Broad band spectral modeling of M87 nucleus

Abstract: We study spectra produced by weakly accreting black hole (BH) systems using the semi-analytic advection dominated accretion flow (ADAF) model and the general-relativistic magentohydrodynamic (GRMHD) simulation. We find significant differences between these two approaches related to a wider spread of the flow parameters as well as a much steeper radial distribution of the magnetic field in the latter. We apply these spectral models to the broad-band spectral energy distribution (SED) of the nucleus of M87 galaxy. The standard (in particular, one-dimensional) formulation of the ADAF model does not allow to explain it; previous claims that this model reproduces the observed SED suffer from an inaccurate treatment of the Compton process. The spectra based on GRMHD simulation are in a much better agreement with the observed data. In our GRMHD model, in which we assumed the BH spin $a=0.9$, bulk of radiation observed between the millimeter and the X-ray range is produced in the disk area within 4 gravitational radii from the BH. In this solution, the synchrotron component easily reproduces the spectral data between the millimeter and the UV range, and the Compton component does not violate the X-rays constraints, for accretion rates not exceeding 0.01 Msun/year and a relatively strong magnetic field, with the plasma $\beta \sim 1$ in the region where radiation is produced. However, the Compton component cannot explain the observed X-ray spectrum. Instead, the X-ray spectrum can be reproduced by a high energy tail of the synchrotron spectrum if electrons have a hybrid energy distribution with a $\sim 5$ per cent nonthermal component.