J. Nousek (PSU), G. Cusumano (IASF-Pa INAF), A. Moretti (INAF-OAB), G.
(INAF-OAB), S. Campana (INAF-OAB), J. Kennea (PSU), D. Burrows (PSU), P. Roming
(PSU), D. VandenBerk (PSU), P. Brown (PSU), N. Gehrels (GSFC), S. Barthelmy
(GSFC), F. Marshall(GSFC), P. Boyd (GSFC), T. Sakamoto (GSFC), J. Osborne
(U. Leicester), P. O'Brien (U. Leicester), G. Chincarini (Univ.
B. Zhang (UNLV) and M. de Pasquale (MSSL)
report on behalf of the Swift Team:
The Swift team now believes that GRB 060218 is most likely an underluminous
GRB. This view is supported by the remarkably long duration of the prompt
emission seen by all three Swift instruments and the probable association
low-z (z=0.033) galaxy (Mirabel GCN 4783; Masetti et al GCN 4803). Although
there are unusual aspects to this event (cf. Gehrels GCN 4787), the high
latitude seems to make a Galactic X-ray transient origin unlikely. Moreover,
the extremely rapid X-ray decay after T+3000 seconds (3 orders of magnitude
in an hour), followed by a slow power law decay in time (alpha = 1.2), looks
very much like a normal GRB XRT lightcurve.
We also draw attention to the chromatic nature of the Swift light curves.
The BAT emission peaked substantially earlier than the XRT emission,
which preceded the UVOT emission peak. Although the low
luminosity inferred from the low-z might suggest a highly off-axis viewing
angle for this burst, an off-axis burst should show achromatic emission
variation, which is not seen here (see predictions by Kumar, P. & Granot, J.
2003, ApJ, 591, 1075, for example.)
The burst continues to be bright in the UVOT, (V= 18.29 +/- 0.07
at 21:29 UT on 20 Feb 2006), so we suggest additional follow-up observations
that might confirm signatures of a host supernova or other evidence that
might clarify the nature of this highly unusual GRB.