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GCN Circular 12641

GRB 111209A: Swift Burst of Interest
2011-12-09T19:04:03Z (12 years ago)
Erik Hoversten at Swift/Penn State <>
E. A. Hoversten (PSU), D.N. Burrows (PSU), S. Campana (INAF-OAB), P.  
A. Evans (U Leicester), N. Gehrels (GSFC), D. Grupe (PSU), M. H.  
Siegel (PSU), report on behalf of the Swift team:

We report an update on Swift observations of the unusual event GRB  
111209A.  BAT triggered twice and is saw extended duration emission  
from this event.  It peaked at 900s and decreased in flux after 1100s,  
but continued well beyond that to >10ks.

We have analyzed 11.9 ks of XRT data for GRB 111209A (Hoversten  et  
al. GCN Circ. 12632), from 425 s to 25.16 ks after the  BAT trigger.  
The data contain 9.4 ks in Windowed Timing (WT) mode data and the  
remainder in photon counting (pc) mode.

The light curve is highly complicated showing multiple flares. The  
afterglow is very bright and XRT remained in WT mode through 4 orbits  
(20ks after the trigger). The light curve after the fourth orbit  
suggests a sharp drop in the count rate from about 14 counts s^-1 at  
the end of the forth orbit to 5 counts s^-1 at the beginning of
the fifth orbit. The data of the 5th orbit were entirely taken in  
photon counting (pc) mode and the decay slope steepens significantly.

The Swift/UVOT began settled observations of the field of GRB 111209A  
427 s after the BAT trigger.  The afterglow is clearly detected in all  
seven UVOT filters.  We find a refined UVOT position of

      RA (J2000)  00:57:22.63  =  14.34429 (deg)
     Dec (J2000) -46:48:03.8   =  -46.80106 (deg)

with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence,  
statistical + systematic).  This position is 0.25 arcseconds from the  
UVOT-enhanced XRT position, which is within the errors.

The white, v, and b filter show a fading source over the first orbit,  
however at the end of the first orbit the white magnitude increases  
abruptly by 0.6 magnitudes corresponding with a significant flare in  
the X-rays.  The v, b, u, uvw1, and uvm2 magnitudes show a  
rebrightening between 5.4 and 7 ks after the burst of 0.3 to 1.1  

The data from all three instrument is similar is several ways to GRB  
060218, the long event associated with SN2006aj, and to GRB 101225A,  
the Christmas burst.  The detection in all UVOT filters implies a low  
redshift of z < 1.6.  We note that at 18 ks after the burst the source  
is still at 18th magnitude in the white filter which is extremely  
unusual.  Given the rare behavior exhibited by GRB 111209A we advocate  
for additional observations at all wavelengths.
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