R. J. White

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.

MULTIWAVELENGTH OBSERVATIONS OF MARKARIAN 421 IN 2005-2006

Since 2005 September, the Whipple 10 m Gamma-ray Telescope has been operated primarily as a blazar monitor. The five northern hemisphere blazars that have already been detected at the Whipple Observatory, Markarian 421 (Mrk 421), H1426+428, Mrk 501, 1ES 1959+650, and 1ES 2344+514, are monitored routinely each night that they are visible. We report on the Mrk 421 observations taken from 2005 November to 2006 June in the gamma-ray, X-ray, optical, and radio bands. During this time, Mrk 421 was found to be variable at all wavelengths probed. Both the variability and the correlations among different energy regimes are studied in detail here. A tentative correlation, with large spread, was measured between the X-ray and gamma-ray bands, while no clear correlation was evident among the other energy bands. In addition to this, the well-sampled spectral energy distribution of Mrk 421 (1101+384) is presented for three different activity levels. The observations of the other blazar targets will be reported separately.

Discovery of very high energy gamma-ray radiation from the BL Lac 1ES 0806+524

The high-frequency-peaked BL Lacertae object 1ES 0806+524, at redshift z = 0.138, was observed in the very high energy (VHE) gamma-ray regime by VERITAS between 2006 November and 2008 April. These data encompass the two- and three-telescope commissioning phases, as well as observations with the full four-telescope array. 1ES 0806+524 is detected with a statistical significance of 6.3 standard deviations from 245 excess events. Little or no measurable variability on monthly timescales is found. The photon spectrum for the period 2007 November to 2008 April can be characterized by a power law with photon index 3.6 ± 1.0stat ± 0.3sys between ~ 300 GeV and ~ 700 GeV. The integral flux above 300 GeV is (2.2 ± 0.5stat ± 0.4sys) × 10–12 cm–2 s–1 which corresponds to 1.8% of the Crab Nebula flux. Non-contemporaneous multiwavelength observations are combined with the VHE data to produce a broadband spectral energy distribution that can be reasonably described using a synchrotron--self-Compton model.

Multiwavelength Observations of LS I +61° 303 with Veritas, Swift, and RXTE

We present results from a long-term monitoring campaign on the TeV binary LSI +61{sup o} 303 with VERITAS at energies above 500 GeV, and in the 2-10 keV hard X-ray bands with RXTE and Swift, sampling nine 26.5 day orbital cycles between 2006 September and 2008 February. The binary was observed by VERITAS to be variable, with all integrated observations resulting in a detection at the 8.8{sigma} (2006/2007) and 7.3{sigma} (2007/2008) significance level for emission above 500 GeV. The source was detected during active periods with flux values ranging from 5% to 20% of the Crab Nebula, varying over the course of a single orbital cycle. Additionally, the observations conducted in the 2007-2008 observing season show marginal evidence (at the 3.6{sigma} significance level) for TeV emission outside the apastron passage of the compact object around the Be star. Contemporaneous hard X-ray observations with RXTE and Swift show large variability with flux values typically varying between 0.5 and 3.0 x 10{sup -11} erg cm{sup -2} s{sup -1} over a single orbital cycle. The contemporaneous X-ray and TeV data are examined and it is shown that the TeV sampling is not dense enough to detect a correlation between the two bands.

Deployment of the VERITAS observatory

The Very Energetic Radiation Imaging Telescope Array System (VERITAS) being constructed in Southern Arizona consists of an array of four Atmospheric Cherenkov 12m Telescopes designed to observe astrophysical gamma rays in the energy range from 100 GeV to tens of TeV. A first telescope has been in operation on a temporary site since the end of 2004 and meets all specifications. The second is being installed on the same site, 85m from the first, in order to test stereoscopic capabilities. The full system of four telescopes is expected to be installed on the definitive site by the end of 2006.

A reinvestigation into the diffuse neutrino flux from the inner Galaxy in light of new very high energy gamma-ray observations

In light of the recent MILAGRO collaborations publication of the detection of diffuse multi‐TeV emission from a region close to the inner Galaxy, we reinvestigate the diffuse neutrino flux calculation from this region. Conventional determinations of the diffuse neutrino flux following cosmic ray propagation through the Galaxy, tuned to reproduce the locally observed cosmic ray spectrum, give rise to diffuse fluxes that are significantly below the detected diffuse flux. Assuming that this excess is hadronic in origin and is emitted by the entire inner Galactic region, we determine the diffuse neutrino flux expected from a region of the Galactic disk with coordinates −40°<l<40°. The calculated diffuse flux of neutrinos, for this hadronic only scenario, is found to be detectable by a km3—scale detector located in the northern hemisphere.

Rise Time of the Simulated VERITAS 12 m Davies‐Cotton Reflector

The Very Energetic Radiation Imaging Telescope Array System (VERITAS) will utilise Imaging Atmospheric Cherenkov Telescopes (IACTs) based on a Davies‐Cotton design with f‐number f/1.0 to detect cosmic gamma‐rays. Unlike a parabolic reflector, light from the Davies‐Cotton does not arrive isochronously at the camera. Here the effect of the telescope geometry on signal rise‐time is examined. An almost square‐pulse arrival time profile with a rise time of 1.7 ns is found analytically and confirmed through simulation.