{
  "bibcode": "2007GCN..6209....1K",
  "body": "D. A. Kann (TLS Tautenburg) reports:\n\nI wish to point out the possibility of the strong optical rebrightening \n(Halpern & Armstrong, GCN 6203) of the afterglow of GRB 070311 \n(Mereghetti et al., GCN 6189) being due to a very late reactivation of the \ncentral engine.\n\nTaking R Band detections (Wren et al., GCN 6198; Halpern & Armstrong, GCN \n6195, GCN 6199, GCN 6203, GCN 6208; Greco et al., GCN 6204, Kann, Filgas \n& Hoegner, GCN 6206), I find that the first three data points (from RAPTOR \nand MDM, up to a day after the GRB) are already not fit well by a single \npower law (alpha = 0.67, chi^2/d.o.f. = 30). Fitting only the first two \npoints from the first hour, I find alpha = 1, and a peak magnitude of the \nrebrightening of at least 3.34 magnitudes above the extrapolation of the \nearly decay.\n\nThis situation is similar to the powerful rebrightening seen for GRB \n050721 (Antonelli et al., A&A, 456, 509).\n\nFitting only the data points after the peak of the rebrightening (beyond \n2.7 days) I find alpha = 3.17 +/- 0.11, which is steeper than the typical \nalpha_2 = p decay seen for post-jet-break afterglows.\n\nOn the other hand, the situation is very similar to the giant X-ray flares \nseen in Swift afterglows (e.g. Burrows et al., Science, 309, 1833), which \nalso often exhibit hard-to-soft evolution. Happening two days after the \nGRB, one could expect such a giant flare to have a peak energy in the \noptical range. There is also possibly a contemporaneous flare followed by \na steep decay detected in the X-rays, as can be seen in the light curve \nposted on Nat Butler's page:\n\nhttp://astro.berkeley.edu/~nat/swift/00020052/bat_xrt.jpg\n\nWhile this is not conclusive evidence that this flare is due to late \ncentral engine activity, further deep optical monitoring of this burst is \nstrongly warranted.\n\nI thank Jules Halpern and Nat Butler for discussions.\n\nThis message may be cited.",
  "circularId": 6209,
  "createdOn": 1173941648000,
  "email": "kann@tls-tautenburg.de",
  "subject": "GRB 070311: Late Central Engine Activity Optical Flare??",
  "submitter": "Alexander Kann at TLS Tautenburg  <kann@tls-tautenburg.de>",
  "eventId": "GRB 070311"
}