TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4805 SUBJECT: GRB 060218: Likely an underluminous GRB DATE: 06/02/21 19:13:26 GMT FROM: John Nousek at Penn State U/Swift J. Nousek (PSU), G. Cusumano (IASF-Pa INAF), A. Moretti (INAF-OAB), G. Tagliaferri (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. Milano-Bicocca), 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 with a 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 galactic 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.