GRB 080721

F. E. Marshall (NASA/GSFC), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), P. A. Evans (U Leicester),
N. Gehrels (NASA/GSFC), C. Gronwall (PSU), C. Guidorzi (INAF-OAB),
S. T. Holland (CRESST/USRA/GSFC), V. Mangano (INAF-IASFPA),
P. T. O'Brien (U Leicester), J. P. Osborne (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL),
D. Perez (U Leicester), M. Perri (ASDC), B. Preger (ASDC),
P. Romano (INAF-IASFPA), M. Stamatikos (NASA/ORAU),
R. L. C. Starling (U Leicester), T. N. Ukwatta (GSFC/GWU),
P. A. Ward (MSSL-UCL) and H. Ziaeepour (UCL-MSSL) report on behalf of
the Swift Team:

At 10:25:16 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080721 (trigger=317508).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 224.471, -11.712 which is 
   RA(J2000) = 14h 57m 53s
   Dec(J2000) = -11d 42' 43"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed at least 4 peaks
structure with a duration of about 35 sec.  The peak count rate
was ~19k counts/sec (15-350 keV), at ~0 sec after the trigger,
during the second peak. 

The XRT began observing the field at 10:27:04.1 UT, 107.2 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 224.4899, -11.7242 which is equivalent to:
   RA(J2000)  = 14h 57m 55.9s
   Dec(J2000) = -11d 43' 27.3"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 79 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

UVOT took a finding chart exposure of nominal 100 seconds with the White
(160-650 nm) filter starting 180 seconds after the BAT trigger. There is a
candidate afterglow in the list of sources generated on-board at
  RA(J2000)  =  14:57:55.8 = 224.48268
  DEC(J2000) = -11:43:25   = -11.72361
with a 1-sigma error radius of about 1.0 arc sec. The estimated white
magnitude is 14.6 with a 1-sigma error of about 0.01 mag. No correction
has been made for Galactic extinction. 

Burst Advocate for this burst is F. E. Marshall (marshall 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.)

C. Guidorzi (INAF-OAB), P. A. Evans and K. L. Page (U Leicester)
report on behalf of the Swift-XRT team:

We note that the correct XRT position of GRB 080721 (Marshall et al.,
GCN Circ. 7988
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 ) is that given in sexagesimal coordinates, which
corresponds to RA, Dec 224.48292, -11.72425 degrees (J2000; 4.7 arcsec
radius, 90%) and is also consistent with the UVOT position.
The XRT position given in degrees in the same circular, which has also
been distributed in the XRT position notice, is not correct.
We apologise for any inconvenient this may have caused.

T-W Chen, L-C Huang, Y-T Chen (NCU)
K.Y Huang (ASIAA) and Y. Urata (Saitama U./ASIAA)
on behalf of the EAFON report;

"We have been monitoring the optical afterglow of GRB080721 (Marshall
et al. GCN Circ 7988
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 ) using the Lulin 1 m telescope after the twilight
(~ 96 min after the burst). Based on Lulin R-band data, the optical
afterglow is located at RA=14:57:55.835, Dec=-11:43:24.76 with less
than 1" position error.  The brightness of the optical afterglow at 98
min after the burst is R~18.1.

Further optical observation is in progress."

S. T. Holland, P. A. Ward (MSSL-UCL), and F. E. Marshall (NASA/GSFC)
report on behalf of the Swift/UVOT Team:

Preliminary UVOT photometry of the afterglow of GRB 080721 (Marshall,
et al. 2008, GCN Circ. 7988
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 ) shows that the afterglow is strongly
detected in the U band, but not in the UVW1, UVM2, or UVW2 bands at
approximately 825 s after the BAT trigger.  This suggests that the
redshift of GRB 080721 may be greater than approximately 2.3.

J. Cummings (GSFC/UMBC), S. D. Barthelmy (GSFC), W. Baumgartner (GSFC/UMBC),
E. Fenimore (LANL), N. Gehrels (GSFC), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), F. E. Marshall (NASA/GSFC), K. McLean (GSFC/UMD),
D. Palmer (LANL), T. Sakamoto (GSFC/UMBC), G. Sato (ISAS),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-240 to T+750 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 080721 (trigger #317508)
(Marshall, et al., GCN Circ. 7988
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 ).  The BAT ground-calculated position is
RA, Dec = 224.481, -11.709 deg, which is 
   RA(J2000)  =  14h 57m 55.4s 
   Dec(J2000) = -11d 42' 33.3" 
with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 4%.
 
The mask-weighted light curve shows approximately six peaks.  The first starts
at ~T-11 sec and peaks at ~T-7 sec.  The second and third peaks are much 
brighter.  They peak at ~T+1 and ~T+7 sec.  The remaining peaks are much smaller
and stretch out to ~T+370 sec.  T90 (15-350 keV) is 16.2 +- 4.5 sec
(estimated error including systematics).
 
The time-averaged spectrum from T-2.6 to T+25.8 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.11 +- 0.08.  The fluence in the 15-150 keV band is 1.2 +- 0.1 x 10^-5 erg/cm2.
The 1-sec peak photon flux measured from T-0.16 sec in the 15-150 keV band
is 20.9 +- 1.8 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/317508/BA/

J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 2722 s of XRT Photon Counting mode data and 6 UVOT
images for GRB 080721, 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 = 224.48231, -11.72363 which is equivalent
to:

RA (J2000): 14h 57m 55.75s
Dec (J2000): -11d 43' 25.1"

with an uncertainty of 1.5 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
http://www.swift.ac.uk/xrt_positions/Goad.pdf), the current algorithm is an
extension of this method.

This circular was automatically generated, and is an official product of the
Swift-XRT team.

C. Guidorzi, F. Pasotti (INAF-OAB), F. E. Marshall (NASA/GSFC)
report on behalf of the Swift-XRT team:

We have analysed the first three orbits of Swift-XRT data obtained for GRB
080721 (Marshall et al., GCN Circ. 7988
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 ), covering 1.3 ks and 2.8 ks
of Windowed Timing (WT) and Photon Counting (PC) modes data, respectively,
between 113 s and 10.6 ks after the trigger.
Notably, the whole first orbit data are taken in WT mode.
The UVOT-enhanced XRT position was given by Osborne et al. in GCN Circ. 
7993
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 .

The light-curve can be modelled by an initial power law with index
0.75 +/- 0.04 followed by a constant rate step from ~260 to 330 s,
after which the curve resumes a power-law decay with an index of
1.06 +/- 0.01 up to 10.6 ks (chisq/dof = 1292/1207).

A spectrum extracted from the first orbit of data can be modelled with an
absorbed power-law, with Gamma = 1.90 +/- 0.02 and
NH = (1.10 +/- 0.05)x1e21 cm^-2, in excess of the Galactic column
in this direction of 6.9e20 cm^-2. We found no evidence for spectral
evolution during this time interval.
The observed (unabsorbed) flux over this time interval (113-1370 s
post-trigger) is 2.2e-9 (2.7e-9) erg cm^-2 s^-1.

If the light-curve continues to decay with alpha ~1.06, the count rate 24
hours after the burst is estimated to be 0.23 count s^-1, which
corresponds to an observed flux of 9.6e-12 erg cm^-2 s^-1.

This is an official product of the Swift-XRT team.

S. Golenetskii, R.Aptekar, E. Mazets, V. Pal'shin, D. Frederiks, P.
Oleynik, M. Ulanov, D. Svinkin, and
T. Cline on behalf of the Konus-Wind team report:

The bright GRB 080721 (Swift-BAT trigger #317508: Marshall et al., GCN
7988
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 , Cummings et al., GCN 7992
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 ) triggered Konus-Wind at T0=37510.927 s 
UT (10:25:10.927).

The burst light shows multipeaked structure with a duration of ~30 s.

As observed by Konus-Wind the burst
had a fluence of 8.38(-0.60, +0.62)x10^-5 erg/cm2,
and a 64-ms peak flux measured from T0+9.648 s
of (1.96 +/- 0.31)x10^-5 erg/cm2/s
(both in the 20 keV - 5 MeV energy range).

The time-integrated spectrum of the burst
(from T0 to T0+23.808 s) is well fitted (in the 20 keV - 5 MeV range)
by GRBM (Band) model for which:
the low-energy photon index is alpha = -0.933(-0.084, +0.106),
the high energy photon index beta = -2.43(-0.42, +0.24),
the peak energy Ep = 485(-59, +67) keV (chi2 = 60.6/76 dof).

All the quoted errors are at the 90% confidence level.

The Konus-Wind light curve of this GRB is available
at http://www.ioffe.rssi.ru/LEA/GRBs/GRB080721_T37510/

P. Ward (MSSL-UCL),  S. T. Holland (CRESST/USRA/GSFC) and F. E. Marshall 
(NASA/GSFC) report on behalf of the Swift/UVOT team:

The Swift Ultra-Violet/Optical Telescope (UVOT) started observing
GRB 080721 (Marshall et al., GCN 7988
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 ) ~118 seconds after the trigger.

A fading afterglow is found in the UVOT White, V, B and U filters. The 
refined Swift/UVOT position of the optical afterglow candidate is:

  RA(J2000)  = 14:57:55.855 =  224.48273
  DEC(J2000) = -11:43:24.54 = -11.723483

with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence).
The following table gives the measured magnitudes:

Filter  T_start T_stop Exp     Mag

white  118    217   99.8   14.58 +/- 0.00
white  858    957   99.8   16.94 +/- 0.03
white 5173   5372  199.8   19.12 +/- 0.08
white 6609   6808  199.8   19.43 +/- 0.11
v      225    624  399.7   14.98 +/- 0.01
v      964   1363  399.8   16.47 +/- 0.03
v     5583   5782  199.8   18.27 +/- 0.17
v     7020   7157  137.7   18.40 +/- 0.23
b      704   6603  409.4   16.56 +/- 0.12
u      680   6398  439.3   16.58 +/- 0.12
uvw1   655   6192  439.2   > 18.01
uvm2   631   5987  245.3   > 17.58
uvw2   734   7014  419.4   > 18.03

Using the measured magnitudes we find a power law decay index of -1.2 and 
-1.01 in the white and V filters respectively. The strong detection in the 
U band, but not in the UVW1, UVM2, or UVW2 bands gives an estimated 
photometric redshift in the range of 2.3 < z < 2.8.

The values quoted above are in the UVOT photometric system
(Poole et al. 2008, MNRAS, 383, 627). No correction has been
made for the expected extinction corresponding to E(B-V) of
0.100 mag in the direction of the burst (Schlegel et al. 1998).

[GCN OPS NOTE(12aug08): Per author's request, the "080703" was changed to
"080721" in the first sentence.]

P. D'Avanzo (INAF-OAB), V. D'Elia (INAF-OAR), G. Adreuzzi, G. Tessicini 
(INAF-TNG), L. A. Antonelli, F. Fiore (INAF-OAR), D. Fugazza, C. 
Guidorzi (INAF-OAB) and G. Chincarini (Univ. Bicocca) report, on behalf 
of the CIBO collaboration:

Following the Swift detection of GRB080721 (Marshall et al., GCN 7988
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 ), 
we triggered observations with the DOLORES low resolution spectrograph, 
mounted at the Italian TNG telescope located in the Canary Islands 
(Spain). The observations were performed using the LR-B grism. We took a 
1800 s spectrum starting around Jul 21 at 21:30 UT (about 11.0 hours 
after the burst) and covering the range 300-780 nm with a resolution of 
R ~ 600.

A preliminary analysis of the spectra shows a broad absorption feature 
at lambda_obs ~ 440 nm (based on a preliminary wavelength calibration). 
Interpreting this feature as Ly-alpha we infer a redshift z = 2.602, in 
agreement with the UVOT photometric redshift estimate (Holland et al., 
GCN 7991
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 ; Ward et al. GCN 7996
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 ). At this redshift we can detect also 
(rest frame) other absorption features like: CII (133.4 nm), SiIV 
doublet (139.3-140.2 nm), SiII (152.6 nm), CIV doublet (154.8-155.0 nm).

We are very grateful for the excellent support of the TNG staff.

P. Jakobsson (U. Hertfordshire), D. Malesani, P. M. Vreeswijk,
J. P. U. Fynbo, B. Milvang-Jensen (DARK/NBI), A. de Ugarte Postigo (ESO),
B. Nordstrom (NBI), E. Stonkute (ITPA, Vilnius) and P. Sorensen (NOT)
report on behalf of a larger collaboration:

We observed the optical afterglow of GRB 080721 (Marshall et al., GCN 7988
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 ;
Chen et al., GCN 7990
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 ) with ALFOSC on the Nordic Optical Telescope at
La Palma. The afterglow is well detected in a single 300 s exposure
starting on 21.875 July 2008 (10.6 hr after the GRB). Comparison with
several USNO-B1 stars (R1 magnitudes) yields R = 20.0 +/- 0.2, where most
of the error comes from the calibration. Using the R-band magnitude
provided by Chen et al., the inferred decay index is alpha ~ 0.9.

Low resolution spectra (3 x 30 min) were also obtained. The combined
spectrum displays a strong absorption feature around 4350 A, with the flux
dropping substantially blueward of this feature. Associating it with
Ly-alpha gives a redshift of z ~ 2.6 for the burst. We find several other
absorption features, including O I, Si II, C II, Si IV, C IV, Fe II
and Al II, corresponding to a redshift of z = 2.591 +/- 0.001, consistent
with the redshift reported by D'Avanzo et al. (GCN 7997
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 ) and the UVOT
photometric redshift (Ward et al., GCN 7996
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 ).

L-C Huang, T-W Chen, Y-T Chen (NCU)
K.Y Huang (ASIAA) and Y. Urata (Saitama U./ASIAA)
on behalf of the EAFON report;

"We made further optical observations for the optical afterglow of GRB
080721 (Marshall et al., GCN 7988
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 ; Chen et al., GCN 7990
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 ) using the
Lulin 1 m telescope. The single R-band image with 300 sec exposure
shows the afterglow well with R=21.3 (mid term of the exposure is
26.19 hours after the burst). Comparison with our previous report
(Chen et al. GCN 7990
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 ), the temporal decay index is ~1.0. This is
consistent with the result reported by Jakobsson et al. (GCN 7998
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 ).

Poonam Chandra (NRAO/UVA) and Dale A. Frail (NRAO) report on
behalf of the Caltech-NRAO-Carnegie GRB Collaboration:

"We used the Very Large Array to observe the field of view toward
optically bright GRB 080721 (GCN 7988
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 ) at a frequency of 8.46 GHz
on 2008 July 24.15 UT. The GRB radio afterglow is undetected and
the peak radio flux at the UVOT afterglow position (GCN 7988
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 ) is
93 � 48 uJy.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation operated under cooperative agreement by Associated
Universities, Inc."