Cosmic Ray Propagation Models Research Articles

The diffuse neutrino flux from the inner Galaxy: Constraints from very high energy gamma-ray observations

Recently, the MILAGRO collaboration reported on the detection of a diffuse multi-TeV emission from a region of the Galactic disk close to the inner Galaxy. The emission is in excess of what is predicted by conventional models for cosmic ray propagation, which are tuned to reproduce the spectrum of cosmic rays observed locally. By assuming that the excess detected by MILAGRO is of hadronic origin and that it is representative for the whole inner Galactic region, we estimate the expected diffuse flux of neutrinos from a region of the Galactic disk with coordinates −40° < l < 40°. Our estimate has to be considered as the maximal expected neutrino flux compatible with all the available gamma-ray data, since any leptonic contribution to the observed gamma-ray emission would lower the neutrino flux. The diffuse flux of neutrinos, if close to the maximum allowed level, may be detected by a km 3-scale detector located in the northern hemisphere. A detection would unambiguously reveal the hadronic origin of the diffuse gamma-ray emission.

The diffuse Galactic γ-rays from dark matter annihilation

The diffuse Galactic γ-rays from EGRET observation shows excesses above 1 GeV in comparison with the expectations from conventional Galactic cosmic ray (CR) propagation model. In the work we try to solve the “GeV excess” problem by dark matter (DM) annihilation in the frame of supersymmetry (SUSY). Compared with previous works, there are three aspects improved in this work: first, the direction-independent “boost factor” for diffuse γ-rays from dark matter annihilation (DMA) is naturally reproduced by taking the DM substructures into account; second, there is no need for renormalization of the diffuse γ-ray background produced by CRs; last but not the least, in this work our new propagation model can give consistent results of both diffuse γ-rays and antiprotons, by directly adding the signals from DMA to the diffuse γ-ray background. This is a self-consistent model among several possible scenarios at present, and can be tested or optimized by the forthcoming experiments such as GLAST, PAMELA and AMS02.

Diffuseγrays andp¯flux from dark matter annihilation: A model for consistent results with EGRET and cosmic ray data

In this work we develop a new propagation model for the Galactic cosmic rays based on the GALPROP code, including contributions from dark matter annihilation. Its predictions of the Galactic diffuse $\ensuremath{\gamma}$-ray spectra are compatible with the EGRET data in all sky regions. It also gives consistent results about the diffuse $\ensuremath{\gamma}$ ray longitude and latitude distributions. The $\mathrm{B}/\mathrm{C}$, $^{10}\mathrm{Be}/^{9}\mathrm{Be}$, proton, electron and antiproton spectra are in agreement with cosmic ray measurements as well. In this model we have taken a universal proton spectrum throughout the Galaxy without introducing large fluctuation, considering the proton energy loss is negligible. The dark matter annihilation signals are boosted after taking the contributions from subhalos into account. Another interesting feature of this model is that it gives a better description of the diffuse $\ensuremath{\gamma}$ rays when taking the source distribution compatible with supernova remnants data, which is different from previous studies.

Diffusive gamma-rays and antiprotons

Based on our three-dimensional cosmic-ray (CR) propagation model in the Galaxy, we study the diffusive gamma-rays (Dγ's) and antiprotons (p̄'s), and compare our numerical results with recent experimental data, particularly those obtained by EGRET and BESS, each for Dγ data and p̄ data respectively. Galactic parameters such as the gas density and diffusion constant are determined from the most recent data for the CR energy spectra, particularly those for protons and helium nuclei from AMS and BESS with excellent accuracy over the 1–100 GeV per nucleon range, and from JACEE and RUNJOB covering the very high energy region up to 500 TeV per particle. Using these parameters, we find that the EGRET data are well reproduced in the energy region Eγ ≲ 2 GeV, in both the spectral shape and the absolute value, and also that the BESS data are compatible with our propagation model in the energy region ≳ 1 GeV. However, there are still two open questions in these components, 1) the intensity of the Dγ's gives a significant enhancement, by approximately the factor 2–3, at ≳ 2 GeV as compared to those expected from the CR spectrum, and 2) we could not confirm whether or not BESS data with the energies less than 1 GeV are consistent or not with those expected from other CR components.

Antiprotons and Cosmic‐Ray Propagation in the Galaxy

Based on our three-dimensional cosmic-ray (CR) propagation model, we study the behavior of CR antiprotons (CR---> 's), and compare our numerical results with recent experimental data, particularly those obtained by the BESS group. There is a common perception that diffusion-reacceleration models produce too low a --> flux below 5 GeV. However, we find that the BESS data are compatible with a model including the reacceleration process in the energy region 1 GeV, if we adopt Galactic parameters that reproduce the CR data for primary components (protons, helium, etc.), secondary components (B, sub-Fe), and CR isotopes (10Be,26Be, etc.). This indicates that most CR---> 's are secondary products of collisions between CRs and the interstellar medium (ISM) gas. However, because of poor statistics, we could not confirm whether or not BESS data with energies less than 1 GeV are consistent with those expected from other CR components, even taking the modulation effect into account. Nevertheless, it is important to determine the intensity of CR---> 's expected from the rich CR data now available, for studies of --> 's of cosmological origin, such as the evaporation of primordial black holes and/or the annihilation of supersymmetiric dark matter particles.

Study of galactic gamma ray sources with Milagro

The diffuse gamma radiation arising from the interaction of cosmic ray particles with matter and radiation in the Galaxy is one of the few probes available to study the origin of the cosmic rays. Milagro is a water Cherenkov detector that continuously views the entire overhead sky. The large field-of-view combined with the long observation time makes Milagro the most sensitive instrument available for the study of large, low surface brightness sources such as the diffuse gamma radiation arising from interactions of cosmic radiation with interstellar matter. In this paper we report our results on diffuse emission from the galactic plane and in particular the Cygnus region. Our observations show at least one new TeV source MGRO J2020+37 as well as correlations with the matter density in the region as would be expected from cosmic-ray proton interactions. However, the TeV gamma-ray flux from the Cygnus region (after excluding MGRO J2020+37) is roughly 5 times that expected from a conventional model of cosmic ray production and propagation.

Discovery of TeV Gamma-Ray Emission from the Cygnus Region of the Galaxy

The diffuse gamma radiation arising from the interaction of cosmic-ray particles with matter and radiation in the Galaxy is one of the few probes available to study the origin of the cosmic rays. Data from the Milagro gamma-ray observatory—a water Cerenkov detector that continuously views ~2 sr of the overhead sky—shows that the brightest extended region in the entire northern sky is the Cygnus region of the Galactic plane. The TeV image of the Cygnus region contains at least one new source, MGRO J2019+37, which is 10.9 σ above the isotropic background, as well as correlations with the matter density in the region. However, the gamma-ray flux from the Cygnus region (after excluding MGRO J2019+37) as measured at ~12 TeV exceeds that predicted from a model of cosmic-ray production and propagation. This observation indicates the existence of either hard-spectrum cosmic-ray sources and/or unresolved sources of TeV gamma rays in the region.

Obtaining Cosmic‐Ray Propagation Parameters from Diffuse Very High Energy Gamma‐Ray Emission from the Galactic Center Ridge

The recent discovery of diffuse, very high energy (VHE) γ-radiation from the Galactic center ridge by the HESS telescope allows for the first time the direct determination of the parameters of Galactic cosmic-ray propagation models. Whereas this discovery showed that the diffuse γ-radiation can be explained by the interaction of VHE cosmic-ray (CR) protons with the interstellar gas located in several giant molecular clouds, we show in this paper that the associated diffusion coefficient for the protons depends on the epoch of activity of the central source of protons: Assuming that the supernova remnant (SNR) Sgr A East was responsible for the particle acceleration, we infer a diffusion coefficient for the Galactic center region of κ = 1-5 kpc2 Myr-1 for a mean proton energy of ~3 TeV. More specifically, for impulsive injection in a 5-10 kyr SNR, we infer a value of κ = 1-2 kpc2 Myr-1, whereas for source activity timescales equal to the age of the SNR, the diffusion coefficient would increase to κ ~ 5 kpc2 Myr-1. These values are smaller than those inferred from local CR abundances. Finally, the above-mentioned values of κ for impulsive injection are equally valid if the required transient source of protons was due to an earlier epoch of stellar infall into the black hole Sgr A*.

A Possible Approach to Three‐dimensional Cosmic‐Ray Propagation in the Galaxy. III. Unstable Nuclei

We extend our three-dimensional propagation model for the stable cosmic rays (CRs) in the Galaxy to that for unstable CRs such as 10Be and 26Al, including the energy gain due to reacceleration and energy loss due to ionization, where the latter is approximated by the cutoff procedure. We assume that there is no boundary in both the radial spread of the disk and the latitudinal spread of the halo, and that the three critical parameters, the diffusion coefficient D, the gas density n, and the CR source density Q, have exponential-type gradients in both Galactic longitude and latitude with scale heights (rD,rn,rQ) for the former and (zD,zn,zQ) for the latter. It is possible to apply the weighted-slab technique to the transport equation, even including the energy change process, as well as the decay effect, if the diffusion coefficient is separable in position r and rigidity R, i.e., D(r; R) = D(r)(v/c)Rα. Combining this with the first-pole approximation in the path-length distribution, we can obtain the analytical solution rather easily. We compare our numerical results with experimental data on 10Be/ 9Be, 26Al/27Al, and so on and obtain a diffusion coefficient D☉ = 1.17 × 1028 cm2 s-1 at E = 1 GeV nucleon-1 and a gas density n☉ = 0.832 H atoms cm-3 at the solar system with scale heights zD = 2.5 kpc and zn = 250 pc. We found the residence time of CRs in the Galaxy to be 3.79 × 107 yr at E = 1 GeV nucleon-1 and 1.14 × 108 yr at E = 100 MeV nucleon-1.

A Possible Approach to Three‐dimensional Cosmic‐Ray Propagation in the Galaxy. II. Stable Nuclei with Energy Change

We extend our model of three-dimensional cosmic-ray propagation without energy change to that including energy change due to reacceleration and ionization energy loss. We assume that there is no boundary in both the radial spread of the disk and the latitudinal spread of the halo, and that the three critical parameters, the diffusion coefficient D, the gas density n, and the cosmic-ray source density Q, depend on both the space position r and the rigidity R of the cosmic-ray particle. It is possible to apply the weighted slab technique to the transport equation, even including the energy change process, if the diffusion coefficient is separable in r and R, i.e., D(; R) = (v/c)RαD(), and combining it with the first-pole approximation in the path length distribution, we can obtain the analytical solution rather easily. We show that the rigidity dependence of the secondary-to-primary ratio behaves as R-α in the high-energy region and as vRα in the low-energy region. We compare our numerical results with experimental data in both the low- and the high-energy regions and find that our model reproduces all components over the wide energy range 1 GeV nucleon-1 to 100 TeV nucleon-1, with the adoption of appropriate scale heights in D, n, and Q. We also confirm that the reacceleration process actually occurs during the propagation in the Galaxy and that its magnitude is comparable with that expected from reasonable numerical choices for three parameters, D, n, and the velocity of the hydromagnetic turbulence vM.

A New Determination of the Extragalactic Diffuse Gamma‐Ray Background from EGRET Data

We use the GALPROP model for cosmic-ray propagation to obtain a new estimate of the Galactic component of γ-rays and show that away from the Galactic plane it gives an accurate prediction of the observed EGRET intensities in the energy range 30 MeV to 50 GeV. On this basis we reevaluate the extragalactic γ-ray background. We find that for some energies previous work underestimated the Galactic contribution at high latitudes and hence overestimated the background. Our new background spectrum shows a positive curvature similar to that expected for models of the extragalactic emission based on the blazar population.

Cosmic-ray antiprotons in the galaxy

A key issue in fundamental physics is the nature of non-baryonic dark matter in the universe. Among the candidates proposed by the particle physics community are neutralinos whose annihilation in the Galactic halo can produce antiprotons with a characteristic spectrum. Several balloon and space borne experiments have been actively looking to detect the neutralino signature in cosmic antiprotons. This task is however hampered by the existence of a secondary “background” flux of antiprotons produced in the interactions of cosmic-ray particles in the interstellar medium. A notable difficulty is that a self-consistent and complete model of cosmic-ray propagation in the Galaxy has thus far been elusive. When subtracting the secondary background from the observed data, it is therefore critical that all viable models of secondary cosmic-ray antiproton propagation be thoroughly examined, and all uncertainties stemming from plurality of the models be duly considered. In this paper we concentrate on three particular models of Diffusion, Galactic Wind, and Distributed Stochastic Reacceleration, and discuss the predicted antiproton spectrum in each model.

Cosmic ray 1H and 2H spectra from BESS 98

The Balloon-borne Experiment with a Super-solenoidal Spectrometer (BESS) instrument has been flown annually from Lynn Lake Manitoba since 1993. The instrument has been upgraded several times to improve its performance. The instalment flown in 1998 was able to detect 2H clearly between 0.13 and 1.78 GeV/n as a result of improvements made on the time-of-flight (TOF) system. The BESS 98 data were analyzed to obtain the ratio and absolute fluxes of 1H and 2H over this energy range. The results were compared with different cosmic ray propagation models and their implications regarding their propagation history are discussed in this paper.

Propagation of secondary antiprotons and cosmic rays in the Galaxy

Recent measurements of the cosmic ray (CR) antiproton flux have been shown to challenge existing CR propagation models. It was shown that the reacceleration models designed to match secondary to primary nuclei ratios (e.g., B/C) produce too few antiprotons. In the present paper, we discuss one possibility to overcome these difficulties. Using the measured antiproton flux and B/C ratio to fix the diffusion coefficient, we show that the spectra of primary nuclei as measured in the heliosphere may contain a fresh local “unprocessed” component at low energies perhaps associated with the Local Bubble, thus decreasing the measured secondary to primary nuclei ratio. The independent evidence for SN activity in the solar vicinity in the last few Myr supports this idea. The model reproduces antiprotons, B/C ratio, and elemental abundances up to Ni ( Z ⩽ 28). Calculated isotopic distributions of Be and B are in perfect agreement with CR data. The abundances of three “radioactive clock” isotopes in CR, 10Be, 26Al, 36Cl, are all consistent and indicate a halo size z h ∼ 4 kpc based on the most accurate data taken by the ACE spacecraft.

Measurement of3He/4He ratio in cosmic rays with the AMS experiment

The cosmic-ray 3He/4He ratios from 0.09 to 1.2 GeV per nucleon are measured by Alpha Magnetic Spectrometer (AMS) at its precursor mission on board the space shuttle Discovery during the flight STS-91 in June 1998. Taking unique advantage of AMS data collected from a large region covering the earth equator, we also studied the magnetic latitude effect on the ratios. Our analysis shows that the relative abundances of 3He and 4He in cosmic rays depend weakly on the magnetic latitude, and given present uncertainties, the observed isotopic composition is generally consistent with the predictions of the standard propagation model of cosmic rays.

The BALLOON-borne and PAMELA experiments for the study of the antimatter component in cosmic rays

The PAMELA experiment is based on a satellite-borne equipment actually in the final integration phase. It will be installed on board of the Russian satellite Resurs DK1 and launched in a quasi-polar orbit from the Baikonur cosmodrom at the beginning of next year. PAMELA will measure the antiproton and positron fluxes in cosmic rays with high statistics and in a large energy range (80 MeV–190 GeV for antiprotons and 50 MeV–270 GeV for positrons), extending to never investigated energies the measurements of several balloon borne experiments performed by the same PAMELA collaboration in last decade. This will make achievable sensitive tests of cosmic ray propagation models in the Galaxy and the search, in an energy range never investigated before, of possible structures in the fluxes. These structures, related to the presence of primary antiparticle sources, could be signals of “new physics”, connected with open problems like dark matter existence and matter/antimatter symmetry in the Universe. The detector consists of a very precise magnetic spectrometer, several scintillation counter hodoscopes to measure the energy losses and times of flight, and a high granularity and deep Si–W calorimeter, augmented by a very compact transition radiation detector and a He3 neutron detector hodoscope, and protected around and on the top by an anticoincidence system.

Formation of the cosmic-ray spectrum due to its propagation in the Galaxy

A model of cosmic ray propagation is proposed to explain the knee of the cosmic ray energy spectrum in the energy range E ∼ 10 14 −10 17 eV. The numerous stellar winds (SW), ionized hydrogen regions (H-II) and supernova remnants (SNR) in the Galaxy are taken into account in this model. The gas density and the magnetic field in these regions are different from the interstellar gas density and the interstellar magnetic field. Therefore they act as scattering centres and magnetic traps for cosmic rays. It is shown that these regions influence cosmic ray propagation in the Galaxy. Our results show that the collision time between cosmic rays and the SNR, SW, and H-II regions is much less than the cosmic ray lifetime in standard models (Berezinskii et al. 1984; Ginzburg & Syrovatskii 1963), in which only the nuclear interaction of the particles with interstellar gas is taken into account. Cosmic ray energies, and thus the cosmic ray spectrum, change due to interactions with these regions. Cosmic ray energy losses in these regions due to adiabatic cooling are comparable to the losses due to nuclear interaction with interstellar gas. It is therefore necessary to take these into account in galactic cosmic ray propagation models.

Challenging Cosmic‐Ray Propagation with Antiprotons: Evidence for a “Fresh” Nuclei Component?

Recent measurements of the cosmic ray (CR) antiproton flux have been shown to challenge existing CR propagation models. It was shown that the reacceleration models designed to match secondary to primary nuclei ratio (e.g., Boron/Carbon) produce too few antiprotons, while the traditional non-reacceleration models can reproduce the antiproton flux but fall short of explaining the low-energy decrease in the secondary to primary nuclei ratio. Matching both the secondary to primary nuclei ratio and antiproton flux requires artificial breaks in the diffusion coefficient and the primary injection spectrum suggesting the need for other approaches. In the present paper we discuss one possibility to overcome these difficulties. Using the measured antiproton flux to fix the diffusion coefficient, we show that the spectra of primary nuclei as measured in the heliosphere may contain a fresh local unprocessed component at low energies, thus decreasing the measured secondary to primary nuclei ratio. A model reproducing antiprotons, B/C ratio, and abundances up to Ni is presented.

The status of the PAMELA experiment

The PAMELA experiment will be launched on-board of a polar orbiting Resurs DK1 satellite in mid-2003. The primary objective of PAMELA is to measure the flux of antiprotons (80 MeV – 190 GeV) and positrons (50 MeV – 270 GeV) in the cosmic radiation. The wide energy range and large statistics (O(10 4) antiprotons and O(10 5) positrons) will allow sensitive tests of cosmic ray propagation models and searches for exotic sources of antiparticles, such as cold dark matter neutralino annihilations. PAMELA is built around a permanent magnet spectrometer equipped with double-sided silicon detectors and surrounded by an active veto shield. This is complemented by a proportional straw tube / carbon fibre radiator transition radiation detector and a silicon-tungsten imaging calorimeter which is augmented by a scintillator shower tail catcher. A time-of-flight system provides the primary experimental trigger. The status of PAMELA is presented along with main results from simulations and test-beam studies.

Heavy nuclei enrichment of the galactic cosmic rays at high energy: astrophysical interpretation

A substantial increase of the mean logarithmic mass 〈ln A〉 of galactic cosmic rays vs energy has been observed. We study three effects that could explain this trend (i) different source spectra for protons and heavy nuclei (ii) a selective nuclear destruction in flight of heavies (iii) a gradient of the source number and chemical composition in the galactic disk. We take advantage of the diffusive cosmic ray propagation model developed at LAPTH to study specifically the geometrical aspects of the propagation and extend it to high energy. Using a simple modeling of the spectral knee around 10 15 eV, a bump in 〈ln A〉 appears. This feature is smoother when the spectral index of protons is steeper than Fe’s. We analyze the effects of the rigidity dependence of the diffusion coefficient and the scale height of the confinement halo and we show that 〈ln A〉 is most sensitive to the first parameter. Pure geometrical effects are less determining than the diffusion coefficient spectral index. Subsequently, we conclude that the physics of cosmic ray confinement is the essential cause of the heavy nuclei enrichment until ∼10 15 eV.