An Accretion-induced X-Ray Flare in Sagittarius A*

Abstract

The recent detection of a three-hour X-ray flare from Sgr A* by Chandra provides very strong evidence for a compact emitting region near this supermassive black hole at the Galactic center. Sgr A*'s mm/sub-mm spectrum and linear polarimetric properties, and its quiescent-state X-ray flux density, are consistent with a model in which low angular momentum gas captured at large radii circularizes to form a hot, magnetized Keplerian flow within tens of Schwarzschild radii of the black hole's event horizon. In Sgr A*'s quiescent state, the X-ray emission appears to be produced by self-Comptonization (SSC) of the mm/sub-mm synchrotron photons emitted in this region. In this paper, we show that the prominent X-ray flare seen in Sgr A* may be due to a sudden enhancement of accretion through the circularized flow. Depending on whether the associated response of the anomalous viscosity is to increase or decrease in tandem with this additional injection of mass, the X-ray photons during the outburst may be produced either via thermal bremsstrahlung (if the viscosity decreases), or via SSC (if the viscosity increases). However, the latter predicts a softer X-ray spectrum than was seen by Chandra, so it appears that a bremsstrahlung origin for the X-ray outburst is favored. A strong correlation is expected between the mm/sub-mm and X-ray fluxes when the flare X-rays are produced by SSC, while the correlated variability is strongest between the sub-mm/far-IR and X-rays when bremsstrahlung emission is dominant during the flare. In addition, we show that future coordinated multi-wavelength observations planned for the 2002 and 2003 cycles may be able to distinguish between the accretion and jet scenarios.

XMM-Newton observation of the brightest X-ray flare detected so far from Sgr A*

“…The present spectral index is consistent with the ones found during the quiescent periods of Sgr A* by Chandra: Γ = 2.5 +0.8 −0.7 in September 1999 (B03), and Γ = 1.8 +0.7 −0.9 in October 2000 (B01). Such a soft X-ray spectrum is predicted by the scenario proposed by Liu & Melia (2002) in case of an increased viscosity of the accretion flow, and by the one proposed by Markoff et al (2001) in case of an increase of accretion rate or jet power. However the predictions of these two latter models are not consistent with observational constraints found at other wavelengths (e.g., Zhao et al 2003;Hornstein et al 2002).…”

X-ray flares from Sgr A*: Star-disk interactions?

“…The model also predicts an extra soft absorption in X-ray if the disk has large enough column depth. Note that such atomic features in Sgr A * spectra are unlikely to be produced in the context of the three other existing models for Sgr A * flares (Markoff et al 2001;Liu & Melia 2002;Narayan 2002, Sect. 5.3).…”

A New X‐Ray Flare from the Galactic Nucleus Detected with theXMM‐NewtonPhoton Imaging Cameras

“…Models that predict emission from the innermost regions near the black hole involve a mechanism acting either at the base of a jet of relativistic particles (Markoff et al 2001) or in the hot Keplerian flow present within the circularization radius of a spherical flow (Melia, Liu, & Coker 2001;Liu & Melia 2002). In both cases, a magnetic field is present in the flow and the submillimeter radiation is attributed to optically thin synchrotron emission from the inner region, while the X-rays are produced by the synchrotron self-Compton (SSC) mechanism whereby radio to millimeter photons are boosted to X-ray energies by the same relativistic or subrelativistic electrons that are producing the synchrotron radiation.…”