General relativistic effects and the near-infrared and X-ray variability of Sgr A* I

TL;DR

This study analyzes NIR and X-ray flares from Sgr A* using relativistic modeling, revealing a common exponential flare shape with implications for the orbital inclination and intrinsic variability timescales.

Contribution

It introduces a relativistic ray-tracing analysis of flare light curves, identifying a universal exponential response shape and constraining the hot spot orbital inclination.

Findings

Flares share a symmetric exponential impulse response with a 15-minute timescale.

The flare shape is inconsistent with edge-on hot-spot orbits.

Constraints on hot spot inclination are approximately 30° to 75°.

Abstract

The near-infrared (NIR) and X-ray emission of Sagittarius A* shows occasional bright flares that are assumed to originate from the innermost region of the accretion flow. We identified $25$ $4.5 \mu m$ and $24$ X-ray flares in archival data obtained with the \textit{Spitzer} and \textit{Chandra} observatories. With the help of general relativistic ray-tracing code, we modeled trajectories of ``hot spots'' and studied the light curves of the flares for signs of the effects of general relativity. Despite their apparent diversity in shape, all flares share a common, exponential impulse response, a characteristic shape that is the building block of the variability. This shape is symmetric, that is, the rise and fall times are the same. Furthermore, the impulse responses in the NIR and X-ray are identical within uncertainties, with an exponential time constant $\tau\sim 15$ minute. The observed characteristic flare shape is inconsistent with hot-spot orbits viewed edge-on. Individually modeling the light curves of the flares, we derived constraints on the inclination of the orbital plane of the hot spots with respect to the observer ($i \sim 30^{\circ} , < 75^{\circ} $) and on the characteristic timescale of the intrinsic variability (tens of minutes).