arXiv subjects

A. W. Strong

Publications and source records attributed to A. W. Strong.

At least 37 records · Page 2Linked to original sources

arXiv2014

Spiral Arms as Cosmic Ray Source Distributions

There is evidence that the distribution of suspected cosmic ray sources are associated with the spiral arm structure of galaxies. It is yet not clearly understood what effect such a cosmic ray source distribution has on the particle transport in our Galaxy. We use the PICARD code to perform high-resolution 3D simulations of electrons and protons in galactic propagation scenarios that include four-arm and two-arm logarithmic spiral cosmic ray source distributions with and without a central bar structure as well as the spiral arm configuration of the NE2001 model for the distribution of free electrons in the Milky Way. Results of these simulation are compared to an axisymmetric radial source distribution. Also, effects on the cosmic ray flux and spectra due to different positions of the Earth relative to the spiral structure are studied. We find that high energy electrons are strongly confined to their sources and the obtained spectra largely depend on the Earth's position relative to the spiral arms. Similar finding have been obtained for low energy protons and electrons albeit at smaller magnitude. We find that even fractional contributions of a spiral arm component to the total cosmic ray source distribution influences the spectra on the Earth. This is apparent when compared to an axisymmetric radial source distribution as well as with respect to the Earth's position relative to the spiral arm structure. We demonstrate that the presence of a Galactic bar manifests itself as an overall excess of low energy electrons at the Earth. Using a spiral arm geometry as a cosmic ray source distributions offers a genuine new quality of modelling and is used to explain features in cosmic ray spectra at the Earth that are else-wise attributed to other propagation effects. We show that realistic cosmic ray propagation scenarios have to acknowledge non-axisymmetric source distributions.[Abbreviated]

astro-ph.HE↗

arXiv2013

A new 3D transport and radiation code for galactic cosmic rays

We show the necessity for a new approach towards comprehensive and consistent simulations of the propagation of galactic cosmic rays. Our developments are optimised for addressing the spatially 3-dimensional inhomogeneous diffusion problem and utilise contemporary numerical methods. We aim to address the transport problem in a full 3-dimensional environment. For that, we test the transition from 2D to 3D simulation results within an existing propagation code. We present sub-kpc scale simulations that allow the investigation of small- scale structures regarding different model conditions such as variety regarding non-axisymmetric cosmic ray source distributions. These results are discussed critically and motivate our development of a new transport code for galactic cosmic rays. The capabilities of this code are outlined.

astro-ph.IM↗

arXiv2013

Determining the Spectrum of Cosmic Rays in Interstellar Space from the Diffuse Galactic Gamma-Ray Emissivity

More than 90% of the Galactic gas-related gamma-ray emissivity above 1 GeV is attributed to the decay of neutral pions formed in collisions between cosmic rays and interstellar matter, with lepton-induced processes becoming increasingly important below 1 GeV. Given the high-quality measurements of the gamma-ray emissivity of local interstellar gas between ~50 MeV and ~4 GeV obtained with the Large Area Telescope on board the Fermi space observatory, it is timely to re-investigate this topic in detail, including the hadronic production mechanisms. The emissivity spectrum will allow the interstellar cosmic-ray spectrum to be determined reliably, providing a reference for origin and propagation studies as well as input to solar modulation models. A method for such an analysis and illustrative results are presented.

astro-ph.HE↗

arXiv2013

On the Physics Connecting Cosmic Rays and Gamma Rays: Towards Determining the Interstellar Cosmic Ray Spectrum

Secondary nuclear production physics is receiving increased attention given the high-quality measurements of the gamma-ray emissivity of local interstellar gas between ~50 MeV and ~40 GeV, obtained with the Large Area Telescope on board the Fermi space observatory. More than 90% of the gas-related emissivity above 1 GeV is attributed to gamma-rays from the decay of neutral pions formed in collisions between cosmic rays and interstellar matter, with lepton-induced processes becoming increasingly important below 1 GeV. The elementary kinematics of neutral pion production and decay are re-examined in light of two physics questions: does isobaric production follow a scaling behavior? and what is the minimum proton kinetic energy needed to make a gamma-ray of a certain energy formed through intermediate pi0 production? The emissivity spectrum will allow the interstellar cosmic-ray spectrum to be determined reliably, providing a reference for origin and propagation studies as well as input to solar modulation models. A method for such an analysis and illustrative results are presented.

astro-ph.HE↗

arXiv2013

Deexcitation nuclear gamma-ray line emission from low-energy cosmic rays in the inner Galaxy

Recent observations of high ionization rates of molecular hydrogen in diffuse interstellar clouds point to a distinct low-energy cosmic-ray component. Supposing that this component is made of nuclei, two models for the origin of such particles are explored and low-energy cosmic-ray spectra are calculated which, added to the standard cosmic ray spectra, produce the observed ionization rates. The clearest evidence of the presence of such low-energy nuclei between a few MeV per nucleon and several hundred MeV per nucleon in the interstellar medium would be a detection of nuclear γ-ray line emission in the range E_ 0.1 - 10 MeV, which is strongly produced in their collisions with the interstellar gas and dust. Using a recent γ-ray cross section compilation for nuclear collisions, γ-ray line emission spectra are calculated alongside with the high-energy γ-ray emission due to π 0 decay, the latter providing normalization of the absolute fluxes by comparison with Fermi-LAT observations of the diffuse emission above E γ= 0.1 GeV. Our predicted fluxes of strong nuclear γ-ray lines from the inner Galaxy are well below the detection sensitivies of INTEGRAL, but a detection, especially of the 4.4-MeV line, seems possible with new-generation γ-ray telescopes based on available technology. We predict also strong γ-ray continuum emission in the 1-8 MeV range, which in a large part of our model space for low-energy cosmic rays exceeds considerably estimated instrument sensitivities of future telescopes.

astro-ph.HE↗

arXiv2012

Continuum Halos in Nearby Galaxies -- an EVLA Survey (CHANG-ES) -- I: Introduction to the Survey

We introduce a new survey to map the radio continuum halos of a sample of 35 edge-on spiral galaxies at 1.5 GHz and 6 GHz in all polarization products. The survey is exploiting the new wide bandwidth capabilities of the Karl G. Jansky Very Large Array (i.e. the Expanded Very Large Array, or EVLA) in a variety of array configurations (B, C, and D) in order to compile the most comprehensive data set yet obtained for the study of radio halo properties. This is the first survey of radio halos to include all polarization products. In this first paper, we outline the scientific motivation of the survey, the specific science goals, and the expected improvements in noise levels and spatial coverage from the survey. Our goals include investigating the physical conditions and origin of halos, characterizing cosmic ray transport and wind speed, measuring Faraday rotation and mapping the magnetic field, probing the in-disk and extraplanar far-infrared - radio continuum relation, and reconciling non-thermal radio emission with high-energy gamma-ray models. The sample size allows us to search for correlations between radio halos and other properties, including environment, star formation rate, and the presence of AGNs. In a companion paper (Paper II) we outline the data reduction steps and present the first results of the survey for the galaxy, NGC 4631.

astro-ph.CO↗

arXiv2012

Continuum Halos in Nearby Galaxies -- an EVLA Survey (CHANG-ES) -- II: First Results on NGC 4631

We present the first results from the CHANG-ES survey, a new survey of 35 edge-on galaxies to search for both in-disk as well as extra-planar radio continuum emission. The motivation and science case for the survey are presented in a companion paper (Paper I). In this paper (Paper II), we outline the observations and data reduction steps required for wide-band calibration and mapping of EVLA data, including polarization, based on C-array test observations of NGC 4631. With modest on-source observing times (30 minutes at 1.5 GHz and 75 minutes at 6 GHz for the test data) we have achieved best rms noise levels of 22 and 3.5 $μ$Jy beam$^{-1}$ at 1.5 GHz and 6 GHz, respectively. New disk-halo features have been detected, among them two at 1.5 GHz that appear as loops in projection. We present the first 1.5 GHz spectral index map of NGC 4631 to be formed from a single wide-band observation in a single array configuration. This map represents tangent slopes to the intensities within the band centered at 1.5 GHz, rather than fits across widely separated frequencies as has been done in the past and is also the highest spatial resolution spectral index map yet presented for this galaxy. The average spectral index in the disk is $\barα_{1.5 GHz}\,=\,-0.84\,\pm\,0.05$ indicating that the emission is largely non-thermal, but a small global thermal contribution is sufficient to explain a positive curvature term in the spectral index over the band. Two specific star forming regions have spectral indices that are consistent with thermal emission. Polarization results (uncorrected for internal Faraday rotation) are consistent with previous observations and also reveal some new features. On broad scales, we find strong support for the notion that magnetic fields constrain the X-ray emitting hot gas.

astro-ph.GA↗

arXiv2012

Galactic annihilation emission from nucleosynthesis positrons

The Galaxy hosts a widespread population of low-energy positrons revealed by successive generations of gamma-ray telescopes through a bright annihilation emission from the bulge region, with a fainter contribution from the inner disk. The exact origin of these particles remains currently unknown. We estimate the contribution to the annihilation signal of positrons generated in the decay of radioactive 26Al, 56Ni and 44Ti. We adapted the GALPROP propagation code to simulate the transport and annihilation of radioactivity positrons in a model of our Galaxy. Using plausible source spatial distributions, we explored several possible propagation scenarios to account for the large uncertainties on the transport of ~1MeV positrons in the interstellar medium. We then compared the predicted intensity distributions to the INTEGRAL/SPI observations. We obtain similar intensity distributions with small bulge-to-disk ratios, even for extreme large-scale transport prescriptions. At least half of the positrons annihilate close to their sources, even when they are allowed to travel far away. In the high-diffusion, ballistic case, up to 40% of them escape the Galaxy. In proportion, this affects bulge positrons more than disk positrons because they are injected further off the plane in a tenuous medium, while disk positrons are mostly injected in the dense molecular ring. The predicted intensity distributions are fully consistent with the observed longitudinally-extended disk-like emission, but the transport scenario cannot be strongly constrained by the current data. Nucleosynthesis positrons alone cannot account for the observed annihilation emission in the frame of our model. An additional component is needed to explain the strong bulge contribution, and the latter is very likely concentrated in the central regions if positrons have initial energies in the 100keV-1MeV range.

astro-ph.HE↗

arXiv2012

Characterizing Cosmic-Ray Propagation in Massive Star-Forming Regions: The Case of 30 Doradus and the Large Magellanic Cloud

Using infrared, radio, and gamma-ray data,we investigate the propagation characteristics of cosmic-ray (CR) electrons and nuclei in the 30 Doradus (30\,Dor) star-forming region in the Large Magellanic Cloud (LMC) using a phenomenological model based on the radio-far-infrared correlation within galaxies. Employing a correlation analysis, we derive an average propagation length of \sim 100-140 pc for \sim 3 GeV CR electrons resident in 30 Dor from consideration of the radio and infrared data. Assuming that the observed gamma-ray emission towards 30 Dor is associated with the star-forming region, and applying the same methodology to the infrared and gamma-ray data, we estimate a \sim 20 GeV propagation length of 200-320 pc for the CR nuclei. This is approximately twice as large as for\sim 3 GeV CR electrons, corresponding to a spatial diffusion coefficient that is \sim 4 times higher, scaling as (R/GV)^δ with δ\approx 0.7-0.8 depending on the smearing kernel used in the correlation analysis. This value is in agreement with the results found by extending the correlation analysis to include \sim 70 GeV CR nuclei traced by the 3-10 GeV gamma-ray data (δ\approx 0.66+/-0.23). Using the mean age of the stellar populations in 30 Dor and the results from our correlation analysis, we estimate a diffusion coefficient D_{R} \approx 0.9-1.0 \times10^27 (R/GV)^{0.7} cm^2 s^-1. We compare the values of the CR electron propagation length and surface brightness for 30 Dor and the LMC as a whole with those of entire disk galaxies. We find that the trend of decreasing average CR propagation distance with increasing disk-averaged star formation activity holds for the LMC, and extends down to single star-forming regions, at least for the case of 30 Dor.

astro-ph.CO↗

arXiv2011

The interstellar cosmic-ray electron spectrum from synchrotron radiation and direct measurements

Context: The relation between Galactic cosmic-ray electrons, magnetic fields and synchrotron radiation. Aims: We exploit synchrotron radiation to constrain the low-energy interstellar electron spectrum, using various radio surveys and connecting with electron data from Fermi-LAT and other experiments. Methods: The GALPROP programme for cosmic-ray propagation, gamma-ray and synchrotron radiation is used. Secondary electrons and positrons are included. Propagation models based on cosmic-ray and gamma-ray data are tested against synchrotron data from 22 MHz to 94 GHz. Results: The synchrotron data confirm the need for a low-energy break in the cosmic-ray electron injection spectrum. The interstellar spectrum below a few GeV has to be lower than standard models predict, and this suggests less solar modulation than usually assumed. Reacceleration models are more difficult to reconcile with the synchrotron constraints. We show that secondary leptons are important for the interpretation of synchrotron emission. We also consider a cosmic-ray propagation origin for the low-energy break. Conclusions: Exploiting the complementary information on cosmic rays and synchrotron gives unique and essential constraints on electrons, and has implications for gamma rays. This connection is especially relevant now in view of the ongoing PLANCK and Fermi missions.

astro-ph.HE↗

arXiv2011

Interstellar gamma rays and cosmic rays: new insights from Fermi-LAT AND INTEGRAL

In over two years of operation Fermi-LAT has revolutionized our knowledge of the gamma-ray sky. Interstellar gamma rays are part of this new era and allow unprecedented tests for models of cosmic rays in the Galaxy. The extension to lower energies with INTEGRAL/SPI data is also evolving. The global multiwavelength luminosity of the Milky Way has been derived, with implications for the Galactic energy balance and the radio-FIR correlation.

astro-ph.HE↗

arXiv2011

Constraints on cosmic-ray propagation models from a global Bayesian analysis

Research in many areas of modern physics such as, e.g., indirect searches for dark matter and particle acceleration in SNR shocks, rely heavily on studies of cosmic rays (CRs) and associated diffuse emissions (radio, microwave, X-rays, gamma rays). While very detailed numerical models of CR propagation exist, a quantitative statistical analysis of such models has been so far hampered by the large computational effort that those models require. Although statistical analyses have been carried out before using semi-analytical models (where the computation is much faster), the evaluation of the results obtained from such models is difficult, as they necessarily suffer from many simplifying assumptions, The main objective of this paper is to present a working method for a full Bayesian parameter estimation for a numerical CR propagation model. For this study, we use the GALPROP code, the most advanced of its kind, that uses astrophysical information, nuclear and particle data as input to self-consistently predict CRs, gamma rays, synchrotron and other observables. We demonstrate that a full Bayesian analysis is possible using nested sampling and Markov Chain Monte Carlo methods (implemented in the SuperBayeS code) despite the heavy computational demands of a numerical propagation code. The best-fit values of parameters found in this analysis are in agreement with previous, significantly simpler, studies also based on GALPROP.

astro-ph.HE↗

arXiv2010

Global cosmic-ray related luminosity and energy budget of the Milky Way

We use the GALPROP code for cosmic-ray (CR) propagation to calculate the broad-band luminosity spectrum of the Milky Way related to CR propagation and interactions in the interstellar medium. This includes gamma-ray emission from the production and subsequent decay of neutral pions, bremsstrahlung, and inverse Compton scattering, and synchrotron radiation. The Galaxy is found to be nearly a CR electron calorimeter, but {\it only} if gamma ray emitting processes are taken into account. Synchrotron radiation alone accounts for only one third of the total electron energy losses with ~10-20% of the total synchrotron emission from secondary CR electrons and positrons. The relationship between far-infrared and radio luminosity that we find from our models is consistent with that found for galaxies in general. The results will be useful for understanding the connection between diffuse emissions from radio through gamma rays in ``normal'' (non-AGN dominated) galaxies, as well as for estimating the broad-band extragalactic diffuse background from these kinds of galaxies.

astro-ph.HE↗

arXiv2009

On possible interpretations of the high energy electron-positron spectrum measured by the Fermi Large Area Telescope

The Fermi-LAT experiment recently reported high precision measurements of the spectrum of cosmic-ray electrons-plus-positrons (CRE) between 20 GeV and 1 TeV. The spectrum shows no prominent spectral features, and is significantly harder than that inferred from several previous experiments. Here we discuss several interpretations of the Fermi results based either on a single large scale Galactic CRE component or by invoking additional electron-positron primary sources, e.g. nearby pulsars or particle Dark Matter annihilation. We show that while the reported Fermi-LAT data alone can be interpreted in terms of a single component scenario, when combined with other complementary experimental results, specifically the CRE spectrum measured by H.E.S.S. and especially the positron fraction reported by PAMELA between 1 and 100 GeV, that class of models fails to provide a consistent interpretation. Rather, we find that several combinations of parameters, involving both the pulsar and dark matter scenarios, allow a consistent description of those results. We also briefly discuss the possibility of discriminating between the pulsar and dark matter interpretations by looking for a possible anisotropy in the CRE flux.

astro-ph.HE↗

arXiv2009

Cosmic-ray electrons, synchrotron and magnetic fields in the Galaxy

The cosmic-ray propagation code GALPROP has recently been enhanced in the areas of Galactic magnetic field and synchrotron radiation. A full 3D treatment is now implemented. We describe these enhancements and show applications to the interpretation of non-thermal radio surveys and gamma rays, to probe the Galactic cosmic-ray electron spectrum and magnetic fields.

astro-ph.GA↗

arXiv2009

The GALPROP Cosmic-Ray Propagation Code

The Galactic cosmic-ray propagation code GALPROP is designed to make predictions of many kinds of data self-consistently, including direct cosmic-ray measurements, gamma rays and synchrotron radiation. In the decade since its conception it has undergone considerable development, which is continuing. A public version was made available a few years ago, supported by a website. We describe the new features of the current version which will become part of the next public release. Plans for future developments are also mentioned.

astro-ph.HE↗

arXiv2009

A Galactic Cosmic-Ray Database

Despite a century of cosmic-ray measurements there seems to have been no attempt to collect these data systematically for use by the CR community. The result is that everyone makes their own collection as required, a large duplication of effort. Therefore we have started a project to place published Galactic CR measurements in a database available online. It currently addresses energies up to 100 TeV, including elemental spectra, secondary/primary nuclei ratios, antiprotons, electrons and positrons. It is updated regularly as data appears in the literature. It is supported by access software. The community is encouraged to participate by providing data, pointing out errors or omissions, and suggestions for improvements.

astro-ph.HE↗

arXiv2009

Large-scale Galactic diffuse gamma rays observed with the Fermi Gamma-Ray Space Telescope

The LAT instrument on the Fermi Gamma-Ray Space Telescope is performing an all-sky survey from 20 MeV to 300 GeV with unprecedented statistics and angular resolution. This is providing a wealth of new information on the non-thermal emission from the Galactic interstellar medium with implications for cosmic rays and Galactic structure. First results at intermediate latitudes have already shown good agreement with predictions based on direct measurements of cosmic rays, suggesting that at least the local (within about 1 kpc from the Sun) gamma-ray emission is understood. We will present the first spectra from regions over the sky using the LAT data, and profiles for selected energies. The aim here is to evaluate the agreement with the models and assess what we can expect to learn as this analysis matures.

astro-ph.HE↗