Galaxies

Research

17 pages, 316 KiB

Open AccessArticle

A Multi-Wavelength View of OJ 287 Activity in 2015–2017: Implications of Spectral Changes on Central-Engine Models and MeV-GeV Emission Mechanism

by Pankaj Kushwaha

Galaxies 2020, 8(1), 15; https://doi.org/10.3390/galaxies8010015 - 14 Feb 2020

Cited by 17 | Viewed by 3144

Abstract

A diverse range of observational results and peculiar properties across the domains of observation have made OJ 287 one of the best-explored BL Lac objects on the issues of relativistic jets and accretion physics as well as the strong theory of gravity. We [...] Read more.

A diverse range of observational results and peculiar properties across the domains of observation have made OJ 287 one of the best-explored BL Lac objects on the issues of relativistic jets and accretion physics as well as the strong theory of gravity. We here present a brief compilation of observational results from the literature and inferences/insights from the extensive studies but focus on the interpretation of its ∼12-yr quasi-periodic optical outbursts (QPOOs) and high energy emission mechanisms. The QPOOs in one model are attributed to the disk-impact related to dynamics of the binary SMBHs while alternative models attribute it to the geometrical effect related to the precession of a single jet or double jets. We discuss implications of the new spectral features reported during the 2015–2017 multi-wavelength high activity of the source—a break in the NIR-optical spectrum and hardening of the MeV-GeV emission accompanied by a shift in the location of its peak, in the context of the two. The reported NIR-optical break nicely fits the description of a standard accretion disk emission from an SMBH of mass

\sim 10^{10} M_{⊙}

while the time of its first appearance at the end of May, 2013 (MJD 56439) is in close coincidence with the time of impact predicted by the disk-impact binary SMBH model. This spectral and temporal coincidence with the model parameters of the disk-impact binary SMBH model provides independent evidence in favor of the model over the geometrical models which argue for a total central-engine mass in the range of

10^{7 - 9} M_{⊙}

. On the other hand, the MeV-GeV spectral change is naturally reproduced by the inverse Compton scattering of photons from the broad-line region and is consistent with the detection of broad emission lines during the previous cycles of quasi-periodic outbursts. Combining this with previous SED studies suggests that in, OJ 287, the MeV-GeV emission results from external Comptonization. Full article

(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)

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9 pages, 909 KiB

Open AccessArticle

PKS 2155-304: A Case Study of Blazar Variability Power Spectrum at the Highest Energies and on the Longest Timescales

by Arti Goyal

Galaxies 2019, 7(3), 73; https://doi.org/10.3390/galaxies7030073 - 16 Aug 2019

Cited by 2 | Viewed by 3277

Abstract

We present the results of our Power Spectral Density (PSD) analysis for the BL Lac object PKS 2155-304, utilizing the nightly-binned long-term light curve from the decade-long monitoring, as well as the minute-binned intra-night light curve from the High Energy Stereoscopic Survey (H.E.S.S.; [...] Read more.

We present the results of our Power Spectral Density (PSD) analysis for the BL Lac object PKS 2155-304, utilizing the nightly-binned long-term light curve from the decade-long monitoring, as well as the minute-binned intra-night light curve from the High Energy Stereoscopic Survey (H.E.S.S.; >200 GeV). The source is unique for exhibiting the shortest flux-doubling timescale at Very High Energy (VHE) among its class and thus provides a rare opportunity to study the particle acceleration on the smallest spatial scales in blazar jets. The light curves are modeled in terms of the Continuous-Time Auto-Regressive Moving Average (CARMA) process. The combined long-term and intra-night PSD extends up to ∼6 decades in the temporal frequency range; unprecedented at the TeV energies for a blazar source. Our systematic approach reveals that PKS 2155-304 shows, on average, a complex shape of variability power spectrum, with more variability power on longer timescales. The long-term variability is best modeled by the CARMA(2,1) process, while the intra-night variability is modeled by a CARMA(1,0) process. We note that the CARMA(1,0) process refers to an Ornstein–Uhlenbeck process where the power-law PSD slope (PSD varies as a function of variability frequency to the power of the negative slope) changes from two to zero, above a certain “characteristic/relaxation” timescale. Even though the derived power spectrum of the intra-night light curve did not reveal a flattening, we speculate such relaxation must occur on timescales longer than a few hours for the source. Full article

(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)

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8 pages, 771 KiB

Open AccessArticle

Millimeter-Wave Monitoring of Active Galactic Nuclei with the Africa Millimetre Telescope

by Michael Backes, Markus Böttcher and Heino Falcke

Galaxies 2019, 7(2), 66; https://doi.org/10.3390/galaxies7020066 - 25 Jun 2019

Cited by 2 | Viewed by 3643

Abstract

Active galactic nuclei are the dominant sources of gamma rays outside our galaxy and are also candidates for the source of ultra-high energy cosmic rays. In addition to being emitters of broad-band non-thermal radiation throughout the electromagnetic spectrum, their emission is highly variable [...] Read more.

Active galactic nuclei are the dominant sources of gamma rays outside our galaxy and are also candidates for the source of ultra-high energy cosmic rays. In addition to being emitters of broad-band non-thermal radiation throughout the electromagnetic spectrum, their emission is highly variable on timescales from years to minutes. Hence, high-cadence monitoring observations are needed to understand their emission mechanisms. The Africa Millimetre Telescope is planned to be the first mm-wave radio telescope on the African continent and one of few in the southern hemisphere. Further to contributing to the global mm-VLBI observations with the Event Horizon Telescope, substantial amounts of observation time will be available for monitoring observations of active galactic nuclei. Here we review the scientific scope of the Africa Millimetre Telescope for monitoring of active galactic nuclei at mm-wavelengths. Full article

(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)

14 pages, 1944 KiB

Open AccessArticle

Fractional Variability—A Tool to Study Blazar Variability

by Bernd Schleicher, Axel Arbet-Engels, Dominik Baack, Matteo Balbo, Adrian Biland, Michael Blank, Thomas Bretz, Kai Bruegge, Michael Bulinski, Jens Buss, Manuel Doerr, Daniela Dorner, Dominik Elsaesser, Sergej Grischagin, Dorothee Hildebrand, Lena Linhoff, Karl Mannheim, Sebastian Achim Mueller, Dominik Neise, Andrii Neronov, Maximilian Noethe, Aleksander Paravac, Wolfgang Rhode, Florian Schulz, Kevin Sedlaczek, Amit Shukla, Vitalii Sliusar, Elan von Willert and Roland Walter Show full author list Hide full author list

Galaxies 2019, 7(2), 62; https://doi.org/10.3390/galaxies7020062 - 30 May 2019

Cited by 26 | Viewed by 6051

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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11 pages, 901 KiB

Open AccessArticle

Flux States of Active Galactic Nuclei

by Daniela Dorner, Axel Arbet-Engels, Dominik Baack, Matteo Balbo, Adrian Biland, Michael Blank, Thomas Bretz, Kai Bruegge, Michael Bulinski, Jens Buss, Manuel Doerr, Dominik Elsaesser, Dorothee Hildebrand, Lena Linhoff, Karl Mannheim, Sebastian Achim Mueller, Dominik Neise, Andrii Neronov, Maximilian Noethe, Aleksander Paravac, Wolfgang Rhode, Bernd Schleicher, Florian Schulz, Kevin Sedlaczek, Amit Shukla, Vitalii Sliusar, Elan von Willert, Roland Walter and FACT Collaboration Show full author list Hide full author list

Galaxies 2019, 7(2), 57; https://doi.org/10.3390/galaxies7020057 - 21 May 2019

Cited by 2 | Viewed by 3594

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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11 pages, 712 KiB

Open AccessArticle

Unbiased Long-Term Monitoring at TeV Energies

by María Magdalena González, Daniela Dorner, Thomas Bretz, José Andrés García-González and on behalf of the FACT, HAWC and M@TE Collaborations

Galaxies 2019, 7(2), 51; https://doi.org/10.3390/galaxies7020051 - 28 Apr 2019

Cited by 3 | Viewed by 2951

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)

11 pages, 979 KiB

Open AccessArticle

The VHE γ-Ray View of the FSRQ PKS 1510-089

by Michael Zacharias, Dijana Dominis Prester, Felix Jankowsky, Elina Lindfors, Manuel Meyer, Mahmoud Mohamed, Heike Prokoph, David Sanchez, Julian Sitarek, Tomislav Terzic, Stefan Wagner, Alicja Wierzcholska and on behalf of the H.E.S.S. and MAGIC Collaborations

Galaxies 2019, 7(1), 41; https://doi.org/10.3390/galaxies7010041 - 20 Mar 2019

Cited by 8 | Viewed by 3884

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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8 pages, 598 KiB

Open AccessArticle

Characterising the Long-Term Variability of Blazars in Leptonic Models

by Hannes Thiersen, Michael Zacharias and Markus Böttcher

Galaxies 2019, 7(1), 35; https://doi.org/10.3390/galaxies7010035 - 5 Mar 2019

Cited by 7 | Viewed by 3052

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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9 pages, 506 KiB

Open AccessArticle

The Long-Lasting Activity in the Flat Spectrum Radio Quasar (FSRQ) CTA 102

by Michael Zacharias, Markus Böttcher, Felix Jankowsky, Jean-Philippe Lenain, Stefan J. Wagner and Alicja Wierzcholska

Galaxies 2019, 7(1), 34; https://doi.org/10.3390/galaxies7010034 - 28 Feb 2019

Viewed by 3045

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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10 pages, 473 KiB

Open AccessArticle

H.E.S.S. Monitoring of PKS 2155-304 in 2015 and 2016

by Alicja Wierzcholska, Michael Zacharias, Felix Jankowsky and Stefan Wagner

Galaxies 2019, 7(1), 21; https://doi.org/10.3390/galaxies7010021 - 19 Jan 2019

Cited by 5 | Viewed by 3115

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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8 pages, 465 KiB

Open AccessEditor’s ChoiceArticle

AMON Multimessenger Alerts: Past and Future

by Hugo Alberto Ayala Solares

Galaxies 2019, 7(1), 19; https://doi.org/10.3390/galaxies7010019 - 16 Jan 2019

Cited by 5 | Viewed by 3563

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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16 pages, 2357 KiB

Open AccessEditor’s ChoiceArticle

The Rate of Short-Duration Gamma-Ray Bursts in the Local Universe

by Soheb Mandhai, Nial Tanvir, Gavin Lamb, Andrew Levan and David Tsang

Galaxies 2018, 6(4), 130; https://doi.org/10.3390/galaxies6040130 - 30 Nov 2018

Cited by 23 | Viewed by 5734

Abstract

Following the faint gamma-ray burst, GRB 170817A, coincident with a gravitational wave-detected binary neutron star merger at

d \sim 40

Mpc, we consider the constraints on a local population of faint short duration GRBs (defined here broadly as

T_{90} < 4

s). [...] Read more.

Following the faint gamma-ray burst, GRB 170817A, coincident with a gravitational wave-detected binary neutron star merger at

d \sim 40

Mpc, we consider the constraints on a local population of faint short duration GRBs (defined here broadly as

T_{90} < 4

s). We review proposed low-redshift short-GRBs and consider statistical limits on a

d ≲ 200

Mpc population using Swift/Burst Alert Telescope (BAT), Fermi/Gamma-ray Burst Monitor (GBM), and Compton Gamma-Ray Observatory (CGRO) Burst and Transient Source Experiment (BATSE) GRBs. Swift/BAT short-GRBs give an upper limit for the all-sky rate of

<

4 y

^{- 1}

at

d < 200

Mpc, corresponding to

<

5% of SGRBs. Cross-correlation of selected CGRO/BATSE and Fermi/GBM GRBs with

d < 100

Mpc galaxy positions returns a weaker constraint of

≲ 12 y^{- 1}

. A separate search for correlations due to SGR giant flares in nearby (

d < 11

Mpc) galaxies finds an upper limit of

< 3 y^{- 1}

. Our analysis suggests that GRB 170817A-like events are likely to be rare in existing SGRB catalogues. The best candidate for an analogue remains GRB 050906, where the Swift/BAT location was consistent with the galaxy IC 0327 at

d \approx 132

Mpc. If binary neutron star merger rates are at the high end of current estimates, then our results imply that at most a few percent will be accompanied by detectable gamma-ray flashes in the forthcoming LIGO/Virgo science runs. Full article

(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)

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Review

Jump to: Research, Other

14 pages, 1318 KiB

Open AccessReview

Long-Term Optical Monitoring of Blazars

by Kosmas Gazeas

Galaxies 2019, 7(2), 58; https://doi.org/10.3390/galaxies7020058 - 21 May 2019

Cited by 1 | Viewed by 3205

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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14 pages, 744 KiB

Open AccessReview

The RoboPol Program: Optical Polarimetric Monitoring of Blazars

by Dmitry Blinov and Vasiliki Pavlidou

Galaxies 2019, 7(2), 46; https://doi.org/10.3390/galaxies7020046 - 10 Apr 2019

Cited by 12 | Viewed by 3870

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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14 pages, 9883 KiB

Open AccessReview

Monitoring and Multi-Messenger Astronomy with IceCube

by René Reimann

Galaxies 2019, 7(1), 40; https://doi.org/10.3390/galaxies7010040 - 19 Mar 2019

Cited by 3 | Viewed by 3855

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

4 π

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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17 pages, 1122 KiB

Open AccessEditor’s ChoiceReview

Gamma-Ray Astrophysics in the Time Domain

by Frank M. Rieger

Galaxies 2019, 7(1), 28; https://doi.org/10.3390/galaxies7010028 - 29 Jan 2019

Cited by 36 | Viewed by 4619

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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11 pages, 535 KiB

Open AccessReview

Monitoring the Extragalactic High Energy Sky

by Jean-Philippe Lenain

Galaxies 2019, 7(1), 9; https://doi.org/10.3390/galaxies7010009 - 3 Jan 2019

Cited by 2 | Viewed by 2922

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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11 pages, 412 KiB

Open AccessEditor’s ChoiceReview

Flux Distribution of Gamma-Ray Emission in Blazars: The Example of Mrk 501

by Carlo Romoli, Nachiketa Chakraborty, Daniela Dorner, Andrew M. Taylor and Michael Blank

Galaxies 2018, 6(4), 135; https://doi.org/10.3390/galaxies6040135 - 6 Dec 2018

Cited by 20 | Viewed by 4072

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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14 pages, 5786 KiB

Open AccessReview

The First- and Second-Order Fermi Acceleration Processes in BL Lacertae Objects

by Bidzina Kapanadze

Galaxies 2018, 6(4), 125; https://doi.org/10.3390/galaxies6040125 - 25 Nov 2018

Cited by 1 | Viewed by 3607

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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15 pages, 6064 KiB

Open AccessFeature PaperEditor’s ChoiceReview

Fermi: Monitoring the Gamma-Ray Universe

by David J. Thompson

Galaxies 2018, 6(4), 117; https://doi.org/10.3390/galaxies6040117 - 16 Nov 2018

Cited by 10 | Viewed by 4524

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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8 pages, 271 KiB

Open AccessConference Report

On the Connection of Radio and γ-Ray Emission in Blazars

by Stella Boula, Maria Petropoulou and Apostolos Mastichiadis

Galaxies 2019, 7(1), 3; https://doi.org/10.3390/galaxies7010003 - 25 Dec 2018

Cited by 4 | Viewed by 3106

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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