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GCN Circular 9066

Subject
GRB 090401B: Swift detection of a burst
Date
2009-04-01T09:02:29Z (15 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
P. Schady (MSSL-UCL), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), O. Godet (U Leicester),
H. A. Krimm (CRESST/GSFC/USRA), J. Mao (INAF-OAB),
P. T. O'Brien (U Leicester), S. R. Oates (UCL-MSSL),
J. P. Osborne (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL), A. M. Parsons (GSFC), P. Romano (INAF-IASFPA),
B. A. Rowlinson (U Leicester) and H. Ziaeepour (UCL-MSSL) report on
behalf of the Swift Team:

At 08:35:25 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090401B (trigger=348152).  The BAT on-board calculated 
location is RA, Dec 95.101d, -8.963d which is 
   RA(J2000)  =  06h 20m 24s
   Dec(J2000) = -8d 57' 47"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows at least four
overlapping peaks with a total duration of ~20 seconds. 
The peak count rate was ~55,000 counts/sec (15-350 keV), at ~8 sec
after the trigger. 

The XRT began observing the field at 08:36:37.9 UT, 73.2 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 95.0889, -8.9734 which is equivalent to:
   RA(J2000)  = 06h 20m 21.33s
   Dec(J2000) = -08d 58' 24.2"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 57 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

UVOT took a finding chart exposure of 150 seconds with the White      
filter  starting 81 seconds after the BAT trigger. No credible      
afterglow candidate has  been found in the automatic analysis. The   
2.7'x2.7' sub-image covers 100% of  the XRT error circle. The typical 
3-sigma upper limit has been about 19.6 mag, although we note that 
there are two bright known sources within the XRT error circle that  
 may be confusing source detection. One of these sources appears to   
be brighter and slightly offset from the DSS position, suggesting the
possible detection of the optical afterglow. However, we shall need
more data to check for any variability. The coverage of the XRT error
circle by the  8'x8' region for the list of  sources generated
on-board is uncertain because the large number of sources  filled the
available telemetry. No correction has been made for the expected 
extinction   corresponding to E(B-V) of 0.67. 

Burst Advocate for this burst is P. Schady (ps AT mssl.ucl.ac.uk). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)
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