Very high energy gamma rays from the direction of Sagittarius A*

TL;DR

This study reports the detection of very high energy gamma-ray emission from the Galactic Centre region using H.E.S.S., identifying a compact source near Sgr A* with a hard power-law spectrum and no observed variability. The measured spectrum is well described by F(E)=F0 E^-α with α ≈ 2.21 and F(>165 GeV) ≈ 1.82×10^-7 m^-2 s^-1, with a flux normalization consistent with a point source of size <3 arcmin. The result conflicts with the steeper spectrum reported by CANGAROO and is not easily reconciled with Whipple's higher-energy flux, implying a steady, localized acceleration site rather than a diffuse TeV excess. The authors discuss hadronic and accretion-jet scenarios near Sgr A*, consider exotic possibilities such as dark matter annihilation, and emphasize the need for continued, deeper observations to resolve the emission's origin and potential extended components.

Abstract

We report the detection of a point-like source of very high energy (VHE) gamma-rays coincident within 1' of Sgr A*, obtained with the H.E.S.S. array of Cherenkov telescopes. The gamma-rays exhibit a power-law energy spectrum with a spectral index of -2.2 +/- 0.09 +/- 0.15 and a flux above the 165 GeV threshold of (1.82 +/- 0.22) * 10^-7 m^-2 s^-1. The measured flux and spectrum differ substantially from recent results reported in particular by the CANGAROO collaboration.

Figures (4)

• Figure 1: Angular distribution of $\gamma$-ray candidates for a 3$^\circ$ field of view centred on Sgr A$^*$. Both data sets ('June/July' and 'July/August') are combined, employing tight cuts to reduce the level of background. The significance of the feature extending along the Galactic Plane is under investigation.
• Figure 2: Centre of gravity of the VHE signal (triangle), superimposed on a 8.5$'$ by 8.5$'$ Chandra X-ray map (Muno et al. chandra_map) of the GC. The location of Sgr A$^*$ is indicated by a cross. The contour lines indicate the 68% and 95% confidence regions for the source position, taking into account systematic pointing errors of 20$"$. The white dashed line gives the 95% confidence level upper limit on the rms source size. The resolution for individual VHE photons - as opposed to the precision for the centre of the VHE signal - is 5.8$'$ (50% containment radius).
• Figure 3: Angular distribution of VHE $\gamma$-rays relative to the location of Sgr A$^*$. Inset: distributions in $\theta^2$ where $\theta$ is the angle between the $\gamma$-ray direction and Sgr A$^*$; a uniform background results in a flat distribution in $\theta^2$. Full points: signal region; open points: background region. The main figure shows background-subtracted excess counts. The solid line indicates the distribution expected for a point source of $\gamma$-rays at the position of Sgr A$^*$.
• Figure 4: Energy spectrum $E^2 dN/dE$ of $\gamma$-rays from the Galactic Centre. Full circles: H.E.S.S. 'July/August 2003' data set. Full triangles: H.E.S.S. 'June/July 2003' data set. The line indicates a power-law fit to the 'July/August' spectrum. Open squares: CANGAROO-II spectrum from Summer 2001 and 2002 (Tsuchiya et al. CANGAROO). Open triangle: Whipple flux from 1995 through 2003 (Kosack et al. VERITAS), converted to a differential flux at the peak detection energy assuming a Crab-like spectrum. The inset shows the EGRET flux from 1991 to 1996 (Mayer-Hasselwander et al. egret) (circles) compared to fits to the CANGAROO-II (dashed line) and H.E.S.S. (solid line) spectra. Due to the poor angular resolution of EGRET ($1^\circ$) the flux shown may include other sources.