Gamma-ray Burst 160530A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN Circular #19472
  P. A. Evans (U. Leicester) reports on behalf of the Swift team:
  Swift has initiated a series of observations, tiled on the sky, of the COSI GRB 160530A. Automated analysis of the XRT data will be presented online at http://www.swift.ac.uk/xrt_products/TILED_GRB00055
  Any uncatalogued X-ray sources detected in this analysis will be reported on this website and via GCN COUNTERPART notices. The probability of finding serendipitous sources, unrelated to the COSI event is high: any X-ray source considered to be a probable afterglow candidate will be reported via a GCN Circular after manual consideration.
  Details of the XRT automated analysis methods are detailed in Evans et al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).
  This circular is an official product of the Swift-XRT team.
• GCN Circular #19473
  John A. Tomsick (UC Berkeley/Space Sciences Laboratory) reports on behalf of the Compton Spectrometer and Imager (COSI) team:
  COSI is a large field of view (1/4-sky) gamma-ray (0.1-10 MeV) instrument on a Superpressure Balloon flight that began on May 16.
  A likely Gamma-ray Burst (GRB 160530A) was detected by COSI in both the cesium iodide shields and the germanium strip detectors (GeDs). The GRB started at 2016 May 30, 07:03:46 UT and lasted until 07:04:23 UT (based on the shield rates). During the burst, the light curve shows several large amplitude fluctuations.
  Based on the GeD rates and assuming a Band function with alpha=1.0, beta=2.5, and E_break = 150 keV, we calculated a preliminary burst fluence of 1.3E-5 erg/cm2. This value may be revised in the future using the spectral capabilities of the GeDs.
  The position of the GRB is R.A. = 120.2 deg, Decl. = -26.2 deg (J2000, 90% confidence error radius = 1.5 degrees), which corresponds to Galactic coordinates of l = 243.9 deg, b = 2.1 deg. Due to the proximity to the Galactic plane, a Galactic origin cannot be ruled out.
  We searched the SIMBAD database for possible counterparts, and three known gamma-ray sources are present in the COSI error region: GRB 080516, 2FGL J0758.0-2615c, and 3FGL J0755.2-2633 are, respectively, 0.4, 0.6, and 1.3 degrees from the center of the COSI error circle.
  Follow-up observations with Swift/XRT are underway (see P.A. Evans et al., GCN Circ. 19472) to search for an afterglow that may allow us to improve the position and provide a definitive classification of the COSI burst.
• GCN Circular #19476
  D. Svinkin, S. Golenetskii, R. Aptekar, D. Frederiks, A. Kozlova, and T. Cline on behalf of the Konus-Wind team,
  K. Hurley, on behalf of the IPN,
  A. von Kienlin, X. Zhang, A. Rau, V. Savchenko, E. Bozzo, and C. Ferrigno, on behalf of the INTEGRAL SPI-ACS GRB team, report:
  The long-duration, very bright GRB 160530A (COSI detection Tomsick GCN Circ. 19473) has been detected by Konus-Wind and INTEGRAL (SPI-ACS), so far, at about 25426 s UT (07:03:46).
  We have triangulated it to a Konus-SPI-ACS annulus centered at RA(2000)=50.332 deg (03h 21m 20s) Dec(2000)=+12.855 deg (+12d 51' 19"), whose radius is 77.066 +/- 0.305 deg (3 sigma).
  The COSI position reported by Tomsick (GCN Circ. 19473) is consistent with the annulus. The annulus combined with the COSI (90% confidence) error circle produces the following preliminary error box:

  ----------------------------------
  RA(2000), deg   Dec(2000), deg
  ----------------------------------
  Center:
  118.942        -25.764
  Corners:
  118.644        -25.659
  118.758        -26.966
  119.239        -25.868
  119.707        -24.768
  ----------------------------------
  

  The error box area is 3404.3 sq. arcmin, and its maximum dimension is 2.36 deg (the minimum one is 18 arcmin). The Sun distance was about 67 deg.
  This box may be improved.
  A triangulation map is posted at http://www.ioffe.ru/LEA/GRBs/GRB160530_T25426/IPN/
  The Konus-Wind time history and spectrum will be given in a forthcoming GCN Circular.
• GCN Circular #19477
  D. Svinkin, S. Golenetskii, R. Aptekar, D. Frederiks, P. Oleynik, M. Ulanov, A. Tsvetkova, A. Lysenko, A. Kozlova, and T. Cline on behalf of the Konus-Wind team, report:
  The long-duration, very intense GRB 160530A (COSI detection: Tomsick, GCN Circ. 19473; IPN triangulation: Svinkin et al., GCN circ. 19476) triggered Konus-Wind at T0=25426.549 s UT (07:03:46.549).
  The burst light curve shows a multipeaked structure started at ~T0-0.1 s with a total duration of ~37 s. The emission is seen up to ~5 MeV.
  The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB160530_T25426/
  As observed by Konus-Wind, the burst had a fluence of 1.30(-0.04,+0.04)x10^-4 erg/cm2, and a 64-ms peak flux, measured from T0+2.416 s, of 3.70(-0.38,+0.38)x10^-5 erg/cm2/s (both in the 20 keV - 10 MeV energy range).
  The time-averaged spectrum of the burst (measured from T0 to T0+38.912 s) is best fit in the 20 keV - 5 MeV range by a power law with exponential cutoff (CPL) model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -0.93(-0.03,+0.03) and Ep = 638(-33,+36) keV (chi2 = 73/73 dof). Fitting by a GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -3.5 (chi2 = 73/72 dof)
  The spectrum near the maximum count rate (measured from T0+1.792 to T0+2.560 s) is best fit in the 20 keV - 7 MeV range by the CPL model with alpha = -0.23(-0.09,+0.10) and Ep = 714(-50,+55) keV (chi2 = 53/62 dof). Fitting by a GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -2.9 (chi2 = 50/61 dof)
  All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.
• GCN Circular #19481
  V. D=E2=80=99Elia (ASDC), L. Izzo (IAA-CSIC), P. A. Evans (U. Leicester), A. A. Breeveld (UCL-MSSL), J.A. Kennea (PSU), D. Malesani (DARK/NBI), C. B. Markwardt (NASA/GSFC) report on behalf of the Swift team:
  Swift has performed follow-up observations of the COSI-detected burst GRB 160530A (Tomsick, GCN Circ. 19473) in a series of observations tiled on the sky. The total exposure time is 24 ks, distributed over 46 tiles; the maximum exposure at a single sky location was 1.4 ks. The data were collected between T0+19.0 ks and T0+158 ks, and are entirely in Photon Counting (PC) mode.
  One uncatalogued X-ray source has been detected by XRT in the intersection of the COSI and the IPN error regions (Svinkin et al., GCN Circ. 19476), however, this source is below the RASS limit at its location, and we cannot currently determine whether it is fading. Therefore, at the present time we cannot state if it is the afterglow. The source was imaged and detected in the time interval T0+145 ks to T0+158 ks and comprises 920 s of exposure time. More XRT data are expected in the forthcoming days. Further details of this source are given below:

  RA (J2000.0):  118.9741  =3D  07:55:53.77
  Dec (J2000.0): -25.4638  =3D  -25:27:49.7
  Error: 6.6 arcsec (radius, 90% conf.)
  Count-rate: 0.0222 +/- 0.0070 ct s^-1
  Flux: (6.3 +/- 2.0)e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)
  

  No optical afterglow consistent with the XRT source position is detected in the initial UVOT exposures. The preliminary 3-sigma upper limit for the summed U-band exposures (926 s) is 20.8 mag.
• GCN Circular #19501
  V. D=E2=80=99Elia (ASDC), L. Izzo (IAA-CSIC), A. A. Breeveld (UCL-MSSL), C. B. Markwardt (NASA/GSFC) report on behalf of the Swift team:
  The source reported in D=E2=80=99Elia et al. (GCN Circ. 19481) as an X-ray afterglow candidate for GRB 160530A (Tomsick, GCN Circ. 19473; Svinkin et al., GCN Circ. 19476) does not show any sign of fading during the Swift-XRT observations. Several X-ray sources have been detected during the XRT follow-up campaign, but again, none of them show evidence of fading. We thus conclude that the X-ray afterglow associated with GRB 160530A was not identified by our observations.
  This is an official product of the Swift-XRT team.