Decay Of Neutral Pions Research Articles

THE ACCRETION WIND MODEL OFFERMIBUBBLES. II. RADIATION

In a previous work, we have shown that the formation of the Fermi bubbles can be due to the interaction between winds launched from the hot accretion flow in Sgr A* and the interstellar medium (ISM). In that work, we focus only on the morphology. In this paper we continue our study by calculating the gamma-ray radiation. Some cosmic ray protons (CRp) and electrons must be contained in the winds, which are likely formed by physical processes such as magnetic reconnection. We have performed MHD simulations to study the spatial distribution of CRp, considering the advection and diffusion of CRp in the presence of magnetic field. We find that a permeated zone is formed just outside of the contact discontinuity between winds and ISM, where the collisions between CRp and thermal nuclei mainly occur. The decay of neutral pions generated in the collisions, combined with the inverse Compton scattering of background soft photons by the secondary leptons generated in the collisions and primary CR electrons can well explain the observed gamma-ray spectral energy distribution. Other features such as the uniform surface brightness along the latitude and the boundary width of the bubbles are also explained. The advantage of this accretion wind model is that the adopted wind properties come from the detailed small scale MHD numerical simulation of accretion flows and the value of mass accretion rate has independent observational evidences. The success of the model suggests that we may seriously consider the possibility that cavities and bubbles observed in other contexts such as galaxy clusters may be formed by winds rather than jets.

Pion and σ-meson Properties in a Strong Magnetic Field**Supported by the National Natural Science Foundation of China under Grant Nos. 10935001, 11175004, and 11435001, and the National Key Basic Research Program of China under Grant Nos. G2013CB834400 and G2015CB856900

With the Nambu–Jona-Lasinio (NJL) model we calculate the properties of pion and σ-meson at finite temperature and finite magnetic field. The obtained temperature and magnetic field strength dependence of the constituent quark mass M, the pion and σ-meson masses and the neutral pion decay constant indicates that, in the simple four fermion interaction model, there exists the magnetic catalysis effect. It also shows that the Gell-Mann–Oakes–Renner relation is violated obviously with the increasing of the temperature, and the effect of the magnetic field becomes pronounced only around the critical temperature. The deviation of the critical temperatures obtained with different criteria indicates that the chiral phase transition driven by the temperature in the magnetic field strength region we have considered is in fact a crossover.

Galactic Propagation of Cosmic Rays from Individual Supernova Remnants

It is widely believed that supernova remnants are the best candidate sources for the observed cosmic ray flux up to the knee, i.e. up to ∼PeV energies. Indeed, the gamma-ray spectra of some supernova remnants can be well explained by assuming the decay of neutral pions which are created in hadronic interactions. Therefore, fitting the corresponding gamma spectra allows us to derive the spectra of cosmic rays at the source which are locally injected into our Galaxy. Using these spectra as a starting point, we propagate the cosmic rays through the Galaxy using the publicly available GALPROP code. Here, we will present first results on the contribution of those SNRs to the total cosmic ray flux and discuss implications.

NEUTRINO CONSTRAINTS TO THE DIFFUSE GAMMA-RAY EMISSION FROM ACCRETION SHOCKS

Accretion of gas during the large scale structure formation has been thought to give rise to shocks that can accelerate cosmic rays. This process then results in an isotropic extragalactic gamma-ray emission contributing to the extragalactic gamma-ray background observed by the Fermi-LAT. Unfortunately this emission has been difficult to constrain and thus presents an uncertain foreground to any attempts to extract potential dark matter signal. Recently, IceCube has detected high-energy isotropic neutrino flux which could be of an extragalactic origin. In general, neutrinos can be linked to gamma rays since cosmic-ray interactions produce neutral and charged pions where neutral pions decay into gamma rays, while charged pions decay to give neutrinos. By assuming that isotropic high-energy IceCube neutrinos are entirely produced by cosmic rays accelerated in accretion shocks during the process of structure formation, we obtain the strongest constraint to the gamma-ray emission from large scale structure formation (strong) shocks and find that they can make at best ~20% of the extragalactic gamma-ray background, corresponding to neutrino flux with spectral index 2, or ~10% for spectral index 2.46. Since typical objects where cosmic rays are accelerated in accretion shocks are galaxy clusters, observed high-energy neutrino fluxes can then be used to determine the gamma-ray emission of a dominant cluster type and constrain acceleration efficiency, and thus probe the process of large scale structure formation.

Systematic search for molecular clouds near supernova remnants as sources of very-high-energy γ-ray emission

Supernova remnants accelerate particles up to energies of at least 100 TeV as established by observations in very-high-energy γ-ray astronomy. Molecular clouds in their vicinity provide an increased amount of target material for proton-proton interaction and subsequent neutral pion decay into γ-rays of accelerated hadrons escaping the remnant. Therefore, these molecular clouds are potential γ-ray sources. The γ-ray emission from these clouds provides a unique environment to derive information on the propagation of very-high-energy particles through the interstellar medium as well as on the acceleration of hadrons in supernova remnants. Current Imaging Atmospheric Cherenkov Telescope systems are suitable to explore a large parameter space of the propagation properties depending on the age of the supernova remnant and the distance between the remnant and the nearby molecular cloud.In this paper we present our strategy and results of a systematic search for γ-ray emitting molecular clouds near supernova remnants which are potentially detectable with current experiments in the TeV energy range and explore the prospects of future experiments.

SEARCHING FOR OVERIONIZED PLASMA IN THE GAMMA-RAY-EMITTING SUPERNOVA REMNANT G349.7+0.2

G349.7$+$0.2 is a supernova remnant (SNR) expanding in a dense medium of molecular clouds and interacting with clumps of molecular material emitting gamma rays. We analyzed the gamma-ray data of Large Area Telescope on board Fermi Gamma Ray Space Telescope and detected G349.7$+$0.2 in the energy range of 0.2$-$300 GeV with a significance of $\sim$13$\sigma$ showing no extended morphology. Modeling of the gamma-ray spectrum revealed that the GeV gamma-ray emission dominantly originates from the decay of neutral pions, where the protons follow a broken power-law distribution with a spectral break at $\sim$12 GeV. To search for features of radiative recombination continua in the eastern and western regions of the remnant, we analyzed the Suzaku data of G349.7$+$0.2 and found no evidence for overionized plasma. In this paper we discuss possible scenarios to explain the hadronic gamma-ray emission in G349.7$+$0.2 and the mixed morphology nature of this SNR.

A particle counting EM calorimeter using MAPS

The availability of full size MAPS sensors makes it possible to construct a calorimeter with pixelsize of a few tens of micrometers. This would be small enough to count individual shower particles and would allow a shower shape analysis on an unprecedented, small scale. Interesting features would be tracking capability for particle flow algorithms and a superior discrimination of single photons from neutral and charged pions at high momenta. A small Molière radius together with high transverse resolution would allow to separate close showers, induced by photons from neutral pion decay. A full scale (4 RM, 28 X0) prototype was constructed to demonstrate this. It features 30 micron pixelsize and a longitudinal sampling at 1 radiation length. We will show results from beam tests of this prototype at electron energies of 2 to 200 GeV.

Quarks and anomalies

A nonperturbative understanding of neutral pion decay was an essential step towards the idea that strong interactions are governed by a color gauge theory for quarks. Some aspects of this work and related problems are still important.

Diffuse Galactic gamma-ray emission with H.E.S.S.

Diffuse $\gamma$-ray emission is the most prominent observable signature of celestial cosmic-ray interactions at high energies. While already being investigated at GeV energies over several decades, assessments of diffuse $\gamma$-ray emission at TeV energies remain sparse. After completion of the systematic survey of the inner Galaxy, the H.E.S.S. experiment is in a prime position to observe large-scale diffuse emission at TeV energies. Data of the H.E.S.S. Galactic Plane Survey are investigated in regions off known $\gamma$-ray sources. Corresponding $\gamma$-ray flux measurements were made over an extensive grid of celestial locations. Longitudinal and latitudinal profiles of the observed $\gamma$-ray fluxes show characteristic excess emission not attributable to known $\gamma$-ray sources. For the first time large-scale $\gamma$-ray emission along the Galactic Plane using imaging atmospheric Cherenkov telescopes has been observed. While the background subtraction technique limits the ability to recover modest variation on the scale of the H.E.S.S. field of view or larger, which is characteristic of the inverse Compton scatter-induced Galactic diffuse emission, contributions of neutral pion decay as well as emission from unresolved $\gamma$-ray sources can be recovered in the observed signal to a large fraction. Calculations show that the minimum $\gamma$-ray emission from $\pi^0$-decay represents a significant contribution to the total signal. This detection is interpreted as a mix of diffuse Galactic $\gamma$-ray emission and unresolved sources.

Cosmic ray antiprotons from nearby cosmic accelerators

The antiproton flux measured by PAMELA experiment might have originated from Galactic sources of cosmic rays. These antiprotons are expected to be produced in the interactions of cosmic ray protons and nuclei with cold protons. Gamma rays are also produced in similar interactions inside some of the cosmic accelerators. We consider a few nearby supernova remnants observed by Fermi LAT. Many of them are associated with molecular clouds. Gamma rays have been detected from these sources which most likely originate in decay of neutral pions produced in hadronic interactions. The observed gamma ray fluxes from these SNRs are used to find out their contributions to the observed diffuse cosmic ray antiproton flux near the earth.

CONSTRAINTS ON THE HADRONIC CONTENT OF GAMMA RAY BURSTS

The IceCube high-energy neutrino telescope has been collecting data since 2006. Conversely, hundreds of Gamma Ray Bursts (GRBs) have been detected by the GBM on board Fermi, since its launch in 2008. So far no neutrino event has been associated with a GRB, despite many models predicting the generation of high energy neutrinos through GRB photon interaction with PeV protons in the GRB jet. We use the non-detection of neutrinos to constrain the hadronic content of GRB jets independent of jet model parameters. Assuming a generic particle spectrum of $E^{-\alpha}$ with $\alpha = 2$, we find that the ratio of the energy carried by pions to that in electrons has to be small $f_\pi / f_e \lesssim 0.24$ at 95\% confidence level. A distribution of spectral slopes can lower $f_\pi / f_e$ by orders of magnitude. Another limit, independent of neutrinos, is obtained if one ascribes the measured Fermi/LAT GeV gamma-ray emission to pair-photon cascades of high-energy photons resulting from (the same photon-hadronic interactions and subsequent) neutral pion decays. Based on the generally observed MeV to GeV GRB fluence ratio of $\approx 10$, we show that $f_\pi / f_e \lesssim 0.3$. In some bursts, where this ratio is as low as unity, $f_\pi / f_e \lesssim 0.03$. These findings add to the mounting doubts regarding the presence of PeV protons in GRB jets.

Lepton-hadronic processes and high-energy neutrinos in NGC 1275

AbstractThe nearby active galaxy NGC 1275, has been widely detected from radio to gamma rays. Its spectral energy distribution (SED) shows a double-peak feature, which is well explained by synchrotron self-Compton (SSC) model. However, recent TeV detections might suggest that very-high-energy γ-rays (E⩾100 GeV) may not have a leptonic origin. We test a lepto-hadronic model to describe the whole SED through SSC emission and neutral pion decay resulting from pγ interactions. Also, we estimate the neutrino events expected in a km3 Cherenkov telescope.

RECOMBINING PLASMA IN THE GAMMA-RAY-EMITTING MIXED-MORPHOLOGY SUPERNOVA REMNANT 3C 391

A group of middle-aged mixed-morphology (MM) supernova remnants (SNRs) interacting with molecular clouds (MC) has been discovered as strong GeV gamma-ray emitters by Large Area Telescope on board Fermi Gamma Ray Space Telescope (Fermi-LAT). The recent observations of the Suzaku X-ray satellite have revealed that some of these interacting gamma-ray emitting SNRs, such as IC443, W49B, W44, and G359.1-0.5, have overionized plasmas. 3C 391 (G31.9+0.0) is another Galactic MM SNR interacting with MC. It was observed in GeV gamma rays by Fermi-LAT as well as in the 0.3 $-$ 10.0 keV X-ray band by Suzaku. In this work, 3C 391 was detected in GeV gamma rays with a significance of $\sim$ 18 $\sigma$ and we showed that the GeV emission is point-like in nature. The GeV gamma-ray spectrum was shown to be best explained by the decay of neutral pions assuming that the protons follow a broken power-law distribution. We revealed radiative recombination structures of silicon and sulfur from 3C 391 using Suzaku data. In this paper we discuss the possible origin of this type of radiative plasma and hadronic gamma rays.

Counting gamma rays in the directions of galaxy clusters

Emission from active galactic nuclei (AGNs) and from neutral pion decay are the two most natural mechanisms that could establish a galaxy cluster as a source of gamma rays in the GeV regime. We revisit this problem by using 52.5 months of Fermi-LAT data above 10 GeV and stacking 55 clusters from the HIFLUCGS sample of the X-ray brightest clusters. The choice of >10 GeV photons is optimal from the point of view of angular resolution, while the sample selection optimizes the chances of detecting signatures of neutral pion decay, arising from hadronic interactions of relativistic protons with an intracluster medium, which scale with the X-ray flux. In the stacked data we detected a signal for the central 0.25 deg circle at the level of 4.3σ. Evidence for a spatial extent of the signal is marginal. A subsample of cool-core clusters has a higher count rate of 1.9 ± 0.3 per cluster compared to the subsample of non-cool core clusters at 1.3 ± 0.2. Several independent arguments suggest that the contribution of AGNs to the observed signal is substantial, if not dominant. No strong support for the large contribution of pion decay was found. In terms of a limit on the relativistic proton energy density, we derived an upper limit of ≃2% relative to the gas thermal energy density, provided that the spectrum of relativistic protons is hard (s = 4.1 in dN ∝ p−sd3p). This estimate assumes that relativistic and thermal components are mixed. For softer spectra the upper limits are weaker and equal to 3% for s = 4.2, 4% for s = 4.3, and 6% for s = 4.4.

Estimates of relativistic electron and proton energy densities in starburst galactic nuclei from radio measurements

The energy density of energetic protons, U_p, in several nearby starburst nuclei (SBNs) has been directly deduced from gamma-ray measurements of the radiative decay of neutral pions produced in interactions with ambient protons. Lack of sufficient sensitivity and spatial resolution makes this direct deduction unrealistic in the foreseeable future for even moderately distant SBNs. A more viable indirect method for determining U_p in star-forming galaxies is to use its theoretically based scaling to the energy density of energetic electrons, U_e, which can be directly deduced from radio synchrotron and possibly also nonthermal hard X-ray emission. In order to improve the quantitative basis and diagnostic power of this leptonic method we reformulate and clarify its main aspects. Doing so we obtain a basic expression for the ratio U_p/U_e in terms of the proton and electron masses and the power-law indices that characterize the particle spectral distributions in regions where the total particle energy density is at equipartition with that of the mean magnetic field. We also express the field strength and the particle energy density in the equipartition region in terms of the region's size, mean gas density, IR and radio fluxes, and distance from the observer, and determine values of U_p in a sample of nine nearby and local SBNs.

SIMULATING THREE-DIMENSIONAL NONTHERMAL HIGH-ENERGY PHOTON EMISSION IN COLLIDING-WIND BINARIES

Massive stars in binary systems have long been regarded as potential sources of high-energy gamma rays.The emission is principally thought to arise in the region where the stellar winds collide and accelerate relativistic particles which ubsequently emit gamma rays. On the basis of a three-dimensional distribution function of high-energy particles in the wind collision region - as obtained by a numerical hydrodynamics and particle transport model - we present the computation of the three-dimensional nonthermal photon emission for a given line of sight. Anisotropic inverse Compton emission is modelled using the target radiation field of both stars. Photons from relativistic bremsstrahlung and neutral pion decay are computed on the basis of local wind plasma densities. We also consider photon photon opacity effects due to the dense radiation fields of the stars. Results are shown for different stellar separations of a given binary system comprising of a B star and a Wolf-Rayet star. The influence of orbital orientation with respect to the line of sight is also studied by using different orbital viewing angles. For the chosen electron-proton injection ratio of 0.01, we present the ensuing photon emission in terms of two-dimensional projections maps, spectral energy distributions and integrated photon flux values in various energy bands. Here, we find a transition from hadron-dominated to lepton-dominated high-energy emission with increasing stellar separations. In addition, we confirm findings from previous analytic modeling that the spectral energy distribution varies significantly with orbital orientation.

IMPULSIVE AND LONG DURATION HIGH-ENERGY GAMMA-RAY EMISSION FROM THE VERY BRIGHT 2012 MARCH 7 SOLAR FLARES

The Fermi Large Area Telescope (LAT) observed two bright X-class solar flares on 2012 March 7, and detected gamma-rays up to 4 GeV. We detected gamma-rays both during the impulsive and temporally-extended emission phases, with emission above 100 MeV lasting for approximately 20 hours. Accurate localization of the gamma-ray production site(s) coincide with the solar active region from which X-ray emissions associated with these flares originated. Our analysis of the >100 MeV gamma-ray emission shows a relatively rapid monotonic decrease in flux during the first hour of the impulsive phase, and a much slower, almost monotonic decrease in flux for the next 20 hours. The spectra can be adequately described by a power law with a high energy exponential cutoff, or as resulting from the decay of neutral pions produced by accelerated protons and ions with an isotropic power-law energy distribution. The required proton spectrum has a number index ~3, with minor variations during the impulsive phase, while during the temporally extended phase the spectrum softens monotonically, starting with index ~4. The >30 MeV proton flux and spectra observed near the Earth by the GOES satellites also show a monotonic flux decrease and spectral softening during the extended phase, but with a harder spectrum, with index ~3. Based on the Fermi-LAT and GOES observations of the flux and spectral evolution of these bright flares, we explore the relative merits of prompt and continuous acceleration scenarios, hadronic and leptonic emission processes, and acceleration at the solar corona by the fast Coronal Mass Ejections (CME) as explanations for the observations. We conclude that the most likely scenario is continuous acceleration of protons in the solar corona which penetrate the lower solar atmosphere and produce pions that decay into gamma-rays.

THE HADRONIC ORIGIN OF THE HARD GAMMA-RAY SPECTRUM FROM BLAZAR 1ES 1101-232

The very hard $\gamma$-ray spectrum from distant blazars challenges the traditional synchrotron self-Compton (SSC) model, which may indicate that there is the contribution of an additional high-energy component beyond the SSC emission. In this paper, we study the possible origin of the hard $\gamma$-ray spectrum from distant blazars. We develop a model to explain the hard $\gamma$-ray spectrum from blazar 1ES 1101-232. In the model, the optical and X-ray radiation would come from the synchrotron radiation of primary electrons and secondary pairs, the GeV emission would be produced by the SSC process, however, the hard $\gamma$-ray spectrum would originate from the decay of neutral pion produced through proton-photon interactions with the synchrotron radiation photons within the jet. Our model can explain the observed SED of 1ES 1101-232 well, especially the very hard $\gamma$-ray spectrum. However, our model requires the very large proton power to efficiently produce the $\gamma$-ray through proton-photon interactions.

FERMI-LAT OBSERVATIONS OF SUPERNOVA REMNANTS INTERACTING WITH MOLECULAR CLOUDS: W41, MSH 17-39, AND G337.7-0.1

We report the detection of gamma-ray emission coincident with three supernova remnants (SNRs) using data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. W41, MSH 17-39, and G337.0-0.1 are SNRs known to be interacting with molecular clouds, as evidenced by observations of hydroxyl (OH) maser emission at 1720 MHz in their directions and other observational information. SNR shocks are expected to be sites of cosmic ray acceleration, and clouds of dense material can provide effective targets for production of gamma-rays from neutral pion-decay. The observations reveal unresolved sources in the direction of G337.0-0.1, and MSH 17-39, and an extended source coincident with W41. We model their broadband emission (radio to gamma-ray) using a simple one-zone model, and after considering scenarios in which the MeV-TeV sources originate from either neutral pion-decay or leptonic emission, we conclude that the gamma-rays are most likely produced through the hadronic channel.

Acceleration of cosmic rays and gamma-ray emission from supernova remnants in the Galaxy

Galactic cosmic rays are believed to be accelerated at supernova remnant shocks. Though very popular and robust, this conjecture still needs a conclusive proof. The strongest support to this idea is probably the fact that supernova remnants are observed in gamma-rays, which are indeed expected as the result of the hadronic interactions between the cosmic rays accelerated at the shock and the ambient gas. However, also leptonic processes can, in most cases, explain the observed gamma-ray emission. This implies that the detections in gamma rays do not necessarily mean that supernova remnants accelerate cosmic ray protons. To overcome this degeneracy, the multi-wavelength emission (from radio to gamma rays) from individual supernova remnants has been studied and in a few cases it has been possible to ascribe the gamma-ray emission to one of the two processes (hadronic or leptonic). Here we adopt a different approach and, instead of a case-by-case study we aim for a population study and we compute the number of supernova remnants which are expected to be seen in TeV gamma rays above a given flux under the assumption that these objects indeed are the sources of cosmic rays. The predictions found here match well with current observational results, thus providing a novel consistency check for the supernova remnant paradigm for the origin of galactic cosmic rays. Moreover, hints are presented for the fact that particle spectra significantly steeper than E^-2 are produced at supernova remnants. Finally, we expect that several of the supernova remnants detected by H.E.S.S. in the survey of the galactic plane should exhibit a gamma-ray emission dominated by hadronic processes (i.e. neutral pion decay). The fraction of the detected remnants for which the leptonic emission dominates over the hadronic one depends on the assumed values of the physical parameters and can be as high as roughly a half.