Gamma-ray Burst 090823

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN Circular #9835
  J. R. Cummings (NASA/GSFC/CRESST) on behalf of the Swift-BAT team,
  S. Golenetskii, R. Aptekar, E. Mazets, V. Pal'shin, D. Frederiks, and T. Cline on behalf of the Konus-Wind team,
  M. Marisaldi, F. Fuschino, C. Labanti, M. Galli, G. Di Cocco on behalf of the AGILE team,
  K. Yamaoka, M. Ohno, Y. Fukazawa, T. Takahashi, M. Tashiro, Y. Terada, T. Murakami, K. Makishima, and Y. Hanabata on behalf of the Suzaku-WAM team,
  and K. Hurley for the IPN
  At 16:11:12 on 2009-08-23, GRB 090823 triggered the Suzaku WAM. The IPN was notified, and the burst was also observed by KONUS-Wind, Agile-MCAL, and (during a slew maneuver) Swift-BAT. A source was found in ground analysis of BAT event data, which is routinely collected

  during many pre-planned slews, and the source location
  was confirmed by IPN triangulation.
  

  
  The BAT position is RA, Dec 228.711, 60.660 which is equivalent to
  RA, Dec (J2000): 15:14:50.6, 60d 39' 36"
  with an uncertainty radius of 4 arcmin (estimated 90% containment, stat+sys).
  We have triangulated GRB 090823 to a Konus-WAM annulus centered at RA(2000)=3D141.472 (09h 25m 53s) Dec(2000)=3D+9.275 (+9d 16' 31"), whose radius is 52.640 =B1 4.346 deg (3 sigma). The BAT source is 0.384 deg (0.27 sigma) from the center line of the annulus.
  As observed in Suzaku-WAM and Agile-MCAL, the burst had two peaks at about T+1 and T+4 seconds, and had emission above 700 keV but not above 1.4 MeV. The BAT lightcurve shows that the emission began with a slow rise beginning about T-3 seconds, and decayed back to background rates by about T+20 seconds. The BAT event data, from which mask-weighted spectra and images are constructed, began at about T+10.5 seconds, missing 99% of the burst.
  A Swift Target of Opportunity has been requested and approved to follow up this burst with the Swift XRT and UVOT about 32 hours after the burst.
• GCN Circular #9836
  S. Golenetskii, R.Aptekar, D. Frederiks, E. Mazets, V. Pal'shin, P. Oleynik, M. Ulanov, D. Svinkin, and T. Cline on behalf of the Konus-Wind team, report:
  The long GRB 090823 (Cummings et al., GCN 9835) triggered Konus-Wind at T0=58270.606 s UT (16:11:10.606) This burst was also observed by the Suzaku WAM, Agile-MCAL, and (during a slew maneuver) Swift-BAT.
  The burst had a smooth structure with a duration of ~10 s. The Konus-Wind light curves of this GRB are available at http://www.ioffe.ru/LEA/GRBs/GRB090823_T58270/
  As observed by Konus-Wind the burst had a fluence of 8.9(-0.3,+0.5)x10-6 erg/cm2, and a 256-ms peak flux, measured from T0+3.072, of 2.8(-0.25,+0.25)x10-6 erg/cm2/s (both in the 20 keV - 2 MeV energy range).
  The time-integrated spectrum of the burst (from T0 to T0+14.848 s) is well fitted (in the 20 keV - 2 MeV range) by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with Alpha = -0.49(-0.15,+0.17), and Ep = 188(-15,+18)keV (chi2 = 56/61 dof). Fitting by the GRB (Band) model yields to the same Alpha and Ep values with the high energy photon index Beta of -7 (< -2.7)
  The spectrum at the maximum count rate, measured from T0 + 0.256s to T0 + 6.656s, is well fitted (in the 20 keV - 2 MeV range) by the GRB (Band) model with:

  Alpha = -0.45(-0.14,+0.16),
  Beta  = -3.0(-2.2,+0.4), and
  Ep    = 182(-18,+18)keV (chi2 = 63/60 dof).
  

  All the quoted values are preliminary. All the quoted errors are at the 90% confidence level.
• GCN Circular #9838
  J. R. Cummings (NASA/GSFC/CRESST)
  Due to a typographical error, the RA of the source quoted for GRB 090823 in GCN #9835 was incorrect by a large amount. The correct BAT position was 128.711, 60.660 which is equivalent to
  RA, Dec (J2000): 8:34:50.7, 60d 39' 36"
  with an uncertainty radius of 4 arcmin (estimated 90% containment, stat+sys).
  I apologize for the error.
• GCN Circular #9839
  A. Copete, J. Grindlay (Harvard) S. Barthelmy, C. Markwardt, N. Gehrels (NASA/GSFC)
  We hereby report confirmation of the Swift-BAT position of GRB 090823 reported by Cummings et al. (GCN 9835, 9838), from an independent analysis of the BAT event-mode data done by the BAT Slew Survey (BATSS), in the slew that started at 16:10:02 UT and lasted 178 sec. The BATSS-calculated position is RA, Dec = 128.671, +60.641 deg, which is:

  RA (J2000) =  08h 34m 41s
  Dec(J2000) = +60d 38' 29"
  

  with an uncertainty of 4.51 arcmin (90% confidence, including systematics). The burst was detected with a significance of 6.2 sigma and 6.7 sigma from imaging in the 15-50 keV and 15-150 keV energy bands, respectively. Consistent with the report in GCN 9835, the lightcurve in the slew data only spans a 15-sec trailing edge of the prompt emission of the burst before it fades into the background.
• GCN Circular #9840
  D. Grupe (PSU), E. Hoversten (PSU) and Jay Cummings (NASA/GSFC) report on behalf of the Swift team:
  We have analysed the XRT and UVOT data of the IPN burst GRB090823 (Cummings et al. GCN Circ. 9835), beginning on 2009-08-24 at 22:17, 30 hours after the Suzaku WAM trigger. The data contain 4.7 ks in XRT photon counting mode. We clearly detect a possible X-ray afterglow within the 4' BAT error circle given by Cummings and Copete et al. (GCN circs. 9838 and 9839, respectively) at RA-2000,Dec-2000=128.67676, +60.65169 which corresponds to

  RA-2000 = 08h 34m 42.42s
  Dec-2000 = +60d 39' 06.1"
  

  with an uncertainty of 5.5"
  During the 4.7 ks observation on 2009-08-24 the X-ray source had a count rate of (5.7+/-1.1)e-3 counts s-1 in the XRT which converts to an observed flux in the 0.3-10 keV band of (2.9+/-0.5)e-13 ergs s-1 cm-2 assuming a typical GRB afterglow spectrum.
  The Swift/UVOT began settled observations of the field of GRB 090823 108380 s after the BAT trigger (Cummings, et al., GCN Circ. 9835). No optical afterglow consistent with the XRT position is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) for the initial exposures are:

  Filter         T_start(s)   T_stop(s)      Exp(s)         Mag
  
  white        109194       115699         1464         >21.8
  v               110009       116640         1680         >20.1
  u               108380       114954         1597         >20.7
  

  The values quoted above are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.06 in the direction of the burst (Schlegel et al. 1998).
  Swift is planning to re-observe the field of GRB 090823 on 2009-08-27 for 10 ks in order to see if the X-ray source has faded.
  This circular is an official product of the Swift team.
• GCN Circular #9902
  D. Grupe (PSU) reports on behalf of the Swift XRT team:
  Swift has observed the field of IPN burst GRB090823 (Cummings et al. GCN Circ. 9835) twice after the initial 4.7 ks observation on 2009-08-24 (Grupe et al., GCN Circ. 9840) for 9.5 ks on 2009-08-27 and 7.5 ks on 2009-09-05. During the 2009-08-27 observation the X-ray source was clearly detected again at a level of (2.3+/-0.6)e-3 counts s-1 (about 1.1e-13 ergs s-1 cm-2), roughly half of the flux during the first observation. During the 2009-09-05 observation however, the X-ray source is definitely gone and we can only derive a 3-sigma upper limit at a level of 1.9e-3 counts s-1 (9.4e-14 ergs s-1 cm-2). We can confirm that the source listed in GCN circ. 9840 is the X-ray afterglow of GRB 090823.
• GCN Report 241.1
  GCN_Report 241.1 has been posted: http://gcn.gsfc.nasa.gov/reports/report_241_1.pdf by D. Grupe at PSU titled: "Final Results on Swift GRB 090823"