GRB 150118B

D. Kocevski (NASA/GSFC), J. Racusin (NASA/GSFC), S. Zhu (UMD) and M. Arimoto (Tokyo Tech) report on behalf of the Fermi-LAT team:

At 09:48:17 UT on Jan 18th, 2015, Fermi-LAT detected high-energy emission from GRB 150118B, which was also detected by Fermi-GBM (trigger 443267300/150118409).

The best LAT on-ground location is found to be

RA, Dec = 240.24, -35.75 (J2000)

with an error radius of 0.5 deg (90% containment, statistical error only). This was ~60 deg from the LAT boresight at the time of the trigger and triggered an autonomous repoint of the spacecraft, although the source was occulted by the Earth within 150s of the GBM trigger and didn't re-enter the LAT field of view until T0+2500s

The data from the Fermi-LAT show an increase in the event rate after the GBM trigger that is temporally correlated with the GBM emission.  The detection above 100 MeV is primarily driven by a single 2 GeV event which is observed within 5 degrees of the GBM position and ~50 seconds after the GBM trigger. Using the LAT Low Energy (LLE) data selection, which is sensitive to > 30 MeV, an increase in the event rate at > 8 sigma significance above background was observed approximately coincident with the time of the GBM emission.

The Fermi-LAT point of contact for this burst is

Makoto Arimoto (arimoto@hp.phys.titech.ac.jp<mailto:arimoto@hp.phys.titech.ac.jp>).

The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.

Matthew Stanbro (UAH) reports on behalf of the Fermi GBM Team:

"At 09:48:17.77 UT on 18 January 2015, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 150118B (trigger 443267300 / 150118409)
which was also detected by the Fermi-LAT (Kocevski et al. 2015, GCN 17307
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The GBM on-ground location is consistent with the LAT position.
The trigger caused GBM to generate an Autonomous Repoint of Fermi.

The GBM light curve consists of several episodes
with a duration (T90) of about 40 s (50-300 keV).
The time-averaged spectrum from T0-4.608 s to T0+49.665 s is
best fit by a power law function with an exponential
high-energy cutoff.  The power law index is -0.89 +/- 0.01 and
the cutoff energy, parameterized as Epeak, is 755 +/- 14 keV

The event fluence (10-1000 keV) in this time interval is
(1.5 +/- 0.007)E-04 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+45.8 s in the 8-1000 keV band
is 40.7 +/- 0.4 ph/s/cm^2.

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

K. Hurley and J. Goldsten, on behalf of the MESSENGER NS GRB team,

I. G. Mitrofanov, D. Golovin, M. L. Litvak, and A. B. Sanin,
on behalf of the HEND-Odyssey GRB team,

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

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

S. Barthelmy, J. Cummings, N. Gehrels, H. Krimm, and D. Palmer,
on behalf of the Swift-BAT team,

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

W. Boynton, C. Fellows, K. Harshman, H. Enos, and R. Starr,
on behalf of the GRS-Odyssey GRB team, report:

The long-duration, bright GRB 150118B (Kocevski et al., GCN Circ. 17307
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has been observed by Fermi (GBM trigger 443267300; Stanbro GCN Circ. 
17308
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 ), Konus-Wind, INTEGRAL (SPI-ACS), Mars-Odyssey (HEND), MESSENGER 
(GRNS), and Swift (BAT), so far, at about 35298 s UT (09:48:18). The 
burst was outside the coded field of view of the BAT.

We have triangulated it to a preliminary, 3 sigma error box whose
coordinates are:
   ---------------------------------------------
    RA(2000), deg                 Dec(2000), deg
   ---------------------------------------------
   Center:
    242.008 (16h 08m 02s) -36.020 (-36d 01' 12")
   Corners:
    242.375 (16h 09m 30s) -34.181 (-34d 10' 51")
    241.798 (16h 07m 12s) -36.507 (-36d 30' 24")
    241.669 (16h 06m 40s) -37.699 (-37d 41' 55")
    242.219 (16h 08m 53s) -35.520 (-35d 31' 13")
   ---------------------------------------------
The error box area is 1139 sq. arcmin, and its maximum
dimension is 3.56 deg (the minimum one is 8.3 arcmin).
The Sun distance was 52 deg.

The box is long because all annuli intersect at grazing incidence due to 
conjunction of the spacecraft involved in the triangulation.

The Fermi (LAT) position (Kocevski et al., GCN Circ. 17307
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 ) is outside 
of the box at 1.5 deg from its center.

This box may be improved.

A triangulation map is posted at
http://www.ioffe.ru/LEA/GRBs/GRB150118_T35302/IPN/

The Konus-Wind time history and spectrum will be given in 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 long-duration, hard-spectrum, intense GRB 150118B
(Fermi-LAT detection: Kocevski et al., GCN 17307
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 ;
Fermi-GBM detection: Stanbro, GCN 17308
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 ;
IPN triangulation: Hurley et al., GCN 17342
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 )
triggered Konus-Wind at T0=35302.394 s UT (09:48:22.394).

The light curve shows multiple emission episodes from ~T0-8 s to ~T0+60 s.
The emission is seen up to ~10 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB150118_T35302/

As observed by Konus-Wind, the burst had a fluence
of 1.87(-0.15,+0.14)x10^-4 erg/cm2, and a 64-ms peak flux,
measured from T0+39.680 s, of 1.88(-0.18,+0.20)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum (measured from T0 to T0+58.368 s)
is best fit in the 20 keV - 15 MeV range
by the cutoff power law with the following model parameters:
the photon index alpha = -0.98(-0.04,+0.04),
and the peak energy Ep = 695(-42,+47) keV,
chi2 = 103/97 dof.
Fitting this spectrum with the Band model yields the same
values of alpha and Ep with an upper limit on beta of -2.8

The spectrum near the peak count rate (measured from T0+35.328 to T0+41.984 s)
is best fit in the 20 keV - 15 MeV range
by the cutoff power law with the following model parameters:
the photon index alpha = -0.84(-0.08,+0.08),
and the peak energy Ep = 614(-55,+64) keV,
chi2 = 115/98 dof.

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