On relativistic particle acceleration in molecular clouds

Dogiel, V. A.; Gurevich, A. V.; Istomin, Ya. N.; Zybin, K. P.

doi:10.1093/mnras/228.4.843

Cited by 42 publications

(47 citation statements)

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“…Ref. [16], [17] and [18]). Calculations of the cosmic-ray positron and electron flux as well as of the fraction of secondary positrons, assuming several propagation models and diffusion setups, have been carried out extensively over the last 30 years (see e.g.…”

Section: Ppb-bets Measured 84 Electron Events Above 100 Gevmentioning

confidence: 99%

Dissecting cosmic-ray electron-positron data with Occam’s razor: the role of known pulsars

Profumo¹

2011

Open Physics

181

159

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Ê Ú ¾¿ Ë ÔØ Ñ Ö ¾¼½½ ÔØ ½ ÇØÓ Ö ¾¼½½ ×ØÖ ØWe argue that both the positron fraction measured by PAMELA and the peculiar spectral features reported in the total electron-positron flux measured by ATIC have a very natural explanation in electron-positron pairs produced by nearby pulsars. While this possibility was pointed out a long time ago, the greatly improved quality of current data potentially allow to reverse-engineer the problem: given the regions of pulsar parameter space favored by PAMELA and by ATIC, are there known pulsars that explain the data with reasonable assumptions on the injected electron-positron pairs? In the context of simple benchmark models for estimating the electron-positron output, we consider all known pulsars, as listed in the most complete available catalogue. We find that it is unlikely that a single pulsar be responsible for both the PAMELA positron fraction anomaly and for the ATIC excess, although two single sources are in principle enough to explain both experimental results. The PAMELA excess positrons likely come from a set of mature pulsars (age ∼ ×10 6 yr), with a distance of 0.8-1 kpc, or from a single, younger and closer source like Geminga. The ATIC data require a larger (and less plausible) energy output, and favor an origin associated to powerful, more distant (1-2 kpc) and younger (age ∼ 5 × 10 5 yr) pulsars. We list several candidate pulsars that can individually or coherently contribute to explain the PAMELA and ATIC data. Although generally suppressed, we find that the contribution of pulsars more distant than 1-2 kpc could contribute for the ATIC excess. Finally, we stress the multi-faceted and decisive role that Fermi-LAT will play in the very near future by (1) providing us with an exquisite measurement of the electron-positron flux, (2) unveiling the existence of as yet undetected gamma-ray pulsars, and (3) searching for anisotropies in the arrival direction of high-energy electrons and positrons.

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Section: Ppb-bets Measured 84 Electron Events Above 100 Gevmentioning

confidence: 99%

Dissecting cosmic-ray electron-positron data with Occam’s razor: the role of known pulsars

Profumo¹

2011

Open Physics

181

159

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“…This acceleration is realized e.g. in low ionized turbulent media (see Dogiel et al 1987) or in extended regions of hot plasma (OB associations) filled with numerous but rather weak shock waves, resulting from SN explosions there (see Bykov & Toptygin 1993). The stochastic acceleration is described in this case as momentum diffusion with the diffusion coefficient α(p)…”

Section: Processes Of Particle Acceleration In the Galactic Ridgementioning

confidence: 99%

“…For stochastic acceleration E max can be derived from the spectrum of electromagnetic turbulence (see e.g. Dogiel et al 1987). However, since we do not specify the mechanism of acceleration its value is uncertain though e.g.…”

Section: Number Of Accelerated Particlesmentioning

confidence: 99%

Nonthermal hard X-ray emission from the Galactic Ridge

Dogiel

Schöenfelder²,

Strong³

2002

A&A

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Abstract. We investigate the origin of the nonthermal X-ray emission from the Galactic ridge in the range 10−200 keV. We consider bremsstrahlung of subrelativistic cosmic ray protons and electrons as production processes. From the solution of the kinetic equations describing the processes of particle in situ acceleration and spatial propagation we derive parameters of the spectra for protons and electrons. It is shown that the spectra must be very hard and have a cut-off at an energy ∼150−500 MeV for protons or ≤300 keV for electrons. For in situ acceleration the flux of accelerated particles consists mainly of protons since the ratio of the accelerated protons to electrons is large and the flux of nuclei with charges Z > 1 is strongly suppressed. We show that the gamma-ray line flux generated by protons does not exceed the upper limit derived from observations if we assume that the X-ray ridge emission is due to proton bremsstrahlung. However, the flux of π• photons produced by the accelerated protons is higher than the observed flux from the Galactic ridge if the cut-off is exponential for ≥150 MeV. If the cut-off in the spectrum is extremely steep its value can be as large as 400 MeV, just near the threshold energy for π• photon production. In this case the flux of gamma-rays is negligible but these protons still produce X-rays up to 200 keV. If a significant part of the hard X-ray emission at energies ∼100 keV is emitted by unresolved sources, then the energy of X-rays produced by the protons does not have to exceed several tens keV. Therefore, the cut-off energy can be as small as 30−50 MeV and in this case the flux of π• photons is negligible too. But for small cutoff energies the flux of nuclear gamma-ray lines exceeds significantly the upper limit derived from the COMPTEL and OSSE data. Hence the cut-off of the proton spectrum has to be somewhere in between 50−150 MeV in order not to exceed both π• and gamma-ray line fluxes. However the energy density of the CR protons would have to be ∼400 eV cm −3 which seems implausible. If on the contrary the hard X-ray emission from the disk is emitted by accelerated electrons we do not have the problems of gamma-ray line and π• fluxes at all, and the required energy density of particles is only ∼0.2 eV cm −3 . But in this case we must assume that acceleration of protons is suppressed. We discuss briefly the possible origin of this effect. We have also estimated the ionization rate produced by the accelerated particles in the interstellar medium, and it is found that ionization of the medium would be very significant for both energetic protons and electrons. In this way we may perhaps resolve the problem of the observed large ionization rate.

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“…• Another limit case is when a strong magnetic turbulence inside the clouds excited by chaotic motions of the neutral gas prevents CR penetration into the clouds (see Dogiel et al, 1987Dogiel et al, , 2005Dogel ′ & Sharov, 1990). In this case the spatial diffusion coefficient inside the clouds, D c , is much smaller than in the intercloud medium that leads to a depletion of CR density inside the clouds due to ionization losses and p − p collisions.…”

Section: Structure Of Magnetic Fields Inside the Clouds And Processesmentioning

confidence: 99%

“…are characteristics of the ionized fraction of the gas and µ in is the frequency of collision between ionized ions and neutral hydrogen. This set of equations was analysed by Dogiel et al (1987Dogiel et al ( , 2005 who showed that the energy of magnetic field fluctuations is concentrated at small scales (L ν ∼ 10 13 cm) where dissipation processes are essential. In this case propagation of magnetized relativistic charged particles along spaghettilike magnetic field lines can be described as diffusion with the coefficient…”

Section: Structure Of Magnetic Fields Inside the Clouds And Processesmentioning

confidence: 99%

On the origin of the 6.4keV line in the Galactic Center region

Dogiel

Chernyshov

Kiselev

et al. 2014

Astroparticle Physics

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We analyse the 6.4 keV iron line component produced in the Galactic Center (GC) region by cosmic rays in dense molecular clouds (MCs) and in the diffuse molecular gas. We showed that this component, in principle, can be seen in several years in the direction of the cloud Srg B2. If this emission is produced by low energy CRs which ionize the interstellar molecular gas the intensity of the line is quite small, < 1%. However, we cannot exclude that local sources of CRs or X-ray photons nearby the cloud may provide much higher intensity of the line from there. Production of the line emission from molecular clouds depends strongly on processes of CR penetration into them. We show that turbulent motions of neutral gas may generate strong magnetic fluctuations in the clouds which prevent free penetration of CRs into the clouds from outside. We provide a special analysis of the line production by high energy electrons. We concluded that these electrons hardly provide the diffuse 6.4 keV line emission from the GC because their density is depleted by ionization losses. We do not exclude that local sources of electrons may provide an excesses of the 6.4 keV line emission in some molecular clouds and even reproduce a relatively short time variations of the iron line emission. However, we doubt whether a single electron source provides the simultaneous short time variability of the iron line emission from clouds which are distant from each other on hundred pc as observed for the GC clouds. An alternative speculation is that local electron sources could also provide the necessary effect of the line variations in different clouds that are seen simultaneously by chance that seems, however, very unlikely.

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On relativistic particle acceleration in molecular clouds

Cited by 42 publications

References 0 publications

Dissecting cosmic-ray electron-positron data with Occam’s razor: the role of known pulsars

Dissecting cosmic-ray electron-positron data with Occam’s razor: the role of known pulsars

Nonthermal hard X-ray emission from the Galactic Ridge

On the origin of the 6.4keV line in the Galactic Center region

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