arXiv subjects

Robert Wagner

Publications and source records attributed to Robert Wagner.

29 records · Page 2Linked to original sources

arXiv2009

Physics Insights from Recent MAGIC AGN Observations

The total set of the 14 active galactic nuclei detected by MAGIC so far includes well-studied bright blazars like Mkn 501, the giant radio galaxy M 87, but also the distant flat-spectrum radio quasar 3C 279, and an intriguing gamma-ray source in the 3C 66A/B region, whose energy spectrum is not compatible with the expectations from 3C 66A. Besides scheduled observations, so far MAGIC succeeded in discovering TeV gamma rays from three blazars following triggers from high optical states. I report selected highlights from recent MAGIC observations of extragalactic TeV gamma-ray sources, emphasizing and discussing the new physics insights the MAGIC observations were able to contribute.

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arXiv2009

Status and recent results of MAGIC

MAGIC is a single-dish Cherenkov telescope located on La Palma (Spain), hence with an optimal view on the Northern sky. Sensitive in the 30 GeV-30 TeV energy band, it is nowadays the only ground-based instrument being able to measure high-energy gamma-rays below 100 GeV. We review the most recent experimental results obtained using MAGIC.

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arXiv2009

Monitoring of Bright Blazars with MAGIC in the 2007/2008 Season

Because of the short duty-cycles and observation-time constraints, studies of bright TeV (E>100 GeV) blazars are mostly restricted to flaring episodes or rather short (days to few weeks) multiwavelength campaigns. At the same time, long-term studies of these objects are essential to gain a more complete understanding of the blazar phenomenon and to constrain theoretical models concerning jet physics. Only unbiased long-term studies are adequate for the determination of flaring state probabilities and for estimating the statistical significance of possible correlations between TeV flaring states and other wavebands or observables, such as neutrino events. Regular observations also provide triggers for multiwavelength ToO observations originating from the TeV waveband. These are particularly needed to identify and study orphan TeV flares, i.e. flares without counterparts in other wavebands. In 2007/8 the MAGIC telescope has monitored three TeV blazars on a regular basis: Mrk 501, Mrk 421, and 1ES 1959+650. We present preliminary results of these observations including the measured light curves and a correlation study for VHE gamma-rays and X-rays and VHE gamma-rays and optical R-band for Mrk 421.

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arXiv2009

Exploring Quantum Gravity with Very-High-Energy Gamma-Ray Instruments - Prospects and Limitations

Some models for quantum gravity (QG) violate Lorentz invariance and predict an energy dependence of the speed of light, leading to a dispersion of high-energy gamma-ray signals that travel over cosmological distances. Limits on the dispersion from short-duration substructures observed in gamma-rays emitted by gamma-ray bursts (GRBs) at cosmological distances have provided interesting bounds on Lorentz invariance violation (LIV). Recent observations of unprecedentedly fast flares in the very-high energy gamma-ray emission of the active galactic nuclei (AGNs) Mkn 501 in 2005 and PKS 2155-304 in 2006 resulted in the most constraining limits on LIV from light-travel observations, approaching the Planck mass scale, at which QG effects are assumed to become important. I review the current status of LIV searches using GRBs and AGN flare events, and discuss limitations of light-travel time analyses and prospects for future instruments in the gamma-ray domain.

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arXiv2008

Scientific Highlights from Observations of Active Galactic Nuclei with the MAGIC Telescope

Since 2004, the MAGIC gamma-ray telescope has newly discovered 6 TeV blazars. The total set of 13 MAGIC-detected active galactic nuclei includes well-studied objects at other wavelengths like Markarian 501 and the giant radio galaxy M87, but also the distant the flat-spectrum radio quasar 3C 279, and the newly discovered TeV gamma-ray emitter S5 0716+71. In addition, also long-term and multi-wavelength studies on well-known TeV blazars and systematic searches for new TeV blazars have been carried out. Here we report selected highlights from recent MAGIC observations of extragalactic TeV gamma-ray sources, emphasizing the new physics insights MAGIC was able to contribute.

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arXiv2008

Correlated Variability in Blazars

Blazars are thought to emit highly-collimated outflows, so-called jets. By their close alignment to our line of sight, relativistic beaming effects enable us to observe these jets over the whole electromagnetic spectrum up to TeV energies, making them ideal laboratories for studying jet physics. In the last years multiwavelength observations of blazars provided us with detailed data sets which helped to characterize the two main components of the non-relativistic emission, peaking in the optical to X-ray and GeV/TeV energy region, respectively. In leptonic acceleration models, they are explained by synchrotron radiation of electrons and inverse-Compton emission from the same electron population and thus, correlations of both emission regimes are expected. We review recent observational results on the presence and absence of such correlations in blazars, and discuss constraints on emission models by quantitative correlation analyses.

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arXiv2008

Synoptic studies of seventeen blazars detected in very high-energy gamma-rays

Since 2002, the number of detected blazars at gamma-ray energies above 100 GeV has more than doubled. I study 17 blazars currently known to emit E>100 GeV gamma rays. Their intrinsic energy spectra are reconstructed by removing extragalactic background light attenuation effects. Luminosity and spectral slope in the E>100 GeV region are then compared and correlated among each other, with X-ray, optical and radio data, and with the estimated black hole (BH) masses of the respective host galaxies. According to expectations from synchrotron self-Compton emission models, a correlation on the 3.6-sigma significance level between gamma-ray and X-ray fluxes is found, while correlations between gamma-ray and optical/radio fluxes are less pronounced. Further, a general hardening of the E>100 GeV spectra with increasing gamma-ray luminosity is observed. This goes in line with a correlation of the gamma-ray luminosity and the synchrotron peak frequency, which is also seen. Tests for possible selection effects reveal a hardening of the spectra with increasing redshift. The gamma-ray emission might depend on the mass of the central BH. The studied blazars show no correlation of the BH masses with the spectral index and the luminosity in the E>100 GeV region. Also temporal properties of the X-ray and E>100 GeV gamma-ray flux are considered. No general trends are found, except that the blazars with the most massive BHs do not show particularly high duty cycles. In general, VHE flare time-scales are not found to scale with the BH mass. As a specific application of the luminosity study, a constraint for the still undetermined redshift of the blazar PG 1553+113 is discussed.

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arXiv2007

Detection of Very-High Energy Gamma-Rays from the BL Lac Object PG 1553+113 with the MAGIC Telescope

The MAGIC telescope has observed very-high energy gamma-ray emission from the BL Lac object PG 1553+113 in 2005 and 2006 at an overall significance is 8.8 sigma. The light curve shows no significant flux variations on a daily timescale. The flux level during 2005 was, however, significantly higher as compared to 2006. The differential energy spectrum between approx. 90 GeV and 500 GeV is well described by a power law with a spectral index of -4.2+-0.3. The photon energy spectrum and spectral modeling allow to pose upper limits of z=0.74 and z=0.56, respectively, on the yet undetermined redshift of PG 1553+113. Recent VLT observations of this blazar show featureless spectra in the near-IR, thus no direct redshift could be determined from these measurements.

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arXiv2007

Discovery of Very High Energy Gamma-Rays from the Distant Flat Spectrum Radio Quasar 3C 279 with the MAGIC Telescope

The quasar 3C 279 is one of the best-studied flat spectrum radio quasars. It is located at a comparatively large redshift of z=0.536: E>100 GeV observations of such distant sources were until recently impossible both due to the expected steep energy spectrum and the expected attenuation of the gamma-rays by the extragalactic background light. Here we present results on the observation of 3C 279 with the MAGIC telescope in early 2006. We report the detection of a significant very high energy gamma-ray signal in the MAGIC energy range on the observation night of 2006 February 23.

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arXiv2007

AGN Observations in the GeV/TeV Energy Range with the MAGIC Telescope

MAGIC currently is the largest imaging atmospheric Cerenkov telescope world-wide. Since 2004, gamma-ray emission from several active galactic nuclei in the GeV/TeV energy range has been detected, some of which were newly discovered as very-high energy gamma-ray sources. The gamma-rays are assumed to originate from particle acceleration processes in the AGN jets. We give an overview of the AGN observed and detected by MAGIC, discuss spectral and temporal properties of these and show physics implications of some selected observations.

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arXiv2007

A First Synoptic Blazar Study Comprising Thirteen Blazars Visible in E>100 GeV Gamma-Rays

Since 2002, the number of detected blazars at E_γ> 100 GeV has more than doubled. I study all currently known BL Lac-type objects with published energy spectra. Their intrinsic energy spectra are reconstructed by removing extragalactic background light attenuation effects. The emission properties are then compared and correlated among each other, with X-ray data, and with the individual black hole masses. In addition, I consider temporal properties of the very high energy gamma-ray flux. Key findings concern the flux--black hole mass and variability scale--black hole mass connections and the correlation of the spectral slope and the luminosity. As a specific application, the study allows to constrain the still undetermined redshift of the blazar PG 1553+113.

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