GRB 081016B

K. L. Page (U Leicester), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), P. A. Evans (U Leicester),
O. Godet (U Leicester), C. Guidorzi (INAF-OAB),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
W.B Landsman (GSFC), C. B. Markwardt (CRESST/GSFC/UMD),
F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester),
J. P. Osborne (U Leicester), D. M. Palmer (LANL),
M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester),
M. C. Stroh (PSU), T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU)
report on behalf of the Swift Team:

At 19:47:14 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 081016B (trigger=331856).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 14.615, -43.543 which is 
   RA(J2000)  =  00h 58m 28s
   Dec(J2000) = -43d 32' 34"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
with a duration of about 5 sec.  The peak count rate
was ~1600 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 19:48:45.1 UT, 90.6 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued, 
fading X-ray source located at RA, Dec 14.5670, -43.5296 which is equivalent
to:
   RA(J2000)  = 00h 58m 16.08s
   Dec(J2000) = -43d 31' 46.6"
with an uncertainty of 3.3 arcseconds (radius, 90% containment). This
location is 134 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
1.96e+20 cm^-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of 150 seconds with the White (160-650 nm)
filter starting 93 seconds after the BAT trigger. No 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 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
0.01. 

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.)

B. E. Schaefer (Louisiana State), E.S. Rykoff (UCSB), and H. Swan (U.  
Mich.) report on behalf of the ROTSE collaboration:

ROTSE-IIIc, located at the H.E.S.S. site at Mt. Gamsberg, Namibia, 
responded to GRB 081016B (Swift trigger 331856), producing images 
beginning 6.4 s after the GCN notice time. An automated response took the 
first image at 19:47:59.4 UT, 44.9 s after the burst, under excellent 
conditions. We took 10 5-sec, 10 20-sec and 20 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; the field is not crowded. Individual 
images have limiting magnitudes ranging from 15.8-17.1; we set the 
following specific limits.

start UT       end UT      t_exp(s)   mlim   t_start-tGRB(s)  Coadd?
--------------------------------------------------------------------
19:47:59.4   19:48:04.4         5     15.8           44.9       N
19:47:59.4   19:49:15.5        76     17.5           44.9       Y

[GCN OPS NOTE(17oct08): Per author's request, the burst name was changed
from "A" to "B".]

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

Using 2096 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 081016B, 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 = 14.56434, -43.53015 which is equivalent
to:

RA (J2000): 00h 58m 15.44s
Dec (J2000): -43d 31' 48.5"

with an uncertainty of 2.1 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.

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

We have analysed the first six orbits of the XRT data obtained for GRB 
081016B (Page et al., GCN Circ. 8381
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 ). After an initial 18 seconds in 
Windowed Timing mode, the remaining 3.5 ks were collected in Photon 
Counting (PC) mode. The UVOT-enhanced position was given by Osborne et al. 
in GCN Circ. 8383
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 .

The first orbit of data shows a decay of alpha = 1.1 +/- 0.3. However, 
after the end of this orbit (~265 s after the trigger), the source is no 
longer detected, indicating a break to a significantly steeper decay 
occurred.

The spectrum of the first orbit of PC data can be modelled with a 
power-law (Gamma = 1.20 +/- 0.18) absorbed at the Galactic value of NH = 
1.96x10^20 cm^-2. The upper limit on the total absorbing column is 
2.2x10^21 cm^-2. The observed (unabsorbed) flux over this time is 
2.43x10^-10 (2.48x10^-10) erg cm^-2 s^-1, corresponding to a counts to 
observed flux conversion of 6.8x10^-11 erg cm^-2 count^-1.

Because of the lack of detection after the first orbit, we cannot 
predict the future count rate.

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

W.Landsman (GSFC) and K. L. Page (U Leicester), on behalf of the 
Swift/UVOT team.

The Swift Ultra-Violet/Optical Telescope (UVOT) began settled 
observations of GRB 081016B (trigger 331856) 93 seconds after the BAT 
trigger (Page et al.,GCN Circ. 8381). No afterglow is detected at the 
enhanced XRT position (Osborne et al., GCN Circ. 8383
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 ) in the initial 
white finding chart or subsequent images. The limiting magnitudes 
(3-sigma in 3" radius 
apertures) in each of the UVOT filters are as follows:

Filter  T_start(s)  T_stop(s)  Exp(s)  Mag UL (3sig)

white       93       243       150     >21.5
b         4710      4823       113     >20.6
v         5373      5572       197     >20.0
u         5988     11724       960     >21.5
uvw1      5783      5982       197     >20.3
uvm2      5577      5777       197     >20.0
uvw2      5168      5368       197     >20.3

The values quoted above are in the UVOT photometric system (Poole et al. 
2008, MNRAS, 383, 627).  They are not corrected for the expected 
Galactic extinction corresponding to a reddening of E(B-V)=0.01 mag in 
the direction of the burst (Schlegel et al. 1998).

M. Stamatikos (GSFC/ORAU), 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/UMD), K. McLean (GSFC/UMD),
K. L. Page (U Leicester),D. M. Palmer (LANL), A. M. Parsons (GSFC),
T. Sakamoto (GSFC/UMBC), G. Sato (ISAS), J. Tueller (GSFC),
T. N. Ukwatta (GWU) (i.e. the Swift-BAT team):

Using the data set from T-240 to T+962 sec from the recent telemetry 
downlink,
we report further analysis of BAT GRB 081016B (trigger #331856)
(Page, et al., GCN Circ. 8381
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 ).  The BAT ground-calculated position is
RA, Dec = 14.582, -43.536 deg which is
  RA(J2000)  =  00h 58m 19.8s
  Dec(J2000) = -43d 32' 07.9"
with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 98%.

The BAT mask-weighted light curve showed a single peak of duration ~2.5 
seconds.
T90 (15-350 keV) is 2.6 +- 0.8 sec (estimated error including systematics).

The time-averaged spectrum from T-0.0 to T+3.0 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
0.92 +- 0.32.  The fluence in the 15-150 keV band is 9.9 +- 1.8 x 10-08 
erg/cm2.
The 1-sec peak photon flux measured from T+0.50 sec in the 15-150 keV band
is 0.5 +- 0.1 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/331856/BA/

B. E. Cobb (UC Berkeley) reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical/IR imaging of the error region of GRB 081016b
(Page et al. GCN 8381
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 ) with a mid-exposure time of 4.3 hours
post-burst (2008-10-17 00:06 UT).  Total summed exposure
times amounted to 36 minutes in I and 30 minutes in J.

No source is detected at the position of the X-ray
afterglow (Osborne et al. GCN 8383
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 ) to approximate
limiting magnitudes of I > 22.1 and J > 19.8. Magnitudes
are calibrated using Landolt standard stars in I, and 2MASS
stars in J.