Probing the spacetime and accretion model for the Galactic Center: Comparison of Kerr and dilaton black hole shadows

Abstract

Context. In the 2017 observation campaign, the Event Horizon Telescope (EHT) for the first time gathered enough data to image the shadow of the super-massive black hole (SMBH) in M 87. Most recently in 2022, the EHT has published the results for the SMBH at the Galactic Center, Sgr A * . In the vicinity of black holes, the influence of strong gravity, plasma physics, and emission processes govern the behavior of the system. Since observations such as those carried out by the EHT are not yet able to unambiguously constrain models for astrophysical and gravitational properties, it is imperative to explore the accretion models, particle distribution function, and description of the spacetime geometry. Our current understanding of these properties is often based on the assumption that the spacetime is well-described by by the Kerr solution to general relativity, combined with basic emission and accretion models. We explore alternative models for each property performing general relativistic magnetohydrodynamic and radiative transfer simulations. Aims. By choosing a Kerr solution to general relativity and a dilaton solution to Einstein-Maxwell-dilaton-axion gravity as exemplary black hole background spacetimes, we aim to investigate the influence of accretion and emission models on the ability to distinguish black holes in two theories of gravity. Methods. We carry out three-dimensional general relativistic magnetohydrodynamics (GRMHD) simulations of both black holes, matched at their innermost stable circular orbit, in two distinct accretion scenarios. Using general-relativistic radiative transfer (GRRT) calculations, we model the thermal synchrotron emission and in the next step apply a non-thermal electron distribution function, exploring representative parameters to compare with multiwavelength observations. We further consider Kerr and dilaton black holes matched at their unstable circular photon orbits, as well as their event horizons. Results. From the comparison of GRMHD simulations, we found a wider jet opening angle and higher magnetisation in the Kerr spacetime. Generally, MAD models showed larger magnetic flux than SANE, as is expected. The GRRT image morphology shows differences between spacetimes due to the Doppler boosting in the Kerr spacetime. However, from pixel-by-pixel comparison we find that in a real-world observation an imaging approach may not be sufficient to distinguish the spacetimes using the current finite resolution of the EHT. From multiwavelength emission and spectral index analysis, we find that accretion model and spacetime have only a small impact on the spectra compared to the choice of emission model. Matching the black holes at the unstable photon orbit or the event horizon further decreases the observed differences.

The Galactic Center as a laboratory for theories of gravity and dark matter

“…Observations of horizon-scale structures in Sgr A*, on one hand gave important constraints for theoretical models [226,252,253], on the other showed the feasibility of 1.3 mm VLBI techniques to peer into the innermost region of Sgr A*. This motivated theoretical advancement that allowed increasingly sophisticated general-relativistic simulations of accretion flows and radiative transfer of SMBH, reproducing realistic images for a great variety of near-horizon emission models, ATGs (see section 3.3) and alternative to BHs [253][254][255][256][257][258][259][260][261][262][263][264][265][266][267][268][269][270][271]. In parallel, VLBI studies of Sgr A* at 1.3 mm wavelength with progressively enhanced arrays, allowed a better characterization of the compact emission, its variability, significant polarization (which suggest the presence of intense magnetic field in the emitting region) and asymmetry in the image structure [272][273][274].…”

Emission Modeling in the EHT–ngEHT Age

“…Additionally, the distribution function can be modified to account for a thermal and a magnetic contribution to the total energy [10,31,32,53]. That way, it is possible to control the amount of magnetically accelerated electrons and the distance of their point of injection into the jet from the central engine.…”

“…Alternative spacetime geometries are rarely investigated in full GRMHD and GRRT, and if so, the emission physics are based on a constant proton-to-electron temperature ratio and purely thermal radiation [68,69]. Only recently have advances been made to study the influence of emission models more akin to reality in an alternative spacetime [33,53]. The fundamental difficulties in finding deviations from the Kerr metric arise from the presence of greater astrophysical uncertainties.…”

Enhanced Blandford Znajek jet in loop quantum black hole

“…GRMHD simulations of stringy black holes have been implemented in several works [24][25][26]. In [24], they demonstrated that a stronger jet is generated via the BZ process in stringy JCAP05(2024)101 rotating black holes.…”