Gamma-ray Burst 180427A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN Circular #22678
  E. Bissaldi (Politecnico & INFN Bari) reports on behalf of the Fermi GBM Team:
  "At 10:37:03.04 UT on 27 April 2018, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 180427A (trigger 546518228 / 180427442). The on-ground calculated location, using the GBM trigger data, is
  RA = 283.33, Dec = +70.30
  (J2000 degrees), with an uncertainty of 1 degree (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 136 degrees.
  The GBM light curve shows a single FRED-like emission episode with a duration (T90) of about 26 s (50-300 keV). The time-averaged spectrum from T0 to T0+26 s is best fit by a Band function with Epeak = 108 +/- 3 keV, alpha = -0.28 +/- 0.04, and beta = -2.81 +/- 0.07.
  The event fluence (10-1000 keV) in this time interval is (4.92 +/- 0.06)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+5.3 s in the 10-1000 keV band is 29.2 +/- 0.8 ph/s/cm^2.
  The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."
• GCN Circular #22679
  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,
  D. Svinkin, S. Golenetskii, R. Aptekar, D. Frederiks, A. Kozlova, 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 180427A (Bissaldi, GCN Circ. 22678) was detected by Fermi (GBM; trigger 546518228), Konus-Wind, Mars-Odyssey (HEND), INTEGRAL (SPI-ACS), and Swift (BAT) at about 38223 s UT (10:37:03). 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:
     271.012 (18h 04m 03s) +68.454 (+68d 27' 14")
    Corners:
     277.258 (18h 29m 02s) +68.858 (+68d 51' 30")
     265.504 (17h 42m 01s) +67.232 (+67d 13' 56")
     265.042 (17h 40m 10s) +67.831 (+67d 49' 51")
     277.094 (18h 28m 23s) +69.480 (+69d 28' 49")
    ---------------------------------------------
  

  The error box area is 2.88 sq. deg, and its maximum dimension is 4.82 deg (the minimum one is 36.96 arcmin). The Sun distance was 86 deg.
  This box may be improved.
  A triangulation map is posted at http://www.ioffe.ru/LEA/GRBs/GRB180427_T38222/IPN
  The Konus-Wind time history and spectrum will be given a forthcoming GCN Circular.
• GCN Circular #22680
  A. Kozlova, S. Golenetskii, R. Aptekar, D. Frederiks, M. Ulanov, D. Svinkin, A. Tsvetkova, A. Lysenko, and T. Cline on behalf of the Konus-Wind team, report:
  The long-duration GRB 180427A (Fermi GBM detection: Bissaldi, GCN Circ. 22678, IPN triangulation: Hurley et al., GCN Circ. 22679) triggered Konus-Wind at T0=38222.928 s UT (10:37:02.928).
  The burst light curve shows a single pulse which starts at ~T0-0.4 s and has a total duration of ~34.5 s. The emission is seen up to ~2 MeV.
  As observed by Konus-Wind, the burst had a fluence of 5.09(-0.19,+0.21)x10^-5 erg/cm2, and a 64-ms peak flux, measured from T0+5.024 s, of 4.93(-0.81,+0.81)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+33.536 s) is best fit in the 20 keV - 2 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.59(-0.07,+0.08), the high energy photon index beta = -3.10(-0.24,+0.17), the peak energy Ep = 116(-4,+4) keV (chi2 = 78/61 dof).
  The spectrum near the maximum count rate (measured from T0+3.840 to T0+7.424 s) is best fit in the 20 keV - 2 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = 0.49(-0.13,+0.14), the high energy photon index beta = -3.78(-0.51,+0.30), the peak energy Ep = 135(-5,+5) keV (chi2 = 78/60 dof).
  The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB180427_T38222/
  There are no data available for the triggered mode 300-1200 keV band.
  All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.
• GCN Circular #22694
  V. Sharma and D. Bhattacharya (IUCAA), V. Bhalerao (IIT-B), A. R. Rao (TIFR) and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:
  Analysis of AstroSat CZTI data showed the detection of a bright GRB 180427A, which was also detected by Fermi-GBM (Bissaldi E. et al., GCN 22678), IPN Triangulation (Hurley K. et al., GCN 22679) and Konus-Wind (Kozlova A. et al., GCN 22680).
  The source was clearly detected in the 40-200 keV energy range. The light curve shows a double-peaked structure in overall FRED-like emission, with strongest emission at 10:37:04.5 UT. The measured peak count rate is 1717 cts/s above the background in combined data of four quadrants, with a total of 18897 cts. The local mean background count rate was 494.8 cts/s. Using cumulative rates, we measure a T90 of 22.4 s. In Preliminary analysis, we find that 981.3 compton events are associated with this event.
  It was also clearly detected in the CsI anticoincidence (Veto) detector 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.