scholarly journals Subtraction of Bright Point Sources from Synthesis Images of the Epoch of Reionization

2011 ◽  
Vol 28 (1) ◽  
pp. 46-57 ◽  
Author(s):  
B. Pindor ◽  
J. S. B. Wyithe ◽  
D. A. Mitchell ◽  
S. M. Ord ◽  
R. B. Wayth ◽  
...  
Keyword(s):  
Active Galactic Nuclei ◽  
Dynamic Range ◽  
Low Frequency ◽  
Neutral Hydrogen ◽  
Galactic Nuclei ◽  
Point Sources ◽  
Field Of View ◽  
Bright Point ◽  
Line Of Sight ◽  
Diffuse Background

AbstractBright point sources associated with extragalactic active galactic nuclei and radio galaxies are an important foreground for low-frequency radio experiments aimed at detecting the redshifted 21-cm emission from neutral hydrogen during the epoch of reionization. The frequency dependence of the synthesized beam implies that the sidelobes of these sources will move across the field of view as a function of observing frequency, hence frustrating line-of-sight foreground subtraction techniques. We describe a method for subtracting these point sources from dirty maps produced by an instrument such as the MWA. This technique combines matched filters with an iterative centroiding scheme to locate and characterize point sources in the presence of a diffuse background. Simulations show that this technique can improve the dynamic range of epoch-of-reionization maps by 2—3 orders of magnitude.

scholarly journals Polarimetric Properties of Blazars Caught by the WEBT

Galaxies ◽  
2021 ◽  
Vol 9 (2) ◽  
pp. 42
Author(s):  
Claudia M. Raiteri ◽  
Massimo Villata
Keyword(s):  
Active Galactic Nuclei ◽  
Galactic Nuclei ◽  
Plasma Jets ◽  
Line Of Sight ◽  
Optical Behaviour ◽  
Relativistic Jets ◽  
Comprehensive Picture ◽  
Multi Wavelength ◽  
Extreme Variability ◽  
Continuous Range

Active galactic nuclei come in many varieties. A minority of them are radio-loud, and exhibit two opposite prominent plasma jets extending from the proximity of the supermassive black hole up to megaparsec distances. When one of the relativistic jets is oriented closely to the line of sight, its emission is Doppler beamed and these objects show extreme variability properties at all wavelengths. These are called “blazars”. The unpredictable blazar variability, occurring on a continuous range of time-scales, from minutes to years, is most effectively investigated in a multi-wavelength context. Ground-based and space observations together contribute to give us a comprehensive picture of the blazar emission properties from the radio to the γ-ray band. Moreover, in recent years, a lot of effort has been devoted to the observation and analysis of the blazar polarimetric radio and optical behaviour, showing strong variability of both the polarisation degree and angle. The Whole Earth Blazar Telescope (WEBT) Collaboration, involving many tens of astronomers all around the globe, has been monitoring several blazars since 1997. The results of the corresponding data analysis have contributed to the understanding of the blazar phenomenon, particularly stressing the viability of a geometrical interpretation of the blazar variability. We review here the most significant polarimetric results achieved in the WEBT studies.

The GLEAM 4-Jy Sample: The brightest radio-sources in the southern sky

2019 ◽  
Vol 15 (S356) ◽  
pp. 375-375
Author(s):  
Sarah White
Keyword(s):  
Radio Emission ◽  
Active Galactic Nuclei ◽  
Visual Inspection ◽  
Low Frequency ◽  
Galactic Nuclei ◽  
Radio Sources ◽  
Relativistic Jets ◽  
The Galaxy ◽  
Murchison Widefield Array ◽  
Past Activity

AbstractLow-frequency radio emission allows powerful active galactic nuclei (AGN) to be selected in a way that is unaffected by dust obscuration and orientation of the jet axis. It also reveals past activity (e.g. radio lobes) that may not be evident at higher frequencies. Currently, there are too few “radio-loud” galaxies for robust studies in terms of redshift-evolution and/or environment. Hence our use of new observations from the Murchison Widefield Array (the SKA-Low precursor), over the southern sky, to construct the GLEAM 4-Jy Sample (1,860 sources at S151MHz > 4 Jy). This sample is dominated by AGN and is 10 times larger than the heavily relied-upon 3CRR sample (173 sources at S178MHz > 10 Jy) of the northern hemisphere. In order to understand how AGN influence their surroundings and the way galaxies evolve, we first need to correctly identify the galaxy hosting the radio emission. This has now been completed for the GLEAM 4-Jy Sample – through repeated visual inspection and extensive checks against the literature – forming a valuable, legacy dataset for investigating relativistic jets and their interplay with the environment.

scholarly journals Co-existence of two plasma phases in solar and AGN coronas

1998 ◽  
pp. 43-53
Author(s):  
A. Kubicela ◽  
J. Arsenijevic ◽  
L.C. Popovic ◽  
N. Trajkovic ◽  
E. Bon
Keyword(s):  
Solar Corona ◽  
Active Galactic Nuclei ◽  
Research Work ◽  
Neutral Hydrogen ◽  
Galactic Nuclei ◽  
Theoretical Explanation ◽  
The Other ◽  
Solar Photosphere ◽  
Hydrogen Lines ◽  
Hot Corona

Here we have juxtaposed two distant cosmic locations of the Sun and AGN where neutral hydrogen appears in a close connection with hot coronas. Besides the solar photosphere, chromosphere and prominences where the presence of neutral hydrogen is well established, its emission quite high in hot solar corona is still puzzling. Some of earlier observations where H? emission in solar corona was detected in eclipse and in daily coronagraphic observations are reviewed. A proper theoretical explanation of this cold chromospheric-type emission in the hot corona does not exist yet. On the other side, a similar emission of hydrogen lines is present in Active Galactic Nuclei (AGNs). Much research work is currently being done in this field. We outline some of the concepts of the AGN structure prevailing in the astrophysics today.

scholarly journals Compton-Thick AGN in the NuSTAR ERA VII. A joint NuSTAR, Chandra, and XMM-Newton Analysis of Two Nearby, Heavily Obscured Sources

2021 ◽  
Vol 922 (2) ◽  
pp. 159
Author(s):  
A. Traina ◽  
S. Marchesi ◽  
C. Vignali ◽  
N. Torres-Albà ◽  
M. Ajello ◽  
...  
Keyword(s):  
Spectral Analysis ◽  
Energy Range ◽  
Active Galactic Nuclei ◽  
Column Density ◽  
Galactic Nuclei ◽  
Seyfert Galaxies ◽  
Line Of Sight ◽  
Broad Energy Range ◽  
Larger Sample ◽  
Broad Energy

Abstract We present the joint Chandra, XMM-Newton, and NuSTAR analysis of two nearby Seyfert galaxies, NGC 3081 and ESO 565-G019. These are the only two having Chandra data in a larger sample of 10 low-redshift (z ≤ 0.05), candidates Compton-thick (CT) Active Galactic Nuclei selected in the 15–150 keV band with Swift-BAT that were still lacking NuSTAR data. Our spectral analysis, performed using physically motivated models, provides an estimate of both the line-of-sight (l.o.s.) and average (N H,S ) column densities of the two torii. NGC 3081 has a Compton-thin l.o.s. column density N H,z = [0.58–0.62] × 1024 cm−2, but the N H,S , beyond the CT threshold (N H,S = [1.41–1.78] × 1024 cm−2), suggests a “patchy” scenario for the distribution of the circumnuclear matter. ESO 565-G019 has both CT l.o.s. and N H,S column densities (N H,z > 2.31 × 1024 cm−2 and N H,S > 2.57 × 1024 cm−2, respectively). The use of physically motivated models, coupled with the broad energy range covered by the data (0.6–70 keV and 0.6–40 keV, for NGC 3081 and ESO 565-G019, respectively) allows us to constrain the covering factor of the obscuring material, which is C TOR = [0.63–0.82] for NGC 3081, and C TOR = [0.39–0.65] for ESO 565-G019.

scholarly journals Detection of a double-peaked H α component from the accretion disc of NGC 4958

2019 ◽  
Vol 486 (1) ◽  
pp. 1138-1145
Author(s):  
T V Ricci ◽  
J E Steiner
Keyword(s):  
Active Galactic Nuclei ◽  
Galactic Nuclei ◽  
Outer Radius ◽  
Accretion Disc ◽  
Line Of Sight ◽  
Broad Component ◽  
Line Region ◽  
Inner Radius ◽  
The Galaxy ◽  
Narrow Line Region

Abstract Active Galactic Nuclei are objects associated with the presence of an accretion disc around supermassive black holes found in the very central region of galaxies with a well-defined bulge. In the optical range of the spectrum, a possible signature of the accretion disc is the presence of a broad double-peaked component that is mostly seen in H α. In this paper, we report the detection of a double-peaked feature in the H α line in the nucleus of the galaxy NGC 4958. The narrow-line region of this object has an emission that is typical of a low-ionization nuclear emission-line region galaxy, which is the usual classification for double-peaked emitters. A central broad component, related to the broad-line region of this object, is seen in H α and also in H β. We concluded that the double-peaked emission is emitted by a circular relativistic Keplerian disc with an inner radius ξi  = 570 ± 83, an outer radius ξo  = 860 ± 170 (both in units of GMSMBH/c2), an inclination to the line of sight i = 27.2 ± 0.7° and a local broadening parameter σ  = 1310 ± 70 km s−1.

scholarly journals Monitoring the Extragalactic High Energy Sky

Galaxies ◽  
2019 ◽  
Vol 7 (1) ◽  
pp. 9 ◽  
Author(s):  
Jean-Philippe Lenain
Keyword(s):  
Active Galactic Nuclei ◽  
Galactic Nuclei ◽  
High Energy ◽  
Line Of Sight ◽  
Data Sets ◽  
High Energies ◽  
Multi Wavelength ◽  
Rich Data ◽  
Very High

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.

scholarly journals The duty cycle of radio galaxies revealed by LOFAR: remnant and restarted radio source populations in the Lockman Hole

2020 ◽  
Vol 496 (2) ◽  
pp. 1706-1717 ◽  
Author(s):  
Stanislav S Shabala ◽  
Nika Jurlin ◽  
Raffaella Morganti ◽  
Marisa Brienza ◽  
Martin J Hardcastle ◽  
...  
Keyword(s):  
Active Galactic Nuclei ◽  
Radio Source ◽  
Low Frequency ◽  
Galactic Nuclei ◽  
Radio Jets ◽  
Modelling Analysis ◽  
Self Consistent ◽  
Large Populations ◽  
Source Populations ◽  
Lockman Hole

ABSTRACT Feedback from radio jets associated with active galactic nuclei (AGNs) plays a profound role in the evolution of galaxies. Kinetic power of these radio jets appears to show temporal variation, but the mechanism(s) responsible for this process are not yet clear. Recently, the LOw Frequency ARray (LOFAR) has uncovered large populations of active, remnant, and restarted radio jet populations. By focusing on LOFAR data in the Lockman Hole, in this work we use the Radio AGNs in Semi-Analytic Environments (RAiSE) dynamical model to present the first self-consistent modelling analysis of active, remnant, and restarted radio source populations. Consistent with other recent work, our models predict that remnant radio lobes fade quickly. Any high (>10 per cent) observed fraction of remnant and restarted sources therefore requires a dominant population of short-lived jets. We speculate that this could plausibly be provided by feedback-regulated accretion.

scholarly journals Galactic neutral hydrogen and the magnetic ISM foreground

2017 ◽  
Vol 12 (S333) ◽  
pp. 146-150
Author(s):  
S.E. Clark
Keyword(s):  
Interstellar Medium ◽  
Dynamic Range ◽  
Low Frequency ◽  
Neutral Hydrogen ◽  
Depth Resolution ◽  
High Dynamic Range ◽  
Neutral Gas ◽  
High Dynamic ◽  
New Generation ◽  
The Relationship

AbstractThe interstellar medium is suffused with magnetic fields, which inform the shape of structures in the diffuse gas. Recent high-dynamic range observations of Galactic neutral hydrogen, combined with novel data analysis techniques, have revealed a deep link between the morphology of neutral gas and the ambient magnetic field. At the same time, an observational revolution is underway in low-frequency radio polarimetry, driven in part by the need to characterize foregrounds to the cosmological 21-cm signal. A new generation of experiments, capable of high angular and Faraday depth resolution, are revealing complex filamentary structures in diffuse polarization. The relationship between filamentary structures observed in radio-polarimetric data and those observed in atomic hydrogen is not yet well understood. Multiwavelength observations will enable new insights into the magnetic interstellar medium across phases.

scholarly journals Combining LOFAR and Apertif Data for Understanding the Life Cycle of Radio Galaxies

Galaxies ◽  
2021 ◽  
Vol 9 (4) ◽  
pp. 88
Author(s):  
Raffaella Morganti ◽  
Nika Jurlin ◽  
Tom Oosterloo ◽  
Marisa Brienza ◽  
Emanuela Orrú ◽  
...  
Keyword(s):  
Active Galactic Nuclei ◽  
Phased Array ◽  
Low Frequency ◽  
Galactic Nuclei ◽  
Radio Galaxies ◽  
Host Galaxy ◽  
Evolutionary Stage ◽  
Spectral Indices ◽  
Feed System ◽  
Lockman Hole

Active galactic nuclei (AGN) at the centres of galaxies can cycle between periods of activity and of quiescence. Characterising the duty-cycle of AGN is crucial for understanding their impact on the evolution of the host galaxy. For radio AGN, their evolutionary stage can be identified from a combination of morphological and spectral properties. We summarise the results we have obtained in the last few years by studying radio galaxies in various crucial phases of their lives, such as remnant and restarted sources. We used morphological information derived from LOw Frequency ARray (LOFAR) images at 150 MHz, combined with resolved spectral indices maps, obtained using recently released images at 1400 MHz from the APERture Tile In Focus (Apertif) phased-array feed system installed on the Westerbork Synthesis Radio Telescope. Our study, limited so far to the Lockman Hole region, has identified radio galaxies in the dying and restarted phases. We found large varieties in their properties, relevant for understanding their evolutionary stage. We started by quantifying their occurrences, the duration of the ‘on’ (active) and ‘off’ (dying) phase, and we compared the results with models of the evolution of radio galaxies. In addition to these extreme phases, the resolved spectral index images can also reveal interesting secrets about the evolution of apparently normal radio galaxies. The spectral information can be connected with, and used to improve, the Fanaroff–Riley classification, and we present one example of this, illustrating what the combination of the LOFAR and Apertif surveys now allow us to do routinely.

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