GRB 100420A

W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
M. M. Chester (PSU), N. Gehrels (NASA/GSFC), C. Guidorzi (U Ferrara),
E. A. Hoversten (PSU), O. M. Littlejohns (U Leicester),
R. Margutti (INAF-OAB), C. B. Markwardt (CRESST/GSFC/UMD),
P. T. O'Brien (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL), A. Rowlinson (U Leicester), M. H. Siegel (PSU)
and E. Troja (NASA/GSFC/ORAU) report on behalf of the Swift Team:

At 05:22:42 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100420A (trigger=419932).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 296.127, +55.793 which is 
   RA(J2000) = 19h 44m 31s
   Dec(J2000) = +55d 47' 33"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual for an image trigger, there is nothing of
note in the BAT light curve. 

The XRT began observing the field at 05:24:52.8 UT, 130.5 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 296.12646, 55.76885 which
is equivalent to:
   RA(J2000)  = 19h 44m 30.35s
   Dec(J2000) = +55d 46' 07.9"
with an uncertainty of 4.0 arcseconds (radius, 90% containment). This
location is 86 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
1.19e+21 cm^-2 (Kalberla et al. 2005). 


UVOT took a finding chart exposure of 150 seconds with the White filter
starting 134 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 none of
the XRT error circle. The coverage of the XRT error circle by the 8'x8' region
for the list of sources generated on-board is uncertain because the large
number of sources filled the available telemetry. No correction has been made
for the expected extinction corresponding to E(B-V) of 0.11. 




Burst Advocate for this burst is W. H. Baumgartner (wayne 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.)

L. Xiao (Louisiana State), W. Rujopakarn (Steward), B. E. Schaefer
(Louisiana State), T. Guver (U Arizona) and S. B. Pandey (U Mich), report
on behalf of the ROTSE collaboration:

ROTSE-IIIb, located at McDonald Observatory, Texas, responded to GRB
100420A (Swift trigger 419932; W. H. Baumgartner et al., GCN 10628
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 ),
producing images beginning 8.8 s after the GCN notice time. An automated
response took the first image at 05:24:07.8 UT, 79.8 s after the burst, and
during the gamma-ray emission. We took 10 5-sec, 10 20-sec and 10 60-sec
exposures. These unfiltered images are calibrated relative to USNO A2.0
(R). Imaging is on going.

Comparison to the DSS (second epoch) reveals no new sources within the
3-sigma Swift/BAT error circle or the XRT error circle, for both single
images and coadding into sets of 10. Individual images have limiting
magnitudes ranging from 15.3-16.8; we set the following specific limits.

start UT end UT t_exp(s) mlim t_start-tGRB(s) Coadd?
--------------------------------------------------------------------
05:24:07.8 05:25:14.9 67 16.7 79.8 Y
05:25:28.5 05:30:11.0 282 17.3 160.5 Y
05:30:20.3 05:41:42.0 681 17.6 452.3 Y

Adria C. Updike, Dieter H. Hartmann (Clemson University), and Chris De
Pree (Agnes Scott College) report:

We observed the field of GRB 100420A (Baumgartner et al., GCN 10628
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 ) using
the 0.9m SARA telescope located at KPNO beginning 3 hours and 4 minutes
after the BAT trigger.  We obtained 22 minutes of observations in the R
band followed by 20 minutes in the I band at high airmass.  At the
location of the XRT error circle, we detect no new sources down to a
limiting magnitude of R > 19.0, I > 18.5 (as compared to the USNO B1.0
catalog).  No further observations are planned.

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

Using 1862 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 100420A, 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 = 296.12761, +55.76926 which is equivalent
to:

RA (J2000): 19h 44m 30.63s
Dec (J2000): +55d 46' 09.3"

with an uncertainty of 2.0 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.

A. Rowlinson (U Leicester) and W. H. Baumgartner (GSFC/UMBC) report on
behalf of the Swift-XRT team:

We have analysed 6.9 ks of XRT data for GRB 100420A (Baumgartner et al.
GCN Circ. 10628
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 ), from 115 s to 13.6 ks after the BAT trigger. The data
comprise 120 s in Windowed Timing (WT) mode (the first 8 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Beardmore et
al. (GCN. Circ 10638).

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.69 (+0.47, -0.39), followed by a break at T+721 s to an
alpha of 1.22 (+0.34, -4.02).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.83 (+0.22, -0.20). The
best-fitting absorption column is consistent with the Galactic value of
1.2 x 10^21 cm^-2 (Kalberla et al. 2005). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum is 4.2 x 10^-11 (5.3 x 10^-11) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of
1.22, the count rate at T+24 hours will be 2.3 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.7 x
10^-15 (1.2 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00419932.

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

M. M. Chester (PSU) and W. H. Baumgartner (GSFC/UMBC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 100420A
134 s after the BAT trigger (Baumgartner et al., GCN Circ. 10628
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 ).
No optical afterglow consistent with the enhanced XRT position  
(Beardmore
et al., GCN Circ. 10638
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 ) is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Poole et al. 2008, MNRAS, 383, 627) for the first finding chart (FC)
exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC           134          284          147         >20.8
u_FC               292          542          246         >20.0
white              134        13589         1503         >22.7
v                  624        30201         1434         >20.5
b                  548        19368         1911         >22.2
u                  292        18659         2409         >21.7
w1                 674        17744         1435         >21.6
m2                 648        30928         1260         >21.5
w2                 598        29287         1434         >21.4

The values quoted above are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.11 in the direction of the burst
(Schlegel et al. 1998).

D. Kuroda, K. Yanagisawa, Y. Shimizu, S. Nagayama, H. Toda
(OAO, NAOJ), M. Yoshida (Hiroshima) and N. Kawai (Tokyo Tech)
report on behalf of the MITSuME collaboration:

We observed the field of GRB 100420A (Baumgartner et al. GCN 10628
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 )
with the optical three color (g', Rc and Ic) CCD camera attached
to the MITSuME 50cm telescope of Okayama Astrophysical Observatory.
The observation started on 2010-04-20 18:03:04 UT (~12.7h after the
burst). We did not find any new point source in the Enhanced XRT
error circle (Beardmore et al., GCNC 10638) in all the three bands.

Three sigma upper limits of the OT are listed below. We used GSC2.3
catalog for flux calibration.


T0+[day]   MID-UT   T-EXP[sec]    g'     Rc     Ic
------------------------------------------------------
0.53748   18:16:41   1320.0      >18.7  >18.6  >18.3
------------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]

J. R. Cummings (GSFC/UMBC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD),
D. M. Palmer (LANL), A. M. Parsons (GSFC), 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-205 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 100420A (trigger #419932)
(Baumgartner, et al., GCN Circ. 10628
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 ).  The BAT ground-calculated position is
RA, Dec = 296.090, 55.755 deg, which is 
   RA(J2000)  =  19h 44m 21.5s 
   Dec(J2000) = +55d 45' 18.7" 
with an uncertainty of 4.6 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 92%.
 
The mask-weighted light curve shows a single peak starting around T-10 sec,
peaking at ~T+10 sec, and ending at ~T+60 sec.  T90 (15-350 keV) is
48 +- 36 sec (estimated error including systematics).
 
The time-averaged spectrum from T-30 to T+70 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.17 +- 0.22.  The fluence in the 15-150 keV band is 5.7 +- 1.0 x 10^-7 erg/cm2.
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/419932/BA/

T. Sakamoto (UMBC/GSFC), D. Donato (ORAU/GSFC), N. Gehrels (GSFC),
T. Okajima (JHU/GSFC), Y. Urata (NCU), C.A. Wallace (FGCU)

We observed the field of GRB 100420A detected by Swift
(trigger #419932; Baumgartner et al., GCN Circ. 10628
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 ) with the 14-inch
Goddard Robotic Telescope (GRT) located at the Goddard Geophysical
and Astronomical Observatory (http://cddisa.gsfc.nasa.gov/ggao/).

Total 435 images of 5 sec (200 images), 30 sec (80 images), 
60 sec (120 images) and 100 sec (35 images) exposures were taken in 
the R filter starting from April 20 05:26:53 (UT) about 4.2 minutes after 
the trigger and stopped on April 20 09:49:15 (UT).  We do not detect 
the optical afterglow both in the individual images and the stucked 
image, which only combined good quality 400 images, inside the XRT 
position (Rowlinson et al., GCN #10639
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 ).  The estimated three sigma 
upper limit of the combined image (total exposure of 10600 sec) is ~18.9  
mag using the USNO-B1 catalog.

M. Andreev,  A. Sergeev (Terskol Branch of  Institute of Astronomy),
A. Volnova (SAI MSU), A. Pozanenko (IKI) report on behalf of larger
GRB follow-up collaboration:


We observed the field of the Swift GRB 100420A (Baumgartner et al.,
GCN 10628
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 ) with the Z-600 telescope of Mt.Terskol observatory in
R-filter between (UT) Apr. 20 23:30 - Apr. 21 00:39 under good weather
conditions with mean seeing of about 1.2 arcsec. We do not detect any
source at the position of the enhanced XRT error circle (Beardmore et
al., GCN 10638
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 ).

The 3 sigma upper limit of the stacked image based on several
USNO-B1.0 field stars is following:


T0+      Filter,  Exposure,  mag.
(mid, d)           (s)

0.7789   R       20x180    >20.2

A.J. Levan (U. Warwick), A. Cucchiara  (PSU), N.R. Tanvir (U.
Leicester), D. Fox (PSU) report on behalf of a larger collaboration:

"We obtained two epochs of observations of GRB 100420A (Baumgartner
et al. GCN 10628
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 ) using Gemini-North/NIRI. Our first epoch consisted
of 540s of observations in each of YJH and K and began at April 20
13:24 UT, approximately 8.2 hours after the burst. Our second epoch
was obtained at April 22 14:27 ~57 hours after the burst.

Within the refined XRT error circle we find a single object in our
first epoch K-band observation. The object has a location of

RA(J2000) 19:44:30.57
Dec(J2000) 55:46:10.0

with an error of 0.3" in each axis. It has a magnitude of K~21 in
our first epoch observation, but has faded below our 3-sigma detection
limit of K~22 in the second epoch. We therefore identify this source
as the infrared afterglow of GRB 100420A.

The source is not significantly detected in our J or H-band observations 
to preliminary limiting magnitudes of H>21.5, J>22.5 (3 sigma). However, 
because of the relative faintness of the K-band observations these limits
are not constraining in terms of high-z and dust extinguished
afterglow models"

Dale A. Frail (NRAO) and Poonam Chandra (RMC) report on behalf of a
larger collaboration:

"We observed the localization of dark burst GRB 100420A (Baumgartner 
et al. GCN 10628
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 ) with the Expanded Very Large Array (EVLA) on May 1.50 UT 
at a center frequency of 8.46 GHz.  We do not detect any radio source
anywhere within the refined XRT error circle (Beardmore et al., GCN 10638
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 )
to a 3-sigma limit of 51 uJy.

The EVLA is still undergoing active commissioning and we caution that
these results should be considered preliminary.  The National Radio
Astronomy Observatory is a facility of the National Science Foundation
operated under cooperative agreement by Associated Universities, Inc."