A brief description of the scientific research done in our group

VERITAS is an array of four imaging air Cherenkov telescopes located at the Whipple Observatory on Mt. Hopkins, about one hour south of Tucson, AZ. VERITAS is dedicated to the study of gamma-ray astrophysics in the energy range from 100 GeV to 30 TeV. This part of the electromagnetic spectrum provides a unique window on the most powerful of the cosm ic particle accelerators, including jets associated with supermassive black holes, shock waves in su pernova remnants and the nebulae surrounding energetic pulsars, and the complex physics of X-ray binary systems. Very-high-energy gamma rays can also probe some of the most important questions in particle physics and cosmology: the search for dark matter, Lorentz invariance violation, and the strength of the magnetic field in intergalactic space.

Supernova Remnants and the Origins of Cosmic Rays

    For nearly 100 years we have known that the earth is bombarded by energetic charged particles, cosmic rays, without a clear understanding of their origins. Cosmic rays can range in energy up to 10^20 eV. Up to 10^15 eV or so, cosmic rays are believed to be accelerated within our galaxy, but the astrophysical accelerators have not yet been unambiguously identified. Growing evidence from VERITAS and similar ground- and space-based observatories (HESS, MAGIC, Fermi-LAT, AGILE) suggests that supernova remnants are one source of Galactic cosmic rays (as long anticipated), but it remains an open question whether they are the sole, or even the primary, source. Deep studies of known Galactic accelerators and surveys of classes of objects will allow us to address this question, as well as explore the details of the acceleration process.

  • GeV/TeV studies of Supernova Remnants (SNRs)
  • IC 443: Classic Example of an SNR interacting with a molecular cloud.
Indirect Searches for Dark Matter

    In many dark matter scenarios, the particles that make up the dark matter can either annihilate with each other, or decay (with a very long lifetime). In either case, one of the final products will be high-energy gamma rays. The mass of the dark matter particle is poorly constrained; above a few hundred GeV, detection of gamma rays from regions of space with unusually high concentrations of dark matter (such as dwarf spheroidal galaxies, or the central region of the Milky Way galaxy) becomes one of the most promising avenues for identifying dark matter.

  • Unidentified Sources
  • Galactic Center
Calibration, Simulation, and Data analysis
  • VERITAS and Fermi-LAT data analysis
  • Calibration of VERITAS instrument

IC 443 (Jellyfish Nebula)

IC 443 is one of the classic examples of a supernova remnant interacting with a molecular cloud. The expanding supernova remnant appears to be surrounded by a ring of gas that cuts across the remnant in the foreground from northwest to southeast. The cloud is dense enough to attenuate the optical emission from IC 443, which is noticeably dimmer across its middle than elsewhere. IC 443 is considered middle-aged, though with large uncertainty in its age (with estimates ranging from a few thousand years to 30 thousand years). It is believed to be ~1.5 kpc away, so its 0.75-degree diameter translates to a size of ~20 pc. In the southeastern part of IC 443, a pulsar wind nebula (PWN) has been identified. This PWN may be powered by the neutron star left behind by the progenitor star whose explosion created the supernova remnant.

VERITAS observed IC 443 for 37.9 hours during 2007 and detected emission above 300 GeV with an excess of 247 events, resulting in a significance of 8.3 standard deviations (sigma) before trials and 7.5 sigma after trials in a point-source search. The emission is centered at 6h16m51s 22 deg 30' 11'' (J2000) +- 0.03 deg stat +- 0.08 deg sys, with an intrinsic extension of 0.16 deg +- 0.03 deg stat +- 0.04 deg sys (assuming a symmetric Gaussian profile). This was the first detection of extended gamma-ray emission from IC 443, and has since been confirmed by the Fermi Gamma-ray Space Telescope. The VHE spectrum is well fit by a power law dN/dE = N0 ⋅ (E/TeV) with a photon index of Γ=2.99 +- 0.38stat +- 0.3sys and an integral flux above 300 GeV of (4.63 +- 0.90stat +- 0.93sys) ⋅ 1012 cm-2 s-1.

IC 443 is interesting to study in TeV gamma rays for two reasons. First, supernova remnants are the leading candidates for the accelerators of cosmic rays up to energies of 1000 TeV or so. Studying TeV gamma rays from IC 443 (and similar supernova remnants) can teach us about how cosmic rays are accelerated in supernova remnants and how efficiently they escape and diffuse into the interstellar medium. IC443 is special in that its expanding shock wave is interacting with the ring of gas that surrounds it. The gas in this cloud is much denser than the average interstellar medium and provides a rich target for cosmic rays to interact with and generate gamma rays. The fact that the TeV gamma rays seen by both VERITAS and MAGIC coincide with the densest regions of this gas supports this picture (as does the observation of GeV gamma rays by the Fermi Gamma-ray Space Telescope). In this scenario, the low flux (~3% that of the Crab Nebula above 300 GeV) and unusually steep spectrum (with a power-law index of Γ=2.99 +- 0.38stat +- 0.3sys) for a supernova remnant are likely a consequence of IC 443's age and the energy-dependent escape of cosmic rays from the immediate environment.

Second, the pulsar wind nebula in IC 443 may also be responsible for at least some of the observed gamma rays. This is a very interesting pulsar wind nebula because it appears to still be embedded in the interior of the supernova remnant and because pulsed emission from the compact object in the nebula has not yet been detected.

In the future, we expect that high-statistics GeV and TeV observations will allow us to assess in more detail whether both of these mechanisms for generating gamma rays are contributing, and learn more about cosmic-ray acceleration and diffusion in this fascinating system.