Characterising the VHE diffuse emission in the central 200 parsecs of our Galaxy with H.E.S.S.

Abstract

The diffuse very high−energy (VHE, > 100 GeV) γ-ray emission observed in the central 200 pc of the Milky Way by H.E.S.S. was found to follow the dense matter distribution in the Central Molecular Zone (CMZ) up to a longitudinal distance of about 130 pc to the Galactic Centre (GC), where the flux rapidly decreases. This was initially interpreted as the result of a burst−like injection of energetic particles 10 4 years ago, but a recent more sensitive H.E.S.S. analysis revealed that the cosmic−ray (CR) density profile drops with the distance to the centre, making data compatible with a steady cosmic PeVatron at the GC. In this paper, we extend this analysis to obtain for the first time a detailed characterisation of the correlation with matter and to search for additional features and individual γ-ray sources in the inner 200 pc. Taking advantage of 250 hours of H.E.S.S. data and improved analysis techniques we perform a detailed morphology study of the diffuse VHE emission observed from the GC ridge and reconstruct its total spectrum. To test the various contributions to the total γ-ray emission, we use an iterative 2D maximum likelihood approach that allows us to build a phenomenological model of the emission by summing a number of different spatial components. We show that the emission correlated with dense matter covers the full CMZ and that its flux is about half the total diffuse emission flux. We also detect some emission at higher latitude likely produced by hadronic collisions of CRs in less dense regions of the GC interstellar medium. We detect an additional emission component centred on the GC and extending over about 15 pc that is consistent with the existence of a strong CR density gradient and confirms the presence of a CR accelerator at the very centre of our Galaxy. We show that the spectrum of the full ridge diffuse emission is compatible with the one previously derived from the central regions, suggesting that a single population of particles fills the entire CMZ. Finally, we report the discovery of a VHE γ-ray source near the GC radio arc and argue that it is produced by the pulsar wind nebula candidate G0.13−0.11.

VERITAS Observations of the Galactic Center Region at Multi-TeV Gamma-Ray Energies

“…of 6.03/5, corresponding to a p value of 0.30. While no significant spectral cutoff is found, consistent with Abdalla et al (2018a), the 95% confidence level lower bound on the ECPL's cutoff energy, assuming a spectral index fixed at 2.22, is 10.3 TeV, consistent with the cutoff observed by MAGIC Collaboration et al (2020). Letting the spectral index vary in the cutoff energy upper limit calculation results in unphysically hard spectral indices.…”

“…The H.E.S.S. (Abdalla et al 2018a) and MAGIC (MAGIC Collaboration et al 2020) measurements of the diffuse ridge spectrum model larger regions than what we use in our analysis. In order to compare our spectra, we can scale up our spectrum to estimate what would be measured in the signal region used by Abdalla et al (2018a).…”

“…To estimate this contribution, we model the 2D excess count map, using King functions for the point sources, and using for the diffuse ridge emission the velocity-integrated CS map times a 2D Gaussian of 1°. 1 width centered at the GC, as in Abdalla et al (2018a), convolved with the VERITAS PSF. The amplitudes of each source component are derived simultaneously from a fit to the correlated excess count map.…”

“…Sources of VHE gamma rays include the strong central source VER J1745-290, coincident with both Sgr A * and PWN 359.95-0.04 (Archer et al 2016), composite SNR G0.9+0.1 , and an unidentified source variously identified as VER 1746-289 (Archer et al 2014(Archer et al , 2016Ahnen et al 2017) or HESS J1746-285 (Abdalla et al 2018a). There is also diffuse emission that extends along the Galactic plane (Aharonian et al 2006a;Abramowski et al 2016;Archer et al 2016;Ahnen et al 2017;Abdalla et al 2018a).…”

“…Sources of VHE gamma rays include the strong central source VER J1745-290, coincident with both Sgr A * and PWN 359.95-0.04 (Archer et al 2016), composite SNR G0.9+0.1 , and an unidentified source variously identified as VER 1746-289 (Archer et al 2014(Archer et al , 2016Ahnen et al 2017) or HESS J1746-285 (Abdalla et al 2018a). There is also diffuse emission that extends along the Galactic plane (Aharonian et al 2006a;Abramowski et al 2016;Archer et al 2016;Ahnen et al 2017;Abdalla et al 2018a). The spectrum of VER J1745-290 (which we consider to be the same object as HESS 1745-290) has a photon index Γ ∼ 2.2, and exhibits a break at ∼10 TeV (Albert et al 2006;Aharonian et al 2009;Archer et al 2016), while the diffuse emission spectrum has Γ ∼ 2.3 with no break or cutoff up to tens of TeV (Aharonian et al 2006b;Abramowski et al 2016).…”