We report the discovery of TeV gamma-ray emission from the Type Ia supernova remnant (SNR) G120.1+1.4, known as Tycho's SNR. Observations performed in the period 2008-2010 with the VERITAS ...ground-based gamma-ray observatory reveal weak emission coming from the direction of the remnant, compatible with a point source located at 00h25m27.s0, + 64?10'50'' (J2000). The TeV photon spectrum measured by VERITAS can be described with a power law dN/dE = C(E/3.42 TeV)-- Delta *G with Delta *G = 1.95 ? 0.51stat ? 0.30sys and C = (1.55 ? 0.43stat ? 0.47sys) X 10--14 cm--2 s--1 TeV--1. The integral flux above 1 TeV corresponds to ~0.9% of the steady Crab Nebula emission above the same energy, making it one of the weakest sources yet detected in TeV gamma rays. We present both leptonic and hadronic models that can describe the data. The lowest magnetic field allowed in these models is ~80 Delta *mG, which may be interpreted as evidence for magnetic field amplification.
We report on observations of very high energy {gamma} rays from the shell-type supernova remnant (SNR) Cassiopeia A with the Very Energetic Radiation Imaging Telescope Array System stereoscopic array ...of four imaging atmospheric Cherenkov telescopes in Arizona. The total exposure time for these observations is 22 hr, accumulated between September and November of 2007. The {gamma}-ray source associated with the SNR Cassiopeia A was detected above 200 GeV with a statistical significance of 8.3{sigma}. The estimated integral flux for this {gamma}-ray source is about 3% of the Crab-Nebula flux. The photon spectrum is compatible with a power law dN/dE {proportional_to} E {sup -{Gamma}} with an index {Gamma} = 2.61 {+-} 0.24{sub stat} {+-} 0.2{sub sys}. The data are consistent with a point-like source. We provide a detailed description of the analysis results and discuss physical mechanisms that may be responsible for the observed {gamma}-ray emission.
We report on the VERITAS discovery of very high energy (VHE) gamma-ray emission above 200 GeV from the high-frequency-peaked BL Lac (HBL) object RX J0648.7+1516 (GB J0648+1516), associated with 1FGL ...J0648.8+1516. The photon spectrum above 200 GeV is fitted by a power law dN/dE = F 0(E/E 0)-- Delta *G with a photon index Delta *G of 4.4 ? 0.8stat ? 0.3syst and a flux normalization F 0 of (2.3 ? 0.5stat ? 1.2sys) X 10--11 TeV--1 cm--2 s--1 with E 0 = 300 GeV. No VHE variability is detected during VERITAS observations of RX J0648.7+1516 between 2010 March 4 and April 15. Following the VHE discovery, the optical identification and spectroscopic redshift were obtained using the Shane 3 m Telescope at the Lick Observatory, showing the unidentified object to be a BL Lac type with a redshift of z = 0.179. Broadband multiwavelength observations contemporaneous with the VERITAS exposure period can be used to subclassify the blazar as an HBL object, including data from the MDM observatory, Swift-UVOT, and X-Ray Telescope, and continuous monitoring at photon energies above 1 GeV from the Fermi Large Area Telescope (LAT). We find that in the absence of undetected, high-energy rapid variability, the one-zone synchrotron self-Compton (SSC) model overproduces the high-energy gamma-ray emission measured by the Fermi-LAT over 2.3 years. The spectral energy distribution can be parameterized satisfactorily with an external-Compton or lepto-hadronic model, which have two and six additional free parameters, respectively, compared to the one-zone SSC model.
Giant X-ray outbursts, with luminosities of about 1037 erg s--1, are observed roughly every five years from the nearby Be/pulsar binary 1A 0535+262. In this article, we present observations of the ...source with VERITAS at very high energies (VHEs; E >100 GeV) triggered by the X-ray outburst in 2009 December. The observations started shortly after the onset of the outburst and provided comprehensive coverage of the episode, as well as the 111 day binary orbit. No VHE emission is evident at any time. We also examined data from the contemporaneous observations of 1A 0535+262 with the Fermi/Large Area Telescope at high-energy photons (E > 0.1 GeV) and failed to detect the source at GeV energies. The X-ray continua measured with the Swift/X-Ray Telescope and the RXTE/PCA can be well described by the combination of blackbody and Comptonized emission from thermal electrons. Therefore, the gamma-ray and X-ray observations suggest the absence of a significant population of non-thermal particles in the system. This distinguishes 1A 0535+262 from those Be X-ray binaries (such as PSR B1259-63 and LS I +61?303) that have been detected at GeV-TeV energies. We discuss the implications of the results on theoretical models.
We report the detection of pulsed gamma rays from the Crab pulsar at energies above 100 giga—electron volts (GeV) with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) array of ...atmospheric Cherenkov telescopes. The detection cannot be explained on the basis of current pulsar models. The photon spectrum of pulsed emission between 100 mega—electron volts and 400 GeV is described by a broken power law that is statistically preferred over a power law with an exponential cutoff. It is unlikely that the observation can be explained by invoking curvature radiation as the origin of the observed gamma rays above 100 GeV. Our findings require that these gamma rays be produced more than 10 stellar radii from the neutron star.
Observations of radio halos and relics in galaxy clusters indicate efficient electron acceleration. We report here on VHE gamma-ray observations of the Coma galaxy cluster with the VERITAS array of ...imaging Cerenkov telescopes, with complementing Fermi Large Area Telescope observations at GeV energies. We use the gamma-ray upper limits to constrain cosmic rays (CRs) and magnetic fields in Coma. Using an analytical approach, the CR-to-thermal pressure ratio is constrained to be <16% from VERITAS data and <1.7% from Fermi data (averaged within the virial radius). These upper limits are starting to constrain the CR physics in self-consistent cosmological cluster simulations and cap the maximum CR acceleration efficiency at structure formation shocks to be <50%. Finally, since galaxy clusters are dark matter (DM) dominated, the VERITAS upper limits have been used to place constraints on the thermally averaged product of the total self-annihilation cross section and the relative velocity of the DM particles, left angle bracket sigmavright angle bracket.
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