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GCN Circular 6985

GRB 071021: R-band detection and analysis
2007-10-24T20:23:32Z (17 years ago)
Silvia Piranomonte at OAR <>
S. Piranomonte (INAF/OAR), P. D'Avanzo (INAF/OABr), D. Malesani
(DARK), A. Fernandez-Soto ( U.Valencia), G. Tagliaferri (INAF/OABr),
G. Chincarini (U.Bicocca), S. Covino (INAF/OABr), F. Fiore (INAF/OAR),
D. Fugazza (INAF/OABr), L. Stella (INAF/OAR), report on behalf of the
MISTICI collaboration:

We observed the field of GRB 071021 (Sakamoto et al., GCN 6958) with the
8.2m VLT+FORS2 equipped with the R filter (effective wavelength:
6550 A; FWHM: 1650 A).

An eight-minute exposure was obtained on Oct 22.06688 , and a second,
thirty-minute exposure, on Oct 22.99604. A weak source is detected in
both images at the position of the candidate afterglow provided by
Castro-Tirado et al. (GCNs 6968, 6971).

The object has not faded significantly between both epochs, although
the very low S/N (~2) in the first image does not allow for a strong
statement regarding its variability.

Combining our own data and all the available optical-NIR data (GCNs
6962, 6973, 6974, 6975, 6976, 6980, 6983) we have measured a
photometric redshift z~5, but with a 2-sigma confidence interval that
includes all values in the z=[0 ... 5.5] range.

An alternative explanation for all the available data is that of a
strongly reddened afterglow (detected as a fading source in the H- and
K-band images), having taken place in a galaxy at a significantly
lower redshift (detected in our deeper, bluer, R- and z-band
images). This model should be checked via spectroscopy of the putative
host galaxy.

Our R-band images and the results of the preliminary photometric
redshift analysis are posted at:

We acknowledge excellent support from the ESO staff, in particular
Emanuela Pompei and Hugues Sana.

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