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

Abstract

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.

Sgr B2 hard X-ray emission with INTEGRAL after 2009: still detectable?

“…Due to the time variability in the Sgr B2 light curve at a few-year scale obtained for the T1 time interval (see Sect. 4), which contradicts the CR scenario (see e.g., Dogiel et al 2014;Tatischeff et al 2012), we applied this model only at the T2 spectrum. The obtained results are in Table 2.…”

“…LECRs can reproduce an X-ray continuum emission and Fe 6.4 keV emission line via bremsstrahlung and fluorescence mechanisms, respectively (see e.g., Tatischeff 2003;Tatischeff et al 2012). The observed fading of the molecular clouds' X-ray emission is in contradiction with the LECR hypothesis (see e.g., Dogiel et al 2014). However, when the front of the X-ray flare finishes its propagation, constant emission caused by LECRs may become visible.…”

“…An alternative hypothesis of the GC molecular clouds X-ray emission is excitation of neutral matter by collisions with low-energy cosmic rays (LECRs, see e.g., Dogiel et al 2009;Dogiel et al 2013Dogiel et al , 2014. LECRs can reproduce an X-ray continuum emission and Fe 6.4 keV emission line via bremsstrahlung and fluorescence mechanisms, respectively (see e.g., Tatischeff 2003;Tatischeff et al 2012).…”

Origin of diffuse X-ray emission and ionization in the GC

“…Simple estimations [22] show that if the ionization rate in Sgr B2 is 4 × 10 −16 s −1 as measured in the GC (see [24]) then the expected 6.4 keV flux produced by CR protons equals or is below 1% of the flux produced by X-ray photons from Sgr A*.…”

“…where m is the electron mass, v is the proton velocity, β = v/c, ln Λ is the Coulomb logarithm. The spectrum of CRs inside the cloud, N p (E, x), can be derived from the equation (see [22])…”