Aims. The accretion of stars onto the central supermassive black hole at the center of the Milky Way is predicted to generate large fluxes of subrelativistic ions in the Galactic center region. We ...analyze the intensity, shape, and spatial distribution of de-excitation gamma-ray lines produced by nuclear interactions of these energetic particles with the ambient medium. Methods. We first estimated the amount and mean kinetic energy of particles released from the central black hole during star disruption. We then calculated the energy and spatial distributions of these particles in the Galactic center region from a kinetic equation. These particle distributions were then used to derive the characteristics of the main nuclear interaction gamma-ray lines. Results. Because the time period of star capture by the supermassive black hole is expected to be shorter than the lifetime of the ejected fast particles against Coulomb losses, the gamma-ray emission is predicted to be stationary. We find that the nuclear de-excitation lines should be emitted from a region with a maximum 5° angular radius. The total gamma-ray line flux below 8 MeV is calculated to be ~10-4 photons cm-2 s-1. The most promising lines for detection are those at 4.44 and ~6.2 MeV, with a predicted flux in each line of ~10-5 photons cm-2 s-1. Unfortunately, it is unlikely that this emission can be detected with the INTEGRAL observatory. But the predicted line intensities appear to be within reach of future gamma-ray space instruments. A future detection of de-excitation gamma-ray lines from the Galactic center region would provide unique information on the high-energy processes induced by the central supermassive black hole and the physical conditions of the emitting region.
The GAMMA-400 telescope will measure the fluxes of gamma rays and cosmic-ray electrons and positrons in the energy range from 100MeV to several TeV. These measurements will allow it to achieve the ...following scientific objectives: search for signatures of dark matter, investigation of gamma-ray point-like and extended sources, study of the energy spectrum of the Galactic and extragalactic diffuse emission, study of gamma-ray bursts and gamma-ray emission from the active Sun, together with high-precision measurements of the high-energy electrons and positrons spectra, protons and nuclei up to the knee.
The bulk of cosmic rays are protons and helium nuclei, whereas the lepton component in the total flux is ∼10−3 at high energy. In the present paper, the simulated capability of the GAMMA-400 telescope to distinguish electrons and positrons from protons in cosmic rays is addressed. The individual contribution to the proton rejection from each detector system of GAMMA-400 is studied separately. The use of the combined information from all detectors allows us to reach a proton rejection of the order of ∼4×105 for vertical incident particles and ∼3×105 for particles with initial inclination of 30° in the electron energy range from 50GeV to 1TeV.
The GAMMA-400 experiment: Status and prospects Topchiev, N. P.; Galper, A. M.; Bonvicini, V. ...
Bulletin of the Russian Academy of Sciences. Physics,
03/2015, Volume:
79, Issue:
3
Journal Article
Peer reviewed
The development of the GAMMA-400 γ-ray telescope continues. The GAMMA-400 is designed to measure fluxes of γ-rays and the electron-positron cosmic-ray component possibly associated with annihilation ...or decay of dark matter particles; and to search for and study in detail discrete γ-ray sources, to measure the energy spectra of Galactic and extragalactic diffuse γ-rays, and to study γ-ray bursts and γ-rays from the active Sun. The energy range for measuring γ-rays and electrons (positrons) is from 100 MeV to 3000 GeV. For 100-GeV γ-rays, the γ-ray telescope has an angular resolution of ∼0.01°, an energy resolution of ∼1%, and a proton rejection factor of ∼5 × 10
5
. The GAMMA-400 will be installed onboard the Russian Space Observatory.
We have investigated the properties of a group of Delta *g-ray-emitting globular clusters (GCs) that have recently been uncovered in our Galaxy. By correlating the observed Delta *g-ray luminosity L ...Delta *g with various cluster properties, we probe the origin of the high-energy photons from these GCs. We report that L Delta *g is positively correlated with the encounter rate Delta *G c and the metallicity Fe/H, which places an intimate link between the Delta *g-ray emission and the millisecond-pulsar population. We also find a tendency that L Delta *g increases with the energy densities of the soft photon at the cluster location. Furthermore, the two-dimensional regression analysis suggests that L Delta *g, soft-photon densities, and Delta *G c /Fe/H possibly span fundamental planes; this finding could potentially provide better predictions for the Delta *g-ray properties of GCs.
In re-analyzing the archival Chandra data of the globular cluster 47 Tucanae, we have detected a new diffuse X-ray emission feature within the half-mass radius of the cluster. The spectrum of the ...diffuse emission can be described by a power-law model plus a plasma component with photon index Gamma ~ 1.0 and plasma temperature kT ~ 0.2 keV. While the thermal component is apparently uniform, the non-thermal contribution falls off exponentially from the core. The observed properties could possibly be explained in the context of multiple shocks resulting from the collisions among the stellar wind in the cluster and the inverse Compton scattering between the pulsar wind and the relic photons.
We show that the well-known discrepancy, known for about two decades, between the radial dependence of the Galactic cosmic ray nucleon distribution, as inferred most recently from EGRET observations ...of diffuse γ-rays above 100 MeV, and of the most likely cosmic ray source distribution (supernova remnants, superbubbles, pulsars) can be explained purely by propagation effects. Contrary to previous claims, we demonstrate that this is possible, if the dynamical coupling between the escaping cosmic rays and the thermal plasma is taken into account, and thus a self-consistent calculation of a Galactic Wind is carried out. Given a dependence of the cosmic ray source distribution on Galactocentric radius r, our numerical wind solutions show that the cosmic ray outflow velocity, $V(r,z) = u_0 + V_{{\rm A} 0}$, also depends both on r, as well as on vertical distance z, with u0 and $V_{{\rm A} 0}$ denoting the thermal gas and the Alfvén velocities, respectively, at a reference level zC. The latter is by definition the transition boundary from diffusion to advection dominated cosmic ray transport and is therefore also a function of r. In fact, the cosmic ray escape time averaged over particle energies decreases with increasing cosmic ray source strength. Thus an increase in cosmic ray source strength is counteracted by a reduced average cosmic ray residence time in the gas disk. This means that pronounced peaks in the radial distribution of the source strength result in mild radial γ-ray gradients at GeV energies, as it has been observed. The effect might be enhanced by anisotropic diffusion, assuming different radial and vertical diffusion coefficients. In order to better understand the mechanism described, we have calculated analytic solutions of the stationary diffusion-advection equation, including anisotropic diffusion in an axisymmetric geometry, for a given cosmic ray source distribution and a realistic outflow velocity field $V(r,z)$, as inferred from the self-consistent numerical Galactic Wind simulations performed simultaneously. At TeV energies the γ-rays from the sources themselves are expected to dominate the observed “diffuse” flux from the disk. Its observation should therefore allow an empirical test of the theory presented.
We investigate the origin of the diffuse 6.4 keV line emission recently detected by Suzaku and the source of H2 ionization in the diffuse molecular gas of the Galactic center (GC) region. We show ...that Fe atoms and H2 molecules in the diffuse interstellar medium of the GC are not ionized by the same particles. The Fe atoms are most likely ionized by X-ray photons emitted by Sgr A* during a previous period of flaring activity of the supermassive black hole. The measured longitudinal intensity distribution of the diffuse 6.4 keV line emission is best explained if the past activity of Sgr A* lasted at least several hundred years and released a mean 2-100 keV luminosity >~ 1038 erg s-1. The H2 molecules of the diffuse gas cannot be ionized by photons from Sgr A*, because soft photons are strongly absorbed in the interstellar gas around the central black hole. The molecular hydrogen in the GC region is most likely ionized by low-energy cosmic rays, probably protons rather than electrons, whose contribution into the diffuse 6.4 keV line emission is negligible.
The origin and properties of the source of positrons annihilating in the Galactic Centre (GC) are still a mystery. One of the criteria that may discriminate between different mechanisms of positron ...production there is the injection energy of positrons. Beacom and Yüksel suggested a method to estimate this energy from the ratio of the 511-keV line to the MeV in-flight annihilation fluxes. From COMPTEL data, they derived that the maximum injection energy of positrons should be about several MeV. This significantly decreased the class of models of the positron origin in the GC, assuming that positrons lose their energy through Coulomb collisions only. However, observations show that the strength of the magnetic field in the GC is much higher than in other parts of the Galaxy; in the GC, it may range from 100 μG to several mG. In these conditions, the synchrotron losses of positrons are significant and this extends the range of acceptable values of the injection energy of positrons. We show that if positron injection in the GC is non-stationary and the magnetic field is higher than 0.4 mG, both radio and gamma-ray restrictions permit the energy to be higher than several GeV.
Aims.The stochastic acceleration of subrelativistic electrons from a background plasma is studied in order to find a possible explanation of the hard X-ray emission detected from the Coma cluster. ...Methods.We calculate the necessary energy supply as a function of the plasma temperature and of the electron energy, and we show that, for the same value of the hard X-ray flux, the energy supply changes gradually from its high value for the case when emitting particle are non-thermal to lower values when the electrons are thermal. The kinetic equations we use include terms describing particle thermalization as well as momentum diffusion due to the Fermi II acceleration. Results.We show that the temporal evolution of the particle distribution function has, at its final stationary stage, a rather specific form. This distribution function cannot be described by simple exponential or power-law expressions. A broad transfer region is formed by Coulomb collisions at energies between the Maxwellian and power-law parts of the distribution functions. In this region the radiative lifetime of a single quasi-thermal electron differs greatly from the lifetime of the distribution function as a whole. For a plasma temperature of 8 keV, the particles emitting bremsstrahlung at $20{-}80$ keV lie in this quasi-thermal regime. We show that the energy supply required by quasi-thermal electrons to produce the observed hard X-ray flux from Coma is one or two orders of magnitude smaller than the value derived from the assumption of a nonthermal origin of the emitting particles. This result may solve the problem of rapid cluster overheating by nonthermal electrons raised by Petrosian (2001): while Petrosian's estimates are correct for nonthermal particles, they are inapplicable in the quasi-thermal range. We finally analyze the change in Coma's Sunyaev-Zeldovich effect caused by the implied distortions of the Maxwellian spectrum of electrons, and we show that evidence for the acceleration of subrelativistic electrons can, in principle, be derived from detailed spectral measurements.
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