GRB 170911A

Y. Takao (RIKEN), H. Negoro (Nihon U.),
S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA),
T. Mihara, M. Sugizaki, S. Nakahira, W. Iwakiri, M. Shidatsu, M. Matsuoka (RIKEN), 
N. Kawai, S. Sugita, T. Yoshii, Y. Tachibana, S. Harita, Y. Muraki, K. Morita (Tokyo Tech), 
A. Yoshida, T. Sakamoto, M. Serino, Y. Kawakubo, Y. Kitaoka, T. Hashimoto (AGU), 
H. Tsunemi, T. Yoneyama (Osaka U.),
M. Nakajima, T. Kawase, A. Sakamaki (Nihon U.),
Y. Ueda, T. Hori, A. Tanimoto, S. Oda (Kyoto U.), 
Y. Tsuboi, Y. Nakamura, R. Sasaki, H. Kawai (Chuo U.), 
M. Yamauchi, C. Hanyu, K, Hidaka (Miyazaki U.), 
T. Kawamuro (NAOJ), and K. Yamaoka (Nagoya U.)


The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source 
at 06:24:21 UT on 2017 September 11.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (72.100 deg, 30.914 deg) = (04 48 23, +30 54 50) (J2000) 
with a statistical 90% C.L. elliptical error region 
with long and short radii of 0.38 deg and 0.32 deg, respectively. 
The roll angle of long axis from the north direction is 8.0 deg counterclockwise. 
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 119 +- 24 mCrab
(4.0-10.0keV, 1 sigma error).
Without the constant flux assumption, we also obtained a rectangular error box 
for the transient source with the following corners:
(R.A., Dec) = (70.630, 31.515) deg = (04 42 31, +31 30 55) (J2000)
(R.A., Dec) = (70.464, 30.944) deg = (04 41 51, +30 56 38) (J2000)
(R.A., Dec) = (73.457, 30.459) deg = (04 53 49, +30 27 31) (J2000)
(R.A., Dec) = (73.640, 31.026) deg = (04 54 33, +31 01 34) (J2000)
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).

There was no significant excess flux in the previous transit at 04:51
and in the next transit at 07:56 with an upper limit of 20 mCrab for each.

Fermi is likely to detected this burst (Trigger number: 526803867).

A. von Kienlin (MPE), R. Hamburg (UAH), and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 06:24:22.79 UT on 11 September 2017, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 170911A (trigger 526803867 / 170911267)
which was also detected by the MAXI GSC
(Takao et al. 2017, GCN 21858
Loading...
 
 )
The GBM on-ground location is consistent with the MAXI position.

The angle from the Fermi LAT boresight at the GBM trigger time is 43 degrees.

The GBM light curve shows a single peak
with a duration (T90) of about 20 s (50-300 keV).
The time-averaged spectrum from T0-3.072 s to T0+4.096 s is
best fit by a power law function with an exponential
high-energy cutoff.  The power law index is -1.2 +/- 0.2 and
the cutoff energy, parameterized as Epeak, is 260 +/- 120 keV


The event fluence (10-1000 keV) in this time interval is
(9.5 +/- 1.7)E-07 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-1.79 s in the 10-1000 keV band
is 1.96 +/- 0.17 ph/s/cm^2.

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