GRB 051016

P. Boyd (GSFC/UMBC), S. Barthelmy (GSFC), A. Beardmore (U. Leicester),
D. Burrows (PSU), N. Gehrels (GSFC), C. Gronwall (PSU), S.T. Holland (GSFC),
J. Kennea (PSU), C. Markwardt (GSFC/UMD), D. Palmer (LANL), T. Sakamoto (GSFC)
on behalf of the Swift team:

At 05:23:31 UT, Swift-BAT triggered and located GRB051016 (trigger=159913).
The BAT on-board calculated location is RA,Dec 122.811,-18.311d
{08h 11m 15s,-18d 18' 39"} (J2000), with an uncertainty of 3 arcmin
(radius, 90% containment, stat+sys).  The BAT light curve shows
a single peak with a total duration of ~15 sec.  The peak count rate
was ~1200 counts/sec (15-350 keV), at ~2 seconds after the trigger.

The spacecraft slewed promptly and the XRT began observing the GRB at
05:25:23 UT, 112 sec after the BAT trigger.  An uncatalogued source
was found by the on-board centroiding algorithm at
 RA(J2000)= 08h 11m 16.3s
DEC(J2000)=-18d 17' 49.2"
with an uncertainty of 8 arcseconds radius (90% containment).
This position lies 56 arcseconds from the centre of the BAT error
circle.  The initial flux is approximately 3.8E-10 ergs/cm2/s (0.3-10 keV).
The count rate decreases during the first 100 seconds.

UVOT began observing the field at 05:25:26 UT.  The small image
includes the BAT and the XRT error circles.  This image contains
no new sources down to a V limiting magnitude of 19.1 (3-sigma).
No new source was seen in the list of sources from the entire
UVOT field of view.  E(B-V) in this direction is 0.088.

M. Nysewander, D. Reichart, A. Crain, A. Foster, J. Haislip report on
behalf of the UNC team of the FUN GRB Collaboration:

Under the control of Skynet, PROMPT observed the localization of GRB 051016
(Boyd et al., GCN 4096
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 ) beginning 6.6 min after the burst, when the field
was still at high airmass, in repeating blocks of BVRcIc (two filters
simultaneously).

We do not detect any new sources within or around the XRT localization. In
a 12 x 80 sec integration of mean epoch 25.6 min after the burst, we
measure a 3-sigma limiting magnitude of Rc = 19.1, based on 3 USNO-B1.0
stars.

PROMPT is still being built and commissioned.

E. Berger (Carnegie Observatories), M. Roth, and S. Gonzalez (Las Campanas
Observatory) report:

"We imaged the error circle of GRB 051016 (GCN #4096
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 ) with the Swope
40-inch telescope in I-band and with the Magellan telescope in J-band.  A
total of 10 min were obtained in I-band on Oct 16.345 UT and 20 min were
obtained in J-band on Oct 16.325 UT (2.9 and 2.4 hours after the burst,
respectively).  We do not detect any sources within the XRT error circle
which are not visible in the DSS.  The 3-sigma upper limits are I>20 mag
and J>21.7 mag."

XMM-Newton will observe GRB051016 (GCN 4096
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 ) at location
(RA=08h 11m 16.3s, DEC=-18d 17' 49.2", J2000), as triggered by D. Fox
via XMM-Newton ToO alert, starting at 12:49 UT, on October 16, 2005,
for an exposure of 30ksec seconds.

A. Beardmore (U. Leicester), D. Burrows, J. Kennea (PSU),
N. Gehrels (GSFC) report on behalf of the Swift XRT team:

We have analysed the first two orbits of data from the Swift XRT
observation of GRB051016 (Boyd et al., GCN4096
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 ). The refined ground
processed coordinates are :

 RA(J2000)  =  08h 11m 16.3s
 Dec(J2000) = -18d 17m 56.3s

with an estimated uncertainty of 8 arcsecs radius (90% containment).
This is 7.1 arcsecs awsy from the XRT position reported in Boyd et al.

The XRT light curve from this interval shows a steep initial decline,
followed by a break to a shallower decay slope. A broken power-law fit
to the light curve gives parameters :

alpha_1 = 4.8 +0.5-0.4
t_break = 247 +/- 15s
alpha_2 = 0.7 +/- 0.1.

The Windowed Timing (WT) and Photon Counting mode spectra from the first
two orbits can be fit with an absorbed power-law with a photon index of
3.14+/-0.45 and a column density of 4.5+/-1.5 E21 cm-2, which is slightly
in excess of the Galactic column density of 1.19E+21 cm-2 in this
direction. The observed 0.2-10 keV flux for the WT data (exposure time
22.2s, centered at T+130s) was 1.31E-10 ergs cm**-2 s**-1, corresponding
to an unabsorbed flux of 9.98E-10 ergs cm**-2 s**-1.

Assuming the X-ray emission from the GRB continues to decline at the
same rate, the predicted XRT count rate at T+24hrs is 0.0036 counts/s,
which  corresponds to  an observed  0.2-10 keV  flux  of 1.1e-13
ergs cm**-2 s**-1.

Nora Loiseau and Ricardo Perez-Martinez report:

Quick-Look-Analysis of the XMM-Newton observation of the GRB051016
field based on an exposure in the EPIC pn camera that started at 13:46 UT,
shows the presence of a source at the SWIFT/XRT position (Boyd et al.,
GCN4096
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 )
within XRT 8 arcsec radius uncertainty.

The estimated EPIC/pn net count rate for the first 5.9 ksec is 0.029
counts/sec.

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

Using the data set from T-300 to T+300 sec from recent telemetry
downlinks, we report further analysis of Swift-BAT GRB 051016 (trigger 159913)
(Boyd, et al., GCN 4096
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 ).  The refined BAT ground position is
(RA,Dec) = 122.820, -18.309 deg {8h 11m 16.9s, -18d 18' 33.2"} (J2000)
+- 2.0 arcmin, (radius, sys+stat, 90% containment).  This position is
38 arcsec from the refined XRT position (Beardmore t al., GCN 4100
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 ).
The partial coding was 18 %.  

The light curve shows a FRED-like peak.  T90 (15-350 keV) is (22 +- 1) sec 
(estimated error including systematics).  

The power law index of the time-averaged spectrum is 1.88 +- 0.27.  
The fluence in the 15-150 keV band is (8.8 +- 1.4) x 10^-7 erg/cm2.  
The 1-sec peak photon flux measured from T0+0.3 sec in the 15-150 keV 
band is (1.6 +- 0.4) ph/cm2/sec.  All the quoted errors are at the 90% 
confidence level.

Andrea De Luca (INAF/IASF, Milano) on behalf of a larger
collaboration reports:

We have analyzed the data from the XMM-Newton observation
of GRB051016, discovered by Swift on 2005, October 16 at
05:23:31 UT (Boyd et al., GCN4096
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 ).

The XMM-Newton observation started on 2005, October 16 at
13:44 UT and lasted for 29.4 ks. 

As reported by Loiseau and Perez-Martinez (GCN4101
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 ), the afterglow of
GRB051016 is clearly detected in the EPIC images and
its position is fully consistent with the refined Swift/XRT one
(Morris et al., GCN3606
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 ).

Extracting source events from a circle of 25 arcsec radius
(containing ~80% of the total counts), the time-averaged,
background-subtracted count rate in the 0.5-8 keV range
is 0.028+/-0.001 cts/s, 0.0092+/-0.0006 cts/s and 0.0100+/-0.0006
cts/s in the pn, MOS1 and MOS2 cameras, respectively.

The afterglow is seen to fade along the XMM-Newton
observation, spanning the time range 30.0-59.4 ks after
the GRB. The pn background-subtracted light curve (0.5-8 keV)
is well fitted (reduced chi2=1.0, 7 d.o.f.) by a power law 
decay with index delta=0.9+/-0.3 (90% c.l.). 
The observed afterglow decay is consistent with the XRT
results (Beardmore et al., GCN4100
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 ).

We extracted the time-averaged spectra and generated ad-hoc
response and effective area files for the pn, MOS1 and MOS2 
cameras. The spectra were fitted simultaneously.
Unless otherwise specified, we quote errors at 90% level for 
a single interesting parameter.

A fit in the energy range 0.5-8 keV with an absorbed power 
law model yields an unacceptable reduced chi2 of 2.5 for 26 d.o.f.,
The resulting NH=(3.0+/-0.2)x10^21 cm^-2 is somewhat higher than 
the expected Galactic value in the burst direction (NH=1.1x10^21
cm^-2, Dickey & Lockman, 1990); the best fitting power law photon 
index is Gamma=2.6+/-0.3.

A better fit may be obtained fixing the NH to the Galactic 
value and adding a redshifted neutral absorber. This yields a
reduced chi2 of 1.74 for 25 d.o.f. The intrinsic gas column 
density and the redshift are not well constrained owing to their
strong correlation. The best fit values are: intrinsic 
NH=6.7x10^22 cm^-2, z=4.1. At 90% confidence level for 2 parameters,
the intrinsic NH is larger than 5x10^21 cm^-2 and the redshift 
is larger than 0.8. The best fit value of the power law photon index 
is Gamma=2.5+/-0.2. The observed flux (0.5-8 keV) is of 7.6x10^-14
erg cm^-2 s^-1, corresponding to an unabsorbed flux of
1.1x10^-13 erg cm^-2 s^-1.