TITLE: GCN CIRCULAR NUMBER: 6980 SUBJECT: GRB 071021: (Relatively shallow) IR non-detections; motivation for optical spectroscopy DATE: 07/10/23 06:05:13 GMT FROM: Josh Bloom at UC Berkeley J. S. Bloom (UC Berkeley), J. X. Prochaska (UCO/Lick), D. A. Perley (UC Berkeley), H.-W. Chen (U Chicago), D. L. Starr (UC Berkeley & LCOGT), M. Modjaz, D. Poznanski (UC Berkeley) report: "We observed the field of GRB 071021 (Sakamoto et al. GCN 6958) with PAIRITEL (*) starting at 2007-10-22 04:53 UT in high wind and poor seeing conditions at Mt Hopkins, Arizona. In the first 1340 second stack, the IR source noted by Castro-Tirado et al. (GCN 6968/6971) is not detected to J=18.35 mag, H=16.87 mag, Ks=16.43 (2.5 sigma upper limit). This is not particularly constraining in light of the deep Subaru imaging (Terada et al. GCN 6976) at a similar epoch. We note that despite the z-band detection (Malesani et al. GCN #6972) the very high-redshift hypothesis of Sakamoto et al. (GCN 6967) is not yet ruled out. In particular, in high z QSO spectra there can be some transmitted flux (in the z-band filter transmission) through the Lyman-alpha forest between ~8000 Ang and 8500 Ang (see, e.g., Fan et al. 2006 **) for z > 6.3 sources. So long as the redshifted Lyman limit at the redshift of the emitting source is blueward of ~8500 Ang, this light would not be entirely suppressed. This consideration yields an upper limit: (1 + z_GRB)*912 Ang < 8500 Ang z_GRB <~ 8.3 At such redshifts, say z_GRB = 8, all reported long-wavelength observations to-date would be naturally accommodated: - the deep non-detections at R and i-band (Berger & Covarrubias, GCNs 6973,6974), being blueward of 912*[1+z_GRB]) - the faint detection at z-band (GCN 6972) due transmitted flux between 912*[1+z_GRB] = 8200 and 8500 Ang is allowed (as above) - the afterglow detections at H and K (GCN 6968; Terada et al. GCN 6976) - the apparent faintness of J-band flux (GCN 6968) relative to H and K given the Gunn-Peterson trough would extend to (1 + z_GRB)*1216 Ang =~ 1.1 micron. All of this assumes that the z-band source is not a foreground galaxy (in which case a higher-z GRB suggested by Castro-Tirado et al. is allowed). If indeed the GRB originated from z > 7, we would expect to see what would resemble emission lines between lam_obs = 912*[1+z_GRB] and ~8500 Ang, motivating the utility of *optical* spectroscopy of this source. Even without spectroscopy, when final magnitudes are reported along with filter transmission curves, a full SED should be illuminating." This message may be cited. (*) http://pairitel.org (**) http://www.journals.uchicago.edu/AJ/journal/issues/ v132n1/205115/205115.web.pdf