Interacting Cosmic Rays With Molecular Clouds: A Bremsstrahlung Origin of Diffuse High-Energy Emission From the Inner 2°×1° of the Galactic Center

Abstract

The high-energy activity in the inner few degrees of the Galactic center is traced by diffuse radio, X-ray, and γ -ray emission. The physical relationship between different components of diffuse gas emitting at multiple wavelengths is a focus of this work. We first present radio continuum observations using the Green Bank Telescope and model the nonthermal spectrum in terms of a broken power-law distribution of ∼GeV electrons emitting synchrotron radiation. We show that the emission detected by Fermi is primarily due to nonthermal bremsstrahlung produced by the population of synchrotron emitting electrons in the GeV energy range interacting with neutral gas. The extrapolation of the electron population measured from radio data to low and high energies can also explain the origin of Fe i 6.4 keV line and diffuse TeV emission, as observed with Suzaku, XMM-Newton, Chandra, and the H.E.S.S. observatories. The inferred physical quantities from modeling multiwavelength emission in the context of bremsstrahlung emission from the inner ∼300×120 pc of the Galactic center are constrained to have the cosmic-ray ionization rate ∼1-10 × 10 −15 s −1 , molecular gas heating rate elevating the gas temperature to 75-200 K, fractional ionization of molecular gas 10 −6 -10 −5 , large-scale magnetic field 10-20 μG, the density of diffuse and dense molecular gas ∼100 and ∼10 3 cm −3 over 300 pc and 50 pc path lengths, and the variability of Fe i Kα 6.4 keV line emission on yearly timescales. Important implications of our study are that GeV electrons emitting in radio can explain the GeV γ -rays detected by Fermi and that the cosmic-ray irradiation model, like the model of the X-ray irradiation triggered by past activity of Sgr A * , can also explain the origin of the variable 6.4 keV emission from Galactic center molecular clouds.

On the origin of the 6.4keV line in the Galactic Center region

“…• The line emission from several clouds like the Arches cluster region, Sgr C, G0.162-0.217, GO.11-0.11 and others (see Fukuoka et al, 2009;Tsuru et al, 2010;Tatischeff et al, 2012;Yusef-Zadeh et al, 2013a) is unlikely due to photoionization because e.g. some of them do not show any time variability as expected in the photoionization model.…”

“…Their fluxes are not time variable and the equivalent width of the 6.4 kev line does not correspond to ionization by photons (see, e.g. Fukuoka et al, 2009;Ponti et al, 2010;Yusef-Zadeh et al, 2013a). A nice example is the molecular cloud nearby the Arches cluster whose 6.4 keV emission may be generated by CRs.…”

“…Therefore, e.g. Morfill (1982); Yusef-Zadeh et al (2013a) assumed that there are no MHD-waves for scattering inside the clouds. Just in this approximation we obtained above the estimations for Sgr B2;…”

“…Parameters of the electron spectrum in the GC region can be estimated from the observed flux of radio emission produced by synchrotron losses of relativistic electrons. Yusef-Zadeh et al (2013a) assumed that these electrons ionize diffuse hydrogen gas and generate also the 6.4 keV line. As follows from Oka et al (2005) the rate of ionization is almost constant throughout the GC region; this means an almost uniform distribution of the electrons there.…”

“…• Although the photon origin of the 6.4 keV line emission is widely accepted, Yusef-Zadeh et al (2013a) developed a model of the line production by relativistic electrons which explained also the flux time variability from the GC clouds.…”