Diffuse Galactic Gamma Rays from Shock-Accelerated Cosmic Rays

A shock-accelerated particle flux is proportional to p(-s), where p is the particle momentum, follows from simple theoretical considerations of cosmic-ray acceleration at nonrelativistic shocks followed by rigidity-dependent escape into the Galactic halo. A flux of shock-accelerated cosmic-ray protons with s≈2.8 provides an adequate fit to the Fermi Large Area Telescope γ-ray emission spectra of high-latitude and molecular cloud gas when uncertainties in nuclear production models are considered. A break in the spectrum of cosmic-ray protons claimed by Neronov, Semikoz, and Taylor [Phys. Rev. Lett. 108, 051105 (2012)] when fitting the γ-ray spectra of high-latitude molecular clouds is a consequence of using a cosmic-ray proton flux described by a power law in kinetic energy.

The work is done within the framework of the PAMELA project [1]. The PAMELA is a space experiment for study of charged particles in the cosmic radiation that has been performed since the launch June 15 2006. This work is dedicated to the high energy cosmic ray proton and helium flux determination (an energy region from 50 GeV/nucleon) based on using a sampling electromagnetic calorimeter. The energy estimation was built on a measurement of a total deposited energy in the calorimeter and an applying of certain event selection criteria connected with an electromagnetic component of a hadronic shower. The helium-proton separation was performed by scintillation detectors of the PAMELA Time of Flight system. This method is indispensable to widen a measurable energy range and to receive additional information in the PAMELA experiment. The obtained energy spectra of protons and helium are presented. 1. Primary Cosmic-ray Helium and Proton Spectra Helium nuclei and Protons are the most abundant elements in cosmic rays. The primary cosmic-ray proton and helium spectrum knowledge is very important to understand the fundamental phenomena of cosmic ray physics. They include cosmic rays acceleration mechanisms, particle propagation, cosmic ray origin and the matter distribution in interstellar space. In addition the proton and helium fluxes contain an essential part of information to be known for a neutrinos absolute flux calculation for example in the movement a neutrino oscillations analysis. The energy range between 1TeV up to 10 TeV remains the most scantily explored one for cosmic ray protons and helium till now. There have been very few measurements for the high energy protons and helium spectra in previous time in balloon borne or satellite experiments, beginning from the PROTON satellites [2] till CREAM [3], but some questions have kept so yet.

Recent measurements of cosmic ray proton and helium spectra show a hardening above a few hundreds of GeV. This excess is hard to understand in the framework of the conventional models of Galactic cosmic ray production and propagation. We propose here to explain this anomaly by the presence of local sources (myriad model). Cosmic ray propagation is described as a diffusion process taking place inside a two-zone magnetic halo. We calculate the proton and helium fluxes at the Earth between 50 GeV and 100 TeV. Improving over a similar analysis, we consistently derive these fluxes by taking into account both local and remote sources for which a unique injection rate is assumed. We find cosmic ray propagation parameters compatible with B/C measurements and for which the proton and helium spectra remarkably agree with the PAMELA and CREAM measurements over four decades in energy.

The observed spectral bump around 10 TV in cosmic-ray (CR) proton and helium spectra, along with the phase and amplitude evolution of CR dipole anisotropy, provide plausible yet indirect evidence for the presence of a nearby CR source. This study investigates the potential of giant molecular clouds (GMCs) located near the solar system to act as natural probes of CRs from the nearby source, with their gamma-ray emissions serving as indicators of spatial variations in CR flux within the solar neighborhood resulting from this source. We show that a nearby source, accounting for the CR data, could imprint distinct features on the $\ensuremath{\gamma}$-ray spectra of different GMCs. We expect that these features are detectable by the Large High Altitude Air Shower Observatory and upcoming high-energy $\ensuremath{\gamma}$-ray observatories, providing a powerful test for the hypothesized nearby source. Notably, we find that determining the energy dependence of the $\ensuremath{\gamma}$-ray spectral index offers a promising approach to investigate the nearby source and constrain its distance. Conversely, if the spectral bump is a widespread Galactic phenomenon, the energy dependence would exhibit uniformity across all GMCs, distinguishing the underlying mechanism accounting for the spectral bump from the scenario involving a nearby source.

During July and August, 1961, the energy spectrum of primary cosmic-ray protons was investigated in the energy range from 80 to 350 MeV. The observations were made in five high-altitude balloon flights at geomagnetic latitudes $\ensuremath{\lambda}\ensuremath{\ge}73$\ifmmode^\circ\else\textdegree\fi{} N. Solar flare and quiet day spectra were obtained. A comparison of the 1960 and 1961 results leads to the following conclusions: (1) A significant flux of low-energy protons is continually present in the primary radiation in the years of high solar activity. (2) This flux decreases with the declining level of solar activity as the galactic cosmic-ray flux increases. It is, therfeore, suggested that it is of solar origin. (3) The time dependence of the observed proton flux suggests the following alternatives: (a) The particles are produced or released more or less continuously by the sun and do not originate only in the large flare events; or (b) The particles are produced in individual large solar flares and subsequently stored over long periods of time. This second alternative would require a new and as yet unknown storage mechanism with a characteristic time of about 30 or more days.

The origin of PeV cosmic rays (CRs) is a long-standing mystery, and ultrahigh-energy gamma-ray observations would play a crucial role in identifying it. Recently, LHAASO reported the discovery of “dark” gamma-ray sources that were detected above 100 TeV without any GeV–TeV gamma-ray counterparts. The origins of these dark gamma-ray sources are unknown. We propose isolated black holes (IBHs) wandering in molecular clouds as the origins of PeV CRs and LHAASO dark sources. An IBH accretes surrounding dense gas, which forms a magnetically arrested disk (MAD) around the IBH. Magnetic reconnection in the MAD can accelerate CR protons up to PeV energies. CR protons of GeV–TeV energies fall to the IBH, whereas CR protons at sub-PeV energies can escape from the MAD, providing PeV CRs into the interstellar medium. The sub-PeV CR protons interact with the surrounding molecular clouds, producing TeV–PeV gamma rays without emitting GeV–TeV gamma rays. This scenario can explain the dark sources detected by LHAASO. Taking into account the IBH and molecular cloud distributions in our Galaxy, we demonstrate that IBHs can provide a significant contribution to the PeV CRs observed on Earth. Future gamma-ray detectors in the southern sky and neutrino detectors would provide a concrete test to our scenario.

This work is a continuation of the work (Isaeva E., 2018) in which a new approximation of the in- tegral energy spectrum of solar cosmic ray (SCR) protons was presented in the range > 1-100 MeV. In this work, the comments and wishes of colleagues working in this field were taken into account. First, the comments were taken into account on the form of the functional for approximat- ing the energy spectrum of protons, which must correspond in form to the functionals predicted by particle acceleration theories. Secondly, the previously obtained functional should be checked for high-energy protons with energies >1000 MeV. For the analysis, we used original records of the intensity of the flux of SCR protons according to the data from GOES. The studied sample contains 349 solar proton events (SPE) for the period from 03-02-1986 to 12-02-2018 years, accompanied by protons with energies in the range > 1-850 MeV. Of 349 SPE, 53 were accompanied by high- energy protons with energies > 850 MeV. In this paper, we present the results of approximation of the integral energy spectrum of SCR protons with energies > 1-850 MeV using a functional containing power and ex- ponential functions. A comparative analysis showed that this functional gives a very good approximation for SCR protons with energies in the range > 1-850 MeV.

Cosmic-ray proton and helium spectra have been measured with the balloon-borne Cosmic Ray Energetics And Mass experiment flown for 42 days in Antarctica in the 2004-2005 austral summer season. High-energy cosmic-ray data were collected at an average altitude of ~38.5 km with an average atmospheric overburden of ~3.9 g cm$^{-2}$. Individual elements are clearly separated with a charge resolution of ~0.15 e (in charge units) and ~0.2 e for protons and helium nuclei, respectively. The measured spectra at the top of the atmosphere are represented by power laws with a spectral index of -2.66 $\pm$ 0.02 for protons from 2.5 TeV to 250 TeV and -2.58 $\pm$ 0.02 for helium nuclei from 630 GeV/nucleon to 63 TeV/nucleon. They are harder than previous measurements at a few tens of GeV/nucleon. The helium flux is higher than that expected from the extrapolation of the power law fitted to the lower-energy data. The relative abundance of protons to helium nuclei is 9.1 $\pm$ 0.5 for the range from 2.5 TeV/nucleon to 63 TeV/nucleon. This ratio is considerably smaller than the previous measurements at a few tens of GeV/nucleon.

The HEGRA system of 4 imaging atmospheric Cherenkov telescopes ~IACTs! has been used to determine the flux and the spectrum of cosmic ray protons over a limited energy range around 1.5 TeV. Although the IACT system is designed for the detection of g-rays with energies above 500 GeV, it has also a large detection area of .10 6 m 2 33 msr for primary protons of energies above 1 TeV and the capability to reconstruct the primary proton energy with a reasonable accuracy DE/E of 50% near this threshold. Furthermore, the principle of stereoscopic detection of air showers permits the effective suppression of air showers induced by heavier primaries already on the trigger level, and in addition on the software level by analysis of the stereoscopic images. The combination of both capabilities permits a determination of the proton spectrum almost independently of the cosmic ray chemical composition. The accuracy of our estimate of the spectral index at 1.5 TeV is limited by systematic uncertainties and is comparable to the accuracy achieved with recent balloon and space borne experiments. In this paper we describe in detail the analysis tools, namely the detailed Monte Carlo simulation, the analysis procedure and the results. We determine the local ~i.e., in the range of 1.5‐3 TeV! differential spectral index to be g p52.7260.02stat60.15syst and obtain an integral flux above 1.5 TeV of F (.1.5 TeV)53.160.6 stat61.2syst310 22 /s sr m 2 . @S0556-2821~99!04107-7#

Measurements of 0.2–20 GeV/n cosmic-ray proton and helium spectra from 1997 through 2002 with the BESS spectrometer

The interstellar molecular clouds are the site of star formation and also the target for the cosmic ray protons to produce \gamma-rays via the hadronic process. The interstellar atomic gas is enveloping the molecular clouds and may also be dense enough to affect the \gamma-ray production. In this Chapter, some of the basic properties of the interstellar gas both in molecular and atomic forms will be reviewed. Then, it is presented that two young TeV \gamma-ray SNRs, RX J1713.7-3946 and RX J0852.0-4622, show good spatial correspondence between the \gamma-rays and the interstellar protons. The good spatial correspondence provides a support for the hadronic origin of the \gamma-rays in these SNRs. It is emphasized that both molecular and atomic hydrogen plays a role as targets for cosmic ray (CR) protons. The clumpy distribution of the target interstellar medium (ISM) protons is crucial in the interaction of the supernova shocks with the ISM, whereas models with uniform ISM distribution are not viable. Finally, it is suggested that the dense atomic gas without molecules may occupy the dominant part of the dark gas in the local ISM.

This thesis presents new observations and analysis of the molecular gas in the Large Magellanic Cloud (LMC). The observations were conducted at the Mopra Telescope as part of the Magellanic Mopra Assessment (MAGMA) project, which has obtained high resolution (45′′) maps of the 12CO(J = 1 → 0) emission from 70% by mass of the LMC’s molecular cloud population. We show that CO emission in the LMC arises predominantly in spatially compact structures with high surface brightness, and that the total CO luminosity of the LMC is two orders of magnitude lower than would be predicted by the correlations between CO luminosity, stellar mass and 1.4 GHz radio continuum that are observed for nearby late-type galaxies. We present a catalogue of giant molecular clouds (GMCs) in the LMC using the MAGMA CO data, and investigate whether the catalogued clouds are similar to GMCs in the Milky Way and other nearby galaxies. We find that GMCs in the LMC roughly follow the scaling relations between radius, velocity dispersion, mass and CO luminosity that have been determined for Galactic GMCs, but that LMC clouds have narrow linewidths and faint CO luminosities relative to their size. The physical properties of the observed GMCs are mostly insensitive to variations in the local interstellar conditions, but there are significant positive correlations between the atomic gas column density and the GMC velocity dispersion, and the stellar mass surface density and both the peak CO brightness and CO surface brightness of the GMCs. Our results are difficult to reconcile with models that posit molecular clouds as equilibrium structures that are regulated by either the interstellar radiation field or the ambient interstellar pressure. Finally, we consider whether molecular gas is relevant for the correlation between the 1.4 GHz radio continuum and far-infrared (FIR) emission within the LMC. We find robust correlations between the non-thermal radio continuum and the gas and dust emission in regions covering half the LMC’s gas disk, and we identify star formation and the neutral gas surface density as the key parameters that determine the strength of these correlations. In regions where the star-formation activity is low relative to the availability of dense gas, the non-thermal radio continuum is more tightly correlated with the gas and dust emission. We demonstrate that coupling between the magnetic field strength and the gas volume density can account for the exponent of the local radio-FIR correlation that we observe in the LMC, adopting plausible assumptions for the LMC’s UV opacity, dust-to-gas ratio and cosmic ray distribution.

We use new HCN(1–0) data from the ACA Large-sample Mapping Of Nearby galaxies in Dense gas (ALMOND) survey to trace the kpc-scale molecular gas density structure and CO(2–1) data from the Physics at High Angular resolution in Nearby GalaxieS–Atacama Large Millimeter/submillimeter Array (PHANGS–ALMA) to trace the bulk molecular gas across 25 nearby star-forming galaxies. At 2.1 kpc scale, we measure the density-sensitive HCN/CO line ratio and the star formation rate (SFR)/HCN ratio to trace the star formation efficiency in the denser molecular medium. At 150 pc scale, we measure structural and dynamical properties of the molecular gas via CO(2–1) line emission, which is linked to the lower resolution data using an intensity-weighted averaging method. We find positive correlations (negative) of HCN/CO (SFR/HCN) with the surface density, the velocity dispersion, and the internal turbulent pressure of the molecular gas. These observed correlations agree with expected trends from turbulent models of star formation, which consider a single free-fall time gravitational collapse. Our results show that the kpc-scale HCN/CO line ratio is a powerful tool to trace the 150 pc scale average density distribution of the molecular clouds. Lastly, we find systematic variations of the SFR/HCN ratio with cloud-scale molecular gas properties, which are incompatible with a universal star formation efficiency. Overall, these findings show that mean molecular gas density, molecular cloud properties, and star formation are closely linked in a coherent way, and observations of density-sensitive molecular gas tracers are a useful tool to analyse these variations, linking molecular gas physics to stellar output across galaxy discs.

Molecular clouds are regions of dense gas and dust in space where new stars and planets are born. There is a strong correlation between the distribution of dust and molecular gas in molecular clouds. The present work focuses on the three-dimensional morphological comparisons between dust and gas in 567 molecular clouds identified in a previously published catalog. We confirm a sample of 112 molecular clouds, where the cloud morphology based on CO observations and dust observations displays good overall consistency. There are up to 334 molecular clouds whose dust distribution might be related to the distribution of gas. We are unable to find gas structures that correlate with the shape of the dust distribution in 24 molecular clouds. For the 112 molecular clouds where the dust distribution correlates very well with the distribution of gas, we use CO observational data to measure the physical properties of these molecular clouds and compare them with the results derived from dust, exploring the correlation between gas and dust in the molecular clouds. We found that the gas and dust in the molecular clouds have a fairly good linear relationship, with a gas-to-dust ratio of GDR=(2.80−0.34+0.37)×1021cm−2mag−1 . The ratio varies considerably among different molecular clouds. We measured the scale height of dust-CO clouds exhibiting strong correlations, finding hZ=43.3−3.5+4.0 pc.

It remains a major challenge to derive a theory of cloud-scale ($\lesssim100$ pc) star formation and feedback, describing how galaxies convert gas into stars as a function of the galactic environment. Progress has been hampered by a lack of robust empirical constraints on the giant molecular cloud (GMC) lifecycle. We address this problem by systematically applying a new statistical method for measuring the evolutionary timeline of the GMC lifecycle, star formation, and feedback to a sample of nine nearby disc galaxies, observed as part of the PHANGS-ALMA survey. We measure the spatially resolved (∼100 pc) CO-to-H α flux ratio and find a universal de-correlation between molecular gas and young stars on GMC scales, allowing us to quantify the underlying evolutionary timeline. GMC lifetimes are short, typically $10\!-\!30\,{\rm Myr}$, and exhibit environmental variation, between and within galaxies. At kpc-scale molecular gas surface densities $\Sigma _{\rm H_2}\ge 8\,\rm {M_\odot}\,{{\rm pc}}^{-2}$, the GMC lifetime correlates with time-scales for galactic dynamical processes, whereas at $\Sigma _{\rm H_2}\le 8\,\rm {M_\odot}\,{{\rm pc}}^{-2}$ GMCs decouple from galactic dynamics and live for an internal dynamical time-scale. After a long inert phase without massive star formation traced by H α (75–90 per cent of the cloud lifetime), GMCs disperse within just $1\!-\!5\,{\rm Myr}$ once massive stars emerge. The dispersal is most likely due to early stellar feedback, causing GMCs to achieve integrated star formation efficiencies of 4–10 per cent. These results show that galactic star formation is governed by cloud-scale, environmentally dependent, dynamical processes driving rapid evolutionary cycling. GMCs and H ii regions are the fundamental units undergoing these lifecycles, with mean separations of $100\!-\!300\,{{\rm pc}}$ in star-forming discs. Future work should characterize the multiscale physics and mass flows driving these lifecycles.