GCN Circular 6209
Subject
GRB 070311: Late Central Engine Activity Optical Flare??
Event
Date
2007-03-15T06:54:08Z (19 years ago)
From
Alexander Kann at TLS Tautenburg <kann@tls-tautenburg.de>
D. A. Kann (TLS Tautenburg) reports:
I wish to point out the possibility of the strong optical rebrightening
(Halpern & Armstrong, GCN 6203) of the afterglow of GRB 070311
(Mereghetti et al., GCN 6189) being due to a very late reactivation of the
central engine.
Taking R Band detections (Wren et al., GCN 6198; Halpern & Armstrong, GCN
6195, GCN 6199, GCN 6203, GCN 6208; Greco et al., GCN 6204, Kann, Filgas
& Hoegner, GCN 6206), I find that the first three data points (from RAPTOR
and MDM, up to a day after the GRB) are already not fit well by a single
power law (alpha = 0.67, chi^2/d.o.f. = 30). Fitting only the first two
points from the first hour, I find alpha = 1, and a peak magnitude of the
rebrightening of at least 3.34 magnitudes above the extrapolation of the
early decay.
This situation is similar to the powerful rebrightening seen for GRB
050721 (Antonelli et al., A&A, 456, 509).
Fitting only the data points after the peak of the rebrightening (beyond
2.7 days) I find alpha = 3.17 +/- 0.11, which is steeper than the typical
alpha_2 = p decay seen for post-jet-break afterglows.
On the other hand, the situation is very similar to the giant X-ray flares
seen in Swift afterglows (e.g. Burrows et al., Science, 309, 1833), which
also often exhibit hard-to-soft evolution. Happening two days after the
GRB, one could expect such a giant flare to have a peak energy in the
optical range. There is also possibly a contemporaneous flare followed by
a steep decay detected in the X-rays, as can be seen in the light curve
posted on Nat Butler's page:
http://astro.berkeley.edu/~nat/swift/00020052/bat_xrt.jpg
While this is not conclusive evidence that this flare is due to late
central engine activity, further deep optical monitoring of this burst is
strongly warranted.
I thank Jules Halpern and Nat Butler for discussions.
This message may be cited.