TITLE: GCN CIRCULAR NUMBER: 7179 SUBJECT: Swift observations of the X-ray transient in NGC 2770 (aka SN2008D) DATE: 08/01/13 20:56:48 GMT FROM: David Burrows at PSU/Swift D. N. Burrows (PSU), J. Cummings (GSFC), H. Krimm (GSFC), N. Gehrels (GSFC), K. L. Page (U. Leicester), S. Barthelmy (GSFC), E. Berger (Princeton), A. Cucchiara (PSU), D. Fox (PSU), S. Immler (GSFC), P. T. O'Brien (U. Leicester), and A. M. Soderberg (Princeton) report on behalf of the Swift team: We report on additional analysis of the initial Swift BAT data and continuing XRT observations of the X-ray transient in NGC 2770 (Berger & Soderberg, GCN Circ. 7159), for which the optical counterpart (Deng & Zhu, GCN 7160) has been spectroscopically identified as a Type Ibc supernova (SN2008D; Malesani et al., GGN Circ. 7169). Quite fortunately, this object was in the BAT field of view in the two previous Swift observations (of BZQ J0618+4620 beginning at 13:04:12.33 and of SN2007ax beginning at 13:12:24.5 UT on 9 Jan 2008). The Swift timeline was: Obs 1: 13:02:02.3: begin slewing to BZQ J0618+4620 (RA, Dec = 94.530, +46.359) 13:04:12.3: end slew, settled on target. NGC 2770 in BAT field of view with 54% coding. Obs 2: 13:12:02.3: begin slewing to SN2007ax (RA, Dec = 125.679, +22.556) 13:13:24.5: end slew, settled on target. NGC 2770 in BAT field of view with ~90% coding Obs 3: 13:32:02.3: begin slewing to SN2007uy in NGC 2770 (RA, Dec = 137.407, +33.126) 13:32:48.9: end slew, settled on target. NGC 2770 in BAT field of view with 100% coding. 13:50:02.1: begin slewing to RX J0923.5+5745 (RA, Dec = 140.887, +57.794) BAT did not trigger on a GRB during the observations of either BZQ J0618 or SN2007ax. An examination of the data from the direction of NGC 2770 during those observations shows no sign of any emission from this source in the BAT energy range, with upper limits of 1.0e-7 erg/cm2 during the BZQ observation (Obs 1) and 1.1e-7 erg/cm2 during the SN2007ax observation (Obs 2). During the observation of SN2007uy in NGC 2770 (Obs 3), when the X-ray transient occurred in the XRT data, the BAT upper limit is 8.9e-8 erg/cm2. These upper limits are for the band 15-150 keV, assuming a power law photon index of 1. Extrapolating the X-ray transient spectrum from Obs 3 (Page et al., GCN Circ. 7170) into the 15-150 keV band, the extrapolated fluence is 5.2e-8, below the BAT fluence limit for this time interval. During the slews, XRT collected no data and BAT collected only count rates. There is no sign of a GRB in the BAT field of view during the slews, based on count rates during those time intervals. To summarize, there is no indication of a GRB in the BAT energy range (15-350 keV) during the 30 minutes before the X-ray transient is seen in the XRT data, or during the observation of SN2007uy, which lasted over 1000 s (i.e., coincident with the X-ray transient). The BAT upper limits are consistent with the extrapolation of the spectrum of the X-ray transient into the 15-150 keV band. The nature and onset time of the X-ray transient are not entirely clear. The source was already quite bright (~2 count/s) in the XRT band (0.3-10 keV) when the first exposure was taken beginning at 13:32:48.9 UT, after which it rose rapidly to a peak count rate of about 5 count/s for ~90 s before decaying rapidly (Page et al., GCN Circ. 7170). We note that the estimate for T0 given by Modjaz et al. (GCN Circ. 7175) is therefore only a limit; an earlier T0 cannot be excluded due to the lack of prior X-ray data. The X-ray transient lightcurve and hard-to-soft spectral evolution are consistent with either a prompt, broad XRF or an X-ray flare (this event is similar to the giant X-ray flare in GRB 050502B); if interpreted as a flare, T0 could be hundreds of seconds earlier. XRT observations are continuing. Analysis of the late XRT data (after the first orbit) is complicated by the presence of a nearby weak source that contaminates the late-time data from the transient. Our analysis, using a very small source extraction region to reduce the contamination from this nearby source, suggests that the late-time light curve has a shallow decay reminiscent of the plateau phase often seen in GRB X-ray afterglows, but lasting much longer than is typical for GRB afterglows (up to at least 200 ks after the first XRT detection). The late-time decay index is -0.67 +0.29/-0.33. The X-ray transient peak could be the onset of an XRF, or it could be an X-ray flare in an XRF that began somewhat earlier. The lack of an associated GRB in the BAT data suggests that there may be no GRB associated with this event (either due to a lack of ultrarelativistic ejecta or because it is not beamed towards us), in which case the X-ray transient could be related to shock breakout. Alternatively this could indeed be a weak XRF with an unusually soft spectrum (Berger & Soderberg, GCN Circ. 7159).