Coupled jet-disk model for Sagittarius A*: explaining the flat-spectrum radio core with GRMHD simulations of jets

Abstract

Context. The supermassive black hole in the center of the Milky Way, Sgr A*, displays a nearly flat radio spectrum that is typical for jets in active galactic nuclei. Indeed, time-dependent magnetized models of radiatively inefficient accretion flows (RIAFs), which are commonly used to explain the millimeter, near-infrared, and X-ray emission of Sgr A*, often also produce jet-like outflows. However, the emission from these models has so far failed to reproduce the flat radio spectrum. Aims. We investigate whether current accretion simulations can produce the compact flat spectrum emission by simply using a different prescription for the heating of the radiating particles in the jet. Methods. We studied the radiative properties of accretion flows onto a black hole produced in time-dependent general-relativistic magnetohydrodynamic (GRMHD) simulations. A crucial free parameter in these models has always been the electron temperature, and here we allowed for variations in the proton-to-electron temperature ratios in the jet and disk. Results. We found that the flat spectrum is readily reproduced by a standard GRMHD model if one has an almost isothermal jet coupled to a two-temperature accretion flow. The low-frequency radio emission comes from the outflowing sheath of matter surrounding the strongly magnetized nearly empty jet. The model is consistent with the radio sizes and spectrum of Sgr A*. Conclusions. Hence, GRMHD models of accreting black holes can in principle naturally reproduce jets that match observed characteristics. For Sgr A* the model fit to the spectrum predicts higher mass-accretion rates when a jet is included than without a jet. Hence, the impact of the recently discovered G2 cloud that is expected to be accreted onto Sgr A* might be less severe than currently thought.

General relativistic magnetohydrodynamical simulations of the jet in M 87

“…Although it is uncertain, the plasma density in the jet spine is most likely very low; as a result, this region would not produce any significant radio or synchrotron emission (see, e.g., Mościbrodzka et al 2011). However, as noted by Mościbrodzka & Falcke (2013) and Mościbrodzka et al (2014), the jet definition should also include the jet sheath, which is a tenuous layer of gas that moves along the jet spine. The jet sheath is less magnetized in comparison to the spine (the plasma β parameter decreases with radius from 50 to 1 in the jet sheath, while β 1 in the spine), but has higher matter content that can be constantly resupplied by an accretion disk.…”

“…Recently, we have phenomenologically found a simplified but natural, location-dependent prescription for electron DFs in the GRMHD models that is able to reproduce observational characteristics of Sgr A* (Mościbrodzka & Falcke 2013;Mościbrodzka et al 2014). In particular, the flat spectrum of the source can be reproduced when we assume that electrons along the jet funnel (i.e., in the jet sheath) produced by the RIAF are hotter than those in the RIAF (accretion disk) itself.…”

Jet Signatures in the Spectra of Accreting Black Holes

“…Physically motivated estimates of mass injection in funnel region suggest that the electron number density is in fact very low ( so should not contribute significantly to the emission (Mościbrodzka & Falcke 2013;Mościbrodzka et al 2014).…”

“…Furthermore, because of the similarities between AGN and the low/hard state in XRBs, we assume that the physics of electron heating and cooling is the same across these systems. We therefore choose a range of values of d  and j  motivated by fitting to Sgr A * and M87, since these are the only sources whose spectra have been fitted to constrain these parameters Mościbrodzka et al 2014;Mościbrodzka & Falcke 2013;Chan et al 2015a;Moscibrodzka et al 2015).…”

“…The disadvantage of this approach is that it is only applicable in regimes in which the radiation is dynamically unimportant. These codes have been used by many authors to calculate the observational signatures of low luminosity systems in which the radiation pressure can be neglected (e.g., Chan et al 2009Chan et al , 2015aChan et al , 2015bMościbrodzka et al 2009;Mościbrodzka et al 2014;Shcherbakov et al 2012;Mościbrodzka & Falcke 2013;Shcherbakov & McKinney 2013). These works mainly focussed on reproducing the spectra and variability properties of Sgr A * , and place important constraints on quantities such as the black hole spin, proton-to-electron temperature ratio, and inclination angle.…”