J/ApJ/720/862 Afterglow light curve of GRB 090926A (Rau+, 2010)
A very metal-poor damped Lyman-α system revealed through the most
energetic GRB 090926A.
Rau A., Savaglio S., Kruhler T., Afonso P., Greiner J., Klose S.,
Schady P., McBreen S., Filgas R., Olivares E.F., Rossi A., Updike A.
<Astrophys. J., 720, 862-871 (2010)>
=2010ApJ...720..862R 2010ApJ...720..862R
ADC_Keywords: Photometry, ugriz ; Gamma rays
Keywords: gamma-ray burst: general - X-rays: individual (GRB 090926A)
Abstract:
We present VLT/FORS2 spectroscopy and Gamma-ray Optical/Near-Infrared
Detector optical/near-IR photometry of the afterglow of the bright
Fermi/LAT GRB 090926A. The spectrum shows prominent Lyman-α
absorption with NHI=1021.73±0.07cm-2 and a multitude of metal
lines at a common redshift of z=2.1062±0.0004, which we associate
with the redshift of the gamma-ray burst (GRB). The metallicity
derived from SII is log(Z/Z☉)~-1.9, one of the lowest values
ever found in a GRB Damped Lyman-α (DLA) system. This confirms
that the spread of metallicity in GRB-DLAs at z∼2 is at least two
orders of magnitude. We argue that this spread in metallicity does not
require a similar range in abundances of the GRB progenitors, since
the neutral interstellar medium probed by the DLA is expected to be at
a significant distance from the explosion site. The hydrogen column
density derived from the Swift/XRT afterglow spectrum (assuming
log(Z/Z☉)~-1.9) is ∼100 times higher than the NHI obtained
from the Lyman-α absorptions. This suggests either a large
column density of ionized gas or a higher metallicity of the
circum-burst medium compared to the gas traced by the DLA. We also
discuss the afterglow light curve evolution and energetics. The
absence of a clear jet-break like steeping until at least 21 days
post-burst suggests a beaming-corrected energy release of
Eγ>3.5x1052erg, indicating that GRB 090926A may have been
one of the most energetic bursts ever detected.
Description:
Our follow-up photometry was initiated on 2009 September 27.03 UT
(73ks post-trigger) once the X-ray afterglow had been localized and
its position became accessible with the seven-band Gamma-ray
Optical/Near-Infrared Detector (GROND) on the MPI/ESO 2.2m telescope
at La Silla, Chile. Observations continued until 28 days post-burst.
The afterglow was detected in all seven bands (g'r'i'z'JHKS), as
expected from the spectroscopic redshift. The best localization was
measured to be RAJ2000=23:33:36.04, DEJ2000=-66:19:26.6 with a
systematic uncertainty of 0.2" compared to USNO-B1.
Optical spectroscopy of the afterglow started on 2009 September 27.08
UT (78ks post-burst) using the FOcal Reducer and low dispersion
Spectrograph 2 (FORS2) mounted on the 8.2m ESO-VLT UT1 telescope in
Paranal, Chile (Program ID: 083.D-0903). The average seeing was ∼0.75"
resulting in an effective spectral resolution of ∼4.5Å FWHM at
4500Å, or ∼300km/s (for a 3600-6300Å coverage).
Objects:
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RA (2000) DE Designation(s)
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23 33 36.04 -66 19 26.6 GRB 090926A = GRB 090926A
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 62 86 Log of GROND optical photometry
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See also:
J/ApJS/199/18 : The Fermi GBM catalog (Paciesas+, 2012)
J/A+A/531/A39 : g'r'i'z'JHKs light curves of GRB 081029 (Nardini+, 2011)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 7 F7.2 ks Tmid Time of mid-exposure minus T0 (1)
9- 14 A6 s Exp Exposure time
16 A1 --- l_gmag Limit flag on gmag
17- 21 F5.2 mag gmag Sloan g' band AB magnitude (2)
23- 26 F4.2 mag e_gmag ? Uncertainty in gmag
28 A1 --- l_rmag Limit flag on rmag
29- 33 F5.2 mag rmag Sloan r' band AB magnitude (2)
35- 38 F4.2 mag e_rmag ? Uncertainty in rmag
40 A1 --- l_imag Limit flag on imag
41- 45 F5.2 mag imag Sloan i' band AB magnitude (2)
47- 50 F4.2 mag e_imag ? Uncertainty in imag
52 A1 --- l_zmag Limit flag on zmag
53- 57 F5.2 mag zmag Sloan z' band AB magnitude (2)
59- 62 F4.2 mag e_zmag ? Uncertainty in zmag
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Note (1): Where T0=2009 September 26 04:20:27 UT.
Note (2): Corrected for Galactic foreground reddening. For the SED fitting, the
additional error of the absolute calibration of 0.05mag was added.
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 27-Jun-2012