[vsnet-grb-info 7594] GRB 090418: Swift detection of a burst with optical afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9149
SUBJECT: GRB 090418: Swift detection of a burst with optical afterglow
DATE:    09/04/18 11:35:24 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

V. Mangano (INAF-IASFPA), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
P.A. Curran (MSSL-UCL), N. Gehrels (NASA/GSFC),
S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU),
J. Mao (INAF-OAB), P. T. O'Brien (U Leicester), D. M. Palmer (LANL),
A. M. Parsons (GSFC), M. Perri (ASDC), B. A. Rowlinson (U Leicester),
B. Sbarufatti (INAF-IASFPA), G. Tagliaferri (INAF-OAB),
T. N. Ukwatta (GSFC/GWU), L. Vetere (PSU) and H. Ziaeepour (UCL-MSSL)
report on behalf of the Swift Team:

At 11:07:40 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090418 (trigger=349510).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 269.334, +33.409 which is 
   RA(J2000)  =  17h 57m 20s
   Dec(J2000) = +33d 24' 31"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows two main peaks
with a duration of about 70 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~45 sec after the trigger. 

The XRT began observing the field at 11:09:16.3 UT, 96.1 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
269.3129, 33.4055 which is equivalent to:
   RA(J2000)  = 17h 57m 15.09s
   Dec(J2000) = +33d 24' 19.9"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 64 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
3.55e+20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 1.05e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the u filter starting
859 seconds after the BAT trigger. An afterglow candidate has been
found in the initial data products at
    RA(J2000) =  17:57:15.1 = 269.31458
   Dec(J2000) = +33:24:21   = +33.40583
with an estimated uncertainty of 1.0 arcsec (radius, 90% confidence, statistical
+ systematic).  This is 1.1 arcsec from the XRT position.  The estimated 
magnitude is u = 17.3 mag. No correction has been made for the expected 
extinction corresponding to E(B-V) of 0.04. 

Burst Advocate for this burst is V. Mangano (vanessa AT ifc.inaf.it). 
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.)