GRB 170830A

T. Yoneyama (Osaka U.), S. Nakahira  (RIKEN), S. Sugita (Tokyo Tech),
T. Mihara, M. Sugizaki, M. Serino,
W. Iwakiri, M. Shidatsu, M. Matsuoka (RIKEN), 
H. Negoro, M. Nakajima, T. Kawase, A. Sakamaki (Nihon U.),
S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA),
N. Kawai, T. Yoshii, Y. Tachibana, S. Harita, Y. Muraki, K. Morita (Tokyo Tech), 
A. Yoshida, T. Sakamoto, Y. Kawakubo, Y. Kitaoka, T. Hashimoto (AGU), 
H. Tsunemi, (Osaka 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), 
K. Yamaoka (Nagoya U.) 


GRB 170830A: MAXI/GSC detection

 The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at UT 03:15:29 UT on 2017 August 30. 
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (267.244 deg, -2.000 deg) = (17 48 58, -02 00 00) (J2000) 
with a statistical 90% C.L. elliptical error region 
with long and short radii of 0.09 deg and 0.05 deg, respectively. 
The roll angle of long axis from the north direction is 41.0 deg counterclockwise. 
We estimated a rectangular error box for the transient source with the following corners:
(266.590, -1.392) deg = (17 46 21, -01 23 31) (J2000) 
(266.518, -1.475) deg = (17 46 04, -01 28 30) (J2000) 
(267.633, -2.210) deg = (17 50 31, -02 12 36) (J2000) 
(267.637, -2.281) deg = (17 50 32, -02 16 50) (J2000).
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).

The X-ray flux averaged over the scan was 1330 +- 75 mCrab
(4.0-10.0keV, 1 sigma error).
There was no significant excess flux in the previous transit at UT 01:42
and in the next transit at UT 04:48 with an upper limit of 20 mCrab for each.

D. Denisenko (Sternberg Astronomical Institute, Moscow State
University) reports:

MAXI/GSC error box of GRB 170830A (T. Yoneyama et al., GCN 21761
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 ) was
observed remotely by iTelescope.Net T32 instrument (0.43-m f/6.8
reflector + FLI Proline 16803 CCD) in Siding Spring, Australia, at
08:55-08:58 UT on 2017 Aug. 30 (5.7 hours after the trigger time). The
telescope was centered at R.A. = 17 48 20, Decl. = -01 50 00 with a
43'x43' field of view covering the entire rectangular error box by
MAXI/GSC. Observations were performed at the zenith distance of 31
degree, only 18 deg from the waxing Moon (phase 0.60).

Visual inspection of the combined image (3x60-sec unfiltered
exposures) against 1991-06-15 DSS Red plate shows no optical transient
to the 3-sigma upper limit of 18.5m (calibrated against USNO-B1.0 Red
magnitudes). In particular, there is no new object in the vicinity of
the unidentified ROSAT X-ray source 1RXS J174835.3-014843 which falls
inside the gamma-ray burst error box.

V. Bhalerao (IITB), V. Sharma and D. Bhattacharya (IUCAA), A. R. Rao (TIFR) and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

Analysis of AstroSat CZTI data showed clear detection of GRB 170830A (MAXI/GSC detection: Yoneyama T. et al., GCN Circ. 21761
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 ) in the 40-200 keV energy range. The light curve shows multiple peaks of emission with the strongest peak at 03:14:00 UT, ~89 s before the MAXI/GSC trigger. The measured peak count rate is 66.3 counts/sec above the background in combined data of four quadrants, with a total of 91.4 counts. The local mean background count rate was 395.1 counts/sec. Using cumulative rates, we measure a T90 of 57.6 secs.

CZTI GRB detections are reported regularly on the payload site at http://astrosat.iucaa.in/czti/?q=grb. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC and PRL. The Indian Space Research Organisation funded, managed and facilitated the project.

M. Stanbro (UAH), E. Bissaldi (Politecnico & INFN Bari)
and C. Meegan (UAH) report on behalf of the Fermi GBM Team:


"At 03:14:01.95 UT on 30 August 2017, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 170830A (trigger 525755646 / 170830135).
This trigger is likely associated with the transient source
reported by MAXI/GSC (Yoneyama et al., GCN 21761
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 ), which was also detected
by AstroSat (Bhalerao et al., GCN 21773
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 ).

The on-ground calculated location, using the GBM trigger
data, is

RA = 275.9, DEC = -4.0,

with an uncertainty of 2.6 degrees (radius, 1-sigma containment,
statistical only; there is additionally a systematic
error which we have characterized as a core-plus-tail model, with 90% of
GRBs having a 3.7 deg error and a small tail suffering a larger than 10 deg
systematic error. [Connaughton et al. 2015, ApJS, 216, 32] ).

The angle from the Fermi LAT boresight at the GBM trigger time is 100 degrees.

The GBM light curve shows two peaks
with a duration (T90) of about 110 s (50-300 keV).
The time-averaged spectrum from T0 to T0+92 s is
adequately fit by a power law function with an exponential
high-energy cutoff.  The power law index is -1.54 +/- 0.07 and
the cutoff energy, parameterized as Epeak, is 180 +/- 40 keV.

The event fluence (10-1000 keV) in this time interval is
(4.20 +/- 0.11)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0 in the 10-1000 keV band
is 4.6 +/- 0.4 ph/s/cm^2.


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