On the Nature of the Variable Infrared Emission from Sagittarius A*

Feng Yuan,Eliot Quataert,Ramesh Narayan

doi:10.1086/383117

Abstract

Recent infrared (IR) observations of the center of our Galaxy indicate that the supermassive black hole (SMBH) source Sgr A* is strongly variable in the IR. The timescale for the variability, ~30 minutes, is comparable to that of the X-ray flares observed by Chandra and XMM-Newton, suggesting a common physical origin. In this paper, we investigate the nature of the IR emission in the context of models recently proposed to interpret the X-ray flares. We show that the IR emission in Sgr A* can be well explained by nonthermal synchrotron emission if a small fraction of the electrons in the innermost region of the accretion flow around the black hole are accelerated into a broken power-law distribution, perhaps through transient events such as magnetic reconnection. The model predicts differences in the variability amplitudes of flares in the IR and X-rays, in general agreement with observations. It also predicts that the IR emission should be linearly polarized, as has indeed been observed during one epoch. IR and X-ray flares analogous to those observed in Sgr A* may be detectable from other accreting SMBHs, provided L 10-8LEdd; at higher luminosities, the flaring emission is dominated by thermal synchrotron self-Compton emission, which is likely to be less variable.

Highlights

The center of our Galaxy provides the best evidence to date for a massive black hole (e.g., Schodel et al 2002; Ghez et al 2003a), associated with the compact radio source, SgrA*
We described a model for the observed emission from Sgr A* (Yuan, Quataert, & Narayan 2003; hereafter YQN03), based on the idea that accretion onto the central BH proceeds via a hot two-temperature radiatively inefficient accretion flow (RIAF; Narayan et al 1995, 1998; see Narayan 2002, Quataert 2003 for reviews; and Melia 1992, Liu & Melia 2001, and Yuan, Markoff & Falcke 2002 for related ideas)
In YQN03 we presented flare models based on the Synchrotron self-Compton (SSC) mechanism that predict roughly the right flux in both the IR and X-ray bands

Summary

Introduction

The center of our Galaxy provides the best evidence to date for a massive black hole (e.g., Schodel et al 2002; Ghez et al 2003a), associated with the compact radio source, Sgr. We described a model for the observed emission from Sgr A* (Yuan, Quataert, & Narayan 2003; hereafter YQN03), based on the idea that accretion onto the central BH proceeds via a hot two-temperature radiatively inefficient accretion flow (RIAF; Narayan et al 1995, 1998; see Narayan 2002, Quataert 2003 for reviews; and Melia 1992, Liu & Melia 2001, and Yuan, Markoff & Falcke 2002 for related ideas) In this model, as in that of Markoff et al (2001), the observed flares are produced when electrons very close to the BH flow are accelerated to ultrarelativistic energies and emit synchrotron or synchrotron self-Compton radiation..

RIAF Models for Sgr A*

Variable IR and X-ray Emission from Accelerated Electrons

Synchrotron models

Summary and Discussion