TITLE: GCN CIRCULAR NUMBER: 3385 SUBJECT: GRB 050509b: Swift-BAT refined analysis of the short hard burst DATE: 05/05/09 07:06:49 GMT FROM: Scott Barthelmy at NASA/GSFC S. Barthelmy (GSFC), L. Barbier (GSFC), J. Cummings (GSFC/NRC), E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (UMD), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/NRC), G. Sato (ISAS), M. Suzuki (Saitama), J. Tueller (GSFC), T. Mitani (ISAS), F. Marshall (GSFC), T. Takahashi (ISAS) on behalf of the Swift/BAT team: At 04:00:19.23 UT Swift-BAT detected GRB 050509b (trigger=118749) (GCN Circ 3381, C. Hurkett et al.). The refined BAT ground position is (RA,Dec) = 189.073,+28.991 {12:36:18,28:59:28} +- 2.8 arcmin [deg; J2000], (95% containment). This is 59 arcsec from the XRT position (Kennea et al., GNC 3383). The burst was in the fully-coded FOV. Using the event-by-event data, the lightcurve shows a single spike with a duration of ~30msec. Further, the hardness ratio S(50-100)/S(25-50) of ~1.5 puts this burst cleanly in the short-hard cluster on the hardness_ratio vs duration scatter-plot. Using a simple power-law model, the photon index of the time-averaged spectrum is 1.5 +- 0.4. The fluence in the 15-350 keV band is (2.3 +- 0.9) x 10^-8 erg/cm2. The 1-s peak photon flux in the 15-350 band is (1.57 +- 0.36) ph/cm2/s. All the quoted errors are at the 90% confidence level. We note that location is consistant with the galaxy cluster NSC J123610+285901 which has a redshift of z=0.22, which roughly translates to a distance of ~1 Gpc. We also note that at this distance and the observed flux, this is is an order of magnitude farther than the 27-Dec-04 SuperFlare event from SGR1806-20 could be seen.