Gamma-ray Burst 090720
(All information courtesy of the instrument teams.)
Previous IAU Circulars
Results of Observations
- 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
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
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
- 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
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:
by E.A. Hoversten
titled: "Final Results on Swift GRB 090720A"
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Jochen Greiner, last update: 31-Jul-2009