62 research outputs found
The Extreme Ultraviolet Spectra of Low Redshift Radio Loud Quasars
This paper reports on the extreme ultraviolet (EUV) spectrum of three low
redshift () radio loud quasars, 3C 95, 3C 57 and PKS 0405-123. The
spectra were obtained with the Cosmic Origins Spectrograph (COS) of the Hubble
Space Telescope. The bolometric thermal emission, , associated with
the accretion flow is a large fraction of the Eddington limit for all of these
sources. We estimate the long term time averaged jet power, , for
the three sources. , is shown to lie along the
correlation of and found in previous
studies of the EUV continuum of intermediate and high redshift quasars, where
the EUV continuum flux density between 1100 \AA\, and 700 \AA\, is defined by
. The high Eddington ratios of the three
quasars extends the analysis into a wider parameter space. Selecting quasars
with high Eddington ratios has accentuated the statistical significance of the
partial correlation analysis of the data. Namely. the correlation of
and is fundamental and the
correlation of and is spurious at a very high
statistical significance level (99.8\%). This supports the regulating role of
ram pressure of the accretion flow in magnetically arrested accretion models of
jet production. In the process of this study, we use multi-frequency and
multi-resolution Very Large Array radio observations to determine that one of
the bipolar jets in 3C 57 is likely frustrated by galactic gas that keeps the
jet from propagating outside the host galaxy.Comment: To appear in MNRA
Extreme radio-wave scattering associated with hot stars
We use data on extreme radio scintillation to demonstrate that this
phenomenon is associated with hot stars in the solar neighbourhood. The ionized
gas responsible for the scattering is found at distances up to 1.75pc from the
host star, and on average must comprise 1.E5 distinct structures per star. We
detect azimuthal velocities of the plasma, relative to the host star, up to 9.7
km/s, consistent with warm gas expanding at the sound speed. The circumstellar
plasma structures that we infer are similar in several respects to the cometary
knots seen in the Helix, and in other planetary nebulae. There the ionized gas
appears as a skin around tiny molecular clumps. Our analysis suggests that
molecular clumps are ubiquitous circumstellar features, unrelated to the
evolutionary state of the star. The total mass in such clumps is comparable to
the stellar mass.Comment: 9 pages, 1 figure, to appear in Ap
Revealing two radio active galactic nuclei extremely near PSR J04374715
Newton's gravitational constant may vary with time at an extremely low
level. The time variability of will affect the orbital motion of a
millisecond pulsar in a binary system and cause a tiny difference between the
orbital period-dependent measurement of the kinematic distance and the direct
measurement of the annual parallax distance. PSR J04374715 is the nearest
millisecond pulsar and the brightest at radio. To explore the feasibility of
achieving a parallax distance accuracy of one light-year, comparable to the
recent timing result, with the technique of differential astrometry, we
searched for compact radio sources quite close to PSR J04374715. Using
existing data from the Very Large Array and the Australia Telescope Compact
Array, we detected two sources with flat spectra, relatively stable flux
densities of 0.9 and 1.0 mJy at 8.4 GHz and separations of 13 and 45 arcsec.
With a network consisting of the Long Baseline Array and the Kunming 40-m radio
telescope, we found that both sources have a point-like structure and a
brightness temperature of 10 K. According to these radio inputs and
the absence of counterparts in the other bands, we argue that they are most
likely the compact radio cores of extragalactic active galactic nuclei rather
than Galactic radio stars. The finding of these two radio active galactic
nuclei will enable us to achieve a sub-pc distance accuracy with the in-beam
phase-referencing very-long-baseline interferometric observations and provide
one of the most stringent constraints on the time variability of in the
near future.Comment: 9 pages, 3 tables, 3 figures. Accepted for publication in MNRA
Time domain studies of Active Galactic Nuclei with the Square Kilometre Array
Variability of radio-emitting active galactic nuclei can be used to probe both intrinsic variations arising from shocks, flares, and other changes in emission from regions surrounding the central supermassive black hole, as well as extrinsic variations due to scattering by structures in our own Galaxy. Such interstellar scattering also probes the structure of the emitting regions, with microarcsecond resolution. Current studies have necessarily been limited to either small numbers of objects monitored over long periods of time, or large numbers of objects but with poor time sampling. The dramatic increase in survey speed engendered by the Square Kilometre Array will enable precision synoptic monitoring studies of hundreds of thousands of sources with a cadence of days or less. Statistics of variability, in particular concurrent observations at multiple radio frequencies and in other bands of the electromagnetic spectrum, will probe accretion physics over a wide range of AGN classes, luminosities, and orientations, as well as enabling a detailed understanding of the structures responsible for radio wave scattering in the Galactic interstellar medium
Finding Extremely Compact Sources Using the ASKAP VAST Survey
VLBI observations of intraday variable (IDV) quasars found in the MASIV (Micro-Arcsecond Scintillation-Induced Variability) 5 GHz VLA Survey of 500 flat-spectrum sources in the northern sky have shown that these sources are extremely compact, often unresolved, on milliarcsecond scales, and more core-dominated than their non-IDV counterparts. VAST: an ASKAP Survey for Variables and Slow Transients, proposes to observe 10,000 square degrees of southern sky daily for 2 years in the VAST-Wide survey component. This is expected to reveal of order 30,000 compact sources brighter than 10 mJy showing refractive interstellar scintillation (the cause of centimeter-wavelength IDV) at the survey frequency of about 1.4 GHz. Many of these sources may be suitable astrometric calibrators for VLBI at higher frequencies
The Highly Self-Absorbed Blazar, PKS\,1351018
PKS\,1351018 at a redshift of is one of the most luminous, steady
synchrotron sources with a luminosity \,erg~s. The
synchrotron luminosity does not seem to vary by more than over 35
years. In order to appreciate this remarkable behavior, if it were at ,
it would have a flux density at 15 GHz in a range of \,Jy over 11
yrs. In spite of this steady behavior, two strong -ray flares \,erg~s were detected in 2011 and 2016. There is blazar-like
behavior coexisting with the steady behavior. This study is aimed at
elucidating the dual nature of this source. We find that the radio source is
extremely compact with a bright core and a steep spectrum secondary component,
12\,mas away, that appears to be constant in position and flux density in six
epochs from 1995 to 2018. We estimate that a jet with a time averaged power of
\,erg~s terminates in this lobe that is
advancing at a deprojected distance of \,kpc from the
central engine. This is the rare case of a young (\,yr), very
powerful radio source that is viewed a few degrees from the jet axis. We find
evidence of a high velocity (4000\,km~s), high ionization wind emanating
form a luminous quasar. The young radio jet appears to experience modest
bending as it navigates through the intense quasar environment.Comment: To appear in Ap
ALMA observations of PKS 1549-79: A case of feeding and feedback in a young radio quasar
We present CO(1-0) and CO(3-2) ALMA observations of the molecular gas in PKS
1549-79, as well as mm and VLBI 2.3-GHz continuum observations of its radio
jet. PKS 1549-79 is one of the closest young, radio-loud quasars caught in an
on-going merger in which the AGN is in the first phases of its evolution. We
detect three structures tracing the accretion and the outflow of molecular gas:
kpc-scale tails of gas accreting onto PKS 1549-79, a circumnuclear disc (CND)
in the inner few hundred parsec, and a very broad (>2300 \kms) component
detected in CO(1-0) at the position of the AGN. Thus, in PKS 1549-79 we see the
co-existence of accretion and the ejection of gas. The line ratio
CO(1-0)/CO(3-2) suggests that the gas in the CND has both high densities and
high kinetic temperatures. We estimate a mass outflow rate of at least 650
msun/yr. This massive outflow is confined to r < 120 pc, which suggests that
the AGN drives the outflow. Considering the amount of molecular gas available
in CND and the observed outflow rate, we estimate a time scale of ~10^5 yr over
which the AGN would be able to destroy the CND, although gas from the merger
may come in from larger radii, rebuilding this disc at the same time. The AGN
appears to self-regulate gas accretion onto the super-massive black hole. From
a comparison with HST data, we find that the ionised gas outflow is more
extended. Nevertheless, the warm outflow is about two orders of magnitude less
massive than the molecular outflow. PKS 1549-79 does not seem to follow the
scaling relation between bolometric luminosity and the relative importance of
warm ionised and molecular outflows claimed to exist for other AGN. We argue
that, although PKS 1549-79 hosts a powerful quasar nucleus and an ultra-fast
outflow, the radio jet plays a significant role in producing the outflow.Comment: Accepted for A&A Main Journa
First geodetic observations using new VLBI stations ASKAP-29 and WARK12M
We report the results of a successful 7 hour 1.4 GHz VLBI experiment using
two new stations, ASKAP-29 located in Western Australia and WARK12M located on
the North Island of New Zealand. This was the first geodetic VLBI observing
session with the participation of these new stations. We have determined the
positions of ASKAP-29 and WARK12M. Random errors on position estimates are
150-200 mm for the vertical component and 40-50 mm for the horizontal
component. Systematic errors caused by the unmodeled ionosphere path delay may
reach 1.3 m for the vertical component.Comment: 11 pages, 6 flgures, 4 table
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