29 research outputs found
The Re-Acceleration of the Shock Wave in the Radio Remnant of SN 1987A
We report on updated radio imaging observations of the radio remnant of
Supernova 1987A (SN 1987A) at 9 GHz, taken with the Australia Telescope Compact
Array (ATCA), covering a 25-year period (1992-2017). We use Fourier modeling of
the supernova remnant to model its morphology, using both a torus model and a
ring model, and find both models show an increasing flux density, and have
shown a continuing expansion of the remnant. As found in previous studies, we
find the torus model most accurately fits our data, and has shown a change in
the remnant expansion at Day 9,300 210 from 2,300 200 km/s to 3,610
240 km/s. We have also seen an increase in brightness in the western lobe
of the remnant, although the eastern lobe is still the dominant source of
emission, unlike what has been observed at contemporary optical and X-ray
wavelengths. We expect to observe a reversal in this asymmetry by the year
2020, and note the south-eastern side of the remnant is now beginning to
fade, as has also been seen in optical and X-ray data. Our data indicate that
high-latitude emission has been present in the remnant from the earliest stages
of the shockwave interacting with the equatorial ring around Day 5,000.
However, we find the emission has become increasingly dominated by the
low-lying regions by Day 9,300, overlapping with the regions of X-ray emission.
We conclude that the shockwave is now leaving the equatorial ring, exiting
first from the south-east region of the remnant, and is re-accelerating as it
begins to interact with the circumstellar medium beyond the dense inner ring.Comment: 22 pages, 14 figures. Accepted to Ap
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
Black Hole Powered Nebulae and a Case Study of the Ultraluminous X-ray Source IC342 X-1
We present new radio, optical, and X-ray observations of three Ultraluminous
X-ray sources (ULXs) that are associated with large-scale nebulae. We report
the discovery of a radio nebula associated with the ULX IC342 X-1 using the
Very Large Array (VLA). Complementary VLA observations of the nebula around
Holmberg II X-1, and high-frequency Australia Telescope Compact Array (ATCA)
and Very Large Telescope (VLT) spectroscopic observations of NGC5408 X-1 are
also presented. We study the morphology, ionization processes, and the
energetics of the optical/radio nebulae of IC342 X-1, Holmberg II X-1 and
NGC5408 X-1. The energetics of the optical nebula of IC342 X-1 is discussed in
the framework of standard bubble theory. The total energy content of the
optical nebula is 6 x 10^52 erg. The minimum energy needed to supply the
associated radio nebula is 9.2 x 10^50 erg. In addition, we detected an
unresolved radio source at the location of IC342 X-1 at VLA scales. However,
our Very Long Baseline Interferometry (VLBI) observations using the European
VLBI Network likely rule out the presence of any compact radio source at
milli-arcsecond (mas) scales. Using a simultaneous Swift X-ray Telescope
measurement, we estimate an upper limit on the mass of the black hole in IC342
X-1 using the "fundamental plane" of accreting black holes and obtain M_BH <
(1.0\pm0.3) x 10^3 M_Sun. Arguing that the nebula of IC342 X-1 is possibly
inflated by a jet, we estimate accretion rates and efficiencies for the jet of
IC342 X-1 and compare with sources like S26, SS433, IC10 X-1.Comment: 11 pages, 8 figures, accepted for publication in Ap
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
The Deep X-ray Radio Blazar Survey (DXRBS). I. Methods and First Results
We have undertaken a survey of archived, pointed ROSAT PSPC data for blazars
by correlating the ROSAT WGACAT database with several publicly available radio
catalogs, restricting our candidate list to serendipitous flat radio spectrum
sources (alpha_r <= 0.70). Here we discuss our survey methods, identification
procedure and first results. Our survey is found to be ~ 95% efficient at
finding flat-spectrum radio-loud quasars (FSRQs, 59 of our first 85 IDs) and BL
Lacertae objects (22 of our first 85 IDs), a figure which is comparable to or
greater than that achieved by other radio and X-ray survey techniques.
The identifications presented here show that all previous samples of blazars
(even when taken together) did not representatively survey the blazar
population, missing critical regions of (L_X,L_R) parameter space within which
large fractions of the blazar population lie. Particularly important is the
identification of a large population of FSRQs (>~ 25% of DXRBS FSRQs) with
ratios of X-ray to radio luminosity >~ 10^-6 (alpha_rx <~ 0.78). In addition,
due to our greater sensitivity, DXRBS has already more than doubled the number
of FSRQs in complete samples with 5 GHz (radio) luminosities between 10^31.5
and 10^33.5 erg/s/Hz and fills in the region of parameter space between X-ray
selected and radio-selected samples of BL Lacs. DXRBS is the very first sample
to contain statistically significant numbers of blazars at low luminosities,
approaching what should be the lower end of the FSRQ luminosity function.Comment: 34 pages, 7 figures, 6 tables, LaTeX file, uses aaspp4.sty. To appear
in the Astronomical Journa
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
An ultra-wide bandwidth (704 to 4 032 MHz) receiver for the Parkes radio telescope
We describe an ultra-wide-bandwidth, low-frequency receiver recently installed on the Parkes radio telescope. The receiver system provides continuous frequency coverage from 704 to 4032 MHz. For much of the band ( ), the system temperature is approximately 22 K and the receiver system remains in a linear regime even in the presence of strong mobile phone transmissions. We discuss the scientific and technical aspects of the new receiver, including its astronomical objectives, as well as the feed, receiver, digitiser, and signal processor design. We describe the pipeline routines that form the archive-ready data products and how those data files can be accessed from the archives. The system performance is quantified, including the system noise and linearity, beam shape, antenna efficiency, polarisation calibration, and timing stability