GRB 070628

S. T. Holland (CRESST/GSFC/USRA), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), P. J. Brown (PSU),
D. N. Burrows (PSU), S. Campana (INAF-OAB), M. L. Conciatore (ASDC),
N. Gehrels (NASA/GSFC), C. Gronwall (PSU),
C. Guidorzi (Univ Bicocca&INAF-OAB), J. A. Kennea (PSU),
H. A. Krimm (GSFC/USRA), V. La Parola (INAF-IASFPA),
V. Mangano (INAF-IASFPA), F. E. Marshall (NASA/GSFC),
P. T. O'Brien (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL), M. Perri (ASDC),
P. Romano (Univ. Bicocca & INAF-OAB), T. Sakamoto (NASA/ORAU),
G. Sato (GSFC/ISAS), M. Stamatikos (NASA/ORAU) and
T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team:

At 14:41:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070628 (trigger=283320).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 115.284, -20.285 which is 
   RA(J2000)  =  07h 41m 08s
   Dec(J2000) = -20d 17' 04"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
on a rising background while entering the SAA with a duration of about 20 sec. 
The peak count rate was ~3000 counts/sec (15-350 keV), at ~5 sec after the
trigger.  Given the galactic latitude of +1.25deg, we can not rule out
a Galactic transient origin of this trigger. 

The XRT began taking data at 14:42:53 UT, 111 seconds after the BAT
trigger. The initial image mode frames show many cosmic ray tracks
due to entrance into the SAA; the XRT on-board centroid algorithm did
not find a source in the image and no prompt position is available. 
Further XRT data are delayed by the SAA passage. We are waiting for
down-linked data to detect and determine a position for the source. 

Because Swift was entering the SAA, there were no prompt UVOT
observations of this source. 

Burst Advocate for this burst is S. T. Holland (sholland 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.)

Swift UVOT Observations of GRB 070628

C. Gronwall (PSU) and S. T. Holland (CRESST/GSFC/USRA)
report on the behalf of the Swift UVOT team:

In response to GRB 070628 (Swift/BAT trigger 283320) at 14:41:02.84 UT,
UVOT took a finding chart exposure of 100 seconds with the White 
(160-650 nm) filter starting 1099 seconds after the BAT trigger. There is 
a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at
    RA(J2000)  = 07:41:06.12 = 115.2755
    DEC(J2000) = -20:16:46.2  = -20.2795
with a 1-sigma error radius of about 0.6 arc sec. This position is 34.9 
arcsec. from the center of the BAT error circle. The estimated magnitude is 
19.1 with a 1-sigma error of about 0.5 mag. No correction has been made 
for extinction.

Gary Melady (University College Dublin), John French (UCD),
Petr Kubanek (GACE Valencia), Martin Jelinek (IAA CSIC Granada)

report on behalf of the Watcher collaboration:

The Watcher 40cm robotic telescope, located at Boyden Observatory,
South Africa, responded to Swift Trigger 283320 (GRB 070628, Holland et al. 
GCN 6584
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 ). when the dome opened at 15:17:29 UT, 36.43 minutes after the
trigger. However the bright evening sky rendered the earliest images
unusable. 

An unfiltered 60s exposure taken under twilight conditions at 16:12:48 UT, 
91.76 minutes after the Swift trigger, showed no new source in the UVOT 
error box (as reported by Gronwall et al., GCN 6585
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 ) down to an R-band 
limiting magnitude of approx 16.2 (in comparison to USNO-B1).

We combined 27 subsequent 60s exposures to create a 1620s unfiltered image with 
an exposure mid-time of 16:28:05 (108.05 minutes after the trigger). The later 
images were taken at high air mass as the target was setting. This shows no new 
source in the error box down to an R-band limiting magnitude of approx 17.2.

The target set at our location at 18:20 UT so no further observations will be made.

This message may be cited.

M. Perri, G. Stratta (ASDC)  and S.T. Holland (CRESST/GSFC/USRA)
report on behalf of the Swift XRT team:

We have analysed the first 3 orbits of Swift XRT data from the possible
burst GRB 070628 (Holland et al., GCN Circ. 6584
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 ). Using an exposure
time of 6.3 ks the refined XRT position is RA (J2000) = 115.275833,
Dec (J2000) = -20.279750, corresponding to:
RA(J2000) = 07h 41m 06.2s
Dec(J2000) = -20d 16' 47.1"
with an uncertainty of 3.5" (90% containment). This is 0.5 arcmin from
the BAT position (GCN Circ. 6584
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 ) and 1.4 arcsec from the UVOT position
(Gronwall et al. GCN Circ. 6585
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 ).

The XRT light-curve in the time interval from T+116s to T+14ks shows
a fading behavior with superimposed flaring activity. More data are being
collected to monitor the light-curve evolution.

The X-ray spectrum covering the time period from  T+1.1ks to T+14ks
is well fit by an absorbed power law with a photon index of  2.2 +/- 0.1
and a total column density of NH=(7.5+/-0.7)e21 cm**-2. We note that
the Galactic column density in the direction of the source is
7.35e21 cm**-2 (Dickey & Lockman 1990).

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

Klotz, A. (CESR-OMP), Boer M. (OHP), Atteia J.L. (LATT-OMP) report:

We imaged the field of GRB 070628 detected by SWIFT
(trigger 283320) with the TAROT robotic telescope (D=25cm)
located at the European Southern Observatory,
La Silla observatory, Chile.

The observations started 8.3h after the GRB trigger
(started at the begining of the night).
The elevation of the field decreased from
from 22 degrees above horizon and weather conditions
were good.

We co-added a series of exposures filtered in R, V and I.
We do not detect any OT at the UVOT location
(Gronwall & Holland GCN Circ. 6585
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 ):

t0+8.3h to t0+9.1h : R > 17.9

Magnitudes were estimated with the nearby USNO-B1 stars
and are not corrected for galactic dust extinction.

N.B. Galactic coordinates are lon=236.5513 lat= +1.2335
and the galactic extinction in R band is 2.4 magnitudes
estimated from D. Schlegel et al. 1998ApJ...500..525S.

This message may be cited.

H. Krimm (GSFC/USRA), L. Barbier (GSFC), S. D. Barthelmy (GSFC),
J. Cummings (GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC),
S. T. Holland (CRESST/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-240 to T+67 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 070628 (trigger #283320)
(Holland, et al., GCN Circ. 6584
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 ).  The BAT ground-calculated position is
RA, Dec = 115.271, -20.281 deg which is 
   RA(J2000)  =   7h 41m 5.1s 
   Dec(J2000) = -20d 16' 50"
with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 58%.
 
The mask-weighted light curve shows pretrigger emission starting at
T-30 sec.  Then comes the main emission peak (roughly symmetrical)
starting at ~T-5 sec and ending at ~T+20 sec.  T90 (15-350 keV) is
39.1 +- 2 sec (estimated error including systematics).  The BAT data
end at T+67 sec because Swift entered the SAA.
 
The time-averaged spectrum from T-31.7 to T+16.2 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.91 +- 0.09.  The fluence in the 15-150 keV band is 3.5 +- 0.2 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+4.90 sec in the 15-150 keV band
is 5.1 +- 0.3 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.

N. Primak, G. Szokoly, J. Greiner, C. Clemens, T. Kruehler, A. Kuepcue-Yoldas, 
A. Yoldas (all MPE Garching), S. Klose, A. Rossi (Tautenburg Observatory), 
and F. Carrier (Instituut voor Sterrenkunde Leuven, Belgium) report:

We observed the field of GRB 070628 (Swift trigger 283320; Holland et al., 
GCN 6584
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 ) with GROND at the 2.2m Max-Planck Institute telescope at the 
ESO La Silla observatory in Chile. The observations started at 22:21:57, 
about 7.7 hrs after the GRB and just after sunset, and lasted 10 min. 
We observed simultaneously in the g,r,i,z,J,H,K bands. The UVOT afterglow
(Cronwall & Holland, GCN 6585
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 ) is clearly detected at H=16.2+-0.3 and 
K=16.3+-0.2 mag. The NIR magnitudes are calibrated against 2MASS. 
Nothing is visible in the J band (limiting magnitude 16.0), and all 
optical frames are saturated due to twilight. 

We note that the afterglow was atypically bright for the given time after
the burst, suggesting a slow fading. We encourage near-IR spectroscopy to
determine the redshift.

W. Landsman (GSFC/SSAI),  F.E. Marshall  (NASA/GSFC) and  S.T. Holland  
(CRESST/GSFC/USRA) report on behalf of the Swift/UVOT team.

The Swift/UVOT observed the field of GRB 070628 starting 1099 sec after 
the trigger (Holland et al., GCN Circ. 6584
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 ).    We strongly detect the 
afterglow in the white and V filters, and have a weak detection in the B 
filter.    The refined afterglow position is

RA(J2000)  =  07h 41m 06.09s
Dec(J2000) = -20d 16' 45.4"

with an uncertainty of 0.7" (radius, 90% containment, including  
systematic uncertainty).  This is within the refined XRT error circle 
(Perri et al. GCN Circ. 6587
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 ).
   
Magnitudes and upper limits are reported below.

Filter   Tstart  Tstop    Exp       Mag
          (s)     (s)     (s)

WHITE    1099    1199      98     19.65 � 0.11
         1202    1302      98     19.62 � 0.11
         6628    6827     197     20.59 � 0.15
        11691   11990     295    >21.2 (3 sigma)


V        1309    1708     393     18.88  � 0.11
         1850    5804     256     19.55  � 0.25
        13207   13506     295    >19.6 (3 sigma)

B        1786    6622      78     20.7 � 0.3
         7856    8055     197    >20.4 (3 sigma)
                        
U        6217    6417     197    >20.0 (3 sigma)

UVW1     6014    6213     197    >19.8 (3 sigma)

UVM2     5809    6008     197    >19.6 (3 sigma)

UVW2     6833    7032     197    >19.9 (3 sigma)


The above magnitudes are not corrected for the large and uncertain 
extinction expected at the low Galactic latitude of GRB 070628.  

A power-law fit  to the white filter observations gives a decay index of 
~0.55 +/- 0.3.

[GCN OPS NOTE(30jun07):  Per author's request, the authors line
was added.]