Nonthermal Electrons in Radiatively Inefficient Accretion Flow Models of Sagittarius A

TLDR: In this paper, the authors investigated radiatively inefficient accretion flow models for Sgr A*, the supermassive black hole in our Galactic center, in light of new observational constraints.

Abstract: We investigate radiatively inefficient accretion flow models for Sgr A*, the supermassive black hole in our Galactic center, in light of new observational constraints. Confirmation of linear polarization in the submillimeter emission argues for accretion rates much less than the canonical Bondi rate. We consider models with low accretion rates and calculate the spectra produced by a hybrid electron population consisting of both thermal and nonthermal particles. The thermal electrons produce the submillimeter emission and can account for its linear polarization properties. As noted in previous work, the observed low-frequency radio spectrum can be explained if a small fraction (≈1.5%) of the electron thermal energy resides in a soft power-law tail. In the innermost region of the accretion flow, turbulence and/or magnetic reconnection events may occasionally accelerate a fraction of the electrons into a harder power-law tail. We show that the synchrotron emission from these electrons, or the Compton upscattering of synchrotron photons by the same electrons, may account for the X-ray flares observed by Chandra.

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Fig. 4.— The effect of different electron power-law indices p on the model shown in Fig. 1. Only the synchrotron emission from power-law and thermal electrons is shown.

Fig. 3.— (a): Synchrotron self-absorption optical depth of thermal electrons as a function of radius at three frequencies for the model shown in Fig. 1. (b): The degree of linear polarization from thermal electrons at four frequencies with (filled circles) and without (open circles) Faraday rotation included. These values are upper limits because they assume a uniform B-field.

Fig. 7.— Two inverse Compton models for the X-ray flare. (a): The dot-dashed line is the quiescent emission from Fig. 1. The dashed line is the synchrotron and SSC emission in the “flaring region” — power-law electrons in a ≈ 2.5RS volume are accelerated with p = 0.5 and η = 120%. The solid line is the total emission during the flare. (b): The thin solid line is the IC flare model from (a). The dashed and thick solid lines are a second IC flare model with similar parameters except that p = 1.1.

Fig. 2.— The radial profiles of electron density ne, electron temperature Te, cooling break Lorentz factor γc, and minimum Lorentz factor γmin for the model shown in Fig. 1.

Fig. 6.— Synchrotron models for the X-ray flares from Sgr A*. The dot-dashed line is the quiescent emission from Fig. 1. The thick dashed line is the synchrotron flare due to power-law electrons with γmax ≈ 106, p = 1, and η = 5.5% (i.e., power-law electrons have 5.5% of the electron energy); the magnetic field in the RIAF close to the BH is ∼ 20 G so there is a cooling break at ∼ 1013 Hz. The thick solid line is the total emission during the flare (quiescent emission plus synchrotron flare). Finally, the thin lines (dashed and solid) correspond to a second synchrotron flare model in which p = 1.2 and the magnetic field is only 0.5 G so that there is no cooling break.

Fig. 1.— A model for the quiescent emission from Sgr A* with s = 0.27, δ = 0.55. The accretion rate close to the BH is Ṁ ≈ 4 × 10−8M⊙ yr−1. The dot-dashed line is the synchrotron and IC emission by thermal electrons. The dashed line is the synchrotron emission by non-thermal electrons; the non-thermal electrons have ≈ 1.5% of the thermal energy with p = 3.5. The dotted line is the total synchrotron and IC emission while the solid line includes the bremsstrahlung emission from the outer parts of the RIAF (shown by the long-dashed line); the latter component explains the extended quiescent X-ray source. The radio data are from Falcke et al. 1998 (open circles) and Zhao et al. 2003 (SMA; filled circles), the IR data are from Serabyn et al. (1997) and Hornstein et al. (2002). The two “bowties” in the X-ray are the quiescent (lower) and flaring (higher) data from Baganoff et al. (2001; 2003).