Modelling the time-dependence of the TeV γ -ray source at the galactic centre

Abstract

The physical mechanism behind the TeV γ -ray source observed at the centre of the Galaxy is still unknown. One intriguing possibility is that the accretion flow on to the central supermassive black hole is responsible for accelerating protons to TeV energies which then diffuse outwards 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 yr and only at energies of 10 TeV. If the accelerator is long-lived, it must have been running for at least 104 yr and have a hard proton injection spectrum of α = 0.75 (where dn/dEinj α E-α) in order to produce the correct amount of high-energy γ -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 yr ago. However, a good fit to the observed HESS data was found with α = 2.7 for the scenario of a brief ( few year long) burst of particle acceleration that occurred ~10 yr ago. If such bursts are common, then they will keep the TeV source energized and will likely produce spectral variability at ≳10 TeV on ≲5-yr time-scales. This model also implies that particle acceleration may be an important mechanism in reducing the radiative efficiency of weakly accreting black holes. © 2010 The Authors. Journal compilation © 2010 RAS.