J/A+A/640/A91 Fitting function for GRB MeV spectra (Yassine+, 2020)
A new fitting function for GRB MeV spectra based on the internal shock
synchrotron model.
Yassine M., Piron F., Daigne F., Mochkovitch R., Longo F., Omodei N.,
Vianello G.
<Astron. Astrophys., 640, A91 (2020)>
=2020A&A...640A..91Y 2020A&A...640A..91Y (SIMBAD/NED BibCode)
ADC_Keywords: Gamma rays ; GRB
Keywords: gamma-ray burst: general - radiation mechanisms: non-thermal
Abstract:
The physical origin of the gamma-ray burst (GRB) prompt emission is
still a subject of debate. Internal shock models have been widely
explored, owing to their ability to explain most of the high-energy
properties of this emission phase. While the Band function or other
phenomenological functions are commonly used to fit GRB prompt
emission spectra, we propose a new parametric function that is
inspired by an internal shock physical model. We use this function as
a proxy of the model to compare it easily to GRB observations.
We built a parametric function that represents the spectral form of
the synthetic bursts provided by our internal shock synchrotron model
(ISSM). We simulated the response of the Fermi instruments to the
synthetic bursts and fit the obtained count spectra to validate the
ISSM function. Then, we applied this function to a sample of 74 bright
GRBs detected by the Fermi GBM, and we computed the width of their
spectral energy distributions around their peak energy. For
comparison, we also fit the phenomenological functions that are
commonly used in the literature. Finally, we performed a time-resolved
analysis of the broadband spectrum of GRB 090926A, which was jointly
detected by the Fermi GBM and LAT. This spectrum has a complex shape
and exhibits a power-law component with an exponential cutoff at high
energy, which is compatible with inverse Compton emission attenuated
by gamma-ray internal absorption.
This work proposes a new parametric function for spectral fitting that
is based on a physical model. The ISSM function reproduces 81% of the
spectra in the GBM bright GRB sample, versus 59% for the Band
function, for the same number of parameters. It gives also relatively
good fits to the GRB 090926A spectra. The width of the MeV spectral
component that is obtained from the fits of the ISSM function is
slightly larger than the width from the Band fits, but it is smaller
when observed over a wider energy range. Moreover, all of the 74
analyzed spectra are found to be significantly wider than the
synthetic synchrotron spectra. We discuss possible solutions to
reconcile the observations with the internal shock synchrotron model,
such as an improved modeling of the shock microphysics or more
accurate spectral measurements at MeV energies.
Description:
An analysis of a GBM sample of 74 bursts with ISSM and Band is
presented.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea4.dat 94 148 Results of the Band and ISSM spectral fits to
GBM data for the prompt emission of 74 GRBs
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See also:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm : GBM catalog
Byte-by-byte Description of file: tablea4.dat
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Bytes Format Units Label Explanations
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1- 12 A12 --- Name GRB name, GRBNNNNNNNNN
14- 17 A4 --- Models [Band/ISSM] Model name
19- 22 I4 --- Ep Peak energy
24- 26 I3 --- e_Ep Peak energy error
28- 32 F5.2 --- alpha Photon index
34- 37 F4.2 --- e_alpha Photon index error
39- 43 F5.2 --- alpha10 Local photon index
45- 48 F4.2 --- e_alpha10 Local photon index error
50- 55 F6.2 --- beta High-energy spectral index
57- 60 F4.2 --- e_beta High-energy spectral index error
62- 66 F5.2 --- betab Photon index at the break energy Eb
of the Band function (1)
68- 71 F4.2 --- e_betab Photon index at the break energy Eb of
the Band function error
73- 78 F6.1 10-4/cm2/s/keV Amp Amplitude parameter
80- 84 F5.1 10-4/cm2/s/keV e_Amp Amplitude parameter error
87- 90 I4 --- PGstat "Poisson-Gauss" fit statistic
91 A1 --- --- [/]
92- 94 I3 --- dof Degree of freedom
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Note (1): By definition, betab is equal to beta for the Band function, while is
it harder than beta for the ISSM function owing to its continuous curvature.
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 02-Nov-2020