[Paper Review] Accretion and ejection in Sgr A*

Ye‐Fei Yuan|arXiv (Cornell University)|Feb 16, 2010

Astrophysical Phenomena and Observations3 citations

TL;DR

This paper proposes that flares in Sgr A* result from episodic ejection of radio-emitting plasma blobs via a magnetohydrodynamic (MHD) mechanism analogous to solar coronal mass ejections. The model explains multiwavelength flares—especially the observed time lags between radio and higher-energy peaks—by linking ejection, magnetic reconnection, and synchrotron/accelerated emission, offering a unified framework for episodic jets in low-luminosity black holes.

ABSTRACT

We review our current understanding to the accretion and ejection processes in Sgr A*. Roughly speaking, they correspond to the quiescent and flare states of the source respectively. The high-resolution {\it Chandra} observations to the gas at the Bondi radius combined with the Bondi accretion theory, the spectral energy distribution from radio to X-ray, and the radio polarization provide us strict constraints and abundant information to the theory of accretion. We review these observational results and describe how the advection-dominated accretion flow model explains these observations. Recently more attentions have been paid to flares in Sgr A*. Many simultaneous multi-wavelength campaigns have been conducted, aiming at uncovering the nature of flares. The main observational properties of flares are briefly reviewed. Especially, the time lag between the peaks of flare at two radio frequencies strongly indicates that the flare is associated with ejection of radio-emitting blobs from the underlying accretion flow. Such kind of episodic jets is distinctive from the continuous jets and are quite common in black hole systems. We introduce the magnetohydrodynamical model for the formation of episodic jets recently proposed based on the analogy with the theory of coronal mass ejection in the Sun. We point out that the various observational appearances of flares should be explained in the framework of this model, since ejection and flare originate from the same physical process.

Motivation & Objective

• To explain the quiescent state of Sgr A* using the advection-dominated accretion flow (ADAF) model, which matches observational constraints on mass accretion rate, temperature, and density at the Bondi radius.
• To investigate the physical origin of Sgr A* flares, particularly the time lags between multiwavelength peaks, which suggest episodic ejection of compact plasma blobs.
• To develop and apply an MHD model for episodic jet formation, inspired by solar coronal mass ejections (CMEs), to explain the ejection process and its observational signatures.
• To unify the interpretation of flares as manifestations of the same physical process underlying blob ejection, rather than separate phenomena.
• To reconcile multiwavelength observations—including spectral energy distribution, polarization, and time-resolved light curves—within a single coherent framework.

Proposed method

• Uses high-resolution Chandra observations to constrain gas density (~100 cm⁻³) and temperature (~2 keV) at the Bondi radius (~0.04 pc), providing outer boundary conditions for accretion models.
• Applies the advection-dominated accretion flow (ADAF) model to explain the low radiative efficiency (η ~ 10⁻⁶) and spectral energy distribution (SED) of Sgr A* from radio to X-ray bands.
• Analyzes simultaneous multiwavelength campaigns to identify time lags between radio and higher-energy flare peaks, indicating that radio flares follow X-ray/IR flares due to delayed blob propagation.
• Adopts an MHD model for episodic jet formation based on solar CME theory: magnetic flux ropes form via reconnection in the accretion flow corona, then eject due to reduced magnetic tension.
• Simulates blob ejection dynamics, showing ejection velocities consistent with light-crossing timescales (~20 s for r_g), and predicts synchrotron emission from the blob and shock fronts.
• Predicts high linear polarization (2–10%) at high radio frequencies due to ordered magnetic fields in the ejected blob, contrasting with lower polarization in the quiescent, tangled-field ADAF.

Experimental results

Research questions

• RQ1 How does the advection-dominated accretion flow (ADAF) model explain the observed low luminosity and spectral energy distribution of Sgr A*?
• RQ2 What causes the time lag between peak emissions at different radio frequencies during Sgr A* flares, and what does it imply about the emission mechanism?
• RQ3 What physical mechanism drives the episodic ejection of compact plasma blobs observed during Sgr A* flares?
• RQ4 How can the observed polarization properties of Sgr A* flares be explained by the magnetic structure of the ejection process?
• RQ5 To what extent can the episodic jet formation in Sgr A* be modeled by analogy with solar coronal mass ejections (CMEs)?

Key findings

• The Bondi accretion rate for Sgr A* is estimated at ~10⁻⁵ M☉ yr⁻¹, consistent with Chandra observations and 3D simulations, providing a robust outer boundary condition.
• The quiescent SED of Sgr A*—with a sub-millimeter bump and power-law radio spectrum—can be naturally explained by the ADAF model, which accounts for low radiative efficiency (η ~ 10⁻⁶).
• Time lags of ~30 minutes between radio flare peaks at different frequencies indicate that radio emission arises from a moving, ejected blob, not a stationary hot spot.
• The MHD model for episodic jets, inspired by solar CMEs, explains blob formation via flux rope evolution and magnetic reconnection, leading to high-velocity ejections (v ~ c/10) on timescales ~20 s.
• Synchrotron emission from the ejected blob and shock fronts in front of it accounts for the radio and sub-millimeter flares, while accelerated electrons in the current sheet produce IR and X-ray flares.
• High linear polarization (2–10%) at high radio frequencies is predicted due to ordered magnetic fields in the ejected plasma, distinguishing it from the lower polarization in the quiescent ADAF state.