arXiv subjects

Z. Paragi

Publications and source records attributed to Z. Paragi.

At least 91 records · Page 5Linked 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

A Comprehensive Radio View of the Extremely Bright Gamma-Ray Burst 130427A

GRB 130427A was extremely bright as a result of occurring at low redshift whilst the energetics were more typical of high-redshift gamma-ray bursts (GRBs). We collected well-sampled light curves at 1.4 and 4.8 GHz of GRB 130427A with the Westerbork Synthesis Radio Telescope (WSRT); and we obtained its most accurate position with the European Very Long Baseline Interferometry Network (EVN). Our flux density measurements are combined with all the data available at radio, optical and X-ray frequencies to perform broadband modeling in the framework of a reverse-forward shock model and a two-component jet model, and we discuss the implications and limitations of both models. The low density inferred from the modeling implies that the GRB 130427A progenitor is either a very low-metallicity Wolf-Rayet star, or a rapidly rotating, low-metallicity O star. We also find that the fraction of the energy in electrons is evolving over time, and that the fraction of electrons participating in a relativistic power-law energy distribution is less than 15%. We observed intraday variability during the earliest WSRT observations, and the source sizes inferred from our modeling are consistent with this variability being due to interstellar scintillation effects. Finally, we present and discuss our limits on the linear and circular polarization, which are among the deepest limits of GRB radio polarization to date.

astro-ph.HE↗

arXiv2014

Constraints on the progenitor system and the environs of SN 2014J from deep radio observations

We report deep EVN and eMERLIN observations of the Type Ia SN 2014J in the nearby galaxy M 82. Our observations represent, together with JVLA observations of SNe 2011fe and 2014J, the most sensitive radio studies of Type Ia SNe ever. By combining data and a proper modeling of the radio emission, we constrain the mass-loss rate from the progenitor system of SN 2014J to $\dot{M} \lesssim 7.0\times 10^{-10}\, {\rm M_{\odot}\, yr^{-1}}$ (3-$σ$; for a wind speed of $100\, {\rm km s^{-1}}$). If the medium around the supernova is uniform, then $n_{\rm ISM} \lesssim 1.3 {\rm cm^3}$ (3-$σ$), which is the most stringent limit for the (uniform) density around a Type Ia SN. Our deep upper limits favor a double-degenerate (DD) scenario--involving two WD stars--for the progenitor system of SN 2014J, as such systems have less circumstellar gas than our upper limits. By contrast, most single-degenerate (SD) scenarios, i.e., the wide family of progenitor systems where a red giant, main-sequence, or sub-giant star donates mass to a exploding WD, are ruled out by our observations. Our estimates on the limits to the gas density surrounding SN 2011fe, using the flux density limits from Chomiuk et al. (2012), agree well with their results. Although we discuss possibilities for a SD scenario to pass observational tests, as well as uncertainties in the modeling of the radio emission, the evidence from SNe 2011fe and 2014J points in the direction of a DD scenario for both.

astro-ph.SR↗

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.

astro-ph.GA↗

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.

astro-ph.GA↗

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.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2013

LLAGN and jet-scaling probed with the EVN

Accreting black holes on all mass scales (from stellar to supermassive) appear to follow a nonlinear relation between X-ray luminosity, radio luminosity and BH mass, indicating that similar physical processes drive the central engines in X-ray binaries and active galactic nuclei (AGN). However, in recent years an increasing number of BH systems have been identified that do not fit into this scheme. These outliers may be the key to understand how BH systems are powered by accretion. Here we present results from EVN observations of a sample of low-luminosity AGN (LLAGN) with known mass that have unusually high radio powers when compared with their X-ray luminosity.

astro-ph.CO↗

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

Broadband monitoring tracing the evolution of the jet and disk in the black hole candidate X-ray binary MAXI J1659-152

MAXI J1659-152 was discovered on 2010 September 25 as a new X-ray transient, initially identified as a gamma-ray burst, but was later shown to be a new X-ray binary with a black hole as the most likely compact object. Dips in the X-ray light curves have revealed that MAXI J1659-152 is the shortest period black hole candidate identified to date. Here we present the results of a large observing campaign at radio, sub-millimeter, near-infrared (nIR), optical and ultraviolet (UV) wavelengths. We have combined this very rich data set with the available X-ray observations to compile a broadband picture of the evolution of this outburst. We have performed broadband spectral modeling, demonstrating the presence of a spectral break at radio frequencies and a relationship between the radio spectrum and X-ray states. Also, we have determined physical parameters of the accretion disk and put them into context with respect to the other parameters of the binary system. Finally, we have investigated the radio-X-ray and nIR/optical/UV-X-ray correlations up to ~3 years after the outburst onset to examine the link between the jet and the accretion disk, and found that there is no significant jet contribution to the nIR emission when the source is in the soft or intermediate X-ray spectral state, consistent with our detection of the jet break at radio frequencies during these states.

astro-ph.HE↗

arXiv2013

Radio and X-ray observations of jet ejection in Cygnus X-2

The ejection of a relativistic jet has been observed in the luminous Galactic low mass X-ray binary Cygnus X-2. Using high resolution radio observations, a directly resolved ejection event has been discovered while the source was on the Horizontal Branch of the Z-track. Contemporaneous radio and X-ray observations were made with the European VLBI Network at 6 cm and the Swift X-ray observatory in the 0.3 - 10 keV band. This has been difficult to achieve because of the previous inability to predict jet formation. Two sets of ~10 hr observations were spaced 12 hr apart, the jet apparently switching on during Day 1. The radio results show an unresolved core evolving into an extended jet. A preliminary value of jet velocity v/c of 0.33 +/- 0.12 was obtained, consistent with previous determinations in Galactic sources. Simultaneous radio and X-ray lightcurves are presented and the X-ray hardness ratio shows the source to be on the Horizontal Branch where jets are expected. The observations support our proposal that jet formation can in future be predicted based on X-ray intensity increases beyond a critical value.

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

VLBI observations of the shortest orbital period black hole binary, MAXI J1659-152

The X-ray transient MAXI J1659-152 was discovered by Swift/BAT and it was initially identified as a GRB. Soon its Galactic origin and binary nature were established. There exists a wealth of multi-wavelength monitoring data for this source, providing a great coverage of the full X-ray transition in this candidate black hole binary system. We obtained two epochs of European VLBI Network (EVN) electronic-VLBI (e-VLBI) and four epochs of Very Long Baseline Array (VLBA) data of MAXI J1659-152 which show evidence for outflow in the early phases. The overall source properties (polarization, milliarcsecond-scale radio structure, flat radio spectrum) are described well with the presence of a compact jet in the system through the transition from the hard-intermediate to the soft X-ray spectral state. The apparent dependence of source size and the radio core position on the observed flux density (luminosity dependent core shift) support this interpretation as well. We see no evidence for major discrete ejecta during the outburst. For the source proper motion we derive 2 sigma upper limits of 115 microas/day in right ascension, and 37 microas/day in declination, over a time baseline of 12 days. These correspond to velocities of 1400 km/s and 440 km/s, respectively, assuming a source distance of 7 kpc.

astro-ph.HE↗

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.

astro-ph.GA↗

arXiv2012

SS433, microquasars, and other transients

X-ray binaries have been an important key in understanding the jet-disc symbiosis in accreting black holes on all mass scales, from stellar-mass to supermassive black holes. SS433 was the first Galactic XRB that has been extensively studied in the radio regime. The radio properties, including the highest angular resolution data can now be better understood in the framework for accretion disc state transitions that is observed in microquasars (black hole X-ray binary systems). SS433 remains unique in various ways to date, and there is still much to learn about black hole accretion phenomena. In the meantime, the electronic very long baseline (e-VLBI) developments at the European VLBI Network (EVN) has allowed us to study microquasars and other transients at milliarcsecond resolutions more flexibly than was possible before. Even more new opportunities will arise as the SKA pathfinders become operational.

astro-ph.HE↗

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↗