GRB 080523

M. C. Stroh (PSU), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
C. Gronwall (PSU), D. Grupe (PSU), S. T. Holland (CRESST/USRA/GSFC),
H. A. Krimm (CRESST/GSFC/USRA), V. Mangano (INAF-IASFPA),
P. T. O'Brien (U Leicester), D. M. Palmer (LANL),
D. Perez (U Leicester), J. L. Racusin (PSU),
M. Stamatikos (NASA/ORAU), T. N. Ukwatta (GSFC/GWU) and
D. E. Vanden Berk (PSU) report on behalf of the Swift Team:

At 21:21:51 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080523 (trigger=312242).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 20.732, -64.053 which is 
   RA(J2000) = 01h 22m 56s
   Dec(J2000) = -64d 03' 12"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
with a duration of about 15 sec.  The peak count rate
was ~700 counts/sec (15-350 keV), at ~5 sec after the trigger. 

The XRT began observing the field at 21:23:17.7 UT, 86.4 seconds after
the BAT trigger. XRT found a bright, uncatalogued, fading X-ray source located
at RA, Dec 20.7992, -64.0322 which is equivalent to:
   RA(J2000)  = 01h 23m 11.8s
   Dec(J2000) = -64d 01' 55.9"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). This
location is 129 arcseconds from the BAT onboard position, within the
BAT error circle. No event data are yet available to assess possible
redshift constraints using X-ray spectroscopy and the nH-z relation
from Grupe et al. (2007). 

UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm)
filter starting 110 seconds after the BAT trigger. No afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 18.5 mag. 
data for the list of sources generated on-board are not available at this time. 
No correction has been made for the expected extinction corresponding to E(B-V)
of 0.03. 

Burst Advocate for this burst is M. C. Stroh (stroh AT astroh.org). 
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.)

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

Using 463 s of overlapping XRT Photon Counting mode and UVOT
data for GRB 080523, 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 = 20.79796, -64.03081 which is equivalent
to:

RA (J2000): 01h 23m 11.51s
Dec (J2000): -64d 01' 50.9"

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.

N. P. M. Kuin (MSSL/UCL), and M. C. Stroh (PSU), report on behalf
of the Swift UVOT team

The Swift UltraViolet/Optical Telescope (UVOT) began observations
of GRB 080523, on May 23, 2008, at  UT,  112 seconds after the initial
Swift BAT trigger (Stroh et al., GCN Circ. 7767
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 ). We detect no source
at the enhanced Swift XRT position (Osborne, GCN Circ.7768
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 ).

The 3-sigma limiting magnitudes for GRB080523 are given below for
the two initial observations, and for summed observations in each
of the UVOT filters:

Filter Tstart(s) Tstop(s) Exp(s) Magnitude

white    112       211     99.8    > 20.78
v        218       617    399.8    > 19.93

white    112     12001   1211.2    > 21.80
v        218      6037    819.2    > 19.86
uvm2     624      6242    439.2    > 20.15
uvw1     649      6448    439.2    > 20.20
u        673      6649    435.9    > 19.92
b        698     11088   1118.9    > 21.56
uvw2     728     12430    841.6    > 20.77


The values quoted above are on the UVOT Photometric System
(Poole et al, 2008, MNRAS 383,627). They are not corrected for the
expected galactic reddening of E(B-V) = 0.025 in the direction of
the burst (Schlegel et al. 1998).

Johan P. U. Fynbo, Daniele Malesani (DARK), Pall Jakobsson (Univ. 
Hertfordshire), Bo Milvang-Jensen and Jens Hjorth (DARK), report on 
behalf of a larger collaboration:

We observed the field of GRB 080523 (Stroh et al., GCN 7767
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 ) with the 
ESO VLT equipped with FORS2. Photometric observations were conducted on 
2008 May 24, between 8:14 and 8:27 UT (about 10.9 hr after the GRB). 
Inside the revised XRT error circle (Osborne et al., GCN 7768
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 ), we 
detect a faint source at coordinates (J2000):

RA = 01:23:11.61
Dec = -64:01:51.8

Lacking any information on variability, we cannot assess whether this 
source is associated with GRB 080523.

A spectrum of the object shows detection of continuum in the blue down 
to 4200 AA, implying a redshift limit z < 2.5. Further analysis is underway.

We note the presence of two further, faint sources close to, but 
slightly outside of, the refined XRT error circle at (RA, Dec) = 
(01:23:11.21, -64:01:53.9) and (01:23:10.88, -64:01:50.4).

We acknowledge excellent support and positive initative from the ESO 
observing staff, particularly Yuri Beletsky and Patricia Guajardo.

D. Palmer (LANL), S. D. Barthelmy (GSFC), W. Baumgartner (GSFC/UMBC),
J. Cummings (GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC),
H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), K. McLean (GSFC/UMD),
T. Sakamoto (GSFC/UMBC), G. Sato (GSFC/ISAS), M. Stamatikos (GSFC/ORAU),
M. C. Stroh (PSU), J. Tueller (GSFC), T. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-239 to T+963 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 080523 (trigger #312242)
(Stroh, et al., GCN Circ. 7767
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 ).  The BAT ground-calculated position is
RA, Dec = 20.769, -64.060 deg, which is 
   RA(J2000)  =  01h 23m 04.4s 
   Dec(J2000) = -64d 03' 36.0" 
with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.
 
The mask-weighted light curve shows a FRED-like peak starting at ~T-10 sec,
peaking at ~T+4, and ending at ~T+80 sec.  There is a weak second peak
starting,ending at 80,110 sec, respectively.  T90 (15-350 keV) is
102 +- 15 sec (estimated error including systematics).
 
The time-averaged spectrum from T-6.1 to T+111.9 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.65 +- 0.18.  The fluence in the 15-150 keV band is 8.8 +- 1.0 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+3.89 sec in the 15-150 keV band
is 0.5 +- 0.1 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/312242/BA/
 
This burst satisfies Sakamoto/Ukwatta Swift-BAT possible high-z criteria
(Ukwatta et al. arXiv:0802.3815).
1) Power Law photon index = 1.65     (PL photon index < 2)
2) 1-s peak photon flux   = 0.51     (1-s peak photon flux < 1.0 ph/cm2/s)
3) Light curve variance   = 1.2e-05  (Variance < 0.0001)
4) T90/(Peak photon flux) = 201      (T90/(Peak photon flux) > 100)
Based on a limited sample of bursts, these criteria yield an 85% chance
it has a redshift greater than 3.5.

M. C. Stroh and D. Grupe (PSU) report on behalf of the Swift-XRT Team:

We have analyzed the first eight orbits of Swift-XRT data obtained for  
GRB 080523 (Stroh et al. GCN Circ. 7767
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 ), totaling 250 s of Windowed  
Timing (WT) data and 11.8ks of Photon Counting (PC) data.

The light curve can be described as a single broken power-law with an  
initial decay index of 2.17 +/- 0.04 followed by a shallower decay  
index of 0.56 +/- 0.27 with a break at T+3.5ks. We predict a count  
rate at T+48 hours of 3.2e-3 cts/s.

The WT data (93 - 343 seconds) can be modeled as an absorbed power law  
with photon index of 2.17 +/- 0.04 and a total absorbing column  
density that is consistent with the Galactic value of 2.1e20 cm^-2.  
The 0.3-10 keV absorbed (unabsorbed) flux during this time is 4.58e-10  
(4.99e-10) erg cm^-2 s^-1.

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

D. Malesani, J. P. U. Fynbo (DARK/NBI), A. de Ugarte Postigo (ESO), P. 
Jakobsson (Univ. Hertfordshire), J. Hjorth, B. Milvang-Jensen 
(DARK/NBI), report on behalf of a larger collaboration:

We observed again the field of GRB 080523 (Stroh et al., GCN 7767
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 ) with 
the ESO VLT equipped with the FORS2 camera. Observations were carried 
out on 2008 May 28, at mean time 9:35 UT (4.5 days after the GRB) in the 
R band.

The optical source identified by Fynbo et al. (GCN 7770
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 ) has clearly 
faded since our previous observation by at least 1.5 mag. This confirms 
that the object is the afterglow of GRB 080523. The refined coordinates 
(J2000, against USNO-B1.0) are

RA = 01:23:11.56
Dec = -64:01:51.59

Residual, marginally significant emission is still detected close to the 
afterglow position. It is unclear whether this is related to the GRB.

A finding chart showing the field in the two epochs is posted at

http://www.astro.ku.dk/~malesani/GRB/080523/080523_finder.jpg

Close inspection of our spectra (Fynbo et al., GCN 7770
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 ) fails to reveal 
any convincing absorption or emission feature. The object has a blue 
continuum, and flux is detected down to ~4070 AA. A conservative limit 
to the redshift is set by the absence of Lyman-alpha forest redward of 
~4900 AA, which implies z < 3. This limit rules out a high redshift as 
suggested by the burst gamma-ray properties (Palmer et al., GCN 7772
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 ).

We acknowledge significant support from the ESO staff at Paranal, 
particularly Linda Schmidtobreick.

Aquib Moin (Curtin Institute of Radio Astronomy / Australia Telescope
National Facility), Steven Tingay (Curtin Institute of Radio Astronomy),
Chris Phillips (Australia Telescope National Facility), Gregory Taylor
(University of New Mexico), Mark Wieringa (Australia Telescope National
Facility) and Ralph Martin (Perth Observatory) report:

We observed the BAT refined position of the GRB 080523 (GCN 7772
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 ) at 4.8
GHz with the Australia Telescope Compact Array (ATCA) on June 01, 2008
between 04:12:15 UT and 06:37:35 UT.

We did not detect a radio source at the BAT position of the GRB 080523
(GCN 7772
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 ). The radio flux density at the GRB position is -0.069 +/-
0.172 mJy at 4.8 GHz (1-sigma).

The Australia Telescope Compact Array (/ Parkes telescope / Mopra
telescope / Long Baseline Array) is part of the Australia Telescope
which is funded by the Commonwealth of Australia for operation as a
National Facility managed by CSIRO.

See field image at:

http://cira.ivec.org/dokuwiki/doku.php/grb/grb_080523_field_image