Gigahertz Peaked Spectrum Radio Sources Research Articles

VLBI position variability of AGNs is inversely correlated with their photometric variability

The stability of the International Celestial Reference Frame (ICRF), realized through geodetic very long baseline interferometry (VLBI) positions of thousands of extragalactic objects, is dependent on the individual positional stability of these objects. It has been recently shown that the prevalence of offsets between the VLBI positions of ICRF objects and their Gaia optical positions, which limit the optical-radio reference frame tie, is inversely correlated with optical photometric variability, suggesting that photometrically variable objects may be more positionally stable. In this work, we determine the relationship between VLBI position stability of ICRF objects and optical-radio position offsets as well as optical photometric variability. We created multi-epoch geodetic VLBI solutions for a sample of 520 ICRF sources that have sufficient data to determine the variability in their VLBI positions over time. We compared this position variability with the fractional photometric variability provided by the Gaia extragalactic source catalog, the Gaia-ICRF optical-radio position offsets, the uncertainty-normalized position offsets, and optical $ BP-RP $ color as well as with possible confounders such as optical magnitude, VLBI/Gaia position error, and redshift. We determined the relationship between VLBI position stability and gamma -ray detection by the Fermi Large Area Telescope (LAT), and we determined how the VLBI position and optical flux variabilities correlate with the spectral classification of our sample, considering flat spectrum radio quasars (FSRQs), quasi-stellar objects, BL Lacs, Seyfert, and gigahertz-peaked spectrum radio sources or compact-steep-spectrum radio sources. We found that VLBI astrometric variability is (i) negatively correlated with optical flux variability, (ii) positively correlated with optical-radio offsets, (iii) negatively correlated with optical color index BP-RP, and (iv) negatively correlated with gamma -ray detection. We also found that the most positionally stable sources are among the FSRQ and BL Lac classes. In other words, redder, photometrically variable sources have the most stable VLBI positions, the smallest optical-radio position offsets, and the highest rate of gamma -ray detection, and these sources tend to be spectrally classified as blazars. Our results are consistent with the most positionally stable sources being blazars, a class of object in which the jet is oriented close to the line of sight and where relativistic beaming increases photometric variability and minimizes the projected offset between the optical and radio positions. Our study should therefore orient future geodetic VLBI observing programs preferentially toward sources with high photometric variability because these sources are predicted to have better VLBI position stabilities and smaller optical-radio position offsets, improving the stability of the celestial reference frame axes.

The relationship between FR0 radio galaxies and gigahertz‐peaked spectrum sources

AbstractWe present the results of our study of the relationship between Fanaroff‐Riley type 0 (FR0) radio galaxies and gigahertz‐peaked spectrum (GPS) sources. Quasi‐simultaneous radio spectra of 34 FR0s were obtained at 2.25–22.3 GHz with the radio telescope RATAN‐600 in 2020–2021 during 2–6 epochs. Most FR0s have flat radio spectra, but we found many spectra with a peaked shape. Due to this fact and the compact nature of FR0s, we suggest their possible relationship with compact steep‐spectrum (CSS)/GPS radio sources. We analyzed broadband radio spectra of the 34 FR0s using the RATAN‐600 measurements and available literature data. There are 14 FR0 objects which can be CSS/GPS radio source candidates. Most FR0s have broader radio spectra than those of genuine GPS sources, with like in blazars. Most spectral indices at the frequencies below and above the peak do not correspond to the values typical of canonical GPS sources. We classified 3 FR0s as low‐power GPS sources according to the canonical criteria. The key issue is the variability properties of FR0s. Some FR0s demonstrate a variability level of up to 25% on a time scale of 1 year according to the RATAN‐600 measurements. The flare phenomena in FR0 objects can imply a relationship between them and blazars.

Resolving the Radio Emission from the Quasar P172+18 at z = 6.82

Abstract We present high angular resolution imaging of the quasar PSO J172.3556+18.7734 at z = 6.82 with the Very Long Baseline Array (VLBA). This source currently holds the record of being the highest redshift radio-loud quasar. These observations reveal a dominant radio source with a flux density of 398.4 ± 61.4 μJy at 1.53 GHz, a deconvolved size of 9.9 × 3.5 mas (52.5 × 18.6 pc), and an intrinsic brightness temperature of (4.7 ± 0.7) × 107 K. A weak unresolved radio extension from the main source is also detected at the ∼3.1σ level. The total flux density recovered with the VLBA at 1.53 GHz is consistent with that measured with the Very Large Array (VLA) at a similar frequency. The quasar is not detected at 4.67 GHz with the VLBA, suggesting a steep spectral index with a limit of 1.55. The quasar is also not detected with the VLBA at 7.67 GHz. The overall characteristics of the quasar suggest that it is a very young radio source similar to lower redshift gigahertz peaked spectrum radio sources, with an estimated kinematic age of ∼103 yr. The VLA observations of this quasar revealed a second radio source in the field 23.″1 away. This radio source, which does not have an optical or IR counterpart, is not detected with the VLBA at any of the observed frequencies. Its nondetection at the lowest observed VLBA frequency suggests that it is resolved out, implying a size larger than ∼0.″17. It is thus likely situated at lower redshift than the quasar.

Hypersensitive Tunable Josephson Escape Sensor for Gigahertz Astronomy

Sensitive photon detection in the gigahertz band constitutes the cornerstone to study different phenomena in astronomy, such as radio burst sources, galaxy formation, cosmic microwave background, axions, comets, gigahertz-peaked spectrum radio sources and supermassive black holes. Nowadays, state of the art detectors for astrophysics are mainly based on transition edge sensors and kinetic inductance detectors. Overall, most sensible nanobolometers so far are superconducting detectors showing a noise equivalent power (NEP) as low as 2x10-20 W/Hz1/2. Yet, fast thermometry at the nanoscale was demonstrated as well with Josephson junctions through switching current measurements. In general, detection performance are set by the fabrication process and limited by used materials. Here, we conceive and demonstrate an innovative tunable Josephson escape sensor (JES) based on the precise current control of the temperature dependence of a fully superconducting one-dimensional nanowire Josephson junction. The JES might be at the core of future hypersensitive in situ-tunable bolometers or single-photon detectors working in the gigahertz regime. Operated as a bolometer the JES points to a thermal fluctuation noise (TFN) NEP_TFN 1x10-25 W/Hz1/2, which as a calorimeter bounds the frequency resolution above 2 GHz, and resolving power below 40 at 50 GHz, as deduced from the experimental data. Beyond the obvious applications in advanced ground-based and space telescopes for gigahertz astronomy, the JES might represent a breakthrough in several fields of quantum technologies ranging from subTHz communications and quantum computing to cryptography and quantum key distribution.

THE RADIO/X-RAY CORRELATION AND BLACK HOLE FUNDAMENTAL PLANE FOR YOUNG RADIO SOURCES: IMPLICATIONS FOR X-RAY ORIGIN AND ACCRETION MODE

ABSTRACT We find that the young radio sources (gigahertz-peaked spectrum and compact steep spectrum radio sources) follow in the radio/X-ray correlation with b = 0.61 ± 0.07 ?> ( L R ∝ L X b ?> ), and the fundamental plane of black hole activity with the form log L R = 0.58 − 0.03 + 0.03 log L X + 0.42 − 0.07 + 0.09 log M BH + 13.83 − 0.97 + 0.91 ?> and the intrinsic scatter &sgr; = 0.29 ?> . The flatter coefficient between radio and X-ray bands denies the jet origin of the X-ray emission in these types of sources. Meanwhile, the higher ratio of X-ray luminosity to Eddington luminosity ( L X / L Edd ?> ) suggests that the X-ray emission is produced by the hot corona coupling with the standard thin disk. The deviation with the radiative efficient fundamental plane proposed by Dong et al. is mainly due to the extended radio emission in young radio sources. This fundamental plane manifests that even the kiloparsec-scaled radio emission has a tight connection with the accretion process, and could be suitable for the radio-loud active galactic nuclei whose radio and X-ray emission are dominated by the extended jets and the radiative efficient accretion flow, respectively. Otherwise, the high-excitation galaxies and low-excitation galaxies do not have obvious distinctions in the radio/X-ray correlation and the fundamental plane.

The multifrequency parsec-scale structure of PKS 2254−367 (IC 1459): a luminosity-dependent break in morphology for the precursors of radio galaxies?

We present the first multi-frequency VLBI images of PKS 2254-367, a Giga-hertz Peaked Spectrum (GPS) radio source hosted by the nearby galaxy IC 1459 (D=20.5 Mpc). PKS 2254-367 and the radio source in NGC 1052 (PKS 0238-084; D=17.2 Mpc) are the two closest GPS radio sources to us, far closer than the next closest example, PKS 1718-649 (D=59 Mpc). As such, IC 1459 and NGC 1052 offer opportunities to study the details of the pc-scale radio sources as well as the environments that the radio sources inhabit, across the electromagnetic spectrum. Given that some models for the origin and evolution of GPS radio sources require a strong connection between the radio source morphology and the gaseous nuclear environment, such opportunities for detailed study are important. Our VLBI images of PKS 2254-367 show that the previously identified similarities between IC 1459 and NGC 1052 continue onto the pc-scale. Both compact radio sources appear to have symmetric jets of approximately the same luminosity, much lower than typically noted in compact double GPS sources. Similarities between PKS 2254-367 and NGC 1052, and differences with respect to other GPS galaxies, lead us to speculate that a sub-class of GPS radio sources, with low luminosity and with jet-dominated morphologies, exists and would be largely absent from radio source surveys with ~1 Jy flux density cutoffs. We suggest that this possible low-luminosity, jet-dominated population of GPS sources could be an analog of the FR-I radio galaxies, with the higher luminosity lobe-dominated GPS sources being the analog of the FR-II radio galaxies.

THE SPECTRAL VARIABILITY OF THE GHZ-PEAKED SPECTRUM RADIO SOURCE PKS 1718-649 AND A COMPARISON OF ABSORPTION MODELS

Using the new wideband capabilities of the Australia Telescope Compact Array (ATCA), we obtain spectra for PKS 1718-649, a well-known gigahertz-peaked spectrum radio source. The observations, between approximately 1 and 10 GHz over three epochs spanning approximately 21 months, reveal variability both above the spectral peak at ~3 GHz and below the peak. The combination of the low and high frequency variability cannot be easily explained using a single absorption mechanism, such as free-free absorption or synchrotron self-absorption. We find that the PKS 1718-649 spectrum and its variability are best explained by variations in the free-free optical depth on our line-of-sight to the radio source at low frequencies (below the spectral peak) and the adiabatic expansion of the radio source itself at high frequencies (above the spectral peak). The optical depth variations are found to be plausible when X-ray continuum absorption variability seen in samples of Active Galactic Nuclei is considered. We find that the cause of the peaked spectrum in PKS 1718-649 is most likely due to free-free absorption. In agreement with previous studies, we find that the spectrum at each epoch of observation is best fit by a free-free absorption model characterised by a power-law distribution of free-free absorbing clouds. This agreement is extended to frequencies below the 1 GHz lower limit of the ATCA by considering new observations with Parkes at 725 MHz and 199 MHz observations with the newly operational Murchison Widefield Array. These lower frequency observations argue against families of absorption models (both free-free and synchrotron self-absorption) that are based on simple homogenous structures.

Variability of GPS Radio Sources at 5 GHz

We carry out flux monitoring on a sample of 169 Gigahertz Peaked Spectrum (GPS) radio sources at 5 GHz and find that about one-third of them show considerable Inter-Month Variability (IMV), and these IMV phenomena are likely to be caused by interstellar scintillation (ISS). Furthermore, we find that those showing IMV tend to be point-like or core-jet structures at VLBI scale.

The B3-VLA CSS sample

Compact steep-spectrum radio sources and giga-hertz peaked spectrum radio sources (CSS/GPS) are generally considered to be mostly young radio sources. In recent years we studied at many wavelengths a sample of these objects selected from the B3-VLA catalog: the B3-VLA CSS sample. Only ~ 60 % of the sources were optically identified. We aim to increase the number of optical identifications and study the properties of the host galaxies of young radio sources. We cross-correlated the CSS B3-VLA sample with the Sloan Digital Sky Survey (SDSS), DR7, and complemented the SDSS photometry with available GALEX (DR 4/5 and 6) and near-IR data from UKIRT and 2MASS. We obtained new identifications and photometric redshifts for eight faint galaxies and for one quasar and two quasar candidates. Overall we have 27 galaxies with SDSS photometry in five bands, for which we derived the ultraviolet-optical spectral energy distribution (UV-O-SED). We extended our investigation to additional CSS/GPS selected from the literature. Most of the galaxies show an excess of ultra-violet (UV) radiation compared with the UV-O-SED of local radio-quiet ellipticals. We found a strong dependence of the UV excess on redshift and analyzed it assuming that it is generated either from the nucleus (hidden quasar) or from a young stellar population (YSP). We also compare the UV-O-SEDs of our CSS/GPS sources with those of a selection of large size (LSO) powerful radio sources from the literature. If the major process of the UV excess is caused by a YSP, our conclusion is that it is the result of the merger process that also triggered the onset of the radio source with some time delay. We do not see evidence for a major contribution from a YSP triggered by the radio sources itself.

The last breath of the young gigahertz-peaked spectrum radio source PKS 1518+047

We present the results from multi-frequency VLBA observations from 327 MHz to 8.4 GHz of the gigahertz-peaked spectrum radio source PKS 1518+047 (4C 04.51) aimed at studying the spectral index distribution across the source. Further multi-frequency archival VLA data were analysed to constrain the spectral shape of the whole source. The pc-scale resolution provided by the VLBA data allows us to resolve the source structure in several sub-components. The analysis of their synchrotron spectra showed that the source components have steep spectral indices, suggesting that no supply/re-acceleration of fresh particles is currently taking place in any region of the source. By assuming the equipartition magnetic field of 4 mG, we found that only electrons with $\gamma$ < 600, are still contributing to the radio spectrum, while electrons with higher energies have been almost completed depleted. The source radiative lifetime we derived is 2700+/-600 years. Considering the best fit to the overall spectrum, we find that the time in which the nucleus has not been active represents almost 20% of the whole source lifetime, indicating that the source was 2150+/-500 years old when the radio emission switched off.

Progress of research on AGNs at the Urumqi Observatory

We report the progress on Very Long Baseline Interferometry (VLBI) observations of Gigahertz Peaked Spectrum (GPS) radio sources, and single-dish observations of active galactic nuclei (AGNs). The GPS sources are a kind of young AGNs observable in radio. From our VLBI observations at 1.6 and 5 GHz with the European VLBI Network (EVN) including the Urumqi and Shanghai stations, most GPS sources show compact doubles with sizes less than 1 kiloparsec. We have classified the sources into double-lobes, core-jets, and complex structures according to the spectral indices as well as images. We also estimated the values of the jet viewing angle for the symmetric objects. In addition, we are monitoring a few samples of AGNs with the Urumqi 25-meter radio telescope, in order to find flux variability. We detected rapid flux variability in quasar 1156+295, and relatively slow variability in a few of the others. The origin of the rapid variability is discussed. Moreover, we launched a radio-optical monitoring program called Fermi-AGN in 2009.

Future investigations of GPS and CSS radio sources with LOFAR

AbstractIn the next few years, the Low Frequency Array (LOFAR) will open up one of the last astronomically unexplored wavelength regimes. While the LOFAR core is currently being erected in the Netherlands, its outer stations will cover a large part of Europe, resulting in an unprecedented angular resolution at &gt; meter wavelengths. Next to many other exciting scientific endeavours, LOFAR will be the first instrument to probe the low frequency spectra of Gigahertz Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) radio sources. It will give new insights into their absorption processes, and probe associated extended emission (possibly linked to earlier epochs of activity) in these enigmatic class of young active galactic nuclei. Furthermore, LOFAR will be sensitive to possibly the most distant GPS and CSS sources, of which their spectral turnovers have redshifted down to the lowest observable radio frequencies (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A Search for Molecular Gas in GHz-peaked Spectrum Radio Sources

We present searches for molecular gas (CO, OH, CS, and NH3) in six gigahertz-peaked spectrum (GPS) radio sources. We do not detect gas in any source and place upper limits on the mass of molecular gas that are generally in the range from ~109 to a few times 1010 M⊙. These limits are consistent with the following interpretations: (1) GPS sources do not require very dense gas in their hosts, and (2) the GPS sources are unlikely to be confined by dense gas and will evolve to become larger radio sources.

Radio Spectral Variability of the Nearby GPS Galaxy PKS 1718–649

AbstractData from new Australia Telescope Compact Array observations of a nearby gigahertz peaked spectrum radio source, PKS 1718–649, show significant variability both below and above the spectral turnover frequency during a nine month period. Simple models of synchrotron self-absorption (SSA) and free–free absorption (FFA) cannot explain the individual spectra or the spectral variability. Novel SSA or FFA models will need to be investigated in order to explain the complex variability in this source. Some potential explanations for the spectral variability are explored here but we can make no strong conclusion about which of SSA or FFA (or alternative mechanisms) causes the peaked spectrum in PKS 1718–649.

ChandraDiscovery of a 300 Kiloparsec X‐Ray Jet in the Gigahertz‐peaked Spectrum Quasar PKS 1127−145

We have discovered an X-ray jet with Chandra imaging of the z=1.187 radio-loud quasar PKS1127-145. In this paper we present the Chandra X-ray data, follow-up VLA observations, and optical imaging using the HST WFPC2. The X-ray jet contains 273+/-5 net counts in 27ksec and extends ~30 arcsec, from the quasar core, corresponding to a minimum projected linear size of ~330/h_50 kpc. The evaluation of the X-ray emission processes is complicated by the observed offsets between X-ray and radio brightness peaks. We discuss the problems posed by these observations to jet models. In addition, PKS1127-145 is a Giga-Hertz Peaked Spectrum radio source, a member of the class of radio sources suspected to be young or ``frustrated'' versions of FRI radio galaxies. However the discovery of an X-ray and radio jet extending well outside the host galaxy of PKS1127-145 suggests that activity in this and other GPS sources may be long-lived and complex.

On the lives of extra-galactic radio sources: the first 100,000 years

In this paper we discuss the early phase of radio source evolution as represented by Gigahertz Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) radio sources. Correlations between their spectral peak and angular size strongly suggest that the spectral turnovers are caused by synchrotron self absorption, and indicate that young radio sources evolve in a self similar way. We argue that the evolution of a radio source during its first 10 5 years is qualitatively very different from that during the rest of its life-time. This may be caused by the difference in the density gradient of the intra-galactic medium inside and outside the core-radius of the host galaxy.

Physical conditions in faint gigahertz-peaked spectrum radio sources

Weak, compact radio sources (∼100 mJy peak flux, L∼1–10 pc) with their spectral peaks at about a gigahertz are studied, based on the complete sample of 46 radio sources of Snellen, drawn from high-sensitivity surveys, including the low-frequency Westerbork catalog. The physical parameters have been estimated for 14 sources: the magnetic field (H⊥), the number density of relativistic particles (ne), the energy of the magnetic field \((E_{H_ \bot } )\), and the energy of relativistic particles (Ee). Ten sources have \(E_{H_ \bot } \ll E_e \), three have approximate equipartition of the energies \((E_{H_ \bot } \sim E_e )\), and only one has \(E_{H_ \bot } \gg E_e \). The mean magnetic fields in quasars (10−3 G) and galaxies (10−2 G) have been estimated. The magnetic field appears to be related to the sizes of compact features as \(H \sim 1/\sqrt L \).

Radio Spectra and Variability of Gigahertz-Peaked Spectrum Radio Sources and Candidates

We have made long-term, high radio frequency observations of southern and equatorial active galactic nuclei (AGNs). After complementing these data with data from the literature, we have constructed the radio spectra of these sources and searched for sources with inverted spectra in the gigahertz range. We have identified 12 new sources with spectral shapes resembling those of the gigahertz-peaked spectrum (GPS) sources and eight other sources with inverted spectral parts in the gigahertz region. Several of these new GPS-source candidates have high (≥10 GHz) peak frequencies in the observer's frame, and they all exhibit strong long-term variability. We have also studied the variability behavior of known GPS sources, mainly quasar-type sources. All of the 14 sources included in our sample show moderate to extremely high radio variability, and at least 12 of them are also variable in the millimeter domain. Long-term monitoring of these sources shows that some of them have spectral shapes resembling those of classical variable flat-spectrum sources, indicating that some of these sources have been misidentified as GPS sources when only sparsely sampled data have been available. On the other hand, four of the variable sources show persistent GPS-type spectra at all stages of activity, suggesting that at least some of the GPS sources can be variable.

Identifications and spectroscopy of Gigahertz Peaked Spectrum sources. II.

We present deep optical and near-infrared imaging as well as optical spectroscopy of as yet unclassified hosts of Gigahertz Peaked Spectrum (GPS) radio sources. Identifications were obtained for 14 objects, several of which are new. Spectroscopic follow-up yielded redshift information on 7 GPS objects. The remaining 18 objects of our sample without redshifts await spectroscopy with 8 m class facilities.

Relationships among some parameters of GPS radio sources

Data on the parameters of the complete sample of gigahertz-peaked spectrum (GPS) radio sources available in the literature are analyzed to find statistical relationships among them. Correlations are found among the following parameters: l)flux density Sm at the turnover point of the spectrum as a function of the corresponding observed frequency vm; 2) the turnover frequency vrest in a fixed coordinate system as a function of the linear size LS in projection; 3 the linear size LS in projection as a function of redshift z; 4 the flux density Sm at the turnover point of a galaxy’s GPS as a function of the corresponding linear size LS in projection.