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.
We report the detection of very high energy -ray emission from the intermediate-frequency-peaked BL Lacertae object W Comae by VERITAS. The source was observed between 2008 January and April. A ...strong outburst of -ray emission was measured in the middle of March, lasting for only 4 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 image. The integral photon flux above image during those two nights corresponds to roughly image 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.
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.
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 250 GeV from M87 in spring 2007 with the VERITAS atmospheric Cerenkov telescope array and discuss its correlation with the X-ray emission. The -ray emission is measured to be pointlike with an intrinsic source radius less than 4.5 super(image ). The differential energy spectrum is fitted well by a power-law function: image m super(-2) s super(-1) TeV super(-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-timescale 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.
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 (10.4s) 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) X 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.
We report our intensive, high angular resolution radio monitoring observations of the jet in M 87 with the VLBI Exploration of Radio Astrometry (VERA) and the European VLBI Network (EVN) from 2011 ...February to 2012 October, together with contemporaneous high-energy (100 MeV < E < 100 GeV) gamma -ray light curves obtained by the Fermi Large Area Telescope. We detected a remarkable (up to ~70%) increase of the radio flux density from the unresolved jetbase (radio core) with VERA at 22 and 43 GHz coincident with the VHE activity. These results in the radio bands strongly suggest that the VHE gamma -ray activity in 2012 originates in the jet base within 0.03 pc or 56 Schwarzschild radii (the VERA spatial resolution of 0.4 mas at 43 GHz) from the central supermassive black hole. This indicates that a new radio-emitting component was created near the black hole in the period of the VHE event, and then propagated outward with progressively decreasing synchrotron opacity.
The authors analyzed and compared 26 VLBI observations of the M87 jet, obtained between 2006 and 2011 with the Very Long Baseline Array at 1.7 GHz and the European VLBI Network at 5 GHz. HST-1 is ...detected at all epochs; they model-fitted its complex structure with two or more components, the two outermost of which display a significant proper motion with a superluminal velocity around similar to 4 c. The motion of a third feature that is detected upstream is more difficult to characterize. The overall position angle of HST-1 has changed during the time of our observations from -65 degree Celsius to -90 degree Celsius, while the structure has moved by over 80 mas downstream. Their results on the component evolution suggest that structural changes at the upstream edge of HST-1 can be related to the very high energy events.
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