On the nature of X-ray flashes

Barraud, C.; Daigne, F.; Mochkovitch, R.; Atteia, J. L.

doi:10.1051/0004-6361:20041572

Cited by 57 publications

(77 citation statements)

References 33 publications

(55 reference statements)

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“…This confirms that a low fraction of accelerated electrons is necessary to have a synchrotron peak in the gamma-ray range (Daigne & Mochkovitch 1998) and that X-ray flashes and X-ray rich gamma-ray bursts can be produced by internal shocks within "clean fireballs", i.e. outflows having a high Lorentz factorΓ (small baryonic pollution) and a small contrast κ (Barraud et al 2005). There are two situations when this scaling for the synchrotron peak is not valid anymore:…”

Section: The Spectral Shape Of Internal Shock Emissionsupporting

confidence: 61%

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Prompt high-energy emission from gamma-ray bursts in the internal shock model

Bošnjak¹,

Daigne

Dubus

2009

A&A

Self Cite

171

229

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Context. Gamma-ray bursts (GRB) are powerful, short duration events with a spectral luminosity peaking in the keV-MeV (BATSE) range. The prompt emission is thought to arise from electrons accelerated in internal shocks propagating within a highly relativistic outflow. Aims. The launch of Fermi offers the prospect of observations with unprecedented sensitivity in high-energy (HE, >100 MeV) gammarays. The aim is to explore the predictions for HE emission from internal shocks, taking into account both dynamical and radiative aspects, and to deduce how HE observations constrain the properties of the relativistic outflow. Methods. The prompt GRB emission is modelled by combining a time-dependent radiative code, solving for the electron and photon distributions, with a dynamical code giving the evolution of the physical conditions in the shocked regions of the outflow. Synthetic lightcurves and spectra are generated and compared to observations. Results. The HE emission deviates significantly from analytical estimates, which tend to overpredict the IC component, when the time dependence and full cross-sections are included. The exploration of the parameter space favors the case where the dominant process in the BATSE range is synchrotron emission. The HE component becomes stronger for weaker magnetic fields. The HE lightcurve can display a prolonged pulse duration due to IC emission, or even a delayed peak compared to the BATSE range. Alternatively, having dominant IC emission in the BATSE range requires most electrons to be accelerated into a steep power-law distribution and implies strong second order IC scattering. In this case, the BATSE and HE lightcurves are very similar. Conclusions. The combined dynamical and radiative approach allows a firm appraisal of GRB HE prompt emission. A diagnostic procedure is presented to identify from observations the dominant emission process and derive constrains on the bulk Lorentz factor, particle density and magnetic field of the outflow.

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Section: The Spectral Shape Of Internal Shock Emissionsupporting

confidence: 61%

“…Rees & Mészáros 1994;Barraud et al 2005;Daigne & Mochkovitch 2007;Kumar & McMahon 2008). We consider the ejection of two equal mass relativistic shells with Lorentz factor Γ 1 and Γ 2 from the central source.…”

Section: Dynamical Evolution During the Internal Shock Phasementioning

confidence: 99%

Prompt high-energy emission from gamma-ray bursts in the internal shock model

Bošnjak¹,

Daigne

Dubus

2009

A&A

Self Cite

171

229

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“…It is then possible to identify the pertinent range of the parameters leading to steep low-energy slopes: the properties of the relativistic outflow (Lorentz factor, kinetic energy) and the parameters describing the microphysics at work in shocked regions (particle acceleration, magnetic field amplification). We consider first collisions between two equal-mass shells (Barraud et al 2005;Bošnjak et al 2009). More realistic outflows are considered in the next subsection.…”

Section: Impact On the Microphysics Parameters In Internal Shocksmentioning

confidence: 99%

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

Daigne

Bošnjak

Dubus

2011

A&A

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206

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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“…Alternatively, under the internal shock model a clean fireball can naturally accommodate the creation of an XRF ( Zhang & Meszaros 2002), especially if the contrast in Lorentz factor between shells is small, which leads to inefficient energy dissipation (e.g., Barraud et al 2005). In this case, the decay index of ¼ 0:82 can also naturally be accommodated, although only if it is observed prior to the jet break.…”

Section: Ground-based Observationsmentioning

confidence: 99%

“…GRBs whose Lorentz factor is modified due to the effects of baryon loading within the jet. In external shock models high baryon loading (the so-called dirty fireball) can create an XRF (Dermer et al 1999;Ramirez-Ruiz & Lloyd-Roming 2002;Huang et al 2002), while in contrast for internal shock models very clean jets produce large X-ray fluxes Barraud et al 2005).…”

Section: Introductionmentioning

confidence: 99%

The FirstSwiftX‐Ray Flash: The Faint Afterglow of XRF 050215B

Levan

Osborne

Tanvir

et al. 2006

ApJ

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We present the discovery of XRF 050215B and its afterglow. The burst was detected by the Swift BAT during the check-out phase, and observations with the X-Ray Telescope began approximately 30 minutes after the burst. These observations found a faint, slowly fading X-ray afterglow near the center of the error box as reported by the BAT. Infrared data obtained at UKIRT after 10 hr also revealed a very faint K-band afterglow. The afterglow appears unusual since it is very faint, especially in the infrared, with K > 20 only 9 hr postburst. The X-ray and infrared light curves exhibit a slow, monotonic decay with $ 0:8 and no evidence for a steepening associated with the jet break to 10 days postburst. We discuss possible explanations for the faintness and slow decay in the context of present models for the production of X-ray flashes.

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On the nature of X-ray flashes

Cited by 57 publications

References 33 publications

Prompt high-energy emission from gamma-ray bursts in the internal shock model

Prompt high-energy emission from gamma-ray bursts in the internal shock model

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

The FirstSwiftX‐Ray Flash: The Faint Afterglow of XRF 050215B

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