Fermi LAT and GBM detection of the X2.1 Solar Flare of September 6 2011
ATel #3635; Masanori Ohno, Hiromitsu Takahashi (Hiroshima U.), Yasuyuki T. Tanaka (ISAS/JAXA), Vlasios Vasileiou(CNRS/IN2P3/LUPM), Nicola Omodei (Stanford University), David Gruber (MPE), Adam Goldstein (UAH), on behalf of the Fermi LAT and GBM collaborations
on 7 Sep 2011; 19:48 UT
Credential Certification: Yasuyuki T. Tanaka (tanaka@astro.isas.jaxa.jp)
The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, detected increased gamma-ray flux in temporal coincidence with the bright X2.1 solar flare on September 6, 2011. The excess is spatially coincident with the Sun at the time of the flare. Gamma-ray emission is detected with a post-trial significance of larger than 30 sigma above background.
Due to pile up of X-rays in the Anti-Coincidence Detector system of the LAT in coincidence with the bright solar flare, the LAT data are not described by the standard instrument response functions from 22:17:43 UTC to 22:34:43 UTC. Standard LAT event data and analysis techniques should not be used in that time interval to evaluate the gamma-ray emission.
Using the LAT P7SOURCE event class and excluding the aforementioned time interval, we calculate that the time-averaged integral flux of >100 MeV gamma rays from 22:34 UTC to 22:46 UTC is approximately 1.9e-4 photons cm^-2 s^-1 with a power-law spectral index of approximately 2.6.
Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of the Sun will continue. For this detection, the Fermi LAT contact person is Masanori Ohno (ohno@hep01.hepl.hiroshima-u.ac.jp).
This solar flare was also independently detected by the Gamma-Ray Burst Monitor (GBM) on Fermi. GBM is composed of 12 thallium-activated sodium iodide (NaI) scintillation detectors (8 keV to 1 MeV) and two bismuth germanate scintillation (BGO) detectors (150 keV - 40 MeV). GBM triggered twice at 22:17:17.88 UTC and 22:29:35.004 UTC. Gamma-ray emission is detected up to ~ 22:41 UTC in the 50 keV - 300 keV energy range. The most energetic emission episode (> 500 keV) started at ~ 22:18 UTC and lasted until ~ 22:23 UTC with gamma-rays detected with energies up to about 5 MeV in the BGO. The peak of the emission is observed at ~ 22:19 UTC.
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.
