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
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:
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.
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
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
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).