[vsnet-grb-info 2617] GRB 050108: a dark burst?

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4454
SUBJECT: GRB 050108: a dark burst?
DATE:    06/01/09 12:48:19 GMT
FROM:    Daniele Malesani at SISSA-ISAS,Trieste,Italy  <malesani at sissa.it>

D. Malesani (SISSA), P. D'Avanzo (INAF/OABr), S. Piranomonte (INAF/OAR), 
G. Chincarini (Univ. Milano-Bicocca), L. Stella (INAF/OAR), and G. 
Tagliaferri (INAF/OABr), report:

Following the revised XRT position (Page et al., GCN 4453), we looked 
again at our VLT images (D'Avanzo et al., GCN 4452) of GRB 050108 (Oates 
et al., GCN 4443; Sakamoto et al., GCN 4445). We still do not detect any 
source inside the revised XRT error circle , down to the limit J > 22.3.

Using the flux and the decay law given by Page et al. (GCN 4452), we 
measure an optical-to-X-ray spectral index alpha_OX < 0.4 (computed 
between 1 keV and 1250 nm, at 0.7 d after the burst). Therefore, GRB 
050108 violates the synchrotron limit and can be classified as a truly 
dark GRB, according to the definition proposed by Jakobsson et al. 
(2004, ApJ, 617, L21).

The lack of absorption in the X-ray spectrum may hint at a high-redshift 
event, rather to a very absorbed one.

Further observations in the near infrared are encouraged.

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