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GCN Circular 9008

GRB 090313, SMARTS optical/IR observations
2009-03-18T01:06:29Z (15 years ago)
Bethany Cobb at UC Berkeley <>
B. E. Cobb (UC Berkeley) reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we obtained
optical/IR imaging of the error region of GRB 090313 (GCN 8980, Mao
et al.) over four epochs (with UT mid-exposure times of: 2009-03-14 04:27,
2009-03-14 06:55, 2009-03-15 06:30 & 2009-03-17 05:07). Total
summed exposure times for each observation amounted to
36 minutes in R and 30 minutes in J.

The GRB afterglow (e.g. GCN 8979, Chornock et al., GCN 8983, Updike et
al. and GCN 8985, Perley et al.) appears in our imaging to be fading 
rapidly after ~19 hours post-burst:

mid-exposure time
(hours post-burst)   R mag           J mag
19.33                19.70+/-0.04    17.38+/-0.15
21.81                19.90+/-0.04    17.62+/-0.15
45.38                21.57+/-0.12    18.91+/-0.16
92.01                > 22.5          > 19.8

These preliminary magnitudes are calibrated using Landolt standard
stars in the optical and a 2MASS star in the IR.

This rapid fading follows the initial plateau phase that was reported by 
Perley et al. (GCN 8985) and de Ugarte Postigo et al. (GCN 8992).  This 
fading is consistent with the light curve break noted by de Ugarte 
Postigo et al. (GCN 8999), though the break appears to occur in this 
imaging somewhat earlier than suggested by Yoshida et al. (GCN 9002).
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