arXiv subjects

P. Cassaro

Publications and source records attributed to P. Cassaro.

8 recordsLinked to original sources

arXiv2025

Twenty years of blazar monitoring with the INAF radio telescopes

The extreme variability of blazars, in both timescale and amplitude, is generally explained as the effect of a relativistic jet closely aligned to the observer's line-of-sight. Due to causality arguments, variability characteristics translate into spatial information about the emitting region of blazars. Since radiation at different wavelengths is emitted in different parts of the jet, multi-frequency observations provide us with a virtual view of the structure of the jet on different scales. Radio--gamma-ray correlations, moreover, are essential to reveal where and how the high-energy radiation is produced. We present here the observations collected within the blazar radio monitoring program that we are running at the Medicina and Noto telescopes. It aims at investigating how the variability characteristics and spectral energy distribution of blazars evolve in time. Since 2004, observation have been performed at 5, 8, 24, and 43 GHz on 47 targets, with monthly cadence; the monitoring program is still active at frequencies of 8 and 24 GHz. The database we built in more than twenty years of activity comprises to date about 21000 flux density measurements. Some basic analysis tools have been applied to the data to characterise the detected variability and offer a first glance at the wealth of information that such a program can provide about blazars.

astro-ph.HE↗

arXiv2024

Jetted subgalactic-size radio sources in merging galaxies -- A jet redirection scenario

Context: The long-standing question concerning jetted subgalactic-size (JSS) radio sources is whether they will evolve into large radio galaxies, die before escaping the host galaxy, or remain indefinitely confined to their compact size. Aims: Our main goal is to propose a scenario that explains the relative number of JSS radio sources and their general properties. Methods: We studied the parsec-scale radio morphology of a complete sample of 21 objects using Very Long Baseline Array (VLBA) observations at various frequencies and analyzed the morphological characteristics of their optical hosts. Results: Many of these radio sources exhibit radio morphologies consistent with transverse motions of their bright edges and are found in dynamically disturbed galaxies. VLBA images suggest the possible presence of large-angle, short-period precessing jets, and an orbital motion of the radio-loud active galactic nucleus (AGN) in a dual or binary system. The majority of JSS radio sources appear to be in systems in different stages of their merging evolution. Conclusions: We propose a scenario where rapid jet redirection, through precession or orbital motion, prevents the jet from penetrating the interstellar medium (ISM) sufficiently to escape the host galaxy. Most JSS radio sources remain compact due to their occurrence in merging galaxies.

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arXiv2017

The high brightness temperature of B0529+483 revealed by RadioAstron and implications for interstellar scattering

The high brightness temperatures, $T_\mathrm{b}\gtrsim 10^{13}$ K, detected in several active galactic nuclei by RadioAstron space VLBI observations challenge theoretical limits. Refractive scattering by the interstellar medium may affect such measurements. We quantify the scattering properties and the sub-mas scale source parameters for the quasar B0529+483. Using RadioAstron correlated flux density measurements at 1.7, 4.8, and 22 GHz on projected baselines up to 240,000 km we find two characteristic angular scales in the quasar core, about 100 $μ$as and 10 $μ$as. Some indications of scattering substructure are found. Very high brightness temperatures, $T_\mathrm{b}\geq 10^{13}$ K, are estimated at 4.8 GHz and 22 GHz even taking into account the refractive scattering. Our findings suggest a clear dominance of the particle energy density over the magnetic field energy density in the core of this quasar.

astro-ph.HE↗

arXiv2017

Single-dish and VLBI observations of Cygnus X-3 during the 2016 giant flare episode

In September 2016, the microquasar Cygnus X-3 underwent a giant radio flare, which was monitored for 6 days with the Medicina Radio Astronomical Station and the Sardinia Radio Telescope. Long observations were performed in order to follow the evolution of the flare on a hourly scale, covering six frequency ranges from 1.5 GHz to 25.6 GHz. The radio emission reached a maximum of 13.2 +/- 0.7 Jy at 7.2 GHz and 10 +/- 1 Jy at 18.6 GHz. Rapid flux variations were observed at high radio frequencies at the peak of the flare, together with rapid evolution of the spectral index: alpha steepened from 0.3 to 0.6 within 5 hours. This is the first time that such fast variations are observed, giving support to the evolution from optically thick to optically thin plasmons in expansion moving outward from the core. Based on the Italian network (Noto, Medicina and SRT) and extended to the European antennas (Torun, Yebes, Onsala), VLBI observations were triggered at 22 GHz on five different occasions, four times prior to the giant flare, and once during its decay phase. Flux variations of 2-hour duration were recorded during the first session. They correspond to a mini-flare that occurred close to the core ten days before the onset of the giant flare. From the latest VLBI observation we infer that four days after the flare peak the jet emission was extended over 30 mas.

astro-ph.HE↗

arXiv2005

Extended emission around GPS radio sources

Extended radio emission detected around a sample of GHz Peaked Spectrum (GPS) radio sources is discussed. Evidence for extended emission which is related to the GPS source is found in 6 objects out of 33. Three objects are associated with quasars with core-jet pc-scale morphology, and three are identified with galaxies with symmetric (CSO) radio morphology. We conclude that the core-jet GPS quasars are likely to be beamed objects with a continuous supply of energy from the core to the kpc scale. It is also possible that low surface brightness extended radio emission is present in other GPS quasars but the emission is below our detection limit due to the high redshifts of the objects. On the other hand, the CSO/galaxies with extended large scale emission may be rejuvenated sources where the extended emission is the relic of previous activity. In general, the presence of large scale emission associated with GPS galaxies is uncommon, suggesting that in the context of the recurrent activity model, the time scale between subsequent bursts is in general longer than the radiative lifetime of the radio emission from the earlier activity.

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arXiv2005

EVN observations of eleven GHz-Peaked-Spectrum radio sources at 2.3/8.4GHz

We present results of EVN observations of eleven GHz-Peaked-Spectrum (GPS) radio sources at 2.3/8.4 GHz. These sources are from the classical ''bright'' GPS source samples with peak flux densities $>$ 0.2 Jy and spectral indices $α< -0.2$ ($S \propto ν^{-α}$) in the optically thick regime of their convex spectra. Most of the target sources did not have VLBI images at the time this project started. The aim of the work is to find Compact Symmetric Object (CSO) candidates from the ''bright'' GPS samples. These CSOs play a key role in understanding the very early stage of the evolution of individual radio galaxies. The reason for investigating GPS source samples is that CSO candidates are more frequently found among this class of radio sources. In fact both classes, GPS and CSO, represent a small fraction of the flux limited and flat-spectrum samples like PR+CJ1 (PR: Pearson-Readhead survey, CJ1: the first Caltech--Jodrell Bank survey) and CJF (the Caltech--Jodrell Bank flat spectrum source survey) with a single digit percentage progressively decreasing with decreasing flux density limit. Our results, with at least 3, but possibly more CSO sources detected among a sample of 11, underline the effectiveness of our approach. The three confirmed CSO sources (1133+432, 1824+271, and 2121$-$014) are characterized by a symmetric pair of resolved components, each with steep spectral indices. Five further sources (0144+209, 0554$-$026, 0904+039, 0914+114 and 2322$-$040) can be considered likely CSO candidates. The remaining three sources (0159+839, 0602+780 and 0802+212) are either of core-jet type or dominated by a single component at both frequencies.

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arXiv2001

Bendings of radio jets in BL Lacertae objects I: EVN and MERLIN observations

Several blazars, and BL Lac objects in particular, show a misalignment between the jet orientation on parsec and kiloparsec scales. Some authors (i.e. Conway & Murphy, 1993) have attempted to explain this behaviour invoking helical jets for misalignment angles around 90$\degr$, showing how in this case there are interesting implications for the understanding of the medium into which the jet is expanding. By comparing sensitive VLA observations (Cassaro et al., 1999) with images available in the literature for the BL Lac objects from the 1-Jy Sample (Stickel et al., 1991), it is clear that there is a wide range of misalignments between the initial jet direction and the kpc-scale jet, when detected. We have carried out VLBI observations of these BL Lac objects, in order to investigate the spatial evolution of the radio jets from few tens to hundreds of mas, and to search for helical jets in this class of sources. We present here the first dataset obtained from EVN+MERLIN observations at 5 GHz for seven objects. From these observations we never have a clear detection of helical jets, we only have a possible signature of their presence in 2 objects. In only one of the sources with a misalignment angle around 90$\degr$ the presence of helical jets can be ruled out. This implies that it is not possible to invoke helical jets to explain the morphology of all the sources showing a misalignment of about 90$\degr$ between the parsec and the kiloparsec scale jets.

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arXiv1999

Extended radio emission in BL Lac objects - I: the images

We have observed 28 sources selected from the 1Jy sample of BL Lac objects (Stickel et al. 1991) with the Very Large Array (VLA) in A, B and D configurations at 1.36, 1.66 and 4.85 GHz, and/or with the Westerbork Synthesis Radio Telescope (WSRT) at 1.40 GHz. In this paper we present high sensitivity images at arcsecond resolution of the 18 objects showing extended structure in our images, and of another source from the FIRST (Faint Images of the Radio Sky at Twenty-cm) survey (Becker et al. 1995). In general our high sensitivity images reveal an amount of extended emission larger than previously reported. In some objects the luminosity of the extended structure is comparable with that of FR~II radio sources. A future paper will be devoted to the interpretation of these results.

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