Cosmic-ray Rate Research Articles

Zenith angle distribution of cosmic ray showers measured with the Yakutsk array and its application to the analysis of arrival directions in equatorial coordinates

The Yakutsk array dataset in the energy interval $(10^{17},10^{19})$ eV is revisited in order to interpret the zenith angle distribution of an extensive air shower event rate of ultra-high energy cosmic rays. The close relation of the distribution to the attenuation of the main measurable parameter of showers, $S_{600}$, is examined. Measured and expected distributions are used to analyze the arrival directions of cosmic rays on an equatorial map including the energy range below $10^{18}$ eV, which was previously avoided due to the reduced trigger efficiency of the array in the range. While the null hypothesis cannot be rejected with data from the Yakutsk array, an upper limit on the fraction of cosmic rays from a separable source in the uniform background is derived as a function of declination and energy.

Unlocking CO Depletion in Protoplanetary Disks. I. The Warm Molecular Layer

Abstract CO is commonly used as a tracer of the total gas mass in both the interstellar medium and in protoplanetary disks. Recently, there has been much debate about the utility of CO as a mass tracer in disks. Observations of CO in protoplanetary disks reveal a range of CO abundances, with measurements of low CO to dust mass ratios in numerous systems. One possibility is that carbon is removed from CO via chemistry. However, the full range of physical conditions conducive to this chemical reprocessing is not well understood. We perform a systematic survey of the time dependent chemistry in protoplanetary disks for 198 models with a range of physical conditions. We vary dust grain size distribution, temperature, comic-ray and X-ray ionization rates, disk mass, and initial water abundance, detailing what physical conditions are necessary to activate the various CO depletion mechanisms in the warm molecular layer. We focus our analysis on the warm molecular layer in two regions: the outer disk (100 au) well outside the CO snowline and the inner disk (19 au) just inside the midplane CO snowline. After 1 Myr, we find that the majority of models have a CO abundance relative to H2 less than 10−4 in the outer disk, while an abundance less than 10−5 requires the presence of cosmic-rays. Inside the CO snowline, significant depletion of CO only occurs in models with a high cosmic-ray rate. If cosmic-rays are not present in young disks, it is difficult to chemically remove carbon from CO. Additionally, removing water prior to CO depletion impedes the chemical processing of CO. Chemical processing alone cannot explain current observations of low CO abundances. Other mechanisms must also be involved.

High energy neutrinos from the sun

The Sun is a main source of high energy neutrinos. These neutrinos appear as secondary particles after the Sun absorbs high-energy cosmic rays, that find there a low-density environment (much thinner than our atmosphere) where most secondary pions, kaons and muons can decay before they lose energy. The main uncertainty in a calculation of the solar neutrino flux is due to the effects of the magnetic fields on the absorption rate of charged cosmic rays. We use recent data from HAWC on the cosmic-ray shadow of the Sun to estimate this rate. We evaluate the solar neutrino flux and show that at 1 TeV it is over ten times larger than the atmospheric one at zenith θz=30∘/150∘. The flux that we obtain has a distinct spectrum and flavor composition: it is harder and richer in antineutrinos and tau/electron neutrinos than the atmospheric background. This solar flux could be detected in current and upcoming neutrino telescopes. KM3NeT, in particular, looks very promising: it will see the Sun high in the sky (the atmospheric flux is lower there than near the horizon) and expects a very good angular resolution (the Sun’s radius is just 0.27°).

Correlations Between Secondary Cosmic Ray Rates and Strong Electric Fields at Lomnický štít

AbstractSince March 2014, there is a continuous measurement of secondary cosmic rays by the detector system SEVAN (Space Environmental Viewing and Analysis Network) at Lomnický štít, altitude 2,634 m above sea level. Starting from June 2016, the count rates (1 s resolution) obtained from the three SEVAN detectors and from their coincidences are available, along with selected meteorological characteristics. Since 30 May 2016 the electric field measurements have been installed at the same site. Several events with clear increase of the count rate in the upper detector of SEVAN were observed during the thunderstorms until 17 September 2016. Examples of these measurements are presented and discussed. Barometric pressure correction and elimination of low‐frequency variability from the signal allow to extract 2 min averaged increases from the data. It is shown that the 2 min averaged increases of count rates measured by SEVAN correspond with periods of high electric field (with higher probability during negative polarity) rather than with the individual discharges (lightning).

Cosmic rays and climate

Climate change and global warming is generally attributed to increases in greenhouse gases in the atmosphere. Other possible contributing effects are constantly being sought. Because of the importance of solar irradiance as a driver of climate, and because the widely known effect of the solar cycle on cosmic rays, it has been speculated for more than 50years that cosmic ray variations may have an impact on climate. The question has been how. A proposed mechanism would be through the effect of ionization from cosmic rays on the rates of nucleation of cloud condensation nuclei. The result would be an impact of the rate of cosmic rays on cloud formation that would subsequently impact the reflection of incoming short wavelengths and the trapping of outgoing long radiation; more cosmic rays would lead to more clouds and a net cooling of the planet (and visa-versa). This paper concludes that while the effect may operate, it is not sufficiently robust to be a significant contributor to the current warming of the planet.

Further gas mixtures with low environment impact

Further gas mixtures with low environment impact have been tested. For the best RPC performance the main effort was to look for a mixture giving a comfortable avalanche-streamer separation and a gas density ensuring sufficient primary ionization. The measurements have been carried out on small size chambers at cosmic ray rates using the waveforms recorded on the oscilloscope. Efficiencies and distributions of amplitude and charge will be presented.

THE DENSE GAS IN THE LARGEST MOLECULAR COMPLEXES OF THE ANTENNAE: HCN AND HCO+ OBSERVATIONS OF NGC 4038/39 USING ALMA

ABSTRACT We present observations of the dense molecular gas tracers , , and in the transition using the Atacama Large Millimeter/submillimeter Array. We supplement our data sets with previous observations of , which trace the total molecular gas content. We separate the Antennae into seven bright regions in which we detect emission from all three molecules, including the nuclei of NGC 4038 and NGC 4039, five super giant molecular complexes in the overlap region, and two additional bright clouds. We find that the ratio of , which traces the dense molecular gas fraction, is greater in the two nuclei ( ) than in the overlap region ( ). We attribute this to an increase in pressure due to the stellar potential within the nuclei; a similar effect to what has been seen previously in the Milky Way and nearby spiral galaxies. Furthermore, the ratio of does not vary by more than a factor of 1.5 between regions. By comparing our measured ratios to photon dominated region (PDR) models including mechanical heating, we find that the ratio of is consistent with mechanical heating contributing ≳5%–10% of the PDR surface heating to the total heating budget. Finally, the ratio of varies from ∼1 in the nucleus of NGC 4038 down to ∼0.5 in the overlap region. The lower ratio in the overlap region may be due to an increase in the cosmic ray rate from the increased supernova rate within this region.

Periodicity analysis of galactic cosmic rays using Fourier, Hilbert, and higher-order spectral methods

Galactic cosmic rays (GCRs) that traverse the heliosphere, in the energy range from several 100 MeV to a few GeV, are subjected to heliospheric modulation. GCRs interact with varying fields of the heliosphere to produce fluctuations in cosmic-ray intensity with variations in solar activity. The effects of modulation are continuously measured by the well-established world-wide neutron monitor network. Solar activity indices and cosmic-ray neutron monitor rates (at different cut-off rigidities) have been used to compare Fourier, Hilbert, and higher-order spectral (bispectral) features of GCR intensity variations at six stations for a period of nearly 50 years. The present study reveals that GCRs exhibit a number of short- and long-term periodicities that vary between 9 days and 22 years. The bispectral analysis shows the characteristic features of nonlinear coupling and complex phase relationships between various harmonics present in GCRs and solar activity proxies. We also offer possible explanations for the observed periodicities with the help of the previous findings.

Astrophysical ZeV acceleration in the relativistic jet from an accreting supermassive blackhole

An accreting supermassive blackhole, the central engine of active galactic nucleus (AGN), is capable of exciting extreme amplitude Alfven waves whose wavelength (wave packet) size is characterized by its clumpiness. The pondermotive force and wakefield are driven by these Alfven waves propagating in the AGN (blazar) jet, and accelerate protons/nuclei to extreme energies beyond Zetta-electron volt (ZeV=1021eV). Such acceleration is prompt, localized, and does not suffer from the multiple scattering/bending enveloped in the Fermi acceleration that causes excessive synchrotron radiation loss beyond 1019eV. The production rate of ZeV cosmic rays is found to be consistent with the observed gamma-ray luminosity function of blazars and their time variabilities.

Cosmic rays and terrestrial life: A brief review

“The investigation into the possible effects of cosmic rays on living organisms will also offer great interest.” – Victor F. Hess, Nobel Lecture, December 12, 1936High-energy radiation bursts are commonplace in our Universe. From nearby solar flares to distant gamma ray bursts, a variety of physical processes accelerate charged particles to a wide range of energies, which subsequently reach the Earth. Such particles contribute to a number of physical processes occurring in the Earth system. A large fraction of the energy of charged particles gets deposited in the atmosphere, ionizing it, causing changes in its chemistry and affecting the global electric circuit. Remaining secondary particles contribute to the background dose of cosmic rays on the surface and parts of the subsurface region. Life has evolved over the past ∼3 billion years in presence of this background radiation, which itself has varied considerably during the period [1–3]. As demonstrated by the Miller–Urey experiment, lightning plays a very important role in the formation of complex organic molecules, which are the building blocks of more complex structures forming life. There is growing evidence of increase in the lightning rate with increasing flux of charged particles. Is there a connection between enhanced rate of cosmic rays and the origin of life? Cosmic ray secondaries are also known to damage DNA and cause mutations, leading to cancer and other diseases. It is now possible to compute radiation doses from secondary particles, in particular muons and neutrons. Have the variations in cosmic ray flux affected the evolution of life on earth? We describe the mechanisms of cosmic rays affecting terrestrial life and review the potential implications of the variation of high-energy astrophysical radiation on the history of life on earth.

Measurement of Cosmic Ray Spectrum and Anisotropy with ARGO-YBJ

The combined measurement of the cosmic ray (CR) energy spectrum and anisotropy in their arrival direction distribution needs the knowledge of the elemental composition of the radiation to discriminate between different origin and propagation models. Important information on the CR mass composition can be obtained studying the EAS muon content through the measurement of the CR rate at different zenith angles. In this paper we report on the observation of the anisotropy of galactic CRs at different angular scales with the ARGO-YBJ experiment. We report also on the study of the primary CR rate for different zenith angles. The light component (p+He) has been selected and its energy spectrum measured in the energy range (5 - 200) TeV for quasi-vertical events. With this analysis for the first time a ground-based measurement of the CR spectrum overlaps data obtained with direct methods for more than one energy decade, thus providing a solid anchorage to the CR spectrum measurements carried out by EAS arrays in the knee region. Finally, a preliminary study of the non-attenuated shower component at a zenith angle $\theta >$ 70$^{\circ}$ (through the observation of the so-called horizantal air showers) is presented.

Magnetic field fluctuations observed in the heliosheath by Voyager 1 at 114 ± 2 AU during 2010

The radial component of the flow speed VR in the heliosheath was decreasing between 2009.0 (108.5 AU) and 2010.3, at which time it reached the “stagnation region” where VR ≈ 0 km/s from 2010.3 to at least to 2011.0 (115.7 AU). The fluctuations of the hour averages of the magnetic field B observed by Voyager 1 (V1) each day throughout 2010 were primarily compressive (longitudinal) fluctuations, along the T (average B) direction in HG coordinates. The yearly average of the daily standard deviations (SD) of the compressive component of B was 〈SDc〉 = 〈SD(BT)〉 ≈ 〈SD(B)〉 ≈ 0.015 nT. The corresponding standard deviations of the transverse components 〈SDt〉 = SD(BR) > ≈ 〈SD(BN)〉 ≈ 0.006 nT. During 2010, 〈SDc〉/〈SDt〉 > 2.6 and 〈SDc〉/〈B〉 = 0.15. The counting rate of cosmic rays >70 MeV/nuc tends to increase with increasing B and SDc, contrary to expectations.

Spectrum and ionization rate of low-energy Galactic cosmic rays

Abstract We consider the rate of ionization of diffuse and molecular clouds in the interstellar medium by Galactic cosmic rays (GCRs) in order to constrain its low-energy spectrum. We extrapolate the GCR spectrum obtained from PAMELA at high energies (≥200 GeV nucleon−1) and a recently derived GCR proton flux at 1–200 GeV from observations of gamma-rays from molecular clouds, and find that the observed average Galactic ionization rate can be reconciled with this GCR spectrum if there is a low-energy cut-off for protons at 10–100 MeV. We also identify the flattening below a few GeV as being due to (a) decrease of the diffusion coefficient and dominance of convective loss at low energy and (b) the expected break in energy spectrum for a constant spectral index in momentum. We show that the inferred CR proton spectrum of Φ ∝ E−1.7±0.2kin for Ekin ≤ few GeV is consistent with a power-law spectrum in momentum p−2.45± 0.4, which we identify as the spectrum at source. Diffusion loss at higher energies then introduces a steepening by E−α with α ∼ 1/3, making it consistent with high-energy measurements.

PROPERTIES OF NEARBY STARBURST GALAXIES BASED ON THEIR DIFFUSE GAMMA-RAY EMISSION

The physical relationship between the far-infrared and radio fluxes of star forming galaxies has yet to be definitively determined. The favored interpretation, the "calorimeter model," requires that supernova generated cosmic ray (CR) electrons cool rapidly via synchrotron radiation. However, this cooling should steepen their radio spectra beyond what is observed, and so enhanced ionization losses at low energies from high gas densities are also required. Further, evaluating the minimum energy magnetic field strength with the traditional scaling of the synchrotron flux may underestimate the true value in massive starbursts if their magnetic energy density is comparable to the hydrostatic pressure of their disks. Gamma-ray spectra of starburst galaxies, combined with radio data, provide a less ambiguous estimate of these physical properties in starburst nuclei. While the radio flux is most sensitive to the magnetic field, the GeV gamma-ray spectrum normalization depends primarily on gas density. To this end, spectra above 100 MeV were constructed for two nearby starburst galaxies, NGC 253 and M82, using Fermi data. Their nuclear radio and far-infrared spectra from the literature are compared to new models of the steady-state CR distributions expected from starburst galaxies. Models with high magnetic fields, favoring galaxy calorimetry, are overall better fits to the observations. These solutions also imply relatively high densities and CR ionization rates, consistent with molecular cloud studies.

Star formation near an obscured AGN

The conditions that affect the formation of stars in radiatively and mechanically active environments are quite different than the conditions that apply to our local interstellar neighborhood. In such galactic environments, a variety of feedback processes can play a significant role in shaping the initial mass function (IMF). Here, we present a numerical study on the effects of an accreting black hole and the influence of nearby massive stars to a collapsing, 800 M_sun, molecular cloud at 10 pc distance from the black hole. We parametrize and study radiative feedback effects of hard X-rays emanating from the black hole broad line region, increased cosmic ray rates due to supernovae in starbursts, and strong UV radiation produced by nearby massive stars. We also investigate the importance of shear from the supermassive, 10^6-10^8 M_sun, black hole as the star-forming cloud orbits around it. We find that thermal pressure from X-rays compresses the cloud, which induces a high star formation rate early on, but reduces the overall star formation efficiency to about 7% due to gas depletion by evaporation. We see that the turn-over mass of the IMF increases up to a factor of 2.3, M_turn = 1-1.5 M_sun, for the model with the highest X-ray flux (160 erg s^-1 cm^-2), while the high-mass slope of the IMF becomes Gamma > -1. This results in more high mass stars and a non-Salpeter IMF. Cosmic rays penetrate deeply into the cloud and increase the gas temperature (50-200 K), which leads to a reduced formation efficiency of low mass stars. High cosmic ray rates increase the average mass of stars, thereby shifting the turn-over mass to higher values, i.e., up to several solar masses. Due to this process, the onset of star formation is also delayed. We conclude that the IMF inside active galaxies is different than the one obtained from local environments.

Modelling the spinning dust emission from dense interstellar clouds

Electric dipole emission arising from PAHs is often invoked to explain the anomalous microwave emission (AME). This assignation is based on an observed tight correlation between the mid-IR emission of PAHs and the AME; and a good agreement between models of spinning dust and the broadband AME spectrum. So far often detected at large scale in the diffuse interstellar medium, the AME has recently been studied in detail in well-known dense molecular clouds with the help of Planck data. While much attention has been given to the physics of spinning dust emission, the impact of varying local physical conditions has not yet been considered in detail. Our aim is to study the emerging spinning dust emission from interstellar clouds with realistic physical conditions and radiative transfer. We use the DustEM code from Compiegne et al. to describe the extinction and IR emission of all dust populations. The spinning dust emission is obtained with SpDust, as described by Silsbee et al., that we have coupled to DustEM. We carry out full radiative transfer simulations and carefully estimate the local gas state as a function of position within interstellar clouds. We show that the spinning dust emission is sensitive to the abundances of the major ions and we propose a simple scheme to estimate these abundances. We also investigate the effect of changing the cosmic-ray rate. In dense media, where radiative transfer is mandatory, we show that the relationship between the spinning and mid-IR emissivities of PAHs is no longer linear and that the spinning dust emission may actually be strong at the centre of clouds where the mid-IR PAH emission is weak. These results provide new ways to trace grain growth from diffuse to dense medium and will be useful for the analysis of AME at the scale of interstellar clouds.

Star formation in extreme environments: the effects of cosmic rays and mechanical heating

Context. The molecular interstellar medium in extreme environments, such as Arp 220, but also NGC 253 appears to have extremely high cosmic ray (CR) rates (103−104 × Milky Way) and substantial mechanical heating from supernova driven turbulence.

ESR dating of laterite from Ban Tha Ta Suea, Kanchanaburi, Thailand

Electron spin resonance (ESR) dating was carried out on laterite samples collected from Ban Tha Ta Suea, the original deposit of the laterite used in the construction of Muang Sing Historical Park. Ban Tha Ta Suea is about 6 km east of Muang Sing Historical Park in Kanchanaburi province (181 km far from Bangkok). The dose response of the g=2.0028 ESR signal was found to be suitable for age determination. An exponential saturation function was fitted to the ESR signal growth on additional γ-irradiation. Extrapolation of this function resulted in the accumulated dose 144±5 Gy. Gamma spectroscopy and neutron activation analysis (NAA) were used to determine concentrations of (238)U, (232)Th and (40)K in the samples from which the annual dose rate 16±2 mGy was calculated. In addition, the dose rate of cosmic rays was taken into account. The ESR age of laterite samples was found to be 9.0±0.9 ka. This age falls within the Holocene epoch of the Quaternary on the geological time scale. Samples from Muang Sing Historical Park were assigned to the same epoch. Both results agree with the geological map.

Cosmic rays from pulsars and magnetars

Abstract We compare the expected abundance of cosmic ray electrons and positrons from pulsars and magnetars. We assume that the distribution of infant pulsars and magnetars follows that of high-mass stars in the Milky Way and that the production rate of cosmic rays is proportional to the spin-down and magnetic-decay power of pulsars and magnetars, respectively. In combination with primary and secondary cosmic ray leptons from other sources (especially supernova remnants), we find that both magnetars and pulsars can easily account for the observed cosmic ray spectrum, in particular the dip seen by HESS (High-Energy Stereoscopic System) at several TeV and the increase in positron fraction found by PAMELA (Payload for Antimatter Exploration and Light Nuclei Astrophysics).

COSMIC RAY SPECTRUM IN SUPERNOVA REMNANT SHOCKS

We perform kinetic simulations of diffusive shock acceleration (DSA) in Type Ia supernova remnants (SNRs) expanding into a uniform interstellar medium (ISM). Bohm-like diffusion assumed, and simple models for Alfvenic drift and dissipation are adopted. Phenomenological models for thermal leakage injection are considered as well. We find that the preshock gas temperature is the primary parameter that governs the cosmic ray (CR) acceleration efficiency and energy spectrum, while the CR injection rate is a secondary parameter. For SNRs in the warm ISM, if the injection fraction is larger than 10^{-4}, the DSA is efficient enough to convert more than 20 % of the SN explosion energy into CRs and the accelerated CR spectrum exhibits a concave curvature flattening to E^{-1.6}. Such a flat source spectrum near the knee energy, however, may not be reconciled with the CR spectrum observed at Earth. On the other hand, SNRs in the hot ISM, with an injection fraction smaller than 10^{-4}, are inefficient accelerators with less than 10 % of the explosion energy getting converted to CRs. Also the shock structure is almost test-particle like and the ensuing CR spectrum can be steeper than E^{-2}. With amplified magnetic field strength of order of 30 microG, Alfven waves generated by the streaming instability may drift upstream fast enough to make the modified test-particle power-law as steep as E^{-2.3}, which is more consistent with the observed CR spectrum.