52 research outputs found
EVN & MERLIN studies of a new sample of BL Lac objects
The recent Deep X-ray Radio Blazar Survey (DXRBS) has identified a sample of BL Lac objects spanning the intermediate range of spectral energy distributions between âclassicalâ X-ray selected and radio-selected samples of BL Lacs. Detailed studies of such samples are needed to answer some of the currently open questions regarding the nature of BL Lacs and their place in a unified model of AGN. High-resolution radio imaging provides direct information on jet evolution and beaming parameters. We present some preliminary results from EVN & MERLIN observations of sources in the DXRBS BL Lac sample for which little or no high-resolution radio data were previously available
On rapid interstellar scintillation of quasars: PKS 1257-326 revisited
The line of sight towards the compact, radio loud quasar PKS 1257-326 passesthrough a patch of scattering plasma in the local Galactic ISM that causes large and rapid,intra-hour variations in the received flux density at centimetre wavelengths. This rapid interstellarscintillation (SS) has been occurring for at least 15 years, implying that the scatteringâscreenâ is at least 100 AU in physical extent. Through observations of the ISS we have measuredmicroarcsecond-scale âcore shiftsâ in PKS 1257-326, corresponding to changing opacityduring an intrinsic outburst. Recent analysis of VLA data of a sample of 128 quasars found 6sources scintillating with a characteristic time-scale of < 2 hours, suggesting that nearby scatteringscreens in the ISM may have a covering fraction of a few percent. That is an importantconsideration for proposed surveys of the transient and variable radio sk
Intraday variability of AGNs in the southern hemisphere
Understanding of the spectral and polarimetric characteristics of rapidly scintillating blazars is fundamental in order to describe both the innermost (sub-pc) regions of these compact objects and the interstellar medium responsible for the scintillation. A multi frequency analysis of the intraday variability in PMN J1326-5256, based on the combination of Australia Telescope Compact Array observations with the data from the monitoring projects at the University of Tasmania, will be described. Some implications concerning the structure of compact radio cores and the properties of the interstellar medium will be discussed
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
Rapid interstellar scintillation of PKS B1257-326: two-station pattern time delays and constraints on scattering and microarcsecond source structure
We report measurements of time delays of up to 8 minutes in the centimeter
wavelength variability patterns of the intra-hour scintillating quasar PKS
1257-326 as observed between the VLA and the ATCA on three separate epochs.
These time delays confirm interstellar scintillation as the mechanism
responsible for the rapid variability, at the same time effectively ruling out
the coexistence of intrinsic intra-hour variability in this source. The time
delays are combined with measurements of the annual variation in variability
timescale exhibited by this source to determine the characteristic length scale
and anisotropy of the quasar's intensity scintillation pattern, as well as
attempting to fit for the bulk velocity of the scattering plasma responsible
for the scintillation. We find evidence for anisotropic scattering and highly
elongated scintillation patterns at both 4.9 and 8.5 GHz, with an axial ratio >
10:1, extended in a northwest direction on the sky. The characteristic scale of
the scintillation pattern along its minor axis is well determined, but the high
anisotropy leads to degenerate solutions for the scintillation velocity. The
decorrelation of the pattern over the baseline gives an estimate of the major
axis length scale of the scintillation pattern. We derive an upper limit on the
distance to the scattering plasma of no more than 10 pc.Comment: 27 pages, 6 figures, accepted for publication 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
Effelsberg Monitoring of a Sample of RadioAstron Blazars: Analysis of Intra-Day Variability
We present the first results of an ongoing intra-day variability (IDV) flux
density monitoring program of 107 blazars, which were selected from a sample of
RadioAstron space very long baseline interferometry (VLBI) targets. The~IDV
observations were performed with the Effelsberg 100-m radio telescope at
4.8\,GHz, focusing on the statistical properties of IDV in a relatively large
sample of compact active galactic nuclei (AGN). We investigated the dependence
of rapid (3 day) variability on various source properties through a
likelihood approach. We found that the IDV amplitude depends on flux density
and that fainter sources vary by about a factor of 3 more than their brighter
counterparts. We also found a significant difference in the variability
amplitude between inverted- and flat-spectrum radio sources, with the former
exhibiting stronger variations. -ray loud sources were found to vary by
up to a factor 4 more than -ray quiet ones, with 4
significance. However a galactic latitude dependence was barely observed, which
suggests that it is predominantly the intrinsic properties (e.g., angular size,
core-dominance) of the blazars that determine how they scintillate, rather than
the directional dependence in the interstellar medium (ISM). We showed that the
uncertainty in the VLBI brightness temperatures obtained from the space VLBI
data of the RadioAstron satellite can be as high as 70\% due to the
presence of the rapid flux density variations. Our statistical results support
the view that IDV at centimeter wavelengths is predominantly caused by
interstellar scintillation (ISS) of the emission from the most compact,
core-dominant region in an AGN.Comment: 23 pages, 9 figures, published online by MDPI Galaxie
Interstellar scintillation as a cosmological probe: Prospects and challenges
The discovery that interstellar scintillation (ISS) is suppressed for compact radiosources at z 2 has enabled ISS surveys to be used as cosmological probes. We discuss brieflythe potential and challenges involved in such an undertaking, based on a dual-frequency surveyof ISS carried out to determine the origin of this redshift dependence
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