GRB 070802

S. D. Barthelmy (GSFC), P. A. Evans (U Leicester),
N. Gehrels (NASA/GSFC), S. Immler (GSFC/UMCP),
V. Mangano (INAF-IASFPA), C. B. Markwardt (CRESST/GSFC/UMD),
P. T. O'Brien (U Leicester), J. P. Osborne (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL),
P. Romano (Univ. Bicocca & INAF-OAB), G. Sato (GSFC/ISAS),
B. Sbarufatti (INAF-IASFPA), M. Stamatikos (NASA/ORAU),
G. Tagliaferri (INAF-OAB), D. E. Vanden Berk (PSU) and
S. D. Vergani (DIAS-DCU) report on behalf of the Swift Team:

At 07:07:25 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070802 (trigger=286809).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 36.851, -55.536 which is 
   RA(J2000)  =  02h 27m 24s
   Dec(J2000) = -55d 32' 10"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a peak
with a duration of roughly 30 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~6 sec after the trigger. 

The XRT began taking data at 07:09:43 UT, 138 seconds after the BAT
trigger.  The XRT on-board centroid algorithm did not find a source
in the image, however using 510s of promptly available data we find
an uncatalogued source at Ra, Dec=36.8987, -55.52781 degrees, which is
equivalent to:
  RA(J2000) =  02h 27m 35.69s
  Dec(J2000) = -55d 31' 40.1"

with an estimated uncertainty of 4.4 arcsec (radius, 90% containment). 
This source is 101.6 arcsec from the BAT position, within the
BAT error circle. 

UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm)
filter starting 141 seconds after the BAT trigger. No afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 25% of
the BAT error circle. The typical 3-sigma upper limit has been about 18.5 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
BAT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for extinction of E(B-V) = 0.026. 

S. Immler is the burst advocate for this burst.  Please contact him
for any questions concerning this burst: immler AT milkyway.gsfc.nasa.gov . 

[GCN OPS NOTE(02aug06): Per author's request, the floating point form of the
XRT Dec location was changed from "-55.2781" to "-55.52781".  We thank
Aybuke Kupcu-Yoldas for pointing out our error.  And see GCN 6697
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  for an refined
XRT position.]

E. Berger and D. Murphy (Carnegie) report:

"We obtained a 180 sec R-band image of the position of GRB 070802 (GCN 
6692
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 ) starting at 2007 August 2, 07:35:13 UT.  We do not find any objects 
within the XRT error circle to a limit of R~21 mag.  Further observations 
are in progress."

J. Greiner, C. Clemens, T. Kruehler, A. Kuepcue-Yoldas, N. Primak, 
G. Szokoly, A. Yoldas (all MPE Garching) and S. Klose (Tautenburg Obs.)
report:

The 7-channel imager GROND at the 2.2 m telescope of the La Silla Observatory 
(ESO/Chile) is observing the field of GRB 070802 (Swift trigger 286809, 
Barthelmy et al. 2007, GCN #6692
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 ). The observation started on August 2 at 
07:17 UT, about 10 min after the GRB trigger. Observations are still ongoing. 
The observation consists of a series of exposures in g', r', i, z', Js, H and 
Ks. The automated data reduction pipeline finds an elongated source in the 
XRT error circle. This source has a magnitude of about Js = 20. This is the 
result of our first four minutes exposure.

E. Berger and D. Murphy (Carnegie) report:

"We also obtained a 360 sec I-band image of the position of GRB 070802 
(GCN 6692
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 ) starting on 2007 August 2, 08:02:19 UT.  Within the XRT error 
circle we detect a faint source with I~21.5 mag located at:
 	RA = 02:27:35.88
 	DEC = -55:31:39.3
with an uncertainty of about 0.5" in each coordinate.  If this is the 
afterglow of GRB 070802 the earlier non-detection in R-band may indicate 
a redshift z~4.5-6, or a highly reddened GRB at a lower redshift."

D. Malesani, J.P.U. Fynbo, P.M. Vreeswijk (DARK) report on behalf of a 
larger collaboration:

We observed the field of GRB 070802 (Barthelmy et al., GCN 6692
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 ) with 
the ESO VLT equipped with FORS2. Inside the XRT error circle, we detect 
the object reported by Berger & Murphy (GCN 6695
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 ) and possibly Greiner 
et al. (GCN 6694
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 ), at the coordinates

RA = 02:27:35.72
Dec = -55:31:39.01

The object is detected in the V, R, and I bands. No clear fading 
behaviour is detected between exposures taken at 8:01 UT and 8:13 UT.

We acknowledge excellent support from the ESO staff, in particular Alain 
Smette.

P.A. Evans (U. Leicester) and V. Mangano (INAF-IASFPA) report on behalf of 
the Swift/XRT team:

We have reanalysed the promptly downlinked XRT data of GRB 070802 
(Barthelmy et al. GCN Circ 6692
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 ) using PSF fitting and exposure maps to 
improve the XRT position. We find an improved position of RA, 
Dec=36.89776, -55.52851 whic is equivalent to:

RA (J2000)  = 02 27 35.46
Dec (J2000) = -55 31 42.7

With an uncertainty of 4.7" (radius, 90% containment). This position is 
3.3" from the original XRT position sent out in GCN Circ. 6692
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 .

This circular is an official product of the Swift team.

[GCN OPS NOTE(02aug07): Per author's request. the Subject line was changed
from "070702" to "070802".]

J.X. Prochaska (UCO/Lick), C.C. Thoene, D. Malesani, J.P.U. Fynbo, P.M. 
Vreeswijk (DARK), report:

We observed the candidate afterglow of GRB 070802 (Barthelmy et al., GCN 
6692
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 ; Berger et al., GCN 6695
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 ) with the ESO VLT + FORS2. A 30-min 
spectrum was acquired starting on Aug 2.378 UT. From the detection of 
several Fe lines, we measure z=2.45.

We acknoweldge excellent support from the VLT staff, in particular Alain 
Smette.

J. Cummings (GSFC/UMBC), L. Barbier (GSFC), S. D. Barthelmy (GSFC),
E. Fenimore (LANL), N. Gehrels (GSFC), S. Immler (GSFC),
H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), D. Palmer (LANL),
A. Parsons (GSFC), T. Sakamoto (GSFC/ORAU), G. Sato (GSFC/ISAS),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-239 to T+776 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 070802 (trigger #286809)
(Barthelmy, et al., GCN Circ. 6692
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 ).  The BAT ground-calculated position is
RA, Dec = 36.903, -55.517 deg, which is 
   RA(J2000)  =   2h 27m 36.7s 
   Dec(J2000) = -55d 31' 01" 
with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 73%.
 
The mask-weighted light curve for this 64-sec image trigger shows a single
peak starting at T+5 sec and ending at ~T+50 sec.  We note that there is
a 3-sigma blip at T-150 lasting about 20 sec.
T90 (15-350 keV) is 16.4 +- 1.0 sec (estimated error including systematics).
 
The time-averaged spectrum from T+4.9 to T+23.2 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.79 +- 0.27.  The fluence in the 15-150 keV band is 2.8 +- 0.5 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+6.15 sec in the 15-150 keV band
is 0.4 +- 0.1 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.

D. Malesani, J.P.U. Fynbo, J. Hjorth, and P.M. Vreeswijk (DARK) report 
on behalf of a larger collaboration:

We report further analysis of our images (Malesani et al., GCN 6696
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 ) of 
GRB 070802 (Barthelmy et al., GCN 6692
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 ). Observations span the range 
08:01 - 10:09 UT, that is 0.9 - 3.0 hr after the GRB. The object 
reported by Greiner et al. (GCN 6694
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 ) and Berger & Murphy (GCN 6695
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 ) is 
well detected in all our images, and is clearly fading following a 
power-law decay with index alpha = 0.6+-0.1. Based on instrumental 
zeropoints, the afterglow looks significantly reddened, with a color R-I 
= 0.55+-0.1.

We acknowledge excellent support from the VLT staff.

N.P.M. Kuin (MSSL) and S. Immler (GSFC)������������� �
report on behalf of the Swift-UVOT team:

The Swift/UVOT observed the field of GRB070802 (trigger=286809) 
starting 2007-08-02 at 07:09:30, 125 s after the BAT trigger (Barthelmy 
et al. 2007, GCN Circ.6692
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 ). We do not find a new source in the UVOT 
observations at the location of the refined XRT position (Evans and 
Mangano, 2007, GCN Circ. 6697
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 ).


The 3-sigma upper limits (in the UVOT photometric system, Breeveld et 
al., GCN Circ. 6614
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 ) for detecting a source inside the XRT error circle 
in the co-added frames (including the settling exposure and finding 
charts) are listed below:

Filter		Tstart	Tstop	Exposure	Magnitude
		(s)		(s)		(s)		(3-sigma UL)
white	142		16875	589.9	>21.25
v		125		12514	773.2	>19.96
uvm2	654		4821	239.4	>19.84
uvw1	679		4969	182.3	>19.77
u		703		6307	219.6	>19.52
b		728		16681	1109.0	>21.27
uvw2	758		10893	1127.5	>21.16

The values quoted above are not corrected for the expected Galactic 
extinction corresponding to a reddening of E_{B-V} = 0.026 mag in the 
direction of the burst (Schlegel et al. 1998).

V. Mangano, B. Sbarufatti, V. La Parola, E. Troja (INAF-IASF PA),
P. Evans (U. Leicester) and S. Immler (GSFC)
report on behalf of the Swift-XRT team:

We have analysed the first three orbits of Swift-XRT data obtained for
GRB 070802 (Barthelmy et al, GCN Circ. 6692
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 ), which include 3.4 ks of
Photon Counting (PC) data.

Using 599 s of overlapping XRT Photon Counting mode and UVOT V-band
data, 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 = 36.89899, -55.52733 which is equivalent to:

RA (J2000):  02 27 35.76
Dec (J2000): -55 31 38.4

with an uncertainty of 2.1 arcsec (radius, 90% confidence).
This is 4.9 arcsec from the initial XRT position (Evans and Mangano,
GCN Circ. 6697
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 ) and 38 arcsec from the BAT ground-calculated position
(Cummings et al, GCN Circ. 6699
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 ). It is also 0.7 arcsec from the
optical position of Malesani et al. (GCN Circ. 6696
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 ) and 1.4 arcsec
from the I band position of Berger and Murphy (GCN Circ. 6695
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 ).

The XRT light-curve decays with slope -2.0+/-0.2 up to about T+500 s,
then flattens to slope -0.25+/-0.08. We do not see a further break up
to T+17 ks.

The spectrum can be fit with an absorbed power law with
a photon index of 1.9+/-0.3 and a column density consistent
with the Galactic value (2.9e20 cm^-2).
The absorbed (unabsorbed) 0.3-10.0keV flux for the spectrum is
(3.9+/-0.8)e-12 ((4.6+/-0.8)e-12) ergs cm^-2 s^-1.

If the decay continues with a slope of ~ 0.2, the count rate at 24
hours is predicted to be 3e-2 count s^-1. Using the spectral fit
described above, this corresponds to an observed flux of about
1.4e-12 ergs cm^-2 s^-1.

This circular is an official product of the XRT team.

E.Palazzi, S. Covino, L.A. Antonelli, L. Calzoletti, 
S. Campana, G.  Chincarini, M.L. Conciatore, S. Cutini, P. 
D'Avanzo,   V. D'Elia, F. Dalessio, F.  Fiore, D. Fugazza, P. 
Goldoni, D. Guetta,  C. Guidorzi, G.L. Israel,  D. Malesani, N. 
Masetti, A. Melandri, E. Meurs, L. Nicastro, E. Pian, S. Piranomonte, D. 
Rizzuto,  L.  Stella, G.  Stratta, G. Tagliaferri, G. Tosti, V. 
Testa, S.D. Vergani,  F. Vitali   report on behalf of the REM team:

"We observed the field of the GRB 070802 (S. D. Barthelmy et al., GCN 
6692
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 )  with the robotic 60-cm REM telescope located at La Silla (Chile).
A set of observations was performed automatically in the R optical filter 
starting at 07:17:44 UT (52 sec after the Swift alert; about 10 minutes 
after the burst) and lasted about 40 minutes.

The analysis of the first 15 minutes exposures do not show 
the optical/NIR afterglow candidate reported by J. Greiner et al (GCN 
6694
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 ), E. Berger et al (GCN. 6695), D. Malesani et al (GCN. 6696). 
The 3-sigma limiting magnitude in the stacked images is R~17.0"