally fast cooling). This requires collisions at small radii and/or with low magnetic fields. Conclusions. Amending the standard fast cooling scenario to account for IC cooling naturally leads to values α up to −1. Marginally fast cooling may also account for values of α up to −2/3, although the conditions required are more difficult to reach. About 20% of GRBs show spectra with slopes α > −2/3. Other effects, not investigated here, such as a thermal component in the electron distribution or pair production by high-energy gamma-ray photons may further affect α. Still, the majority of observed GRB prompt spectra can be reconciled with a synchrotron origin, constraining the microphysics of mildly relativistic internal shocks.

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“…integrating the scattering cross-section over the electron energy spectrum assuming fast cooling electrons Kumar & McMahon 2008), substitutes for γm in terms of ǫe and γc in terms of ǫB, and arrives at eq-5.…”

Section: For the First Order Scattering : Y Thmentioning

confidence: 99%

“…Nonetheless, synchrotron self-Compton (SSC) process has been also suggested (e.g. (Panaitescu & Mészáros 2000;Kumar & McMahon 2008)). For such models, one would expect a synchrotron component peaking in the lower energy band, and prompt optical emission is expected.…”

Section: Introductionmentioning

confidence: 99%

Gamma Ray Burst Prompt Emission Variability in Synchrotron and Synchrotron Self-Compton Lightcurves

Resmi

Zhang

McEnery³

et al. 2011

AIP Conference Proceedings

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Gamma Ray Burst prompt emission is believed to originate from electrons accelerated in a highly relativistic outflow. "Internal shocks" due to collisions between shells ejected by the central engine is a leading candidate for electron acceleration. While synchrotron radiation is generally invoked to interpret prompt gamma-ray emission within the internal shock model, synchrotron self-Compton (SSC) is also considered as a possible candidate of radiation mechanism. In this case, one would expect a synchrotron emission component at low energies, and the naked-eye GRB 080319B has been considered as such an example. In the view that the gamma-ray lightcurve of GRB 080319B is much more variable than its optical counterpart, in this paper we study the relative variability between the synchrotron and SSC components. We develop a "top-down" formalism by using observed quantities to infer physical parameters, and subsequently to study the temporal structure of synchrotron and SSC components of a GRB. We complement the formalism with a "bottom-up" approach where the synchrotron and SSC lightcurves are calculated through a Monte-Carlo simulations of the internal shock model. Both approaches lead to the same conclusion. Small variations in the synchrotron lightcurve can be only moderately amplified in the SSC lightcurve. The SSC model therefore cannot adequately interpret the gamma-ray emission properties of GRB 080319B.

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A general scheme for modelling γ-ray burst prompt emission

Cited by 109 publications

References 83 publications

GRB060614: a “fake” short GRB from a merging binary system

GRB060614: a “fake” short GRB from a merging binary system

Reconciling observed gamma-ray burst prompt spectra with synchrotron radiation?

Gamma Ray Burst Prompt Emission Variability in Synchrotron and Synchrotron Self-Compton Lightcurves

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