scholarly journals Radio Galaxies—The TeV Challenge

Galaxies ◽  
2019 ◽  
Vol 7 (1) ◽  
pp. 23 ◽  
Author(s):  
Bindu Rani
Keyword(s):  
Gamma Ray ◽  
Radio Galaxies ◽  
High Energy ◽  
Energy Emission ◽  
Long Baseline ◽  
Multi Wavelength ◽  
Emission Models ◽  
Γ Ray ◽  
Radiation Processes ◽  
High Energy Emission

Over the past decade, our knowledge of the γ -ray sky has been revolutionized by ground- and space-based observatories by detecting photons up to several hundreds of tera-electron volt (TeV) energies. A major population of the γ -ray bright objects are active galactic nuclei (AGN) with their relativistic jets pointed along our line-of-sight. Gamma-ray emission is also detected from nearby misaligned AGN such as radio galaxies. While the TeV-detected radio galaxies ( T e V R a d ) only form a small fraction of the γ -ray detected AGN, their multi-wavelength study offers a unique opportunity to probe and pinpoint the high-energy emission processes and sites. Even in the absence of substantial Doppler beaming T e V R a d are extremely bright objects in the TeV sky (luminosities detected up to 10 45 erg s − 1 ), and exhibit flux variations on timescales shorter than the event-horizon scales (flux doubling timescale less than 5 min). Thanks to the recent advancement in the imaging capabilities of high-resolution radio interferometry (millimeter very long baseline interferometry, mm-VLBI), one can probe the scales down to less than 10 gravitational radii in T e V R a d , making it possible not only to test jet launching models but also to pinpoint the high-energy emission sites and to unravel the emission mechanisms. This review provides an overview of the high-energy observations of T e V R a d with a focus on the emitting sites and radiation processes. Some recent approaches in simulations are also sketched. Observations by the near-future facilities like Cherenkov Telescope Array, short millimeter-VLBI, and high-energy polarimetry instruments will be crucial for discriminating the competing high-energy emission models.

scholarly journals Radio Galaxies - The TeV Challenge

2018 ◽  
Author(s):  
Bindu Rani
Keyword(s):  
Gamma Ray ◽  
Radio Galaxies ◽  
High Energy ◽  
Energy Emission ◽  
Long Baseline ◽  
Multi Wavelength ◽  
Emission Models ◽  
Radiation Processes ◽  
High Energy Emission ◽  
Near Future

Over the past decade, our knowledge of the $\gamma$-ray sky has been revolutionized by ground- and space-based observatories by detecting photons up to several hundreds of tera-electron volt (TeV) energies. A major population of the $\gamma$-ray bright objects are active galactic nuclei (AGN) with their relativistic jets pointed along our line-of-sight. Gamma-ray emission is also detected from nearby mis-aligned AGN such as radio galaxies. While the TeV-detected radio galaxies ($TeVRad$) only form a small fraction of the $\gamma$-ray detected AGN, their multi-wavelength study offers a unique opportunity to probe and pinpoint the high-energy emission processes and sites. Even in the absence of substantial Doppler beaming $TeVRad$ are extremely bright objects in the TeV sky (luminosities detected up to $10^{45}~erg~s^{-1}$), and exhibit flux variations on timescales shorter than the event-horizon scales (flux doubling timescale less than 5 minutes). Thanks to the recent advancement in the imaging capabilities of high-resolution radio interferometry (millimeter very long baseline interferometry, mm-VLBI), one can probe the scales down to less than 10 gravitational radii in $TeVRad$, making it possible not only to test jet launching models but also to pinpoint the high-energy emission sites and to unravel the emission mechanisms. This review provides an overview of the high-energy observations of $TeVRad$ with a focus on the emitting sites and radiation processes. Some recent approaches in simulations are also sketched. Observations by the near-future facilities like Cherenkov Telescope Array, short millimeter-VLBI, and high-energy polarimetry instruments will be crucial for discriminating the competing high-energy emission models.

scholarly journals Gamma-ray emission in radio galaxies under the VLBI scope

2019 ◽  
Vol 627 ◽  
pp. A148 ◽  
Author(s):  
R. Angioni ◽  
E. Ros ◽  
M. Kadler ◽  
R. Ojha ◽  
C. Müller ◽  
...  
Keyword(s):  
Gamma Ray ◽  
Monitoring Program ◽  
Radio Galaxies ◽  
High Energy ◽  
Viewing Angle ◽  
Long Baseline ◽  
Superluminal Motion ◽  
Multi Wavelength ◽  
The Individual ◽  
The Relationship

Aims. In the framework of the multi-wavelength and very long baseline interferometry (VLBI) monitoring program TANAMI (Tracking Active Nuclei with Austral Milliarcsecond Interferometry), we study the evolution of the parsec-scale radio emission in radio galaxies in the southern hemisphere and their relationship to the γ-ray properties of the sources. Our study investigates systematically, for the first time, the relationship between the two energy regimes in radio galaxies. In this first paper, we focus on Fermi-LAT-detected sources. Methods. The TANAMI program monitors a large sample of radio-loud AGN at 8.4 GHz and 22.3 GHz with the Australian long baseline array (LBA) and associated telescopes in Antarctica, Chile, New Zealand and South Africa. We performed a kinematic analysis for five γ-ray detected radio galaxies using multi-epoch 8.4 GHz VLBI images, deriving limits on intrinsic jet parameters such as speed and viewing angle. We analyzed 103 months of Fermi-LAT data in order to study possible connections between the γ-ray properties and the pc-scale jets of Fermi-LAT-detected radio galaxies, both in terms of variability and average properties. We discuss the individual source results and draw preliminary conclusions on sample properties including published VLBI results from the MOJAVE (Monitoring Of Jets in Active galactic nuclei with VLBA Experiments) survey, with a total of fifteen sources. Results. We find that the first γ-ray detection of Pictor A might be associated with the passage of a new VLBI component through the radio core, which appears to be a defining feature of high-energy emitting Fanaroff-Riley type II radio galaxies. We detect subluminal parsec-scale jet motions in the peculiar AGN PKS 0521−36, and we confirm the presence of fast γ-ray variability in the source down to timescales of six hours, which is not accompanied by variations in the VLBI jet. We robustly confirm the presence of significant superluminal motion, up to βapp ∼ 3, in the jet of the TeV radio galaxy PKS 0625−35. Our VLBI results constrain the jet viewing angle to be θ <  53°, allowing for the possibility of a closely aligned jet. Finally, by analyzing the first pc-scale multi-epoch images of the prototypical compact symmetric object (CSO) PKS 1718−649, we place an upper limit on the separation speed between the two mini-lobes. This in turn allows us to derive a lower limit on the age of the source. Conclusions. We can draw some preliminary conclusions on the relationship between pc-scale jets and γ-ray emission in radio galaxies, based on Fermi-LAT-detected sources with available multi-epoch VLBI measurements. We find that the VLBI core flux density correlates with the γ-ray flux, as seen in blazars. On the other hand, the γ-ray luminosity does not show any dependence on the core brightness temperature and core dominance, which are two common indicators of jet Doppler boosting. This seems to indicate that γ-ray emission in radio galaxies is not driven by orientation-dependent effects, as in blazars, in accordance with the unified model of jetted AGN.

scholarly journals High Energy Pulsars: Pulses and Populations

1996 ◽  
Vol 160 ◽  
pp. 331-338 ◽  
Author(s):  
Roger W. Romani
Keyword(s):  
High Energy ◽  
Sky Surveys ◽  
X Ray ◽  
Energy Emission ◽  
Emission Models ◽  
Γ Ray ◽  
High Energy Emission

AbstractRecent X-ray and γ-ray observations have detected a number of isolated spin-powered pulsars. Studies of the pulse profiles of these objects are providing a useful guide to the site of the high energy emission. The γ-ray pulses, which are closely tied to the primary population of radiating particles, seem to be an especially useful discriminant between pulsar models. With an understanding of how the pulsar luminosity and beaming factors evolve with spin parameters, pulsar population syntheses can also be used to check the predictions of high energy emission models. In turn, comparison with γ-ray sky surveys constrains the properties of the young pulsar population.

scholarly journals THE 2010 M 87 VHE FLARE AND ITS ORIGIN: THE MULTI-WAVELENGTH PICTURE

2012 ◽  
Vol 08 ◽  
pp. 184-189 ◽  
Author(s):  
◽  
M. RAUE ◽  
L. STAWARZ ◽  
D. MAZIN ◽  
P. COLIN ◽  
...  
Keyword(s):  
Gamma Ray ◽  
Galactic Nuclei ◽  
High Energy ◽  
Massive Black Hole ◽  
Long Baseline ◽  
Multi Wavelength ◽  
Very High Energy ◽  
Γ Ray ◽  
Very Long Baseline Array ◽  
Innermost Region

The giant radio galaxy M 87, with its proximity (16 Mpc) and its very massive black hole ((3-6) × 109 M⊙), provides a unique laboratory to investigate very high energy (E>100 GeV; VHE) gamma-ray emission from active galactic nuclei and, thereby, probe particle acceleration to relativistic energies near supermassive black holes (SMBH) and in relativistic jets. M 87 has been established as a VHE γ-ray emitter since 2005. The VHE γ-ray emission displays strong variability on timescales as short as a day. In 2008, a rise in the 43 GHz Very Long Baseline Array (VLBA) radio emission of the innermost region (core; extension of < 100 R s ; Schwarzschild radii) was found to coincide with a flaring activity at VHE. This had been interpreted as a strong indication that the VHE emission is produced in the direct vicinity of the SMBH. In 2010 a flare at VHE was again detected triggering further multi-wavelength (MWL) observations with the VLBA, Chandra, and other instruments. At the same time, M 87 was also observed with the Fermi-LAT telescope at MeV/GeV energies, the European VLBI Network (EVN), and the Liverpool Telescope (LT). Here, preliminary results from the 2010 campaign will be reported.

scholarly journals Gamma-ray pulsar light curves as probes of magnetospheric structure

2016 ◽  
Vol 82 (3) ◽  
Author(s):  
A. K. Harding
Keyword(s):  
Magnetic Field ◽  
Gamma Ray ◽  
High Energy ◽  
Field Structure ◽  
Light Curves ◽  
Pulsar Magnetosphere ◽  
Magnetic Field Structure ◽  
Energy Emission ◽  
Emission Models ◽  
High Energy Emission

The large number of ${\it\gamma}$-ray pulsars discovered by the Fermi Gamma-Ray Space Telescope since its launch in 2008 dwarfs the handful that were previously known. The variety of observed light curves makes possible a tomography of both the ensemble-averaged field structure and the high-energy emission regions of a pulsar magnetosphere. Fitting the ${\it\gamma}$-ray pulsar light curves with model magnetospheres and emission models has revealed that most of the high-energy emission, and the particles acceleration, takes place near or beyond the light cylinder, near the current sheet. As pulsar magnetosphere models become more sophisticated, it is possible to probe magnetic field structure and emission that are self-consistently determined. Light curve modelling will continue to be a powerful tool for constraining the pulsar magnetosphere physics.

scholarly journals CONSTRAINING THE HIGH-ENERGY EMISSION FROM GAMMA-RAY BURSTS WITHFERMI

2012 ◽  
Vol 754 (2) ◽  
pp. 121 ◽  
Author(s):  
◽  
M. Ackermann ◽  
M. Ajello ◽  
L. Baldini ◽  
G. Barbiellini ◽  
...  
Keyword(s):  
Gamma Ray ◽  
High Energy ◽  
Gamma Ray Bursts ◽  
Energy Emission ◽  
High Energy Emission

scholarly journals Upper limits on the high-energy emission from gamma-ray bursts observed by AGILE-GRID

2012 ◽  
Vol 547 ◽  
pp. A95 ◽  
Author(s):  
F. Longo ◽  
E. Moretti ◽  
L. Nava ◽  
R. Desiante ◽  
M. Olivo ◽  
...  
Keyword(s):  
Gamma Ray ◽  
High Energy ◽  
Gamma Ray Bursts ◽  
Energy Emission ◽  
Upper Limits ◽  
High Energy Emission

scholarly journals High-energy emission from gamma-ray bursts

2018 ◽  
Vol 27 (13) ◽  
pp. 1842003 ◽  
Author(s):  
Lara Nava
Keyword(s):  
Gamma Ray ◽  
Theoretical Models ◽  
High Energy ◽  
Gamma Ray Bursts ◽  
Quality Of Data ◽  
Large Area ◽  
High Energies ◽  
Energy Emission ◽  
High Energy Emission ◽  
Very High

The number of gamma-ray bursts (GRBs) detected at high energies ([Formula: see text][Formula: see text]GeV) has seen a rapid increase over the last decade, thanks to observations from the Fermi-Large Area Telescope. The improved statistics and quality of data resulted in a better characterization of the high-energy emission properties and in stronger constraints on theoretical models. In spite of the many achievements and progresses, several observational properties still represent a challenge for theoretical models, revealing how our understanding is far from being complete. This paper reviews the main spectral and temporal properties of [Formula: see text][Formula: see text]GeV emission from GRBs and summarizes the most promising theoretical models proposed to interpret the observations. Since a boost for the understanding of GeV radiation might come from observations at even higher energies, the present status and future prospects for observations at very-high energies (above [Formula: see text][Formula: see text]100[Formula: see text]GeV) are also discussed. The improved sensitivity of upcoming facilities, coupled to theoretical predictions, supports the concrete possibility for future ground GRB detections in the high/very-high energy domain.

scholarly journals Observational Manifestations of Young Neutron Stars: Spin-powered Pulsars

2004 ◽  
Vol 218 ◽  
pp. 13-20 ◽  
Author(s):  
Michael Kramer
Keyword(s):  
Neutron Stars ◽  
Gamma Ray ◽  
High Energy ◽  
Electromagnetic Spectrum ◽  
Crab Pulsar ◽  
Pulsar Emission ◽  
Energy Emission ◽  
Radio Frequencies ◽  
High Energy Emission

The largest number of known young neutron stars axe observed as spin-powered pulsars. While the majority of those are detected at radio frequencies, an increasing number can be studied in other parts of the electromagnetic spectrum as well. The Crab pulsar is the prototype of a young pulsar which can be observed from radio to gamma-ray frequencies, providing a red thread of discussion during a tour through the pulsar properties observed across the electromagnetic spectrum. The basic observational features of pulsar emission are presented, preparing the ground for more detailed reviews given in these proceedings. Here, particular attention will be paid to those emission features which may provide a link between the radio and high-energy emission processes.

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