Cosmic-ray Density Research Articles

Radiative cooling rates, ion fractions, molecule abundances, and line emissivities including self-shielding and both local and metagalactic radiation fields

ABSTRACT We use the spectral synthesis code cloudy to tabulate the properties of gas for an extensive range in redshift (z = 0–9), temperature (log T[K] = 1–9.5), metallicity (log Z/Z⊙ = −4 – +0.5, Z = 0), and density ($\log n_{\mathrm{H}}[\, \mathrm{cm}^{-3}] = -8$ − +6). This therefore includes gas with properties characteristic of the interstellar, circumgalactic, and intergalactic media. The gas is exposed to a redshift-dependent UV/X-ray background, while for the self-shielded lower-temperature gas (i.e. ISM gas), an interstellar radiation field and cosmic rays are added. The radiation field is attenuated by a density- and temperature-dependent column of gas and dust. Motivated by the observed star formation law, this gas column density also determines the intensity of the interstellar radiation field and the cosmic ray density. The ionization balance, molecule fractions, cooling rates, line emissivities, and equilibrium temperatures are calculated self-consistently. We include dust, cosmic rays, and the interstellar radiation field step-by-step to study their relative impact. These publicly available tables are ideal for hydrodynamical simulations. They can be used stand alone or coupled to a non-equilibrium network for a subset of elements. The release includes a C routine to read in and interpolate the tables, as well as an easy-to-use python graphical user interface to explore the tables.

On the Gamma-Ray Emission of W44 and Its Surroundings

Abstract We present the analysis of 9.7 yr Fermi-LAT data of the middle-aged supernova remnant W44 and the massive molecular gas complex that surrounds it. We derived a high-quality spectral energy distribution of gamma-radiation of the shell over three decades. The very hard spectrum below 1 GeV supports the earlier claims regarding the hadronic origin of radiation. We also confirm the presence of two extended γ-ray structures located at two opposite edges of the remnant along its major axis. Based on the high-resolution gas maps, we demonstrate that the gamma-ray structures are caused by the enhanced cosmic-ray density rather than the gradient of the gas distribution. We argue that the revealed cosmic-ray “clouds” suggest an anisotropic character of the escape of high-energy particles from the shell along the magnetic field of the remnant.

All-sky angular power spectrum – I. Estimating brightness temperature fluctuations using the 150-MHz TGSS survey

ABSTRACT Measurements of the Galactic synchrotron emission are important for the 21-cm studies of the epoch of reionization. The study of synchrotron emission is also useful for quantifying the fluctuations in the magnetic field and the cosmic-ray electron density of the turbulent interstellar medium (ISM) of our Galaxy. Here, we present the all-sky angular power spectrum (Cℓ) measurements of the diffuse synchrotron emission obtained using the TIFR GMRT Sky Survey (TGSS) at 150 MHz. We estimate Cℓ using visibility data both before and after subtracting the modelled point sources. The amplitude of the measured Cℓ decreases significantly after subtracting the point sources, and it is slightly higher in the Galactic plane for the residual data. The residual Cℓ is most likely to be dominated by the Galactic synchrotron emission. The amplitude of the residual Cℓ decreases significantly away from the Galactic plane. We find that the measurements are quite symmetric in the Northern and Southern hemispheres except in the latitude range 15°−30°, which is the transition region from the disc-dominated to the diffuse halo-dominated region. A comparison between this interferometric measurement and the scaled version of the Haslam rms map at 150 MHz shows that the correlation coefficient (r) is greater than 0.5 for most of the latitude ranges considered here. This indicates that the TGSS is quite sensitive to the diffuse Galactic synchrotron radiation.

Probing the sea of galactic cosmic rays with Fermi-LAT

High energy $\gamma$ rays from Giant Molecular Clouds (GMCs) carry direct information about the spatial and energy distributions of Galactic Cosmic Rays (CRs). The recently released catalogs of GMCs contain sufficiently massive clouds to be used as barometers for probing, through their $\gamma$-ray emission, the density of CRs throughout the Galactic Disk. Based on the data of \fermi{}, we report the discovery of $\gamma$-ray signals from nineteen GMCs located at distances up to 12.5 kpc. The galactocentric radial distribution of the CR density derived from the $\gamma$-ray and CO observations of these objects, as well as from some nearby clouds that belong to the Gould Belt complex, unveil a homogeneous \textquotedblleft sea" of CRs with a constant density and spectral shape close to the flux of directly (locally) measured CRs. We found noticeable deviations from the \textquotedblleft sea level" only in some locations characterized by enhanced CR density in the galactocentric 4--6 kpc ring. Furthermore, we found a hint for fluctuations of the CR density in different locations within the same 4--6 kpc ring. The confirmation of this result with the next-generation $\gamma$-ray detectors based on the higher quality data and denser coverage of galactocentric distances, would have dramatic implications for the understanding of the origin of Galactic CRs.

Constraints on the Emission of Gamma-Rays from M31 with HAWC

Abstract Cosmic rays, along with stellar radiation and magnetic fields, are known to make up a significant fraction of the energy density of galaxies such as the Milky Way. When cosmic rays interact in the interstellar medium, they produce gamma-ray emission which provides an important indication of how the cosmic rays propagate. Gamma-rays from the Andromeda galaxy (M31), located 785 kpc away, provide a unique opportunity to study cosmic-ray acceleration and diffusion in a galaxy with a structure and evolution very similar to the Milky Way. Using 33 months of data from the High Altitude Water Cherenkov Observatory, we search for teraelectronvolt gamma-rays from the galactic plane of M31. We also investigate past and present evidence of galactic activity in M31 by searching for Fermi bubble-like structures above and below the galactic nucleus. No significant gamma-ray emission is observed, so we use the null result to compute upper limits on the energy density of cosmic rays >10 TeV in M31.

A more detailed look at Galactic magnetic field models: using free–free absorption in HII regions

Context. Cosmic rays (CRs) and the Galactic magnetic field (GMF) are fundamental actors in many processes in the Milky Way. The observed interaction product of these actors is Galactic synchrotron emission integrated over the line of sight (LOS). A comparison to simulations can be made with this tracer using existing GMF models and CR density models. This probes the GMF strength and morphology and the CR density. Aims. Our aim is to provide insight into the Galactic CR density and the distribution and morphology of the GMF strength by exploring and explaining the differences between the simulations and observations of synchrotron intensity. Methods. At low radio frequencies HII regions become opaque due to free–free absorption. Using these HII regions we can measure the synchrotron intensity over a part of the LOS through the Galaxy. The measured intensity per unit path length, that is, the emissivity, for HII regions at different distances, allows us to probe the variation in synchrotron emission not only across the sky but also in the third dimension of distance. Performing these measurements on a large scale is one of the new applications of the window opened by current low-frequency arrays. Using a number of existing GMF models in conjunction with the Galactic CR modeling code GALPROP, we can simulate these synchrotron emissivities. Results. We present an updated catalog, compiled from the literature, of low-frequency absorption measurements of HII regions, their distances, and electron temperatures. We report a simulated emissivity that shows a compatible trend for HII regions that are near the observer. However, we observe a systematically increasing synchrotron emissivity for HII regions that are far from the observer, which is not compatible with the values simulated by the GMF models and GALPROP. Conclusions. Current GMF models plus a GALPROP generated CR density model cannot explain low-frequency absorption measurements. One possibility is that distances to all HII regions catalogued at the kinematic “far” distance are erroneously determined, although this is unlikely since it ignores all evidence for far distances in the literature. However, a detection bias due to the nature of this tracer requires us to keep in mind that certain sources may be missed in an observation. The other possibilities are an enhanced emissivity in the outer Galaxy or a diminished emissivity in the inner Galaxy.

Radiation thermo-chemical models of protoplanetary disks

Context. Disks around pre-main-sequence stars evolve over time by turbulent viscous spreading. The main contender to explain the strength of the turbulence is the magnetorotational instability model, whose efficiency depends on the disk ionization fraction. Aims. Our aim is to compute self-consistently the chemistry including polycyclic aromatic hydrocarbon (PAH) charge chemistry, the grain charging, and an estimate of an effective value of the turbulence α parameter in order to find observational signatures of disk turbulence. Methods. We introduced PAH and grain charging physics and their interplay with other gas-phase reactions in the physico-chemical code PRODIMO. Non-ideal magnetohydrodynamics effects such as ohmic and ambipolar diffusion are parametrized to derive an effective value for the turbulent parameter αeff. We explored the effects of turbulence heating and line broadening on CO isotopologue submillimeter lines. Results. The spatial distribution of αeff depends on various unconstrained disk parameters such as the magnetic parameter βmag or the cosmic ray density distribution inside the protoplanetary disk s. The inner disk midplane shows the presence of the so-called dead zone where the turbulence is almost inexistent. The disk is heated mostly by thermal accommodation on dust grains in the dead zone, by viscous heating outside the dead zone up to a few hundred astronomical units, and by chemical heating in the outer disk. The CO rotational lines probe the warm molecular disk layers where the turbulence is at its maximum. However, the effect of turbulence on the CO line profiles is minimal and difficult to distinguish from the thermal broadening. Conclusions. Viscous heating of the gas in the disk midplane outside the dead zone is efficient. The determination of α from CO rotational line observations alone is challenging.

CHANG-ES

Context. Cosmic-ray electrons (CREs) originating from the star-forming discs of spiral galaxies frequently form extended radio haloes that are best observable in edge-on galaxies, where their properties can be directly investigated as a function of vertical height above the disc.Aims. For the present study, we selected two nearby edge-on galaxies from the Continuum Halos in Nearby Galaxies – an EVLA Survey (CHANG-ES), NGC 891 and 4565, which differ largely in their detectable halo extent and their star-formation rates (SFRs). Our aim is to figure out how such differences are related to the (advective and/or diffusive) CRE transport in the disc and in the halo.Methods. We use wide-band 1.5 and 6 GHz Very Large Array (VLA) observations obtained in the B, C, and D configurations, and combine the 6 GHz images with Effelsberg observations to correct for missing short spacings. After subtraction of the thermal emission, we investigate the spatially resolved synchrotron spectral index distribution in terms of CRE spectral ageing. We further compute total magnetic field strengths assuming equipartition between the cosmic-ray (CR) energy density and the magnetic field, and measure synchrotron scale heights at both frequencies. Based on the fitted vertical profiles of the synchrotron intensity and on the spectral index profile between 1.5 and 6 GHz, we create purely advective and purely diffusive CRE transport models by numerically solving the 1D diffusion–loss equation. In particular, we investigate for the first time the radial dependence of synchrotron and magnetic field scale heights, advection speeds, and diffusion coefficients, whereas previous studies of these two galaxies only determined global values of these quantities.Results. We find that the overall spectral index distribution of NGC 891 is mostly consistent with continuous CRE injection. In NGC 4565, many of the local synchrotron spectra (even in the disc) feature a break between 1.5 and 6 GHz and are thus more in line with discrete-epoch CRE injection (Jaffe–Perola (JP) or Kardashev–Pacholczyk (KP) models). This implies that CRE injection time-scales are lower than the synchrotron cooling time-scales. The synchrotron scale height of NGC 891 increases with radius, indicating that synchrotron losses are significant. NGC 891 is probably dominated by advective CRE transport at a velocity of ≳150 km s−1. In contrast, NGC 4565 is diffusion-dominated up toz = 1 kpc or higher, with a diffusion coefficient of ≥2 × 1028 cm2 s−1.

Inflation of 430-parsec bipolar radio bubbles in the Galactic Centre by an energetic event.

The Galactic Centre contains a supermassive black hole with a mass of four million Suns1 within an environment that differs markedly from that of the Galactic disk. Although the black hole is essentially quiescent in the broader context of active galactic nuclei, X-ray observations have provided evidence for energetic outbursts from its surroundings2. Also, although the levels of star formation in the Galactic Centre have been approximately constant over the past few hundred million years, there is evidence of increased short-duration bursts3, strongly influenced by the interaction of the black hole with the enhanced gas density present within the ring-like central molecular zone4 at Galactic longitude |l|<0.7degrees and latitude |b|<0.2degrees. The inner 200-parsec region is characterized by large amounts of warm molecular gas5, a high cosmic-ray ionization rate6, unusual gas chemistry, enhanced synchrotron emission7,8, and a multitude of radio-emitting magnetized filaments9, the origin of which has not been established. Here we report radio imaging that reveals a bipolar bubble structure, with an overall span of 1degree by 3degrees (140parsecs×430parsecs), extending above and below the Galactic plane and apparently associated with the Galactic Centre. The structure is edge-brightened and bounded, with symmetry implying creation by an energetic event in the Galactic Centre. We estimate the age of the bubbles to be a few million years, with a total energy of 7×1052ergs. We postulate that the progenitor event was a major contributor to the increased cosmic-ray density in the Galactic Centre, and is in turn the principal source of the relativistic particles required to power the synchrotron emission of the radio filaments within and in the vicinity of the bubble cavities.

Asymmetric Latitudinal Gradients of Galactic Cosmic Rays at Low and High Cutoff Rigidities in Two Negative Solar Magnetic Cycles: Solar Cycles 21/22 and 23/24

The south–north asymmetry of the Galactic cosmic-ray (GCR) density [ $n$ ] denoted as $\delta n_{\text{S-N}}/n$ with respect to the heliospheric current sheet (HCS), observed by neutron monitors at low (≈ 1 GV) and high (≈ 13 and ≈ 17 GV) cutoff rigidities [ $P_{\text{c}}$ ], is investigated in two negative solar magnetic polarities when the dominant polar field is toward the northern hemisphere of the Sun. Using a new simple correction method for secular changes due to solar activity for the $\delta n_{\text{S-N}}/n$ reveals that it is inversely rigidity dependent. Annual values of the corrected $\delta n_{\text{S-N}}/n$ are close to zero in both solar magnetic cycles and exhibit substantial deviations due to their temporal 27-day variations. After corrections for Earth’s excursions in helio-latitude for the north–south (NS) differences of HCS tilt angle [ $\alpha$ ] as $\Delta\alpha_{ \text{N-S}}^{\text{E}}$ , the number of 27.3-day Carrington rotations that possess correspondences between the corrected $\delta n _{\text{S-N}}/n$ and $\Delta\alpha_{\text{N-S}}^{\text{E}}$ in both solar cycles (SCs) is significantly increased. Correlations of the two parameters are considerably increased in both SCs, particularly for SC 21/22. The asymmetric or unidirectional latitudinal gradients [ $G _{\perp}$ ] derived from the consistencies between corrected $\delta n_{\text{S-N}}/n$ and $\Delta\alpha_{\text{N-S}}^{ \text{E}}$ for the low and high $P_{\text{c}}$ in the SC 21/22 are 1.38% AU−1 and 0.69% AU−1, respectively, which are a few times larger than of SC 23/24. This suggests that asymmetric cosmic-ray modulation by the asymmetric HCS is more pronounced in SC 21/22 than in SC 23/24. Although the corrections also provide results that are fairly consistent with the drift model for the symmetric or bidirectional latitudinal gradient [ $G_{z}$ ], the effects of the SN asymmetric modulation of the GCR density are important as well. It is found that $G_{\perp}$ , $G_{z}$ , and SN asymmetric GCR modulation by NS asymmetric solar-wind speed are also inversely dependent on rigidity. The interplay between SN asymmetric modulation of the GCR density by the asymmetries in HCS and solar-wind speed, together with the effects of diffusion, are discussed for both of the negative solar magnetic cycles.

Massive stars as major factories of Galactic cosmic rays

The identification of the main contributors to the locally observed fluxes of cosmic rays is a prime objective in the resolution of the long-standing enigma of the source of cosmic rays. We report on a compelling similarity of the energy and radial distributions of multi-TeV cosmic rays extracted from observations of very-high-energy γ-rays towards the Galactic Centre and two prominent clusters of young massive stars, Cygnus OB2 and Westerlund 1. We interpret this resemblance as evidence that cosmic rays responsible for the diffuse very-high-energy γ-ray emission from the Galactic Centre are accelerated by the ultracompact stellar clusters located in the heart of the Galactic Centre. The derived 1/r decrement of the cosmic ray density with the distance from a star cluster is a distinct signature of continuous cosmic ray injection into the interstellar medium over a few million years. The lack of brightening of the γ-ray images towards the stellar clusters excludes the leptonic origin of γ-ray radiation. The hard, ∝E−2.3-type, power-law energy spectra of parent protons continues up to ~1 PeV. The efficiency of conversion of the kinetic energy of stellar winds to cosmic rays can be as high as 10%, implying that young massive stars may operate as proton PeVatrons with a dominant contribution to the flux of the highest-energy Galactic cosmic rays. Ultracompact stellar clusters in the Galactic Centre are likely to be major contributors to the Galactic cosmic ray flux in the multi-TeV energy range. Observations of the diffuse gamma-ray emission from the Galactic Centre and two young massive star clusters correlate with the cosmic-ray distribution.

Energetics of high-energy cosmic radiations

The luminosity densities of high-energy cosmic radiations are studied to find connections among the various components, including high-energy neutrinos measured with IceCube and gamma rays with the Fermi satellite. Matching the cosmic-ray energy generation rate density in a GeV-TeV range estimated for Milky Way with the ultrahigh-energy component requires a power-law index of the spectrum, $s_{\rm cr}\approx2.1-2.2$, somewhat harder than $s_{\rm cr}\approx2.3-2.4$ for the local index derived from the AMS-02 experiment. The soft GeV-TeV cosmic-ray spectrum extrapolated to higher energies can be compatible with PeV cosmic rays inferred from neutrino measurements, but overshoots the CR luminosity density to explain GeV-TeV gamma rays. The extrapolation from ultrahigh energies with a hard spectrum, on the other hand, can be consistent with both neutrinos and gamma-rays. These point towards either reacceleration of galactic cosmic rays or the presence of extragalactic sources with a hard spectrum. We discuss possible cosmic-ray sources that can be added.

On the Background-gyroresonant Character of Bell’s Instability in the Large-current Regime

Abstract We show that the Bell instability, which is widely considered potentially important for cosmic-ray (CR) acceleration, is the low-frequency limit of a gyroresonant interaction between the protons of the interstellar medium and shear-Alfvén waves. At large CR current densities, its growth rate is therefore limited by the proton gyrofrequency, and two modes emerge from the cold-beam dispersion relation. A third mode driven by electron gyroresonance is only weakly unstable at low current densities. We discuss implications for magnetic field amplification and its saturation in the vicinity of supernova remnants.

Database capabilities for studying Forbush-effects and interplanetary disturbances

For a comprehensive study of Forbush-effects and their relationship to solar, interplanetary and geomagnetic disturbances, IZMIRAN researchers created (and continuously replenishes) a unique database of transient phenomena in cosmic rays and the interplanetary environment. In it, variations in the density and anisotropy of cosmic rays are combined with solar, interplanetary and geomagnetic parameters. The database includes a large number of different characteristics for about 7500 Forbush-effects, covering more than half a century of observations (1957-2017). In the work presented, some of the features of this tool are demonstrated.

Cosmic-ray propagation in the bi-stable interstellar medium

Context. Cosmic rays propagate through the galactic scales down to the smaller scales at which stars form. Cosmic rays are close to energy equipartition with the other components of the interstellar medium and can provide a support against gravity if pressure gradients develop. Aims. We study the propagation of cosmic rays within the turbulent and magnetised bi-stable interstellar gas. The conditions necessary for cosmic-ray trapping and cosmic-ray pressure gradient development are investigated. Methods. We derived an analytical value of the critical diffusion coefficient for cosmic-ray trapping within a turbulent medium, which follows the observed scaling relations. We then presented a numerical study using 3D simulations of the evolution of a mixture of interstellar gas and cosmic rays, in which turbulence is driven at varying scales by stochastic forcing within a box of 40 pc. We explored a large parameter space in which the cosmic-ray diffusion coefficient, the magnetisation, the driving scale, and the amplitude of the turbulence forcing, as well as the initial cosmic-ray energy density, vary. Results. We identify a clear transition in the interstellar dynamics for cosmic-ray diffusion coefficients below a critical value deduced from observed scaling relations. This critical diffusion depends on the characteristic length scale L of Dcrit ≃ 3.1 × 1023 cm2 s−1(L/1 pc)q+1, where the exponent q relates the turbulent velocity dispersion σ to the length scale as σ ~ Lq. Hence, in our simulations this transition occurs around Dcrit ≃ 1024–1025 cm2 s−1. The transition is recovered in all cases of our parameter study and is in very good agreement with our simple analytical estimate. In the trapped cosmic-ray regime, the induced cosmic-ray pressure gradients can modify the gas flow and provide a support against the thermal instability development. We discuss possible mechanisms that can significantly reduce the cosmic-ray diffusion coefficients within the interstellar medium. Conclusions. Cosmic-ray pressure gradients can develop and modify the evolution of thermally bi-stable gas for diffusion coefficients D0 ≤ 1025 cm2 s−1 or in regions where the cosmic-ray pressure exceeds the thermal one by more than a factor of ten. This study provides the basis for further works including more realistic cosmic-ray diffusion coefficients, as well as local cosmic-ray sources.

The rigidity spectrum of the long-term cosmic ray variations during solar activity cycles 19–24

The monthly rigidity spectrum of cosmic ray (CR) variations in the 19-24 cycles of solar activity was obtained by the global survey method using the data of continuous ground and near-Earth monitoring of CRs, exempted from atmospheric and local effects.The changes of the spectrum, first obtained for such a long period, made it possible to reveal the features of large-scale effects in CR modulation, the presence of 22-year and 11-year CR variations in the spectrum, and confirm an abnormal spectrum change in the 70s. The paper assumes a rigidity spectrum of CRs, given in a three-parameter form. Analysis of the obtained long-term CR variations for particles with rigidity 10 GV shows that the amplitude of the 22-year wave in the CR intensity increases from cycle to cycle and reaches its maximum value at the minimum of 23/24th solar activity cycle. Softening of the spectrum at the cycle minima has been revealed for the negative polarity of the solar magnetic field (qA<0). The reasons for the abnormally high CR density at the minimum of the 24th cycle and the spectrum features in the 70s are discussed. The spectrum of long-term CR variations in the 19-24 solar activity cycles, determined from the experimental data, makes it possible to verify some conclusions of the theory of heliospheric CR modulation concerning the role of the magnetic drift of particles in cycles with the different polarity of the solar magnetic field. In particular, we propose the explanation for the observed R−2 spectrum of the variations in the minima of the negative solar activity cycles, related with the scattering of particles in the vicinity of the neutral current sheet.

The Orion Region: Evidence of enhanced cosmic-ray density in a stellar wind forward shock interaction with a high density shell

Context. In recent years, an in-depth γ-ray analysis of the Orion region has been carried out by the AGILE and Fermi/LAT (Large Area Telescope) teams with the aim of estimating the H2–CO conversion factor, XCO. The comparison of the data from both satellites with models of diffuse γ-ray Galactic emission unveiled an excess at (l, b)=[213.9, −19.5], in a region at a short angular distance from the OB star κ-Ori. Possible explanations of this excess are scattering of the so-called “dark gas”, non-linearity in the H2–CO relation, or cosmic-ray (CR) energization at the κ-Ori wind shock. Aims. Concerning this last hypothesis, we want to verify whether cosmic-ray acceleration or re-acceleration could be triggered at the κ-Ori forward shock, which we suppose to be interacting with a star-forming shell detected in several wavebands and probably triggered by high energy particles. Methods. Starting from the AGILE spectrum of the detected γ-ray excess, showed here for the first time, we developed a valid physical model for cosmic-ray energization, taking into account re-acceleration, acceleration, energy losses, and secondary electron contribution. Results. Despite the characteristic low velocity of an OB star forward shock during its “snowplow” expansion phase, we find that the Orion γ-ray excess could be explained by re-acceleration of pre-existing cosmic rays in the interaction between the forward shock of κ-Ori and the CO-detected, star-forming shell swept-up by the star expansion. According to our calculations, a possible contribution from freshly accelerated particles is sub-dominant with respect the re-acceleration contribution. However, a simple adiabatic compression of the shell could also explain the detected γ-ray emission. Futher GeV and TeV observations of this region are highly recommended in order to correctly identify the real physical scenario.

Low-frequency measurements of synchrotron absorbing HII regions and modeling of observed synchrotron emissivity

Context. Cosmic rays (CRs) and magnetic fields are dynamically important components in the Galaxy, and their energy densities are comparable to that of the turbulent interstellar gas. The interaction of CRs and Galactic magnetic fields (GMF) produces synchrotron radiation clearly visible in the radio regime. Detailed measurements of synchrotron radiation averaged over the line-of-sight (LOS), so-called synchrotron emissivities, can be used as a tracer of the CR density and GMF strength. Aims. Our aim is to model the synchrotron emissivity in the Milky Way using a three-dimensional dataset instead of LOS-integrated intensity maps on the sky. Methods. Using absorbed HII regions, we measured the synchrotron emissivity over a part of the LOS through the Galaxy, changing from a two-dimensional to a three-dimensional view. Performing these measurements on a large scale is one of the new applications of the window opened by current low-frequency arrays. Using various simple axisymmetric emissivity models and a number of GMF-based emissivity models, we were able to simulate the synchrotron emissivities and compare them to the observed values in the catalog. Results. We present a catalog of low-frequency absorption measurements of HII regions, their distances and electron temperatures, compiled from literature. These data show that the axisymmetric emissivity models are not complex enough, but the GMF-based emissivity models deliver a reasonable fit. These models suggest that the fit can be improved by either an enhanced synchrotron emissivity in the outer reaches of the Milky Way or an emissivity drop near the Galactic center. Conclusions. Current GMF models plus a constant CR density model cannot explain low-frequency absorption measurements, but the fits improved with slight (ad hoc) adaptations. It is clear that more detailed models are needed, but the current results are very promising.

Cosmic rays and galactic rotation curves

A theoretical basis for modifying Newtonian dynamics on a galactic scale can be obtained by postulating that cosmic rays interact with graviton exchanges between distant masses. This assumes that these charged particles move under the influence of local electromagnetic fields rather than the weak gravitational fields of distant matter. It leads to an enhancement of graviton exchanges between distant masses via an additional gravitational force term inversely proportional to distance. At planetary and local interstellar distances this predicts an extremely small additional gravitational force, but it can become significant on a galactic scale. The model is used here to predict rotational velocities in a wide range of galaxies including the Milky Way, Andromeda (M31) and some galaxies in the THINGS study. Results are obtained assuming a galactic cosmic ray density consistent with observations in the solar system. This approach is compared with the dark matter hypothesis and with Modified Newtonian Dynamics (MOND), the two primary postulates used to explain the constant rotational velocities observed in most galaxies.

Magnetic Mirroring and Focusing of Cosmic Rays

Abstract We study the combined impact of magnetic mirroring and focusing on the ionization by cosmic rays (CRs) in dense molecular clouds and circumstellar disks. We show that for effective column densities of up to ∼1025 cm−2 (where ionization is the main mechanism of energy loss by CRs) the two effects practically cancel each other out, provided the magnetic field strength has a single peak along field lines. In this case the ionization rate at a given location is controlled solely by attenuation of interstellar CRs due to energy losses. The situation is very different in the presence of magnetic pockets—local minima of the field strength, where the CR density and thus ionization can be reduced drastically. We obtain simple analytical expressions allowing accurate calculation of the ionization rate in these regions.