Restrictions on the injection energy of positrons annihilating near the Galactic center

Topic: An INTEGRAL View of Compact ObjectsBeacom and Yüksel (2006) suggested an elegant method to estimate an initial energy of annihilating positrons from the ratio of 511 keV line and in-flight annihilation fluxes. The idea was that in the case of stationary injection of positrons at high energies the fluxes of 511 keV line emission (generated by thermalized positrons) and of in-flight annihilation (produced by fast positrons) were proportional to each other. For the delta-function injection spectrum they showed that the initial energy of annihilating positrons cannot exceed 3 MeV. Otherwise the expected in-flight annihilation flux of fast positrons in the range 1 to 10 MeV is higher than observed by COMPTEL. However, the conclusion may be changed significantly if one analyzes the spatial distribution of the 511 keV emission and 1-10 MeV emission. In addition if the magnetic field in Galactic center region is 1 mG or higher the injection energy of positrons produced by central compact source can be unrestricted.published_or_final_versio

X-Ray afterglow of SWIFT J1644+57: a Compton echo?

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Origin of thermal and non-thermal hard X-ray emission from the Galactic center

Topic: An INTEGRAL View of Compact ObjectsWe analyse new results of CHANDRA and SUZAKU which found a flux of hard X-ray emission from the compact region around Sgr A* (r∼ 100 pc). We propose that this emission is a consequence of a special transient accretion process when a part of captured star obtains an additional angular momentum. As a result a flux of subrelativistic protons is ejected from the Galactic black hole, which heats up the background plasma in the Galactic center up to temperature about 6-10 keV and produces by inverse bremsstrahlung a flux of non-thermal X-ray emission in the energy range above 10 keV.published_or_final_versio

Galactic Center Research: Manifestations of the Central Black Hole

This review summarizes a few of the frontiers of Galactic center research that are currently the focus of considerable activity and attention. It is aimed at providing a necessarily incomplete sketch of some of the timely work being done on phenomena taking place in, or originating in, the central few parsecs of the Galaxy, with particular attention to topics related to the Galactic black hole (GBH). We have chosen to expand on the following exciting topics: 1) the characterization and the implications for the variability of emission from the GBH, 2) the strong evidence for a powerful X-ray flare in the Galactic center within the past few hundred years, and the likelihood that the GBH is implicated in that event, 3) the prospects for detecting the "shadow" of the GBH, 4) an overview of the current state of research on the central S-star cluster, and what has been learned from the stellar orbits within that cluster, and 5) the current hypotheses for the origin of the G2 dust cloud that is projected to make a close passage by the GBH in 2013.Comment: Review article, in press with Review of Astronomy and Astrophysic

High Energy Radiation from the Galactic Black Hole Sgr A*

We suggest that the energy source of the observed TeV, GeV and 511 KeV annihilation lines from the direction of the Galactic Center is the Galactic black hole Sgr A*, which becomes active when a captured star is tidally disrupted and matter is accreted into the black hole. During the active phase relativistic protons are ejected. GeV-TeV photons result from the decay of neutral pions produced by proton-proton collisions. Positrons lost most of their energies in the central region and diffuse to 500 pc region, where they cool to several eV via ionization loss. 511 KeV lines are produced by these thermalized positrons through the decay of positronium in warm gas. It is extremely important to notice that the energy injection is not continuous because the capture event only takes place once every 105yr and the gas density in the Galactic Center is non-uniform. We compare the observed data with model predictions. © 2007 Elsevier B.V. All rights reserved.link_to_subscribed_fulltex

High energy radiation from the direction of the galactic black hole Sgr A*

X-ray observations indicate that the Galactic black hole Sgr A* is inactive now, however, we suggest that Sgr A* can become active when a captured star is tidally disrupted and matter is accreted into the black hole. Consequently the Galactic black hole could be a powerful source of relativistic protons with a characteristic energy ∼1052 erg per capture. The diffuse GeV and TeV γ-rays emitted in the direction of the Galactic Center (GC) are the direct consequences of p-p collisions of such relativistic protons ejected by very recent capture events occurred ≤105 yr ago. On the other hand, the extended electron-positron annihilation line emission observed from GC is a phenomenon related to a large population of thermalized positrons, which are produced, cooled down and accumulated through hundreds of past capture events during a period of ∼107 yr. In addition to explaining GeV, TeV and 511 keV annihilation emissions we also estimate the photon flux of several MeV resulting from in-flight annihilation process. © 2007 COSPAR.link_to_subscribed_fulltex

On the origin of annihilation emission from the Galactic Center

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 50M ⊙star at several tens million years ago. An alternative possibility is that the black hole continues to capture stars with ∼1M ⊙ every hundred thousand years. Secondary positrons produced by p -p collisions at energies ≥ 30 MeV are cooled down to thermal energies by Coulomb collisions, and annihilate 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 ten million years for the positrons to cool down to thermal temperatures so they can diffuse into a very large extended region around the Galactic center. A much more recent star capture may be also able to account for recent TeV observations within 10 pc of the galactic center as well as for the unidentified GeV gamma-ray sources found by EGRET at GC. The spectral difference between the GeV flux and the TeV flux could be explained naturally in this model as well.link_to_subscribed_fulltex

Restrictions on the injection energy of positrons annihilating near the Galactic Centre

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. © 2010 The Authors. Journal compilation © 2010 RAS.link_to_subscribed_fulltex

X- and gamma-ray emission from the galactic center

Session V. High-Energy Diagnostics of Galactic Nuclear ActivityASP Conference Series v. 43