Nonthermal hard X-ray emission from the Galactic Ridge

¹ Max-Planck Institut für extraterrestrische Physik, Postfach 1603, 85740 Garching, Germany e-mail: vos@mpe.mpg.de,aws@mpe.mpg.de
² P.N. Lebedev Physical Institute, 117924 Moscow, Russia

Corresponding author: V. A. Dogiel, vad@lpi.ru

Received: 28 February 2001
Accepted: 15 November 2001

Abstract

We investigate the origin of the nonthermal X-ray emission from the Galactic ridge in the range  keV. We consider bremsstrahlung of subrelativistic cosmic ray protons and electrons as production processes. 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-off at an energy  MeV for protons or 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 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-off is exponential for  MeV. If the cut-off 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 significant part of the hard X-ray emission at energies  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-off energy can be as small as  MeV and in this case the flux of photons is negligible too. But for small cutoff energies the flux of nuclear gamma-ray lines exceeds significantly the upper limit derived from the COMPTEL and OSSE data. Hence the cut-off of the proton spectrum has to be somewhere in between  MeV in order not to exceed both and gamma-ray line fluxes. However the energy density of the CR protons would have to be  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 only  eV cm^-3. But in this case we must assume that acceleration of protons is suppressed. We discuss briefly the possible origin of this effect. 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 significant for both energetic protons and electrons. In this way we may perhaps resolve the problem of the observed large ionization rate.