Special Issue "Monitoring the Non-Thermal Universe"

A special issue of Galaxies (ISSN 2075-4434).

Deadline for manuscript submissions: closed (28 February 2019)

Special Issue Editors

Guest Editor
Dr. Daniela Dorner

Department of Astronomy, University Würzburg, 97074 Würzburg, Germany
Website | E-Mail
Interests: active galactic nuclei; Gamma-ray astronomy; multi-wavelength studies; Cherenkov telescopes
Guest Editor
Prof. Dr. Thomas Bretz

RWTH Aachen University, 52062 Aachen, Germany
Website | E-Mail
Interests: cosmic-rays; blazars; variability; TeV astronomy

Special Issue Information

Dear Colleagues,

Monitoring is the key to understand the physics governing the non-thermal Universe. Although many sources are studied already in great detail over the entire electromagnetic spectrum, more information is required to further constrain the wide parameter space of emission models. Time-resolved energy spectra in all wavebands are crucial. This highlights the importance of any monitoring program, or more generally: Time-domain astrophysics.

The workshop “Monitoring the Non-Thermal Universe” brings together experts from multi-wavelength and multi-messenger astronomy working on monitoring, observations, variability analysis, astro-statistics, data analysis and interpretation, and theoretical modeling.

With this Special Issue of Galaxies, the current status is summarized, providing a unique and comprehensive reference on the importance of monitoring for the insight into source physics. It will serve as a common basis for future studies and projects.

Prof. Dr. Thomas Bretz,

Dr. Daniela Dorner

Guest Editor

Manuscript Submission Information

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Keywords

  • Monitoring
  • Active Galactic Nuclei
  • Blazar variability
  • Multi-Wavelength
  • Multi-Messenger
  • Data Analysis and processing
  • Timeseries analysis
  • Blazar modeling

Published Papers (17 papers)

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Research

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Open AccessArticle
Fractional Variability—A Tool to Study Blazar Variability
Received: 8 February 2019 / Revised: 17 May 2019 / Accepted: 23 May 2019 / Published: 30 May 2019
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Abstract
Active Galactic Nuclei emit radiation over the whole electromagnetic spectrum up to TeV energies. Blazars are one subtype with their jets pointing towards the observer. One of their typical features is extreme variability on timescales, from minutes to years. The fractional variability is [...] Read more.
Active Galactic Nuclei emit radiation over the whole electromagnetic spectrum up to TeV energies. Blazars are one subtype with their jets pointing towards the observer. One of their typical features is extreme variability on timescales, from minutes to years. The fractional variability is an often used parameter for investigating the degree of variability of a light curve. Different detection methods and sensitivities of the instruments result in differently binned data and light curves with gaps. As they can influence the physics interpretation of the broadband variability, the effects of these differences on the fractional variability need to be studied. In this paper, we study the systematic effects of completeness in time coverage and the sampling rate. Using public data from instruments monitoring blazars in various energy ranges, we study the variability of the bright TeV blazars Mrk 421 and Mrk 501 over the electromagnetic spectrum, taking into account the systematic effects, and compare our findings with previous results. Especially in the TeV range, the fractional variability is higher than in previous studies, which can be explained by the much longer (seven years compared to few weeks) and more complete data sample. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
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Open AccessArticle
Flux States of Active Galactic Nuclei
Received: 21 February 2019 / Revised: 24 April 2019 / Accepted: 30 April 2019 / Published: 21 May 2019
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Abstract
Blazars are known to show variability on time scales from minutes to years covering a wide range of flux states. Studying the flux distribution of a source allows for various insights. The shape of the flux distribution can provide information on the nature [...] Read more.
Blazars are known to show variability on time scales from minutes to years covering a wide range of flux states. Studying the flux distribution of a source allows for various insights. The shape of the flux distribution can provide information on the nature of the underlying variability processes. The level of a possible quiescent state can be derived from the main part of the distribution that can be described by a Gaussian distribution. Dividing the flux states into quiescent and active, the duty cycle of a source can be calculated. Finally, this allows alerting the multi-wavelength and multi-messenger community in case a source is in an active state. To get consistent and conclusive results from flux distributions, unbiased long-term observations are crucial. Only like this is a complete picture of the variability and flux states, e.g., an all-time quiescent state, possible. In seven years of monitoring of bright TeV blazars, the first G-APD Cherenkov telescope (FACT) has collected a total of more than 11,700 hours of physics data with 1500 hours to 3000 hours per source for Mrk 421, Mrk 501, 1ES 1959+650, and 1ES 2344+51. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
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Open AccessArticle
Unbiased Long-Term Monitoring at TeV Energies
Received: 28 February 2019 / Revised: 19 April 2019 / Accepted: 20 April 2019 / Published: 28 April 2019
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Abstract
For the understanding of the variable, transient and non-thermal universe, unbiased long-term monitoring is crucial. To constrain the emission mechanisms at the highest energies, it is important to characterize the very high energy emission and its correlation with observations at other wavelengths. At [...] Read more.
For the understanding of the variable, transient and non-thermal universe, unbiased long-term monitoring is crucial. To constrain the emission mechanisms at the highest energies, it is important to characterize the very high energy emission and its correlation with observations at other wavelengths. At very high energies, only a limited number of instruments is available. This article reviews the current status of monitoring of the extra-galactic sky at TeV energies. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
Open AccessArticle
The VHE γ-Ray View of the FSRQ PKS 1510-089
Received: 31 January 2019 / Revised: 13 March 2019 / Accepted: 15 March 2019 / Published: 20 March 2019
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Abstract
The flat spectrum radio quasar PKS 1510-089 is a monitored target in many wavelength bands due to its high variability. It was detected as a very-high-energy (VHE) γ-ray emitter with H.E.S.S. in 2009, and has since been a regular target of VHE [...] Read more.
The flat spectrum radio quasar PKS 1510-089 is a monitored target in many wavelength bands due to its high variability. It was detected as a very-high-energy (VHE) γ-ray emitter with H.E.S.S. in 2009, and has since been a regular target of VHE observations by the imaging Cherenkov observatories H.E.S.S. and MAGIC. In this paper, we summarize the current state of results focusing on the monitoring effort with H.E.S.S. and the discovery of a particularly strong VHE flare in 2016 with H.E.S.S. and MAGIC. While the source has now been established as a weak, but regular emitter at VHE, no correlation with other energy bands has been established. This is underlined by the 2016 VHE flare, where the detected optical and high-energy γ-ray counterparts evolve differently than the VHE flux. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
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Open AccessArticle
Characterising the Long-Term Variability of Blazars in Leptonic Models
Received: 18 January 2019 / Revised: 22 February 2019 / Accepted: 28 February 2019 / Published: 5 March 2019
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Abstract
Most research on blazar variability focuses on individual flares to explain acceleration and radiation mechanisms and improve on current models. These short-time events (being minutes, hours, or days) might not be representative of the underlying mechanisms causing small-amplitude variability and/or continuous emission which [...] Read more.
Most research on blazar variability focuses on individual flares to explain acceleration and radiation mechanisms and improve on current models. These short-time events (being minutes, hours, or days) might not be representative of the underlying mechanisms causing small-amplitude variability and/or continuous emission which is present most of the time. We will therefore investigate long-term (months to years) variability of blazar emission in the framework of current leptonic blazar models. For this purpose, we introduce generated time-dependent parameter variations which are based on typical Power Spectral Densities (PSDs) associated with the variability of accretion flows. The PSDs from the resulting light curves are analyzed and compared to one another, as well as the PSD of the input variation. Correlations between light curves are also investigated to aid identification of characteristic variation patterns associated with leptonic models. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
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Open AccessArticle
The Long-Lasting Activity in the Flat Spectrum Radio Quasar (FSRQ) CTA 102
Received: 11 January 2019 / Revised: 11 February 2019 / Accepted: 14 February 2019 / Published: 28 February 2019
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Abstract
The flat spectrum radio quasar CTA 102 (z=1.032) went through a tremendous phase of variability. Since early 2016 the gamma-ray flux level has been significantly higher than in previous years. It was topped by a four month long giant [...] Read more.
The flat spectrum radio quasar CTA 102 ( z = 1.032 ) went through a tremendous phase of variability. Since early 2016 the gamma-ray flux level has been significantly higher than in previous years. It was topped by a four month long giant outburst, where peak fluxes were more than 100 times higher than the quiescence level. Similar trends are observable in optical and X-ray energies. We have explained the giant outburst as the ablation of a gas cloud by the relativistic jet that injects additional matter into the jet and can self-consistently explain the long-term light curve. Here, we argue that the cloud responsible for the giant outburst is part of a larger system that collides with the jet and is responsible for the years-long activity in CTA 102. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
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Open AccessArticle
H.E.S.S. Monitoring of PKS 2155-304 in 2015 and 2016
Received: 24 November 2018 / Revised: 3 January 2019 / Accepted: 14 January 2019 / Published: 19 January 2019
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Abstract
PKS 2155-304 is one of the brightest blazar located in Southern Hemisphere, monitored with H.E.S.S. since the first light of the experiment. Here we report multiwavelength monitoring observations collected during the period of 2015–2016 with H.E.S.S., Fermi-LAT, Swift-XRT, Swift-UVOT, and ATOM. Two [...] Read more.
PKS 2155-304 is one of the brightest blazar located in Southern Hemisphere, monitored with H.E.S.S. since the first light of the experiment. Here we report multiwavelength monitoring observations collected during the period of 2015–2016 with H.E.S.S., Fermi-LAT, Swift-XRT, Swift-UVOT, and ATOM. Two years of multiwavelength data with very good temporal coverage allowed to characterize broadband emission observed from the region of PKS 2155-304 and study potential multifrequency correlations. During the period of monitoring, PKS 2155-304 revealed complex multiwavelength variability with two outbursts characterized by completely different multiband properties. The 2015 activity of the blazar is characterized by a flare observed at all wavelengths studied. The broadband emission observed during the outburst is well correlated without any time lags. Contrary to 2015, in 2016, only orphan outburst in the optical and ultraviolet wavelengths was observed. Such an orphan activity is reported for the first time for the blazar PKS 2155-304. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
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Open AccessArticle
AMON Multimessenger Alerts: Past and Future
Received: 30 November 2018 / Revised: 8 January 2019 / Accepted: 12 January 2019 / Published: 16 January 2019
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Abstract
The Astrophysical Multimessenger Observatory Network (AMON) was founded to tie the world’s high-energy and multimessenger observatories into a single network, with the purpose to enable the discovering of multimessenger sources, to exploit these sources for purposes of astrophysics, fundamental physics, and cosmology, and [...] Read more.
The Astrophysical Multimessenger Observatory Network (AMON) was founded to tie the world’s high-energy and multimessenger observatories into a single network, with the purpose to enable the discovering of multimessenger sources, to exploit these sources for purposes of astrophysics, fundamental physics, and cosmology, and to explore archival datasets for evidence of multimessenger source populations. Contributions of AMON to date include the GCN prompt alerts for likely-cosmic neutrinos, multiple follow-up campaigns for likely-cosmic neutrinos including the IceCube-170922A event, and several archival searches for transient and flaring γ + ν and ν + CR multimessenger sources. Given the new dawn of multimessenger astronomy recently realized with the detection of the neutron binary star merger and the possible γ + ν coincidence detection from the blazar TXS0506+056, in 2019, we are planning to commission several multimessenger alert streams, including GW + γ and high-energy γ + ν coincidence alerts. We will briefly summarize the current status of AMON and review our monitoring plans for high-energy and multimessenger AMON alerts during what promises to be a very exciting year for multimessenger astrophysics. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
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Review

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Open AccessReview
Long-Term Optical Monitoring of Blazars
Received: 28 February 2019 / Revised: 20 April 2019 / Accepted: 14 May 2019 / Published: 21 May 2019
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Abstract
Systematic monitoring of specific targets in the optical regime was historically applied on a very narrow sample of known variable stars. The discovery of blazars in the 20th century brought to the foreground the need for new global sky surveys, covering the entire [...] Read more.
Systematic monitoring of specific targets in the optical regime was historically applied on a very narrow sample of known variable stars. The discovery of blazars in the 20th century brought to the foreground the need for new global sky surveys, covering the entire sky and fainter sources. Full-sky surveys are conducted more easily from space observatories, while radio telescopes perform follow up observations from the ground. Blazars are detected in a wide range of energies, while they exhibit strong variability in various wavelengths from γ-rays and X-rays to the optical and radio domain. This results in a detailed classification, according to their emission properties in each region. The rapid variability in optical domain makes blazars interesting targets for optical sky surveys, offering a new opportunity to study their variability in the time domain. Digital sky surveys in optical and near-IR found a fertile ground with the aid of sensitive sensors. Only a few dedicated programs are focusing on blazar variability, a trend which evolved rapidly in the last decade. Modern techniques, in combination with dedicated sky survey programs lead towards a new era of long-term monitoring of blazars, aiming towards the search or variability on various time scales. In this work, an overview of blazar optical surveys and monitoring projects is given, addressing the major points of each one, and highlighting the constraints that the long-term study of blazars will bring through future international campaigns. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
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Open AccessReview
The RoboPol Program: Optical Polarimetric Monitoring of Blazars
Received: 28 February 2019 / Revised: 4 April 2019 / Accepted: 8 April 2019 / Published: 10 April 2019
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Abstract
After three years of polarimetric monitoring of blazars, the RoboPol project has uncovered several key characteristics of polarimetric rotations in the optical for these most variable sources. The most important of these is that polarization properties of the synchrotron emission in the optical [...] Read more.
After three years of polarimetric monitoring of blazars, the RoboPol project has uncovered several key characteristics of polarimetric rotations in the optical for these most variable sources. The most important of these is that polarization properties of the synchrotron emission in the optical appear to be directly linked with gamma-ray activity. In this paper, we discuss the evidence for this connection, as well as the broader features of polarimetric behavior in blazars that are key in making progress with theoretical modeling of blazar emission. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
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Open AccessReview
Monitoring and Multi-Messenger Astronomy with IceCube
Received: 27 February 2019 / Revised: 12 March 2019 / Accepted: 13 March 2019 / Published: 19 March 2019
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Abstract
IceCube currently is the largest neutrino observatory with an instrumented detection volume of 1 km3 buried in the ice-sheet close to the antarctic South Pole station. With a 4π field of view and an up-time of >99%, it is continuously monitoring [...] Read more.
IceCube currently is the largest neutrino observatory with an instrumented detection volume of 1 km3 buried in the ice-sheet close to the antarctic South Pole station. With a 4 π field of view and an up-time of >99%, it is continuously monitoring the full sky to detect astrophysical neutrinos. With the detection of an astrophysical neutrino flux in 2013, IceCube opened a new observation window to the non-thermal Universe. The IceCube collaboration has a large program to search for astrophysical neutrinos, including measurements of the energy spectrum of the diffuse astrophysical flux, auto- and cross-correlation studies with other multi-messenger particles, and a real-time alert and follow-up system. On 22 September 2017, the IceCube online system sent out an alert reporting a high-energy neutrino event. This alert triggered a series of multi-wavelength follow-up observations that revealed a spatially-coincident blazar TXS 0506+056, which was also in an active flaring state. This correlation was estimated at a 3 σ level. Further observations confirmed the flaring emission in the very-high-energy gamma-ray band. In addition, IceCube found an independent 3.5 σ excess of a time-variable neutrino flux in the direction of TXS 0506+056 two years prior to the alert by examining 9.5 years of archival neutrino data. These are the first multi-messenger observations of an extra-galactic astrophysical source including neutrinos since the observation of the supernova SN1987A. This review summarizes the different detection and analysis channels for astrophysical neutrinos in IceCube, focusing on the multi-messenger program of IceCube and its major scientific results. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
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Open AccessReview
Gamma-Ray Astrophysics in the Time Domain
Received: 17 December 2018 / Revised: 22 January 2019 / Accepted: 24 January 2019 / Published: 29 January 2019
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Abstract
The last few years have seen gamma-ray astronomy maturing and advancing in the field of time-domain astronomy, utilizing source variability on timescales over many orders of magnitudes, from a decade down to a few minutes and shorter, depending on the source. This review [...] Read more.
The last few years have seen gamma-ray astronomy maturing and advancing in the field of time-domain astronomy, utilizing source variability on timescales over many orders of magnitudes, from a decade down to a few minutes and shorter, depending on the source. This review focuses on some of the key science issues and conceptual developments concerning the timing characteristics of active galactic nuclei (AGN) at gamma-ray energies. It highlights the relevance of adequate statistical tools and illustrates that the developments in the gamma-ray domain bear the potential to fundamentally deepen our understanding of the nature of the emitting source and the link between accretion dynamics, black hole physics, and jet ejection. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
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Open AccessReview
Monitoring the Extragalactic High Energy Sky
Received: 27 November 2018 / Revised: 21 December 2018 / Accepted: 23 December 2018 / Published: 3 January 2019
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Abstract
Blazars are jetted active galactic nuclei with a jet pointing close to the line of sight, hence enhancing their intrinsic luminosity and variability. Monitoring these sources is essential in order to catch them flaring and promptly organize follow-up multi-wavelength observations, which are key [...] Read more.
Blazars are jetted active galactic nuclei with a jet pointing close to the line of sight, hence enhancing their intrinsic luminosity and variability. Monitoring these sources is essential in order to catch them flaring and promptly organize follow-up multi-wavelength observations, which are key to providing rich data sets used to derive e.g., the emission mechanisms at work, and the size and location of the flaring zone. In this context, the Fermi-LAT has proven to be an invaluable instrument, whose data are used to trigger many follow-up observations at high and very high energies. A few examples are illustrated here, as well as a description of different data products and pipelines, with a focus given on FLaapLUC, a tool in use within the H.E.S.S. collaboration. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
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Open AccessReview
Flux Distribution of Gamma-Ray Emission in Blazars: The Example of Mrk 501
Galaxies 2018, 6(4), 135; https://doi.org/10.3390/galaxies6040135
Received: 7 November 2018 / Revised: 3 December 2018 / Accepted: 4 December 2018 / Published: 6 December 2018
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Abstract
Flux distribution is an important tool to understand the variability processes in active galactic nuclei. We now have available a great deal of observational evidences pointing towards the presence of log-normal components in the high energy light curves, and different models have been [...] Read more.
Flux distribution is an important tool to understand the variability processes in active galactic nuclei. We now have available a great deal of observational evidences pointing towards the presence of log-normal components in the high energy light curves, and different models have been proposed to explain these data. Here, we collect some of the recent developments on this topic using the well-known blazar Mrk 501 as example of complex and interesting aspects coming from its flux distribution in different energy ranges and at different timescales. The observational data we refer to are those collected in a complementary manner by Fermi-LAT over multiple years, and by the First G-APD Cherenkov Telescope (FACT) telescope and the H.E.S.S. array in correspondence of the bright flare of June 2014. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
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Open AccessReview
The First- and Second-Order Fermi Acceleration Processes in BL Lacertae Objects
Galaxies 2018, 6(4), 125; https://doi.org/10.3390/galaxies6040125
Received: 19 October 2018 / Revised: 15 November 2018 / Accepted: 21 November 2018 / Published: 25 November 2018
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Abstract
BL Lacertae objects constitute a rare class of active galactic nuclei with extreme observational features attributed to the Doppler-boosted emission from a relativistic jet, closely aligned to our line-of-sight. Their spectral energy distribution, extending over 17–19 orders of frequency from radio to the [...] Read more.
BL Lacertae objects constitute a rare class of active galactic nuclei with extreme observational features attributed to the Doppler-boosted emission from a relativistic jet, closely aligned to our line-of-sight. Their spectral energy distribution, extending over 17–19 orders of frequency from radio to the TeV energy range, is of non-thermal origin and shows a typical two-component structure. The lower-energy component, ranging from the radio to X-rays in the high-energy peaked BL Lacertae sources, is widely accepted to be a synchrotron radiation emitted by ultra-relativistic charged particles, to be initially accelerated via the Blandford–Znajek mechanism or magneto-hydrodynamic processes in the vicinity of the central super-massive black hole. However, the accelerated particles should lose the energy, sufficient for the emission of the keV-GeV photons, very quickly and the source can maintain its flaring state on the daily-weekly timescales only if some additional acceleration mechanisms are continuously at work. According to different studies and simulations, the particles can gain tremendous energies due to the propagation of relativistic shocks through the jet: By means of first-order Fermi mechanism at the shock front, or they undergo an efficient stochastic (second-order Fermi) acceleration close to the shock front, in the turbulent jet medium. Our intensive X-ray spectral study of TeV-detected, bright BL Lacertae objects (Mrk 421, 1ES 1959+650, Mrk 501) often show the signatures of the stochastic acceleration, while those related to the first-order Fermi process arefound relatively rarely. The TeV-undetected sources (1H 1516+660, BZB J1341+3959, BZB J1237+6258) mostly do not show the signatures of the efficient stochastic acceleration in their jets. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
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Open AccessFeature PaperReview
Fermi: Monitoring the Gamma-Ray Universe
Galaxies 2018, 6(4), 117; https://doi.org/10.3390/galaxies6040117
Received: 17 October 2018 / Revised: 2 November 2018 / Accepted: 6 November 2018 / Published: 16 November 2018
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Abstract
Since 2008, the Large Area Telescope and the Gamma-ray Burst Monitor on the Fermi Gamma-ray Space Telescope have been monitoring the entire sky at energies from about 8 keV to more than 1 TeV. Photon-level data and high-level data products are made publicly [...] Read more.
Since 2008, the Large Area Telescope and the Gamma-ray Burst Monitor on the Fermi Gamma-ray Space Telescope have been monitoring the entire sky at energies from about 8 keV to more than 1 TeV. Photon-level data and high-level data products are made publicly available in near-real time, and efforts continue to improve the response time. This long-duration, all-sky monitoring has enabled a broad range of science, from atmospheric phenomena on Earth to signals from high-redshift sources. The Fermi instrument teams have worked closely with multiwavelength and multi-messenger observers and theorists to maximize the scientific return from the observatory, and they look forward to continued cooperative efforts as Fermi moves into its second decade of operation. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
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Other

Jump to: Research, Review

Open AccessConference Report
On the Connection of Radio and γ-Ray Emission in Blazars
Received: 20 November 2018 / Revised: 14 December 2018 / Accepted: 21 December 2018 / Published: 25 December 2018
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Abstract
Blazars are a sub-category of radio-loud active galactic nuclei with relativistic jets pointing towards to the observer. They are well-known for their non-thermal variable emission, which practically extends over the whole electromagnetic spectrum. Despite the plethora of multi-wavelength observations, the issue about the [...] Read more.
Blazars are a sub-category of radio-loud active galactic nuclei with relativistic jets pointing towards to the observer. They are well-known for their non-thermal variable emission, which practically extends over the whole electromagnetic spectrum. Despite the plethora of multi-wavelength observations, the issue about the origin of the γ -ray and radio emission in blazar jets remains unsettled. Here, we construct a parametric leptonic model for studying the connection between the γ -ray and radio emission in both steady-state and flaring states of blazars. Assuming that relativistic electrons are injected continuously at a fixed distance from the black hole, we numerically study the evolution of their population as it propagates to larger distances while losing energy due to expansion and radiative cooling. In this framework, γ -ray photons are naturally produced at small distances (e.g., 10 3 pc) when the electrons are still very energetic, whereas the radio emission is produced at larger distances (e.g., 1 pc), after the electrons have cooled and the emitting region has become optically thin to synchrotron self-absorption due to expansion. We present preliminary results of our numerical investigation for the steady-state jet emission and the predicted time lags between γ -rays and radio during flares. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
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