GCN Circular 2432

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
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) 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
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 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.