TITLE:   GCN CIRCULAR
NUMBER:  14508
SUBJECT: GRB 130427A: Fermi-LAT refined analysis
DATE:    13/04/29 21:30:22 GMT
FROM:    Judith Racusin at GSFC  <judith.racusin@nasa.gov>

S. Zhu, J. Racusin (NASA/GSFC), J. Chiang (SLAC), G. Vianello (Stanford) report on behalf of the Fermi LAT team:

Using LAT source class events >100 MeV between T0+0 and 700 seconds after the GBM trigger, we find a LAT localization of RA = 173.148, Dec = +27.709, with a 68% containment radius of 0.068 degrees (statistical only). This localization is consistent with other reported positions.

The >100 MeV emission spectrum during the GBM T90 (T0+0 to 138 seconds) is fit by a power law with an index of -1.96 +/- 0.07. The fluence during this time is (1.1 +/- 0.1)E-4 erg/cm^2, making this the highest fluence LAT-detected burst in the LAT energy range (Fermi LAT Collaboration, arXiv:1303.2908). The >100 MeV peak flux, measured from 11.52 to 37.33 seconds, is (1.4 +/- 0.2)E-3 ph/cm^2/s.

The LAT Low Energy (LLE) emission during the bright structured peak (0 to 20 seconds) is roughly correlated with the GBM emission. A spike at T0+0 seconds is coincident in both LAT and LLE, but precedes the onset of the GBM emission. There are peaks in the LAT light curve at approximately 13 and 22 seconds; neither peak is coincident with the LLE or GBM.

Significant emission >100 MeV was detected throughout the first orbit until ~735 seconds, at which point the burst became occulted by the Earth. The LAT emission was still significantly detected when the burst emerged from occultation at ~3000 seconds, and remained detectable for about a day. The extended emission light curve can be fit by a broken power-law that has a power-law index = -0.89 +/- 0.04 at early times, and at late times, it has an index in the range -1.3 to -1.5; the temporal break occurs around 550-800 seconds after the GBM trigger.

We clarify that the Swift-BAT trigger time (Barthelmy et al., GCN 14470) is ~51 seconds after the GBM trigger, so the emission detected by RAPTOR beginning ~50 seconds before the Swift trigger with a peak magnitude of R~7.4 (Wren et al., GCN 14476) is therefore coincident with the GBM and LAT emission onset.

The Fermi LAT point of contact for this burst is Sylvia Zhu (s.jc.zhu@gmail.com).

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.