GRB 100425A

D. Grupe (PSU), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (NASA/UMBC), S. T. Holland (CRESST/USRA/GSFC),
C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the
Swift Team:

At 02:50:45 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100425A (trigger=420398).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 299.187, -26.441 which is 
   RA(J2000) = 19h 56m 45s
   Dec(J2000) = -26d 26' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed two peaks, one with a 
duration of about 8 sec at T0 and one of about 6 sec at T+38 sec.  The 
peak count rate was ~500 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 02:52:03.9 UT, 78.8 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 299.19593,
-26.43039 which is equivalent to:
   RA(J2000)  = 19h 56m 47.02s
   Dec(J2000) = -26d 25' 49.4"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 47 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
8.59e+20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 3.11e-09 erg cm^-2 s^-1 (0.2-10
keV). 



UVOT took a finding chart exposure of 150 seconds with the White filter
starting 88 seconds after the BAT trigger. No credible 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 19.6 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 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.15. 

Burst Advocate for this burst is D. Grupe (grupe AT astro.psu.edu). 
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.)

D. Malesani (DARK/NBI), A. J. Levan (Univ. Warwick), N. R. Tanvir (Univ. 
Leicester), J. P. U. Fynbo (DARK/NBI), P. Goldoni (APC/Univ. Paris 7 and 
SAp/CEA), report on behalf of a larger collaboration:

We are observing the field of GRB 100425A (Grupe et al., GCN 10673
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 ) with 
the ESO VLT equipped with the X-shooter spectrograph.

In the R-band acquisition image, taken starting around 06:51 UT 
(approximately 4 hr after the GRB), we detect two sources within the 
refined XRT error circle (http://www.swift.ac.uk/sper/), at coordinates:

A: RA(J2000) = 19:56:47.30, Dec(J2000) = -26:25:49.3
B: RA(J2000) = 19:56:47.13, Dec(J2000) = -26:25:50.1

Source A is visible in the DSS, and has a similar magnitude to its 
archival value. Source B is not readily visible in the DSS and has a 
magnitude R = 20.55 +- 0.2, which is at the limit of the DSS 
sensitivity. The large error comes mostly from calibration scatter in 
comparison with USNO-B1 stars.

In a second image taken starting at 7:44 UT (4.9 hr after the GRB), 
source B has faded by 0.17 +- 0.08 mag.

Given the consistency of its position with the XRT error circle and the 
moderate evidence for variability, we propose source B as the optical 
afterglow of GRB 100425A.

A finding chart from the X-shooter image can be seen at the following URL:

http://www.astro.ku.dk/~malesani/GRB/100425A/GRB100425A_finder.png

Further observations and analysis are ongoing. We acknowledge a 
particularly helpful and attentive support from the ESO staff in 
Paranal, especially Jonathan Smoker, Lorena Faundez, Manuel Olivares 
and Steffen Mieske.

F. Olivares, T. Kruehler and J. Greiner (all MPE Garching) report on 
behalf of the GROND team:

We observed the field of GRB 100425A (Swift trigger 420398; Grupe et 
al., GCN 10673
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 ) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 
2008, PASP 120, 405) mounted at the 2.2 m MPI/ESO telescope at La Silla 
Observatory (Chile).

Observations started at 05:17 UT on April 25, 2.4 hours after the GRB 
trigger, and were initially performed through thin cloud coverage.

The optical afterglow candidate reported by Malesani et al. (GCN 10680
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 , 
source 'B') is found to be clearly variable. Between 2.9 and 5.9 hours 
after the burst, the source decayed roughly 0.5 mag in the r' band. At a 
midtime of 08:45 UT we measure the following preliminary magnitudes (all 
in the AB system) in stacked images with a total integration time of 24 
min in g'r'i'z' and 20 min in JHK:

g = 21.5 +- 0.2
r = 21.0 +- 0.1
i = 20.8 +- 0.1
z = 20.5 +- 0.1
J = 20.1 +- 0.2
H = 19.9 +- 0.2
K = 19.6 +- 0.2

calibrated against the GROND zeropoints and 2MASS field stars, and not 
corrected for the expected foreground extinction of E(B-V)=0.15 
(Schlegel et al. 1998).

P. Goldoni (APC/Univ. Paris 7 and SAp/CEA), H. Flores (Paris Obs.), D. 
Malesani (DARK/NBI), A. J. Levan (Univ. Warwick), N. R. Tanvir (Univ. 
Leicester), J. P. U. Fynbo (DARK/NBI), report on behalf of a larger 
collaboration:

We observed the optical afterglow of GRB 100425A (Grupe et al., GCN 
10673
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 ; Malesani et al., GCN 10680
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 ; Olivares et al., GCN 10683
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 ) with th 
ESO VLT equipped with the X-shooter spectrograph. Four spectra lasting 
10 minutes each were obtained, covering the spectral range 3000-25000 
AA, starting on 2010 April 25, at 6:53 UT (4.0 hr after the GRB).

We detect the traces of both objects lying within the XRT error circle. 
Preliminary inspection of the spectrum of the afterglow reveals several 
absorption features which we interpret as due to Mg II and Fe II, at a 
common redshift z = 1.755.

We caution that the data reduction was carried out using archival 
calibration files. We acknowledge a particularly helpful and attentive 
support from the ESO staff in Paranal, especially Jonathan Smoker, 
Lorena Faundez, Manuel Olivares and Steffen Mieske.

C. B. Markwardt (GSFC/UMD), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC), D. Grupe (PSU),
H. A. Krimm (GSFC/USRA), D. M. Palmer (LANL), T. Sakamoto (GSFC/UMBC),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. N. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-61 to T+242 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 100425A (trigger #420398)
(Grupe, et al., GCN Circ. 10673
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 ).  The BAT ground-calculated position is
RA, Dec = 299.161, -26.463 deg, which is 
   RA(J2000)  =  19h 56m 38.7s 
   Dec(J2000) = -26d 27' 47.8" 
with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 48%.
 
The mask-weighted light curve shows three peaks.  A first, precursor peak
at ~T-45 sec, then the main peak at T-2 sec to ~T+10 sec, and the third peak
at T+35 to ~T+65 sec.  T90 (15-350 keV) is 37.0 +- 2.4 sec (estimated error
including systematics).
 
The time-averaged spectrum from T-2.1 to T+39.4 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.42 +- 0.32.  The fluence in the 15-150 keV band is 4.7 +- 0.9 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+0.27 sec in the 15-150 keV band
is 1.4 +- 0.2 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/420398/BA/

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

Using 5331 s of XRT Photon Counting mode data and 8 UVOT
images for GRB 100425A, 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 = 299.19651, -26.43069 which is equivalent
to:

RA (J2000): 19h 56m 47.16s
Dec (J2000): -26d 25' 50.5"

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) and Evans
et al. (2009, MNRAS, 397, 1177).

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

S. R. Oates (MSSL-UCL) and D. Grupe (PSU) report on behalf of the  
Swift/UVOT team:

The Swift/UVOT began settled observations of the field of
GRB 100425A 88s after the BAT trigger (Grupe et al., GCN 10673
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 ).
We do not detect a new source at the enhanced Swift XRT position
(Goad et al. GCN 10686
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 ) and at the location of source B detected
by the VLT telescope (Malesani  et al. GCN 10680
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 ) and GROND
telescope (Olivares et al. GCN 10683
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 ). Photometry is complicated
by the prescence of the VLT source A within the source aperture.

The 3-sigma upper limits for the finding chart exposures (FC)
and summed images are:

Filter   T_start   T_stop   Exp(s)  Mag (3-sigma upper limit)
-------------------------------------------------------------
white (FC)  88      238      147         >19.37
white       88      5778     513         >19.59
u     (FC)  301     551      246         >18.92
u	    301     6796     650         >19.08
v           631     11961    1317        >19.76
b           556     5573     236         >19.71
uvw1        680     6600     432         >19.44
uvm2        4757    12580    997         >20.87
uvw2        606     11048    1318        >21.26
-------------------------------------------------------------

The quoted upper limits have not been corrected for the expected
Galactic extinction along the line of sight of E_(B-V) = 0.15 mag.
All photometry is on the UVOT photometric system described in Poole et
al. (2008, MNRAS, 383, 627).

D. Grupe (PSU) reports on behalf of the Swift-XRT team:

We have analyzed 14.2 ks of XRT data for GRB 100425A (Grupe et al.
GCN Circ. 10673
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 ), from 85 s to 81 ks after the BAT trigger. The
data consist of 106s in Windowed Timing mode (WT) and 14.1 ks
in  Photon Counting (PC) mode. The current XRT light curve can be
fit by a broken power law model with an initial decay slope of
5.3+/-0.3 with a beak at 330+/-20 s followed by a shallow decay
slope of 0.55+/- 0.05. The last data point at 77 ks after the
burst suggests a break at about 60ks. However, this is based on
one data point. Therefore, a prediction on the future behavior of
the  X-ray afterglow can not be given at this point.


A spectrum formed from the WT mode data can be fitted with an
absorbed power-law with a photon spectral index of 4.00 (+/-0.15).
The best-fitting absorption column density is
1.49 (+0.18, -0.17) x 10^22 cm^-2, at a redshift of 1.755
(Goldini et al. GCN Circ 10684
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 ), in addition to the Galactic value
of 8.6 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum
has a photon index of 2.30+/-0.13 and a best-fitting
absorption column density consistent with the Galactic value.
Based on this spectrum, the counts to observed (unabsorbed)
0.3-10 keV flux conversion  factor deduced from this spectrum is
3.77 x 10^-11 (5.17 x 10^-11) erg cm^-2 count^-1.

Swift will continue observing the X-ray afterglow of GRB 100425A
over the next few days.


The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00420398.

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