Modelling the time dependence of the TeV Gamma-ray source at the Galactic Centre

TL;DR

This study models the time and energy dependence of the TeV gamma-ray emission at the Galactic Centre, suggesting that variability in particle acceleration affects the observed gamma-ray flux with specific delays and energy thresholds.

Contribution

It introduces a detailed time-dependent model of proton diffusion and gamma-ray production at the Galactic Centre, linking variability to acceleration timescales and spectral features.

Findings

Variability in acceleration appears after ~10 years at >10 TeV energies.

A long-lived accelerator requires at least 10^4 years and a hard spectrum to match observations.

Brief acceleration bursts can explain spectral variability and recent gamma-ray observations.

Abstract

The physical mechanism behind the TeV gamma-ray source observed at the centre of the Galaxy is still unknown. One intriguing possibility is that the accretion flow onto the central supermassive black hole is responsible for accelerating protons to TeV energies which then diffuse outward to interact with molecular gas at distances of ~1 pc. Here, we build on our earlier detailed calculations of the proton transport to consider the time and energy dependence of the TeV signal following a burst of particle acceleration at Sgr A*. We find that, due to the strong energy dependence of the proton diffusion, any variability in the particle acceleration rate will only be visible in the TeV signal after a delay of ~ 10 yrs, and only at energies >~ 10 TeV. If the accelerator is long-lived it must have been running for at least 10^4 yrs and have a hard proton injection spectrum of \alpha=0.7 (where dn/dE_inj \propto E^{-\alpha}) in order to produce the correct amount of high energy gamma-ray flux. This rapid diffusion of high energy protons also rules out the possibility that the observed TeV source is directly related to the period of increased activity of Sgr A* that ended ~ 100 yrs ago. However, a good fit to the observed H.E.S.S. data was found with \alpha=2.7 for the scenario of a brief (~few year long) burst of particle acceleration that occurred ~ 10 yrs ago. If such bursts are common then they will keep the TeV source energised and will likely produce spectral variability at >~ 10 TeV on <~ 5 yr timescales. This model also implies that particle acceleration may be an important mechanism in reducing the radiative efficiency of weakly accreting black holes.