GRB 051001

E.S. Rykoff (U Mich), H. Swan (U Mich), B. Schaefer (Louisiana State), 
S.A. Yost (U Mich), F. Yuan (U Mich) report on behalf of the ROTSE 
collaboration:

ROTSE-IIIa, located at Siding Spring Observatory, Australia, responded 
to GRB 051001 (Swift trigger 157870), producing images beginning 7.9 s 
after the GCN notice time. An automated response took the first image at 
11:13:01.9 UT, 85.6 s after the burst, under fair conditions. We took 10 
5-sec, 10 20-sec and 20+ 60-sec eposures. Observations are ongoing. 
These unfiltered images are calibrated relative to USNO A2.0 (R).

Comparison to the DSS (second epoch) reveals no new sources within the 
3-sigma BAT error circle (or the possible X-ray position from the GCN 
notice), for both single images and coadding into sets of 10. Individual 
images have limiting magnitudes ranging from 16.1-17.4; we set the 
following specific limits.

start UT       end UT      t_exp(s)   mlim   t_start-tGRB(s)  Coadd?
--------------------------------------------------------------------
11:13:01.9   11:13:06.9         5     16.2           85.6       N
11:13:01.9   11:15:25.9       144     17.7           85.6       Y
11:15:26.6   11:20:19.6       293     18.1          230.3       Y

A. Moretti (OAB), J. Cummings (GSFC-NRC),
N. Gehrels (GSFC), S. Hunsberger (PSU), H. Krimm (GSFC-USRA),
C. Markwardt (GSFC-UMD), F. Marshall (GSFC), D. Palmer (LANL),
P. Romano (OAB),  on behalf of the Swift team:

GRB 051001:  Swift Detection of a Burst

At 11:11:36 UT, Swift-BAT triggered and located GRB051001
(trigger=157870). The spacecraft slewed immediately, which in
this case was 75 seconds after the image trigger.  The BAT
on-board calculated location is RA,Dec 350.973d, -31.528d
{23h 23m 53s, -31d 31' 39"} (J2000), with an uncertainty
of 3 arcmin (radius, 90% containment, stat+sys).  Since this
was an image trigger there is no obvious information in the light curve.

XRT began observing the field at 11:14:42, 185 seconds after the
BAT trigger.  Onboard centroiding found a bright fading uncatalogued
X-ray source in the field of view at the following coordinates:
RA(J2000):   23h 23m 49.1
Dec(J2000):  -31d 31' 15.6"
We estimate the uncertainty of this position to be 6 arcseconds
(radius, 90%  containment).  This position is 61 arcseconds
from the BAT position above.

UVOT began observing the field 189 seconds after the trigger.
The small image does not overlap the XRT error circle.
No new source was seen in the list of sources from the entire
UVOT field of view.

We are currently in the portion of the orbits where the spacecraft
does not pass over the Malindi downlink station.  Therefore, it will
be at least four hours before we have access to the full data set
for the refined analyses.

T. Sakamoto (GSFC/NRC), L. Angelini (GSFC-JHU), L. Barbier (GSFC),
S. Barthelmy (GSFC), M. Chester (PSU), 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), G. Sato (ISAS), J. Tueller (GSFC),
on behalf of the Swift-BAT team:

Using the data set from T-300 to T+300 sec from the recent telemetry
downlink, we report further analysis of Swift-BAT GRB 051001
(trigger #157870)  (Moretti, et al., GCN 4051
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 ).  The refined BAT
ground position is (RA,Dec) = 350.990,-31.523 {23h 23m 57.7s,
-31d 31' 23.6"} [deg; J2000] +-2.3 arcmin, (radius, sys+stat,
90% containment).  This is 1.8 arcmin from the XRT flight centroid
and 0.9 arcmin from the BAT position derived onboard.  The partial
coding was 74 %.

The mask-weighted light curve shows very broad emission starting at
~10 sec before the trigger and extending to T+195 sec.  There are
two main peaks: from T-10 to T+60 sec, and a stronger one from T+80
to T+180 and weak peaks ~T+185 to T+195 and T+230 to T+240 sec.
There is no significant emission in the BAT energy range after T+240 out
to T+1400 sec. With such a low level light curve it is difficult to
estimate the duration, but our best estimate of T90 (15-350 keV) is
190 +- 10 sec (estimated error including systematics).

The power law index of the time-averaged spectrum is 2.10 +- 0.15.
The fluence in the 15-150 keV band is (1.8 +- 0.15) x 10^6 erg/cm2.
The 1-sec peak photon flux measured from T0+153.3 sec in the 15-150 keV
band is (0.51 +- 0.12) ph/cm2/sec.  All the quoted errors are at the 90%
confidence level.

E. Rol (U. of Leicester), A. Levan, N. Tanvir (U. of Hertfordshire),
report on behalf of a larger collaboration:

We have observed the field of GRB 051001 (Moretti, GCN 4051
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 ) with the
IRIS2 instrument on the Anglo-Australian Telescope. Two epochs of
observations were performed in each of the ZJHKs bands, roughly 2
hours and 6 hours after the trigger, respectively. Our observations
covered the entire BAT error box, albeit with a small section (<5%) on
the edge of the FOV. Comparison between the two epochs shows no
variable sources.

Inside the XRT error circle, one source is visible, which appears to
be extended. Its location is, calibrated to the 2MASS:

  RA    23:23:49.04    (J2000)
  Dec  -31:31:18.8     (J2000)

with an estimated error of 0.16" in each coordinate.

Magnitude estimates for the first epoch, calibrated with respect to
the 2MASS, are as follows:

 filter   mid-time      exposure     magnitude        limiting
         after trigger    time      of extended        magnitude
           (hours)      (minutes)    source           (3 sigma)
   Ks       1.40           20         18.7 (0.1)        20.3
   H        1.85           20         20.0 (0.1)        20.7
   J        2.36           10         20.9 (0.3)        20.7

The first epoch Ks image can be seen at
http://www.star.le.ac.uk/~er45/grb051001/

D.B. Fox (Penn State), N.E.D. Noel (Instituto de Astrofisica de
Canarias), and S.L. Prior (RSAA/ANU), with B.A. Peterson and
B.P. Schmidt (RSAA/ANU) report:

"We have observed the Swift XRT localization region for GRB 051001
(Swift Trigger 157870; Moretti et al., GCN 4051
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 ) with the 40-inch
telescope at Siding Spring Observatory.  A coadded I-band image with
mean epoch 1 October 11:53 UT (52 minutes after the burst) reveals no
sources within this region, to a limiting magnitude of I>20.8 mag as
calibrated against the photometry of the USNO-B1.0 catalog.

In particular, we do not detect the extended object of Rol, Levan &
Tanvir (GCN 4053
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 ) to the quoted limit."

P. Tristram (Univ. of Canterbury),  M. Jelinek,
A. J. Castro-Tirado, A. de Ugarte Postigo,
S. Guziy, J. Gorosabel (IAA-CSIC, Granada)
and Ph. Yock (Univ. of Auckland)

report:

"Following the detection of GRB 051001 by SWIFT 
(Krimm et al. GCN 4052
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 ), we have imaged the              
entire error box with the 0.6-m (+MOA camera) at 
Mt. John Observatory in wide R- and B-band filters 
(8 x 300-s each) starting on Oct 1.48 UT (i.e. 
45 min after the event).  We do not detect the
object reported by Rol et al. (GCNC 4053) within 
the SWIFT/XRT error box (Moretti et al. GCN 4051
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 ) 
down to R about 21.5 ."

A. Moretti (INAF-OAB), P. Romano (INAF-OAB), G. Tagliaferri (INAF-OAB),
S. Campana (INAF-OAB), G. Chincarini (INAF-OAB), M. Chester (PSU),
P. Boyd (GSFC-UMBC), on behalf of the Swift-XRT Team report.

We have analyzed the Swift XRT data from the first day of observation of
GRB 051001 (Moretti et al., GCN  4051
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 ). 
As already reported an uncatalogued, fading X-ray source was detected by
XRT. The refined position of the source is 
RA(J2000): 23 23 48.8   
Dec(J2000):-31 31 17.0
We estimate an uncertainty of 6 arcseconds radius (90% containment).

The observations started in Windowed Timing (WT) 192 seconds from the BAT trigger.
The light curve shows a fast decay in the first orbit with a slope of
-2.9+-0.1. Since the second orbit onward the decay 
has been flatter, with a power law slope of -0.65+-0.10.   
The spectral fit (simple absorbed power-law) to the WT data yields a
photon index of 1.48-/+0.02 in the [0.2-10] keV band. The derived NH is
(1.3)E21 cm^-2, which is higher than the Galactic value  (1.4E+20
cm-2; Dickey & Lockman 1990). 

The unabsorbed 0.5-10.0 keV flux of the afterglow at 24 hours after the
burst is then estimated to be 2E-13 ergs/s/cm2.

F. Marshall(GSFC), A. Moretti (OAB), J. Nousek (PSU), W. Voges (MPE),
N. Gehrels (GSFC) report on behalf of the Swift/UVOT team.

The Swift/UVOT instrument began observing the field of the burst GRB051001
at 11:14:45 UT, 189 seconds after the BAT trigger (Moretti et al., GCN 4051
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 ).

Within the 6 arcsec radius XRT error circle (GCN 4051
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 ), no new optical/UV
source is detected. We derive the following 3-sigma upper limit magnitudes
on any optical/UV counterpart in the summed images in each UVOT filter
(from 6" radius apertures centered on the XRT location, which is
representative of the local background).

     Filter    T_range(sec)  Exp(sec)  3-sig UL

       V           189-289       100     18.9
       V         189-11522      1189     20.1
       B          335-6646      1077     21.2
       U          321-5738      1106     21.0
       UVW1      307-13221       952     20.9
       UVW2    11530-12429       900     21.4
       UVM2      6655-7435       780     21.5