Issue |
A&A
Volume 452, Number 3, June IV 2006
|
|
---|---|---|
Page(s) | 819 - 825 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20054435 | |
Published online | 06 June 2006 |
X-ray flares in the early Swift observations of the possible naked gamma-ray burst 050421
1
X-ray and Observational Astronomy Group, Department of Physics & Astronomy, University of Leicester, LE1 7RH, UK e-mail: og19@star.le.ac.uk
2
Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
3
Universities Space Research Association, 10211 Wincopin Circle, Suite 500, Columbia, MD, 21044-3432, USA
4
Department of Physics, University of Nevada, Box 454002, Las Vegas, NV 89154-4002, USA
5
ASI Science Data Center, via G. Galilei, 00044 Frascati, Italy
6
National Research Council, 2101 Constitution Ave NW, Washington, DC 20418, USA
7
Department of Astronomy & Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802, USA
8
INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica Sezione di Palermo, Via U. La Malfa 153, 90146 Palermo, Italy
9
INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807, Merate (LC), Italy
Received:
28
October
2005
Accepted:
26
February
2006
We present the Swift observations of the faint burst
GRB 050421. The X-ray light-curve shows at least two flares: the
first flare peaking at ~110 s after the BAT trigger (T0) and the
second one peaking at ~154 s. The first flare presents a flux
variation of and a short timescale
ratio
. The second flare is smaller
and presents a flux variation of
and a
short timescale ratio
. We argue that
the mechanism producing these flares is probably late internal shocks. The
X-ray light-curve is consistent with a rapid decline with a temporal index
, which decays from ~10-9 erg cm-2 s-1 at
T0+100 s to <
erg cm-2 s-1 at
T0+900 s. A possible spectral softening is also observed with time,
from
to ∼1.2. A good joint fit to the BAT and XRT
spectra before T0+171 s with
indicates
that the early X-ray and Gamma-ray emissions are likely produced by the same
mechanism. We argue that the X-ray spectral softening, if any, is due to a
shift of the peak of the prompt emission spectrum down to lower energies,
and that the rapid decline of the X-ray emission is probably the tail of the
prompt emission. This suggests that the X-ray emission is completely
dominated by high latitude radiation and the external shock, if any, is
extremely faint and below the detection threshold. GRB 050421 is
likely the first “naked burst” detected by Swift.
Key words: gamma ray: bursts
© ESO, 2006
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