High-Resolution Study of Selected Intraday Variable Sources

Radio waves are affected by the ionized interstellar medium through which they pass. Wide ranges of scattering effects can occur and influence the observed characteristics of compact radio objects. Distinguishing between scattering-induced and source-intrinsic phenomena is very important both in studying the interstellar matter of the Milky Way and in investigating the physical properties of the background source itself. 20-25% of flat spectrum extragalactic radio sources exhibit Intraday Variability, which is thought to be at least partly due to interstellar scintillations. This phenomenon is still not understood completely. Can propagation effect alone account for all of the observed variability characteristics? How important is the contribution from the intrinsic structure of the source? What is the mixing ratio between extrinsic and intrinsic effects? In what extent are the radio observations of Active Galactic Nuclei influenced by the interstellar matter of the Milky Way? In this thesis, these questions are investigated through different observational approaches: IDV observations of selected samples of flat spectrum radio sources are presented. The dependence of variability characteristics on the galactic latitude is discussed. The interplay between extrinsic and source intrinsic effects is studied in detail for a specially selected scatter broadened quasar using single dish and interferometric measurements. Investigating the milliarcsecond to sub-milliarcsecond structure of an IDV source at the highest angular resolution in closely-spaced epochs in time can reveal the possible changes in the source structure on IDV time-scales. Space-VLBI data of total and polarized intensity of an IDV blazar are analyzed and discussed

Radio and mid-infrared properties of the blazar J1419−-0838

The radio quasar NVSS J141922$-$083830 (J1419$-$0838) was initially classified as an uncategorised blazar-type object, following its detection in the $\gamma$-ray band with the Fermi space telescope. Later, using multi-waveband observations and modeling, its was found to be a flat-spectrum radio quasar (FSRQ). However, its radio emission has never been discussed in depth in the literature. Here we present a detailed analysis on the radio properties of J1419$-$0838 using archival interferometric imaging data at pc and kpc scales. We conclude that the flux density variations, the flat radio spectrum, the compact nature of the quasar structure at all scales, and the relativistic Doppler enhancement of the radio emission all support the previous classification as an FSRQ. We also investigated the short- and long-term mid-infrared (MIR) light curve of the quasar based on observations by the Wide-field Infrared Survey Explorer satellite, and found that there is significant variability on time-scales of days as well as years. Comparison of the MIR light curve to the times of previously reported $\gamma$-ray and optical flares shows no clear correlation between the events at different wavebands.Comment: 15 pages, 5 figures, 3 tables, accepted for publication in Astrophysics and Space Scienc

Sub-mJy radio emission from high-redshift active galactic nuclei in the footprint of the VLA Sky Survey

Using empty-field `Quick Look' images from the first two epochs of the VLA Sky Survey (VLASS) observations, centred on the positions of $\sim3700$ individually radio-non-detected active galactic nuclei (AGNs) at $z\ge4$, we performed image stacking analysis to examine the sub-mJy emission at $3$ GHz. We found characteristic monochromatic radio powers of $P_\mathrm{char}=(2-13) \times 10^{24}$ W Hz$^{-1}$, $P_\mathrm{char}=2\times10^{24}-1.3\times10^{25}$ W Hz$^{-1}$, indicating that AGN-related radio emission is widespread in the sample. The signal-to-noise ratios of the redshift-binned median stacked maps are between $4-6$, and we expect that with the inclusion of the yet to be completed third-epoch VLASS observations, the detection limit defined as signal-to-noise ratio $\mathrm{SNR}\ge6$ could be reached, and the redshift dependence can be determined. To obtain information on the general spectral properties of the faint radio emission in high-redshift AGNs, we confined the sample to $\sim3000$ objects covered by both the VLASS and the Faint Images of the Radio Sky at Twenty-centimeters (FIRST) survey. We found that the flux densities from the median stacked maps show a characteristic spectral index of $\alpha^*=-0.30\pm0.15$, which is in agreement with the median spectral index of the radio-detected $z\ge4$ AGNs from our high-redshift AGN catalogue. The three-band mid-infrared colour--colour diagram based on Wide-field Infrared Survey Explorer observations provides further support regarding the AGN contribution to the radio emission in the sub-mJy sample.Comment: 9 pages,7 figures, 3 tables. Accepted for publication in MNRA

Radio emission from dust-obscured galaxies

The coevolution of galaxies and their central supermassive black holes is a subject of intense research. A class of objects, the dust-obscured galaxies (DOGs) are particularly interesting in this respect as they are thought to represent a short evolutionary phase when violent star formation activity in the host galaxy may coexist with matter accretion onto the black hole powering the active nucleus. Here we investigate different types of DOGs classified by their mid-infrared spectral energy distributions to reveal whether they can be distinguished by their arcsec-scale radio properties. Radio emission is unaffected by dust obscuration and may originate from both star formation and an active nucleus. We analyse a large sample of 661 DOGs complied from the literature and find that only a small fraction of them ($\sim 2$ per cent) are detected with flux densities exceeding $\sim 1$ mJy in the Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey. These radio-detected objects are almost exclusively `power-law' DOGs. Stacking analysis of the FIRST image cutouts centred on the positions of individually radio-undetected sources suggests that weak radio emission is present in `power-law' DOGs. On the other hand, radio emission from `bump' DOGs is only marginally detected in the median-stacked FIRST image.Comment: 7 pages, 3 figures, accepted to MNRA

Nagy szögfelbontású rádió-interferometria - űreszközökkel

A világűrből érkező rádiósugárzás legnagyobb része – csakúgy, mint a látható fény – akadálytalanul áthatol a Föld légkörén. Így rádiócsillagászati megfigyeléseket földi rádióteleszkópokkal is kitűnően lehet végezni. Miért célszerű akkor mégis a Föld körül keringő űreszközökre telepíteni rádiótávcsövet? Ennek megértéséhez felidézzük, hogy a rádiócsillagászatban használatos műszerek szögfelbontása – a látható fényéhez viszonyítva sok nagyságrenddel hosszabb elektromágneses hullámhosszak miatt – igencsak korlátozott. Hatalmas rádióteleszkópok építése lehetetlen vállalkozás volna, ezért fejlődött ki az interferometrikus megfigyelési módszer. Az egymástól távol elhelyezett antennák adatainak kombinálásával egy akkora képzeletbeli rádióteleszkópot lehet létrehozni, amelynek a szögfelbontását nem az egyedi antennák átmérője, hanem a köztük levő távolság határozza meg. Az 1960-as évek végén kifejlesztett nagyon hosszú bázisvonalú interferometria (VLBI) alkalmas arra, hogy a Föld különböző pontjain elhelyezett rádióteleszkópokból alkosson egyetlen hálózatot. Így centiméteres hullámhosszakon az ívmásodperc ezred részének megfelelő, rendkívül finom szögfelbontással lehet vizsgálni a kompakt rádiósugárzó égitestek szerkezetét. Vannak azonban olyan csillagászati objektumok, amelyek még így is felbontatlanok maradnak! Ezeknek a megfigyelésére szolgál az űr-VLBI technika. A földi rádiótávcső-hálózatot egy műholdon elhelyezett antennával kiegészítve, akár bolygónk átmérőjénél is jóval hosszabb bázisvonalak s így még finomabb felbontás érhető el. Bemutatjuk az űr-VLBI technika alapelvét, és szólunk az első űr-VLBI mesterséges holdról, a Japánból 1997-ben felbocsátott HALCA-ról. Összefoglaljuk az új űrcsillagászati megfigyelési technikával az elmúlt években elért tudományos eredményeket, kiemelve közülük egyes hazai vonatkozásúakat is. Végül szót ejtünk az űr-VLBI jövőjéről, a technika előtt álló műszaki és tudományos kihívásokról, a közeli években felbocsátandó új űr-VLBI műholdak – az orosz RadioAstron és a második generációs japán ASTRO-G – programjáról is

Unveiling the weak radio quasar population at z≥4

We applied image stacking on empty-field Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey maps centred on optically identified high-redshift quasars at $z\geq4$ to uncover the hidden $\mu$Jy radio emission in these active galactic nuclei (AGN). The median stacking procedure for the full sample of $2229$ optically identified AGN uncovered an unresolved point source with an integrated flux density of 52$~\mu$Jy, with a signal-to-noise ratio $\sim10$. We co-added the individual image centre pixels to estimate the characteristic monochromatic radio power at $1.4~$GHz considering various values for the radio spectral index, revealing a radio population with $P_\mathrm{1.4GHz}\sim10^{24}~$W Hz$^{-1}$. Assuming that the entire radio emission originates from star-forming (SF) activity in the nuclear region of the host galaxy, we obtained an upper limit on the characteristic star formation rate, $\sim4200~$M$_\odot~$yr$^{-1}$. The angular resolution of FIRST images is insufficient to distinguish between the SF and AGN origin of radio emission at these redshifts. However, a comparison with properties of individual sources from the literature indicates that a mixed nature is likely. Future very long baseline interferometry radio observations and ultra-deep Square Kilometre Array surveys are expected to be sensitive enough to detect and resolve the central $1-10~$kpc region in the host galaxies, and thus discriminate between SF and AGN related emission.Comment: 9 pages, 3 figures, 3 tables, accepted for publication in MNRA

A self-lensing supermassive binary black hole at radio frequencies: the story of Spikey continues

The quasar J1918+4937 was recently suggested to harbour a milliparsec-separation binary supermassive black hole (SMBH), based upon modeling the narrow spike in its high-cadence Kepler optical light curve. Known binary SMBHs are extremely rare, and the tight constraints on the physical and geometric parameters of this object are unique. The high-resolution radio images of J1918+4937 obtained with very long baseline interferometry (VLBI) indicate a rich one-sided jet structure extending to 80 milliarcseconds. Here we analyse simultaneously-made sensitive 1.7- and 5-GHz archive VLBI images as well as snapshot 8.4/8.7-GHz VLBI images of J1918+4937, and show that the appearance of the wiggled jet is consistent with the binary scenario. We develop a jet structural model that handles eccentric orbits. By applying this model to the measured VLBI component positions, we constrain the inclination of the radio jet, as well as the spin angle of the jet emitter SMBH. We find the jet morphological model is consistent with the optical and radio data, and that the secondary SMBH is most likely the jetted one in the system. Furthermore, the decade-long 15-GHz radio flux density monitoring data available for J1918+4937 are compatible with a gradual overall decrease in the the total flux density caused by a slow secular change of the jet inclination due to the spin-orbit precession. J1918+4937 could be an efficient high-energy neutrino source if the horizon of the secondary SMBH is rapidly rotating.Comment: 12 pages, 5 figures, 4 tables, accepted for publication in MNRA

Kompakt extragalaktikus objektumok vizsgálata rádió-interferométeres technikával = Radio interferometric studies of compact extragalactic objects

A nagyon hosszú bázisvonalú interferométeres (VLBI) megfigyelési módszer extragalaktikus asztrofizikai és asztrometriai alkal­mazásaival foglalkoztunk. A legfinomabb felbontással tanulmányoztuk a ma ismert két legtávolabbi rádiókvazár szerkezetét. A J1427+3312 kvazár (vöröseltolódása z=6,12) két komponensből áll, egy igen fiatal objektum lehet. Vizsgáltunk más távoli (z>3) rádiókvazárokat, valamint egyéb különleges aktív galaxismagokat is. Tanulmányoztunk napi időskálán jelentős fényességváltozásokat mutató kvazárokat. A jelenséget a Tejútrendszer látóirányba eső csillagközi anyagában fellépő szcintilláció okoz­hatja. Felfedeztük, hogy az optikai emissziós színképvonalai alapján kettősnek sejtett J1425+3231 kvazár valóban két kompakt rádióforrásból áll. Sikerült relativisztikus jet jelenlétére utaló tágulást felismerni egy Ic típusú szupernóvában (SN 2007gr). Az égi vonatkoztatási rendszert kijelölő aktív galaxismagok igen kis látszó sajátmozgásait összefüggésbe hoztuk ezred-ívmásodperces rádiószerkezetükkel. Rádió és optikai tartományban mért koordinátáik összefüggését vizs­gálva a vártnál több jelentős pozíciós eltérést találtunk. A készülő nemzetközi űr-VLBI programok számára új kalibrátor rádióforrásokat és halvány potenciális célpontokat kerestünk. Bekapcsolódtunk egy kísérleti javaslat kidolgozásába, amelynek célja egy a Jupiter környezetébe küldendő űrszonda navigációjának pontosítása a kvazárok jelentette égi háttér és a VLBI technika segítségével. | We applied Very Long Baseline Interferometry (VLBI) for studies in extragalactic astrophysics and astrometry. We revealed the radio structure of the two most distant radio quasars known at present, with the finest angular resolution. The quasar J1427+3312 (redshift z=6.12) is resolved into two components, and is probably a very young object. We also studied other high-redshift (z>3) quasars, and various peculiar active galactic nuclei, including those with intra-day flux-density variability. This effect is caused by the scintillation in the Galactic interstellar clouds in the line of sight. We found that J1425+3231, a dual active galactic nucleus suspected from its double-peaked narrow optical emission lines, is indeed composed of two compact radio sources. A mildly relativistic radio jet was inferred in the Type Ic Supernova 2007gr. We studied the connection between the tiny apparent proper motions and the milliarcsecond-scale VLBI structures of compact radio sources that define the celestial reference frame. By comparing the radio and optical coordinates of a large sample of radio reference frame sources, we found a significant number of positional outliers. For future international Space VLBI missions, we selected new potential calibrator and weak target objects. We took part in an experiment proposal with the aim of the precise VLBI navigation of prospective spacecraft sent to Jupiter, using the compact extragalactic radio sources in the background

Is There a Blazar Nested in the Core of the Radio Galaxy 3C411?

Previous spectral energy distribution modeling based on XMM-Newton X-ray observation of the classical double-lobed radio galaxy 3C 411 left the possibility open for the presence of a blazar-like core. We investigated this scenario by characterizing the radio brightness distribution in the inner ~10-pc region of the source. We applied the very long baseline interferometry (VLBI) technique at four different frequencies from 1.7 to 7.6 GHz. We analyzed archival data from the Very Long Baseline Array (VLBA) taken in 2014, and data from the European VLBI Network (EVN) obtained in 2017. The VLBI images reveal pc-scale extended structure in the core of 3C 411 that can be modeled with multiple jet components. The measured core brightness temperatures indicate no Doppler enhancement that would be expected from a blazar jet pointing close to the line of sight. While there is no blazar-type core in 3C 411, we found indication of flux density variability. The overall morphology of the source is consistent with a straight jet with ~50{\deg} inclination angle at all scales from pc to kpc.Comment: 9 pages, 6 figures, 2 tables, accepted for publication in Ap