{
"subject": "GRB 031026 (=H2882): Evidence for High Redshift from Prompt",
"bibcode": "2003GCN..2432....1A",
"createdOn": 1067436391000,
"circularId": 2432,
"submitter": "George Ricker at MIT ",
"email": "grr@space.mit.edu",
"body": "GRB 031026 (=H2882): Evidence for High Redshift from Prompt Emission\n\nJ-L. Atteia, G. Ricker, K. Hurley, J. G. Jernigan, N. Kawai, D. \nLamb, and S. Woosley, on behalf of the HETE Science Team;\n\nC. Barraud, M. Boer, J-F Olive, and J-P Dezalay, on behalf of the \nHETE FREGATE Team;\n\nT. Donaghy, C. Graziani, E. Fenimore, M. Galassi, M. Matsuoka, T. \nSakamoto, Y. Shirasaki, M. Suzuki, T. Tamagawa, K. Torii, A. Yoshida, \nY. Nakagawa, R. Satoh, Y. Urata, T. Yamazaki and Y. Yamamoto, on \nbehalf of the HETE WXM Team;\n\nJ. Villasenor, N. Butler, G. Crew, J. Doty, A. Dullighan, G. \nPrigozhin, R. Vanderspek, G. Azzibrouck, J. Braga, R. Manchanda, and \nG. Pizzichini, on behalf of the HETE Operations and HETE Optical-SXC \nTeams;\n\nreport:\n\nCareful spectral analysis of the prompt X-ray and gamma-ray emission \nof GRB 031026 (Butler et al; GCN 2429) confirms that this burst has a \nvery hard spectrum. This is unusual for relatively faint, long GRBs \nlike GRB 030126.\n\nWe have computed a \"pseudo-redshift\" for GRB 031026, based on the \nspectral properties of its prompt high energy emission, using the \nprescription of Atteia (2003) (see A&A, 407, L1, for the definition \nof pseudo-redshift, or pseudo-z). We determine a value of 14 for the \npseudo-z. This value is the highest one amongst the 40 HETE GRBs for \nwhich pseudo-z's have been calculated; only 3 of the 40 have \npseudo-z's greater than 4:\n\nGRB 020305, pseudo-z = 5.9\nGRB 010612, pseudo-z = 9.5\nGRB 031026, pseudo-z = 14.\n\nFor the 10 HETE GRBs with spectroscopic redshifts for which \ncalculation of a pseudo-z has been possible, the pseudo-z is always \nwithin a factor of two of the spectroscopic redshift. However, this \nagreement reflects to some degree the fact that the prescription used \nto calculate the pseudo-z's was optimized for these 10 bursts, and it \nhas not been possible as yet to predict the pseudo-z's of an \nindependent set of GRBs. Furthermore, the 10 HETE bursts included in \nthis fit span the redshift range z = 0.2 to z= 3.5, and thus no \ncalibration of the pseudo-z prescription has yet been possible for a \nredshift as large as the pseudo-z value we find for GRB 031026. \nNevertheless, we believe that it is important to report the pseudo-z \nwe find for GRB 031026, given the possible importance of this burst.\n\nConsidering the unusually hard spectrum yet small peak flux of GRB \n031026, potentially revealing a source at very high redshift, we urge \nobservers to perform deep observations of the HETE localization error \ncircle for this burst (see GCN 2429 for more details) at other than \noptical wavelengths. If the source of this burst truly lies at a \nvery high redshift (z > 7), its optical afterglow could be \nundetectable as a result of absorption by the Lyman alpha forest, yet \nX-ray and/or IR observations might reveal a counterpart. Furthermore, \nat a fixed time of observation after the GRB, the effect of \ncosmological time dilation for a very high redshift could in \nprinciple increase the spectral flux in a given frequency band to \nsuch a degree that the afterglow would appear dramatically brighter \nthan might otherwise be anticipated. Thus, a z=7 afterglow observed \nat 8 days after the burst could be as bright (e.g. in a fixed X-ray \nor IR band) as would a z=1 afterglow observed at 1 day after the \nburst (Lamb and Reichart 2000; Ciardi and Loeb 2000).\n\nThis result may be cited."
}