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GRB spectrum from gradual dissipation in a magnetized outflow

Gill, Ramandeep and Granot, Jonathan and Beniamini, Paz (2020) GRB spectrum from gradual dissipation in a magnetized outflow. Monthly Notices of the Royal Astronomical Society, 499 (1). pp. 1356-1372. ISSN 0035-8711. doi:10.1093/mnras/staa2870.

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Modelling of many gamma-ray burst prompt emission spectra sometimes requires a (quasi) thermal spectral component in addition to the Band function that sometimes leads to a double-hump spectrum, the origin of which remains unclear. In photospheric emission models, a prominent thermal component broadened by sub-photospheric dissipation is expected to be released at the photospheric radius, r_(ph) ∼10¹² cm. We consider an ultra-relativistic strongly magnetized steady outflow with a striped-wind magnetic-field structure undergoing gradual and continuous magnetic energy dissipation at r < r_s that heats and accelerates the flow to a bulk Lorentz factor Γ(r) = Γ_∞min [1, (r/r_s)^(1/3)], where typically r_(ph) < r_s. Similar dynamics and energy dissipation rates are also expected in highly variable magnetized outflows without stripes/field-reversals. Two modes of particle energy injection are considered: (a) power-law electrons, e.g. accelerated by magnetic reconnection, and (b) distributed heating of all electrons (and e±-pairs), e.g. due to magnetohydrodynamic instabilities. Steady-state spectra are obtained using a numerical code that evolves coupled kinetic equations for a photon-electron-positron plasma. We find that (i) the thermal component consistently peaks at (1+z)E_(pk) ∼ 0.2−1 MeV, for a source at redshift z, and becomes sub-dominant if the total injected energy density exceeds the thermal one, (ii) power-law electrons cool mainly by synchrotron emission whereas mildly relativistic and almost monoenergetic electrons in the distributed heating scenario cool by Comptonization on thermal peak photons, (iii) both scenarios can yield a low-energy break, and (iv) the ∼0.5(1+z)⁻¹ keV X-ray emission is suppressed in scenario (a), whereas it is expected in scenario (b). Energy-dependent linear polarization can differentiate between the two particle heating scenarios.

Item Type:Article
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URLURL TypeDescription Paper
Gill, Ramandeep0000-0003-0516-2968
Granot, Jonathan0000-0001-8530-8941
Beniamini, Paz0000-0001-7833-1043
Additional Information:© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2020 September 15. Received 2020 September 15; in original form 2020 August 24. RG thanks Sylvain Guiriec for useful discussions. RG and JG’s research was supported by the ISF-NSFC joint research programme (grant no. 3296/19). PB’s research was funded in part by the Gordon and Betty Moore Foundation through grant no. GBMF5076. DATA AVAILABILITY. No new data were generated or analysed in support of this research.
Funding AgencyGrant Number
Israel Science FoundationUNSPECIFIED
National Natural Science Foundation of China3296/19
Gordon and Betty Moore FoundationGBMF5076
Subject Keywords:acceleration of particles, magnetic reconnection, MHD, radiation mechanisms: non-thermal, relativistic processes, gamma-ray burst: general
Issue or Number:1
Record Number:CaltechAUTHORS:20201204-161632328
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Official Citation:Ramandeep Gill, Jonathan Granot, Paz Beniamini, GRB spectrum from gradual dissipation in a magnetized outflow, Monthly Notices of the Royal Astronomical Society, Volume 499, Issue 1, November 2020, Pages 1356–1372,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:106927
Deposited By: George Porter
Deposited On:05 Dec 2020 01:54
Last Modified:16 Nov 2021 18:58

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