GCN Circular 7864
Subject
GRB 080605: TLS 3rd Epoch - Source Confusion
Date
2008-06-09T01:24:01Z (16 years ago)
From
Alexander Kann at TLS Tautenburg <kann@tls-tautenburg.de>
D. A. Kann, U. Laux & S. Ertel (TLS Tautenburg) report:
We obtained further images of the Swift GRB 080605 (Sbarufatti et al., GCN
7828). A 300 second Ic frame obtained under excellent observing conditions
shows the field remains unchanged in comparison to the second epoch as
well as the final images of the first epoch (Kann et al., GCN 7845).
Recently, Yoshida et al. (GCN 7863) also claimed detection of a plateau
phase. Clemens et al. (GCN 7851), on the other hand, report a clear fading
of the afterglow, as well as only a moderately red color in comparison to
Kann et al. (GCN 7845), which can possibly be explained by the moderate
Galactic reddening alone.
Comparison with a multicolor finding chart taken by GROND (C. Clemens,
priv. comm.) reveals the culprit: source confusion. Near the position of
the afterglow, there are in total three sources, all seemingly stellar in
GROND images, along a line. The star to the southeast (Source #1) was
immediately reported (Sbarufatti et al., GCN 7828, Kann et al., GCN 7829)
and is clearly visible in the DSS. To the northwest, there is another
source (Source #2), also clearly visible in the DSS. An inspection of the
DSS IR frame reveals that #2 is hardly visible at all, but a new source
(Source #3), inbetween #1 and #2, now becomes clearly visible. This source
is extremely red, possibly a red dwarf, and it is THIS source which begins
to dominate the photometry reported in Kann et al. (GCN 7845) (and
presumably Yoshida et al., GCN 7863), which is mostly I band. The small
offset from source #1 lead us to believe that this is the afterglow (and
explained the puzzling observation that it seemed to slightly shift
position).
Therefore, we retract the identification of a plateau phase of the
afterglow of GRB 080605, as well as the claim of extreme redness and the
resulting high lumonisity around 1 day in the observer frame. On the other
hand, the afterglow IS clearly detected on the early I band frames of the
TLS RRM observation, as well as in the R band and a stacked V band frame.
The early photometry, which leads to a decay slope of alpha ~ 1, is still
valid. Image subtraction will have to be performed to see how many
detections we have actually gotten.
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