GRB 070227

C. Pagani (PSU), M. M. Chester (PSU), J. R. Cummings (NASA/UMBC),
D. M. Palmer (LANL), T. Sakamoto (NASA/ORAU) and L. Vetere (PSU)
report on behalf of the Swift Team:

At 2007-02-27 22:21:59 Swift-BAT detected GRB 070227 (trigger 262347). 
The burst was not found automatically onboard, because a planned 
slew terminated the trigger prematurely. The burst was found later 
in ground analysis. Swift did not slew to the burst, and a target 
of opportunity observation for this burst will be uploaded at 
the next command opportunity (around 1400 UT). 

The BAT position is RA, Dec 120.566, -46.305 (J2000)
  RA:  +08h 02m 16s
  Dec: -46d 18' 17"
with an estimated error circle of 3 arcmin. The burst was 
in the extreme partially coded field of view, the coding 
was 3.4%. 

The burst as seen by BAT was a single peak at least 8 seconds 
long. We have detailed information for only T-2 to T+8 sec. 
Since the burst was weak and on the edge of the FOV, and
because of the subsequent slew, we can not determine if 
additional emission occurred.

C. Pagani, D. Grupe, D.N. Burrows (PSU) report on behalf of the Swift/XRT 
Team:

The Swift BAT detected GRB 070227 at 22:21:59 UT (GCN 6156
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 ).  The Swift 
observatory did not slew promptly because a planned slew terminated the 
trigger prematurely. The burst was found later in ground analysis.
The XRT began taking data at 2007-02-28 13:51:31 UT, approximately 15 hours 
and 30 minutes after the trigger.  
In a preliminary data analysis of the first 2.6ks of PC data we detect an 
uncataloged X-ray source located at RA, Dec 120.5808 -46.3135 which is

RA(J2000)  =   08h 02m 19.4s
Dec(J2000) =  -46d 18' 48.7"

We estimate an uncertainty of 8 arcseconds radius (90% containment). 
This position is 48 arcseconds from the BAT position (GCN 6156
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 ).
The source count rate is approximately 0.005 counts/s.
Observations are still underway and further ground analysis will be 
performed to determine if this X-ray source is fading.

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

L. Barbier (GSFC), S. D. Barthelmy (GSFC), J. Cummings (GSFC/UMBC),
E. Fenimore (LANL), N. Gehrels (GSFC), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), C. Pagani (PSU), D. Palmer (LANL),
A. Parsons (GSFC), T. Sakamoto (GSFC/ORAU), G. Sato (GSFC/ISAS),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC)
on behalf of the Swift-BAT team:
 
Using the data set from T-2.0 to T+8.1 sec, we report further analysis 
of BAT GRB 070227 (trigger #262347)  (Pagani, et al., GCN Circ. 6156
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 ).  
The BAT ground-calculated position is RA, Dec = 120.566, -46.305 (J2000)
  RA:  +08h 02m 16s
  Dec: -46d 18' 17"
with an uncertainty of 2.5 arcmin, (radius, sys+stat, 90% containment).  
The partial coding was 3%.  Because this was a failed trigger on-board, 
this is all the detailed BAT data we are going to get on this burst.
 
The burst had two weak overlapping peaks with spectral evolution. The
larger peak was at T+1 and the smaller at T+6 sec.  There is no 
additional strong emission at times later than T+8, but we cannot rule 
out weak emission.  T90 (15-350 keV) is 7 +- 2 sec (estimated error 
including systematics).
 
The time-averaged spectrum from T+0.0 to T+8.0 is best fit by a simple 
power-law model.  The power law index of the time-averaged spectrum is 
1.54 +- 0.27.  The fluence in the 15-150 keV band is 
1.6 +- 0.2 x 10^-06 erg/cm2.  The 1-sec peak photon flux measured from 
T+0.00 sec in the 15-150 keV band is 2.7 +- 0.4 ph/cm2/sec.  The quoted 
errors are at the 90% confidence level considering the statistical and
usual systematic effects.  However, because of the extreme partial coding
of this burst, and particularly the reduced sensitivity to low energy
photons at the edge of the FOV, we expect an extra systematic uncertainty
contribution of about 10%.

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

We imaged the field of GRB 070227 detected by SWIFT
(trigger 262347) with the TAROT robotic telescope (D=25cm)
located at the European Southern Observatory,
La Silla observatory, Chile.

The observations started 7.76 hours after the GRB trigger
(5.2s after the notice). The elevation of the field decreased
from 41 degrees above horizon, weather conditions
were excellent and the moon elevation was less than 10 degrees.

The date of trigger : t0 = 2007-02-27T22:21:58.752

We co-added a series of 26 long exposures
taken between t0+7.76h and t0+8.98h. The co-added
image has a limiting magnitude R~20.2.
The field is very crowded.

We do not detect any OT in the BAT refined
error box (Barbier et al. GCNC 6158)
nor in the XRT one (Pagani et al. GCNC 6157).
Concerning the XRT error box, we detected clearly
all stars present in the USNO-B1 catalog or in
the DSS-R image.

Magnitudes are not corrected for galactic dust extinction.

N.B. Galactic coordinates are lon=261.2574 lat= -8.2038
and the galactic extinction in R band is 0.7 magnitudes
estimated from D. Schlegel et al. 1998ApJ...500..525S.

This message may be cited.

S. B. Pandey (UCL-MSSL) and C. Pagani (PSU) report on behalf of
the Swift/UVOT Team:

The Swift/UVOT started observing the field of GRB 070227 (trigger 262347)
56680 s after the BAT trigger (Pagani et al., GCN Circ. 6156
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 ). UVOT
observations started late because the BAT observation did not result
in an automated response, but was found during the ground analysis.

The 3-sigma upper limits for detecting a source inside the XRT error 
circle (Pagani, Grupe & Burrows, GCN Circ. 6157
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 ) in the co-added
exposures are:

Filter     T_start   T_stop   Exp(s)  Mag (3-sigma upper limit)
---------------------------------------------------------------
     V       56680     57586    885    20.90
     B       63392     64113    702    21.58
     U       62480     63386    885    21.31
   UVW1      61573     62473    886    21.17
   UVM2      57592     58329    726    21.34
   UVW2      55773     56673    886    21.04
---------------------------------------------------------------

Caution should be taken due to the presence of a nearby bright star. The 
XRT error circle lies lies within the wings of the PSF of the bright star 
in all the UVOT frames. This complicates the photometry, so these limits 
could possibly be underestimated.

The values quoted above are not corrected for the expected Galactic
extinction E_{B-V} = 0.34 mag towards the direction of the burst
(Schlegel et al. 1998).

A. C. Gilmore and P. M. Kilmartin (Mt John Observatory of the University of 
Canterbury) report:

We stacked 20 x 60 second clear exposures with an Apogee U-47 CCD on a 0.6-m 
f/13.5 reflector at Mt John Observatory, centred on February 28 09:54 UT (11.5 
hours after the burst.) Our images do not show any new object in or near the 
Swift-BAT error circle when compared with DSS-2 red and DSS-2 infra-red images.  


This message may be cited.

[GCN OPS NOTE (28feb07): This message was delayed ~20 hrs due to unknown
internet delivery problems.]

A. C. Gilmore and P. M. Kilmartin (Mt John Observatory of the University of 
Canterbury) report:

We stacked 20 x 60 second clear exposures with an Apogee U-47 CCD on a 0.6-m 
f/13.5 reflector at Mt John Observatory, centred on February 28 09:54 UT (11.5 
hours after the burst.) Our images do not show any new object in or near the 
Swift-BAT error circle when compared with DSS-2 red and DSS-2 infra-red images.  

This message may be cited.

C. Pagani, J. L. Racusin, L. Vetere, D. Grupe, D. N. Burrows (PSU), report on 
behalf of the Swift XRT team:

We have analysed the Swift-XRT Photon Counting data of GRB 070227 (GCN Circ. 
6156
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 ) from T+56ks to T+386ks (where T is the BAT trigger time). The candidate 
X-ray afterglow reported in GCN Circ. 6157
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  has faded from the initial count 
rate of ~ 0.01 counts/s to a count rate of 0.002 counts/s. 
Using 39ks of PC data the refined XRT position is RA,Dec= 120.5805, -46.3135 
which is:

RA(J2000): 08h 02m 19.33s
Dec(J2000): -46d 18' 48.7"

with an uncertainty of 4 arcseconds (90% containment).  This position is 47 
arcseconds from the refined BAT position given in GCN Circ. 6158
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  and 0
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 .7 
arcseconds from the XRT position given in GCN Circ. 6157
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 . 

The light-curve shows an initial flat phase during the first 20ks of XRT 
obervations ending at about 80ks after the burst, followed by a decay that can 
be fitted with a power law with index 0.9+/-0.2.

The spectral fit to the PC data is not very well constrained due to the low 
number of afterglow events.  The fit with an absorbed power law, applying 
C-statistics, gives a photon index of 2.9 (+0.8/-0.5) in the [0.3-10] keV 
band, and a column density of (0.57 +0.26/-0.18)e22 cm^-2 (compared with the 
Galactic value in this direction of 0.31e22 cm^-2).  
The average estimated unabsorbed 0.3-10 keV flux from T+56ks to T+386ks of XRT 
Photon Counting observations was 3.6e-13 erg cm^-2 s^-1.

This Circular is an official product of the Swift XRT Team.