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

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 α=0.7 (where dn/dE_inj \propto E^{-α}) 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 α=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.

6 pages, 5 figures (1 in colour). Accepted by MNRAS

References in corpus (9)

• An Unambiguous Detection of Faraday Rotation in Sagittarius A*
• A radio-emitting outflow in the quiescent state of A0620-00: implications for modelling low-luminosity black hole binaries
• Fading hard X-ray emission from the Galactic Centre molecular cloud Sgr B2
• Discovery of Variable Iron Fluorescence from Reflection Nebulae in the Galactic Center
• Inverse Compton Scenarios for the TeV Gamma-Ray Emission of the Galactic Centre
• Jet-lag in Sgr A*: What size and timing measurements tell us about the central black hole in the Milky Way
• A Possible Link Between the Galactic Center HESS Source and Sgr A*
• Constraining Radiatively Inefficient Accretion Flows with Polarization
• Radio Synchrotron Emission from Secondary Leptons in the Vicinity of Sgr A*

Cited by in corpus (6)

• VERITAS Observations of the Galactic Center Region at Multi-TeV Gamma-Ray Energies
• Observations of Sagittarius A* during the pericenter passage of the G2 object with MAGIC
• Very-high energy observations of the Galactic center region by VERITAS in 2010-2012
• Gamma-rays from millisecond pulsar population within the central stellar cluster in the Galactic Center
• Fermi LAT observations of Sagittarius A*: Imaging Analysis
• The Galactic Center Region Imaged by VERITAS