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GCN Circular 6209

GRB 070311: Late Central Engine Activity Optical Flare??
2007-03-15T06:54:08Z (17 years ago)
Alexander Kann at TLS Tautenburg <>
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:

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.

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