Context. Very high-energy (VHE, E > 100 GeV) observations of the blazar Mrk 501 with MAGIC in 2014 provided evidence for an unusual narrow spectral feature at about 3 TeV during an extreme X-ray ...flaring activity. The one-zone synchrotron-self Compton scenario, widely used in blazar broadband spectral modeling, fails to explain the narrow TeV component. Aims. Motivated by this rare observation, we propose an alternative model for the production of narrow features in the VHE spectra of flaring blazars. These spectral features may result from the decay of neutral pions ( π 0 bumps) that are in turn produced via interactions of protons (of tens of TeV energy) with energetic photons, whose density increases during hard X-ray flares. Methods. We explored the conditions needed for the emergence of narrow π 0 bumps in VHE blazar spectra during X-ray flares reaching synchrotron energies ∼100 keV using time-dependent radiative transfer calculations. We focused on high-synchrotron peaked (HSP) blazars, which comprise the majority of VHE-detected extragalactic sources. Results. We find that synchrotron-dominated flares with peak energies ≳100 keV can be ideal periods for the search of π 0 bumps in the VHE spectra of HSP blazars. The flaring region is optically thin to photopion production, its energy content is dominated by the relativistic proton population, and the inferred jet power is highly super-Eddington. Application of the model to the spectral energy distribution of Mrk 501 on MJD 56857.98 shows that the VHE spectrum of the flare is described well by the sum of a synchrotron self-Compton (SSC) component and a distinct π 0 bump centered at 3 TeV. Spectral fitting of simulated SSC+ π 0 spectra for the Cherenkov Telescope Array (CTA) show that a π 0 bump could be detected at a 5 σ significance level with a 30-min exposure. Conclusions. A harder VHE γ -ray spectrum than the usual SSC prediction or, more occasionally, a distinct narrow bump at VHE energies during hard X-ray flares, can be suggestive of a relativistic hadronic component in blazar jets that otherwise would remain hidden. The production of narrow features or spectral hardenings due to π 0 decay in the VHE spectra of blazars is testable with the advent of CTA.
The first statistically significant detection of the cosmic gamma -ray horizon (CGRH) that is independent of any extragalactic background light (EBL) model is presented. The CGRH is a fundamental ...quantity in cosmology. It gives an estimate of the opacity of the universe to very high energy (VHE) gamma -ray photons due to photon-photon pair production with the EBL. The only estimations of the CGRH to date are predictions from EBL models and lower limits from gamma -ray observations of cosmological blazars and gamma -ray bursts. Here, we present homogeneous synchrotron/synchrotron self-Compton (SSC) models of the spectral energy distributions of 15 blazars based on (almost) simultaneous observations from radio up to the highest energy gamma -rays taken with the Fermi satellite. These synchrotron/SSC models predict the unattenuated VHE fluxes, which are compared with the observations by imaging atmospheric Cherenkov telescopes. This comparison provides an estimate of the optical depth of the EBL, which allows us a derivation of the CGRH through a maximum likelihood analysis that is EBL-model independent. We find that the observed CGRH is compatible with the current knowledge of the EBL.
Context
. We are operating an elastic light detecting and ranging system (LIDAR) for the monitoring of atmospheric conditions during regular observations of the MAGIC telescopes.
Aims
. We present ...and evaluate methods for converting aerosol extinction profiles, obtained with the LIDAR, into corrections of the reconstructed gamma-ray event energy and instrument response functions of Imaging Atmospheric Cherenkov Telescopes.
Methods
. We assess the performance of these correction schemes with almost seven years of Crab Nebula data obtained with the MAGIC telescopes under various zenith angles and different aerosol extinction scenarios of Cherenkov light.
Results
. The methods enable the reconstruction of data taken under nonoptimal atmospheric conditions with aerosol transmissions down to ~0.65 with systematic uncertainties comparable to those for data taken under optimal conditions. For the first time, the correction of data affected by clouds has been included in the assessment. The data can also be corrected when the transmission is lower than 0.65, but the results are less accurate and suffer from larger systematics.
ABSTRACT We report the first hard X-ray observations with NuSTAR of the BL Lac-type blazar PKS 2155-304, augmented with soft X-ray data from XMM-Newton and γ-ray data from the Fermi Large Area ...Telescope, obtained in 2013 April when the source was in a very low flux state. A joint NuSTAR and XMM spectrum, covering the energy range 0.5-60 keV, is best described by a model consisting of a log-parabola component with curvature and a (local) photon index 3.04 0.15 at photon energy of 2 keV, and a hard power-law tail with photon index 2.2 0.4. The hard X-ray tail can be smoothly joined to the quasi-simultaneous γ-ray spectrum by a synchrotron self-Compton component produced by an electron distribution with index p = 2.2. Assuming that the power-law electron distribution extends down to γmin = 1 and that there is one proton per electron, an unrealistically high total jet power of Lp ∼ 1047 erg s−1 is inferred. This can be reduced by two orders of magnitude either by considering a significant presence of electron-positron pairs with lepton-to-proton ratio , or by introducing an additional, low-energy break in the electron energy distribution at the electron Lorentz factor γbr1 ∼ 100. In either case, the jet composition is expected to be strongly matter-dominated.
Context. High-resolution radio observations are ideal for constraining the value of physical parameters in the inner regions of active-galactic-nucleus jets and complement results on multiwavelength ...(MWL) observations. This study is part of a wider multifrequency campaign targeting the nearby TeV blazar Markarian 421 (z = 0.031), with observations in the sub-mm (SMA), optical/IR (GASP), UV/X-ray (Swift, RXTE, MAXI), and γ rays (Fermi-LAT, MAGIC, VERITAS). Aims. We investigate the jet’s morphology and any proper motions, and the time evolution of physical parameters such as flux densities and spectral index. The aim of our wider multifrequency campaign is to try to shed light on questions such as the nature of the radiating particles, the connection between the radio and γ-ray emission, the location of the emitting regions and the origin of the flux variability. Methods. We consider data obtained with the Very Long Baseline Array (VLBA) over twelve epochs (one observation per month from January to December 2011) at 15 GHz and 24 GHz. We investigate the inner jet structure on parsec scales through the study of model-fit components for each epoch. Results. The structure of Mrk 421 is dominated by a compact (~0.13 mas) and bright component, with a one-sided jet detected out to ~10 mas. We identify 5–6 components in the jet that are consistent with being stationary during the 12-month period studied here. Measurements of the spectral index agree with those of other works: they are fairly flat in the core region and steepen along the jet length. Significant flux-density variations are detected for the core component. Conclusions. From our results, we draw an overall scenario in which we estimate a viewing angle 2° < θ < 5° and a different jet velocity for the radio and the high-energy emission regions, such that the respective Doppler factors are δr ~ 3 and δh.e. ~ 14.
High-resolution radio observations are ideal for constraining the value of physical parameters in the inner regions of active-galactic- nucleus jets and complement results on multiwavelength ...observations. The aim of their wider multifrequency campaign is to try to shed light on questions such as the nature of the radiating particles, the connection between the radio and gamma -ray emission, the location of the emitting regions and the origin of the flux variability. The authors consider data obtained with the Very Long Baseline Array over twelve epochs at 15 GHz and 24 GHz. The structure of Mrk 421 is dominated by a compact similar to 0.13 mas and bright component, with a one-sided jet detected out to similar to 10 mas. The authors identify 5-6 components in the jet that are consistent with being stationary during the 12-month period studied here. Measurements of the spectral index agree with those of other works: they are fairly flat in the core region and steepen along the jet length.
Context. High-resolution radio observations allow us to directly image the innermost region of active galactic nuclei. The Very Long Baseline Array (VLBA) data analyzed in this paper were obtained ...during a multiwavelength (MWL) campaign, carried out in 2011, from radio to very high energy gamma rays, on the TeV blazar Markarian 421 (Mrk 421). Aims. Our aim was to obtain information on the jet structure in Mrk 421 during the MWL campaign at the highest possible angular resolution and with high temporal frequency observations, in order to compare structural and flux density evolution with higher energy variations. Methods. We consider data obtained with the VLBA at 43 GHz through two sets of observations: one is part of a dedicated multi-frequency monitoring campaign, in which we observed Mrk 421 once a month from January to December 2011 at three frequencies; the other is extracted from the Boston University monitoring program, which observes 34 blazars at 43 GHz about once per month. We model-fit the data in the visibility plane, study the proper motion of jet components, the light curve, and the spectral index of the jet features. We compare the radio data with optical light curves obtained at the Steward Observatory, considering also the optical polarization information. Results. Mrk 421 has a bright nucleus and a one-sided jet extending towards the north-west for a few parsecs. The model-fits show that brightness distribution is well described using 6–7 circular Gaussian components, four of which are reliably identified at all epochs; all components are effectively stationary except for component D, at ~0.4 mas from the core, whose motion is, however, subluminal. Analysis of the light curve shows two different states, with the source being brighter and more variable in the first half of 2011 than in the second half. The highest flux density is reached in February. A comparison with the optical data reveals an increase of the V magnitude and of the fractional polarization simultaneous with the enhancement of the radio activity.
We report the Fermi Large Area Telescope (LAT) detection of two very-high-energy (VHE, E > 100 GeV) gamma -ray photons from the directional vicinity of the distant (redshift, z = 1.1) blazar PKS ...0426-380. The null hypothesis that both the 134 and 122 GeV photons originate from unrelated sources can be rejected at the 5.5sigma confidence level. We therefore claim that at least one of the two VHE photons is securely associated with PKS 0426?380, making it the most distant VHE emitter known to date. The results are in agreement with recent Fermi-LAT constraints on the extragalactic background light (EBL) intensity, which imply a z Asymptotically = to 1 horizon for Asymptotically = to100 GeV photons. The LAT detection of the two VHE gamma -rays coincided roughly with flaring states of the source, although we did not find an exact correspondence between the VHE photon arrival times and the flux maxima at lower gamma -ray energies. Modeling the gamma -ray continuum of PKS 0426-380 with daily bins revealed a significant spectral hardening around the time of the first VHE event detection (LAT photon index Gamma Asymptotically = to 1.4) but on the other hand no pronounced spectral changes near the detection time of the second one. This combination implies a rather complex variability pattern of the source in gamma -rays during the flaring epochs. An additional flat component is possibly present above several tens of GeV in the EBL-corrected Fermi-LAT spectrum accumulated over the ~8 month high state.
ABSTRACT
We present a measurement of the extragalactic background light (EBL) based on a joint likelihood analysis of 32 gamma-ray spectra for 12 blazars in the redshift range z = 0.03–0.944, ...obtained by the MAGIC telescopes and Fermi-LAT. The EBL is the part of the diffuse extragalactic radiation spanning the ultraviolet, visible, and infrared bands. Major contributors to the EBL are the light emitted by stars through the history of the Universe, and the fraction of it that was absorbed by dust in galaxies and re-emitted at longer wavelengths.
The EBL can be studied indirectly through its effect on very high energy photons that are emitted by cosmic sources and absorbed via γγ interactions during their propagation across cosmological distances. We obtain estimates of the EBL density in good agreement with state-of-the-art models of the EBL production and evolution. The 1σ upper bounds, including systematic uncertainties, are between 13 per cent and 23 per cent above the nominal EBL density in the models. No anomaly in the expected transparency of the Universe to gamma-rays is observed in any range of optical depth. We also perform a wavelength-resolved EBL determination, which results in a hint of an excess of EBL in the 0.18–0.62 $\mu\mathrm{ m}$ range relative to the studied models, yet compatible with them within systematics.
The sensitivity of photomultipliers (PMT) can be substantially increased by applying a light scattering lacquer doped with a Wavelength Shifter. Applying this method to the Electron Tubes 9116/17A ...PMT resulted in a good sensitivity in the short wave UV range as well as a 15–20% increase in quantum efficiency above
350
nm
. Details of the procedure and a simple model for the explanation of the enhancement are presented.
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