Bulletin of the AAS (BAAS), volume 40, number 1, 2008 (10.12)

The high mass X-ray binary LS I +61 303 has been observed over several months in 2006 and 2007 with the VERITAS array of imaging air-Cherenkov telescopes. A signal of high energy gamma rays with energies above 300 GeV is detected in several orbital cycles of the binary system. The detected flux of gamma rays is strongly variable over the orbital period of 26.5 days, while the maximum flux (corresponding to about 10% of the flux of the Crab Nebula) is always found at approximately apastron. This suggests a strong dependence of particle acceleration and propagation on the relative position of the two objects in the system. In this talk, results from these recent VERITAS observations are discussed in the context of contemporaneous X-ray observations.

Bulletin of the AAS (BAAS), volume 40, number 1, 2008 (12.09)

Observations of the Crab Nebula, the standard candle in TeV astronomy, provide a convenient method to calibrate and to characterize the performance of a Cherenkov telescope. Demonstrating the consistency of these observations with the well established properties of Crab is an important test of a new instrument and builds confidence that the results from other observations with the telescope array are correct. Scientifically, it is interesting to measure the energy spectrum of the Crab Nebula close to the inverse-Compton peak where a deviation from the power law seen at energies above 300 GeV is expected. Additionally, it is important to search for pulsed emission from the Crab Pulsar at energies beyond 10 GeV, the highest energy at which EGRET detected pulsed emission from the Crab. With these motivations, the Crab has been observed extensively during the 2-, 3-, and 4-telescope phases of the comissioning of VERITAS. The energy spectrum of the Crab Nebula between 200 GeV and 7 TeV is constructed from these data sets. A search for pulsed emission from the Crab Pulsar at gamma-ray energies above 100 GeV is also performed. The Crab data set from these observations does not show any significant pulsed excess, so an upper limit on the pulsed emission is obtained.

Bulletin of the AAS (BAAS), volume 40, number 1, 2008 (18.06)

If dark matter (DM) is composed of massive, weakly-interacting particles such as the neutralino predicted by supersymmetry, pair annihilation to gamma rays or secondary particles ultimately producing a continuum spectrum of gamma rays may take place in gravitationally clustered DM. Due to their large mass-to-light ratios and the absence of conventional gamma-ray sources in their vicinity, dwarf spheroidal galaxies of the Local Group are obvious targets to search for such annihilation. We report here on gamma-ray observations taken with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) during the 2007/8 season of the dwarf galaxies Ursa Minor, Draco, and Willman I and the local group galaxy M33. We discuss the implications of these measurements for models of DM clustering and DM particle properties.

The Astrophysical Journal, Volume 679, Issue 2, pp. 1427-1432.

arXiv: astro-ph/0802.2363 

Abstract: LS I +61 303 is one of only a few high-mass X-ray binaries currently detected at high significance in very high energy gamma-rays. The system was observed over several orbital cycles (between September 2006 and February 2007) with the VERITAS array of imaging air-Cherenkov telescopes. A signal of gamma-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 Gamma=2.40+-0.16_stat+-0.2_sys and a flux above 300 GeV corresponding to 15-20% of the flux from the Crab Nebula.

Additional Information:

Differential energy spectrum of VHE photons above 300 GeV for LS I +61 303 around apastron (orbital phases 0.5–0.8). Taken from Fig.2 in V.A.Acciari et al, The Astrophysical Journal, Volume 679, Issue 2, pp. 1427-1432 (arXiv:0802.2363)):

Energy            dN/dE
[TeV]        [cm^{-2}s^{-1}TeV^{-1}]
----------------------------------------------
0.37          2.83e-11 +- 1.14e-11
0.53         1.25e-11 +- 3.66e-12
0.74         6.70e-12 +- 1.49e-12
1.05         2.23e-12 +- 6.37e-13
1.48         1.24e-12 +- 3.17e-13
2.09         6.62e-13 +- 1.93e-13
2.99       <1.96e-13
4.18         7.00e-14 +- 3.48e-14
5.96       <4.14e-14