Gamma-ray Burst 170921

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #21902
E. Bissaldi (Politecnico & INFN Bari) report on behalf of the Fermi GBM Team:
"At 04:02:11.51 UT on 21 September 2017, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 170921B (trigger 527659336 / 170921168).
The on-ground calculated location, using the GBM trigger data, is
RA = 134.38, DEC = +46.46
(J2000 degrees, equivalent to 08 h 57m, 46d 27'), with an uncertainty of 1.6 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 101 degrees.
The GBM light curve consists of a bright FRED-like peak with a duration (T90) of about 34 s (50-300 keV). The time-averaged spectrum from T0 to T0+30 s is adequately fit by a Band function with Epeak = 100 +/- 10 keV, alpha = -1.2 +/- 0.1, and beta = -2.3 +/- 0.1.
The event fluence (10-1000 keV) in this time interval is (5.4 +/- 0.1)E-5 erg/cm^2. The 1-sec peak photon flux measured starting from T0+8.1 s in the 10-1000 keV band is 38.7 +/- 0.6 ph/s/cm^2.
The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."
GCN Circular #21918
K. Hurley, on behalf of the IPN,
I. G. Mitrofanov, D. Golovin, M. L. Litvak, and A. B. Sanin, on behalf of the HEND-Odyssey GRB team,
A. Kozlova, S. Golenetskii, R. Aptekar, D. Frederiks, D. Svinkin, and T. Cline on behalf of the Konus-Wind team,
V. Connaughton, M. S. Briggs, C. Meegan, V. Pelassa, and A. Goldstein, on behalf of the Fermi GBM team,
A. von Kienlin, X. Zhang, A. Rau, V. Savchenko, E. Bozzo, and C. Ferrigno, on behalf of the INTEGRAL SPI-ACS GRB team,
S. Barthelmy, J. Cummings, H. Krimm, and D. Palmer, on behalf of the Swift-BAT 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 GRB 170921B (Bissaldi, GCN 21902) was detected by Fermi (GBM trigger 527659336), Konus-Wind, INTEGRAL (SPI-ACS), Mars-Odyssey (HEND), and Swift (BAT) at about 14531 s UT (04:02:11). 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:
   132.375 (08h 49m 30s) +57.057 (+57d 03' 24")
  Corners:
   112.726 (07h 30m 54s) +39.192 (+39d 11' 30")
   113.512 (07h 34m 03s) +41.297 (+41d 17' 50")
   162.356 (10h 49m 25s) +62.282 (+62d 16' 56")
   160.088 (10h 40m 21s) +62.030 (+62d 01' 49")
  ---------------------------------------------
```
The error box area is 8.7 sq. deg, and its maximum dimension is 37.5 deg (the minimum one is 14.4 arcmin). The Sun distance was 63 deg.
This box may be improved.
A triangulation map is posted at http://www.ioffe.ru/LEA/GRBs/GRB170921_T14528/IPN
The Konus-Wind time history and spectrum will be given in a forthcoming GCN Circular.
GCN Circular #21922
V. Sharma (IUCAA), D. Bhattacharya (IUCAA), V. Bhalerao (IITB), 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 short GRB 170921B (Fermi GBM detection: Bissaldi E. et al, GCN Circ. 21902) in the 40-200 keV energy range. The light curve shows a bright FRED-like emission with peak at 04:02:11.51 UT, coinciding with Fermi/GBM trigger. The measured peak count rate is 473.5 counts/s above the background in combined data of four quadrants, with a total 4475 counts. The local mean background count rate was 332 counts/s. Using cumulative rates, we measure a T90 of 30.5 s.
It was clearly detected in the CsI anticoincidence detector (Veto) also as bright detection in the 100-500 keV energy range.
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.
GCN Circular #21926
A. Kozlova, S. Golenetskii, R. Aptekar, D. Frederiks, P. Oleynik, M. Ulanov, D. Svinkin, A. Tsvetkova, A. Lysenko, and T. Cline on behalf of the Konus-Wind team, report:
The long-duration GRB 170921B (Fermi GBM detection: Bissaldi, GCN Circ. 21902; IPN Triangulation: Hurley et al., GCN Circ. 21918) triggered Konus-Wind at T0=14528.886 s UT (04:02:08.886).
The burst light curve shows a single pulse which starts at ~T0-0.2 s and has a total duration of ~25 s. This pulse passes into a weaker decaying tail, which can be traced out to ~T0+164 s. The emission is seen up to ~4 MeV.
As observed by Konus-Wind, the burst had a fluence of 5.44(-0.22,+0.22)x10^-5 erg/cm2, and a 64-ms peak flux, measured from T0+0.704 s, of 6.75(-1.20,+1.21)x10^-6 erg/cm2/s (both in the 20 keV - 10 MeV energy range).
The time-averaged spectrum of the burst (measured from T0 to T0+22.528 s) is best fit in the 20 keV - 4 MeV range by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -1.42(-0.05,+0.05) and Ep = 138(-6,+7) keV (chi2 = 76/72 dof). Fitting by a GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -3.3 (chi2 = 76/71 dof)
The spectrum near the maximum count rate (measured from T0+0.256 to T0+2.048 s) is best fit in the 20 keV - 4 MeV range by a power law with exponential cutoff model with alpha = -0.33(-0.13,+0.14) and Ep = 318(-22,+25) keV (chi2 = 52/67 dof). Fitting by a GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -3.7 (chi2 = 52/66 dof)
The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB170921_T14528/
All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.

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Jochen Greiner, last update: 25-Sep-2017 [Disclaimer]