Gamma-ray Burst 090720

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN Circular #9688
  Arne Rau (MPE) reports on behalf of the Fermi GBM Team:
  "At 06:38:08.28 UT on 20 July 20009, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 090720 (trigger 269764690 / 090720276).
  The on-ground calculated location, using the GBM trigger data, is RA = 199.5, DEC = -16.4 (J2000 degrees, equivalent to 13 h 18 m, -16 d 24 '), with an uncertainty of 5.0 degrees (radius, 1-sigma containment, statistical only; there is additionally a systematic error which is currently estimated to be 2 to 3 degrees).
  The angle from the Fermi LAT boresight is 113 degrees.
  This burst was also independently detected (but not triggered) by INTEGRAL SPI-ACS.
  The GBM light curve shows a single peak with a duration (T90) of about 7.0 s (8-1000 keV). The time-averaged spectrum from T0-1.856 s to T0+5.312 s is best fit by a power law function with an exponential high energy cutoff. The power law index is -0.75 +/- 0.10 and the cutoff energy, parameterized as Epeak, is 117.50 +/- 7.12 keV (chi squared 507 for 507 d.o.f.).
  The event fluence (8-1000 keV) in this time interval is (2.9 +/- 0.1)E-06 erg/cm^2. The 1.024-sec peak photon flux measured starting from T0+0.002 s in the 8-1000 keV band is 10.9 +/- 0.3 ph/s/cm^2.
  The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."
• GCN Circular #9694
  J. R. Cummings (NASA/GSFC/CRESST), S. Holland (NASA/GSFC/CRESST), M. Stamatikos (NASA/ORAU) on behalf of the Swift Team
  At 06:38:08 on July 20, 2009 Swift-BAT rate-triggered on GRB 090720 (BAT trigger #357811). The burst was also detected by the Fermi GBM (trigger #269764690, Rau, GCN #9688). No source was found onboard in automated processing. A significant source on the edge of the BAT field of view was found in ground processing. The position was RA, Dec 203.694, -10.335, which is equivalent to:

  RA (J2000) 13h 34m 46.6s
  Dec (J2000) -10d 20m 06s
  

  with an error radius of 3 arcmin (estimated 90% containment). This was 6 degrees from the Fermi GBM ground position, just outside the GBM 1-sigma radius.
  As seen in BAT, the burst was a single weak pulse about 3 seconds long.
  A Swift TOO has been requested and approved.
  The Swift Burst Advocate for this burst is Erik Hoversten (hoversten at astro dot psu dot edu). 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.)
• GCN Circular #9697
  D. Grupe (PSU), S. R Oates (MSSL-UCL), E. A. Hoversten (PSU), P.A. Evans and M.R. Goad (U. Leicester) report on behalf of the Swift team:
  Swift XRT and UVOT started observing the field of GRB 090720 11 hours after the Fermi GBM and Swift/BAT triggers (Rau et al. GCN circ 9688, Cummings et al. GCN circ 9694).
  Using 1233 s of XRT Photon Counting mode data and one UVOT image, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 203.68020, -10.33480 which is equivalent to:

  RA (J2000):  13 34 43.25
  Dec (J2000): -10 20 05.3
  

  with an uncertainty of 3.0 arcsec (radius, 90% confidence).
  This position may be improved as more data are received. The latest position can be viewed at http://www.swift.ac.uk/xrt_positions. Position enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, arXiv:0812.3662).
  Swift has continued observing the field of GRB 090720 and the source is clearly fading. Currently it is at a level of less than 1.6e-3 counts/s (or 5.0 e-14 ergs/s/cm2). Due to the low number of counts, no spectral analysis can be performed.
  With the Swift/UVOT, we do not detect any source at the Swift XRT position. The 3-sigma upper limits for the finding chart exposure is:

  Filter   T_start(s)   T_stop(s)   Exp(s)  Mag (3-sigma upper limit)
  -------------------------------------------------------------
  white      41,776      63,952      579          > 21.28
  u          39,573      63,855      3513         > 21.10
  -------------------------------------------------------------
  

  The quoted upper limits have not been corrected for the expected Galactic extinction along the line of sight of E_(B-V) = 0.05 mag. All photometry is on the UVOT photometric system described in Poole et al. (2008, MNRAS, 383, 627).
  This circular is an official product of the Swift team.
• GCN Circular #9699
  A. Cucchiara (PSU), S. B. Cenko (UC Berkeley), D. B. Fox (PSU), E. Ofek (Caltech), N. Tanvir (U. Leicester) report:
  "On April 21.26 UT and 22.24 UT we obtained two R-band images of the Fermi/Swift GRB 090720 (GCN 9688, 9694) with GMOS on the Gemini-North telescope for a total exposure time, at each epoch, of 15 minutes.
  In the coadded images we identify a possible R counterpart inside the XRT error circle (Grupe et al., GCN 9697) at the coordinates:

  RA (J2000): 13:34:43.33
  Dec (J2000): -10:20:04.9
  

  with an uncertainty of 0.5".
  The estimated magnitude at the two epochs are:

  R = 21.50 +- 0.03 mag
  R = 22.34 +- 0.04 mag
  

  The implied fading, by 0.84 +- 0.05 mag between our two epochs, is consistent with a power-law decay of index alpha=1.12 +- 0.11 referenced to the burst trigger time. We conclude from this fading behavior that the source is the optical afterglow of GRB 090720.
  Magnitude values are calibrated using the USNO B1 star at

  RA= 13:34:39.9
  Dec= -10:20:17.06
  

  which has tabulated magnitude R = 19.79. We thank the Gemini-N staff for the support in performing this observations, in particular Tom Geballe and Paul Hirst."
• GCN Report 233.1
  GCN_Report 233.1 has been posted: http://gcn.gsfc.nasa.gov/reports/report_233_1.pdf by E.A. Hoversten at PSU titled: "Final Results on Swift GRB 090720A"