GRB 090418B

I. Bikmaev, R.Zhuchkov, N. Sakhibullin (KSU/AST),
I. Khamitov, Z. Eker (TUG), U. Kiziloglu (METU), E. Gogus (Sabanci Uni.),
R. Burenin, M. Pavlinsky, R. Sunyaev (IKI) 

report:

The field of GRB 090418B (Copete et al., GCN 9159
Loading...
 
 ) was observed with
Russian-Turkish 1.5-m telescope (RTT150, Bakirlitepe, TUBITAK National
Observatory, Turkey). We obtained series of 3x300s exposures in Rc filter,
centered on April 19, 01:04 UT, i.e. 16 hours after the burst.  In addition,
a second epoch of 4x300 sec exposures was obtained in Rc filter on April 20,
0:17 UT.

Similarly to Guidorzi et al. (GCN 9176
Loading...
 
 ), Afonso et al. (GCN 9178
Loading...
 
 ), Nissinen
et al. (GCN 9181
Loading...
 
 ) we do not detect any obvious fading optical source inside
the BAT error circle. The upper limit for the first epoch of our
observations is Rc > 22.7 mag.

K. Kono, E. Sonoda, M. Yamauchi, N. Ohmori, H. Hayashi,
K. Noda, A. Daikyuji, Y. Nishioka (Univ. of Miyazaki),
M. Ohno, M. Suzuki, M. Kokubun, T. Takahashi (ISAS/JAXA),
W. Iwakiri, M. Tashiro, Y. Terada, A. Endo, K. Onda,
T. Sugasahara (Saitama U.), Y. Urata (NCU),
T. Enoto, K. Nakazawa, K. Makishima (Univ. of Tokyo),
K. Yamaoka, S. Sugita (Aoyama Gakuin U.), Y. E. Nakagawa, T. Tamagawa
(RIKEN), 
T. Uehara, T. Takahashi, Y. Fukazawa, Y. Hanabata (Hiroshima U.),
S. Hong (Nihon U.), N. Vasquez (Tokyo Tech.),
on behalf of the Suzaku WAM team, report:

The long GRB 090418B 
was detected by the Suzaku Wide-band  All-sky Monitor (WAM)
which covers an energy range of 50 keV - 5 MeV at 08:54:21.299 UT (=T0).
The observed light curve shows a multi-peaked structure
starting at T0-3 s, ending at T0+35 s,
with a duration (T90) of about 29 seconds.

The fluence in 100 - 1000 keV was 1.22 (-0.14, +0.06) x10-5 erg/cm2.
The 1-s peak flux measured from T0+32 s was 2.13 (-0.58, +0.35) photons/cm2/s 
in the same energy range.

Preliminary result shows that the time-averaged spectrum from T0+14 s to
T0+52 s is well fitted by a single power-law with a photon index
of 2.09 (-0.20, +0.22) (chi2/d.o.f = 13/17).

All the quoted errors are at statistical 90% confidence level,
in which the systematic uncertainties are not included.

The light curves for this burst will be available at:
http://www.astro.isas.jaxa.jp/suzaku/HXD-WAM/WAM-GRB/grb/untrig/grb_table.html

Markku Nissinen (Taurus Hill Observatory) and Veli-Pekka Hentunen (Taurus

Hill Observatory) report:



Global-rent-a-scope GRAS002 Tak Mewlon 0.30m telescope with SBIG ST-8E

CCD at Mayhill (New Mexico, USA) was used for follow-up observations of 

GRB090418B. The observations were started on April 19, at 03:55 UTC (19 hours

after the burst) and stopped at 05:35 UTC. Four unfiltered 600 seconds images

were taken.  We do not detect any new object with respect to POSSII F/USNO-B1.0

inside the BAT error circle. Upper limit for the observations is > 20.5 mag (3UL). Quoted

upper limit has been derived using POSSII F and USNO-B1.0 field stars as reference.

 

Quoted upper limit has been derived using POSSII F and USNO-B1.0 field stars 
as reference.

P. Afonso (MPE), S. Klose (Tautenburg), T. Kruehler and J. Greiner (both
MPE Garching), report on behalf of the GROND team:

GROND (Greiner et al. 2008, PASP 120, 405), mounted at the 2.2m ESO/MPI
telescope at La Silla Observatory (Chile) started follow-up observations of
GRB 090418B (Copete et al., GCN #9150
Loading...
 
 ) on April 19, at 03:00 UTC (18.0 h
after the burst). In addition, a second epoch of imaging was obtained,
which started on April 20, 04:05 UTC.

Similar to Guidorzi et al. (2009, GCN #9176
Loading...
 
 ) we do not detect any new object 
with respect to SDSS/2MASS inside the BAT error circle. Image subtraction 
between the two epochs does not reveal any obvious variable source. Upper 
limits from the first observation are as follows (all in the AB system):

g' > 23.8 mag
r' > 24.0 mag
i' > 23.2 mag
z' > 23.0 mag
J > 22.0 mag
H > 21.4 mag
K > 20.8 mag

Quoted upper limits have been derived using SDSS and 2MASS field stars as
reference.

C. Guidorzi (U. Ferrara), R.J. Smith, I.A. Steele, C.G. Mundell,
D. Bersier (Liverpool JMU), A. Gomboc (U. Ljubljana), P. O'Brien,
N. Tanvir (U. Leicester) on behalf of a large collaboration report:

On 2009 April 19 at 12:53:50 UT we observed the Swift-BAT Slew Survey
GRB 090418B (Copete et al., GCN Circ. 9159
Loading...
 
 ) with the 2-m Faulkes 
Telescope North using the i' and R filters.

 From a set of 6x300 s frames in each filter, we could not find any
uncatalogued object inside the BAT error circle with respect to the SDSS
field stars, down to the following limiting magnitudes:

Mid Time   Exposure  Filter   Limit
(days)      (s)               (mag)
-------------------------------------
1.17        6x300      i'      > 21
1.20        6x300      R       > 21.5
-------------------------------------

S. Golenetskii, R.Aptekar, E. Mazets, V. Pal'shin, D. Frederiks, P.
Oleynik, M. Ulanov, D. Svinkin, and T. Cline on behalf of the Konus-Wind
team report:

The long Swift-BATSS GRB 090418B (Copete et al., GCN 9159
Loading...
 
 )
triggered Konus-Wind at T0=32366.325 s UT (08:59:26.325).

The burst light curve shows a soft weak precursor at ~T0-20 s, the main 
multipeaked pulse at T0 with a duration of ~45 s, and a weak soft pulse 
at ~T0+70 with a duration of ~30 s (the same lc structure was seen by 
BAT: Copete et al., GCN 9159
Loading...
 
 ).

As observed by Konus-Wind the burst
had a fluence of (2.31 +/- 0.09)x10^-5 erg/cm2,
and a 256-ms peak flux measured from T0+15.840 s
of (1.54 +/- 0.20)x10^-6 erg/cm2/s
(both in the 20 keV - 1 MeV energy range).

The time-integrated spectrum of the burst
(from T0 to T0+40.704 s) can be fitted (in the 20 keV-1 MeV range)
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -1.23(-0.09, +0.010),
and Ep = 117(-7, +8) keV (chi2 = 62.0/56 dof).
Fitting by GRB (Band) model yields the same alpha and Ep,
and only an upper limit on the high energy
photon index: beta < -3.18 (chi2 = 62.0/55 dof).

All the quoted errors are at the 90% confidence level.

The Konus-Wind light curve of this GRB is available
at http://www.ioffe.ru/LEA/GRBs/GRB090418_T32366/

A. Copete, J. Grindlay (Harvard)
S. Barthelmy, C. Markwardt, N. Gehrels (NASA/GSFC)

We report the detection of a bright GRB by the BAT Slew Survey (BATSS)  
in the slew that started at 08:59:02 and lasted 151 sec. The ground- 
calculated position is RA, Dec = 225.910, +17.224 deg, which is
RA (J2000) = 15h 03m 38s
Dec(J2000) = +17d 13' 26"
with an uncertainty of 1.91 arcmin (90% confidence, including  
systematics). The detection was triggered by simultaneous independent  
detections of 86.0 sigma and 41.8 sigma from imaging in the 15-50 keV  
and 50-150 keV energy bands, respectively. The burst mask-tagged  
lightcurve lightcurve in the 15-150 keV band shows a strong and broad  
multi-peaked emission structure starting at T+25sec as the burst  
enters the BAT field-of-view, lasting 48 sec, and peaking 6 times at T 
+29, T+35, T+38, T+47, T+58 and T+62sec. This is followed by a weaker  
broad peak starting at T+87sec, lasting 28sec and peaking at T+100sec.  
T90 = 65.0sec and T50 = 17.0sec.

The time-averaged spectrum over the 120 sec exposure time of the  
source is best fit by a power law with an exponential cutoff, with  
photon index 1.42 and high-energy cut at 319 keV. The fluence in the  
15-150 keV band is 1.6 x 10^-5 erg/cm^2, and the peak 1-sec flux is  
6.5 x 10^-7 erg/cm^2/sec at T+38sec. A follow-up Swift ToO observation  
for this burst has been scheduled.