1,721,376 research outputs found
Variability and parsec-scale radio structure of candidate compact symmetric objects
Full text linkWe report results on multiepoch Very Large Array (VLA) and pc-scale Very Long Baseline Array (VLBA) observations of candidate compact symmetric objects (CSOs) from the faint sample of high-frequency peakers. New VLBA observations could resolve the radio structure in about 42 per cent of the observed sources, showing double components that may be either mini-lobes or core-jet structures. Almost all the sources monitored by the VLA show some variability on time-scale of a decade, and only one source does not show any significant variation. In 17 sources, the flux density changes randomly as it is expected in blazars, and in four sources the spectrum becomes flat in the last observing epoch, confirming that samples selected in the GHz regime are highly contaminated by beamed objects. In 16 objects, the pc-scale and variability properties are consistent with a young radio source in adiabatic expansion, with a steady decrease of the flux density in the optically thin part of the spectrum, and a flux density increase in the optically thick part. For these sources, we estimate dynamical ages between a few tens to a few hundred years. The corresponding expansion velocity is generally between 0.1c and 0.7c, similar to values found in CSOs with different approaches. The fast evolution that we observe in some CSO candidates suggests that not all the objects would become classical Fanaroff-Riley radio sources
High-energy properties of the high-redshift flat spectrum radio quasar PKS 2149-306
Full text linkWe investigate the gamma-ray and X-ray properties of the flat spectrum radio quasar PKS 2149-306 at redshift z = 2.345. A strong gamma-ray flare from this source was detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope satellite in 2013 January, reaching on January 20 a daily peak flux of (301 +/- 36) x 10(-8) ph cm(-2) s(-1) in the 0.1-100 GeV energy range. This flux corresponds to an apparent isotropic luminosity of (1.5 +/- 0.2) x 10(50) erg s(-1), comparable to the highest values observed by a blazar so far. During the flare the increase of flux was accompanied by a significant change of the spectral properties. Moreover significant flux variations on a 6-h time-scale were observed, compatible with the light crossing time of the event horizon of the central black hole. The broad-band X-ray spectra of PKS 2149-306 observed by Swift-XRT and NuSTAR are well described by a broken power-law model, with a very hard spectrum (Gamma(1) similar to 1) below the break energy, at E-break = 2.5-3.0 keV, and Gamma(2) similar to 1.4-1.5 above the break energy. The steepening of the spectrum below similar to 3 keV may indicate that the soft X-ray emission is produced by the low-energy relativistic electrons. This is in agreement with the small variability amplitude and the lack of spectral changes in that part of the X-ray spectrum observed between the two NuSTAR and Swift joint observations. As for the other high-redshift FSRQ detected by both Fermi-LAT and Swift-BAT, the photon index of PKS 2149-306 in hard X-ray is 1.6 or lower and the average gamma-ray luminosity higher than 2 x 10(48) erg s(-1)
Parsec-scale view of CSS/GPS sources at 327 MHz
Full text linkGlobal very long baseline interferometry (VLBI) observations at low frequencies are of great importance since they allow us to test the oldest part of the relativistic electron populations in young radio sources. At 327 MHz ((Formula presented.) 92 cm), the most compact regions are self-absorbed, and the number of cores detected is very small. This could provide useful measurements of the magnetic field in the relativistic plasma, to be compared with the equipartition field derived from minimum energy arguments. Partial self-absorption takes place also in hot-spots. However, disentangling the hot-spot emission from that of the lobe turned out to be rather difficult. On the other hand, a clearer and more complete characterization of the volumes occupied by the magnetized relativistic plasma and its total energy content can be obtained at 327 MHz. In turn, an independent estimate of the source age can be inferred, on the basis of simple assumptions on the jet power. Here, we show that the ages derived in this way (between (Formula presented.) and (Formula presented.) year) are consistent with the radiative ages determined from the break frequency in the radio spectrum
Radio spectra of High Frequency Peakers
No full textNew radio spectra of High Frequency Peakers (HFP) obtained from the Jansky Very Large Array (JVLA) show that variability is common among this class of sources. A subsample of sources have been observed with a nearly continuous spectral sampling between 1 and 10 GHz. The observed HFP sources were previously classified as F (flat), H (HFP profile with little or no flux density variability) and V (variable, but preserving a peaked spectrum). In general, sources classified as V and H show a decrease of the flux density measured in the optically thin part of the spectrum, while there is a moderate increment in the optically thick region, resulting into a progressive shift of the spectral peak to lower frequencies. This is consistent with the idea of an expanding bubble of radio plasma. The sources with an F classification instead show substantial variability, both in spectral shape and in time evolution. In these HFP sources an irregular production of energy is best observed since the radio emission is dominated by recently generated relativistic plasma, and the contribution of mini lobes, in which old plasma accumulates, is marginal if not absent at all, given the short radiative life of electrons in strong magnetic fields (tens of mG) found in these objects
Young radio sources: From newly born to short‐lived objects
Full text linkThe evolutionary stage of a powerful radio source originated by an active galactic nucleus is related to its linear size. Following the evolution models, intrinsically compact objects would evolve into the population of classical radio galaxies. However, the fraction of young radio sources in flux density-limited samples is much larger than what is expected from the number counts of large radio sources, suggesting the existence of short-lived objects. We present results of multi-epoch very large array and very long baseline array observations of a sub-sample of extreme GPS sources that should be at the very beginning of their radio evolution. We also briefly introduce a new project aiming at determining the incidence of short-lived objects among a statistically complete sample of young radio sources
The physics of the radio emission in the quiet side of the AGN population with the SKA
No full textSuper massive black holes (SMBH) are thought to be ubiquitously hosted in massive galaxies. They may be either quiescent, like the case of Sgr A* in our Galaxy, or active, and they are at the basis of the phenomena known as Active Galactic Nuclei (AGN). In this case they often manifest their presence by releasing a huge amount of energy which usually overwhelms the star-related contribution of the entire host galaxy. Although they have been targets of many multiwavelength campaigns, the main physical processes at work in AGN are still under debate. In particular the origin of the radio emission and the mechanisms involved are among the open questions in astrophysics. The radio-loud AGN population and their radio emission is linked to the presence of bipolar outflows of relativistic jets. However, the large majority of the AGN population do
not form powerful highly-relativistic jets on kpc scales, like those observed in radio galaxies and radio quasars. This does not mean that they are radio-silent objects. On the contrary, these systems are characterized by radio luminosity up to 10
23 W/Hz at 1.4 GHz, challenging our knowledge on the physical processes at the basis of the radio emission in radio-quiet objects. The main mechanisms proposed so far are synchrotron radiation from mildly relativistic mini-jets, thermal cyclo-synchrotron emission by low-efficiency accretion flow (like ADAF or ADIOS), or thermal free-free emission from the X-ray heated corona or wind. The difficulty in understanding the main mechanism involved is related to the weakness of these objects, which precludes the study of non- local radio-quiet AGN. Multifrequency, high-sensitivity polarimetric radio observations are, thus, crucial to constrain the nature of the power engine, and they may help in distinguishing between
the contribution from star formation and AGN activity. The advent of the Square Kilometer Array (SKA), with its sub-arcsecond resolution and unprecedented sensitivity will allow us to investigate these processes in radio-quiet AGN, even at high redshift for the first time. Both the broad-band radio spectrum and the polarization information will help us in disentangling between non-thermal and thermal origin of the radio emission. The jump in sensitivity of a few order of
magnitudes at the (sub-) mJy level will enable us to detect radio emission from a large number of radio-quiet AGN at high redshift, providing a fundamental step in our understanding of their cosmological evolution
VLBI images at 327 MHz of compact steep spectrum and GHz-peaked spectrum sources from the 3C and PW samples
Full text linkWe present results on global very long baseline interferometry (VLBI) observations at 327 MHz of 18 compact steep-spectrum (CSS) and GHz-peaked spectrum (GPS) radio sources from the 3C and the Peacock & Wall catalogues. About 80 per cent of the sources have a 'double/triple' structure. The radio emission at 327 MHz is dominated by steep-spectrum extended structures, while compact regions become predominant at higher frequencies. As a consequence, we could unambiguously detect the core region only in three sources, likely due to self-absorption affecting its emission at this low frequency. Despite their low surface brightness, lobes store the majority of the source energy budget, whose correct estimate is a key ingredient in tackling the radio source evolution. Low-frequency VLBI observations able to disentangle the lobe emission from that of other regions are therefore the best way to infer the energetics of these objects. Dynamical ages estimated from energy budget arguments provide values between 2 × 103 and 5 × 104 yr, in agreement with the radiative ages estimated from the fit of the integrated synchrotron spectrum, further supporting the youth of these objects. A discrepancy between radiative and dynamical ages is observed in a few sources where the integrated spectrum is dominated by hotspots. In this case the radiative age likely represents the time spent by the particles in these regions, rather than the source age
Gamma-ray emitting narrow-line Seyfert 1 galaxies. New discoveries and open questions
No full textThe discovery of γ-ray emission from 5 radio-loud narrow-line Seyfert 1 galaxies revealed the presence of a possible emerging third class of AGNs with relativistic jets, in addition to blazars and radio galaxies. The existence of relativistic jets also in this subclass of Seyfert galaxies opened an unexplored research space for our knowledge of the radio-loud AGNs. Here, we discuss the radio-to-γ-rays properties of the γ-ray emitting narrow-line Seyfert 1 galaxies, also in comparison with the blazar scenario
Relativistic jets in narrow-line Seyfert 1 galaxies. New discoveries and open questions
No full textBefore the launch of the Fermi satellite only two classes of AGNs were known to produce relativistic jets and thus emit up to the γ-ray energy range: blazars and radio galaxies, both hosted in giant elliptical galaxies. The first four years of observations by the Large Area Telescope on board Fermi confirmed that these two are the most numerous classes of identified sources in the extragalactic γ-ray sky, but the discovery of γ-ray emission from 5 radio-loud narrow-line Seyfert 1 galaxies revealed the presence of a possible emerging third class of AGNs with relativistic jets. Considering that narrow-line Seyfert 1 galaxies seem to be typically hosted in spiral galaxy, this finding poses intriguing questions about the nature of these objects, the onset of production of relativistic jets, and the cosmological evolution of radio-loud AGN. Here, we discuss the radio-to-γ-rays properties of the γ-ray emitting narrow-line Seyfert 1 galaxies, also in comparison with the blazar scenario
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