Andrzej Marecki

FIRST-based survey of compact steep spectrum sources. V. Milliarcsecond-scale morphology of CSS objects

Aims. Multifrequency VLBA observations of the final group of ten objects in a sample of FIRST-based compact steep spectrum (CSS) sources are presented. The sample was selected to investigate whether objects of this kind could be relics of radio-loud AGNs switched off at very early stages of their evolution or possibly to indicate intermittent activity. Methods. Initial observations were made using MERLIN at 5 GHz. The sources have now been observed with the VLBA at 1.7, 5 and 8.4 GHz in a snapshot mode with phase-referencing. The resulting maps are presented along with unpublished 8.4-GHz VLA images of five sources. Results. Some of the sources discussed here show a complex radio morphology and therefore a complicated past that, in some cases, might indicate intermittent activity. One of the sources studied - 1045+352 - is known as a powerful radio and infrared-luminous broad absorption line (BAL) quasar. It is a young CSS object whose asymmetric two-sided morphology on a scale of several hundred parsecs, extending in two different directions, may suggest intermittent activity. The young age and compact structure of 1045+352 is consistent with the evolution scenario of BAL quasars. It has also been confirmed that the submillimetre flux of 1045+352 can be seriously contaminated by synchrotron emission.

Signatures of restarted activity in core-dominated, triple radio sources selected from the FIRST survey

Signatures of the re-occurrence of activity in radio-loud AGNs, indicated either by the so-called double-double or X-shaped structures, have been observed in a number of radio sources. All such objects known to date have linear sizes of the order of a megaparsec. A number of the sources that are appreciably more compact than this, but that exhibit hints of a past phase of activity, were found in the VLA FIRST survey. Their structures show symmetric relic lobes straddling relatively bright, unresolved cores. Observations of the cores of 15 such structures with MERLIN at 5 GHz have shown that four of them are doubles or core-jets on the subarcsecond scale. Misalignments of ∆PA > 30 ◦ between the axis of the inner structure and the line connecting the fitted maxima of the arcminute-scale relic lobes are clearly visible in three of the four sources. From these results, we can infer that a rapid repositioning of the central engine in each of these three radio sources is the most plausible interpretation of the observed morphology and that a merger is most likely the original cause of such a repositioning. In the case of TXS 1033+026, the optical image extracted from the SDSS archives clearly suggests that two objects separated by only 2.7 kpc (projected onto the sky plane) are indeed merging. The inner parts of TXS 0818+214 and TXS 1312+563 could be interpreted as double-lobed, and consequently, these sources could be of the double-double type; but further multifrequency observations are necessary to provide support for such an interpretation.

FIRST-based survey of Compact Steep Spectrum sources IV. Multifrequency VLBA observations of very compact objects

Context. Evidence has been mounting recently that activity in some radio-loud AGNs (RLAGNs) can cease shortly after ignition and that perhaps even a majority of very compact sources may be short-lived phenomena because of a lack of stable fuelling from the black hole. Thus, they can fade out before having evolved to large, extended objects. Re-ignition of the activity in such objects is not rul ed out. Aims. With the aim of finding more examples of these objects and to in vestigate if they could be RLAGNs switched off at very early stages of their evolution, multifrequency VLBA observations of six sources with angular sizes significantly less than an arcse cond, yet having steep spectra, have been made. Methods. Observations were initially made at 1.65 GHz using the VLBA with the inclusion of Effelsberg telescope. The sources were then re-observed with the VLBA at 5, 8.4 and 15.4 GHz. All the observations were carried out in a snapshot mode with phase referencing. Results. One of the sources studied, 0809+404, is dominated by a compact component but also has diffuse, arcsecond-scale emission visible in VLA images. The VLBI observations of the “core” structure have revealed that this is also diffuse and fading away at higher frequencies. Thus, the inner component of 0809+404 could be a compact fading object. The remaining five sourc es presented here show either core-jet or edge-brightened double-lobed structures indicating that they are in an active phase. Conclusions. The above result is an indication that the activity of the hos t galaxy of 0809+404 may be intermittent. Previous observations obtained from the literature and those presented here indicate that activity had ceased once in the past, then restarted , and has recently switched off again.

The transition from quasar radio-loud to radio-quiet state in the framework of the black hole scalability hypothesis

Aims. There are several lines of evidence that active galactic nuclei (AGN) can be regarded as scaled-up X-ray binaries (XRB). The timescales of the evolutionary phenomena in these two classes are proportional to the black hole (BH) masses. Consequently, unlike in the case of XRBs, the evolution of AGNs is too slow to be followed directly. What could be done, however, is to assign particular types of active galaxies to different evolutionary stages observable in XRBs. We studied such an assignment for three quasars with clear signatures of a recent transition from the radio-loud to the radio-quiet state. Methods. The quasars we investigated have large-scale radio lobes that are clearly asymmetric – one lobe is of Fanaroff-Riley II type, while the other one is a diffuse relic devoid of a hotspot. We suggest that the prime cause of the asymmetry of these radio sources is that the nuclei of their host galaxies currently produce no jets. To prove that, we observed them with milliarcsecond resolution to check if they are similar to those in radio-quiet quasars. Results. The observations carried out with the EVN revealed that the nuclei of the quasars under investigation are not of a core-jet type that is characteristic for radio-loud, lobe-dominated quasars. It follows that the lobes are no longer fuelled and that the apparent asymmetry results from the orientation, which causes a time lag of the order of 10 6 years between their images: the lobe perceived as a relic is nearer than the lobe with a hotspot and so it is observed in a later stage of the decay. Conclusions. The three AGNs under investigation were radio-loud earlier, but now they have switched to the radio-quiet state. In the framework of the XRB/AGN unification, the above means that they have left the very high state and have moved now to the high/soft state. If this scenario is correct it poses a challenge to the so-called spin paradigm. While a radio-loud AGN must have a spinning BH in its centre, the BH of a radio-quiet AGN does not necessarily have low spin; AGNs with high-spin BHs, like those we deal with here, may become radio-quiet.

Activity restart - a key to explaining the morphology of J1211+743

J1211+743 is a giant radio galaxy with a one-sided jet and two asymmetric lobes, one of which is of Fanaroff-Riley (FR) type II with a hotspot and the other is a diffuse relic devoid of a hotspot. The jet points towards the latt er lobe, which is diffi cult to explain in a standard way within the double-lobed radio source paradigm. Here, I propose to assume that the nucleus of J1211+743 has undergone a re-ignition of activity and its lobes, presumably both originally of FR II type, represent an earlier active phase, while the jet represents the current one. The asymmetry of the lobes is a consequence of the orientation of the source combined with an activity switch-off that occurred between two active periods. The relic lobe is on the near side with regard to the observer, whereas the radiation from the far-side lobe arrives significantly l ater owing to its longer distance to the observer. The far-si de lobe is thus perceived to have not yet decayed. On the other hand, the jet behaves in a standard way, i.e. its projected orientation refl ects the near side of the source. Hence, we are able to explain why the location of the relic lobe correlates with the direction of the jet .

Evidence of a double-double morphology in B 0818+214

The so-called double-double structure in radio sources is the most conspicuous signature of their restarted activity. Observations indicate that in the majority of double-double radio sources (DDRS), the span of the radio lobes is larger than 0.7 Mpc. This lower limit is also suggested by theory. However, it seemed likely that the apparent core of B 0818+214, a radio galaxy with an overall linear size of its radio structure below that limit, could harbour a compact double well aligned with the outer lobes so that the whole object would fulfil the criteria of a DDRS. Here, we present evidence that the central component of B 0818+214, when magnified through the EVN+MERLIN 18-cm observations, shows two FR II-like lobes. As the separation of the inner lobes is not greater than 5.7 kpc, they are immersed in the ISM of the host galaxy. This circumstance is the likely reason why the inner double has become visible, despite the predictions of the theory according to which B 0818+214 as a whole is too small for a new double to develop inside the cocoon inflated during the previous active phase. Moreover, we speculate that its host galaxy is not active at the moment and so the inner double may be in the coasting phase often observed in other medium-sized symmetric objects with intermittent activity. It could be, therefore, that two different mechanisms of accretion disk instabilities, ionisation and radiation-pressure driven, may be independently responsible for triggering active phases, manifesting as the outer and the inner doubles, respectively.

First-based survey of compact steep spectrum sources. III. MERLIN and VLBI observations of subarcsecond-scale objects

Context. According to a generally accepted paradigm, small intrinsic sizes of Compact Steep Spectrum (CSS) radio sources are a direct consequence of their youth, but in later stages of their evolution they are believed to become large-scale sources. However, this notion was established mainly for strong CSS sources. Aims. In this series of papers we test this paradigm on 60 weaker objects selected from the VLA FIRST survey. They have 5-GHz flux densities in the range 150 < S 5 GHz < 550 mJy and steep spectra in the range 0.365 ≤ ν ≤ 5 GHz. The present paper is focused on sources that fulfill the above criteria and have angular sizes in the range ∼0. �� 2–1 �� . Methods. Observations of 19 such sources were obtained using MERLIN in “snapshot” mode at 5 GHz. They are presented along with 1.7-GHz VLBA and 5-GHz EVN follow-up snapshot observations made for the majority of them. For one of the sources in this subsample, 1123+340, a full-track 5-GHz EVN observation was also carried out. Results. This study provides an important element to the standard theory of CSS sources, namely that in a number of them the activity of their host galaxies probably switched off quite recently and their further growth has been stopped because of that. In the case of 1123+340, the relic of a compact “dead source” is particularly well preserved by the presence of intracluster medium of the putative cluster of galaxies surrounding it. Conclusions. The observed overabundance of compact sources can readily be explained in the framework of the scenario of “premature” cessation of the activity of the host galaxy nucleus. It could also explain the relatively low radio flux densities of many such sources and, in a few cases, their peculiar, asymmetric morphologies. We propose a new interpretation of such asymmetries based on the light-travel time argument.

Are 3C 249.1 and 3C 334 restarted quasars

This Research Note follows up a Letter in which I posit that J1211+743 is a restarted radio source. This means that its structure, where the jet points to the relic lobe, is only apparently paradoxical. Here, I propose the same scenario and apply the same mathematical model to 3C249.1 and 3C334. The ultimate result of my investigation is that these two well-known radio-loud quasars can be understood best so far if it was assumed that they, too, had been restarted.

Fibre-optic delivery of time and frequency to VLBI station

The quality of Very Long Baseline Interferometry (VLBI) radio observations predominantly relies on precise and ultra-stable time and frequency (T&F) standards, usually hydrogen masers (HM), maintained locally at each VLBI station. Here, we present an operational solution in which the VLBI observations are routinely carried out without use of a local HM, but using remote synchronization via a stabilized, long-distance fibre-optic link. The T&F reference signals, traceable to international atomic timescale (TAI), are delivered to the VLBI station from a dedicated timekeeping laboratory. Moreover, we describe a proof-of-concept experiment where the VLBI station is synchronized to a remote strontium optical lattice clock during the observation.

Multi-epoch VLBA observations of radio galaxy 0932+075: is this a compact symmetric object?

A part of the radio structure of the galaxy 0932+075 emerged as a possible compact symmetric object (CSO) after the observation with the Very Long Baseline Array (VLBA) at 5 GHz in 1997. More than a decade later, we carried out observations at 5, 15.4, and 22.2 GHz using the VLBA to test this possibility. We report here that we have found a component whose spectrum is inverted in the whole range from 5 GHz to 22 GHz and we label it a high-frequency peaker (HFP). Using a set of 5 GHz images from two epochs separated by 11.8 years and a set of 15.4 GHz images separated by 8.2 years, we were able to examine the proper motions of the three components of the CSO candidate with respect to the HFP. We found that their displacements cannot be reconciled with the CSO paradigm. This has led to the rejection of the hypothesis that the western part of the arcsecond-scale radio structure of 0932+075 is a CSO anchored at the HFP. Consequently, the HFP cannot be labelled a core and its role in this system is unclear.