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Global millimeter VLBI array survey of ultracompact extragalactic radio sources at 86 GHz
Nair, Dhanya G.,Lobanov, Andrei P.,Krichbaum, Thomas P.,Ros, Eduardo,Zensus, Johann Anton,Kovalev, Yuri Y.,Lee, Sang-Sung,Mertens, Florent,Hagiwara, Yoshiaki,Bremer, Michael,Lindqvist, Michael,de Vice Springer-Verlag 2019 Astronomy and astrophysics Vol.622 No.-
<P><I>Context</I>. Very long baseline interferometry (VLBI) observations at 86 GHz (wavelength, <I>λ</I> = 3 mm) reach a resolution of about 50 <I>μ</I>as, probing the collimation and acceleration regions of relativistic outflows in active galactic nuclei (AGN). The physical conditions in these regions can be studied by performing 86 GHz VLBI surveys of representative samples of compact extragalactic radio sources.</P><P><I>Aims</I>. To extend the statistical studies of compact extragalactic jets, a large global 86 GHz VLBI survey of 162 compact radio sources was conducted in 2010-2011 using the Global Millimeter VLBI Array (GMVA).</P><P><I>Methods</I>. The survey observations were made in a snapshot mode, with up to five scans per target spread over a range of hour angles in order to optimize the visibility coverage. The survey data attained a typical baseline sensitivity of 0.1 Jy and a typical image sensitivity of 5 mJy beam<SUP>−1</SUP>, providing successful detections and images for all of the survey targets. For 138 objects, the survey provides the first ever VLBI images made at 86 GHz. Gaussian model fitting of the visibility data was applied to represent the structure of the observed sources and to estimate the flux densities and sizes of distinct emitting regions (components) in their jets. These estimates were used for calculating the brightness temperature (<I>T</I>b) at the jet base (core) and in one or more moving regions (jet components) downstream from the core. These model-fit-based estimates of <I>T</I>b were compared to the estimates of brightness temperature limits made directly from the visibility data, demonstrating a good agreement between the two methods.</P><P><I>Results</I>. The apparent brightness temperature estimates for the jet cores in our sample range from 2.5 × 10<SUP>9</SUP> K to 1.3 × 10<SUP>12</SUP> K, with the mean value of 1.8 × 10<SUP>11</SUP> K. The apparent brightness temperature estimates for the inner jet components in our sample range from 7.0 × 10<SUP>7</SUP> K to 4.0 × 10<SUP>11</SUP> K. A simple population model with a single intrinsic value of brightness temperature, <I>T</I>0, is applied to reproduce the observed distribution. It yields <I>T</I>0 = (3.77−0.14<SUP>+0.10</SUP>) × 10<SUP>11</SUP> K for the jet cores, implying that the inverse Compton losses dominate the emission. In the nearest jet components, <I>T</I>0 = (1.42−0.19<SUP>+0.16</SUP>) × 10<SUP>11</SUP> K is found, which is slightly higher than the equipartition limit of ∼5 × 10<SUP>10</SUP> K expected for these jet regions. For objects with sufficient structural detail detected, the adiabatic energy losses are shown to dominate the observed changes of brightness temperature along the jet.</P>
ACCELERATION OF COMPACT RADIO JETS ON SUB-PARSEC SCALES
Lee, Sang-Sung,Lobanov, Andrei P.,Krichbaum, Thomas P.,Zensus, J. Anton American Astronomical Society 2016 The Astrophysical journal Vol.826 No.2
<P>Jets of compact radio sources are highly relativistic and Doppler boosted, making studies of their intrinsic properties difficult. Observed brightness temperatures can be used to study the intrinsic physical properties of relativistic jets, and constrain models of jet formation in the inner jet region. We aim to observationally test such inner jet models. The very long baseline interferometry (VLBI) cores of compact radio sources are optically thick at a given frequency. The distance of the core from the central engine is inversely proportional to the frequency. Under the equipartition condition between the magnetic field energy and particle energy densities, the absolute distance of the VLBI core can be predicted. We compiled the brightness temperatures of VLBI cores at various radio frequencies of 2, 8, 15, and 86 GHz. We derive the brightness temperature on sub-parsec scales in the rest frame of the compact radio sources. We find that the brightness temperature increases with increasing distance from the central engine, indicating that the intrinsic jet speed (the Lorentz factor) increases along the jet. This implies that the jets are accelerated in the (sub-)parsec regions from the central engine.</P>
THE RADIO PROPERTIES OF RADIO-LOUD NARROW-LINE SEYFERT 1 GALAXIES ON PARSEC SCALES
Gu, Minfeng,Chen, Yongjun,Komossa, S.,Yuan, Weimin,Shen, Zhiqiang,Wajima, Kiyoaki,Zhou, Hongyan,Zensus, J. A. IOP Publishing 2015 The Astrophysical journal Supplement series Vol.221 No.1
<P>We present the detection of the compact radio structures of 14 radio-loud narrow-line Seyfert 1 (NLS1) galaxies from Very Long Baseline Array (VLBA) observations at 5 GHz performed in 2013. While 50% of the sources of our sample show a compact core only, the remaining 50% exhibit a core-jet structure. The measured brightness temperatures of the cores range from 10(8.4) to 10(11.4) K with a median value of 10(10.1) K, indicating that the radio emission is from non-thermal jets, and that, likely, most sources are not strongly beamed, thus implying a low jet speed in these radio-loud NLS1 galaxies. In combination with archival data taken at multiple frequencies, we find that seven sources show flat or even inverted radio spectra, while steep spectra are revealed in the remaining seven objects. Although all of these sources are very radio-loud with R > 100, their jet properties are diverse in terms of their milliarcsecond (mas) scale (parsec scale) morphology and their overall radio spectral shape. The evidence for slow jet speeds (i.e., less relativistic jets), in combination with the low kinetic/radio power, may offer an explanation for the compact VLBA radio structure in most sources. The mildly relativistic jets in these high accretion rate systems are consistent with a scenario where jets are accelerated from the hot corona above the disk by the magnetic field and the radiation force of the accretion disk. Alternatively, a low jet bulk velocity can be explained by low spin in the Blandford-Znajek mechanism.</P>