K. É. Gabányi

A spinning supermassive black hole binary model consistent with VLBI observations of the S5 1928+738 jet

Very Long Baseline Interferometry (VLBI) allows for high-resolution and high-sensitivity observations of relativistic jets, that can reveal periodicities of several years in their structure. We perform an analysis of long-term VLBI data of the quasar S5 1928+738 in terms of a geometric model of a helical structure projected onto the plane of the sky. We monitor the direction of the jet axis through its inclination and position angles. We decompose the variation of the inclination of the inner 2 milliarcseconds of the jet of S5 1928+738 into a periodic term with amplitude of ~0.89 deg and a linear decreasing trend with rate of ~0.05 deg/yr. We also decompose the variation of the position angle into a periodic term with amplitude of ~3.39 deg and a linear increasing trend with rate of ~0.24 deg/yr. We interpret the periodic components as arising from the orbital motion of a binary black hole inspiraling at the jet base and derive corrected values of the mass ratio and separation from the accumulated 18 years of VLBI data. Then we identify the linear trends in the variations as due to the slow reorientation of the spin of the jet emitter black hole induced by the spin-orbit precession and we determine the precession period T_SO=4852+/-646 yr of the more massive black hole, acting as the jet emitter. Our study provides indications, for the first time from VLBI jet kinematics, for the spinning nature of the jet-emitting black hole.

Constraining the parameters of the putative supermassive binary black hole in PG 1302–102 from its radio structure

We investigate the pc-scale kinematics and kpc-scale radio morphology of the quasar PG 1302-102, which may harbour a sub-pc separation supermassive binary black hole system at its centre as inferred from optical variability. High-resolution radio interferometric measurements obtained with the Very Long Baseline Array (VLBA) in the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE) programme at 15 GHz at 20 epochs spanning 17 years were analysed to investigate the pc-scale radio structure. Archival observations with the Very Large Array (VLA) at 1.4 GHz and 5 GHz were obtained to study the kpc-scale morphology. We find that the pc-scale jet is inclined within ~2.2 deg to the line of sight and has a half-opening angle of about 0.2 deg. The parameters derived from the pc-scale radio jet are qualitatively consistent with those obtained from the analysis of the optical light curve of PG 1302-102. We obtain at least 0.08 for the mass ratio of the two black holes in the system. We find some indication for a helical jet structure on kpc-scale, but the directions of the inner and the extended radio jets are significantly different, obstructing a straightforward connection of the pc- and kpc-scale jets within the binary scenario.

Two in one? A possible dual radio-emitting nucleus in the quasar SDSS J1425+3231

The radio-emitting quasar SDSS J1425+3231 (z = 0.478) was recently found to have double-peaked narrow [O III] optical emission lines. Based on the analysis of the optical spectrum, Peng et al. suggested that this object harbours a dual active galactic nucleus (AGN) system, with two supermassive black holes (SMBHs) separated on a kpc scale. SMBH pairs should be ubiquitous according to hierarchical galaxy formation scenarios in which the host galaxies and their central black holes grow together via interactions and eventual mergers. Yet the number of presently confirmed dual SMBHs on kpc or smaller scales remains small. A possible way to obtain direct observational evidence for duality is to conduct high-resolution radio interferometric measurements, provided that both AGN are in an evolutionary phase when some activity is going on in the radio. We used the technique of very long baseline interferometry (VLBI) to image SDSS J1425+3231. Observations made with the European VLBI Network at 1.7- and 5-GHz frequencies in 2011 revealed compact radio emission at sub-mJy flux density levels from two components with a projected linear separation of similar to 2.6 kpc. These two components support the possibility of a dual AGN system. The weaker component remained undetected at 5 GHz due to its steep radio spectrum. Further study will be necessary to securely rule out a jetshock interpretation of the less dominant compact radio source. Assuming the dual AGN interpretation, we discuss black hole masses, luminosities and accretion rates of the two components using available X-ray, optical and radio data. While high-resolution radio interferometric imaging is not an efficient technique to search blindly for dual AGN, it is an invaluable tool to confirm the existence of selected candidates.

The radio structure of 3C 316, a galaxy with double-peaked narrow optical emission lines

The galaxy 3C 316 is the brightest in the radio band among the optically selected candidates exhibiting double-peaked narrow optical emission lines. Observations with the Very Large Array, Multi-Element Remotely Linked Interferometer Network (e-MERLIN), and the European Very Long Baseline Interferometry Network (EVN) at 5 GHz have been used to study the radio structure of the source in order to determine the nature of the nuclear components and to determine the presence of radio cores. The e-MERLIN image of 3C 316 reveals a collimated coherent east-west emission structure with a total extent of about 3 kpc. The EVN image shows seven discrete compact knots on an S-shaped line. However, none of these knots could be unambiguously identified as an active galactic nucleus (AGN) core. The observations suggest that the majority of the radio structure belongs to a powerful radio AGN, whose physical size and radio spectrum classify it as a compact steep-spectrum source. Given the complex radio structure with radio blobs and knots, the possibility of a kpc-separation dual AGN cannot be excluded if the secondary is either a naked core or radio quiet.

A single radio-emitting nucleus in the dual AGN candidate NGC 5515

The Seyfert galaxy NGC 5515 has double-peaked narrow-line emission in its optical spectrum, and it has been suggested that this could indicate that it has two active nuclei. We observed the source with high-resolution very long baseline interferometry at two radio frequencies, reduced archival Very Large Array data, and re-analysed its optical spectrum. We detected a single, compact radio source at the position of NGC 5515, with no additional radio emission in its vicinity. The optical spectrum of the source shows that the blue and red components of the double-peaked lines have very similar characteristics. While we cannot rule out unambiguously that NGC 5515 harbours a dual AGN, the assumption of a single AGN provides a more plausible explanation for the radio observations and the optical spectrum.

Four dual AGN candidates observed with the VLBA

According to hierarchical structure formation models, merging galaxies are expected to be seen in different stages of their coalescence. However, currently there are no straightforward observational methods neither to select nor to confirm a large number of dual active galactic nuclei (AGN) candidates. Most attempts involve the better understanding of double-peaked narrow emission line sources, to distinguish the objects where the emission lines originate from narrow-line kinematics or jet-driven outflows from those which might harbour dual AGN. We observed four such candidate sources with the Very Long Baseline Array (VLBA) at 1.5 GHz with

Parsec-scale jet properties of the quasar PG 1302−102

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On the nature of bright compact radio sources at z > 4.5

10 milli-arcsecond angular resolution where spectral profiles of AGN optical emission suggested the existence of dual AGN. In SDSS J210449.13-000919.1 and SDSS J23044.82-093345.3, the radio structures are aligned with the optical emission features, thus the double-peaked emission lines might be the results of jet-driven outflows. In the third detected source SDSS J115523.74+150756.9, the radio structure is less extended and oriented nearly perpendicular to the position angle derived from optical spectroscopy. The fourth source remained undetected with the VLBA but it has been imaged with the Very Large Array at arcsec resolution a few months before our observations, suggesting the existence of extended radio structure. In none of the four sources did we detect two radio-emitting cores, a convincing signature of duality.

Radio properties of the γ -ray emitting CSO candidate 2234+282

The quasar PG 1302-102 is believed to harbour a supermassive binary black hole (SMBBH) system. Using the available 15 GHz and 2-8 GHz, multi-epoch Very Long Baseline Array data, we constrain the pc-scale jet properties based on the inferred mean proper motion, including a bulk Lorentz factor >= 5.1 +/- 0.8, jet inclination angle <= 11.degrees 4 +/- 1.degrees 7, projected position angle = 31.degrees 8, intrinsic half opening angle <= 0.degrees 9 +/- 0.degrees 1 and a mean 2-8 GHz spectral index of 0.31. A general relativistic helical jet model is presented and applied to predict quasi-periodic oscillations of similar to 10 d, power-law power spectrum shape and a contribution of up to similar to 53 per cent to the observed variable core flux density. The model is used to make a case for high resolution, moderately sampled, long duration radio interferometric observations to reveal signatures due to helical knots and distinguish them from those due to SMBBH orbital activity including a phase difference similar to pi and an amplitude ratio (helical light-curve amplitude/SMBBH light-curve amplitude) of 0.2-3.3. The prescription can be used to identify helical kinematic signatures from quasars, providing possible candidates for further studies with polarization measurements. It can also be used to infer promising SMBBH candidates for the study of gravitational waves if there are systematic deviations from helical signatures.

J0906+6930: a radio-loud quasar in the early Universe

High-redshift radio-loud quasars are used to, among other things, test the predictions of cosmological models, set constraints on black hole growth in the early universe and understand galaxy evolution. Prior to this paper, 20 extragalactic radio sources at redshifts above 4.5 have been imaged with very long baseline interferometry (VLBI). Here we report on observations of an additional ten z>4.5 sources at 1.7 and 5 GHz with the European VLBI Network (EVN), thereby increasing the number of imaged sources by 50%. Combining our newly observed sources with those from the literature, we create a substantial sample of 30 z>4.5 VLBI sources, allowing us to study the nature of these objects. Using spectral indices, variability and brightness temperatures, we conclude that of the 27 sources with sufficient information to classify, the radio emission from one source is from star formation, 13 are flat-spectrum radio quasars and 13 are steep-spectrum sources. We also argue that the steep-spectrum sources are off-axis (unbeamed) radio sources with rest-frame self-absorption peaks at or below GHz frequencies and that these sources can be classified as gigahertz peaked-spectrum (GPS) and megahertz peaked-spectrum (MPS) sources.