222 research outputs found
A Flare in the Jet of Pictor A
A Chandra X-ray imaging observation of the jet in Pictor A showed a feature
that appears to be a flare that faded between 2000 and 2002. The feature was
not detected in a follow-up observation in 2009. The jet itself is over 150 kpc
long and a kpc wide, so finding year-long variability is surprising. Assuming a
synchrotron origin of the observed high-energy photons and a minimum energy
condition for the outflow, the synchrotron loss time of the X-ray emitting
electrons is of order 1200 yr, which is much longer than the observed
variability timescale. This leads to the possibility that the variable X-ray
emission arises from a very small sub-volume of the jet, characterized by
magnetic field that is substantially larger than the average over the jet.Comment: 12 pages, 3 figures, to appear in Ap. J. Letter
The Sub-parsec Scale Radio Properties of Southern Starburst Galaxies. I. Supernova Remnants, the Supernova Rate, and the Ionised Medium in the NGC 253 Starburst
Wide-field, very long baseline interferometry (VLBI) observations of the
nearby starburst galaxy NGC 253, obtained with the Australian Long Baseline
Array (LBA), have produced a 2.3 GHz image with a maximum angular resolution of
15 mas (0.3 pc). Six sources were detected, all corresponding to sources
identified in higher frequency (>5 GHz) VLA images. One of the sources,
supernova remnant 5.48-43.3, is resolved into a shell-like structure
approximately 90 mas (1.7 pc) in diameter. From these data and data from the
literature, the spectra of 20 compact radio sources in NGC 253 were modelled
and found to be consistent with free-free absorbed power laws. Broadly, the
free-free opacity is highest toward the nucleus but varies significantly
throughout the nuclear region (tau_0 ~ 1->20), implying that the overall
structure of the ionised medium is clumpy. Of the 20 sources, nine have flat
intrinsic spectra associated with thermal radio emission and the remaining 11
have steep intrinsic spectra, associated with synchrotron emission from
supernova remnants. A supernova rate upper limit of 2.4 yr^-1 is determined for
the inner 320 pc region of the galaxy at the 95% confidence level, based on the
lack of detection of new sources in observations spanning almost 17 years and a
simple model for the evolution of supernova remnants. A supernova rate of >0.14
(v/10^4) yr^-1 is implied from estimates of supernova remnant source counts,
sizes and expansion rates, where v is the radial expansion velocity of the
supernova remnant in km s^-1. A star formation rate of 3.4 (v/10^4) <
SFR(M<=5Msun) < 59 Msun yr^-1 has been estimated directly from the supernova
rate limits and is of the same order of magnitude as rates determined from
integrated FIR and radio luminosities.Comment: Accepted by the Astronomical Journal. 34 pages, 6 figures; fixed
typos in assumed expansion velocit
Deep Chandra observations of Pictor A
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. ©: 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.We report on deep Chandra observations of the nearby broad-line radio galaxy Pictor A, which we combine with new Australia Telescope Compact Array (ATCA) observations. The new X-ray data have a factor 4 more exposure than observations previously presented and span a 15-year time baseline, allowing a detailed study of the spatial, temporal and spectral properties of the AGN, jet, hotspot and lobes. We present evidence for further time variation of the jet, though the flare that we reported in previous work remains the most significantly detected time-varying feature. We also confirm previous tentative evidence for a faint counterjet. Based on the radio through X-ray spectrum of the jet and its detailed spatial structure, and on the properties of the counterjet, we argue that inverse-Compton models can be conclusively rejected, and propose that the X-ray emission from the jet is synchrotron emission from particles accelerated in the boundary layer of a relativistic jet. For the first time, we find evidence that the bright western hotspot is also time-varying in X-rays, and we connect this to the small-scale structure in the hotspot seen in high-resolution radio observations. The new data allow us to confirm that the spectrum of the lobes is in good agreement with the predictions of an inverse-Compton model and we show that the data favour models in which the filaments seen in the radio images are predominantly the result of spatial variation of magnetic fields in the presence of a relatively uniform electron distribution.Peer reviewe
A matched-filter approach to radio variability and transients: searching for orphan afterglows in the VAST Pilot Survey
Radio transient searches using traditional variability metrics struggle to
recover sources whose evolution timescale is significantly longer than the
survey cadence. Motivated by the recent observations of slowly evolving radio
afterglows at gigahertz frequency, we present the results of a search for radio
variables and transients using an alternative matched-filter approach. We
designed our matched-filter to recover sources with radio light curves that
have a high-significance fit to power-law and smoothly broken power-law
functions; light curves following these functions are characteristic of
synchrotron transients, including "orphan" gamma-ray burst afterglows, which
were the primary targets of our search. Applying this matched-filter approach
to data from Variables and Slow Transients Pilot Survey conducted using the
Australian SKA Pathfinder, we produced five candidates in our search.
Subsequent Australia Telescope Compact Array observations and analysis revealed
that: one is likely a synchrotron transient; one is likely a flaring active
galactic nucleus, exhibiting a flat-to-steep spectral transition over
months; one is associated with a starburst galaxy, with the radio emission
originating from either star formation or an underlying slowly-evolving
transient; and the remaining two are likely extrinsic variables caused by
interstellar scintillation. The synchrotron transient, VAST J175036.1181454,
has a multi-frequency light curve, peak spectral luminosity and volumetric rate
that is consistent with both an off-axis afterglow and an off-axis tidal
disruption event; interpreted as an off-axis afterglow would imply an average
inverse beaming factor , or equivalently, an average jet opening angle of deg.Comment: 20 pages, 6 figures; accepted for publication in MNRA
High-velocity OH megamasers in IRAS 20100-4156: Evidence for a Supermassive Black Hole
We report the discovery of new, high-velocity narrow-line components of the
OH megamaser in IRAS 20100-4156. Results from the Australian Square Kilometre
Array Pathfinder (ASKAP)'s Boolardy Engineering Test Array (BETA) and the
Australia Telescope Compact Array (ATCA) provide two independent measurements
of the OH megamaser spectrum. We found evidence for OH megamaser clumps at
409 and 562 km/s (blue-shifted) from the systemic velocity of the galaxy,
in addition to the lines previously known. The presence of such high velocities
in the molecular emission from IRAS 201004156 could be explained by a ~50 pc
molecular ring enclosing an approximately 3.8 billion solar mass black hole. We
also discuss two alternatives, i.e. that the narrow-line masers are dynamically
coupled to the wind driven by the active galactic nucleus or they are
associated with two separate galactic nuclei. The comparison between the BETA
and ATCA spectra provides another scientific verification of ASKAP's BETA. Our
data, combined with previous measurements of the source enabled us to study the
variability of the source over a twenty-six year period. The flux density of
the brightest OH maser components has reduced by more than a factor of two
between 1988 and 2015, whereas a secondary narrow-line component has more than
doubled in the same time. Plans for high-resolution VLBI follow-up of this
source are discussed, as are prospects for discovering new OH megamasers during
the ASKAP early science program.Comment: Accepted to MNRAS. Seven pages, three figure
Periodic Radio Emission from the T8 Dwarf WISE J062309.94-045624.6
We present the detection of rotationally modulated, circularly polarized
radio emission from the T8 brown dwarf WISE J062309.94-045624.6 between 0.9 and
2.0 GHz. We detected this high proper motion ultracool dwarf with the
Australian SKA Pathfinder in GHz imaging data from the Rapid ASKAP
Continuum Survey. We observed WISE J062309.94-045624.6 to have a time and
frequency averaged Stokes I flux density of mJy beam, with
an absolute circular polarization fraction of , and calculated a
specific radio luminosity of erg s Hz. In
follow-up observations with the Australian Telescope Compact Array and MeerKAT
we identified a multi-peaked pulse structure, used dynamic spectra to place a
lower limit of kG on the dwarf's magnetic field, and measured a
h periodicity which we concluded to be due to rotational
modulation. The luminosity and period we measured are comparable to those of
other ultracool dwarfs observed at radio wavelengths. This implies that future
megahertz to gigahertz surveys, with increased cadence and improved
sensitivity, are likely to detect similar or later-type dwarfs. Our detection
of WISE J062309.94-045624.6 makes this dwarf the coolest and latest-type star
observed to produce radio emission.Comment: Accepted for publication in ApJ Letters; 11 pages, 3 figures and 2
table
Classical Novae in the ASKAP Pilot Surveys
We present a systematic search for radio counterparts of novae using the
Australian Square Kilometer Array Pathfinder (ASKAP). Our search used the Rapid
ASKAP Continuum Survey, which covered the entire sky south of declination
( square degrees) at a central frequency of 887.5
MHz, the Variables and Slow Transients Pilot Survey, which covered
square degrees per epoch (887.5 MHz), and other ASKAP pilot surveys, which
covered square degrees with 2-12 hour integration times. We
crossmatched radio sources found in these surveys over a two-year period, from
April 2019 to August 2021, with 440 previously identified optical novae, and
found radio counterparts for four novae: V5668 Sgr, V1369 Cen, YZ Ret, and RR
Tel. Follow-up observations with the Australian Telescope Compact Array confirm
the ejecta thinning across all observed bands with spectral analysis indicative
of synchrotron emission in V1369 Cen and YZ Ret. Our light-curve fit with the
Hubble Flow model yields a value of
for the mass ejected in V1369 Cen. We also derive a peak surface brightness
temperature of K for YZ Ret. Using Hubble Flow model simulated radio
lightcurves for novae, we demonstrate that with a 5 sensitivity limit
of 1.5 mJy in 15-min survey observations, we can detect radio emission up to a
distance of 4 kpc if ejecta mass is in the range , and
upto 1 kpc if ejecta mass is in the range . Our
study highlights ASKAP's ability to contribute to future radio observations for
novae within a distance of 1 kpc hosted on white dwarfs with masses
, and within a distance of 4 kpc hosted on white dwarfs
with masses .Comment: This paper has been accepted for publication in PASA. It consists of
13 pages, 5 figures and 4 table
Discovery of a young, highly scattered pulsar PSR J1032-5804 with the Australian SKA Pathfinder
We report the discovery of a young, highly scattered pulsar in a search for
highly circularly polarized radio sources as part of the Australian Square
Kilometre Array Pathfinder (ASKAP) Variables and Slow Transients (VAST) survey.
In follow-up observations with Murriyang/Parkes, we identified PSR J1032-5804
and measured a period of 78.7 ms, dispersion measure (DM) of 8194 pc
cm, rotation measure of -20001 rad m, and a characteristic
age of 34.6 kyr. We found a pulse scattering timescale at 3 GHz of ~22 ms,
implying a timescale at 1 GHz of ~3845 ms, which is the third most scattered
pulsar known and explains its non-detection in previous pulsar surveys. We
discuss the identification of a possible pulsar wind nebula and supernova
remnant in the pulsar's local environment by analyzing the pulsar spectral
energy distribution and the surrounding extended emission from multiwavelength
images. Our result highlights the possibility of identifying extremely
scattered pulsars from radio continuum images. Ongoing and future large-scale
radio continuum surveys will offer us an unprecedented opportunity to find more
extreme pulsars (e.g., highly scattered, highly intermittent, highly
accelerated), which will enhance our understanding of the characteristics of
pulsars and the interstellar medium.Comment: 15 pages, 10 figures, 2 tables. Accepted for publication in Ap
Calibration and Stokes Imaging with Full Embedded Element Primary Beam Model for the Murchison Widefield Array
15 pages, 11 figures. Accepted for publication in PASA. © Astronomical Society of Australia 2017The Murchison Widefield Array (MWA), located in Western Australia, is one of the low-frequency precursors of the international Square Kilometre Array (SKA) project. In addition to pursuing its own ambitious science program, it is also a testbed for wide range of future SKA activities ranging from hardware, software to data analysis. The key science programs for the MWA and SKA require very high dynamic ranges, which challenges calibration and imaging systems. Correct calibration of the instrument and accurate measurements of source flux densities and polarisations require precise characterisation of the telescope's primary beam. Recent results from the MWA GaLactic Extragalactic All-sky MWA (GLEAM) survey show that the previously implemented Average Embedded Element (AEE) model still leaves residual polarisations errors of up to 10-20 % in Stokes Q. We present a new simulation-based Full Embedded Element (FEE) model which is the most rigorous realisation yet of the MWA's primary beam model. It enables efficient calculation of the MWA beam response in arbitrary directions without necessity of spatial interpolation. In the new model, every dipole in the MWA tile (4 x 4 bow-tie dipoles) is simulated separately, taking into account all mutual coupling, ground screen and soil effects, and therefore accounts for the different properties of the individual dipoles within a tile. We have applied the FEE beam model to GLEAM observations at 200 - 231 MHz and used false Stokes parameter leakage as a metric to compare the models. We have determined that the FEE model reduced the magnitude and declination-dependent behaviour of false polarisation in Stokes Q and V while retaining low levels of false polarisation in Stokes U.Peer reviewedFinal Accepted Versio
A Strong Jet Signature in the Late-Time Lightcurve of GW170817
We present new 0.6-10 GHz observations of the binary neutron star merger
GW170817 covering the period up to 300 days post-merger, taken with the Karl G.
Jansky Very Large Array, the Australia Telescope Compact Array, the Giant
Metrewave Radio Telescope and the MeerKAT telescope. We use these data to
precisely characterize the decay phase of the late-time radio light curve. We
find that the temporal decay is consistent with a power-law slope of t^-2.2,
and that the transition between the power-law rise and decay is relatively
sharp. Such a slope cannot be produced by a quasi-isotropic (cocoon-dominated)
outflow, but is instead the classic signature of a relativistic jet. This
provides strong observational evidence that GW170817 produced a successful jet,
and directly demonstrates the link between binary neutron star mergers and
short-hard GRBs. Using simple analytical arguments, we derive constraints on
the geometry and the jet opening angle of GW170817. These results are
consistent with those from our companion Very Long Baseline Interferometry
(VLBI) paper, reporting superluminal motion in GW170817.Comment: 11 pages, 3 figures, 3 tables. Accepted for publication in ApJ
Letter
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