GCN Circular 6980
Subject
GRB 071021: (Relatively shallow) IR non-detections; motivation for optical spectroscopy
Date
2007-10-23T06:05:13Z (17 years ago)
From
Josh Bloom at UC Berkeley <jbloom@astron.berkeley.edu>
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