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Register a new userCosmic-ray Particles in the Galactic Center: Blowing in the Wind
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Recent results from multi-wavelength observations of the inner few hundred pc of the Galactic center have added two new characteristics to the ISM in this unique region. One is the cosmic ray ionization rate derived from H$_3^+$ measurements is at least two orders of magnitudes higher than in the disk of the Galaxy. The other is the bipolar thermal X-ray and synchrotron emission from this region, suggesting a relic of past activity. We propose that the high cosmic ray pressure drives a large-scale wind away from the Galactic plane and produces the bipolar emission as well as highly blue-shifted diffuse gas detected in H$_3^+$ absorption studies. We then discuss the interaction of large-scale winds with a number of objects, such as cloudlets and stellar wind bubbles, to explain the unusual characteristics of the ISM in this region including the nonthermal radio filaments. One of the implications of this scenario is the removal of gas driven by outflowing winds may regulate star formation or black hole accretion.
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Measurements of the velocity of interstellar HeI inside of the heliosphere have been conducted over the past forty years. These historical data suggest that the ecliptic longitude of the direction of the interstellar flow has increased at an average rate of about 0.19 degrees per year over time. Possible astronomical explanations for these short-term variations in the interstellar gas entering the heliosphere are presented.
We present new developments on the Cosmic--Ray driven, galactic dynamo, modeled by means of direct, resistive CR--MHD simulations, performed with ZEUS and PIERNIK codes. The dynamo action, leading to the amplification of large--scale galactic magnetic fields on galactic rotation timescales, appears as a result of galactic differential rotation, buoyancy of the cosmic ray component and resistive dissipation of small--scale turbulent magnetic fields. Our new results include demonstration of the global--galactic dynamo action driven by Cosmic Rays supplied in supernova remnants. An essential outcome of the new series of global galactic dynamo models is the equipartition of the gas turbulent energy with magnetic field energy and cosmic ray energy, in saturated states of the dynamo on large galactic scales.
We report the discovery of a $1^circ$ scale X-ray plume in the northern Galactic Center (GC) region observed with Suzaku. The plume is located at ($l$, $b$) $sim$ ($0mbox{$.!!^circ$}2$, $0mbox{$.!!^circ$}6$), east of the radio lobe reported by previous studies. No significant X-ray excesses are found inside or to the west of the radio lobe. The spectrum of the plume exhibits strong emission lines from highly ionized Mg, Si, and S that is reproduced by a thin thermal plasma model with $kT sim 0.7$ keV and solar metallicity. There is no signature of non-equilibrium ionization. The unabsorbed surface brightness is $3times10^{-14}$ erg cm$^{-2}$ s$^{-1}$ arcmin$^{-2}$ in the 1.5-3.0 keV band. Strong interstellar absorption in the soft X-ray band indicates that the plume is not a foreground source but is at the GC distance, giving a physical size of $sim$100 pc, a density of 0.1 cm$^{-3}$, thermal pressure of $1times10^{-10}$ erg cm$^{-3}$, mass of 600 $M_odot$ and thermal energy of $7times10^{50}$ erg. From the apparent association with a polarized radio emission, we propose that the X-ray plume is a magnetized hot gas outflow from the GC.
One important prediction of acceleration of particles in the supernova caused shock in the magnetic wind of exploding Wolf Rayet and Red Super Giant stars is the production of an energetic particle component with an E^-2 spectrum, at a level of a few percent in flux at injection. After allowing for transport effects, so steepening the spectrum to E^-7/3, this component of electrons produces electromagnetic radiation and readily explains the WMAP haze from the Galactic Center region in spectrum, intensity and radial profile. This requires the diffusion time scale for cosmic rays in the Galactic Center region to be much shorter than in the Solar neighborhood: the energy for cosmic ray electrons at the transition between diffusion dominance and loss dominance is shifted to considerably higher particle energy. We predict that more precise observations will find a radio spectrum of u^-2/3, at higher frequencies u^-1, and at yet higher frequencies finally u^-3/2.
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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