GRB 050418

L. Barbier (GSFC), S. Barthelmy (GSFC), J. Cummings (GSFC/NRC), 
E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (GSFC/UMD), 
K. Hurley (UC Berkeley), H. Krimm (GSFC/USRA), 
C. Markwardt (GSFC/UMD), F. Marshall (GSFC),  D. Palmer (LANL), 
A. Parsons (GSFC), T. Sakamoto (GSFC/NRC), G. Sato (ISAS), 
M. Suzuki (Saitama), J. Tueller (GSFC) 
on behalf of the Swift-BAT team: 

At 11:00:34.6 UT Swift-BAT triggered on burst GRB 050418 
(trigger=114893).  The BAT flight position is RA,Dec=44.341,-18.538 
(J2000).  We note this is 34 deg from the Sun, hence the s/c did not 
slew because of the Sun observing constraint.  The burst was 39 deg 
off the BAT boresight.  The lightcurve shows a main peak of duration 
15-20 seconds, followed by apparent peaks at up to 70-80 seconds after 
the trigger.  The peak count rate measured by BAT was 4500 counts/sec 
in the 15-350 keV band.

T. Mitani (ISAS), L. Barbier (GSFC), S. Barthelmy (GSFC), 
J. Cummings (GSFC/NRC), E. Fenimore (LANL), N. Gehrels (GSFC), 
D. Hullinger (UMD), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD),
D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/NRC), 
G. Sato (ISAS), M. Suzuki (Saitama), J. Tueller (GSFC), 
on behalf of the Swift/BAT team:

At 11:00:34.6 UT Swift-BAT detected GRB 050418 (trigger=114893)
(GCN Circ 3288
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 , Barbier et al.).  The refined BAT ground position
is (RA,Dec) = 44.346, -18.538, [deg; J2000] +- 3 arcmin, (95%
containment).  The partial coding was 48 %.  

The BAT mask-weighted light curve shows an initial triangular peak
(~12 s rise, ~8 s fall time) followed by a quartet of softer peaks
from 50 to 80 seconds after the maximum of the first peak, with
the last and brightest of the four being as intense as the
initial peak in 15-50 keV, but less than a quarter the height
in 50-100 keV.  T90 (15-350 keV) is (83 +- 5) seconds 
(estimated error including systematics).  

The photon index of the time-averaged spectrum is 1.7 +- 0.1.  The 
fluence in the 15-350 keV band are (8.6 +- 0.4) x 10^-6 erg/cm2.  
The 1-s peak photon flux measured from T0+0.75 second in the 15-350 
band is (5.0 +- 0.3) ph/cm2/s.  All the quoted errors are at the 90% 
confidence level.

B. Monard and A. Price report on behalf of the AAVSO International High
Energy Network on optical observations of the field of GRB050418 (Sakamoto
et al,; GCN #3288
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 ).

Observations by Monard do not reveal ay new source compared with the
POSS-II red plates down to an unfiltered limiting magnitude of 16 at an
observation midpoint time of 16:50 (UT). As previously reported in GCN
#3288
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 , the field was relatively close to the Sun therefore close to the
horizon.

Details of the observation are below.

Name: Berto Monard
email: bmonard@mweb.co.za
Site: Bronberg Observatory
Location: -25 =BA 54' 48", 28=BA 26' 44"E
Elevation: 1590m
Scope: LX200 12"
ScopeFocalRatio: f/3.7
CCDVendor: SBIG, ST-7XME
CCDPixelScale: about 1.8 arcsec /pixel
CCDFOV: 21 arcmin (E/W) x 14 arcmin (N/S)
Object: 050418
ObsDate: 050418
ObsMidPointTime: 16 50 UT
Exposure per frame: 28 sec
NumberOfFrames: 33
Filters: none
Processing: dark/flat
Seeing: good (about 3 arcsec)
LimitingMag: 16CR for stack of 13 images
Sky: partly cloudy / 1/2 moon / target field close to horizon
afterglowmag: NA
Report: the observed star field was centered on 02h 57m 22s -18d 32' 15"
(2000). The obtained deep image was compared visually to a DSS-2 red image
centered on the above coordinates.

No new object showed in the quoted error box down to a magnitude of 16CR.

Note: The actual observation period was very short due to the
circumstances and no further observations were possible.