Issue |
A&A
Volume 528, April 2011
|
|
---|---|---|
Article Number | A15 | |
Number of page(s) | 8 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/201015891 | |
Published online | 18 February 2011 |
Fermi/GBM observations of the ultra-long GRB 091024
A burst with an optical flash
1
Max Planck Institute for extraterrestrial Physics,
Giessenbachstrasse, Postfach
1312, 85748
Garching, Germany
e-mail: dgruber@mpe.mpg.de
2
Universe Cluster, Technische Universität München,
Boltzmannstraße 2, 85748
Garching,
Germany
3
University College, Dublin, Belfield, Stillorgan
Road, Dublin 4,
Ireland
4
University of Alabama in Huntsville, NSSTC, 320 Sparkman Drive, Huntsville, AL
35805,
USA
5
Space Science Office, VP62, NASA/Marshall Space Flight Center Huntsville,
AL
35812,
USA
6
Jacobs Technology, Inc., Huntsville, Alabama
7
Los Alamos National Laboratory, PO Box 1663, Los Alamos, NM
87545,
USA
8
Universities Space Research Association,
NSSTC, 320 Sparkman
Drive, Huntsville,
AL
35805,
USA
Received:
8
October
2010
Accepted:
26
December
2010
Aims. In this paper we examine gamma-ray and optical data of GRB 091024, a gamma-ray burst (GRB) with an extremely long duration of T90 ≈ 1020 s, as observed with the Fermi Gamma-ray Burst Monitor (GBM).
Methods. We present spectral analysis of all three distinct emission episodes using data from Fermi/GBM. Because of the long nature of this event, many ground-based optical telescopes slewed to its location within a few minutes and thus were able to observe the GRB during its active period. We compare the optical and gamma-ray light curves. Furthermore, we estimate a lower limit on the bulk Lorentz factor from the variability and spectrum of the GBM light curve and compare it with that obtained from the peak time of the forward shock of the optical afterglow.
Results. From the spectral analysis we note that, despite its unusually
long duration, this burst is similar to other long GRBs, i.e. there is spectral evolution
(both the peak energy and the spectral index vary with time) and spectral lags are
measured. We find that the optical light curve is highly anti-correlated to the prompt
gamma-ray emission, with the optical emission reaching the maximum during an epoch of
quiescence in the prompt emission. We interpret this behavior as the reverse shock
(optical flash), expected in the internal-external shock model of GRB emission but
observed only in a handful of GRBs so far. The lower limit on the initial Lorentz factor
deduced from the variability time scale ()
is consistent within the error to the one obtained using the peak time of the forward
shock (Γ0 = 120) and is also consistent with Lorentz factors of other long
GRBs.
Key words: gamma-ray burst: general / gamma-ray burst: individual: GRB 091024
© ESO, 2011
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