GRB 051016B

A. M. Soderberg, E. Berger and E. Ofek report on behalf of the
Caltech-NRAO-Carnegie collaboration:

"We observed the optical afterglow position (GCN 4105
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 ) of GRB 051016B (GCN
4103
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 ) with the Low Resolution Imaging Spectrometer on the Keck I telescope
on 2005 October 31.6 UT.  We detect an extended source in the g and r
filters which we identify as the host galaxy.  In a 900 sec spectrum we
detect emission lines of [O II] and Ne III, at a redshift of z=0.9364."

D. Sharapov, M. Ibrahimov (MAO), A.Pozanenko (IKI),V.Rumyantsev (CrAO) on 
behalf of larger GRB follow up collaboration report:

We observed the error box  of GRB051016B (Parsons et al, GCN 4103
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 ) with 1.5m 
telescope of Maidanak Astronomical Observatory. Set of  R images were taken 
between Oct.16 (UT) 23:34 - 23:58. Afterglow candidate proposed by  Chen 
(GCN 4105
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 )  and Kinoshita (GCN 4111
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 ) is detected in a stacked image. 
Preliminary photometry of the stacked  image against of  USNO A2.0 is 
following:

Mid time,     Exposure, Filter, Mag.
(UT)            (s)

Oct.16.990   6x180   R       21.43 +/- 0.24

The message may be cited.

K. Torii (Osaka U.) reports on behalf of the ART collaboration:

 The error region of the soft GRB 051016b (Parsons et al. GCN 4103
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 )
was observed by the 14 inch Automated Response Telescope. VRcIc
imaging started at 2005 October 16, 18:29:54 UT (105s after the
trigger) and 60s integration was repeated.

 The optical afterglow candidate (Chen et al. GCN 4105
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 ) is not
detected in our frames and the following limiting magnitudes for
single and stacked frames are derived.

------------------------------------------
StartUT	Filter	Limit	Nframes
------------------------------------------
18:29:54 Rc	15.7R	1
18:30:58 Ic	15.2I	1
18:29:54 V+Rc+Ic 18.3R	9V+10Rc+10Ic
------------------------------------------

D. Kinoshita, Y.T. Chen, Y.L. Wu, K-Y. Huang, W-H. Ip(NCU), 
Y. Urata (RIKEN) on behalf of EAFON report:

" We have observed the field of GRB 051016B (Parsons et al. GCN 4103
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 )
with Lulin 1m telescope at 17.845 (1.08 days after the burst).  The
R-band stacked image made from 5 x 300sec exposure do not show the
candidate source reported by Chen et al (GCN 4105
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 ). These results
suggest that the source located at RA=08:48:27.81 DEC=+13:39:20.0 is
the afterglow of GRB 051016B.

These images are available at
 http://140.115.34.82/~qq/grb/GRB051016B_LOT.jpg

Our light curve around 1.5-2 hour do not show power-law decay as we
reported (Chen et al GCN 4105
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 ). However, several early afterglows show
platue phase such as GRB 041006 (Urata et al. ApJL submitted).

This message may be cited."

J. S. Bloom (UCB), H.-W. Chen, (Chicago), R. J. Foley (UCB), J. X.
Prochaska (UCSC), report on behalf of a larger collaboration:

"We observed the position of GRB 051016b (GCN 4103
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 ) with the NIRI
instrument on the Gemini North Telescope beginning 20051017.59 UTC.
In a stack of 45 minutes of exposure we detect several faint sources
consistent with the XRT position (GCN 4108
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 ):

   A: 08:48:27.85 13:39:20.4
   B: 08:48:27.32 13:39:18.1
   C: 08:48:27.64 13:39:13.6
   D: 08:48:27.42 13:39:23.9

The astrometry is derived assuming the nominal platescale of NIRI
and based on the assumption that a bright galaxy (2MASS 1036.075258)
is at position (J2000) 08:48:27.607 +13:39:02.67. Source A is 0.6" E
and 0.4" N of an optical source reported by Maeno et al. (GCN 4106
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 )
and we thus associate this apparent IR point source (A) with the
Lulin/UVOT source. We measure an isophotal mag of Ks[A] = 20.52
+/- 0.12 mag, assuming the 2MASS galaxy has magnitude of Ks = 15.209
mag. We make no claim of variability but source A is the most
point-like of the 4 objects in the XRT error circle, so it is still
a plausible afterglow candidate."

We thank the Gemini observing staff for performing this ToO.

This message may be cited.

O. Godet (U. Leicester), A. Parsons (GSFC) K. Page, J. Osborne, A.
Beardmore (U. Leicester), D. Burrows (PSU), N. Gehrels (GSFC) report on
behalf the Swift XRT team:

We have analysed the first four orbits of data from the Swift XRT
observation of GRB051016B (Parsons et al., GCN 4103
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 ). The refined ground
processed coordinates are :
 RA(J2000)  =  08h 48m 27.5s
 Dec(J2000) = +13d 39m 20.8s
with an estimated uncertainty of 8 arcsecs radius (90% containment).
This is 4.9 arcsecs away from the XRT position reported in Parsons et
al.

The XRT light curve shows a steep initial decline with a decay slope
of 5.3+0.6/-0.7, followed by a break at ~T+ 200s to a shallower decay
slope with a flare at ~T+409s superposed. At ~T+6000s, another break can
be seen in the X-ray light-curve with a steeper decay slope of
0.9+/-0.2.

The X-ray spectra show strong evolution in the early phase.  The
Windowed Timing (WT) data from the first orbit (from T+82s to T+107s)
can be fit with an absorbed power-law with a photon index of
6.2+2.4/-1.5 and a column density excess of 4.8+/-2.0 E21 cm-2 over
the Galactic column density of 3.66E+20 cm-2 in this direction. The
Photon Counting (PC) spectrum from the first orbit (containing the
flare) can be fit with an absorbed power-law with a photon index of
1.8+0.2/-0.4 and no absorption excess over the Galactic absorption. The
PC spectrum of the second, third and fourth orbits can be fit by an
absorbed power-law with a photon index of 1.5+/-0.1 and no absorption
excess over the Galactic absorption.

The observed 0.2-10 keV flux for the PC data from the second, third
and fourth orbits (exposure time ~7600s, starting at ~T+3950s) was
3.82E-12 ergs cm**-2 s**-1, corresponding to an unabsorbed flux of
4.15E-12 ergs cm**-2 s**-1.

Assuming the X-ray emission from the GRB continues to decline at the
same rate (alpha~0.94), the predicted XRT count rate at T+24hrs is
0.0077 counts/s, which  corresponds to an observed 0.2-10 keV flux of
4.2e-13 ergs cm**-2 s**-1.

A. J. Blustin (UCL-MSSL), A. Parsons (GSFC), S. T. Holland (GSFC),
P. Meszaros, M. Chester (PSU), N. Gehrels (GSFC) on behalf of
the Swift/UVOT team report:

The Swift/UVOT began taking a settled exposure of the field of
GRB 051016B 78 s after the trigger (Parsons et al. GCN 4103
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 ).
A faint optical/UV source (not visible in the DSS) is detected
in summed images at the position reported by Chen et al. (GCN 4105
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 )
at greater than 3 sigma in four of the UVOT filters; the
magnitudes and upper limits are as follows:

Filter  T_range(sec)  Exp(sec)  Mag/3-sig upper limit

V       78-29720      2418      20.5 (upper limit)
B       445-28020     2748      22.0 +/- 0.5
U       391-35505     2507      20.9 +/- 0.3
UVW1    337-34740     2796      21.7 +/- 0.4
UVM2    283-33833     2798      21.0 (upper limit)
UVW2    554-28928     2748      22.8 +/- 0.6

The detection in UVW2 would imply that the redshift of the source
is less than ~1.3.

There was a further uncatalogued nearby UV-only source at
(J2000 08:48:27.7 +13:39:12.0), outside the XRT error circle,
with the following magnitudes:

Filter   Mag

UVW1     21.9 +/- 0.4
UVM2     22.3 +/- 0.4
UVW2     22.0 +/- 0.3

Further data will be required to tell whether either source is
variable.











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S.Maeno,E.Sonoda,Y.Tokunaga,M.Yamauchi
(University of Miyazaki)

"We have observed the field covering the error circle of
GRB 051016B (GCN4103
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 ) with the unfiltered CCD camera on the 30-cm
telescopeat University of Miyazaki.
The observation was started 19:16:07 UT on Oct.16.
After co-adding a set of 25 images (19:16:07 - 19:59:25 UT)
of 30 sec exposures, we have compared with the USNO A2.0 catalog.
Preliminary analysis shows there is no new source brighter than
17.8 mag."

GRB 051016B : Optical follow-up at Lulin 

Y.T. Chen, Y.L. Wu, K-Y. Huang, W-H. Ip(NCU), 
Y. Urata (RIKEN) on behalf of EAFON report: 

" We have observed entire error region of GRB 051016B (Parsons et 
al. GCN 4103
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 ) with 1-m telescope at Lulin Observatory, 
Taiwan. Compare with DSS-2 images, we find a source in the 
Swift/XRT error region. The position is RA=08:48:27.81 
DEC=+13:39:20.0. The source do not show significant variability during 
our observational period (1.5-2 hours after the burst).  These 3-sigma 
limiting magnitudes derived from the USNO-B1.0 catalog are R=21.5. 

This message may be cited."

L. Barbier (GSFC), S. Barthelmy (GSFC), J. Cummings (GSFC/NRC),
E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (GSFC/UMD),
H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), F. Marshall (GSFC),
D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/NRC),
G. Sato (ISAS), T. Takahashi (ISAS), J. Tueller (GSFC)
on behalf of the Swift-BAT team:

Using the full data set from the recent telemetry downlink, we report
further analysis of Swift-BAT GRB 051016B (trigger #159994)
(Parsons, et al., GCN 4103
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 ).  The ground-analysis position is
RA,Dec 132.120,+13.627 {08h 48m 28.8s,+13d 37' 38.4"} (J2000)
with an uncertainty of 1.6 arcmin (radius, 90%, stat+sys).
The parital coding fraction is 79 %.  T90 is 4.0 +- 0.1 sec.
The lightcurve consists of two overlapping peaks at T+0.3 and T+3.0 sec.
Fitting a simple power law over the full interval from T-0.0 to T+4.6 sec,
the photon index is 2.38 +/- 0.23 with a fluence of 1.7 +/- 0.2 X 10^-7
erg/cm^2.  The peak flux in a 1-sec wide window starting at T+0.11 sec
is 1.32 +/- 0.17 ph/cm^2/sec.  All values are in the 15-150 keV band
at the 90% confidence level.

A. Parsons (GSFC), S. Barthelmy (GSFC), A. Beardmore (U. Leicester),
P. Boyd (GSFC), D. Burrows (PSU), J. Cummings (GSFC/NRC), N. Gehrels (GSFC),
O. Godet (U. Leicester), C. Gronwall (PSU), J. Kennea (PSU),
H. Krimm (GSFC/USRA), F. Marshall (GSFC), D. Palmer (LANL), T. Sakamoto (GSFC)
on behalf of the Swift team:

At 18:28:09 UT, Swift-BAT triggered and located GRB 051016B (trigger=159994).
The BAT on-board calculated location is RA,Dec 132.067d,+13.658d
{08h 48m 16s,+13d 39' 27"} (J2000), with an uncertainty of 3 arcmin
(radius, 90% containment, stat+sys).  The BAT light curve shows a double peak
structure with a total duration of at least 15 sec -- the slew started at
T+15 sec, so determination of the full duration will have to wait until
after the full data set is downlinked.  The peak count rate was ~2200
counts/sec (15-150 keV), at ~1 and 3 seconds after the trigger.  Initial
inspection of the 4-channel lightcurves shows this burst to be soft.

The spacecraft slewed promptly and the XRT began observing the GRB
at 18:29:24 UT, 75 sec after the BAT trigger.  An uncatalogued source
was found by the on-board centroiding algorithm at
RA(J2000)=08h 48m 27.6s
Dec(J2000)=+13d 39' 25.5"
with an uncertainty of 8 arcsec radius (90% containment).
This position lies 168 arcsec from the center of the BAT error circle.
The initial flux is approximately 4.6e-10 ergs/cm2/s.
The count rate decreases during the first 100 seconds.

UVOT began observing the field at 18:29:26 UT, 77 sec after the BAT trigger.
The small image contains the entire XRT error circle.  This image contains
no new sources down to a limiting magnitude of 18.7 (3-sigma).
No new source was seen in the list of sources from the entire UVOT
field of view.  E(B-V) in the direction of the XRT position is 0.037.