Gamma-ray blazars are among the most extreme astrophysical sources, harboring phenomena far more energetic than those attainable by terrestrial accelerators. These galaxies are understood to be active galactic nuclei that are powered by accretion ont
o supermassive black holes and have relativistic jets pointed along the Earth line of sight. The emission displayed is variable at all wavelengths and timescales probed thus far, necessitating contemporaneous broadband observations to disentangle the details of the emission processes within the relativistic jets. The very high energy (VHE; $Ege$100 GeV) photons emitted by these sources are detectable with ground-based imaging atmospheric Cherenkov telescopes such as VERITAS. As these photons propagate extragalactic distances, the interaction with the diffuse starlight that pervades the entire Universe results in a distance and energy dependent gamma-ray opacity, offering a unique method for probing photon densities on cosmological scales. These galaxies have also been postulated to be potential sources of ultra-high-energy cosmic rays, a theory which can be examined through deep gamma-ray observations of sources which probe moderate gamma-ray opacities. Within this work, I will highlight ongoing research regarding the broadband emission from VERITAS-observed VHE blazars, as well as the potential to use them for cosmological insight.
Due to its extraordinarily high concentration of known relativistic particle accelerators such as pulsar wind nebula, supernova remnants, dense molecular cloud regions, and the supermassive black hole (Sgr A*); the center of the Milky Way galaxy has
long been an ideal target for high energy (HE, 0.1-100 GeV) and very high energy ( VHE, 50 GeV-50 TeV) gamma-ray emission. Indeed, detections of Sgr A* and other nearby regions of gamma-ray emission have been reported by EGRET and Fermi-LAT in the HE band, as well as CANGAROO, Whipple, HESS, VERITAS, and MAGIC in the VHE band. Here we report on the results of extended observations of the region with VERITAS between 2010-2014. Due to the visibility of the source for VERITAS in the Northern Hemisphere, these observations provide the most sensitive probe of gamma-ray emission above 2 TeV in one of the most complicated and interesting regions of our home galaxy.
The role pulsar wind nebulae play in producing our locally observed cosmic ray spectrum remains murky, yet intriguing. Pulsar wind nebulae are born and evolve in conjunction with SNRs, which are favored sites of Galactic cosmic ray acceleration. As a
result they frequently complicate interpretation of the gamma-ray emission seen from SNRs. However, pulsar wind nebulae may also contribute directly to the local cosmic ray spectrum, particularly the leptonic component. This paper reviews the current thinking on pulsar wind nebulae and their connection to cosmic ray production from an observational perspective. It also considers how both future technologies and new ways of analyzing existing data can help us to better address the relevant theoretical questions. A number of key points will be illustrated with recent results from the VHE (E > 100 GeV) gamma-ray observatory VERITAS.
The Galactic center is an interesting region for high-energy (0.1-100 GeV) and very-high-energy (E > 100 GeV) gamma-ray observations. Potential sources of GeV/TeV gamma-ray emission have been suggested, e.g., the accretion of matter onto the supermas
sive black hole, cosmic rays from a nearby supernova remnant (e.g. SgrA East), particle acceleration in a plerion, or the annihilation of dark matter particles. The Galactic center has been detected by EGRET and by Fermi/LAT in the MeV/GeV energy band. At TeV energies, the Galactic center was detected with moderate significance by the CANGAROO and Whipple 10 m telescopes and with high significance by H.E.S.S., MAGIC, and VERITAS. We present the results from three years of VERITAS observations conducted at large zenith angles resulting in a detection of the Galactic center on the level of 18 standard deviations at energies above ~2.5TeV. The energy spectrum is derived and is found to be compatible with hadronic, leptonic and hybrid emission models discussed in the literature. Future, more detailed measurements of the high-energy cutoff and better constraints on the high-energy flux variability will help to refine and/or disentangle the individual models.
In March 2013, a flaring episode from the Crab Nebula lasting ~2 weeks was detected by the Fermi-LAT (Large Area Telescope on board the Fermi Gamma-ray Space Telescope). VERITAS provides simultaneous observations throughout this period. During the fl
are, the Fermi-LAT detected a 20-fold increase in flux above the average synchrotron flux >100 MeV seen from the Crab Nebula. Simultaneous measurements with VERITAS are consistent with the non-variable long-term average Crab Nebula flux at TeV energies. Assuming a linear correlation between the very-high-energy flux change >1 TeV and the flux change seen in the Fermi-LAT band >100 MeV during the period of simultaneous observations, the linear correlation factor can be constrained to be at most 8.6 * 10^-3 with 95% confidence.
The VERITAS VHE gamma-ray observatory recently completed a major upgrade of its camera and pattern triggering systems. Bias curve testing of the upgraded VERITAS Observatory under dark sky conditions indicates a 50% increase in photon detection effic
iency, and a 30% reduction in triggering threshold. Optimization of analysis of the Crab nebula observations performed in late 2012 and early 2013 is ongoing. A comparison of these results with pre-upgrade Crab observations can provide the most direct method for quantifying the impact of the upgrade on VERITAS sensitivity and energy threshold.
Since the commissioning of the array in Spring 2007, the VERITAS array (sensitive in the 0.1-50 TeV energy range) has acquired over 300 hours of observations investigating the TeV emission from X-ray binary star systems, in particular focusing on the
known TeV binary targets LS I +61 303 and HESS J0632+057. Both TeV binaries have been monitored by VERITAS for several years and the resulting dataset is continuing to yield important results in the characterization of these poorly understood systems. We present these results, as well as the contemporaneous observations of these sources taken with Fermi-LAT and Swift-XRT. In the case of LS I +61 303, simultaneous observations taken with VERITAS and Fermi-LAT reveal a break in emission in the 10-200 GeV range. For HESS J0632 057, the extended VERITAS observations have allowed for the first identification of a binary system through TeV gamma-ray observations.
The study of $gamma$-ray emission from galactic sources such as supernova remnants (SNR) may provide key insights into their potential role as accelerators of cosmic rays up to the knee ($sim 10^{15}$ eV). The VERITAS Observatory is sensitive to gala
ctic and extragalactic $gamma$-ray sources in the 100 GeV to 30 TeV energy range. We report here on VERITAS observations of the vicinity of the cocoon of freshly accelerated cosmic rays reported by Fermi, which lies between potential accelerators in the Cygnus OB2 association and the $gamma$-Cygni SNR. A particular focus is placed on the source VER J2019 +407 in $gamma$-Cygni.
Here we present highlights from VERITAS observations of high-frequency-peaked BL Lac objects (HBLs). We discuss the key science motivations for observing these sources -- including performing multiwavelength campaigns critical to understanding the em
ission mechanisms at work in HBLs, constraining the intensity and spectra shape of the extragalactic background light, and placing limits on the strength of the intergalactic magnetic field.
The Fermi space telescope has detected over 100 pulsars. These discoveries have ushered in a new era of pulsar astrophysics at gamma-ray energies. Gamma-ray pulsars, regardless of whether they are young, old, radio-quiet etc, all exhibit a seemingly
unifying characteristic: a spectral energy distribution which takes the form of a power law with an exponential cut-off occurring between ~1 and ~10 GeV. The single known exception to this is the Crab pulsar, which was recently discovered to emit pulsed gamma rays at energies exceeding a few hundred GeV. Here we present an update on observations of the Crab pulsar above 100 GeV with VERITAS. We show some new results from a joint gamma-ray/radio observational campaign to search for a correlation between giant radio pulses and pulsed VHE emission from the Crab pulsar. We also present some preliminary results on Lorentz invariance violation tests performed using Fermi and VERITAS observations of the Crab pulsar.
