{
  "bibcode": "2018GCN.23509....1G",
  "body": "J.D. Gropp (PSU), J. A. Kennea (PSU), S. J. LaPorte (PSU),\nA. Y. Lien (GSFC/UMBC), B. Sbarufatti (PSU), K. K. Simpson (PSU) and\nA. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift\nObservatory Team:\n\nAt 20:28:17 UT, the Swift Burst Alert Telescope (BAT) triggered detected a\nmarginal-significance peak in an untriggered image near the\nline of sight to a nearby galaxy (trigger=874754).  \nSwift slewed immediately to the burst. \nThe BAT on-board calculated location is \nRA, Dec 188.083, +16.309 which is \n   RA(J2000) = 12h 32m 20s\n   Dec(J2000) = +16d 18' 32\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  As is usual with an image trigger, the available\nBAT light curve shows no significant structure. \n\nThe XRT began observing the field at 20:30:40.8 UT, 142.8 seconds after\nthe BAT trigger. No source was detected in 368 s of promptly downlinked\ndata. We are waiting for the full dataset to detect and localise the\nXRT counterpart. \n\nUVOT took a finding chart exposure of 150 seconds with the White filter\nstarting 147 seconds after the BAT trigger. No credible afterglow candidate has\nbeen found in the initial data products. The 2.7'x2.7' sub-image covers 25% of\nthe BAT error circle. The typical 3-sigma upper limit has been about 19.6 mag. \nThe 8'x8' region for the list of sources generated on-board covers 100% of the\nBAT error circle. The list of sources is typically complete to about 18 mag. No\ncorrection has been made for the expected extinction corresponding to E(B-V) of\n0.03. \n\nSwift alerted and followed up on this event due to a program of\nlowering thresholds for image peaks in the vicinity of nearby galaxies,\ntrading an increased false event rate for greater sensitivity. \n\nGiven to the lack of a BAT rate increase, the marginal significance\nof the image peak (5.99 sigma), the distance between the\nimage peak and the putative associated galaxy (11.1 arcmin),\nand the lack of a new source in the XRT data, we believe that\nthis event is a spurious statistical fluctuation in the image. \n\nWe do not believe that this is an astrophysical event.  Unless\nthe full downlinked dataset refutes this belief, no further\nobservations are planned.",
  "circularId": 23509,
  "createdOn": 1543956293000,
  "email": "amy.y.lien@nasa.gov",
  "subject": "Swift Trigger 874754 is likely not an astrophysical event",
  "submitter": "Amy Lien at GSFC  <amy.y.lien@nasa.gov>"
}