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GCN Circular 2432

Subject
GRB 031026 (=H2882): Evidence for High Redshift from Prompt
Date
2003-10-29T14:06:31Z (21 years ago)
From
George Ricker at MIT <grr@space.mit.edu>
GRB 031026 (=H2882): Evidence for High Redshift from Prompt Emission

J-L. Atteia, G. Ricker, K. Hurley, J. G. Jernigan,  N. Kawai, D. 
Lamb, and S. Woosley, on behalf of the HETE Science Team;

C. Barraud, M. Boer, J-F Olive, and J-P Dezalay, on behalf of the 
HETE FREGATE Team;

T. Donaghy, C. Graziani, E. Fenimore, M. Galassi, M. Matsuoka, T. 
Sakamoto, Y. Shirasaki, M. Suzuki, T. Tamagawa, K. Torii, A. Yoshida, 
Y. Nakagawa, R. Satoh, Y. Urata, T. Yamazaki and Y. Yamamoto, on 
behalf of the HETE WXM Team;

J. Villasenor, N. Butler, G. Crew, J. Doty, A. Dullighan, G. 
Prigozhin, R. Vanderspek, G. Azzibrouck, J. Braga, R. Manchanda, and 
G. Pizzichini, on behalf of the HETE Operations and HETE Optical-SXC 
Teams;

report:

Careful spectral analysis of the prompt X-ray and gamma-ray emission 
of GRB 031026 (Butler et al; GCN 2429) confirms that this burst has a 
very hard spectrum.  This is unusual for relatively faint, long GRBs 
like GRB 030126.

We have computed a "pseudo-redshift" for GRB 031026, based on the 
spectral properties of its prompt high energy emission, using the 
prescription of Atteia (2003) (see A&A, 407, L1, for the definition 
of pseudo-redshift, or pseudo-z).  We determine a value of 14 for the 
pseudo-z.  This value is the highest one amongst the 40 HETE GRBs for 
which pseudo-z's have been calculated; only 3 of the 40 have 
pseudo-z's greater than 4:

GRB 020305, pseudo-z = 5.9
GRB 010612, pseudo-z = 9.5
GRB 031026, pseudo-z = 14.

For the 10 HETE GRBs with spectroscopic redshifts for which 
calculation of a pseudo-z has been possible, the pseudo-z is always 
within a factor of two of the spectroscopic redshift.  However, this 
agreement reflects to some degree the fact that the prescription used 
to calculate the pseudo-z's was optimized for these 10 bursts, and it 
has not been possible as yet to predict the pseudo-z's of an 
independent set of GRBs.  Furthermore, the 10 HETE bursts included in 
this fit span the redshift range z = 0.2 to z= 3.5, and thus no 
calibration of the pseudo-z prescription has yet been possible for a 
redshift as large as the pseudo-z value we find for GRB 031026. 
Nevertheless, we believe that it is important to report the pseudo-z 
we find for GRB 031026, given the possible importance of this burst.

Considering the unusually hard spectrum yet small peak flux of GRB 
031026, potentially revealing a source at very high redshift, we urge 
observers to perform deep observations of the HETE localization error 
circle for this burst (see GCN 2429 for more details) at other than 
optical wavelengths.  If the source of this burst truly lies at a 
very high redshift (z > 7), its optical afterglow could be 
undetectable as a result of absorption by the Lyman alpha forest, yet 
X-ray and/or IR observations might reveal a counterpart. Furthermore, 
at a fixed time of observation after the GRB, the effect of 
cosmological time dilation for a very high redshift could in 
principle increase the spectral flux in a given frequency band to 
such a degree that the afterglow would appear dramatically brighter 
than might otherwise be anticipated. Thus, a z=7 afterglow observed 
at 8 days after the burst could be as bright (e.g. in a fixed X-ray 
or IR band) as would a z=1 afterglow observed at 1 day after the 
burst (Lamb and Reichart 2000; Ciardi and Loeb 2000).

This result may be cited.
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