Evidence For Cosmic Ray Acceleration Research Articles

Possible Evidence for Cosmic-Ray Acceleration in the Type Ia SNR RCW 86: Spatial Correlation between TeV Gamma-Rays and Interstellar Atomic Protons

Abstract We present a detailed morphological study of TeV gamma-rays, synchrotron radiation, and interstellar gas in the young Type Ia supernova remnant (SNR) RCW 86. We find that the interstellar atomic gas shows good spatial correlation with the gamma-rays, indicating that the TeV gamma-rays from RCW 86 are likely predominantly of hadronic origin. In contrast, the spatial correlation between the interstellar molecular cloud and the TeV gamma-rays is poor in the southeastern shell of the SNR. We argue that this poor correlation can be attributed to the low-energy cosmic rays (∼1 TeV) not penetrating into the dense molecular cloud due to an enhancement of the turbulent magnetic field around the dense cloud of ∼10–100 μG. We also find that the southwestern shell, which is bright in both synchrotron X-ray and radio continuum radiation, shows a significant gamma-ray excess compared with the interstellar proton column density, suggesting that leptonic gamma-rays via inverse Compton scattering possibly contribute alongside the hadronic gamma-rays. The total cosmic-ray energies of the young TeV gamma-ray SNRs—RX J1713.7−3946, Vela Jr, HESS J1731−347, and RCW 86—are roughly similar, which indicates that cosmic rays can be accelerated in both the core-collapse and Type Ia supernovae. The total energy of cosmic rays derived using the gas density, ∼1048–1049 erg, gives a safe lower limit due mainly to the low filling factor of interstellar gas within the shell.

ON WEAK REDSHIFT DEPENDENCE OF GAMMA-RAY SPECTRA OF DISTANT BLAZARS

Line-of-sight interactions of cosmic rays provide a natural explanation of the hard gamma-ray spectra of distant blazars, which are believed to be capable of producing both gamma rays and cosmic rays. For sources with redshifts z> 0.1, secondary gamma rays produced in cosmic-ray interactions with background photons close to an observer can dominate over primary gamma rays originating at the source. The transition from one component to another is accompanied by a change in the spectral index depending on the source redshift. We present theoretical predictions and show that they agree with the data from Fermi Large Area Telescope. This agreement, combined with the spectral data from Atmospheric Cherenkov Telescopes, provides evidence of cosmic ray acceleration by active galactic nuclei and opens new opportunities for studying photon backgrounds and intergalactic magnetic fields.

CORRELATION OF SUPERNOVA REMNANT MASERS AND GAMMA-RAY SOURCES

Supernova remnants interacting with molecular clouds are potentially exciting systems in which to detect evidence of cosmic ray acceleration. Prominent gamma-ray emission is produced via the decay of neutral pions when cosmic rays encounter the nearby dense clouds. In many of the supernova remnants coincident with gamma-ray sources, the presence of OH(1720 MHz) masers is used to identify interaction with dense gas and to provide a kinematic distance to the system. In this paper we use statistical tests to demonstrate that there is a correlation between these masers and a class of GeV- to TeV-energy gamma-ray sources coincident with interacting remnants. For pion decay, the gamma-ray luminosity provides a direct estimate of the local cosmic ray density. We find the cosmic ray density is enhanced by one to two orders of magnitude over the local solar value, comparable to X-ray-induced ionization in these remnants. The inferred ionization rates are sufficient to explain non-equilibrium chemistry in the post-shock gas, where high columns of hydroxyl are observed.

Prospects of identifying the sources of the Galactic cosmic rays with IceCube

The Milagro detector has identified several TeV gamma-ray sources with fluxes consistent with typical cosmic-ray generating supernova remnants interacting with the interstellar medium. In this contribution we show that IceCube, a kilometer-scale neutrino detector under construction at the South Pole, can provide incontrovertible evidence of cosmic-ray acceleration in these sources by detecting neutrinos. The sensitivity is maximal if only events with energies above about 40 TeV are considered where the atmospheric neutrino background is largely reduced.

Publisher’s Note: Neutrino flux from cosmic ray accelerators in the Cygnus spiral arm of the Galaxy [Phys. Rev. D76, 067301 (2007)

Intriguing evidence has been accumulating for the production of cosmic rays in the Cygnus region of the Galactic plane. We here show that the IceCube experiment can produce incontrovertible evidence for cosmic ray acceleration by observing neutrinos from the decay of charged pions accompanying the TeV photon flux observed in the HEGRA, Whipple, Tibet and Milagro experiments. Our assumption is that the TeV photons observed are the decay products of neutral pions produced by cosmic ray accelerators in the nearby spiral arm of the Galaxy. Because of the proximity of the sources, IceCube will obtain evidence at the 5sigma level in 15 years of observation.

Neutrino flux from cosmic ray accelerators in the Cygnus spiral arm of the Galaxy

Intriguing evidence has been accumulating for the production of cosmic rays in the Cygnus region of the galactic plane. We here show that the IceCube experiment can produce incontrovertible evidence for cosmic ray acceleration by observing the neutrinos from the decay of charged pions accompanying the TeV photon flux observed in the HEGRA, Whipple, Tibet, and Milagro experiments. Our assumption is that the TeV photons observed are the decay products of neutral pions produced by cosmic ray accelerators in the nearby spiral arm of the galaxy. Because of the proximity of the sources, IceCube will obtain evidence at the $5\ensuremath{\sigma}$ level in 15 years of observation.

Evidence for cosmic-ray acceleration in supernova remnants from X-ray observations

Abstract Spatially-resolved X-ray spectroscopic observations over the past several years have led to the discovery of non-thermal X-ray emission arising in the shells of most young Galactic supernova remnants, most notably SN 1006 and Cas A. In addition, the X-ray emission from the shells of a few newly-discovered supernova remnants is dominated by a non-thermal component. This emission is thought to be synchrotron emission from electrons shock accelerated to hundreds of TeV, and thus represents strong evidence that cosmic rays are accelerated in SNR shocks. The inferences made using the X-ray observations are corroborated by the detection of TeV γ-rays from two of these remnants. We review the status of the X-ray observations and describe how they can be used to provide insight into the shock-acceleration process.

Search for X-ray evidence of cosmic ray acceleration in SNR shocks

The discovery using ASCA that the featureless X-ray spectrum from the shell of SN 1006 is produced by shock-accelerated TeV electrons, and is thus an indicator of cosmic ray acceleration, has sparked a search for X-ray spectral evidence for cosmic ray acceleration in other remnants, as well as much theoretical work. Thus far evidence for cosmic ray acceleration, in the form of synchrotron continuum components, has been found in several additional remnants, including Cas A, G347.5-0.5, and IC 443. The information provided by both detections and non-detections of these components can be used to investigate the shock acceleration process. We review the latest results of the continuing search for X-ray indicators of cosmic ray acceleration in Galactic and Magellanic Cloud SNR's and what we have learned thus far from it.