A. Allafort

SEARCH FOR SPATIALLY EXTENDED FERMI LARGE AREA TELESCOPE SOURCES USING TWO YEARS OF DATA

Spatial extension is an important characteristic for correctly associating γ-ray-emitting sources with their counterparts at other wavelengths and for obtaining an unbiased model of their spectra. We present a new method for quantifying the spatial extension of sources detected by the Large Area Telescope (LAT), the primary science instrument on the Fermi Gamma-ray Space Telescope (Fermi). We perform a series of Monte Carlo simulations to validate this tool and calculate the LAT threshold for detecting the spatial extension of sources. We then test all sources in the second Fermi-LAT catalog for extension. We report the detection of seven new spatially extended sources.

FERMI LARGE AREA TELESCOPE STUDY OF COSMIC RAYS AND THE INTERSTELLAR MEDIUM IN NEARBY MOLECULAR CLOUDS

We report an analysis of the interstellar γ -ray emission from the Chamaeleon, R Coronae Australis (R CrA), and Cepheus and Polaris flare regions with the Fermi Large Area Telescope. They are among the nearest molecular cloud complexes, within ∼300 pc from the solar system. The γ -ray emission produced by interactions of cosmic rays (CRs) and interstellar gas in those molecular clouds is useful to study the CR densities and distributions of molecular gas close to the solar system. The obtained γ -ray emissivities above 250 MeV are (5.9 ± 0.1stat +0.9 −1.0sys) × 10−27 photons s−1 sr−1 H-atom−1, (10.2 ± 0.4stat +1.2 −1.7sys) × 10−27 photons s−1 sr−1 H-atom−1, and (9.1 ± 0.3stat +1.5 −0.6sys) × 10−27 photons s−1 sr−1 H-atom−1 for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively. Whereas the energy dependences of the emissivities agree well with that predicted from direct CR observations at the Earth, the measured emissivities from 250 MeV to 10 GeV indicate a variation of the CR density by ∼20% in the neighborhood of the solar system, even if we consider systematic uncertainties. The molecular mass calibrating ratio, XCO = N(H2)/WCO, is found to be (0.96 ± 0.06stat +0.15 −0.12sys) × 1020 H2-molecule cm−2 (K km s−1)−1, (0.99 ± 0.08stat +0.18 −0.10sys) × 1020 H2-molecule cm−2 (K km s−1)−1, and (0.63 ± 0.02stat +0.09 −0.07sys) × 1020 H2-molecule cm−2 (K km s−1)−1 for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively, suggesting a variation of XCO in the vicinity of the solar system. From the obtained values of XCO, the masses of molecular gas traced by WCO in the Chamaeleon, R CrA, and Cepheus and Polaris flare regions are estimated to be ∼5 × 103M , ∼103M , and ∼3.3 × 104M , respectively. A comparable amount of gas not traced well by standard Hi and CO surveys is found in the regions investigated.We report an analysis of the interstellar γ-ray emission from the Chamaeleon, R Coronae Australis (R CrA), and Cepheus and Polaris flare regions with the Fermi Large Area Telescope. They are among the nearest molecular cloud complexes, within ∼ 300 pc from the solar system. The γ-ray emission produced by interactions of cosmicrays (CRs) and interstellar gas in those molecular clouds is useful to study the CR densities and distributions of molecular gas close to the solar system. The obtained γ-ray emissivities above 250 MeV are (5.9 ± 0.1stat +0.9 −1.0sys) × 10 −27 photons s−1 sr−1 H-atom−1, (10.2 ± 0.4stat +1.2 −1.7sys) × 10 −27 photons s−1 sr−1 H-atom−1, and (9.1 ± 0.3stat +1.5 −0.6sys) × 10 −27 photons s−1 sr−1 H-atom−1 for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively. Whereas the energy dependences of the emissivities agree well with that predicted from direct CR observations at the Earth, the measured emissivities from 250 MeV to 10 GeV indicate a variation of the CR density by ∼ 20 % in the neighborhood of the solar system, even if we consider systematic uncertainties. The molecular mass calibrating ratio, XCO = N(H2)/WCO, is found to be (0.96 ± 0.06stat +0.15 −0.12sys) ×10 20 H2-molecule cm −2 (K km s−1)−1, (0.99 ± 0.08stat +0.18 −0.10sys) ×10 20 H2-molecule cm −2 (K km s−1)−1, and (0.63 ± 0.02stat +0.09 −0.07sys) ×10 20 H2-molecule cm −2 (K km s−1)−1 for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively, suggesting a variation of XCO in the vicinity of the solar system. From the obtained values of XCO, the masses of molecular gas traced by WCO in the Chamaeleon, R CrA, and Cepheus and Polaris flare regions are estimated to be ∼ 5×103 M⊙, ∼ 10 3 M⊙, and ∼ 3.3×10 4 M⊙, respectively. A comparable amount of gas not traced well by standard H I and CO surveys is found in the regions investigated. University of California at Santa Cruz, Santa Cruz, CA 95064, USA Institut für Astround Teilchenphysik and Institut für Theoretische Physik, Leopold-Franzens-Universität Innsbruck, A-6020 Innsbruck, Austria Department of Physics, University of Washington, Seattle, WA 98195-1560, USA NYCB Real-Time Computing Inc., Lattingtown, NY 11560-1025, USA Department of Chemistry and Physics, Purdue University Calumet, Hammond, IN 46323-2094, USA Institut für Theoretische Physik and Astrophysik, Universität Würzburg, D-97074 Würzburg, Germany Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain Consorzio Interuniversitario per la Fisica Spaziale (CIFS), I-10133 Torino, Italy INTEGRAL Science Data Centre, CH-1290 Versoix, Switzerland NASA Postdoctoral Program Fellow, USA Dipartimento di Fisica, Università di Roma “Tor Vergata”, I-00133 Roma, Italy Institut Universitaire de France, France

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.

Fermi-LAT Observations of High-energy Behind-the-limb Solar Flares

We report on the Fermi-LAT detection of high-energy emission from the behind-the-limb (BTL) solar flares that occurred on 2013 October 11, and 2014 January 6 and September 1. The Fermi-LAT observations are associated with flares from active regions originating behind both the eastern and western limbs, as determined by STEREO. All three flares are associated with very fast coronal mass ejections (CMEs) and strong solar energetic particle events. We present updated localizations of the >100 MeV photon emission, hard X-ray (HXR) and EUV images, and broadband spectra from 10 keV to 10 GeV, as well as microwave spectra. We also provide a comparison of the BTL flares detected by Fermi-LAT with three on-disk flares and present a study of some of the significant quantities of these flares as an attempt to better understand the acceleration mechanisms at work during these occulted flares. We interpret the HXR emission to be due to electron bremsstrahlung from a coronal thin-target loop top with the accelerated electron spectra steepening at semirelativistic energies. The >100 MeV gamma-rays are best described by a pion-decay model resulting from the interaction of protons (and other ions) in a thick-target photospheric source. The protons are believed to have been accelerated (to energies >10 GeV) in the CME environment and precipitate down to the photosphere from the downstream side of the CME shock and landed on the front side of the Sun, away from the original flare site and the HXR emission.

Gamma-ray observations of the supernova remnant RX J0852.0-4622 with the Fermi large area telescope

We report on gamma-ray observations of the supernova remnant (SNR) RX J0852.0–4622 with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. In the Fermi-LAT data, we find a spatially extended source at the location of the SNR. The extension is consistent with the SNR size seen in other wavelengths such as X-rays and TeV gamma rays, leading to the identification of the gamma-ray source with the SNR. The spectrum is well described as a power law with a photon index of Γ = 1.85 ± 0.06 (stat)+0.18 – 0.19 (sys), which smoothly connects to the H.E.S.S. spectrum in the TeV energy band. We discuss the gamma-ray emission mechanism based on multiwavelength data. The broadband data can be fit well by a model in which the gamma rays are of hadronic origin. We also consider a scenario with inverse Compton scattering of electrons as the emission mechanism of the gamma rays. Although the leptonic model predicts a harder spectrum in the Fermi-LAT energy range, the model can fit the data considering the statistical and systematic errors.

Gamma-Ray Observations of the Supernova Remnant RX J0852.0-4622 with the Fermi LAT

We report on gamma-ray observations of the supernova remnant (SNR) RX J0852.0–4622 with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. In the Fermi-LAT data, we find a spatially extended source at the location of the SNR. The extension is consistent with the SNR size seen in other wavelengths such as X-rays and TeV gamma rays, leading to the identification of the gamma-ray source with the SNR. The spectrum is well described as a power law with a photon index of Γ = 1.85 ± 0.06 (stat)+0.18 – 0.19 (sys), which smoothly connects to the H.E.S.S. spectrum in the TeV energy band. We discuss the gamma-ray emission mechanism based on multiwavelength data. The broadband data can be fit well by a model in which the gamma rays are of hadronic origin. We also consider a scenario with inverse Compton scattering of electrons as the emission mechanism of the gamma rays. Although the leptonic model predicts a harder spectrum in the Fermi-LAT energy range, the model can fit the data considering the statistical and systematic errors.

Fermi-LAT Observations of the 2017 September 10 Solar Flare

The Fermi-Large Area Telescope (LAT) detection of the X8.2 GOES class solar flare of 2017 September 10 provides for the first time observations of a long duration high-energy gamma-ray flare associated with a Ground Level Enhancement (GLE). The >100 MeV emission from this flare lasted for more than 12 hours covering both the impulsive and extended phase. We present the localization of the gamma-ray emission and find that it is consistent with the active region (AR) from which the flare occurred over a period lasting more than 6 hours contrary to what was found for the 2012 March 7 flares. The temporal variation of the proton index inferred from the gamma-ray data seems to suggest two phases in acceleration of the proton population. Based on timing arguments we interpret the second phase to be tied to the acceleration mechanism powering the GLE, believed to be particle acceleration at a coronal shock driven by the CME.

Fermi Large Area Telescope observations of the Sun: The first ten years

The Fermi Large Area Telescope (LAT) observations of the active Sun provide the largest sample of detected solar flares with emission greater than 30 MeV to date. These include detections of impulsive and sustained emission, extending up to ~20 hours in the case of the 2012 March 7 X-class flares. Of particular interest is the first detection of >100 MeV gamma-ray emission from three solar flares whose positions behind the limb were confirmed by the STEREO spacecrafts. The LAT data provides a new observational channel that, when combined with observations from across the electromagnetic spectrum, provide a unique opportunity to diagnose the mechanisms of high-energy emission and particle acceleration in solar flares. We will present an overview of these observations including the emission of the Sun in its quiescent state and discuss how these observations provide constrains on different emission mechanisms.

Fermi Large Area Telescope observation of high-energy solar flares: constraining emission scenarios

The Fermi Large Area Telescope (LAT) is the most sensitive instrument ever de10 ployed in space for observing gamma-ray emission >100 MeV. This has also been demonstrated 11 by its detection of quiescent gamma-ray emission from pions produced by cosmic-ray protons 12 interacting in the solar atmosphere, and from cosmic-ray electron interactions with solar optical 13 photons. The Fermi-LAT has also detected high-energy gamma-ray emission associated with 14 GOES M-class and X-class solar flares, each accompanied by a coronal mass ejection and a 15 solar energetic particle event, increasing the number of detected solar flares by almost a factor 16 of 10 with respect to previous space observations. During the impulsive phase, gamma rays with 17 energies up to several hundreds of MeV have been recorded by the LAT. Emission up to GeV 18 energies lasting several hours after the flare has also been detected by the LAT. Of particu19 lar interest are the recent detections of three solar flares whose position behind the limb was 20 confirmed by the STEREO satellites. While gamma-ray emission up to tens of MeV resulting 21 from proton interactions has been detected before from occulted solar flares, the significance of 22 these particular events lies in the fact that these are the first detections of >100 MeV gamma-ray 23 emission from footpoint-occulted flares. We will present the Fermi-LAT, RHESSI and STEREO 24 observations of these flares and discuss the various emission scenarios for these sources. 25

GAMMA-RAY OBSERVATIONS OF THE SUPERNOVA REMNANT RX J0852.0–4622 WITH THEFERMILARGE AREA TELESCOPE

We report on gamma-ray observations of the supernova remnant (SNR) RX J0852.0–4622 with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. In the Fermi-LAT data, we find a spatially extended source at the location of the SNR. The extension is consistent with the SNR size seen in other wavelengths such as X-rays and TeV gamma rays, leading to the identification of the gamma-ray source with the SNR. The spectrum is well described as a power law with a photon index of Γ = 1.85 ± 0.06 (stat)+0.18 – 0.19 (sys), which smoothly connects to the H.E.S.S. spectrum in the TeV energy band. We discuss the gamma-ray emission mechanism based on multiwavelength data. The broadband data can be fit well by a model in which the gamma rays are of hadronic origin. We also consider a scenario with inverse Compton scattering of electrons as the emission mechanism of the gamma rays. Although the leptonic model predicts a harder spectrum in the Fermi-LAT energy range, the model can fit the data considering the statistical and systematic errors.