29 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
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
Milliarcsecond-Scale Structure in the Gamma-Ray Loud Quasar PKS 1622-297
We have made a high-resolution VLBI observation of the gamma-ray loud quasar
PKS 1622-297 with the HALCA spacecraft and ground radio telescopes at 5 GHz in
1998 February, almost three years after the source exhibited a spectacular GeV
gamma-ray flare. The source shows an elongated structure toward the west on the
parsec scale. The visibility data are well modeled by three distinct
components; a bright core and two weaker jet components. Comparison with
previous observations confirms that the jet components have an apparent
superluminal motion up to 12.1 h^{-1}c, with the inner jet components having
lower superluminal speeds. We apply the inverse Compton catastrophe model and
derive a Doppler factor, \delta, of 2.45, which is somewhat lower than that of
other gamma-ray loud active galactic nuclei (AGNs), suggesting the source was
in a more quiescent phase at the epoch of our observation. As an alternative
probe of the sub-parsec scale structure, we also present the results from
multi-epoch ATCA total flux monitoring, which indicate the presence of
persistent intraday variability consistent with refractive interstellar
scintillation. We examine the gamma-ray emission mechanism in the light of
these observations.Comment: 10 pages, 6 figures, 3 tables, to appear in PASJ, Vol.58, No.
Optical study of PKS B1322-110, the intra-hour variable radio source
Observations with the Australia Telescope Compact Array revealed intra-hour variations in the radio source PKS B1322-110 (Bignall et al. 2019). As part of an optical follow-up, we obtained Gemini Hα and Hα continuum (HαC) images of the PKS B1322-110 field. A robust 19 Ï detection of PKS B1322- 110 in the HαâHαC image prompted us to obtain the first optical spectrum of PKS B1322-110. With the Gemini spectrum we determine that PKS B1322-110 is a flat-spectrum radio quasar at a redshift of z = 3.007 ± 0.002. The apparent flux detected in the Hα filter is likely to originate from He ii emission redshifted precisely on the Galactic Hα narrow-band filter. We set upper limits on the emission measure of the Galactic plasma, for various possible cloud geometries
Optical study of PKS B1322-110, the intra-hour variable radio source
Observations with the Australia Telescope Compact Array revealed intra-hour
variations in the radio source PKS B1322-110 (Bignall et al. 2019). As part of
an optical follow-up, we obtained Gemini Halpha and Halpha continuum (HalphaC)
images of the PKS B1322-110 field. A robust 19-sigma detection of PKS B1322-110
in the Halpha-HalphaC image prompted us to obtain the first optical spectrum of
PKS B1322-110. With the Gemini spectrum we determine that PKS B1322-110 is a
flat-spectrum radio quasar at a redshift of z=3.007 +/- 0.002. The apparent
flux detected in the Halpha filter is likely to originate from HeII emission
redshifted precisely on the Galactic Halpha narrow-band filter. We set upper
limits on the emission measure of the Galactic plasma, for various possible
cloud geometries
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
The Radio Variability of the Gravitational Lens PMN J1838-3427
We present the results of a radio variability study of the gravitational lens
PMN J1838-3427. Our motivation was to determine the Hubble constant by
measuring the time delay between variations of the two quasar images. We
monitored the system for 4 months (approximately 5 times longer than the
expected delay) using the Australia Telescope Compact Array at 9 GHz. Although
both images were variable on a time scale of a few days, no correlated
intrinsic variability could be identified, and therefore no time delay could be
measured. Notably, the fractional variation of the fainter image (8%) was
greater than that of the brighter image (4%), whereas lensed images of a point
source would have the same fractional variation. This effect can be explained,
at least in part, as the refractive scintillation of both images due to the
turbulent interstellar medium of the Galaxy.Comment: To appear in AJ (8 pages, including 4 figures