V. A. Dogiel

Annihilation Emission from the Galactic Black Hole

Both diffuse high-energy gamma rays and an extended electron-positron annihilation line emission have been observed in the Galactic Center (GC) region. Although X-ray observations indicate that the Galactic black hole Sgr A* is inactive now, we suggest that Sgr A* can become active when a captured star is tidally disrupted and matter is accreted into the black hole. As a consequence the Galactic black hole could be a powerful source of relativistic protons. We are able to explain the current observed diffuse gamma rays and the very detailed 511 keV annihilation line of secondary positrons by p-p collisions of such protons, with appropriate injection times and energy. Relativistic protons could have been injected into the ambient material if the black hole captured a 50 M☉ star at several tens times 106 yr ago. An alternative possibility is that the black hole continues to capture stars with ~1 M☉ every 105 yr. Secondary positrons produced by p-p collisions at energies 30 MeV are cooled down to thermal energies by Coulomb collisions and are annihilated in the warm neutral and ionized phases of the interstellar medium with temperatures about several eV, because the annihilation cross section reaches its maximum at these temperatures. It takes about 10 million years for the positrons to cool down to thermal temperatures so that they can diffuse into a very large extended region around the GC. A much more recent star capture may also be able to account for recent TeV observations within 10 pc of the GC, as well as for the unidentified GeV gamma-ray sources found by EGRET at GC. The spectral difference between the GeV and TeV flux could be explained naturally in this model as well.

THE ORIGIN OF GAMMA RAYS FROM GLOBULAR CLUSTERS

Fermi has detected gamma-ray emission from eight globular clusters (GCs). It is commonly believed that the energy sources of these gamma rays are millisecond pulsars (MSPs) inside GCs. Also it has been standard to explain the spectra of most Fermi Large Area Telescope pulsars including MSPs resulting from the curvature radiation (CR) of relativistic electrons/positrons inside the pulsar magnetosphere. Therefore, gamma rays from GCs are expected to be the collection of CR from all MSPs inside the clusters. However, the angular resolution is not high enough to pinpoint the nature of the emission. In this paper, we calculate the gamma rays produced by the inverse Compton (IC) scattering between relativistic electrons/positrons in the pulsar wind of MSPs in the GCs and background soft photons including cosmic microwave/relic photons, background star lights in the clusters, the galactic infrared photons, and the galactic star lights. We show that the gamma-ray spectrum from 47 Tucanae can be explained equally well by upward scattering of either the relic photons, the galactic infrared photons, or the galactic star lights, whereas the gamma-ray spectra from the other seven GCs are best fitted by the upward scattering of either the galactic infrared photons or the galactic star lights. We also find that the observed gamma-ray luminosity is correlated better with the combined factor of the encounter rate and the background soft photon energy density. Therefore, the IC scattering may also contribute to the observed gamma-ray emission from GCs detected by Fermi in addition to the standard CR process. Furthermore, we find that the emission region of high-energy photons from GCs produced by the IC scattering is substantially larger than the cores of GCs with a radius >10 pc. The diffuse radio and X-rays emitted from GCs can also be produced by the synchrotron radiation and IC scattering, respectively. We suggest that future observations including radio, X-rays, and gamma rays with energy higher than 10 GeV and better angular resolution can provide better constraints for the models.

Diffuse gamma-ray emission from the Galactic center - A multiple energy injection model

We suggest that the energy source of the observed diffuse gamma-ray emission from the direction of the Galactic center is the Galactic black hole Sgr A*, which becomes active when a star is captured at a rate of ∼10 −5 yr −1 . Subsequently the star is tidally disrupted and its matter is accreted into the black hole. During the active phase relativistic protons with a characteristic energy ∼6 × 10 52 erg per capture are ejected. Over 90% of these relativistic protons disappear due to proton-proton collisions on a timescale τpp ∼ 10 4 years in the small central bulge region with radius ∼50 pc within Sgr A*, where the density is ≥10 3 cm −3 . The gamma-ray intensity, which results from the decay of neutral pions produced by proton-proton collisions, decreases according to e −t/τ pp ,w heret is the time after last stellar capture. Less than 5% of relativistic protons escaped from the central bulge region can survive and maintain their energy for >10 7 years due to much lower gas density outside, where the gas density can drop to ∼ 1c m −3 . They can diffuse to a ∼500 pc region before disappearing due to proton-proton collisions. The observed diffuse GeV gamma-rays resulting from the decay of neutral pions produced via collision between these escaped protons and the gas in this region is expected to be insensitive to time in the multiinjection model with the characteristic injection rate of 10 −5 yr −1 . Our model calculated GeV and 511 keV gamma-ray intensities are consistent with the observed results of EGRET and INTEGRAL, however, our calculated inflight annihilation rate cannot produce sufficient intensity to explain the COMPTEL data.

The Origin of Diffuse X-Ray Emission from the Galactic Ridge. II. Nonequilibrium Emission Due to In Situ Accelerated Electrons

The origin of the Galactic ridge X-ray emission has been investigated from various points of view, such as the iron K line, the hard part of the continuum, and energetics. We propose a ridge plasma model based on stochastic particle acceleration in the interstellar medium to explain the properties of the soft and hard X-rays consistently. In situ accelerated electrons form a spectrum that consists of three components: bulk thermal (Maxwellian), quasi-thermal, and nonthermal (power law) through diffusion in momentum space. For the bulk temperature of a few hundred eV, the quasi-thermal component extends up to a few tens of keV. While nonthermal electrons are collisionless, so as to stay in the acceleration regime, quasi-thermal electrons interact with the bulk electrons through Coulomb collisions. Thus, the interaction with the bulk plasma significantly alters the X-ray emission, and the resultant spectrum can explain the observed features that resemble the emission from a multitemperature or nonequilibrium plasma of order of keV. From a comparison of the model predictions with the observed spectrum, we found that the Galactic ridge X-ray emission is explained by electron acceleration in interstellar gas with temperature 0.3-0.6 keV and density (6.5–3.4) × 10-3 cm-3, which can be bound by Galactic gravity. This model can also solve the energetics problem: since a substantial part of the X-ray flux is accounted for by nonequilibrium emission due to quasi-thermal electrons of a small fraction of the medium, we need neither hot plasmas of the order of keV nor higher rates of supernova explosion in the Galaxy to explain the ridge X-ray emission.

The origin of diffuse X-ray emission from the galactic ridge. I. Energy output of particle sources

We analyze processes for the hard X-ray emission from the Galactic disk, whose origin has remained enigmatic for many years. Up until now, no model has been able to explain the physical origin of this emission. Even the most plausible mechanism of bremsstrahlung radiation requires an energy output in emitting particles higher than the luminosity provided by known Galactic sources. We show that this energy enigma can be resolved if the emission comes directly from regions of particle acceleration. In this case, a broad quasi-thermal transition region of particle excess is formed between the thermal and nonthermal energy regions. The necessary energy output for production of electrons emitting 10 keV X-rays is of the order of 1041 ergs s-1, which can definitely be supplied by supernovae or other known Galactic sources of energy. The temperature of the accelerating region is restricted to a value of a few 100 eV, and plasmas with these temperatures are hydrostatically stable in the Galaxy. Since only background electrons are supposed to be accelerated, the acceleration process does not violate the state of hydrostatic equilibrium in the Galactic disk.

Nonthermal hard X-ray emission from the Galactic Ridge

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 pro- cesses. 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-o at an energy150 500 MeV for protons or300 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 photons produced by the accelerated protons is higher than the observed flux from the Galactic ridge if the cut-o is exponential for 150 MeV. If the cut-o in the spectrum is extremely steep its value can be as large as 400 MeV, just near the threshold energy for 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 signicant part of the hard X-ray emission at energies100 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-o energy can be as small as 30 50 MeV and in this case the flux of photons is negligible too. But for small cuto energies the flux of nuclear gamma-ray lines exceeds signicantly the upper limit derived from the COMPTEL and OSSE data. Hence the cut-o of the proton spectrum has to be somewhere in between 50 150 MeV in order not to exceed both and gamma-ray line fluxes. However the energy density of the CR protons would have to be400 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 fluxes at all, and the required energy density of particles is only0: 2e V cm 3 . But in this case we must assume that acceleration of protons is suppressed. We discuss briefly the possible origin of this eect. 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 signicant for both energetic protons and electrons. In this way we may perhaps resolve the problem of the observed large ionization rate.

THE FUNDAMENTAL PLANE OF GAMMA-RAY GLOBULAR CLUSTERS

We have investigated the properties of a group of γ-ray-emitting globular clusters (GCs) that have recently been uncovered in our Galaxy. By correlating the observed γ-ray luminosity L γ with various cluster properties, we probe the origin of the high-energy photons from these GCs. We report that L γ is positively correlated with the encounter rate Γ c and the metallicity [Fe/H], which places an intimate link between the γ-ray emission and the millisecond-pulsar population. We also find a tendency that L γ increases with the energy densities of the soft photon at the cluster location. Furthermore, the two-dimensional regression analysis suggests that L γ, soft-photon densities, and Γ c /[Fe/H] possibly span fundamental planes; this finding could potentially provide better predictions for the γ-ray properties of GCs.

Multi-wavelength Emission from the Fermi Bubbles. I. Stochastic Acceleration from Background Plasma

We analyze processes of electron acceleration in the Fermi bubbles in order to define parameters and restrictions of the models, which are suggested for the origin of these giant radio and gamma-ray structures. In the case of the leptonic origin of the nonthermal radiation from the bubbles, these electrons should be produced somehow in situ because of the relatively short lifetime of high-energy electrons, which lose their energy by synchrotron and inverse-Compton processes. It has been suggested that electrons in bubbles may be accelerated by shocks produced by tidal disruption of stars accreting onto the central black hole or a process of re-acceleration of electrons ejected by supernova remnants. These processes will be investigated in subsequent papers. In this paper, we focus on in situ stochastic (Fermi) acceleration by a hydromagnetic/supersonic turbulence, in which electrons can be directly accelerated from the background plasma. We showed that the acceleration from the background plasma is able to explain the observed fluxes of radio and gamma-ray emission from the bubbles, but the range of permitted parameters of the model is strongly restricted.

A past capture event at Sagittarius A* inferred from the fluorescent X-ray emission of Sagittarius B clouds

The fluorescent X-ray emission from neutral iron in the molecular clouds [Sagittarius (Sgr) B] indicates that the clouds are being irradiated by an external X-ray source. The source is probably associated with the Galactic central black hole (Sgr A*), which triggered a bright outburst 100 yr ago. We suggest that such an outburst could be due to a partial capture of a star by Sgr A*, during which a jet was generated. Using constraints from the observed flux and the time variability (∼10 yr) of the Sgr B fluorescent emission, we find that the shock produced by the interaction of the jet with the dense interstellar medium represents a plausible candidate for the X-ray source emission.

MULTI-WAVELENGTH EMISSION FROM THE FERMI BUBBLE. III. STOCHASTIC (FERMI) RE-ACCELERATION OF RELATIVISTIC ELECTRONS EMITTED BY SNRs

We analyse the model of stochastic re-acceleration of electrons, which are emitted by supernova remnants (SNRs) in the Galactic Disk and propagate then into the Galactic halo, in order to explain the origin on nonthermal (radio and gamma-ray) emission from the Fermi Bubbles (FB). We assume that the energy for re-acceleration in the halo is supplied by shocks generated by processes of star accretion onto the central black hole. Numerical simulations show that regions with strong turbulence (places for electron re-acceleration) are located high up in the Galactic Halo about several kpc above the disk. The energy of SNR electrons that reach these regions does not exceed several GeV because of synchrotron and inverse Compton energy losses. At appropriate parameters of re-acceleration these electrons can be re-accelerated up to the energy 10E12 eV which explains in this model the origin of the observed radio and gamma-ray emission from the FB. However although the model gamma-ray spectrum is consistent with the Fermi results, the model radio spectrum is steeper than the observed by WMAP and Planck. If adiabatic losses due to plasma outflow from the Galactic central regions are taken into account, then the re-acceleration model nicely reproduces the Planck datapoints.