Fermi Large Area Telescope observations of high-energy gamma-ray emission from behind-the-limb solar flares

Fermi-LAT >30 MeV observations have increased the number of detected solar flares by almost a factor of 10 with respect to previous space observations. These sample both the impulsive and long duration phases of GOES M and X class flares. Of particular interest is the recent detections of three solar flares whose position behind the limb was confirmed by the STEREO-B spacecraft. While gamma-ray emission up to tens of MeV resulting from proton interactions has been detected before from occulted solar flares, the significance of these particular events lies in the fact that these are the first detections of >100 MeV gamma-ray emission from footpoint-occulted flares. We will present the Fermi-LAT, RHESSI and STEREO observations of these flares and discuss the various emission scenarios for these sources and implications for the particle acceleration mechanisms.

First detection of >100 MeV gamma rays associated with a behind-the-limb solar flare

We report the first detection of >100 MeV gamma rays associated with a behind-the-limb solar flare, which presents a unique opportunity to probe the underlying physics of high-energy flare emission and particle acceleration. On 2013 October 11 a GOES M1.5 class solar flare occurred ~ 9.9 degrees behind the solar limb as observed by STEREO-B. RHESSI observed hard X-ray emission above the limb, most likely from the flare loop-top, as the footpoints were occulted. Surprisingly, the Fermi Large Area Telescope (LAT) detected >100 MeV gamma-rays for ~30 minutes with energies up to GeV. The LAT emission centroid is consistent with the RHESSI hard X-ray source, but its uncertainty does not constrain the source to be located there. The gamma-ray spectra can be adequately described by bremsstrahlung radiation from relativistic electrons having a relatively hard power-law spectrum with a high-energy exponential cutoff, or by the decay of pions produced by accelerated protons and ions with an isotropic pitch-angle distribution and a power-law spectrum with a number index of ~3.8. We show that high optical depths rule out the gamma rays originating from the flare site and a high-corona trap model requires very unusual conditions, so a scenario in which some of the particles accelerated by the CME shock travel to the visible side of the Sun to produce the observed gamma rays may be at work.

New Fermi-LAT event reconstruction reveals more high-energy gamma rays from Gamma-ray bursts

Based on the experience gained during the four and a half years of the mission, the Fermi -LAT collaboration has undertaken a comprehensive revision of the event-level analysis going under the name of Pass 8. Although it is not yet finalized, we can test the improvements in the new event reconstruction with the special case of the prompt phase of bright Gamma-Ray Bursts (GRBs), where the signal to noise ratio is large enough that loose selection cuts are sufficient to identify gamma- rays associated with the source. Using the new event reconstruction, we have re-analyzed ten GRBs previously detected by the LAT for which an x-ray/optical follow-up was possible and found four new gamma rays with energies greater than 10 GeV in addition to the seven previously known. Among these four is a 27.4 GeV gamma-ray from GRB 080916C, which has a redshift of 4.35, thus making it the gamma ray with the highest intrinsic energy (147 GeV) detected from a GRB. We present here the salient aspects of the new event reconstruction and discuss the scientific implications of these new high-energy gamma rays, such as constraining extragalactic background light models, Lorentz invariance violation (LIV) tests, the prompt emission mechanism and the bulk Lorentz factor of the emitting region.

In-flight measurement of the absolute energy scale of the Fermi Large Area Telescope

The Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope is a pair-conversion telescope designed to survey the gamma-ray sky from 20 MeV to several hundreds of GeV. In this energy band there are no astronomical sources with sufficiently well known and sharp spectral features to allow an absolute calibration of the LAT energy scale. However, the geomagnetic cutoff in the cosmic ray electron-plus-positron (CRE) spectrum in low earth orbit does provide such a spectral feature. The energy and spectral shape of this cutoff can be calculated with the aid of a numerical code tracing charged particles in the Earths magnetic field. By comparing the cutoff value with that measured by the LAT in different geomagnetic positions, we have obtained several calibration points between ~6 and ~13 GeV with an estimated uncertainty of ~2%. An energy calibration with such high accuracy reduces the systematic uncertainty in LAT measurements of, for example, the spectral cutoff in the emission from gamma ray pulsars.

Extending the Galactic Cosmic Ray electron + positron spectrum measured by the Fermi LAT

Launched on the 11th of June 2008, the Fermi Large Area Telescope (LAT) has made several outstanding scientific contributions to the high energy astrophysics community. One of these contributions was the high statistics measurement of the Galactic Cosmic Ray (GCR) electron + positron spectrum from 20 GeV to 1 TeV. The Fermi satellite is in a nearly circular orbit with an inclination of 25.6 degrees at an altitude of 565 km. Given this orbit it is possible to measure the GCR electrons + positrons down to roughly 5 GeV. However, this lower limit in energy is highly dependent on the orbital position of the LAT in geomagnetic coordinates due to the rigidity cutoff. In order to measure the spectrum down to these energies it is necessary to sample the population of electrons + positrons in several different geomagnetic positions. In this poster we present the analysis performed to extend the lower limit in energy of the GCR electron + positron spectrum measured by the Fermi LAT.

First results on Cosmic Ray electron spectrum below 20 GeV from the Fermi LAT

Designed to be a successor of the previous flown space based gamma ray detectors, the Fermi Large Area Telescope (LAT) is also an electron detector. Taking advantage of its capability to separate electromagnetic and hadronic signals it is possible to accurately measure the Cosmic Ray electron spectrum. The spectra of primary cosmic ray electrons below 20 GeV is influenced by many local effects such as solar modulation and the geomagnetic cutoff. For energies below a few GeV it is possible to observe the albedo population of electrons which are controlled by the local magnetic field. In this paper we present the LAT electron analysis in particular event selection and validation as well as the first results on the measurement of the electron spectrum below 20 GeV.