arXiv subjects

S. Frey

Publications and source records attributed to S. Frey.

At least 73 records · Page 4Linked to original sources

arXiv2014

The first estimate of radio jet proper motion at z>5

The extremely high redshift (z=5.3) radio source SDSS J102623.61+254259.5 (J1026+2542) is among the most distant and most luminous radio-loud active galactic nuclei (AGN) known to date. Its one-sided radio jet structure on milli-arcsecond (mas) and ~10-mas scales typical for blazars was first imaged at 5 GHz with very long baseline interferometry (VLBI) in 2006. Here we report on our dual-frequency (1.7 and 5 GHz) imaging observations performed with the European VLBI Network (EVN) in 2013. The prominent jet structure allows us to identify individual components whose apparent displacement can be detected over the time span of 7.33 yr. This is the first time when jet proper motions are directly derived in a blazar at z>5. The small values of up to ~0.1 mas/yr are consistent with what is expected in a relativistic cosmological model if redshift is a measure of distance. The apparent superluminal jet speeds, considered tentative because derived from two epochs only, exceed 10c for three different components along the jet. Based on modeling its spectral energy distribution, J1026+2542 is known to have its X-ray jet oriented close to the line of sight, with significant Doppler boosting and a large bulk Lorentz factor (Gamma~13). The new VLBI observations, indicating ~2.3 x 10^12 K lower limit to the core brightness temperature, are consistent with this picture. The spectral index in the core region is -0.35.

astro-ph.GA↗

arXiv2014

Future mmVLBI Research with ALMA: A European vision

Very long baseline interferometry at millimetre/submillimetre wavelengths (mmVLBI) offers the highest achievable spatial resolution at any wavelength in astronomy. The anticipated inclusion of ALMA as a phased array into a global VLBI network will bring unprecedented sensitivity and a transformational leap in capabilities for mmVLBI. Building on years of pioneering efforts in the US and Europe the ongoing ALMA Phasing Project (APP), a US-led international collaboration with MPIfR-led European contributions, is expected to deliver a beamformer and VLBI capability to ALMA by the end of 2014 (APP: Fish et al. 2013, arXiv:1309.3519). This report focuses on the future use of mmVLBI by the international users community from a European viewpoint. Firstly, it highlights the intense science interest in Europe in future mmVLBI observations as compiled from the responses to a general call to the European community for future research projects. A wide range of research is presented that includes, amongst others: - Imaging the event horizon of the black hole at the centre of the Galaxy - Testing the theory of General Relativity an/or searching for alternative theories - Studying the origin of AGN jets and jet formation - Cosmological evolution of galaxies and BHs, AGN feedback - Masers in the Milky Way (in stars and star-forming regions) - Extragalactic emission lines and astro-chemistry - Redshifted absorption lines in distant galaxies and study of the ISM and circumnuclear gas - Pulsars, neutron stars, X-ray binaries - Testing cosmology - Testing fundamental physical constants

astro-ph.IM↗

arXiv2014

A close-pair binary in a distant triple supermassive black-hole system

Galaxies are believed to evolve through merging, which should lead to multiple supermassive black holes in some. There are four known triple black hole systems, with the closest pair being 2.4 kiloparsecs apart (the third component is more distant at 3 kiloparsecs), which is far from the gravitational sphere of influence of a black hole with mass $\sim$10$^9$ M$_\odot$ (about 100 parsecs). Previous searches for compact black hole systems concluded that they were rare, with the tightest binary system having a separation of 7 parsecs. Here we report observations of a triple black hole system at redshift z=0.39, with the closest pair separated by $\sim$140 parsecs. The presence of the tight pair is imprinted onto the properties of the large-scale radio jets, as a rotationally-symmetric helical modulation, which provides a useful way to search for other tight pairs without needing extremely high resolution observations. As we found this tight pair after searching only six galaxies, we conclude that tight pairs are more common than hitherto believed, which is an important observational constraint for low-frequency gravitational wave experiments.

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arXiv2014

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 (VLBI) 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.

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arXiv2014

Probing the Active Massive Black Hole Candidate in the Center of NGC 404 with VLBI

Recently Nyland et al. (2012) argued that the radio emission observed in the center of the dwarf galaxy NGC 404 originates in a low-luminosity active galactic nucleus (LLAGN) powered by a massive black hole ($M\sim<10^6$ M$_{\odot}$). High-resolution radio detections of MBHs are rare. Here we present sensitive, contemporaneous Chandra X-ray, and very long baseline interferometry (VLBI) radio observations with the European VLBI Network (EVN). The source is detected in the X-rays, and shows no long-term variability. If the hard X-ray source is powered by accretion, the apparent low accretion efficiency would be consistent with a black hole in the hard state. Hard state black holes are known to show radio emission compact on the milliarcsecond scales. However, the central region of NGC 404 is resolved out on 10 milliarcsecond (0.15-1.5 pc) scales. Our VLBI non-detection of a compact, partially self-absorbed radio core in NGC 404 implies that either the black hole mass is smaller than $3^{+5}_{-2}\times10^5$ M$_{\odot}$, or the source does not follow the fundamental plane of black hole activity relation. An alternative explanation is that the central black hole is not in the hard state. The radio emission observed on arcsecond (tens of pc) scales may originate in nuclear star formation or extended emission due to AGN activity, although the latter would not be typical considering the structural properties of low-ionization nuclear emission-line region galaxies (LINERs) with confirmed nuclear activity.

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arXiv2014

VLBI observations of the radio quasar J2228+0110 at z=5.95 and other field sources in multiple-phase-centre mode

A patch of sky in the SDSS Stripe 82 was observed at 1.6 GHz with Very Long Baseline Interferometry (VLBI) using the European VLBI Network (EVN). The data were correlated at the EVN software correlator at JIVE (SFXC). There are fifteen known mJy/sub-mJy radio sources in the target field defined by the primary beam size of a typical 30-m class EVN radio telescope. The source of particular interest is a recently identified high-redshift radio quasar J222843.54+011032.2 (J2228+0110) at redshift z=5.95. Our aim was to investigate the milli-arcsecond (mas) scale properties of all the VLBI-detectable sources within this primary beam area with a diameter of 20 arcmin. The source J2228+0110 was detected with VLBI with a brightness temperature T_b>10^8 K, supporting the active galactic nucleus (AGN) origin of its radio emission, which is conclusive evidence that the source is a radio quasar. In addition, two other target sources were also detected, one of them with no redshift information. Their brightness temperature values (T_b >10^7 K) measured with VLBI suggest a non-thermal synchrotron radiation origin for their radio emission. The detection rate of 20% is broadly consistent with other wide-field VLBI experiments carried out recently. We also derived the accurate equatorial coordinates of the three detected sources using the phase-referencing technique. This experiment is an early attempt of a wide-field science project with SFXC, paving the way for the EVN to conduct a large-scale VLBI survey in the multiple-phase-centre mode.

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arXiv2013

Unveiling recurrent jets of the ULX Holmberg II X-1: evidence for a massive stellar-mass black hole?

We report on the discovery of an apparent triple radio structure hidden inside the radio bubble of the ultraluminous X-ray source Holmberg II X-1. The morphology is consistent with a collimated jet structure, which is observed to emit optically thin synchrotron radiation. The central component has a steep radio spectrum and is brighter than the outer components indicating a renewed radio activity. We estimate a minimum time-averaged jet power of 2 x 10^{39} erg/s that is associated with a time-averaged isotropic X-ray luminosity of at least 4 x 10^{39} erg/s. Our results suggest that Holmberg II X-1 is powered by a black hole of M_BH \geq 25 M_sun, that is inferred to be accreting at a high Eddington rate with intermittent radio activity.

astro-ph.HE↗

arXiv2013

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 (VLA), Multi-Element Remotely Linked Interferometer Network (e-MERLIN), and the European VLBI 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 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.

astro-ph.CO↗

arXiv2013

Optical-radio positional offsets for active galactic nuclei

Context. It will soon become possible to directly link the most accurate radio reference frame with the Gaia optical reference frame using many common extragalactic objects. It is important to know the level of coincidence between the radio and optical positions of compact active galactic nuclei (AGN). Aims. Using the best catalogues available at present, we investigate how many AGN with significantly large optical-radio positional offsets exist as well as the possible causes of these offsets. Methods. We performed a case study by finding optical counterparts to the International Celestial Reference Frame (ICRF2) radio sources in the Sloan Digital Sky Survey (SDSS) Data Release 9 (DR9). The ICRF2 catalogue was used as a reference because the radio positions determined by Very Long Baseline Interferometry (VLBI) observations are about two orders of magnitude more accurate than the optical positions. Results. We find 1297 objects in common for ICRF2 and SDSS DR9. Statistical analysis of the optical-radio differences verifies that the SDSS DR9 positions are accurate to ~55 mas in both coordinates, with no systematic offset with respect to ICRF2. We find 51 sources (~4% of the sample) for which the positional offset exceeds 170 mas (~3σ). Astrophysical explanations must exist for most of these outliers. There are 3 known strong gravitational lenses among them. Dual AGN or recoiling supermassive black holes may also be possible. Conclusions. The most accurate Gaia-VLBI reference frame link will require a careful selection of a common set of objects by eliminating the outliers. On the other hand, the significant optical-radio positional non-coincidences may offer a new tool for finding e.g. gravitational lenses or dual AGN candidates. Detailed follow-up radio interferometric and optical spectroscopic observations are encouraged to investigate the outlier sources found in this study.

astro-ph.CO↗

arXiv2013

A compact radio source in the high-redshift soft gamma-ray blazar IGR J12319-0749

Context. Blazars are powerful active galactic nuclei (AGNs) radiating prominently in the whole electromagnetic spectrum, from the radio to the X-ray and gamma-ray bands. Their emission is dominated by synchrotron and inverse-Compton radiation from a relativistic jet originating from an accreting central supermassive black hole. The object IGR J12319-0749 has recently been identified as a soft gamma-ray source with the IBIS instrument of the INTEGRAL satellite, coincident with a quasar at high redshift (z=3.12). Aims. We studied the radio structure of IGR J12319-0749 to strengthen its blazar identification by detecting a compact radio jet on the milli-arcsecond (mas) angular scale, and to measure its astrometric position accurate to mas level. Methods. We used the technique of electronic very long baseline interferometry (e-VLBI) to image IGR J12319-0749 with the European VLBI Network (EVN) at 5 GHz on 2012 June 19. Results. IGR J12319-0749 (J1231-0747) is a compact radio source, practically unresolved on interferometric baselines up to ~136 million wavelengths. The estimated brightness temperature (at least ~2 x 10^12 K) indicates that the radio emission of its jet is Doppler-boosted. The accurate position of the compact radio source is consistent with the positions measured at higher energies.

astro-ph.CO↗

arXiv2013

On the Doppler boosting in the compact radio jet of the distant blazar J1026+2542 at z=5.3

Based on its broad-band spectral energy distribution, and the X-ray spectrum in particular, the radio-loud active galactic nucleus (AGN) SDSS J102623.61+254259.5 (J1026+2542) has recently been classified as a blazar. The extremely high redshift of the source, z=5.3, makes it one of the most distant and most luminous radio-loud AGN known to date. From published 5-GHz very long baseline interferometry (VLBI) imaging data obtained in 2006, the source has a typical blazar appearance on mas scales, with a prominent one-sided jet extending to ~20 mas. We estimate the brightness temperature of J1026+2542 and find no strong evidence for Doppler boosting. The jet viewing angle is possibly at least ~20 deg. The bulk Lorentz factor and the viewing angle of the jet could reliably be determined in the near future from multi-epoch VLBI observations.

astro-ph.CO↗

arXiv2012

VLBI search for the radio counterpart of HESS J1943+213

HESS J1943+213, a TeV point source close to the Galactic plane recently discovered by the H.E.S.S. collaboration, was proposed to be an extreme BL Lacertae object, though a pulsar wind nebula (PWN) nature could not be completely discarded. To investigate its nature, we performed high-resolution radio observations with the European Very Long Baseline Interferometry Network (EVN) and reanalyzed archival continuum and H {\sc i} data. The EVN observations revealed a compact radio counterpart of the TeV source. The low brightness temperature and the resolved nature of the radio source are indications against the beamed BL Lacertae hypothesis. The radio/X-ray source appears immersed in a $\sim$ 1\arcmin elliptical feature suggesting a possible galactic origin (PWN nature) for the HESS source. We found that HESS\,J1943+213 is located in the interior of a $\sim1\degr$ diameter H {\sc i} feature, and explored the possibility of they being physically related.

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arXiv2012

Redshift, Time, Spectrum - the most distant radio quasars with VLBI

The highest-redshift quasars are still rare and valuable objects for observational astrophysics and cosmology. They provide important constraints on the growth of the earliest supermassive black holes in the Universe, and information on the physical conditions in their environment. Among the nearly 60 quasars currently known at redshifts z>5.7, only a handful are "strong" emitters in radio continuum. These can be targets of sensitive high-resolution Very Long Baseline Interferometry (VLBI) observations to reveal their innermost structure, down to ~10 pc linear scales. We review the results of our earlier European VLBI Network (EVN) experiments on three of the most distant radio quasars known to date, and give a preliminary report on the EVN detection of a fourth one. The results obtained so far suggest that we see really young active galactic nuclei - not just in a cosmological sense but also in terms of their active life in radio.

astro-ph.CO↗

arXiv2012

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. (2011) suggested that this object harbours a dual active galactic nucleus (AGN) system, with two supermassive black holes (SMBHs) separated on the 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 (EVN) at 1.7 GHz 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 \sim2.6 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 jet--shock 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.

astro-ph.CO↗

arXiv2012

Radio-optical outlier quasars - a case study with ICRF2 and SDSS

With Gaia, it will become possible to directly link the radio and optical reference frames using a large number of common objects. For the most accurate radio-optical link, it is important to know the level of spatial coincidence between the quasars' optical positions, and the radio positions determined by Very Long Baseline Interferometry (VLBI) observations. The "outlier" objects, for which the positions are significantly offset at the two different electromagnetic wavebands, may be of astrophysical interest as well. Here we present a case study to compare the radio positions of ~800 quasars common in the second realization of the International Celestial Reference Frame (ICRF2) and in the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) catalogue. Compared to the radio ICRF2, the SDSS provides two orders of magnitude less accurate astrometric data in the optical. However, its extensive sky coverage and faint magnitude limit allow us to directly relate the positions of a large sample of radio sources. This way we provide an independent check of the overall accuracy of the SDSS positions and confirm that the astrometric calibration of the latest Data Release 8 (DR8) is poorer than that of the DR7. We find over 20 sources for which the optical and radio brightness peaks are apparently not coincident at least at the 3-sigma level of SDSS DR7 positional accuracy, and briefly discuss the possible causes, including dual active galactic nuclei.

astro-ph.CO↗

arXiv2011

WMAP extragalactic point sources as potential Space VLBI calibrators

The point source list of the Wilkinson Microwave Anisotropy Probe (WMAP) is a uniform, all-sky catalogue of bright sources with flux density measurements at high (up to 94 GHz) radio frequencies. We investigated the five-year WMAP list to compile a new catalogue of bright and compact extragalactic radio sources to be potentially studied with Very Long Baseline Interferometry at millimeter wavelengths (mm-VLBI) and Space VLBI (SVLBI). After comparing the WMAP data with the existing mm-VLBI catalogues, we sorted out the yet unexplored sources. Using the 41, 61 and 94 GHz WMAP flux densities, we calculated the spectral indices. By collecting optical identifications, lower-frequency radio flux densities and VLBI images from the literature, we created a list of objects which have not been investigated with VLBI at 86 GHz before. With total flux density at least 1 Jy and declination above -40 degree, we found 37 suitable new targets. It is a nearly 25% addition to the known mm-VLBI sources. Such objects are also potentially useful as phase-reference calibrators for the future Japanese SVLBI mission ASTRO-G at its highest observing frequency (43 GHz). The phase-referencing capability of ASTRO-G would allow long integrations and hence better sensitivity for observing faint target sources close to suitable phase calibrators in the sky.

astro-ph.CO↗

arXiv2011

Into the central 10 pc of the most distant known radio quasar. VLBI imaging observations of J1429+5447 at z=6.21

Context: There are about 60 quasars known at redshifts z>5.7 to date. Only three of them are detected in the radio above 1 mJy flux density at 1.4 GHz frequency. Among them, J1429+5447 (z=6.21) is the highest-redshift radio quasar known at present. These rare, distant, and powerful objects provide important insight into the activity of the supermassive black holes in the Universe at early cosmological epochs, and on the physical conditions in their environment. Aims: We studied the compact radio structure of J1429+5447 on the milli-arcsecond (mas) angular scale, in order to compare the structural and spectral properties with those of other two z~6 radio-loud quasars, J0836+0054 (z=5.77) and J1427+3312 (z=6.12). Methods: We performed Very Long Baseline Interferometry (VLBI) imaging observations of J1429+5447 with the European VLBI Network (EVN) at 1.6 GHz on 2010 June 8, and at 5 GHz on 2010 May 27. Results: Based on its observed radio properties, the compact but somewhat resolved structure on linear scales of <100 pc, and the steep spectrum, the quasar J1429+5447 is remarkably similar to J0836+0054 and J1427+3312. To answer the question whether the compact steep-spectrum radio emission is a "universal" feature of the most distant radio quasars, it is essential to study more, yet to be discovered radio-loud active galactic nuclei at z>6.

astro-ph.CO↗

arXiv2011

The most distant radio quasars at the highest resolution

There are about 50 quasars known at redshifts z>5.7 to date. Only three of them are detected in the radio (J0836+0054, z=5.77; J1427+3312, z=6.12; J1429+5447, z=6.21). The highest-redshift quasars are in the forefront of current astrophysical and cosmological research since they provide important constraints on the growth of the earliest supermassive black holes in the Universe, and on the physical conditions in their environment. These sources are indeed associated with active galactic nuclei as revealed by high-resolution Very Long Baseline Interferometry (VLBI) observations. It is still unclear whether the physical properties of the few z~6 radio quasars are in general similar to those of their lower-redshift cousins. In the case of J1427+3312, the 100-pc scale double morphology suggests a young radio source. Here we report on the recent European VLBI Network (EVN) imaging observations of J1429+5447, the most distant radio quasar known. Based on its milli-arcsecond-scale structural and spectral properties, this quasar is similar to J0836+0054 and J1427+3312. This raises the question if the compact steep-spectrum radio emission is a "universal" feature of the most distant radio quasars. It could only be answered after many more objects at z>6 are discovered and studied.

astro-ph.CO↗