{
  "bibcode": "2008GCN..7179....1B",
  "editedOn": 1777053113356,
  "circularId": 7179,
  "email": "dnburrows@gmail.com",
  "submitter": "David Burrows at PSU/Swift  <dnburrows@gmail.com>",
  "version": 2,
  "subject": "Swift observations of the X-ray transient in NGC 2770 (aka SN2008D)",
  "body": "D. N. Burrows (PSU), J. Cummings (GSFC), H. Krimm (GSFC), N. Gehrels \n(GSFC), K. L. Page (U. Leicester), S. Barthelmy (GSFC), E. Berger \n(Princeton), A. Cucchiara (PSU), D. Fox (PSU), S. Immler (GSFC), P. \nT. O'Brien (U. Leicester), and A. M. Soderberg (Princeton) report on \nbehalf of the Swift team:\n\nWe report on additional analysis of the initial Swift BAT data and \ncontinuing XRT observations of the X-ray transient in NGC 2770 \n(Berger & Soderberg, GCN Circ. 7159), for which the optical \ncounterpart (Deng & Zhu, GCN 7160)  has been spectroscopically \nidentified as a Type Ibc supernova (SN2008D;  Malesani et al., GGN \nCirc. 7169).\n\nQuite fortunately, this object was in the BAT field of view in the \ntwo previous Swift observations (of BZQ J0618+4620 beginning at \n13:04:12.33 and of SN2007ax beginning at 13:12:24.5 UT on 9 Jan \n2008).  The Swift timeline was:\n\nObs 1:\n13:02:02.3: begin slewing to BZQ J0618+4620 (RA, Dec = 94.530, +46.359)\n13:04:12.3: end slew, settled on target.  NGC 2770 in BAT field of \nview with 54% coding.\nObs 2:\n13:12:02.3: begin slewing to SN2007ax (RA, Dec = 125.679, +22.556)\n13:13:24.5: end slew, settled on target.  NGC 2770 in BAT field of \nview with ~90% coding\nObs 3:\n13:32:02.3: begin slewing to SN2007uy in NGC 2770 (RA, Dec = 137.407, +33.126)\n13:32:48.9: end slew, settled on target.  NGC 2770 in BAT field of \nview with 100% coding.\n\n13:50:02.1: begin slewing to RX J0923.5+5745 (RA, Dec = 140.887, +57.794)\n\nBAT did not trigger on a GRB during the observations of either BZQ \nJ0618 or SN2007ax.  An examination of the data from the direction of \nNGC 2770 during those observations shows no sign of any emission from \nthis source in the BAT energy range, with upper limits of 1.0e-7 \nerg/cm2 during the BZQ observation (Obs 1) and 1.1e-7 erg/cm2 during \nthe SN2007ax observation (Obs 2).  During the observation of SN2007uy \nin NGC 2770 (Obs 3), when the X-ray transient occurred in the XRT \ndata, the BAT upper limit is 8.9e-8 erg/cm2.  These upper limits are \nfor the band 15-150 keV, assuming a power law photon index of \n1.  Extrapolating the X-ray transient spectrum from Obs 3 (Page et \nal., GCN Circ. 7170) into the 15-150 keV band, the extrapolated \nfluence is 5.2e-8, below the BAT fluence limit for this time interval.\n\nDuring the slews, XRT collected no data and BAT collected only count \nrates.   There is no sign of a GRB in the BAT field of view during \nthe slews, based on count rates during those time intervals.\n\nTo summarize, there is no indication of a GRB in the BAT energy range \n(15-350 keV) during the 30 minutes before the X-ray transient is seen \nin the XRT data, or during the observation of SN2007uy, which lasted \nover 1000 s (i.e., coincident with the X-ray transient).  The BAT \nupper limits are consistent with the extrapolation of the spectrum of \nthe X-ray transient into the 15-150 keV band.\n\nThe nature and onset time of the X-ray transient are not entirely \nclear.  The source was already quite bright (~2 count/s) in the XRT \nband (0.3-10 keV) when the first exposure was taken beginning at \n13:32:48.9 UT, after which it rose rapidly to a peak count rate of \nabout 5 count/s for ~90 s before decaying rapidly (Page et al., GCN \nCirc. 7170).  We note that the estimate for T0 given by Modjaz et al. \n(GCN Circ. 7175) is therefore only a limit; an earlier T0 cannot be \nexcluded due to the lack of prior X-ray data.  The X-ray transient \nlightcurve and hard-to-soft spectral evolution are consistent with \neither a prompt, broad XRF or an X-ray flare (this event is similar \nto the giant X-ray flare in GRB 050502B); if interpreted as a flare, \nT0 could be hundreds of seconds earlier.\n\nXRT observations are continuing.  Analysis of the late XRT data \n(after the first orbit) is complicated by the presence of a nearby \nweak source that contaminates the late-time data from the \ntransient.  Our analysis, using a very small source extraction region \nto reduce the contamination from this nearby source, suggests that \nthe late-time light curve has a shallow decay reminiscent of the \nplateau phase often seen in GRB X-ray afterglows, but lasting much \nlonger than is typical for GRB afterglows (up to at least 200 ks \nafter the first XRT detection).  The late-time decay index is -0.67 \n+0.29/-0.33.\n\nThe X-ray transient peak could be the onset of an XRF, or it could be \nan X-ray flare in an XRF that began somewhat earlier.  The lack of an \nassociated GRB in the BAT data suggests that there may be no GRB \nassociated with this event (either due to a lack of ultrarelativistic \nejecta or because it is not beamed towards us), in which case the \nX-ray transient could be related to shock breakout.  Alternatively \nthis could indeed be a weak XRF with an unusually soft spectrum \n(Berger & Soderberg, GCN Circ. 7159).",
  "eventId": "SN 2008D",
  "editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov>",
  "createdOn": 1200257808000
}