Journal Article

Locating the very high energy source in the Galactic Centre with milliarcsecond accuracy

A. Abramowski, S. Gillessen, D. Horns and H.-S. Zechlin

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 402, issue 2, pages 1342-1348
Published in print February 2010 | ISSN: 0035-8711
Published online February 2010 | e-ISSN: 1365-2966 | DOI:
Locating the very high energy source in the Galactic Centre with milliarcsecond accuracy

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Very high energy (VHE) γ-rays (E > 100 GeV) have been detected from the direction of the Galactic Centre up to energies E > 10 TeV. Up to now, the origin of this emission is unknown due to the limited positional accuracy of the observing instruments. One of the counterpart candidates is the supermassive black hole (SMBH) Sgr A*. If the VHE emission is produced within ≈1015 cm ≈1000 rG(rG=G M/c2 is the Schwarzschild radius) of the SMBH, a decrease of the VHE photon flux in the energy range 100–300 GeV is expected whenever an early-type or giant star approaches the line of sight within ≈ milliarcseconds (mas). The dimming of the flux is due to absorption by pair-production of the VHE photons in the soft photon field of the star, an effect we refer to as pair-production eclipse (PPE). Based upon the currently known orbits of stars in the inner arcsecond of the Galaxy, we find that PPEs lead to a systematic dimming in the 100–300 GeV band at the level of a few per cent and lasts for several weeks. Since the PPE affects only a narrow energy band and is well correlated with the passage of the star, it can be clearly discriminated against other systematic or even source-intrinsic effects. While the effect is too small to be observable with the current generation of VHE detectors, upcoming high count-rate experiments like the Cherenkov telescope array (CTA) will be sufficiently sensitive. Measuring the temporal signature of the PPE bears the potential to locate the position and size of the VHE-emitting region within the inner 1000 rG or in the case of a non-detection exclude the immediate environment of the SMBH as the site of γ-ray production altogether.

Keywords: Galaxy: centre; Galaxy: nucleus; gamma-rays: observations

Journal Article.  4725 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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