TITLE: GCN CIRCULAR NUMBER: 9438 SUBJECT: GRB 090530: Swift detection of a burst with an optical counterpart DATE: 09/05/30 03:30:01 GMT FROM: Scott Barthelmy at NASA/GSFC J. K. Cannizzo (NASA/UMBC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (NASA/UMBC), C. Gronwall (PSU), S. T. Holland (CRESST/USRA/GSFC), V. Mangano (INAF-IASFPA), C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), T. Sakamoto (NASA/UMBC), B. Sbarufatti (INAF-IASFPA), M. H. Siegel (PSU), M. Stamatikos (NASA/ORAU) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 03:18:18 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090530 (trigger=353567). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 179.395, +26.584 which is RA(J2000) = 11h 57m 35s Dec(J2000) = +26d 35' 01" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single peak with a duration of about 10 sec. The peak count rate was ~5000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 03:19:33.1 UT, 74.7 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 179.41927, 26.59282 which is equivalent to: RA(J2000) = 11h 57m 40.62s Dec(J2000) = +26d 35' 34.2" with an uncertainty of 3.9 arcseconds (radius, 90% containment). This location is 84 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (1.78e+20 cm^-2, Kalberla et al. 2005), with an excess column of 2.1 (+2.01/-1.71) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 6.42e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 83 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 11:57:40.50 = 179.41876 DEC(J2000) = +26:35:38.6 = 26.59405 with a 90%-confidence error radius of about 0.75 arc sec. This position is 4.7 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.39 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.02. Burst Advocate for this burst is J. K. Cannizzo (cannizzo AT milkyway.gsfc.nasa.gov). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/too.html.)