GRB 120401A

V. Sudilovsky, J. Elliott, J. Greiner (all MPE Garching), T. Kruehler (DARK) and A. Rau (also MPE Garching), report on behalf of the GROND team:

We observed the field of GRB120401A (Swift trigger 519043; Beardmore et al., GCN #13182
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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 MPG/ESO telescope at La Silla Observatory (Chile).

First observations started at 23:32 UT on 2012-04-01, 18.1 hours after the GRB trigger. For an exposure time of 18.76 min in g'r'i'z' and 15 min in JHK performed at an average seeing of 1".8 and at an average airmass of 2.1, we find a single point source within the 1".6 enhanced Swift-XRT error circle reported by Evans et al. (GCN #13184
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 ) at

RA (J2000.0)   = 03:52:19.82 = 58.08258
Dec. (J2000.0) = -17:38:08.5 = -17.63569

with an uncertainty of 0.3" in each coordinate.

We estimate preliminary magnitudes (all in AB) of

g' > 23,
r' = 22.7 +/- 0.2
i' = 21.5 +/- 0.1
z' = 21.0 +/- 0.1
J  = 20.7 +/- 0.2,
H  > 20.7, and
K  > 18.5

The SED, constructed from the Galactic foreground extinction (E_(B-V)=0.053 mag, Schlegel et al. 1998) corrected magnitudes of the object gives a photometric redshift of z=4.5 �� 0.5, while intrinsic or host galaxy extinction alone cannot account for the r'-i' colour. 

A second observation began at 23:29 UT on 2012-04-02, 1.75 days after the GRB trigger. Due to telescope pointing problems, JHK were the only filters to cover the GRB location. For an exposure time of 38.3 min in JHK at an average seeing of 1".4 and at an average airmass of 2.0, we do not detect the source. With an upper limit of J (AB) > 21.4, it suggests a decay of at least 0.7 mag.

Given magnitudes are calibrated against GROND zeropoints as well as 2MASS field stars.

P.A. Evans (U. Leicester), O.M. Littlejohns (U. Leicester), A. Maselli 
(INAF-IASFPA), V. Mangano (INAF-IASFPA), A. Melandri (INAF-OAB), J.A.
Kennea (PSU), M.C. Stroh (PSU), D.N. Burrows (PSU) and A.P. Beardmore
report on behalf of the Swift-XRT team:

We have analysed 10 ks of XRT data for GRB 120401A (Beardmore  et al.
GCN Circ. 13182
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 ), from 124 s to 24.8 ks after the  BAT trigger. The
data comprise 519 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Evans et al. (GCN. Circ 13183).

The late-time light curve (from T0+5.0 ks) can be modelled with  a
power-law decay with a decay index of alpha=1.2 (+0.4, -0.3).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.29 (+0.07, -0.06). The
best-fitting absorption column is  1.27 (+0.13, -0.12) x 10^21 cm^-2,
in excess of the Galactic value of 3.2 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 1.75 (+0.11, -0.10)
and a best-fitting absorption column consistent with the Galactic
value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 4.0 x 10^-11 (4.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     0 (+1.5, -0) x 10^20 cm^-2
Galactic foreground: 3.2 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.75 (+0.11, -0.10)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00519043.

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

D. M. Palmer (LANL), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
A. P. Beardmore (U Leicester), J. R. Cummings (GSFC/UMBC), E. E. Fenimore (LANL),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC),
T. Sakamoto (GSFC/UMBC), G. Sato (ISAS), M. Stamatikos (OSU),
J. Tueller (GSFC), T. N. Ukwatta (MSU)
(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 120401A (trigger #519043)
(Beardmore, et al., GCN Circ. 13182
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 ).  The BAT ground-calculated position is
RA, Dec = 58.057, -17.658 deg, which is 
   RA(J2000)  =  03h 52m 13.8s 
   Dec(J2000) = -17d 39' 29.6" 
with an uncertainty of 2.5 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.
 
The mask-weighted light curve shows the burst was occuring when the location
came into the BAT FoV at T-100 sec during a preplanned slew.  There was a decline,
then a increase around T+0, and the a decline again returning to baseline
around T+180 sec.  We are not quoting a T90 because we did not observe
the burst in its entirety.
 
The time-averaged spectrum from T-92.97 to T+52.72 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.66 +- 0.23.  The fluence in the 15-150 keV band is 9.1 +- 1.4 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T-85 sec in the 15-150 keV band
is 0.12 +- 0.04 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/519043/BA/

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

Using 2418 s of XRT Photon Counting mode data and 3 UVOT
images for GRB 120401A, 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 = 58.08251, -17.63577 which is equivalent
to:

RA (J2000): 03h 52m 19.80s
Dec (J2000): -17d 38' 08.8"

with an uncertainty of 1.6 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.

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Using  promptly downlinked XRT event data for GRB 120401A, we find an
enhanced XRT position of the afterglow: RA, Dec: 58.0819, -17.6359
which is equivalent to:
   RA (J2000)  = 03 52 19.67
   Dec (J2000) = -17 38 09.2
with an uncertainty of 2.1 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/519043.

Position enhancement is is described by Goad et al. (2007, A&A, 476,
1401) and Evans et al. (2009, MNRAS, 397, 1177).

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

A. P. Beardmore (U Leicester), W. H. Baumgartner (GSFC/UMBC),
V. D'Elia (ASDC), A. Maselli (INAF-IASFPA),
C. J. Mountford (U Leicester), C. Pagani (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL),
T. Sakamoto (NASA/UMBC), M. H. Siegel (PSU),
R. L. C. Starling (U Leicester) and B.-B. Zhang (PSU) report on behalf
of the Swift Team:

At 05:24:15 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 120401A (trigger=519043).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 58.068, -17.671 which is 
   RA(J2000) = 03h 52m 16s
   Dec(J2000) = -17d 40' 15"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for an image trigger, no
activity is apparent in the immediately available light curve. 

The XRT began observing the field at 05:26:14.1 UT, 118.3 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 58.0835, -17.6349 which is equivalent to:
   RA(J2000)  = +03h 52m 20.04s
   Dec(J2000) = -17d 38' 05.6"
with an uncertainty of 5.4 arcseconds (radius, 90% containment). This
location is 140 arcseconds from the BAT onboard position, within the
BAT error circle. No event data are yet available to determine the
column density using X-ray spectroscopy. 

The initial flux in the 2.5 s image was 1.35e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 127 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
0.05. 

Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.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.)