J/MNRAS/449/L6 Gamma-ray burst Eiso/Lopt (Coward+, 2015)
Selection biases in the gamma-ray burst Eiso-Lopt, X correlation.
Coward D.M., Howell E.J., Wan L., Macpherson D.
<Mon. Not. R. Astron. Soc., 449, L6-L10 (2015)>
=2015MNRAS.449L...6C 2015MNRAS.449L...6C (SIMBAD/NED BibCode)
ADC_Keywords: Gamma rays ; X-ray sources
Keywords: methods: statistical - gamma-ray burst: general
Abstract:
Gamma-ray burst (GRB) optical and X-ray afterglow luminosity is
expected to correlate with the GRB isotropic equivalent kinetic energy
of the outflow in the standard synchrotron model for GRB afterglows.
Previous studies, using prompt GRB isotropic equivalent energy
(Eiso) as a proxy for isotropic equivalent kinetic energy, have
generally confirmed a correlation between X-ray and optical afterglow
luminosities. Assuming that GRB afterglow luminosity does not evolve
strongly with redshift, we identify a strong Malmquist bias in GRB
optical and X-ray afterglow luminosity data. We show that selection
effects dominate the observed Eiso-Lopt, X correlations, and have
likely been underestimated in other studies. The bias is strongest for
a subset of optically faint bursts m>24 at 24h with z>2. After
removing this optical selection bias, the Eiso-Lopt, X correlation
for long GRBs is not statistically significant, but combining both
long and short GRB luminosity data the correlation is significant.
Using the median of the Eiso and Lopt, X distributions, we apply
the synchrotron model assuming the same power-law index for short and
long GRBs, but different microphysical parameter distributions.
Comparing the ratio of optical and X-ray luminosities, we find
tentative evidence that the fraction of post-shock energy in magnetic
fields, εB, could be systematically higher in short GRBs
compared to long GRBs.
Description:
LGRB and SRGB optical luminosities, are obtained from Kann et al.
(2010, Cat. J/ApJ/720/1513, hereafter K10) and references therein. Our
optical data selection taken from K10 includes 61 LGRB optical
afterglow absolute magnitudes measured at 24h in the rest frame. Based
on Norris et al. (2011ApJ...735...23N 2011ApJ...735...23N), we classify a subset of SGRBs
as SGRB-EE in this sample.
To ensure a consistent sample of Eiso data, we use the Butler online
catalogue Swift BAT Integrated Spectral Parameters
(http://butler.lab.asu.edu/Swift/bat_spec_table.html). This catalogue,
an extension of Butler et al. (2007A&A...469..379E 2007A&A...469..379E), Butler, Bloom &
Poznanski (2010, Cat. J/ApJ/711/495), circumvents the nominal BAT
upper energy of 150keV to produce values of Eiso through a Bayesian
approach. We note that Butler provides no Eiso data for GRBs 061006,
061210 and 071112C, so we use estimates provided by K10.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table.dat 38 61 LGRB optical data used in the analysis
fig3.dat 71 67 Figure 3
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See also:
J/ApJ/711/495 : Durations of Swift/BAT GRBs (Butler+, 2010)
J/ApJ/720/1513 : The afterglows of Swift-era GRBs. I. (Kann+, 2010)
Byte-by-byte Description of file: table.dat
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Bytes Format Units Label Explanations
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1- 7 A7 --- LGRB LGRB name
9- 14 F6.4 10+39W Lum Optical luminosity @ 24 hrs (1046erg/s) (1)
17- 23 F7.4 mag Flux Optical flux @ 24 hrs in magnitude (1)
25- 30 F6.4 10+54 Eiso Isotropic equivalent energy (2)
33- 38 F6.4 --- z Redshift (3)
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Note (1): taken directly from Kann et al., 2010, Cat. J/ApJ/720/1513.
Note (2): taken from http://butler.lab.asu.edu/Swift/bat_spec_table.html .
Note (3): taken from http://www.mpe.mpg.de/~jcg/grbgen.html5 .
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Byte-by-byte Description of file: fig3.dat
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Bytes Format Units Label Explanations
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1- 7 A7 --- GRB GRB name
10- 13 F4.2 --- z Redshift
15- 21 F7.2 10+34W Lopt Optical luminosity (1043erg/s)
23- 28 F6.2 10+34W B_Lopt ? Lopt upper interval value
31- 36 F6.2 10+34W b_Lopt ? Lopt lower interval value
38- 43 F6.2 10+34W e_Lopt ? rms uncertainty on Lobt
46- 53 F8.2 10+50 Eiso Isotropic equivalent energy
55- 62 F8.2 10+50 B_Eiso Eiso upper interval value
64- 71 F8.2 10+50 b_Eiso Eiso Lower interval value
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 10-Nov-2015