Gamma-ray Burst 150721A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN Circular #18041
  E. Bissaldi (INFN Bari), V. Connaughton (USRA) and A. von Kienlin (MPE) report on behalf of the Fermi GBM Team:
  At 05:49:08.93 UT on 21 July 2015, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 150721A (trigger 459150552/150721242).
  The on-ground calculated location, using the GBM trigger data, is
  RA= 334.1, DEC = +7.8 (J2000 degrees, equivalent to 22h 16m, 07d 48')
  with an uncertainty of 1.5 degrees (radius, 1-sigma containment, statistical only; there is additionally a systematic error which is currently estimated to be 2 to 3 degrees).
  The trigger resulted in an Autonomous Repoint Request (ARR) by the GBM Flight Software owing to the high peak flux of the GRB. This was accepted and the spacecraft slewed to the GBM in-flight location. The initial angle from the Fermi LAT boresight is 63 degrees.
  This burst was also independently detected by INTEGRAL SPI-ACS.
  The GBM light curve consists of a first peak at trigger time followed by a very soft peak around 100 s. The duration (T90) is about 20 s (50-300 keV). The time-averaged spectrum from T0 to T0+20.2 s is best fit by a smoothly broken power law function. The low-energy index is -0.58 +/- 0.06, the high-energy index is -2.90 +/- 0.06 and the break energy is 45.7 +/- 2.3 keV.
  The event fluence (10-1000 keV) in this time interval is (1.52 +/- 0.08)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+5.4 s in the 10-1000 keV band is 16.7 +/- 0.3 ph/s/cm^2.
  A Band function fits the spectrum equally well with Epeak = 56.26 +/- 1.15 keV, alpha = -0.16 +/- 0.07, and beta = -2.74 +/- 0.05.
  The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog.
• GCN Circular #18105
  S. B. Cenko, L. P. Singer (NASA/GSFC), M. M. Kasliwal (Carnegie), and V. Bhalerao (IUCAA) report on behalf of a larger collaboration:
  We imaged 12 fields covering an area of 87 deg^2 in and around the 1-sigma statistical+systematic uncertainty region of the final Fermi GBM localization. We estimate a 72% a priori probability that these fields contain the true location of the source. Sifting through candidate variable sources using image subtraction and standard iPTF vetting procedures, we detected the following optical transient candidates:
  iPTF15bud, at the coordinates:

    RA(J2000) = 22h 20m 08.68s (335.036172 deg)
    Dec(J2000) = +08d 59' 01.9" (+8.983848 deg)
  

  The source is coincident with the nucleus of the resolved source SDSS J222008.67+085901.7 (photo-z = 0.20 +/- 0.02) The following P48 photometry indicates no evidence for intra-night variability:

    +5.25 hr: R = 19.88 +/- 0.04
    +5.97 hr: R = 19.91 +/- 0.08
  

  iPTF15buc, at the coordinates:

    RA(J2000) = 22h 20m 26.17s (335.109057 deg)
    Dec(J2000) = +05d 59' 31.1" (+5.991972 deg)
  

  The source is coincident with the nucleus of the resolved source SDSS J222026.17+055930.4 (photo-z = 0.08 +/- 0.01) The following P48 photometry indicates no evidence for intra-night variability:

    +5.09 hr: R = 19.64 +/- 0.04
    +5.83 hr: R = 19.57 +/- 0.04
  

  iPTF15bub, at the coordinates:

    RA(J2000) = 22h 21m 17.29s (335.322031 deg)
    Dec(J2000) = +06d 52' 24.7" (+6.873532 deg)
  

  The source is coincident with the nucleus of the resolved source SDSS J222117.28+065224.4 (photo-z = 0.32 +/- 0.07) The following P48 photometry indicates no evidence for intra-night variability:

    +5.26 hr: R = 20.40 +/- 0.07
    +6.00 hr: R = 20.43 +/- 0.20
  

  iPTF15bua, at the coordinates:

    RA(J2000) = 22h 00m 00.79s (330.003312 deg)
    Dec(J2000) = +11d 25' 14.0" (+11.420547 deg)
  

  The source is coincident with the nucleus of the resolved source SDSS J220000.78+112513.9 (photo-z = 0.25 +/- 0.06) The following P48 photometry indicates no evidence for intra-night variability:

    +5.54 hr: R = 20.94 +/- 0.12
    +5.56 hr: R = 21.22 +/- 0.20
  

  iPTF15btz, at the coordinates:

    RA(J2000) = 21h 57m 06.00s (329.274989 deg)
    Dec(J2000) = +12d 06' 45.8" (+12.112722 deg)
  

  The source is near the nucleus of a resolved source in SDSS. The following P48 photometry indicates marginal evidence for fading:

    +5.01 hr: R = 20.26 +/- 0.07
    +5.56 hr: R = 20.50 +/- 0.12
  

  Times are relative to the GBM trigger. Magnitudes are in the Mould R filter and in the AB system, calibrated with respect to point sources in SDSS as described in Ofek et al. (2012, http://dx.doi.org/10.1086/664065).
  Given the likely nuclear associations and the lack of fading, we do not consider any of these sources to be likely candidates for the optical afterglow of GRB 150721A.
  The diagram http://www.its.caltech.edu/~lsinger/iptf/Fermi459150552.pdf shows the locations of our candidates and the P48 fields in relation to the Fermi GBM 1- and 2-sigma statistical+systematic contours.