GRB 150811B

P. Jenke (UAH) reports on behalf of the Fermi GBM Team:

"At 20:22:13.75 UT on August 11 2015, the Fermi Gamma-Ray Burst 
Monitor triggered and located 
GRB 150811B (trigger 461017337/150811849). 
The GBM on-ground location, using the Fermi GBM trigger data, 
is RA = 188.0, Dec = -13.5 (J2000 degrees, equivalent to 
12h 32m 00s, -13d 30'), with an uncertainty of 8.7 
degree (radius, 1-sigma containment,
statistical only; there is additionally a systematic error which is
currently estimated to be 2 to 3 degrees).

The angle of the burst direction to the Fermi LAT boresight is 
108 degrees.

The GBM light curve consists of a single peak with a duration
(T90) of about 0.6 s (50-300 keV).
The time-averaged spectrum from T0-0.13 s to T0+0.64 s is
adequately fit by a Band function with Epeak = 1300 +/- 100 keV,
Alpha =-0.23 +/- 0.07 and Beta = -3.0 +/- 0.2.

The event fluence (10-1000 keV) in this time interval is
(1.6 +/- 0.1)E-07 erg/cm^2. The 64 ms peak photon flux measured
starting from T0-0.13 s in the 10-1000 keV band
is 8.6 +/- 0.3 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."

S. Golenetskii, R. Aptekar, V. Pal'shin, D. Frederiks,
D. Svinkin, and T. Cline on behalf of the Konus-Wind team,

K. Hurley, on behalf of the IPN,

V. Connaughton, M. S. Briggs, C. Meegan, V. Pelassa,
and A. Goldstein, on behalf of the Fermi GBM team, and

A. von Kienlin, X. Zhang, A. Rau, V. Savchenko, E. Bozzo,
and C. Ferrigno, on behalf of the INTEGRAL SPI-ACS GRB team,
report:

The short-duration GRB 150811B has been detected by Fermi (GBM: Jenke, 
GCN Circ. 18130
Loading...
 
 ), Konus-Wind, and INTEGRAL (SPI-ACS), so far, at about 
73334 s UT (20:22:14).

We have triangulated it to a preliminary, 3 sigma error box whose 
coordinates are:
---------------------------------------------
   RA(2000), deg                 Dec(2000), deg
  ---------------------------------------------
  Center:
   186.348 (12h 25m 23s) -14.105 (-14d 06' 20")
  Corners:
   184.572 (12h 18m 17s) -16.337 (-16d 20' 12")
   188.337 (12h 33m 21s) -12.037 (-12d 02' 14")
   187.888 (12h 31m 33s) -11.720 (-11d 43' 10")
   184.110 (12h 16m 26s) -16.010 (-16d 00' 36")
  ---------------------------------------------
The error box area is 3.1 sq. deg, and its maximum
dimension is 5.7 deg (the minimum one is 32.5 arcmin).
The Sun distance was 52 deg.

This box may be improved.

A triangulation map is posted at
http://www.ioffe.ru/LEA/GRBs/GRB150811_T73329/IPN/

The Konus-Wind time history and spectrum will be given in a forthcoming 
GCN Circular.

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

The short-duration, hard-spectrum, intense GRB 150811B
(Fermi GBM detection: Jenke, GCN 18130
Loading...
 
 ;
IPN triangulation: Golenetskii et al., GCN 18133
Loading...
 
 )
triggered Konus-Wind at T0=73329.515 s UT (20:22:09.515).

The burst light curve shows a single pulse with a duration of ~0.7 s.
The emission is seen up to ~10 MeV.

As observed by Konus-Wind, the burst
had a fluence of 9.08(-1.11,+1.22)x10^-6 erg/cm2,
and a 16-ms peak energy flux, measured from T0+0.232 s,
of 2.89(-0.92,+0.95)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the burst
(measured from T0 to T0+0.256 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -0.35(-0.24,+0.29),
and Ep = 1197(-220,+299) keV (chi2 = 28/27 dof).
Fitting by the GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index:
beta < -2.51 (chi2 = 28/26 dof).

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB150811_T73329/

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