J/ApJ/880/76 6 GRBs with Swift XRT and Fermi GBM obs. (Ahlgren+, 2019)
Investigating subphotospheric dissipation in gamma-ray bursts using joint
Fermi-Swift observations.
Ahlgren B., Larsson J., Valan V., Mortlock D., Ryde F., Pe'er A.
<Astrophys. J., 880, 76 (2019)>
=2019ApJ...880...76A 2019ApJ...880...76A
ADC_Keywords: GRB; Models; Gamma rays; X-ray sources
Keywords: gamma-ray burst: general ; methods: data analysis ;
radiation mechanisms: thermal
Abstract:
The jet photosphere has been proposed as the origin for the gamma-ray
burst (GRB) prompt emission. In many such models, characteristic
features in the spectra appear below the energy range of the Fermi
Gamma-ray Burst Monitor (GBM) detectors, so joint fits with X-ray data
are important in order to assess the photospheric scenario. Here we
consider a particular photospheric model which assumes localized
subphotospheric dissipation by internal shocks in a non-magnetized
outflow. We investigate it using Bayesian inference and a sample of
eight GRBs with known redshifts which are observed simultaneously with
Fermi GBM and the Swift X-ray Telescope (XRT). This provides us with
an energy range of 0.3keV-40MeV and much tighter parameter
constraints. We analyze 32 spectra and find that 16 are well described
by the model. We also find that the estimates of the bulk Lorentz
factor, Γ, and the fireball luminosity, L0,52, decrease while
the fraction of dissipated energy, εd, increases in the joint
fits compared to GBM-only fits. These changes are caused by a small
excess of counts in the XRT data, relative to the model predictions
from fits to GBM-only data. The fact that our limited implementation
of the physical scenario yields 50% accepted spectra is promising, and
we discuss possible model revisions in the light of the new data.
Specifically, we argue that the inclusion of significant
magnetization, as well as removing the assumption of internal shocks,
will provide better fits at low energies.
Description:
We examine all gamma-ray bursts (GRBs) with a known redshift which
have overlapping observations in the Swift X-ray Telescope (XRT) and
Fermi GBM detectors, up until 2018 November 1.
Our sample consists of eight GRBs which all have at least one spectrum
with significant signal in both the XRT and GBM (S/N>3). Binning the
data with Bayesian blocks and performing the S/N cut, we obtain 32 spectra.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 100 21 Point estimates of model parameters and rd for
all 21 fits which passed the posterior predictive
checks (PPC) for 6 GRBs in our sample
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See also:
J/ApJ/609/935 : Gamma-ray burst formation rate (Yonetoku+, 2004)
J/A+A/564/A125 : AGN Torus model comparison of AGN in CDFS (Buchner+, 2014)
J/MNRAS/454/L31 : GRB prompt emission fitted with DREAM model (Ahlgren+, 2015)
J/A+A/609/A112 : Bulk Lorentz factors of gamma-ray bursts (Ghirlanda+, 2018)
Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 7 A7 --- GRB GRB name
9- 13 F5.1 s Tstart [72.5/147] Time bin, start
15- 19 F5.1 s Tstop [90.2/156] Time bin, stop
21- 24 F4.2 --- ed [0.02/0.4] Mean dissipated energy εd
26- 29 F4.2 --- e_ed [0.02/0.4] 1σ lower uncertainty in ed
31- 34 F4.2 --- E_ed [0.03/0.4] 1σ upper uncertainty in ed
36- 39 F4.1 10+45W L [1.2/89.3] Mean fireball luminosity, L0,52
41- 44 F4.1 10+45W e_L [1.1/79.3] 1σ lower uncertainty in L
46- 49 F4.1 10+45W E_L [1.3/99.5] 1σ upper uncertainty in L
51- 55 F5.1 --- Gamma [77.5/162] Mean coasting bulk Lorentz factor
57- 61 F5.1 --- e_Gamma [71.3/149] 1σ lower uncertainty in Gamma
63- 67 F5.1 --- E_Gamma [82.5/175] 1σ upper uncertainty in Gamma
69- 72 F4.1 10+12cm rd [0.9/31.5] Mean dissipation radius
74- 77 F4.1 10+12cm e_rd [0.9/27] 1σ lower uncertainty in rd
79- 82 F4.1 10+12cm E_rd [1/36.2] 1σ upper uncertainty in rd
84- 87 F4.2 --- Nr [0.9/1.4] Mean free relative normalization
parameter, cross calibration constant
89- 92 F4.2 --- e_Nr [0.8/1.25] 1σ lower uncertainty in Nr
94- 97 F4.2 --- E_Nr [1/1.4] 1σ upper uncertainty in Nr
99-100 I2 --- Fig Figure part number;
column added by CDS from author
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Acknowledgements:
Bjorn Ahlgren [bjornah kth.se --KTH Royal Institute of Technology, Sweden]
History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 14-Jan-2021