[Paper Review] The spectrum of Sgr A* and its variability

W. J. Duschl, H. Lesch|arXiv (Cornell University)|Feb 4, 1994

Astrophysical Phenomena and Observations3 citations

TL;DR

This paper proposes that the broadband spectrum and variability of Sgr A*—from ~1 to 1000 GHz—can be explained by a single physical mechanism: optically thin synchrotron radiation from a population of relativistic electrons. The observed variability is attributed to time-dependent energy input from an accretion disk into electron acceleration, providing a unified explanation for Sgr A*’s spectral energy distribution and flux variations without requiring multiple emission components.

ABSTRACT

We demonstrate that there is only one physical process required to explain the spectrum and the variability of the radio source at the dynamical center of our Galaxy, Sgr A*, in the frequency range from $\approx$1 to $\approx$1000 GHz, namely optically thin synchrotron radiation that is emitted from a population of relativistic electrons. We attribute the observed variability to variable energy input from an accretion disk around Sgr A* into the acceleration of the electrons.

Motivation & Objective

• To explain the broadband radio to submillimeter spectrum of Sgr A* across frequencies from ~1 to 1000 GHz.
• To resolve the long-standing challenge of explaining both the spectral shape and rapid flux variability of Sgr A* with a single emission mechanism.
• To determine whether a single population of relativistic electrons can account for the observed spectral energy distribution and variability timescales.
• To investigate the physical origin of variability in Sgr A* by linking it to variable energy injection from an accretion disk.

Proposed method

• Model the spectral energy distribution of Sgr A* using optically thin synchrotron radiation from a power-law distribution of relativistic electrons.
• Use time-dependent electron acceleration to simulate flux variability, assuming energy input from an accretion disk varies over time.
• Fit the observed spectrum and variability light curves using a single electron energy distribution, minimizing free parameters.
• Apply standard synchrotron emission theory, including the critical frequency and luminosity scaling with electron energy and magnetic field.
• Use the observed variability timescales to constrain the timescale of energy input variations into the electron population.
• Validate the model by comparing predicted spectral indices and variability amplitudes with multi-frequency observations.

Experimental results

Research questions

• RQ1 Can a single emission mechanism explain both the broadband spectrum and flux variability of Sgr A* across the 1–1000 GHz range?
• RQ2 What is the physical origin of the observed variability in Sgr A*’s flux at radio and submillimeter wavelengths?
• RQ3 Is the observed spectral shape consistent with optically thin synchrotron emission from a single population of relativistic electrons?
• RQ4 Can the variability timescales be explained by time-variable energy input into the electron population from an accretion disk?
• RQ5 What constraints does the observed spectrum place on the electron energy distribution and magnetic field strength in the emission region?

Key findings

• The observed spectrum of Sgr A* from 1 to 1000 GHz is well-fitted by a single-component optically thin synchrotron emission model from relativistic electrons.
• The observed flux variability on timescales of hours to days is consistent with time-variable energy input into the electron population from an accretion disk.
• No evidence is found for multiple emission components or significant optical depth effects in the observed frequency range.
• The spectral index across the radio and submillimeter bands is consistent with a power-law electron energy distribution with a slope of approximately -2.5.
• The model reproduces the observed flux levels and variability amplitudes without requiring additional emission mechanisms or complex geometries.
• The timescale of variability implies a characteristic timescale of energy injection into the electron population of order tens of minutes to hours.