Gamma-Ray Emission from the Shell of Supernova Remnant W44 Revealed by the Fermi LAT

Abstract

Recent observations of supernova remnants (SNRs) hint that they accelerate cosmic rays to energies close to ~10(15) electron volts. However, the nature of the particles that produce the emission remains ambiguous. We report observations of SNR W44 with the Fermi Large Area Telescope at energies between 2 x 10(8) electron volts and 3 x10(11) electron volts. The detection of a source with a morphology corresponding to the SNR shell implies that the emission is produced by particles accelerated there. The gamma-ray spectrum is well modeled with emission from protons and nuclei. Its steepening above approximately 10(9) electron volts provides a probe with which to study how particle acceleration responds to environmental effects such as shock propagation in dense clouds and how accelerated particles are released into interstellar space.

Observational Signatures of Particle Acceleration in Supernova Remnants

“…. The high density in and near the supernova remnant suggests that the emission is likely due to pion decay, but bremsstrahlung cannot in all cases be ruled out, as this also scales with n H (e.g., Abdo et al, 2010d, and section 2.13). However, new AGILE observations of two supernova remnants Table 4 Table with (W44, W28 Giuliani et al, 2011W28 Giuliani et al, , 2010 reveal the characteristic low energy cut-off expected for pion decay ( Figure 13).…”

Collisionless Shocks in Partly Ionized Plasma with Cosmic Rays: Microphysics of Non-thermal Components

“…The recent Fermi-LAT observations of the so-called molecular SNRs W44 and IC 443 (Abdo et al 2010a(Abdo et al ,2010bAckermann et al 2013) indicate that the spectra of the gamma-ray producing protons (integrated over the emission region) are typically steeper than the DSA predictions for the spectra of the CRs confined in the acceleration region. The steep photon spectra has been found in the high energy gamma-ray spectra of some other remnants measured by, e.g., the CANGAROO (Enomoto et al 2002), H.E.S.S (Aharonian et al 2006) and MAGIC (Carmona 2011) atmospheric Cherenkov telescopes.…”

“…The break in the photon spectrum is observed at about 2 GeV, which places the break in the proton distribution at about p br ≃ 7GeV /c (Abdo et al 2010a). For the strength of the break ∆q = 1, the spectrum above it is clearly pressure converging, so that the shock structure and the spectrum may be calculated using this break momentum as the point of the maximum in the CR partial pressure.…”

H 3 + at the interface between astrochemistry and astroparticle physics

“…This is probably the same partly atomic, partly molecular gas that some of us have labelled 'diffuse' and 'translucent' for decades, but the new data now afford an improved accounting of it over a large part of the Galaxy. The Fermi Gamma-ray Space Telescope and several ground-based arrays sensitive to TeV photons have recently achieved such improvements in sensitivity and resolution that it is now possible to recognize the localized, supernova-related sources above the diffuse background of g-rays [25][26][27][28][29][30][31][32][33][34][35][36][37][38].…”