297 research outputs found
Rapidly Evolving Circularly Polarized Emission during the 1994 Outburst of GRO J1665-40
We report the detection of circular polarization during the 1994 outburst of
the Galactic microquasar GRO J1655-40. The circular polarization is clearly
detected at 1.4 and 2.4GHz, but not at 4.8 and 8.4GHz, where its magnitude
never exceeds 5 mJy. Both the sign and magnitude of the circular polarization
evolve during the outburst. The time dependence and magnitude of the polarized
emission can be qualitatively explained by a model based on synchrotron
emission from the outbursts, but is most consistent with circular polarization
arising from propagation effects through the relativistic plasma surrounding
the object.Comment: 8 pages, 3 figs., A&A accepte
All Transients, All the Time: Real-Time Radio Transient Detection with Interferometric Closure Quantities
We demonstrate a new technique for detecting radio transients based on
interferometric closure quantities. The technique uses the bispectrum, the
product of visibilities around a closed-loop of baselines of an interferometer.
The bispectrum is calibration independent, resistant to interference, and
computationally efficient, so it can be built into correlators for real-time
transient detection. Our technique could find celestial transients anywhere in
the field of view and localize them to arcsecond precision. At the Karl G.
Jansky Very Large Array (VLA), such a system would have a high survey speed and
a 5-sigma sensitivity of 38 mJy on 10 ms timescales with 1 GHz of bandwidth.
The ability to localize dispersed millisecond pulses to arcsecond precision in
large volumes of interferometer data has several unique science applications.
Localizing individual pulses from Galactic pulsars will help find X-ray
counterparts that define their physical properties, while finding host galaxies
of extragalactic transients will measure the electron density of the
intergalactic medium with a single dispersed pulse. Exoplanets and active stars
have distinct millisecond variability that can be used to identify them and
probe their magnetospheres. We use millisecond time scale visibilities from the
Allen Telescope Array (ATA) and VLA to show that the bispectrum can detect
dispersed pulses and reject local interference. The computational and data
efficiency of the bispectrum will help find transients on a range of time
scales with next-generation radio interferometers.Comment: Accepted to ApJ. 8 pages, 5 figures, 2 tables. Revised to include
discussion of non-Gaussian statistics of techniqu
The Rotation Measure and 3.5mm Polarization of Sgr A*
We report the detection of variable linear polarization from Sgr A* at a
wavelength of 3.5mm, the longest wavelength yet at which a detection has been
made. The mean polarization is 2.1 +/- 0.1% at a position angle of 16 +/- 2 deg
with rms scatters of 0.4% and 9 deg over the five epochs. We also detect
polarization variability on a timescale of days. Combined with previous
detections over the range 150-400GHz (750-2000 microns), the average
polarization position angles are all found to be consistent with a rotation
measure of -4.4 +/- 0.3 x 10^5 rad/m^2. This implies that the Faraday rotation
occurs external to the polarized source at all wavelengths. This implies an
accretion rate ~0.2 - 4 x 10^-8 Msun/yr for the accretion density profiles
expected of ADAF, jet and CDAF models and assuming that the region at which
electrons in the accretion flow become relativistic is within 10 R_S. The
inferred accretion rate is inconsistent with ADAF/Bondi accretion. The
stability of the mean polarization position angle between disparate
polarization observations over the frequency range limits fluctuations in the
accretion rate to less than 5%. The flat frequency dependence of the inter-day
polarization position angle variations also makes them difficult to attribute
to rotation measure fluctuations, and suggests that both the magnitude and
position angle variations are intrinsic to the emission.Comment: Ap.J.Lett. accepte
The Performance and Calibration of the CRAFT Fly's Eye Fast Radio Burst Survey
Since January 2017, the Commensal Real-time ASKAP Fast Transients survey
(CRAFT) has been utilising commissioning antennas of the Australian SKA
Pathfinder (ASKAP) to survey for fast radio bursts (FRBs) in fly's eye mode.
This is the first extensive astronomical survey using phased array feeds
(PAFs), and a total of 20 FRBs have been reported. Here we present a
calculation of the sensitivity and total exposure of this survey, using the
pulsars B1641-45 (J1644-4559) and B0833-45 (J0835-4510, i.e.\ Vela) as
calibrators. The design of the survey allows us to benchmark effects due to PAF
beamshape, antenna-dependent system noise, radio-frequency interference, and
fluctuations during commissioning on timescales from one hour to a year.
Observation time, solid-angle, and search efficiency are calculated as a
function of FRB fluence threshold. Using this metric, effective survey
exposures and sensitivities are calculated as a function of the source counts
distribution. The implied FRB rate is significantly lower than the
\,sky\,day calculated using nominal exposures and
sensitivities for this same sample by \citet{craft_nature}. At the Euclidean
power-law index of , the rate is \,sky\,day above a threshold of \,Jy\,ms, while for the best-fit index for this sample of , it is
\,sky\,day above a threshold of \,Jy\,ms. This strongly suggests that these calculations be performed
for other FRB-hunting experiments, allowing meaningful comparisons to be made
between them.Comment: 21 pages, 15 figures, 2 tables, accepted for publication in PAS
Circularly polarized resonant soft x-ray diffraction study of helical magnetism in hexaferrite
Magnetic spiral structures can exhibit ferroelectric moments as recently
demonstrated in various multiferroic materials. In such cases the helicity of
the magnetic spiral is directly correlated with the direction of the
ferroelectric moment and measurement of the helicity of magnetic structures is
of current interest. Soft x-ray resonant diffraction is particularly
advantageous because it combines element selectivity with a large magnetic
cross-section. We calculate the polarization dependence of the resonant
magnetic x-ray cross-section (electric dipole transition) for the basal plane
magnetic spiral in hexaferrite Ba0.8Sr1.2Zn2Fe12O22 and deduce its domain
population using circular polarized incident radiation. We demonstrate there is
a direct correlation between the diffracted radiation and the helicity of the
magnetic spiral.Comment: 4 pages, 4 figure
Dual-Frequency Observations of 140 Compact, Flat-Spectrum Active Galactic Nuclei for Scintillation-Induced Variability
The 4.9 GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey
detected a drop in Interstellar Scintillation (ISS) for sources at redshifts z
> 2, indicating an apparent increase in angular diameter or a decrease in flux
density of the most compact components of these sources, relative to their
extended emission. This can result from intrinsic source size effects or
scatter broadening in the Intergalactic Medium (IGM), in excess of the expected
(1+z)^0.5 angular diameter scaling of brightness temperature limited sources
due to cosmological expansion. We report here 4.9 GHz and 8.4 GHz observations
and data analysis for a sample of 140 compact, flat-spectrum sources which may
allow us to determine the origin of this angular diameter-redshift relation by
exploiting their different wavelength dependences. In addition to using ISS as
a cosmological probe, the observations provide additional insight into source
morphologies and the characteristics of ISS. As in the MASIV Survey, the
variability of the sources is found to be significantly correlated with
line-of-sight H-alpha intensities, confirming its link with ISS. For 25
sources, time delays of about 0.15 to 3 days are observed between the
scintillation patterns at both frequencies, interpreted as being caused by a
shift in core positions when probed at different optical depths. Significant
correlation is found between ISS amplitudes and source spectral index; in
particular, a large drop in ISS amplitudes is observed at spectral indices of <
-0.4 confirming that steep spectrum sources scintillate less. We detect a
weakened redshift dependence of ISS at 8.4 GHz over that at 4.9 GHz, with the
mean variance at 4-day timescales reduced by a factor of 1.8 in the z > 2
sources relative to the z < 2 sources, as opposed to the factor of 3 decrease
observed at 4.9 GHz. This suggests scatter broadening in the IGM.Comment: 30 pages, 14 figures, accepted for publication in the Astronomical
Journa
Why Do Compact Active Galactic Nuclei at High Redshift Scintillate Less?
The fraction of compact active galactic nuclei (AGNs) that exhibit
interstellar scintillation (ISS) at radio wavelengths, as well as their
scintillation amplitudes, have been found to decrease significantly for sources
at redshifts z > 2. This can be attributed to an increase in the angular sizes
of the \muas-scale cores or a decrease in the flux densities of the compact
\muas cores relative to that of the mas-scale components with increasing
redshift, possibly arising from (1) the space-time curvature of an expanding
Universe, (2) AGN evolution, (3) source selection biases, (4) scatter
broadening in the ionized intergalactic medium (IGM) and intervening galaxies,
or (5) gravitational lensing. We examine the frequency scaling of this redshift
dependence of ISS to determine its origin, using data from a dual-frequency
survey of ISS of 128 sources at 0 < z < 4. We present a novel method of
analysis which accounts for selection effects in the source sample. We
determine that the redshift dependence of ISS is partially linked to the
steepening of source spectral indices ({\alpha}^8.4_4.9) with redshift, caused
either by selection biases or AGN evolution, coupled with weaker ISS in the
{\alpha}^8.4_4.9 < -0.4 sources. Selecting only the -0.4 < {\alpha}^8.4_4.9 <
0.4 sources, we find that the redshift dependence of ISS is still significant,
but is not significantly steeper than the expected (1+z)^0.5 scaling of source
angular sizes due to cosmological expansion for a brightness temperature and
flux-limited sample of sources. We find no significant evidence for scatter
broadening in the IGM, ruling it out as the main cause of the redshift
dependence of ISS. We obtain an upper limit to IGM scatter broadening of <
110\muas at 4.9 GHz with 99% confidence for all lines of sight, and as low as <
8\muas for sight-lines to the most compact, \sim 10\muas sources.Comment: 38 pages, 13 figures, accepted for publication in The Astrophysical
Journa
A Compact Extreme Scattering Event Cloud Towards AO 0235+164
We present observations of a rare, rapid, high amplitude Extreme Scattering
Event toward the compact BL-Lac AO 0235+164 at 6.65 GHz. The ESE cloud is
compact; we estimate its diameter between 0.09 and 0.9 AU, and is at a distance
of less than 3.6 kpc. Limits on the angular extent of the ESE cloud imply a
minimum cloud electron density of ~ 4 x 10^3 cm^-3. Based on the amplitude and
timescale of the ESE observed here, we suggest that at least one of the
transients reported by Bower et al. (2007) may be attributed to ESEs.Comment: 11 pages, 2 figure
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