{
  "bibcode": "2013GCN.14448....1M",
  "body": "A. Maselli (INAF-IASFPA), A. P. Beardmore (U Leicester),\nA. Y. Lien (NASA/GSFC/ORAU), V. Mangano (INAF-IASFPA),\nC. J. Mountford (U Leicester), K. L. Page (U Leicester),\nD. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the\nSwift Team:\n\nAt 07:47:57 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 130427A (trigger=554620).  Swift slewed immediately to the burst. \nThe BAT on-board calculated location is \nRA, Dec 173.139, +27.692 which is \n   RA(J2000) = 11h 32m 33s\n   Dec(J2000) = +27d 41' 29\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  The BAT light curve shows an extremely bright\ncomplex peak about 20 seconds long starting at T-50, while Swift was \nslewing from the previous pre-planned target, followed by a smaller \npeak during the slew to the burst with emission through at least T+200. \nThe peak count rate was ~ 100,000 counts/sec (15-350 keV), \nat T-40 sec, before the trigger. \n\nThe XRT began observing the field at 07:50:17.7 UT, 140.2 seconds after\nthe BAT trigger. XRT found a bright, uncatalogued X-ray source located\nat RA, Dec 173.1362, 27.7129 which is equivalent to:\n   RA(J2000)  = +11h 32m 32.69s\n   Dec(J2000) = +27d 42' 46.4\"\nwith an uncertainty of 4.7 arcseconds (radius, 90% containment). This\nlocation is 75 arcseconds from the BAT onboard position, within the BAT\nerror circle. No event data are yet available to determine the column\ndensity using X-ray spectroscopy. \n\nUVOT took a finding chart exposure of 150 seconds with the White filter\nstarting 147 seconds after the BAT trigger.  A blurred bright source\nappears to be located near the XRT position in the initial 2.7'x2.7'\nsub-image.  However, the lack of reference stars and lack of star tracker\nlock prevents a definitive identification and the measurement of a position\nor magnitude. \n\nThis is an extremely bright burst in all three instruments,\nand it was also seen by Fermi/GBM. \n\nAfter the slew to the burst, the star trackers had trouble\nlocking on to give a spacecraft attitude, so the XRT position\nmay be inaccurate at the arcminute level. \n\nThe apparent optical counterpart appears extremely blurred, \npossibly due to the lack of star tracker lock, but more likely to be \ndue to instrumental effects on a very bright optical counterpart. \n\n\nFurther follow-up is warranted for all ground-based observatories. \n\nBurst Advocate for this burst is A. Maselli (maselli AT ifc.inaf.it). \nPlease contact the BA by email if you require additional information\nregarding Swift followup of this burst. In extremely urgent cases, after\ntrying the Burst Advocate, you can contact the Swift PI by phone (see\nSwift TOO web site for information: http://www.swift.psu.edu/too.html.)",
  "circularId": 14448,
  "createdOn": 1367051053000,
  "email": "palmer@lanl.gov",
  "subject": "GRB 130427A: Swift detection of a very bright burst with a likely bright optical counterpart",
  "submitter": "David Palmer at LANL  <palmer@lanl.gov>",
  "eventId": "GRB 130427A"
}