[Paper Review] Low angular momentum accretion flow model of Sgr A* activity

B. Czerny, Monika Mościbrodzka+3 more|ArXiv.org|Oct 12, 2007

Astrophysical Phenomena and Observations20 references3 citations

TL;DR

This paper proposes a low angular momentum accretion flow (LALF) model for Sgr A* to explain its variable, low-luminosity emission. The model features inflowing material from nearby stars with minimal angular momentum, leading to direct infall and natural variability near the innermost stable orbit. It reproduces the observed broad-band spectrum and Faraday rotation constraints when a small fraction of energy is channeled into non-thermal electrons.

ABSTRACT

Sgr A* is a source of strongly variable emission in several energy bands. It is generally agreed that this emission comes from the material surrounding the black hole which is either falling in or flowing out. The activity must be driven by accretion but the character of accretion flow in this object is an open question. We suggest that the inflow is dominated by the relatively low angular momentum material originating in one of the nearby group of stars. Such material flows in directly towards the black hole up to the distance of order of ten Schwarzschild radii or less, where it hits the angular momentum barrier which leads naturally to a flow variability. We study both the analytical and the numerical solutions for the flow dynamics, and we analyze the radiation spectra in both cases using the Monte Carlo code to simulate the synchrotron, bremsstrahlung and the Compton scattering. Our model roughly reproduces the broad band spectrum of Sgr A* and its variability if we allow for a small fraction of energy to be converted to non-thermal population of electrons. It is also consistent (for a range of viewing angles) with the strong constraints on the amount of circumnuclear material imposed by the measurements of the Faraday rotation.

Motivation & Objective

• Explain the variable, low-luminosity emission of Sgr A* through a low angular momentum accretion flow model.
• Address the lack of direct evidence for a cold accretion disk in Sgr A* by proposing a flow that avoids disk formation.
• Reconcile the observed X-ray and radio spectra with accretion physics while satisfying tight constraints from Faraday rotation.
• Investigate the role of non-thermal electron populations in shaping the observed radiation spectrum.
• Assess the impact of general relativistic effects and dynamical variability on spectral and polarization signatures.

Proposed method

• Use analytical and numerical solutions for low angular momentum flow dynamics, including shock formation and radial velocity profiles.
• Simulate radiation spectra using a Monte Carlo code to model synchrotron, bremsstrahlung, and Compton scattering processes.
• Compute the Faraday rotation measure as a function of viewing angle to compare with observational limits from Marrone et al. (2006).
• Integrate photon paths in Kerr spacetime to estimate escape probabilities and assess GR effects on emissivity and spectra.
• Perform MHD simulations with improved temporal resolution to capture fast variability (e.g., 17-minute QPO-like features).
• Compare model predictions with multi-wavelength observations, particularly the broad-band spectrum and polarization angle variations.

Experimental results

Research questions

• RQ1 Can a low angular momentum accretion flow reproduce the observed broad-band spectrum of Sgr A*?
• RQ2 Does the model remain consistent with the tight observational constraints on Faraday rotation measure?
• RQ3 How do general relativistic effects influence the escape probability and emissivity of photons in the flow?
• RQ4 Can the model naturally produce the observed fast variability (e.g., 17-minute timescale) without requiring a standard accretion disk?
• RQ5 What role does a non-thermal electron population play in matching the observed luminosity and spectral shape?

Key findings

• The low angular momentum accretion flow model produces a natural variability pattern due to the angular momentum barrier near the innermost stable orbit.
• The model reproduces the observed broad-band spectrum of Sgr A* when a small fraction of energy is converted into a non-thermal electron population.
• For a range of viewing angles, the model's predicted Faraday rotation measure is consistent with observational limits from Marrone et al. (2006).
• General relativistic effects significantly alter photon escape probabilities, especially in shocked flow configurations, indicating that GR must be included in spectral modeling.
• New MHD simulations show that fast variability on timescales of ~17 minutes can be naturally reproduced in the dynamical evolution of the flow.
• The effective emissivity is higher in shocked flows compared to smooth flows, even without explicit shock emission, highlighting the importance of dynamical structure in spectral modeling.