GRB 061027

M. Stamatikos (NASA/ORAU), S. D. Barthelmy (GSFC),
D. N. Burrows (PSU), S. Campana (INAF-OAB), M. M. Chester (PSU),
G. Cusumano (INAF-IASFPA), N. Gehrels (NASA/GSFC),
C. Guidorzi (Univ Bicocca&INAF-OAB), K. M. McLean (LANL/UTD),
T. Mineo (INAF-IASFPA), P. T. O'Brien (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL),
S. B. Pandey (UCL-MSSL), P. Romano (Univ. Bicocca & INAF-OAB) and
T. Sakamoto (NASA/ORAU) report on behalf of the Swift Team:

At 10:15:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located possible GRB 061027 (trigger=235645).  Swift slewed immediately
to the burst.  The BAT on-board calculated location is 
RA,Dec 270.812, -82.228 {18h 03m 15s, -82d 13' 39"} (J2000)
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  This is a 64-sec image trigger and as is typical
the BAT lightcurve does not show any significant emission.  The image
peak is barely above the significance threshold for automatic
detection, and therefore this may be a statistical fluctuation. 

The XRT began taking data at 10:17:29 UT, 147 seconds after the BAT
trigger. The XRT on-board centroid algorithm did not find a source in
the image and no prompt position is available. Initial down-linked
data products also show no evidence for any X-ray source in the image
in ~200 s of data. This gives a very rough upper limit of 
2E-12 erg/cm2/s for any source in the field of view.  We note 
that only one long burst has had no XRT detection following a 
prompt slew, out of 119 long GRBs observed by XRT with prompt 
slews to date.  Further analysis awaits the full dowloaded 
data products. 

UVOT took a finding chart exposure of 100 seconds with the White
(160-650 nm) filter starting 151.7 seconds after the BAT trigger. No
afterglow candidate has been found in the initial data products. The
limiting magnitude of the finding chart image is approximately 19.0. 
No correction has been made for the expected extinction.

A. Parsons (GSFC), L. Barbier (GSFC), S.D. Barthelmy (GSFC),
J. Cummings (GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC),
D. Hullinger (BYU-Idaho), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD),
D. Palmer (LANL), T. Sakamoto (GSFC/ORAU), G. Sato (GSFC/ISAS),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC),  D.N. Burrows (PSU),
V. Mangano, T. Mineo, G. Cusumano, B. Sbarufatti (INAF-IASFPA),
S.B. Pandey (UCL-MSSL)
on behalf of the Swift team:
 
Using the data set from T-239 to T+963 sec from recent telemetry downlinks,
we report further analysis of BAT possible GRB 061027 (trigger #235645)
(Stamatikos, et al., GCN Circ. 5760
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 ).  The BAT ground-calculated position
is RA,Dec = 270.992, -82.239 deg {18h 3m 58.1s, -82d 14' 19.9"} (J2000)
+- 3.7 arcmin, (radius, sys+stat, 90% containment).  The partial coding was 68%.
 
The event starts ~100 sec before the trigger and extends to ~T+70 sec.
All of the emission is in the 15-50 keV band -- there is nothing above 50 keV.
Because this event is very weak, we had to use 10-sec binning for the lightcurve.
There appears to be no statistically significant structure in the lightcurve.
T90 (15-350 keV) is 150 +- 20 sec (estimated error including systematics).
 
The time-averaged spectrum from T-100 to T+70 is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.91 +- 0.48.  The fluence in the 15-150 keV band is 4.7 +- 1.5 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+0.00 sec in the 15-150 keV
band is 0.08 +- 0.02 ph/cm2/sec.  All the quoted errors are at the 90%
confidence level. 

Given the very low flux, the somewhat low detection significance, and
the soft spectrum for this event, we can not determine if this is a
long soft GRB or some hard x-ray transient from an unknown source.
The position in galactic coordinates is l, b = 311, -25, so the source
is off the plane but still possibly a galactic transient.  We can not rule out
the possibility that this event is due to a chance statistical fluctuation
in the image and rate domains.

XRT observed the field of GRB061027 for 1.5 ks in PC mode starting
at 10:17:37 UT (152 seconds after the BAT trigger).  No source has been
detected within the BAT error circle down to a limiting flux
of 3.6e-13 erg cm^-2 s^-1.  This is unusually low for a GRB afterglow
at this point after a trigger, and suggests that this event may be
either an unusual GRB or due to some other source, either transient or noise.

UVOT began observing the field around 133 sec after the BAT trigger.
No afterglow candidate is found within the BAT error circle.
The 3-sigma upper limit of the UVOT filters are given below.

Filter    T_start   T_stop   Exposure(s)   Mag (3-sig Upper limit)
V         133       6777     805           20.41
B         735       6163     206           20.40
U         711       5958     216           20.04
W1        687       5753     235           19.96
M2        663       6839     292           19.95
W2        763       6573     216           20.29
White     152       6367     501           21.10

Klotz, A. (CESR-OMP), Boer M. (OHP), Atteia J.L. (LATT-OMP) report:

We imaged the field of GRB 061027 detected by SWIFT
(trigger 235645) with the TAROT robotic telescope (D=25cm)
located at the European Southern Observatory,
La Silla observatory, Chile.

This trigger was long, had no high-energy emission,
it was detected by "image trigger" (Stamatikos et al. GCNC 5760)
and UVOT limiting mag. is V=20.4 until two hours
after the burst. This is a good high-redshift candidate, so
we shot a series of filtered R and I exposures centered
on the BAT refined position (Parsons et al. GCNC 5761).

The observations started 14.0h after the GRB trigger.
The elevation of the field decreased from from 33 degrees
above horizon and weather conditions were excellent.
We obtained a second set of exposures on 2006 nov. 7 for
comparison and to extract fading sources.

The date of trigger : t0 = 2006-10-27T10:15:02.304

We co-added series of filtered exposures, and we do
not detect any OT:

t0+14.0h to t0+18.6h : R > 20.0 and I > 18.9

Magnitudes were estimated with the nearby USNO-B1 stars
and are not corrected for galactic dust extinction.

N.B. Galactic coordinates are lon=311.3409 lat=-25.2566
and the galactic extinction in R band is 0.1 magnitudes
estimated from D. Schlegel et al. 1998ApJ...500..525S.

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