Gamma-ray Burst 130821A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN Circular #15113
  P. Jenke (UAH) reports on behalf of the Fermi GBM Team:
  "At 16:10:28.011 UT on August 21 2013, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 130821 (trigger 398794231/130821674), which resulted in an Autonomous Repoint Request (ARR) that was accepted and the LAT slewed to the GBM position. The GBM on-ground location, using the Fermi GBM trigger data, is RA = 308.2, Dec = -15.5 (J2000 degrees, equivalent to J2000 20h 32m, -15d 30'), with a statistical uncertainty of 1 degree (radius, 1-sigma containment, statistical only; there is additionally a systematic error which is currently estimated to be 2 to 3 degrees).
  The angle of the burst direction to the Fermi LAT boresight is 37 degrees.
  The GBM light curve consists of multiple peaks with a duration (T90) of about 84 s (50-300 keV). The time-averaged spectrum from T0-3 s to T0+49 s is well fit by a Band function with Epeak of 165 +/- 5 keV, Alpha = -0.54 +/- 0.03 and Beta = -2.01 +/- 0.02.
  The event fluence (10-1000 keV) in this time interval is (6.60 +/- 0.08)E-05 erg/cm^2. The 1.0-sec peak photon flux measured starting from T0+32 s in the 10-1000 keV band is 27.6 +/- 0.4 ph/s/cm^2.
  The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."
• GCN Circular #15115
  D. Kocevski (NASA/GSFC), N. Omodei (Stanford), J. Racusin (NASA/GSFC), S. Zhu (U. of Maryland), S. Guiriec (NASA/GSFC), Eleonora Troja (NASA/GSGC/UMCP) report on behalf of the Fermi-LAT team:
  At 16:10:28.011 UT on 21 Aug 2013, Fermi-LAT detected high energy emission from GRB 130821A, which was also detected by Fermi-GBM (trigger 398794231/130821674 -- GCN 15113). The GBM detection triggered an autonomous repoint of the spacecraft.
  The best LAT on-ground location is found to be RA=314.1, DEC=-12.0 (J2000) with an error radius of 0.1 deg (68% containment, statistical error only).
  The burst was about 37 deg from the LAT boresight at the time of the trigger and the spacecraft slew brought the source within the LAT field of view for the next 2400 seconds. The data from the Fermi-LAT show long lasting emission with >40 photons above 100 MeV observed out to 2000s seconds with a TS of >170. Multi-peaked emission lasting roughly 40 seconds can be seen using the non-standard LAT Low Energy (LLE) with a significance of ~13 sigma. The highest energy LAT photon has an energy of ~6 GeV arriving 219 seconds after the trigger.
  A Swift TOO request has been submitted.
  The Fermi LAT point of contact for this burst is Daniel Kocevski (daniel.kocevski@nasa.gov).
  The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.
• GCN Circular #15123
  K.L. Page, P.A. Evans (U. Leicester), V. D'Elia (ASDC), J. Gelbord (Eureka Scientific) and J.L. Racusin (NASA/GSFC) report on behalf of the Swift-XRT team:
  Swift perfomed a Target of Opportunity observation of the Fermi GBM and LAT-detected GRB 130821A (GCN Circ. 15113, 15115), between 100.7 and 136.5 ks after the trigger. Within the LAT error circle (0.1 degrees at 1-sigma; statistical error only), in 9.9 ks of data there is a weak source with a count rate of (1.3 +0.4/-0.3) x 10^-3 count s^-1 with a position of RA, Dec 314.1601, -12.043481, which is equivalent to

  RA(J2000)  =  20h 56m 38.44s
  Dec(J2000) = -12d 02' 36.53"
  

  (5.5 arcsec uncertainty, 90% containment). We cannot determine whether the source is fading.
  We note, however, that in a ~10 ks observation faint background sources are expected, and there are two other detections (one uncatalogued, one a possible quasar) of a similar count rate outside the LAT 1-sigma error circle. The logN-logS work by Mateos et al. (2008, A&A, 492, 51) indicate we would expect to find around one source of this brightness within the ~0.03 deg^2 of the LAT error circle, while Elvis et al. (2009, ApJS, 184, 158) provide a slightly higher sky density, suggesting up to 3 sources might be expected. It is therefore likely that the source detected in this XRT observation is a serendipitous background object.
  Excluding this possible, weak source, the 3-sigma upper limit on the XRT count rate within the LAT error circle is 1.8 x 10^-3 count s^-1 (equivalent to an observed flux of 7.3 x 10^-14 erg cm^-2 s^-1 over 0.3-10 keV, assuming a typical counts to flux conversion factor of 4 x 10^-11 erg cm^-2 count^-1).
  We also note that, within the XRT field of view, although outside the LAT 1-sigma error circle, there is an X-ray source with a UVOT-enhanced position of RA, Dec 313.92706, -11.96634, which is equivalent to

  RA(J2000)  =  20h 55m 42.49s
  Dec(J2000) = -11d 57' 58.8"
  

  (2.3 arcsec uncertainty, 90% containment). This position is 4 arcsec away from the catalogued XMM Slew Survey object XMMSL1 J205542.2-115756. The 1-sigma uncertainty on the positions in the slew catalogue is 8 arcsec; thus, this X-ray source is consistent with being XMMSL1 J205542.2-115756 and is unrelated to GRB 130821A.
  This circular is an official product of the Swift-XRT team.
• GCN Circular #15124
  D. Xu (DARK/NBI), D.-Y. Ren, C. Cao, S.-M. Hu (SDU) report:
  We observed the XRT field of GRB 130821A (Jenke et al., GCN 15113; Kocevski et al., GCN 15115; Page et al., GCN 15123) centered at RA(J2000) = 20h 56m 38.44s and Dec(J2000) = -12d 02' 36.53", using the 1m telescope located in Weihai, Shandong, China. We obtained 9x300s SDSS r'-band frames under cloudy conditions at a mean time of 14:20 UT on 2013-08-24 (i.e., 2.997 days after the burst).
  No optical source is detected within the XRT error circle in the stacked image, down to r'>20.8 mag, calibrated with nearby SDSS field.
• GCN Circular #15125
  S. Golenetskii, R.Aptekar, D. Frederiks, V. Pal'shin, P. Oleynik, M. Ulanov, D. Svinkin, and T. Cline on behalf of the Konus-Wind team, report:
  The long-duration intense GRB 130821A (Fermi-GBM detection: Jenke, GCN 15113; Fermi-LAT detection: Kocevski, et al., GCN 15115) triggered Konus-Wind at T0=3D58258.022 s UT (16:10:58.022).
  The light curve shows multiple pulses from ~T0-40 to ~T0+80 s. The emission is seen up to ~9 MeV.
  The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB130821_T58258/
  As observed by Konus-Wind, the burst had a fluence of (9.9 =B1 0.9)x10-5 erg/cm2, and a 64-ms peak flux, measured from T0+6.784 s, of (1.3 =B1 0.1)x10-5 erg/cm2/s (both in the 20 keV - 10 MeV energy range).
  The time-averaged spectrum (measured from T0 to T0+78.080 s) is best fit in the 20 keV - 18 MeV range by the GRB (Band) function with the following model parameters: the low-energy photon index alpha =3D -1.33 =B1 0.11, the high energy photon index beta =3D -2.25 =B1 0.19, the peak energy Ep =3D 260 =B1 47 keV, chi2 =3D 84.6/97 dof.
  The spectrum at the maximum count rate (measured from T0 to T0+12.544 s) is best fit in the 20 keV - 18 MeV range by the GRB (Band) function with the following model parameters: the low-energy photon index alpha =3D -1.11 =B1 0.07, the high energy photon index beta =3D -2.33 =B1 0.10, the peak energy Ep =3D 255 =B1 22 keV, chi2 =3D 96.9/97 dof.
  All the quoted results are preliminary.
• GCN Circular #15127
  K. Hurley on behalf of the Mars Odyssey GRB team,
  S. Golenetskii, R. Aptekar, E. Mazets, V. Pal'shin, D. Frederiks, D. Svinkin, and T. Cline on behalf of the Konus-Wind team,
  I. G. Mitrofanov, D. Golovin, M. L. Litvak, and A. B. Sanin, on behalf of the HEND-Odyssey GRB team,
  W. Boynton, C. Fellows, K. Harshman, H. Enos, and R. Starr, on behalf of the GRS-Odyssey GRB team,
  D. M. Smith, J. McTiernan, R. Schwartz, and W. Hajdas, on behalf of the RHESSI GRB team,
  A. von Kienlin, X. Zhang, A. Rau, V. Savchenko, E. Bozzo, and C. Ferrigno, on behalf of the INTEGRAL SPI-ACS GRB team,
  and
  V. Connaughton, M. S. Briggs, C. Meegan, and V. Pelassa, on behalf of the Fermi GBM team, report:
  GRB 130821A, observed by the Fermi GBM (GCN 15113) and LAT (GCN 15115), was also observed by Konus-Wind (GCN 15125), RHESSI, INTEGRAL SPI-ACS, and Mars Odyssey-HEND. We have triangulated it to an annulus centered at RA(2000)=297.840 deg (19h 51m 22s) Dec(2000)=-21.890 deg (-21d 53' 24"), whose radius is 18.673 +/- 0.049 deg (3 sigma). The minimum distance between the center of the LAT one sigma error circle (GCN 15115) and the IPN annulus center line is 0.266 degrees; the circle lies entirely outside the annulus. The two Swift XRT sources reported by Page et al. (GCN 15123) also lie outside the IPN annulus, and are therefore unlikely to be associated with the GRB. A map has been posted at ssl.berkeley.edu/ipn3/130821.
  Only minor improvement in the IPN localization is possible.
• 1312.2662 from 11 Dec 13
  Yun-Feng Liang et al.: High energy emission of GRB 130821A: constraining the density profile of the circum-burst medium as well as the initial Lorentz factor of the outflow
  GRB 130821A was detected by Fermi-GBM/LAT, Konus-Wind, SPI-ACS/INTEGRAL, RHESSI and Mars Odyssey-HEND. Although the data of GRB 130821A are very limited, we show in this work that the high energy gamma-ray emission (i.e., above 100 MeV) alone imposes tight constraint on the density profile of the circum-burst medium as well as the initial Lorentz factor of the outflow. The temporal behavior of the high energy gamma-ray emission is consistent with the forward shock synchrotron radiation model and the circum-burst medium likely has a constant-density profile. The Lorentz factor is about a few hundred, similar to other bright GRBs.