We report the successful development of a novel methodology of energy reconstruction for very high energy gamma rays detected with Imaging Atmospheric Cherenkov Telescopes (IACTs). This methodology, ...based on the machine learning algorithm Random Forest, and named RF-Erec, has been adjusted for being used with data from the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) stereo telescope system, which is a worldwide leading instrument for gamma-ray astronomy in the energy range from about 20GeV to beyond 100TeV.
The RF-Erec has been evaluated using different realistic scenarios with Monte Carlo simulated data and real observations from the Crab Nebula (the standard candle for the VHE gamma-ray community). This new methodology has been validated by the MAGIC software board, and it is implemented and ready-to-use in the MAGIC Analysis and Reconstruction Software (MARS). This new methodology, validated by the MAGIC software board, has been implemented and is ready for use in the MAGIC Analysis and Reconstruction Software (MARS). We demonstrate that, in comparison to the previous energy reconstruction methodology for MAGIC data, which relied on Look-Up-Tables (LUTs- Erec) and has been utilized in over 100 scientific publications over the last decade, RF-Erec significantly enhances the energy reconstruction of gamma rays. This improvement extends the capabilities of the MAGIC telescopes.
Specifically, when quantifying the energy resolution with the width of a Gaussian fitted to the error distribution (resolution-σ), the RF-Erec energy resolution-σ is 20% at 100 GeV and 11% above 1 TeV for Zenith distances (Zd) below 35 degrees, while it is 20% at 1TeV and 13% above 10TeV for Zd above 55 degrees. For a wide range of the observable energies, the improvement of energy resolution-σ, compared to LUTs-Erec, reaches roughly a factor of two, and the improvement is even larger for high Zd observations. Differently to many other works in the literature, our evaluation also considers the energy dispersion and the actual energy migration of events, where RF-Erec improves the performance of LUTs-Erec by factors of a few. The manuscript also demonstrates the importance of energy reconstruction methods with a small energy migration in order to prevent the appearance of artificial spectral features. These artifacts are particularly important at the high end of the gamma-ray spectra, where a few extra high-energy photons could have dramatic consequences for studies related to the EBL attenuation, Lorentz invariance violation, or searches for Axion-like-particles.
We investigate the precise location of the radio core in the nearby blazar Mrk 501 for the first time during its X-ray and TeV γ-ray active state in 2012 by revisiting from the perspective of ...astrometry the six-epoch observations with the Very Long Baseline Array at 43 GHz reported by Koyama et al. We find that the position of the radio core seen at 43 GHz remained stable during our observations from 2012 to 2013 February within 42 as in the southeast jet direction and 56 as along the northeast jet direction. This implies that the location of the 43 GHz radio-emitting core was limited within the deprojected scale of 4.6 × 103 Schwarzschild radii (Rs) during the high-energy active state. This result is a contrast to another case of the astrometric observation of the famous nearby TeV blazar Mrk 421, which showed a clear radio core position change soon after the large X-ray flare in 2011, reported by Niinuma et al. We compare the two cases and discuss possible origins of the different results of the radio core astrometry in the high-energy active states between the nearby blazars. Based on the internal shock model for blazars, the Lorentz factors of the ejecta explaining the stability of the radio core in Mrk 501 are expected to be a few times smaller than those for the wandering core in Mrk 421.
Mrk 501 is one of the most prominent TeV-emitting blazars and belongs to the class of high synchrotron peaked (HSP) blazars. The Doppler factors derived from the jet kinematics are much too low to ...provide sufficient beaming for the detected high-energy emission (the so-called Lorentz factor crisis). This BL Lac object is also a prime example of a misaligned AGN with an approximately 90∘ difference in orientation between the inner parsec-scale jet and the kpc-scale jet structure. We have performed a detailed analysis of the pc-scale jet kinematics, based on 23 years of VLBA observations (at 15 GHz) and find, in addition to robustly consolidating the already claimed stationary jet features and a hinted absence of component ejections, a significant drift of the outer nuclear jet. The two outermost jet features move with somewhat higher but still subluminal speeds. Albeit, they move orthogonally to the inner jet, which itself does not partake in the drifting motion. The effect of this intriguing kinematics is that the jet appears strongly curved at first (1995) but then appears to straighten out (2018). To our knowledge, this is the first time that the orthogonal swing of just the outer part of a nuclear jet has been observed. We discuss the possible physical nature of this turning maneuver. In addition, we report evidence for jet emission, which most likely originates in a spine–sheath structure.
Context.
An electron-positron beam escaping from the magnetospheric vacuum gap of an accreting black hole interacts with recombination-line photons from surrounding gas clouds. Inverse-Compton ...scattering and subsequent pair production initiate unsaturated electromagnetic cascades exhibiting a characteristic spectral energy distribution.
Aims.
By modelling the interactions of beam electrons (positrons) with hydrogen and helium recombination-line photons, we seek to describe the spectral signature of beam-driven cascades in the broad emission-line region of blazar jets.
Methods.
Employing coupled kinetic equations for electrons (positrons) and photons including an escape term, we numerically obtained their steady-state distributions and the escaping photon spectrum.
Results.
We find that cascade emission resulting from beam interactions can produce a narrow spectral feature at TeV energies. Indications of such an intermittent feature, which defies an explanation in the standard shock-in-jet scenario, have been found at ≈ 4
σ
confidence level at an energy of ≈ 3 TeV in the spectrum of the blazar Mrk 501.
Conclusions.
The energetic requirements for explaining the intermittent 3 TeV bump with the beam-interaction model are plausible: Gap discharges that lead to multi-TeV beam electrons (positrons) carrying ≈ 0.1% of the Blandford-Znajek luminosity, which interact with recombination-line photons from gas clouds that reprocess ≈ 1% of the similar accretion luminosity are required.
Experimental Gamma-Ray Astronomy Paneque, David
Journal of physics. Conference series,
07/2012, Letnik:
375, Številka:
5
Journal Article
Recenzirano
Odprti dostop
Our knowledge of the γ-ray sky has dramatically changed due to the advent of the new ground-based Imaging Atmospheric Cherenkov Telescopes (H.E.S.S., MAGIC and VEPJTAS) and the satellite-borne ...instruments (AGILE and Fermi). These facilities boosted the number of γ-ray sources by one order of magnitude in the last 6 years, providing us with about 2000 sources detected above 100 MeV (from space) and about 100 sources detected above 100 GeV (from the ground). The combination of this large leap in experimental capabilities together with the fact that the Universe is still quite unexplored at these extreme energies is evidence of a large scientific discovery potential that will surely make the decade 2010–2020 a golden age for γ-ray astronomy. In this manuscript I provide a subjective review of some of the most exciting observations from this rapidly evolving field during the last two years.
Our knowledge of the gamma -ray sky has dramatically changed due to the advent of the new ground-based Imaging Atmospheric Cherenkov Telescopes (H.E.S.S., MAGIC and VERITAS) and the satellite-borne ...instruments (AGILE and Fermi). These facilities boosted the number of gamma -ray sources by one order of magnitude in the last 6 years, providing us with about 2000 sources detected above 100 MeV (from space) and about 100 sources detected above 100 GeV (from the ground). The combination of this large leap in experimental capabilities together with the fact that the Universe is still quite unexplored at these extreme energies is evidence of a large scientific discovery potential that will surely make the decade 2010-2020 a golden age for gamma -ray astronomy. In this manuscript I provide a subjective review of some of the most exciting observations from this rapidly evolving field during the last too years.
We present a catalog of gamma-ray sources at energies above 10 GeV based on data from the Large Area Telescope (LAT) accumulated during the first 3 yr of the Fermi Gamma-ray Space Telescope mission. ...The first Fermi-LAT catalog of >10 GeV sources (1FHL) has 514 sources. For each source we present location, spectrum, a measure of variability, and associations with cataloged sources at other wavelengths. We found that 449 (87%) could be associated with known sources, of which 393 (76% of the 1FHL sources) are active galactic nuclei. Of the 27 sources associated with known pulsars, we find 20 (12) to have significant pulsations in the range >10 GeV (>25 GeV). In this work we also report that, at energies above 10 GeV, unresolved sources account for 27% +/- 8% of the isotropic γ -ray background, while the unresolved Galactic population contributes only at the few percent level to the Galactic diffuse background. We also highlight the subset of the 1FHL sources that are best candidates for detection at energies above 50-100 GeV with current and future ground-based γ -ray observatories.
We present a new gamma ray energy reconstruction method based on Random Forest to be commonly used for the data analysis of the MAGIC Telescopes, a system of two Imaging Atmospheric Cherenkov ...Telescopes. The energy resolution with the new energy reconstruction improves compared to the one obtained with the LUTs method. For standard observations i.e. dark conditions with pointing zenith (Zd) less than 35 deg for a point-like source, the energy resolution goes from \(\sim 20\%\) at 100 GeV to \(\sim 10\%\) at a few TeV. In addition, the new method suppresses the outlier population in the energy error distribution, which is thus better described by a Gaussian distribution. The new energy reconstruction method enhances the reliability especially for the sources with steep spectra, in higher energies and/or in observations at higher Zd pointings. We validate the new method in different ways and demonstrate some cases of its remarkable benefit in spectral analysis with simulated observation data.
Millisecond pulsars (MSPs) are old neutron stars that spin hundreds of times per second and appear to pulsate as their emission beams cross our line of sight. To date, radio pulsations have been ...detected from all rotation-powered MSPs. In an attempt to discover radio-quiet gamma-ray MSPs, we used the aggregated power from the computers of tens of thousands of volunteers participating in the Einstein@Home distributed computing project to search for pulsations from unidentified gamma-ray sources in Fermi Large Area Telescope data. This survey discovered two isolated MSPs, one of which is the only known rotation-powered MSP to remain undetected in radio observations. These gamma-ray MSPs were discovered in completely blind searches without prior constraints from other observations, raising hopes for detecting MSPs from a predicted Galactic bulge population.
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