J/A+A/609/A112 Bulk Lorentz factors of gamma-ray bursts (Ghirlanda+, 2018)
Bulk Lorentz factors of gamma-ray bursts.
Ghirlanda G., Nappo F., Ghisellini G., Melandri A., Marcarini G., Nava L.,
Salafia O.S., Campana S., Salvaterra R.
<Astron. Astrophys., 609, A112 (2018)>
=2018A&A...609A.112G 2018A&A...609A.112G (SIMBAD/NED BibCode)
ADC_Keywords: Gamma rays ; GRB
Keywords: gamma-ray burst: general - radiation mechanisms: non-thermal -
relativistic processes
Abstract:
Knowledge of the bulk Lorentz factor Γ0 of gamma-ray bursts
(GRBs) allows us to compute their comoving frame properties shedding
light on their physics. Upon collisions with the circumburst matter,
the fireball of a GRB starts to decelerate, producing a peak or a
break (depending on the circumburst density profile) in the light
curve of the afterglow. Considering all bursts with known redshift and
with an early coverage of their emission, we find 67 GRBs (including
one short event) with a peak in their optical or GeV light curves at a
time tp. For another 106 GRBs we set an upper limit tpUL. The
measure of tp provides the bulk Lorentz factor Γ0 of the
fireball before deceleration. We show that tp is due to the dynamics
of the fireball deceleration and not to the passage of a
characteristic frequency of the synchrotron spectrum across the
optical band. Considering the tp of 66 long GRBs and the 85 most
constraining upper limits, we estimate Γ0 or a lower limit
Γ0LL. Using censored data analysis methods, we reconstruct
the most likely distribution of tp. All tp are larger than the
time Tp,γ when the prompt γ-ray emission peaks, and are
much larger than the time Tph when the fireball becomes transparent,
that is, tp>Tp,γ>Tph. The reconstructed distribution of
Γ0 has median value ∼300 (150) for a uniform (wind)
circumburst density profile. In the comoving frame, long GRBs have
typical isotropic energy, luminosity, and peak energy
<Eiso≥3(8)x1050erg, <Liso≥3(15)x1047erg/s, and
<Epeak≥1(2)keV in the homogeneous (wind) case. We confirm that the
significant correlations between Γ0 and the rest frame
isotropic energy (Eiso), luminosity (Liso), and peak energy (Ep)
are not due to selection effects. When combined, they lead to the
observed Ep-Eiso and Ep-Liso correlations. Finally, assuming a
typical opening angle of 5 degrees, we derive the distribution of the
jet baryon loading which is centered around a few 10-6M☉.
Description:
We consider GRBs with measured redshift z and well constrained
spectral parameters of the prompt emission. For these events it is
possible to estimate the isotropic energy Eiso and luminosity Liso and
the rest frame peak spectral energy Ep (i.e. the peak of the νFν
spectrum).
Table A.1 lists all the GRBs we collected. The "Gold" sample is
composed of sources with a complete set of information, namely
measured tp (Col. 6) and spectral parameters (Cols. 3-5). It
contains 49 events: 48 long GRBs plus the short event 090510. GRBs of
the Gold sample have the label "(g)" at the end of their names
reported in Col. 1 of Table A.1. The redshift z, rest frame peak
energy Ep, isotropic energy and luminosity (Eiso and Liso,
respectively) are given in Table A.1. Eight out of 49 GRBs have their
tp measured from the GeV light curve as observed by the Fermi-LAT
(labelled "L" or "SL" for the short GRB 090510).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 98 172 *Sample of GRBs with measured tp (69 events) from
the peak of the light curve
refs.dat 72 108 References
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Note on tablea1.dat: those from the LAT light curve are labelled "L".
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Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 7 A7 --- GRB GRB designation (NNNNNNA)
9- 10 A2 --- n_GRB [gsb *] Note on GRB (1)
12- 13 A2 --- f_GRB [SL ] L for LAT light curve,
SL for for the short GRB 090510
15- 18 F4.2 --- z Redshift
20- 24 F5.3 [keV] logEp Peak energy
26- 31 F6.4 [keV] e_logEp rms uncertainty on logEp
33- 38 F6.3 [10-7J] logEiso Isotropic energy
40- 45 F6.4 [10-7J] e_logEiso rms uncertainty on logEiso
47- 52 F6.3 [10-7W] logLiso Isotropic luminosity
54- 59 F6.4 [10-7W] e_logLiso rms uncertainty on logLiso
61- 62 A2 --- l_logtp [≤ ] Limit flag on logtp
63- 68 F6.3 [s] logtp Afterglow peak time
70- 98 A29 --- Ref Reference, in refs.dat file
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Note (1): Notes as follows:
g = Gold sample
s = Silver sample
b = for GRB 100901A, not explained in the paper
* = for GRB 071112C and GRB 100901A, not explained in the paper
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Byte-by-byte Description of file: refs.dat
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Bytes Format Units Label Explanations
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1- 29 A29 --- Ref Reference code
32- 50 A19 --- BibCode BibCode
52- 72 A21 --- Com Comments
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 19-Mar-2018