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
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.