GRB 080426

A. de Ugarte Postigo (ESO), D. A. Kann (TLS Tautenburg),
A.J. Castro-Tirado (IAA-CSIC, Granada), A. Aguirre (CAHA),
C. Jordi (U. Barcelona) report on behalf of a larger collaboration:

We have observed the XRT error circle (Beardmore et al. GCN 7641
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 )
of the possible intermediate GRB 080426 (H. Ziaeepour et al.
GCN 7639
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 , J. Cummings et al. GNC 7640) with the 2.2m telescope
at Calar Alto Observatory (Almer�a, Spain) at high airmass but
good conditions.

The observations started at 26 April 19:53:33 UT and were carried
out in B, V, R and I bands. In an I-band combined frame comprised
of 11x300s with mean epoch 7.50 hours after the burst, we do not
detect any source within the XRT error box down to a limiting
magnitude of I~22.5.

Further analysis is ongoing. This message may be cited.

A.P. Beardmore (U. Leicester) and H. Ziaeepour (UCL-MSSL) report on behalf
of the Swift-XRT team :

The Swift-XRT began observing GRB 080426 (trigger=310219, Ziaeepour et
al.  GCN Circ. 7639
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 ) in Photon Counting mode, 230 s after the BAT
trigger.  The best XRT position for this burst is the UVOT-enhanced
position reported by Beardmore et al. (GCN Circ. 7641
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 ).

Three orbits of data out to T+13 ks, totalling 5.7 ks of exposure,
reveal a light curve which follows a simple power-law decay, with an
index of -1.07 +/- 0.08.

The spectrum from the first three orbits is well fit by an absorbed
power-law model, with a photon index of 2.01 +/- 0.25 and column density
of (5.3 +/- 1.3)e21 cm^-2, which is slightly in excess of the average
Galactic column density in this direction of 3.7e21 cm^-2. The
observed (unabsorbed) 0.3-10 keV flux is 5.1e-12 (9.1e-12)
erg cm^-2 s^-1.

If the burst continues to decay at the same rate, we predict an
XRT count rate of 2.0e-3 count/s at T+24 hours, which corresponds
to an observed 0.3-10 keV flux of approximately 1.2e-13 erg cm^-2 s^-1.

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

S.R. Oates (UCL-MSSL) and H. Ziaeepour (UCL-MSSL) report on behalf of
the Swift/UVOT team.

The Swift/UVOT began settled observations the field of GRB 080426
(trigger 310219) 226 seconds after the BAT trigger (Ziaeepour, et al.,
GCN Circ. 7639
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 ). No afterglow is detected at the enhanced XRT position
(Beardmore, et al., GCN Circ. 7641
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 ) in any of the UVOT filters. The 3 
sigma upper limits are given below:

Filter   T_start  T_stop    Exp.    Mag UL (3sig)
            (s)    (s)
------------------------------------------------
white      226     326       98        >20.38
white      446     7457     362        >20.86
v          333     7868    1316        >19.98
b          431     7253     303        >20.01
u          407     7047     333        >19.79
uvw1       382     6843     352        >19.99
uvm2       357     7969     407        >19.94
uvw2       462     7663     510        >20.26
-------------------------------------------------

These upper limits are not corrected for the Galactic extinction in
the line of sight, corresponding to a reddening of E(B-V)= 0.798 mag
(Schlegel et al., 1998, ApJS, 500, 525). The photometry is based on
the UVOT photometric system described in Poole et al. (2008, MNRAS, 383, 
627).

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

Using 894 s of overlapping XRT Photon Counting mode and UVOT
data for GRB 080426, 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 = 26.49890, +69.46823 which is equivalent
to:

RA (J2000): 01h 45m 59.74s
Dec (J2000): +69d 28' 05.6"

with an uncertainty of 1.8 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.

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), K. McLean (GSFC/UMD), D. Palmer (LANL),
T. Sakamoto (GSFC/UMBC), G. Sato (GSFC/ISAS), M. Stamatikos (GSFC/ORAU),
J. Tueller (GSFC), T. Ukwatta (GWU), H. Ziaeepour (UCL-MSSL)
(i.e. the Swift-BAT team):
 
Using the data set from T-119 to T+183 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 080426 (trigger #310219)
(Ziaeepour, et al., GCN Circ. 7639
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 ).  The BAT ground-calculated position is
RA, Dec = 26.510, 69.469 deg which is 
   RA(J2000)  =  01h 46m 02.4s 
   Dec(J2000) = +69d 28' 07.9" 
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 43%.
 
The mask-weighted light curve shows a single spike starting at T+0 and
ending at T+2.0 sec.  The lag analysis yields:
  Lag31 = 88 +/- 9 ms  (50-100 to 15-25 keV bands)
  Lag21 = 74 +/- 6 ms  (25-50  to 15-25 keV bands)
T90 (15-350 keV) is 1.7 +- 0.4 sec (estimated error including systematics).
While the T90 value puts this burst in the middle of the long and Short burst
T90-distribution peaks, the lag value puts this burst in the Long burst class.
 
The time-averaged spectrum from T+0.1 to T+2.3 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.98 +- 0.13.  The fluence in the 15-150 keV band is 3.7 +- 0.3 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T-0.08 sec in the 15-150 keV band
is 4.8 +- 0.3 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level. 

We note that this is an unusual burst.  The T90 for the 50-300 keV  
BATSE range is 1.3 sec which would put it in the short class in that  
era.  However, the long lag and a spectrum that is not hard lead us  
to believe it is in the long class.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/310219/BA/

H. Ziaeepour (UCL-MSSL), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), N. Gehrels (NASA/GSFC), C. Guidorzi (INAF-OAB),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
N. P. M. Kuin (MSSL), J. Mao (INAF-OAB),
C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC),
P. T. O'Brien (U Leicester), S. R. Oates (UCL-MSSL), C. Pagani (PSU),
D. M. Palmer (LANL), D. Perez (U Leicester),
R. L. C. Starling (U Leicester), G. Tagliaferri (INAF-OAB),
T. N. Ukwatta (GSFC/GWU) and P. A. Ward (MSSL-UCL) report on behalf
of the Swift Team:

At 13:23:22 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080426 (trigger=310219).  Swift slewed to the burst
after a brief delay due to Earth limb constraint. 
The BAT on-board calculated location is 
RA, Dec 26.574, +69.492 which is 
   RA(J2000) = 01h 46m 18s
   Dec(J2000) = +69d 29' 30"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 2 sec.  The peak count rate
was ~7500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 13:27:05.2 UT, 222.5 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 26.49931, 69.46856
which is equivalent to:
   RA(J2000)  = 01h 45m 59.83s
   Dec(J2000) = +69d 28' 06.8"
with an uncertainty of 2.8 arcseconds (radius, 90% containment). This
location is 126 arcseconds from the BAT onboard position, within the
BAT error circle. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
3.65e+21 cm^-2 (Kalberla et al. 2005), so we cannot constrain the
redshift at this time using the relation from Grupe et al. (2007). A
summary of the promptly downlinked data is given at
http://www.swift.ac.uk/sper/310219/. 

UVOT took a finding chart exposure of 100 seconds with the White  
(160-650 nm)filter starting 226 seconds after the BAT trigger. No  
afterglow candidate has  been found in the initial data products. 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 XRT error circle. The list of sources is typically complete to 
about 18mag. No correction has been made for the expected 
extinction corresponding to E(B-V) of 0.79. 

Burst Advocate for this burst is H. Ziaeepour (hz AT mssl.ucl.ac.uk). 
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.)