TITLE: GCN CIRCULAR NUMBER: 9417 SUBJECT: GRB 090520 Swift-BAT detection of a burst in ground analysis DATE: 09/05/21 21:04:47 GMT FROM: Jay R. Cummings at NASA/GSFC/Swift J. R. Cummings (GSFC/UMBC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), E. E. Fenimore (LANL), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL), A. M. Parsons (GSFC), T. Sakamoto (GSFC/UMBC), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. N. Ukwatta (GWU) (i.e. the Swift-BAT team): On May 20, 2009 at 01:37:16 UT, BAT triggered on GRB 090520 (trigger #352671). The burst was weak, and the instrument failed to find a source in onboard analysis. A significant source was found in ground analysis. The source is located at RA, Dec 11.613, -8.000 deg, which is RA(J2000) = 00h 46m 27.1s Dec(J2000) = -08d 00' 00" with an uncertainty of 3 arcmin (radius, sys+stat, 90% containment). The partial coding was 48%. The burst consisted of a single FRED pulse, starting about T-2 seconds, with a T90 of 20 ± 5 seconds (estimated error including systematics). The BAT event data for failed triggers is limited to 10 seconds. Using 10 seconds of event data from T-2 to T+8 seconds, the best fit to the spectrum is a simple powerlaw with a photon index of 0.8 +- 0.2. The fluence for this interval in the 15-150 keV band was 3.4 ± 0.5 x 10-7 ergs/cm2 (68% confidence). The BAT source position was about 3 arcmin from a low-redshift galaxy, 6dFGS gJ004613.4-080000. This is an emission-line galaxy at z = 0.0850. A Swift TOO was requested, but initially the source was in the Swift Moon constraint. In any case was 52 degrees (~2.5 hours in RA) from the Sun, not in constraint, but still an unfavorable angle, so no Swift followup observation is planned.