J/MNRAS/485/474 Model for subphotospheric dissipation in GRBs (Ahlgren+, 2019)
Testing a model for subphotospheric dissipation in GRBs: fits to Fermi data
constrain the dissipation scenario.
Ahlgren B., Larsson J., Ahlberg E., Lundman C., Ryde F., Pe'er A.
<Mon. Not. R. Astron. Soc., 485, 474-497 (2019)>
=2019MNRAS.485..474A 2019MNRAS.485..474A (SIMBAD/NED BibCode)
ADC_Keywords: GRB ; Models
Keywords: radiation mechanisms: thermal - gamma-ray burst: general
Abstract:
It has been suggested that the prompt emission in gamma-ray bursts
(GRBs) could be described by radiation from the photosphere in a hot
fireball. Such models must be tested by directly fitting them to data.
In this work we use data from the Fermi Gamma-ray Space Telescope and
consider a specific photospheric model, in which the kinetic energy of
a low-magnetization outflow is dissipated locally by internal shocks
below the photosphere. We construct a table model with a physically
motivated parameter space and fit it to time-resolved spectra of the
36 brightest Fermi GRBs with a known redshift. We find that about
two-thirds of the examined spectra cannot be described by the model,
as it typically underpredicts the observed flux. However, since the
sample is strongly biased towards bright GRBs, we argue that this
fraction will be significantly lowered when considering the full
population. From the successful fits we find that the model can
reproduce the full range of spectral slopes present in the sample. For
these cases we also find that the dissipation consistently occurs at a
radius of ∼1012cm and that only a few per cent efficiency is
required. Furthermore, we find a positive correlation between the
fireball luminosity and the Lorentz factor. Such a correlation has
been previously reported by independent methods. We conclude that if
GRB spectra are due to photospheric emission, the dissipation cannot
only be the specific scenario we consider here.
Description:
We have considered a model for subphotospheric dissipation as the
origin of GRB prompt emission and fitted it to Fermi GRB data. Our
sample contains the brightest GRBs with known redshifts observed by
Fermi (Bhat et al. 2016ApJS..223...28B 2016ApJS..223...28B, Cat. J/ApJS/223/28) before
2016-06-01. The known redshift1 helps us fix the normalization
parameter of the model spectrum via the corresponding luminosity
distance, instead of leaving it as a free parameter. This is important
because we want to be able to test the model's ability to correctly
predict the GRB flux. We chose a fluence cut of >10-5erg/cm2, in
order to allow ourselves to perform a time-resolved analysis with good
signal strength.
The dissipation is localized and we assume internal shocks as the
dissipation mechanism. Additionally, the model does not take into
account geometric effects, a fuzzy photosphere, or jet hydrodynamics.
We consider the scenario where there are no significant magnetic
fields presents. A table model, DREAM1.2, was created from simulations
using the numerical code of Pe'er & Waxman (2005ApJ...628..857P 2005ApJ...628..857P).
Using DREAM1.2, we performed a time-resolved analysis of 36 bursts,
using Bayesian blocks as a binning method. We analysed a total of 634
time-resolved spectra. Out of these, we find that 171 spectra are well
described by our model, passing the GOF test and having no parameters
on the boundaries of the parameter space. This corresponds to an
acceptance rate of about 27 per cent, with 10 bursts having at least
50 per cent accepted fits.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 280 171 Best-fitting parameter values for accepted fits
with DREAM1.2
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See also:
J/ApJS/223/28 : The third Fermi/GBM GRB catalog (6yr) (Bhat+, 2016)
Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Burst Burst identifier (Fermi bnYYMMDDddd in Simbad)
11- 31 F21.17 s tstart Start time of time bin
33- 53 F21.17 s tstop Stop time of time bin
55- 74 F20.18 --- ed Fraction of the kinetic energy of the protons
76- 95 F20.18 --- b_ed ?=0.0 Minimal value of ed (1)
97-116 F20.18 --- B_ed ?=0.0 Maxmal value of ed (1)
118-137 F20.16 --- L052 Fireball luminosity (2)
139-158 F20.16 --- b_L052 Minimal value of L052 (1)
160-179 F20.16 --- B_L052 ?=0.0 Maximal value of L052
181-198 F18.14 --- Gamma Bulk Lorentz factor
200-217 F18.14 --- b_Gamma ?=0.0 Minimal value of Gamma (1)
219-236 F18.14 --- B_Gamma ?=0.0 Maximal value of Gamma (1)
238-257 F20.17 10+12cm rd Dissipation radius (3)
259-280 F22.18 10+12cm e_rd Error on rd
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Note (1): A missing uncertainty means that the uncertainty is unconstrained at
the 1σ level
Note (2): In the fireball model (see e.g. Pe'er 2015AdAst2015E..22P 2015AdAst2015E..22P for a recent
review), an isotropic equivalent luminosity, L0=L052x1052erg/s, is
emitted from the central engine in the form of baryons, electrons,
B fields, and photons
Note (3): We define the dissipation radius as rd=rph/tau, where tau is the
optical depth due to the baryonic electrons
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History:
From electronic version of the journal
(End) Ana Fiallos [CDS] 12-Sep-2022