Gamma-ray Burst 100427A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN Circular #10699
  J. R. Cummings (GSFC/UMBC), H. A. Krimm (GSFC/USRA), and R. L. C. Starling (U Leicester) on behalf of the Swift team:
  At 08:31:55 UT Swift-BAT detected a rate increase. Onboard analysis was terminated prematurely by a pre-planned slew, but in later ground analysis a significant source was found at RA, Dec 89.171, -3.461, which is

  RA  (J2000)    05h 56m 41.1s
  Dec (J2000)   -03d 27' 40"
  

  with an estimated uncertainty of 2.5 arcmin radius (90% containment).
  The burst had two main peaks, the first much weaker, from about T-3 to T+3 sec and from T+11 to T+23 sec. We have full data for only the first, weaker peak. The peak non-maskweighted count rate (15-350 keV) was about 3000 counts/sec at 16 seconds after the trigger.
  Since the source was not detected onboard, there are no automated data products for this burst.
  A Swift TOO has started.
• GCN Circular #10702
  A. Goldstein (UAH) and D. Gruber (MPE) report on behalf of the Fermi GBM Team:
  "At 08:32:08.71 UT on 27 April 2010, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 100427A (trigger 294049930 / 100427356), which was also detected by the Swift-BAT (J.R. Cummings et al. 2010, GCN 10699). The GBM on-ground location is consistent with the Swift-BAT position. The angle from the Fermi LAT boresight is 29 degrees.
  The GBM light curve consists of a single pulse with multiple peaks with a duration (T90) of about 24 s (8-1000 keV). The time-averaged spectrum from T0-4.1 s to T0+11.3 s is best fit by a power law function with an exponential high energy cutoff. The power law index is -1.02 +0.12/-0.11 and the cutoff energy, parameterized as Epeak, is 121.0 +15.4/-11.8 keV (Castor C-Stat 519 for 484 d.o.f.).
  The event fluence (8-1000 keV) in this time interval is (3.01 +/- 0.13)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+5.18 s in the 8-1000 keV band is 4.79 +/- 0.24 ph/s/cm^2.
  The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."
• GCN Circular #10703
  A. Breeveld (MSSL/UCL), A. Rowlinson, N. Lyons and R.L.C. Starling (U. Leicester) report on behalf of the Swift/UVOT team:
  Swift began settled observations of the field of GRB 100427A 110220 s after the BAT rate increase detected in ground analysis (Cummings et al., GCN Circ. 10699). No afterglow consistent with the BAT position given in GCN Circ. 10699 is detected in the initial XRT or UVOT exposures.
  Swift-XRT obtained 5.9 ks of Photon Counting (PC) mode data, from T0 + 110 ks to T0 + 144 ks. No source is found in the BAT error circle, with a 3-sigma upper limit of 1.3 x 10-3 count s-1. For a typical GRB, this would correspond to an upper limit on the 0.3 - 10 keV flux of 5 x 10-14 erg cm-2 s-1.
  For the UVOT a preliminary 3-sigma upper limit for the initial summed exposures, using the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627), is:

  Filter         T_start(s)   T_stop(s)      Exp(s)         Mag
  u               110220       143784         5836         >21.2
  

  The values quoted above are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.69 in the direction of the burst (Schlegel et al. 1998).