Radio variability of Sagittarius A^* due to an orbiting star

¹ Korea Institute for Advanced Study, 207-43 Cheongryangri-dong Dongdaemun-gu, Seoul 130-012, Korea
² Korea Astronomy Observatory, 36-1 Hwaam-dong, Yusong-gu, Taejon 305-348, Korea e-mail: cschoi@kao.re.kr

Corresponding author: Heon-Young Chang, hyc@ns.kias.re.kr

Received: 5 June 2003
Accepted: 23 July 2003

Abstract

Recently, unprecedentedly accurate data on the orbital motion of stars in the vicinity of Sgr  have become available. Such information can be used not only to constrain the mass of the supermassive black hole (SMBH) in the Galactic center but also to study the source of the radio emission. Two major competing explanations of the radio spectrum of Sgr  are based on two different models, that is, hot accretion disk and jet. Hence, independent observational constraints are required to resolve related issues. It has been suggested that a star passing by a hot accretion disk may cool the hot accretion disk by Comptonization and consequently cause the radio flux variation. We explore the possibility of using the observational data of the star S2, currently closest to the Galactic center, to distinguish physical models for the radio emission of Sgr , by applying the stellar cooling model to Sgr  with the orbital parameters derived from the observation. The relative difference in the electron temperature due to stellar cooling by S2 is a few parts of a thousand and the consequent relative radio luminosity difference is of the order of 10^-4. Therefore, one might expect to observe the radio flux variation with a periodic or quasi-periodic modulation in the frequency range  if radiatively inefficient hot accretion flows are indeed responsible for the radio emission, contrary to the case of a jet. According to our findings, even though no periodic radio flux variations have been reported up to date a radiatively inefficient hot accretion disk model cannot be conclusively ruled out. This is because the current available sensitivity is insufficient and because the energy bands that have been studied are too high to observe the effect of the star S2 even if it indeed interacts with the hot disk.