//////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9454 SUBJECT: GRB 090531: Swift detection of a burst DATE: 09/05/31 01:56:55 GMT FROM: Scott Barthelmy at NASA/GSFC M. Stamatikos (NASA/ORAU), S. D. Barthelmy (GSFC), C. Gronwall (PSU), S. T. Holland (CRESST/USRA/GSFC), V. Mangano (INAF-IASFPA) and D. M. Palmer (LANL) report on behalf of the Swift Team: At 01:45:17 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090531 (trigger=353627). Swift could not immediately slew to the burst location. The BAT on-board calculated location is RA, Dec 178.687, +7.799 which is RA(J2000) = 11h 54m 45s Dec(J2000) = +07d 47' 58" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peaked structure with a duration of about 50 sec. The peak count rate was ~1500 counts/sec (15-350 keV), at ~10 sec after the trigger. Because of a Moon constraint, Swift could not slew, and there will be no XRT or UVOT observations until June 3. Burst Advocate for this burst is M. Stamatikos (michael 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.) //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9455 SUBJECT: GRB 090531: P60 Observations DATE: 09/05/31 07:07:45 GMT FROM: S. Bradley Cenko at Caltech S. B. Cenko (UC Berkeley) reports on behalf of a larger collaboration: We have imaged the field of GRB090531 (Stamatikos et al., GCN 9454) with the automated Palomar 60 inch telescope. Observations began at 4:47 UT on 2009 May 31 (~ 4.03 hr after the burst trigger) and were taken in the Sloan r', i', and z' filters. In coadded images, we find no new sources inside the BAT error circle to the following limits (calculated with reference to the SDSS DR7): Filter Magnitude Time Since Burst (hours) ---------------------------------------------------------------------- r' > 21.5 4.22 i' > 21.3 4.25 z' > 20.5 4.30 //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9456 SUBJECT: GRB 090531: GROND upper limits DATE: 09/05/31 08:07:23 GMT FROM: Andrea Rossi at TLS Tautenburg A. Rossi (Tautenburg), F. Olivares, and J. Greiner (both MPE Garching) report on behalf of the GROND team: We observed the field of GRB 090531 (Swift trigger 353627, Stamatikos et al., GCN #9454) simultaneously in g'r'i'z'JHK with GROND mounted at the 2.2m MPI/ESO telescope at La Silla Observatory (Chile). Observations started on 31 May 2009 at 01:47 UT, 2 minutes after the burst and continued until 03:29 UT. We do not find any new source within the BAT error circle. Image subtraction between the first and last images of each band does not reveal any variable source. The first image , consisting of 4.4 min exposure in g'r'i'z' and 4 min in JHK, yields the following upper limits (all AB): g' > 22.6 r' > 22.8 i' > 22.4 z' > 22.0 J > 21.5 H > 20.9 K > 20.2 which were obtained using the SDSS and 2MASS field stars as reference. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9460 SUBJECT: GRB 090531: Swift-BAT refined analysis DATE: 09/05/31 16:36:42 GMT FROM: Scott Barthelmy at NASA/GSFC T. Sakamoto (GSFC/UMBC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (GSFC/UMBC), E. E. Fenimore (LANL), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL), G. Sato (ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. N. Ukwatta (GWU) (i.e. the Swift-BAT team): Using the data set from T-60 to T+243 sec from recent telemetry downlinks, we report further analysis of BAT GRB 090531 (trigger #353627) (Stamatikos, et al., GCN Circ. 9454). The BAT ground-calculated position is RA, Dec = 178.672, 7.824 deg, which is RA(J2000) = 11h 54m 41.4s Dec(J2000) = +07d 49' 27.0" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 73%. The mask-weighted light curve shows a single peak starting at ~T-25 sec, peaking at T+9 sec, and ending at ~T+60 sec. There is a possible (2.5 sigma CL) episode of emission from T+80 to ~T+140 sec. T90 (15-350 keV) is 32 +- 6 sec (estimated error including systematics). The time-averaged spectrum from T-12.6 to T+29.4 sec is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.86 +- 0.42, and Epeak of 64.7 +- 14.3 keV (chi squared 51.91 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.3 +- 0.1 x 10^-6 erg/cm2 and the 1-sec peak flux measured from T+9.29 sec in the 15-150 keV band is 1.3 +- 0.2 ph/cm2/sec. A fit to a simple power law gives a photon index of 1.68 +- 0.09 (chi squared 65.50 for 57 d.o.f.). All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/353627/BA/ //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9461 SUBJECT: GRB 090531B: Swift detection of a burst DATE: 09/05/31 18:44:29 GMT FROM: Scott Barthelmy at NASA/GSFC J. R. Cummings (NASA/UMBC), S. D. Barthelmy (GSFC), N. Gehrels (NASA/GSFC), C. Gronwall (PSU), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), V. Mangano (INAF-IASFPA), C. B. Markwardt (CRESST/GSFC/UMD), P. T. O'Brien (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), T. Sakamoto (NASA/UMBC) and B. Sbarufatti (INAF-IASFPA) report on behalf of the Swift Team: At 18:35:56 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090531B (trigger=353728). Swift will execute a delayed slew to the burst. The BAT on-board calculated location is RA, Dec 252.064, -36.025 which is RA(J2000) = 16h 48m 15s Dec(J2000) = -36d 01' 27" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows an initial bright spike at T_zero, then two much smaller spikes at T+21 sec and T+35 sec. The peak count rate was ~7000 counts/sec (15-350 keV), at ~0 sec after the trigger. Due to an observing constraint, Swift will not slew until T0+50.1 minutes. There will be no XRT or UVOT data until this time. Burst Advocate for this burst is J. R. Cummings (jayc 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.) //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9462 SUBJECT: GRB 090531B: ROTSE-III Optical Limits DATE: 09/05/31 19:20:55 GMT FROM: Brad Schaefer at LSU B. E. Schaefer (Louisiana State), T. Guver (U Arizona), S. B. Pandey (U Mich), report on behalf of the ROTSE collaboration: ROTSE-IIIc, located at the H.E.S.S. site at Mt. Gamsberg, Namibia, responded to GRB 090531B (Swift trigger 353728; J. R. Cummings et al., GCN 9461), producing images beginning 7.3 s after the GCN notice time. An automated response took the first image at 18:36:22.3 UT, 26.1 s after the burst, and during the gamma-ray emission, under excellent conditions. We took 10 5-sec, 10 20-sec and 50 60-sec exposures. These unfiltered images are calibrated relative to USNO A2.0 (R). Imaging is on going. Comparison to the DSS (second epoch) reveals no new sources within the 3-sigma Swift/BAT error circle, for both single images and coadding into sets of 10; however, we are limited as the field is somewhat crowded. Individual images have limiting magnitudes ranging from 15.3-16.0; we set the following specific limits. start UT end UT t_exp(s) mlim t_start-tGRB(s) Coadd? -------------------------------------------------------------------- 18:36:22.3 18:36:27.3 5 15.3 26.1 N 18:47:31.3 18:52:11.6 280 16.3 695.1 Y [GCN OPS NOTE(31may09): Per author's request, the "B" was added to the burst name.] //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9463 SUBJECT: GRB 090531B: Swift-XRT candidate afterglow position DATE: 09/05/31 20:27:50 GMT FROM: Boris Sbarufatti at INAF-IASF-Pa B. Sbarufatti, V. Mangano (INAF-IASFPA) and J. R. Cummings (NASA/UMBC) report on behalf of the Swift-XRT team The XRT began observing the field of GRB 090531B (trigger 353728, Cummins, et al., GCN Circ 9461) at 19:26:22 UT, 3026 seconds after the BAT trigger, as soon as the burst went outside the Earth limb observing constraint. Using promptly downlinked data we find an X-ray source with an enhanced position: RA,Dec = 252.0748, -36.0350 (degrees) which is equivalent to: RA (J2000.0) = 16 48 17.95 DEC (J2000.0) = -36 02 06.0 with an uncertainty of 3.4 arcsec (radius, 90% containment). This location is 47.8 arcseconds from the BAT onboard position, inside the BAT error circle. This circular is an official product of the Swift-XRT team. [GCN OPS NOTE(03sep09): Per operator's desire, the Subject line was corrected from "090831B" to "090531B". F.Marshall and H.Krimm noticed this.] //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9464 SUBJECT: Initial Swift/UVOT observations of GRB 090531B DATE: 09/05/31 20:49:26 GMT FROM: Caryl Gronwall at PSU/Swift-UVOT C. Gronwall (PSU) and J. R. Cummings (NASA/UMBC) report on behalf of the UVOT Team: In response to GRB090531B (Swift/BAT trigger 353728; Cummings et al., GCN Circ. 9461) at 18:35:56 UT, UVOT took a finding chart exposure of 150 seconds with the White filter starting 3848 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle (Sbarufatti et al., GCN Circ. 9463) The typical 3-sigma upper limit has been about 19.6 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 1.12. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9465 SUBJECT: GRB 090531B: Swift-BAT refined analysis DATE: 09/06/01 00:03:02 GMT FROM: Scott Barthelmy at NASA/GSFC T. N. Ukwatta (GWU), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (GSFC/UMBC), E. E. Fenimore (LANL), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL), T. Sakamoto (GSFC/UMBC), G. Sato (ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC) (i.e. the Swift-BAT team): Using the data set from T-61 to T+242 sec from the recent telemetry downlink, we report further analysis of BAT GRB 090531B (trigger #353728) (Cummings, et al., GCN Circ. 9461). The BAT ground-calculated position is RA, Dec = 252.070, -36.015 deg, which is RA(J2000) = 16h 48m 16.8s Dec(J2000) = -36d 00' 53.4" with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%. The mask-weighted light curve shows an initial bright spike with a total width of 1.3 sec. It returns to baseline and then starts (!T+15 sec) a long smooth rise to a peak around T+40 sec and a smooth decline ending at ~T+110 sec. The spike is spectrally harder than the long smooth peak. This lightcurve is consistance with a short hard burst with an extended emission. T90 (15-350 keV) is 80 +- 23 sec (estimated error including systematics). The time-averaged spectrum from T-13.0 to T+67.0 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.51 +- 0.22. The fluence in the 15-150 keV band is 7.1 +- 0.9 x 10^-7 erg/cm2. The 1-sec peak photon flux measured from T+0.2 sec in the 15-150 keV band is 1.25 +- 0.1 ph/cm2/sec. All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/353728/BA/ //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9469 SUBJECT: GRB 090531B: Swift/UVOT Upper Limits DATE: 09/06/01 10:06:42 GMT FROM: Patricia Schady at MSSL/Swift P. Schady (MSSL-UCL) and J. R. Cummings (NASA/UMBC) report on behalf of the Swift/UVOT team: The Swift/UVOT observed the field of GRB 090531B starting 3924 s after the BAT trigger (Cummings et al., GCN Circ. 9461), once the GRB came out of Earth limb observing constraint. No optical afterglow is detected in any of the UVOT exposures at the position of the X-ray afterglow (Sbarufatti et al., GCN Circ. 9463), with the following three-sigma upper limits: Filter T_mid(s) Exp(s) 3-sig Mag UL white 3934 74 > 20.52 v 4105 98 > 19.08 b 4926 98 > 20.06 u 4721 98 > 19.74 uvw1 4515 98 > 19.65 uvm2 4310 98 > 19.32 uvw2 5336 98 > 19.61 The values quoted above are not corrected for the large Galactic extinction due to the reddening of E(B-V)=1.12 in the direction of the burst (Schlegel et al. 1998). The photometry is on the UVOT photometric system described in Poole et al. (2008, MNRAS, 383, 627). //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9470 SUBJECT: GRB 090531B, RIMOTS optical upper limits DATE: 09/06/01 13:32:06 GMT FROM: Eri Sonoda at U of Miyazaki/Japan E. Sonoda, K. Noda, N. Ohmori, K. Kono, H. Hayasi, A. Daikyuji, Y. Nisioka, M. Yamauchi (University of Miyazaki) We have observed the field covering the error circle of GRB090531B (Swift trigger 353728, GCN 9461(J. R. Cummings et al.)) with the unfiltered CCD camera on the 30-cm telescope at University of Miyazaki. The observation was started 18:37:28 UT (92 s after the Swift trigger), under cloudy condition. First image was obtained at 18:41:05 UT (309 s after the Swift trigger). We have compared our data of 30 sec exposures with the USNO-A2.0 catalog. There is no new source at the reported position. (GCN 9462(B. E. Schaefer et al.), GCN 9464(C. Gronwall et al.) GCN 9465(T. N. Ukwatta et al.), GCN 9469(P. Schady et al.)) Preliminary analysis shows there is no new source brighter than 14.6 mag. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9471 SUBJECT: Corrections to GRB 090531B Swift/UVOT Upper Limits DATE: 09/06/01 14:29:44 GMT FROM: Patricia Schady at MSSL/Swift P. Schady (MSSL-UCL) and J. R. Cummings (NASA/UMBC) report on behalf of the Swift/UVOT team: We report a mistake in a few of the times provided in GCN 9469, including the start time of the observations, which was in fact 3849 s after the BAT trigger (Cummings et al., GCN Circ. 9461). Also, the exposure times of the UVOT observations should have been two times the values given. The corrected 3-sigma upper limits for the UVOT observations of the field of GRB 090531B are provided below. Filter T_mid(s) Exp(s) 3-sig Mag UL white 3924 147 > 20.52 v 4105 197 > 19.08 b 4926 197 > 20.06 u 4721 197 > 19.74 uvw1 4515 197 > 19.65 uvm2 4310 197 > 19.32 uvw2 5336 197 > 19.61 The values quoted above are not corrected for the large Galactic extinction due to the reddening of E(B-V)=1.12 in the direction of the burst (Schlegel et al. 1998). The photometry is on the UVOT photometric system described in Poole et al. (2008, MNRAS, 383, 627). //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9475 SUBJECT: GRB 090531B: Swift/BAT spectral lag results DATE: 09/06/01 15:55:43 GMT FROM: Tilan Ukwatta at GSFC/GWU T. N. Ukwatta (GWU/GSFC), S. D. Barthelmy (GSFC), J. R. Cummings (NASA/UMBC), T. Sakamoto (GSFC/UMBC), M. Stamatikos (GSFC/ORAU), N. Gehrels (GSFC) (for the Swift-BAT team): For GRB 090531B (Cummings et al. GCN 9461), the BAT team has analyzed spectral lags for the data from T-1.0 sec to T+2.0 sec using non-mask weighted light curves with 16 msec time binning. The spectral lags were measured between standard canonical BAT energy bands: channel 1 (15-25 keV), 2 (25-50 keV), 3 (50-100 keV) and 4 (100-350 keV) are given below. Lag Ch3-Ch2 : -2 +/- 9 msec Lag Ch4-Ch2 : 11 +/- 5 msec Lag Ch4-Ch3 : 8 +/- 3 msec The signal-to-noise ratio in channel 1 is too weak to make a lag measurement. These lag values are consistent with lags observed for short hard burst with extended emission. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9476 SUBJECT: GRB 090531A: SARA upper limit DATE: 09/06/01 17:27:55 GMT FROM: Adria C. Updike at Clemson U Gary Henson, Shannon Hall, Mark Gorski (ETSU/SARA REU), Adria Updike, Dieter Hartmann (Clemson University) report: The 0.9m SARA telescope located at Kitt Peak National Observatory began imaging the field of GRB 090531A (Stamatikos et al., GCN 9454) about 2.5 hours after the burst under decent weather conditions. In one hour of stacked exposures, we detect no new sources down to a limiting magnitude of R > 20.5 (with respect to the USNO B.1 catalog). No further observations are planned. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9480 SUBJECT: GRB 090531B: GROND observation DATE: 09/06/02 11:58:24 GMT FROM: Jochen Greiner at MPI A. Rossi (Tautenburg), F. Olivares, T. Kruehler and J. Greiner (all MPE Garching) report on behalf of the GROND team: We observed the field of GRB 090531B (Swift trigger 353728, Cummings et al. 2009, GCN #9461) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPI/ESO telescope at La Silla Observatory (Chile). Observations started on 1 June 2009 at 00:03 UT, 5.5 hours after the burst and continued until 00:50 UT under mediocre seeing (~1.5), cirrus and Moon light. A second epoch started at 04:33 UT under better conditions, consisting of 72 min exposure in g'r'i'z' and 60 min in JHK. We find three objects within the XRT error circle (Sbaruffatti et al. 2009, GCN #9463), and another three at the outer circumference of the error circle (see finding chart on http://www.mpe.mpg.de/~jcg/grb090531B.html). Within +-0.2 mag, none of these objects shows any variability between 5.5 and 10 hrs post-burst, and thus are most likely foreground objects. In empty regions of the error circle, the first epoch yields the following upper limits (all AB): g' > 22.9 r' > 22.6 i' > 22.1 z' > 22.8 J > 20.1 H > 19.8 K > 19.2 which were obtained using the USNO and 2MASS field stars as reference. However, given the poor conditions and preliminary photometric calibration, a variable object might be hidden behind one of the foreground objects, and masked in the difference image due to the distorted PSF from the first epoch. In that case, the upper limits would be about 2 mag brighter. No correction has been applied for the strong foreground reddening of E(B-V)=1.11 mag (Schlegel et al. 1998). //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9481 SUBJECT: GRB 090531b, SMARTS optical/IR observations DATE: 09/06/02 21:50:32 GMT FROM: Bethany Cobb at UC Berkeley B. E. Cobb (UC Berkeley) reports: Using the ANDICAM instrument on the 1.3m telescope at CTIO, we obtained optical/IR imaging of the error region of GRB 090531b (GCN 9461, Cummings et al.) over two epochs with mid-exposure times of 2009-06-01 02:56 UT and 2009-06-02 05:26 UT (8.3 and 34.8 hours post-burst). Total summed exposure times for each observation amounted to 36 minutes in R and 30 minutes in J. While there are a number of sources in and around the position of the GRB X-ray afterglow (GCN 9463, Sbarufatti et al.; GCN 9480, Rossi et al.), image differencing of the two epochs using the ISIS image subtraction program does not reveal any significant variable source. The sky limiting magnitude of the images is I>21.1 and J>18.5 (where optical photometry is calibrated using USNO-B1 stars and IR photometry with 2MASS; these limits have not been corrected for Galactic reddening). //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9501 SUBJECT: GRB 090531B: Fermi GBM detection DATE: 09/06/10 01:07:56 GMT FROM: Sylvain Guiriec at UAH S. Guiriec (UAH) reports on behalf of the Fermi GBM Team: "At 18:35:56.49 UT on 31 May 2009, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 090531B (trigger 265487758 / 090531775) which was also detected by the SWIFT-BAT (Cummings et al. 2008, GCN 9461). The GBM on-ground location is consistent with the Swift position. The angle from the Fermi LAT boresight is 25 degrees. The GBM light curve consists of a single peak with a duration (T90) of about 2 s (8-1000 keV). The time-averaged spectrum from T0-0.23 s to T0+0.77 s is adequately fit by a Band function with Epeak = 2166(+675-408) keV, alpha = -0.71(+0.10-0.10), and beta = -2.47(+0.25-0.46) (Castor C-Stat 780 for 602 d.o.f.). The event fluence (8-1000 keV) in this time interval is (6.20 +/- 0.18)E-07 erg/cm^2. The 1-sec peak photon flux measured starting from T0-0.23 s in the 8-1000 keV band is 1.49 +/- 0.04 ph/s/cm^2. The spectral and temporal analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog." //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 9525 SUBJECT: Swift XRT observations of GRB 090531A DATE: 09/06/18 16:21:56 GMT FROM: Boris Sbarufatti at INAF-IASF-Pa B. Sbarufatti, V. Mangano (INAF-IASFPA) on behalf of the Swift-XRT team. The XRT onboard Swift observed the field of GRB 090531A on 2009 June 10 and 2009 June 17, after it came outside the Moon observing contstraint, for a total exposure of 35.5 ks in Photon Counting mode. Three significant sources are detected inside the initial BAT error circle (Stamatikos, et al., GCN circ. 9454). Two of them, detected at a level of 1.2E-3 counts/s and 9.0E-4 counts/s, are compatible with a stable count-rate and are found outside the BAT refined error circle (Sakamoto, et al., GCN circ. 9460). The third source, detected at a level of 6.5E-4 counts/s is too faint to estimate if it is fading or not, and it is found just inside the BAT refined error circle, at a distance of 64.8 arcsec from the BAT refined position. Its coordinates are RA,Dec = 178.65539, +7.81671 which is: RA(J2000) = 11h 54m 37.29s Dec(J2000) = +07d 49' 00.2" with an uncertainty of 4.8 arcsec (radius, 90% confidence). This circular is an official product of the Swift-XRT team. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 10016 SUBJECT: GRB 090531A: Skynet/PROMPT Observations DATE: 09/10/11 21:32:42 GMT FROM: Aaron LaCluyze at U.North Carolina A. LaCluyze, D. Reichart, J. Haislip, K. Ivarsen, A. Foster, J. Moore, A. Oza, M. Schubel, J. Styblova, A. Trotter, J. A. Crain, and M. Nysewander report: Skynet observed the Swift/BAT localization of GRB 090531A (Stamatikos et al., GCN 9454) with three of the 16" PROMPT telescopes at CTIO beginning 6.1 minutes after the trigger in UVRI. We do not detect the afterglow (Sbarufatti & Mangano, GCN 9525). Stacking only images that increase the limiting magnitude yields: mean 1-sig. 1-sig. time 3-sig. sys. stat. since lim. cal. cal. cal. trig. tel. exp. fil. mag. stars* unc. unc. (m) (# x s) (mag) (mag) 6.5 PROMPT-5 1 x 40 I 17.6 32 SDSS 7 0.055 0.002 15.3 PROMPT-5 11 x 80 I 19.4 32 SDSS 7 0.055 0.000 84.0 PROMPT-3 75 x 80 U 19.3 7 SDSS 7 0.037 0.001 92.6 PROMPT-5 85 x 80 I 20.5 32 SDSS 7 0.055 0.000 + 1 x 40 94.1 PROMPT-4 43 x 80 V 21.1 37 SDSS 7 0.007 0.000 104.8 PROMPT-4 54 x 80 R 21.3 40 SDSS 7 0.018 0.000 * Transformed using Jester et al., 2005, ApJ, 130, 873.