21 research outputs found
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
Optical and near-infrared spectroscopy of the black hole swift J1753.5-0127
We report on a multiwavelength observational campaign of the black hole (BH) X-ray binary Swift J1753.5-0127 that consists of an ESO/X-shooter spectrum supported by contemporaneous Swift/X-ray Telescope+Ultra-Violet/Optical Telescope (UVOT) and Australia Telescope Compact Array data. Interstellar medium absorption lines in the X-shooter spectrum allow us to determine E(B-V)=0.45+/- 0.02 along the line of sight to the source. We also report detection of emission signatures of He ii λ 4686, Hα, and, for the first time, H i λ 10906 and PaÎČ. The double-peaked morphology of these four lines is typical of the chromosphere of a rotating accretion disk. Nonetheless, the paucity of disk features points toward a low level of irradiation in the system. This is confirmed through spectral energy distribution modeling, and we find that the UVOT+X-shooter continuum mostly stems from the thermal emission of a viscous disk. We speculate that the absence of reprocessing is due to the compactness of an illumination-induced envelope that fails to reflect enough incoming hard X-ray photons back to the outer regions. The disk also marginally contributes to the Compton-dominated X-ray emission and is strongly truncated, with an inner radius about 1000 times larger than the BH's gravitational radius. A near-infrared excess is present, and we associate it with synchrotron radiation from a compact jet. However, the measured X-ray flux is significantly higher than what can be explained by the optically thin synchrotron jet component. We discuss these findings in the framework of the radio-quiet versus X-ray-bright hypothesis, favoring the presence of a residual disk, predicted by evaporation models, that contributes to the X-ray emission without enhancing the radio flux
TANAMI: Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry. III. First-epoch S band images
With the emergence of very high energy astronomy (VHE; E>100 GeV), new open
questions were presented to astronomers studying the multi-wavelength emission
from blazars. Answers to these open questions, such as the Doppler crisis, and
finding the location of the high-energy activity have eluded us thus far.
Recently, quasi-simultaneous multi-wavelength monitoring programs have shown
considerable success in investigating blazar activity. After the launch of the
Fermi Gamma-ray Space Telescope in 2008, such quasi-simultaneous observations
across the electromagnetic spectrum became possible. In addition, with very
long baseline interferometry (VLBI) observations we can resolve the central
parsec region of active galactic nuclei (AGN) and compare morphological changes
to the gamma-ray activity to study high-energy emitting blazars. To achieve our
goals, we need sensitive, long-term VLBI monitoring of a complete sample of VHE
detected AGN. We performed VLBI observations of TeV-detected AGN and high
likelihood neutrino associations as of December of 2021 with the Long Baseline
Array (LBA) and other southern hemisphere radio telescopes at 2.3 GHz. In this
paper we present first light TANAMI S-band images, focusing on the TeV-detected
sub-sample of the full TANAMI sample. Apart from these very high
energy-detected sources, we also show images of the two flux density
calibrators and two additional sources included in the observations. We study
the redshift, 0.1-100 GeV photon flux and S-band core brightness temperature
distributions of the TeV-detected objects, and find that flat spectrum radio
quasars and low synchrotron peaked sources on average show higher brightness
temperatures than high-synchrotron-peaked BL Lacs. Sources with bright GeV
gamma-ray emission also show higher brightness temperature values than
gamma-low sources
Upper Limits on Central Black Hole Masses of Globular Clusters from Radio Emission and a Possible Black Hole Detection in the Ursa Minor Dwarf Galaxy
Intermediate mass black holes have been alternatively predicted to be quite
common in the centers of globular clusters or nearly impossible to form and
retain in the centers of globular clusters. As it has been recently shown that
radio observations are currently the most sensitive observational technique for
detecting such objects, we have obtained new deep radio observations of Omega
Cen, and have re-analyzed older observations of M~15 in hope of constraining
the masses of possible black holes in their centers. In both cases, upper
limits of about 100 Jy are found at GHz frequencies. We find that if the
Bondi-Hoyle accretion rate truly represents the spherical accretion rate onto a
black hole, then the masses of the black holes in the centers of these two
galaxies are severely constrained - with mass limits of less than about 100
solar masses in both cases. If more realistic assumptions are made based on
recent work showing the Bondi rate to be a severe overestimate, then the data
for Omega Cen are marginally consistent with a black hole of about 1/1000 the
cluster's mass (i.e. about 1000 ). The data for M~15 then are only
marginally consistent with previous reports of a solar mass black
hole, and we note that there is considerable hope for either detecting the
black hole or improving this upper limit with current instrumentation. Finally,
we discuss the possibility that the radio source near the core of the Ursa
Minor dwarf spheroidal galaxy is a black hole.Comment: 6 pages, no figures, accepted to MNRAS Letter
Discovery of a Sub-Parsec Radio Counterjet in the Nucleus of Centaurus A
A sub-parsec scale radio counterjet has been detected in the nucleus of the
closest radio galaxy, Centaurus A (NGC 5128), with VLBI imaging at 2.3 and 8.4
GHz. This is one of the first detections of a VLBI counterjet and provides new
constraints on the kinematics of the radio jets emerging from the nucleus of
Cen A. A bright, compact core is seen at 8.4 GHz, along with a jet extending
along P.A. 51 degrees. The core is completely absorbed at 2.3 GHz. Our images
show a much wider gap between the base of the main jet and the counterjet at
2.3 GHz than at 8.4 GHz and also that the core has an extraordinarily inverted
spectrum. These observations provide evidence that the innermost 0.4-0.8 pc of
the source is seen through a disk or torus of ionized gas which is opaque at
low frequencies due to free-free absorption.Comment: 3 pages, 2 postscript figures, scheduled for publication in August 1,
1996 issue of Ap.J. Letter
Overview of the coordinated ground-based observations of Titan during the Huygens mission
Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titan's atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titan's atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Satumian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental imnortance for the interpretatinn of results from the Huygens mission
Identification of the new low-redshift GHz-peaked spectrum radio source and implications for the GHz-peaked spectrum class
The extragalactic radio source PKS B2254-367, associated with the galaxy IC 1459, is identified as a GHz-peaked spectrum (GPS) radio source. At a distance of 19 Mpc, PKS B2254-367 is one of the closest known GPS radio sources. IC 1459, NGC 1052 (18 Mpc) and NGC 6328 (56 Mpc) are the only GPS radio source host galaxies for which the detailed kinematics of the host galaxies can be studied. All three galaxies present strong evidence for merger activity, an actively fuelled black hole and high-density environments with which the radio sources interact. Although radio luminosity evolution is generally invoked to explain the apparent overabundance of GPS sources relative to radio galaxies, such strong evidence for merger-induced activity in the nuclei of nearby GPS host galaxies argues that at least some GPS radio sources are limited in their development by the effects of merger activity and the resulting likely sporadic fuelling of the central black holes and accretion discs that power the radio sources. The radio structure associated with NGC 1052 may directly suggest such a scenario
The Australian experience with the PC-EVN recorder
We report on our experiences using the MetsĂ€hovi Radio Observatory's (MRO) VLBI Standard Interface (VSI, Whitney 2002) recorder in a number of astronomical applications. The PC-EVN device is a direct memory access (DMA) interface which allows 512 megabit per second (Mbps) or better recording to âoff the shelfâ PC components. We have used this setup to record at 640 Mbps for a pulsar coherent dispersion system and at 256 Mpbs for a global VLBI session. We have also demonstrated recording at 512 Mbps and will shortly form cross correlations between the CPSR-II and the PC-EVN systems
Rapid interstellar scintillation of quasar PKS 1257-326
PKS 1257-326 is one of three quasars known to show unusually large and rapid, intra-hour intensity variations, as a result of scintillation in the turbulent Galactic interstellar medium. We have measured time delays in the variability pattern arrival times at the VLA and the ATCA, as well as an annual cycle in the time-scale of variability for this source. Results of the two-station time delay observations are presented here. Implications for the, scintillation of this source axe discussed in the light of these results, together with results from two years of monitoring with the ATCA