T. Ergin

VERITAS Observations of the γ-Ray Binary LS I +61 303

LS I +61 303 is one of only a few high-mass X-ray binaries currently detected at high significance in very high energy γ-rays. The system was observed over several orbital cycles (between 2006 September and 2007 February) with the VERITAS array of imaging air Cerenkov telescopes. A signal of γ-rays with energies above 300 GeV is found with a statistical significance of 8.4 standard deviations. The detected flux is measured to be strongly variable; the maximum flux is found during most orbital cycles at apastron. The energy spectrum for the period of maximum emission can be characterized by a power law with a photon index of -->Γ = 2.40 ± 0.16stat± 0.2sys and a flux above 300 GeV corresponding to 15%-20% of the flux from the Crab Nebula.

Observation of Gamma-Ray Emission from the Galaxy M87 above 250 GeV with VERITAS

The multiwavelength observation of the nearby radio galaxy M87 provides a unique opportunity to study in detail processes occurring in Active Galactic Nuclei from radio waves to TeV -rays. Here we report the detection of -ray emission above 250GeV from M87 in spring 2007 with the VERITAS atmospheric Cherenkov telescope array and discuss its correlation with the X-ray emission. The -ray emission is measured to be point-like with an intrinsic source radius less than 4.5 arcmin. The differential energy spectrum is fitted well by a power-law function: d�/dE=(7.4±1.3stat±1.5sys)× (E/TeV) (−2.31±0.17 stat±0.2sys) 10 −9 m −2 s −1 TeV −1 . We show strong evidence for a year-scale correlation between the -ray flux reported by TeV experiments and the X-ray emission measured by the ASM/RXTE observatory, and discuss the possible short-time-scale variability. These results imply that the -ray emission from M87 is more likely associated with the core of the galaxy than with other bright X-ray features in the jet.

VERITAS Discovery of >200 GeV Gamma-Ray Emission from the Intermediate-Frequency-Peaked BL Lacertae Object W Comae

We report the detection of very high-energy -ray emission from the intermediate-frequencypeaked BLLacertae object WComae (z = 0.102) by VERITAS, an array of four imaging atmospheric-Cherenkov telescopes. The source was observed between January and April 2008. A strong outburst of -ray emission was measured in the middle of March, lasting for only four days. The energy spectrum measured during the two highest flare nights is fit by a power-law and is found to be very steep, with a differential photon spectral index of = 3 .81±0.35stat±0.34syst. The integral photon flux above 200GeV during those two nights corresponds to roughly 9% of the flux from the Crab Nebula. Quasi-simultaneous Swift observations at X-ray energies were triggered by the VERITAS observations. The spectral energy distribution of the flare data can be described by synchrotron-self-Compton (SSC) or external-Compton (EC) leptonic jet models, with the latter offering a more natural set of parameters to fit the data.

Veritas observations of the BL Lac object 1ES 1218+304

The VERITAS collaboration reports the detection of very-high-energy gamma-ray emission from the high-frequency-peaked BL Lac object 1ES 1218+304 located at a redshift of z = 0.182. A gamma-ray signal was detected with a statistical significance of 10.4 standard deviations (10.4?) for the observations taken during the first three months of 2007, confirming the discovery of this object made by the MAGIC collaboration. The photon spectrum between ~160?GeV and ~1.8?TeV is well described by a power law with an index of ? = 3.08 ? 0.34stat ? 0.2sys. The integral flux is ?(E>200GeV) = (12.2 ? 2.6) ? 10?12 cm-2 s?1, which corresponds to ~6% of that of the Crab Nebula. The light curve does not show any evidence for very high energy flux variability. Using lower limits on the density of the extragalactic background light in the near to mid-infrared, we are able to limit the range of intrinsic energy spectra for 1ES?1218+304. We show that the intrinsic photon spectrum has an index that is harder than ? = 2.32 ? 0.37stat. When including constraints from the spectra of 1ES?1101-232 and 1ES?0229+200, the spectrum of 1ES?1218+304 is likely to be harder than ? = 1.86 ? 0.37stat.

Status report from VERITAS

VERITAS is a ground-based gamma-ray observatory covering energies between 100 GeV and 50 TeV and will start operating by January 2007. We give a brief report of the construction status and performance characteristics of the telescopes.

VERITAS: Status c.2005

VERITAS is a ground-based gamma-ray observatory that uses the imaging atmospheric Cherenkov technique and operates in the very high-energy (VHE) region of the gamma- ray spectrum from 100 GeV to 50 TeV. The observatory consists of an array of four 12m-diameter imaging atmospheric Cherenkov telescopes located in southern Arizona, USA. The four-telescope array has been fully operational since September 2007, and over the last two years, VERITAS has been operating with high reliability and sensitivity. It is currently one of the most sensitive VHE observatories. This paper summarizes the status of VERITAS as of October, 2009, and describes the detection of several new VHE gamma-ray sources.