GRB 090123

D. Grupe (PSU), P. A. Evans (U Leicester), C. Gronwall (PSU),
C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC),
J. L. Racusin (PSU), M. Stamatikos (NASA/ORAU) and L. Vetere (PSU)
report on behalf of the Swift Team:

At 07:51:56 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090123 (trigger=340895).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 6.775, -23.514 which is 
   RA(J2000) = 00h 27m 06s
   Dec(J2000) = -23d 30' 49"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 30 sec, beginning at about T-10 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 07:53:36.2 UT, 99.9 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 6.7863, -23.5007 which is equivalent to:
   RA(J2000)  = 00h 27m 08.71s
   Dec(J2000) = -23d 30' 02.5"
with an uncertainty of 5.1 arcseconds (radius, 90% containment). This
location is 60 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. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 107 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	00:27:08.74 =  6.78641
  DEC(J2000) = -23:30:03.9  = -23.50108
with a 90%-confidence error radius of about 0.75 arc sec. This position is 1.4
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.71 with a 1-sigma error of about 0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.02. 

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.)

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

Using 1885 s of XRT Photon Counting mode data and 1 UVOT
images for GRB090123, 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 = 6.78599, -23.50156 which is equivalent
to:

RA (J2000): 00h 27m 8.64s
Dec (J2000): -23d 30' 05.6"

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
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.

T. N. Ukwatta (GWU), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), E. E. Fenimore (LANL), N. Gehrels (GSFC),
D. Grupe (PSU), H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD),
D. M. Palmer (LANL), T. Sakamoto (GSFC/UMBC), G. Sato (ISAS),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC)
(i.e. the Swift-BAT team):
 
Using the data set from T-240 to T+962 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 090123 (trigger #340895)
(Grupe, et al., GCN Circ. 8842
Loading...
 
 ).  The BAT ground-calculated position is
RA, Dec = 6.794, -23.503 deg, which is 
   RA(J2000)  =  00h 27m 10.5s 
   Dec(J2000) = -23d 30' 12.6" 
with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 36%.
 
The mask-weighted light curve shows multiple overlapping peaks starting
at ~T-48 sec, with the max peak at ~T+3 sec, and ending at ~T+120 sec.
T90 (15-350 keV) is 131 +- 23 sec (estimated error including systematics).
 
The time-averaged spectrum from T-49.6 to T+117.9 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.74 +- 0.12.  The fluence in the 15-150 keV band is 2.9 +- 0.2 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+0.08 sec in the 15-150 keV band
is 1.7 +- 0.3 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/340895/BA/

F.E. Marshall (NASA/GSFC) and D. Grupe (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT observed the field of GRB 090123 starting 107 s after
the BAT trigger (Grupe et al., GCN Circ. 8842
Loading...
 
 ). We detect the optical
afterglow (Grupe et al.) in the white, v, b, u, uvw1, uvm2, and
uvw2 filters at the position:

  RA(J2000)  =  00:27:08.73 =   6.78639
  DEC(J2000) = -23:30:04.1  = -23.50113

with an estimated uncertainty of 0.5 arc sec. (radius, 90% confidence).
There is a broad peak in the light curve ~600 sec. after the trigger.
The 3.4-sigma detection in the uvw2 filter indicates that the redshift 
is less than ~1.4.

The magnitudes in the UVOT photometric system 
(Poole et al. 2008, MNRAS, 383, 627) in the first finding chart (FC) 
exposure and subsequent exposures are:

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

white (FC)         107          257          147         17.8 +/- 0.1
v                  649          669           20         15.4 +/- 0.1
b                  575          595           20         15.8 +/- 0.1
u                  320          570          250         16.4 +/- 0.1
uvw1               698          718           20         17.3 +/- 0.23
uvm2              1251         8112          294         19.6 +/- 0.25
uvw2              1547         7843          413         20.5 +/- 0.29

The values quoted above are not corrected for the Galactic extinction due
to the reddening of E(B-V) = 0.02 in the direction of the burst 
(Schlegel et al. 1998).

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

We have analyzed 7.7 ks of XRT data for GRB090123 (Grupe et al. GCN 
Circ. 8842
Loading...
 
 ),
beginning 106 s after the BAT trigger. The data comprise 334 s in Windowed
Timing (WT) mode with the remainder in Photon Counting (PC) mode.
The enhanced XRT position for this burst was given by Osborne et al.
(GCN. Circ 8843).

The light curve can be modeled with an initial power-law decay with an
index of alpha=1.78 (+0.06, -0.08), followed by a break at T+1530 
(+720,-220) s to
an alpha of 1.20 (+0.08, -0.10).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.65+/-0.05 with an absorption 
column fixed to the Galactic
value of 1.79 x 1020 cm-2 (Kalberla et al. 2005). The PC mode spectrum 
has a
photon index of 1.81+/-0.08 and an absorption column density also 
consistent
with the Galactic value.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 4.2 x 10-11 (4.5 x 10-11) erg cm-2 count-1.


If the light curve continues to decay with a power-law decay index of 1.20,
the count rate at T+24 hours will be 0.0031 counts/s, corresponding to an
observed (unabsorbed) 0.3-10 keV flux of 1.3 x 10-13 (1.4 x 10-13) erg cm-2
s-1.

A. Rossi (Tautenburg Obs.) and J. Greiner (MPE Garching) report on behalf 
of the GROND team:

GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m ESO/MPI
telescope at La Silla Observatory (Chile) imaged the field of GRB 090123
(Swift trigger 340895, Grupe et al. 2009, GCN #8842
Loading...
 
 ) simultaneously in
g'r'i'z'JHK. Observation started on 2009-01-24 at 01:22 UTC, about 17 hours 
after the burst. The total exposure time was 25 minutes, the average seeing 
1.3 at airmass 2.5.

We detect the afterglow at the position reported by SWIFT UVOT (Marshall 
et al 2009, GCN #8846
Loading...
 
 ) in g'r'i'z', but not in JHK. We estimate the following 
observed magnitudes and upper limits (all in AB system):

g' = 22.77 +- 0.05
r' = 22.45 +- 0.05
i' = 21.95 +- 0.08
z' = 22.09 +- 0.12
J > 21.5
H > 21.0
K > 20.5

calibrated against GROND zeropoints and 2MASS field stars.