arXiv:0907.4975

The reflector used by an imaging atmospheric Cherenkov telescope (IACT) consists of a tessellated array of mirrors mounted on a large frame. This arrangement allows for a very large reflecting surface with sufficient optical quality for the implementation of the IACT technique at a moderate price. The main challenge presented by such a reflector is maintaining the optical quality, which depends on the individual alignment of several hundred mirror facets. We describe a method of measuring and correcting the alignment of the mirror facets of the reflectors used by the VERITAS telescopes. This method employs a CCD camera, placed at the focal point of the reflector, which acquires a series of images of the reflector while the telescope performs a raster scan about a star. Well-aligned facets appear bright when the telescope points directly at the star while misaligned facets appear bright when the angle between the telescope pointing direction and the star is twice the misalignment angle of the mirror. Data from these scans can therefore be used to produce a set of corrections which can be applied to the facets. In this contribution we report on initial experience with an alignment system based on this principle.

arXiv:0907.4792

M87 is a nearby radio galaxy and because of its misaligned jet, it is possible to correlate detailed spatially-resolved emission regions in the radio, optical to X-ray waveband with unresolved but contemporaneous flux measurements in the TeV regime. Hence, M87 provides a unique opportunity to reveal the emission mechanisms responsible for high energy gamma-ray emission from active galactic nuclei. Observations with VERITAS since 2007 have resulted in 90 hours of data while 2008 observations were part of a concerted effort involving the three major atmospheric Cherenkov observatories: H.E.S.S., MAGIC and VERITAS. As a result of the TeV campaign, a high flux state of M87 was detected in February 2008 showing multiple flares with rapid variability. We will present the comprehensive results from VERITAS observations since 2007 and also show preliminary results from the 2009 campaign.

Science Magazine, Volume 325, pp 444-448 (2009)

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arXiv/0908.0511

The accretion of matter onto a massive black hole is believed to feed the relativistic plasma jets found in many active galactic nuclei (AGN). Although some AGN accelerate particles to energies exceeding 10¹² electron volts and are bright sources of very-high-energy (VHE) -ray emission, it is not yet known where the VHE emission originates. Here we report on radio and VHE observations of the radio galaxy Messier 87, revealing a period of extremely strong VHE -ray flares accompanied by a strong increase of the radio flux from its nucleus. These results imply that charged particles are accelerated to very high energies in the immediate vicinity of the black hole.

arXiv:0907.3958

HESS J0632+057 is one of only two unidentified high energy gamma-ray sources which appear to be point-like in nature. It is possibly associated with the massive star MWC 148 and has been suggested to resemble known TeV binary systems like LS I +61 303 or LS 5039. These binaries are rare and extreme (only three TeV binaries are known to date), and their gamma-ray emission mechanism has not been understood. HESS J0632+057 was observed by VERITAS, an array of four 12 m imaging atmospheric Cherenkov telescopes, in 2006, 2008 and 2009. Based on these observations we present evidence for variability in the high energy gamma-ray emission from HESS J0632+057.