arXiv subjects

Nectaria A. B. Gizani

Publications and source records attributed to Nectaria A. B. Gizani.

13 recordsLinked to original sources

arXiv2023

On the large scale morphology of Hercules A: destabilized hot jets?

Extragalactic jets are generated as bipolar outflows at the nuclei of active galaxies. Depending on their morphology, they are classified as Fanaroff-Riley type I (centre-brightened) and Fanaroff-Riley type II (edge-brightened) radio jets. However, this division is not sharp and observations of these sources at large scales often show intermediate jet morphologies or even hybrid jet morphologies with a FRI type jet on one side and a FRII type jet on the other. A good example of a radio galaxy that is difficult to classify as FRI or FRII is Hercules~A. This source shows jets with bright radio lobes (a common feature of FRII type jets) albeit without the hotspots indicative of the violent interaction between the jet and the ambient medium at the impact region, because the jets seem to be disrupted inside the lobes at a distance from the bow shocks surrounding the lobes. In this paper, we explore the jet physics that could trigger this peculiar morphology by means of three-dimensional relativisitic hydrodynamical simulations. Our results show that the large-scale morphological features of Hercules A jets and lobes can be reproduced by the propagation of a relativistically hot plasma outflow that is disrupted by helical instability modes, and generates a hot lobe that expands isotropically against the pressure-decreasing intergalactic medium. We also discuss the implications that this result may have for the host active nucleus in terms of a possible transition from high-excitation to low-excitation galaxy modes.

astro-ph.HE↗

arXiv2012

Implications of pc and kpc jet asymmetry to the cosmic ray acceleration

We probe the role that the directional asymmetry, between relativistic outflows and kilo-parsec scale jets, play in the acceleration of cosmic rays. For this reason we use two powerful, nearby Active Galactic Nuclei (AGNs). These radio galaxies are atypical compared to the usual AGN as they contain ring-like features instead of hotspots. Our VLBI radio data have revealed a substantial misalignment between their small and large scale jets. Taking into account the overall information we have obtained about the AGNs themselves (VLA and VLBI radio data at 18 cm) and their clusters (X-ray observations) our study supports the present ideas of powerful radiogalaxies (radio quiet and radio loud) being sources of cosmic rays as well as their ability to accelarate the latter to ultra high energies.

astro-ph.HE↗

arXiv2012

Extended reagions of radio emission not associated with the AGN phenomenon as sources of acceleration of cosmic rays: The case of cD galaxies

Diffuse, non-thermal extended emission not associated with the AGN phenomenon, found in many clusters of galaxies hosted by an AGN, are related to the acceleration of cosmic rays. In the current work we present preliminary evidence of absence of such formations in clusters of galaxies hosted by optically identified cD galaxies. Our subsample consists of three powerful low redshift radiogalaxies, centered in poor clusters of galaxies. We have searched for radio relics and (mini)halos which could be forming as a result of the confinement of cosmic rays by bubbles creayed by the AGN. We report on the work in progress.

astro-ph.HE↗

arXiv2012

Alternative Astronomical FITS imaging

Astronomical radio maps are presented mainly in FITS format. Astronomical Image Processing Software (AIPS) uses a set of tables attached to the output map to include all sorts of information concerning the production of the image. However this information together with information on the flux and noise of the map is lost as soon as the image of the radio source in fits or other format is extracted from AIPS. This information would have been valuable to another astronomer who just uses NED, for example, to download the map. In the current work, we show a method of data hiding inside the radio map, which can be preserved under transformations, even for example while the format of the map is changed from fits to other lossless available image formats.

astro-ph.IM↗

arXiv2011

On the origin and acceleration of cosmic rays: Cooling flow clusters and AGN hosts

We are looking for radio `relics' and `halos' in an X-ray selected sample of clusters of galaxies. These radio features are not a product of the Active Galactic Nuclei (AGN)-mechanism, but more likely are associated with past cluster merger events. AGN hosts of cooling flow clusters contain particle bubbles that show non-thermal radio emission. These bubbles could explain the presence of radio relics and halos if they can restrict cosmic rays efficiently. Intracluster magnetic fields and cluster environments can reveal the acceleration mechanisms of cosmic rays. Using radio/X-ray data and analytical methods we examine three AGN hosts out of our 70 clusters, namely Hercules A, 3C310 and 3C388. We found that none of these clusters contain relics and/or halos.

astro-ph.CO↗

arXiv2011

Impact of solar activity on climate changes in Athens region, Greece

The scope of this work is to study the role that the solar weather plays in terrestrial weather. For this reason we study the effect of the solar activity on the climate changes in Greece. In the current work we look for possible correlation between the solar activity data spanning the years from 1975 to 2000 and the meteorological data from two weather stations based inside the city of Athens, Greece (New Philadelphia) and in greater Athens in the north of Attica (Tatoi area). We examine the annual variations of the average values of six meteorological parameters: temperature, atmospheric pressure, direction and intensity of wind, rainfall and relative air humidity. The solar data include decade variations, within the above period, of the solar irradiance, mean sunspot number between two solar cycles, magnetic cycle influence, and solar UV driving of climate (radio flux).

physics.ao-ph↗

arXiv2011

Searching for radio relics and halos. Their role in the formation and acceleration of extragalactic cosmic rays

We search for extended regions of radio emission not associated with Active Galactic Nuclei, known as 'relics', 'halos' and 'mini halo's, in a sample of 70 Abell clusters for which we have radio, optical and X-ray data. AGN can produce particle bubbles of non-thermal emission, which can restrict cosmic rays. Hence radio relics and (mini) halos could be forming as a result of the confinement of cosmic rays by these bubbles. We are probing the role that intracluster mag- netic fields (using Faraday rotation measure and inverse compton arguments), mergers (through radio/X-ray interactions), cooling flows (X-ray data), radio jets/shocks as well as radio (mini) halos/relics play in the formation, acceleration and propagation of cosmic rays. For the current study we have selected two powerful nearby radio galaxies from our sample: Hercules A and 3C 388. We report on the work in progress and future plans.

astro-ph.CO↗

arXiv2005

First Results of the 74 MHz VLA-Pie Town Link. Hercules A at Low Frequencies

We present the results of the first successful observations of the Pie Town link with the Very Large Array (VLA) at 74 MHz on Hercules A. The improvement in resolution from 25 arcsec to 10 arcsec resolves the helical- and ring-like features seen at higher frequencies. We also present new high dynamic range images of this powerful radio galaxy at 325 MHz. Our low frequency observations confirm the multiple outburst interpretation of the spectral index differences at high frequencies. Comparison between our radio and ROSAT X-ray data does not reveal any association between the X-ray emission from the cluster and the radio lobes. There are no extra regions of radio emission at 74 MHz.

astro-ph↗

arXiv2004

A multiband study of Hercules A. I. ROSAT observations of the intracluster medium

We have made ROSAT PSPC and HRI X-ray observations to study the intracluster gas surrounding the powerful radio source Hercules A. The cluster is luminous in X-rays (L_bol = 4.8x10^37 W), and the host of the radio source is the central dominating galaxy of the cluster. The X-ray surface brightness profile is well fitted by a modified King beta model, but the cluster is elongated parallel to the radio source, especially on the scale of the radio lobes. Fits to individual quadrants give a core radius 50 per cent larger along the radio axis. Part of this elongation appears to be associated with enhanced X-ray emission superimposed on the outer radio lobes. There are no obvious depressions in the X-ray emission coincident with the radio lobes, as expected if the relativistic plasma displaces the ICM. However, we show that these depressions may be quite weak. From the surface brightness profile for the PSPC data the X-ray emission extends out to ~ 2.2 Mpc radius. In the absence of the powerful jets, we would expect a cooling flow at the centre of the cluster, but currently it must be substantially disturbed by the expansion of the radio lobes. The PSPC spectrum reveals a cool component of the ICM with 0.5 < kT < 1 keV. The central cooling time could be as low as 2 Gyr if the cool component is centrally concentrated, otherwise it is around 6 Gyr. The modelled central electron density is typical for modest cooling flows. Finally, we have detected faint X-ray emission from a compact central source.

astro-ph↗

arXiv2003

A multiband study of Hercules A. II. Multifrequency VLA imaging

We have mapped the powerful radio galaxy Hercules A at six frequencies spanning 1295 to 8440 MHz using the VLA in all four configurations. Here we discuss the structure revealed in total intensity, spectral index, polarization, and projected magnetic field. Our observations clearly reveal the relation between the bright jets, radio source. The jets and rings form a coherent structure with a dramatically flatter spectrum than the surrounding lobes and bridge, strongly suggesting that they represent a recently renewed outburst from the active nucleus. The spectrum of the lobes is also steeper than in typical radio sources, and steepens further towards the centre. The compact core is optically thin and also has a remarkably steep spectrum (alpha \~ -1.2). There is some evidence that the old lobe material has been swept up and compressed ahead of the new outburst. We interpret the dramatic asymmetry in the bright structure, and more subtle differences between diffuse lobe structures, in terms of relativistic beaming combined with front-to-back light-travel delays which mean that we view the two lobes at different stages of the outburst. After correcting for Faraday rotation the projected magnetic field closely follows the edge of the lobes, the jets, and the rings; the field pattern in the two lobes is broadly similar. We confirm a strong asymmetry in depolarization and Faraday rotation, with the jet side the less depolarized and the flatter spectrum, consistent with general correlations between these asymmetries. The spectral index asymmetry is clearly present in the `old' lobe material and so, at least in this case, is not due to beaming; but it can be understood in terms of the light-travel delay.

astro-ph↗

arXiv2001

Probing the pc- and kpc-scale environment of the powerful radio galaxy Hercules A

We present the kpc-scale behaviour of the powerful extragalactic radio source Hercules A and the behaviour of the intracluster gas in which the radio source is situated. We have found that Hercules A exhibits a strong Laing-Garrington effect. The X-ray observations have revealed an extended X-ray emission elongated along the radio galaxy axis. The estimated temperature of the cluster is kT = 2.45 keV and the central electron density is no~7.8 x 10^(-3) cm^(-3) which reveals a hot, dense environment in which Hercules A is situated. From the combined study of the radio and X-ray data we have estimated a central value of 3<Bo (muG)<9. We also present the most recent results from the analysis of the radio data on the pc-scale structure of the radio galaxy, observed at 18 cm by the EVN-MERLIN array. A faint but compact radio source, coincident with the optical centre of Hercules A was detected by the EVN at 18 mas resolution. The total flux density of the EVN core is 14.6 mJy. Its angular size is 18 x 7 mas with a position angle of ~139 degrees. There is also evidence for extended emission in the NW-SE direction, most probably from the eastern pc-scale jet. If this is true then there is a misalignment between the direction of the pc-eastern and the aligned kpc-scale jets of ~35 degrees.

astro-ph↗

arXiv1999

The kpc-scale radio source population

We are conducting a multi-wavelength (radio, optical, and X-ray) observational campaign to classify, morphologically and physically, a sample of 55 flat-spectrum radio sources dominated by structure on kpc-scales. This sample contains 22 compact-/medium-sized symmetric object candidates, a class of objects thought to be the early stages of the evolution of radio galaxies. The vast majority of the remaining objects have core-plus-one-sided-jet structures, half of which present sharply bent jets, probably due to strong interactions with the interstellar medium of the host galaxies. Once the observational campaign is completed, we will constrain evolutionary theories of radio galaxies at their intermediate stages and possibly understand the physics of the hypothesized narrow line region in active galactic nuclei, given our advantageous statistical position.

astro-ph↗

arXiv1999

Testing minimum energy with powerful radio sources in clusters of galaxies

We analyze ROSAT data for cluster gas surrounding powerful radio galaxies, which is well fitted by a ``beta-model'' gas distribution, after allowing for a compact central source. The cluster thermal pressure at the distance of the radio lobes is typically an order of magnitude larger than the lobe minimum pressure. Since radio lobes are sharply-bounded, the missing pressure is not simply entrained intra-cluster gas. Thus the minimum energy in the lobes is a severe underestimate of the actual energy content. We argue that the extra energy is mostly in the form of particles, so that the magnetic field is below equipartition and thus not a major factor in the lobe dynamics. The large departure from minimum energy has far-reaching implications for the nature of AGN central engines and the supply of mechanical energy to the cluster gas.

astro-ph↗