Gamma-ray Burst 071028B

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN Circular #7019
  J. R. Cummings (CRESST/NASA/GSFC) and J. Greiner (MPE) for the Swift team At 02:43:46 UT on October 28, the Swift Burst Alert Telescope (BAT) triggered on GRB 071028B (trigger #295492). The burst was on the edge of the field of view (4.1% coded) and no source was found automatically onboard. A source was found during later ground analysis at a location RA, Dec 354.163, -31.630 which is

     RA(J2000)  =  23h 36m 39s
     Dec(J2000) = -31d 37' 47"
  

  with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows two FRED peaks, the first at T+1 sec about 4 seconds long, the second at T+48 sec about 9 seconds long. The peak count rate was about 1500 counts/sec (15-350 keV), at ~1 sec after the trigger.
  Because the burst was detected on the ground, there are no prompt XRT or UVOT data. A Swift TOO has been scheduled for October 30, at approximately 1400 UT.
  There have been several ground-detected Swift-BAT bursts lately: GRB 071006 (GCN circ #6858), GRB 071010C (GCN circ #6906) and this one. We have not changed our procedures substantially, and we do not believe that we have missed many such bursts in the past. It apparently is merely an odd statistical fluctuation of rare events: Two weak bursts on the edge of the BAT field of view and another immediately before a preplanned slew maneuver.
  Five such bursts have been noted before, one occurring just before a preplanned slew, GRB 051012 (GCN circ #4093), one combining both factors, GRB 070227 (GCN circ #6156), and the other three being very weak/short, GRB 051114 (GCN circ #4272), GRB 060505 (GCN circ #5076), and GRB 070406 (GCN circ #6247). A fifth nearly similar case was a very long and weak burst, GRB 060123 (GCN circ #4608), producing a significant detection onboard though automatic long-image triggers were not enabled at that time.
• GCN Circular #7040
  Dirk Grupe (PSU) and Patricia Schady (MSSL/UCL) report on behalf of the Swift team:
  The Swift XRT began observing the field of GRB 071028B (GCN 7019; Cummings 2007) on 2007 October 30 13:36 UT approximately 212 ks after the burst. Note that this burst was discovered in ground processing of the BAT data. Therefore no immediate observations after the burst were performed. There are clearly three sources within the 3' BAT error circle. The brightest of these (0.01 counts/s) coincides with a known source listed in the USNO catalogue. There is one source nearly at the center of the BAT error circle at RA-2000, Dec-2000 354.16300, -31.62050 which corresponds to

  RA-2000 = 23 36 39.1
  Dec-2000 = -31 37 13.8
  

  with a 4.8" XRT error circle. The second source is located at 354.11579, -31.63948 corresponding to

  RA-2000: 23 36 27.8
  Dec-2000: -31 38 22.1
  

  with a 4.8" XRT error circle.
  UVOT observed the field of GRB 071028B for a total of 6443s in the V-filter. There is no optical counterpart within the XRT error circles of the first and second source. The 3-sigma upper limits are V > 21.3mag and V >21.6 mag, respectively. These values are not corrected for the expected Galactic extinction corresponding to a reddening of E(B-V) = 0.02 mag in the direction of the burst (Schlegel et al. 1998). There are no counterparts at either of the source positions in the USNO catalogue within the XRT error radius (90% containment).
  This circular is an official product of the Swift team
• GCN Circular #7064
  Dirk Grupe (PSU) reports on behalf of the Swift XRT team:
  The Swift XRT re-observed the field of GRB 071028B (GCN 7019; Cummings 2007) on 2007 November 07 for 9 ks and 2007 November 10 for 11ks. Clearly the first source listed in GCN Circ 7040 (Grupe & Schady, 2007) had faded while the second source is still visible at a similar level as reported in GCN 7040. It is therefore clear that this first source was the X-ray afterglow of GRB 070128, since its flux decreased. The position of the X-ray afterglow of GRB 071028B is RA-2000, Dec-2000 354.16300, -31.62050 which corresponds to

  RA-2000 = 23 36 39.1
  Dec-2000 = -31 37 13.8
  

  with a 4.8" XRT error circle. The 3-sigma upper limit at the position of the burst combining both data sets (20ks) is 6.7e-4 counts/s which converts roughly to 3e-14 ergs/s/cm2.
  This circular is an official product of the Swift team
• GCN Report 105.1
  GCN_Report 105.1 has been posted: http://gcn.gsfc.nasa.gov/reports/report_105_1.pdf by D. Grupe at PSU titled: "Final Swift Observations of GRB 071028B"
• 1103.6130 from 1 Apr 11
  C. Clemens et al.: GRB 071028B, a burst behind large amounts of dust in an unabsorbed galaxy
  We report on the discovery and properties of the fading afterglow and underlying host galaxy of GRB 071028B, thereby facilitating a detailed comparison between these two. Observations were performed with the Gamma-ray Burst Optical and Near-infrared Detector at the 2.2 m telescope on the La Silla Paranal Observatory in Chile. We conducted five observations from 1.9 d to 227.2 d after the trigger and obtained deep images in the g'r'i'z' and JHKs bands. Based on accurate seven-channel photometry covering the optical to near-infrared wavelength range, we derive a photometric redshift of z = 0.94 +0.05 -0.10 for the unabsorbed host galaxy of GRB 071028B. In contrast, we show that the afterglow with an intrinsic extinction of AV(SB) = (0.70 +/- 0.11) mag is moderately absorbed and requires a relatively flat extinction curve. According to the reported Swift/BAT observations, the energetics yield an isotropic energy release of E(gamma,iso.,rest) = (1.4 +2.4 -0.7) x 10^51 erg.