GRB 120121A

K. L. Page (U Leicester), A. P. Beardmore (U Leicester),
P. A. Evans (U Leicester), J. A. Kennea (PSU), D. M. Palmer (LANL),
M. H. Siegel (PSU), R. L. C. Starling (U Leicester) and
B.-B. Zhang (PSU) report on behalf of the Swift Team:

At 09:42:19 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 120121A (trigger=512164).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 249.336, -23.954 which is 
   RA(J2000) = 16h 37m 21s
   Dec(J2000) = -23d 57' 14"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
FRED structure with a duration of about 40 sec.  The peak count rate
was ~800 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 09:44:18.7 UT, 119.4 seconds after
the BAT trigger. No source was detected in the 2.5-s promptly available
image. We are waiting for the full dataset to detect and localise the
XRT counterpart. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 123 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 25% of
the BAT 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
BAT 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
0.84. 

Burst Advocate for this burst is K. L. Page (kpa 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.)

K.L. Page and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT 
team:

Using promptly downlinked data we find an X-ray source with an enhanced 
position: RA,Dec = 249.3547, -23.9618 (degrees) which is equivalent to:

RA  (J2000.0) = 16 37 25.13
DEC (J2000.0) = -23 57 42.4

with an uncertainty of 3.2 arcsec (radius, 90% containment). This location 
is 67.6 arcseconds from the BAT onboard position, inside the BAT error 
circle. It is not yet possible to determine whether the source is fading.

L. Elenin (KIAM), A. Pozanenko (IKI), A. Volnova (SAI MSU), I. Molotov
(KIAM) report on behalf of larger GRB  follow-up collaboration:

We observed the field of the Swift GRB 120121A  (Page et al.,  GCN 12885
Loading...
 
 )
with 0.45-m telescope ISON-NM observatory on Jan. 21. We took several
unfiltered images of  60 s exposure on Jan. 21, starting (UT) 12:29:25.  We
do not detect any object within the XRT error circle (Page et al., GCN 12886
Loading...
 
 
), down to 19.7m (calibrated against the USNO B1.0 catalog).

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

Using 1511 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 120121A, 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 = 249.35428, -23.96156 which is equivalent
to:

RA (J2000): 16h 37m 25.03s
Dec (J2000): -23d 57' 41.6"

with an uncertainty of 1.7 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. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (GSFC/UMBC),
E. E. Fenimore (LANL), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
C. B. Markwardt (GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL),
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 120121A (trigger #512164)
(Page, et al., GCN Circ. 12885
Loading...
 
 ).  The BAT ground-calculated position is
RA, Dec = 249.363, -23.963 deg which is 
   RA(J2000)  =  16h 37m 27.1s 
   Dec(J2000) = -23d 57' 45.9" 
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 70%.
 
The mask-weighted light curve shows several overlapping peaks starting at
~T-10 sec, peaking at ~T+2 sec, and ending at ~T+70 sec.
T90 (15-350 keV) is 26.1 +- 3.1 sec (estimated error including systematics).
 
The time-averaged spectrum from T-11.00 to T+18.33 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.23 +- 0.13.  The fluence in the 15-150 keV band is 1.1 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+2.86 sec in the 15-150 keV band
is 0.7 +- 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/512164/BA/

K. L. Page (U Leicester) reports on behalf of the Swift-XRT team:

We have analysed 9.8 ks of XRT data for GRB 120121A (Page  et al. GCN
Circ. 12885
Loading...
 
 ), from 130 s to 23.1 ks after the  BAT trigger. The data
are entirely in Photon Counting (PC) mode. The enhanced XRT position
for this burst was given by Evans et al. (GCN. Circ 12888).

The light curve can be modelled with  a power-law decay with a decay
index of alpha=0.88 (+0.09, -0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.0 (+0.7, -0.6). The
best-fitting absorption column is  2.4 (+1.3, -0.9) x 10^22 cm^-2, in
excess of 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 7.5 x 10^-11 (2.0 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.4 (+1.3, -0.9) x 10^22 cm^-2
Galactic foreground: 1.2 x 10^21 cm^-2
Excess significance: 4.3 sigma
Photon index:	     2.0 (+0.7, -0.6)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00512164.

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

F. E. Marshall (NASA/GSFC) and K. L. Page (U. Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 120121A
124 s after the BAT trigger (Page et al., GCN Circ. 12885
Loading...
 
 ).
No optical afterglow consistent with the XRT position
(Evans et al., GCN Circ. 12888
Loading...
 
 )
is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

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

white_FC           124          273          147         >20.2
u_FC               336          585          246         >19.7
white              124         6777          673         >21.1
v                  664         7188          510         >18.9
b                  591        13219         1160         >20.2
u                  336        18998         2078         >20.2
w1                 714        18281         2108         >20.4
m2               16474        17374          886         >20.1
w2                1020        23198         1298         >21.1

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.87 in the direction of the burst
(Schlegel et al. 1998).