93 research outputs found
Disentangling interstellar plasma screens with pulsar VLBI: Combining auto- and cross-correlations
Pulsar scintillation allows a glimpse into small-scale plasma structures in
the interstellar medium, if we can infer their properties from the
scintillation pattern. With Very Long Baseline Interferometry and working in
delay-delay rate space, where the contributions of pairs of images to the
interference pattern become localized, the scattering geometry and distribution
of scattered images on the sky can be determined if a single,
highly-anisotropic scattering screen is responsible for the scintillation.
However, many pulsars are subject to much more complex scattering environments
where this method cannot be used. We present a novel technique to reconstruct
the scattered flux of the pulsar and solve for the scattering geometry in these
cases by combining interferometric visibilities with cross-correlations of
single-station intensities. This takes advantage of the fact that, considering
a single image pair in delay-delay rate space, the visibilities are sensitive
to the sum of the image angular displacements, while the cross-correlated
intensities are sensitive to the difference, so that their combination can be
used to localize both images of the pair. We show that this technique is able
to reconstruct the published scattering geometry of PSR B0834+06, then apply it
to simulations of more complicated scattering systems, where we find that it
can distinguish features from different scattering screens even when the
presence of multiple screens is not obvious in the Fourier transform of the
dynamic spectrum. This technique will allow us to both better understand the
distribution of scattering within the interstellar medium and to apply current
scintillometry techniques, such as modelling scintillation and constraining the
location of pulsar emission, to sources for which a current lack of
understanding of the scattering environment precludes the use of these
techniques. (abridged)Comment: Submitted to MNRAS; comments welcom
Multiepoch VLBI of L Dwarf Binary 2MASS J0746+2000AB: Precise Mass Measurements and Confirmation of Radio Emission from Both Components
Surveys have shown that up to 1/10th of all ultracool dwarfs (UCDs) are appreciable radio emitters, with their emission attributed to a combination of gyrosynchrotron radiation and the electron cyclotron maser instability. 2M J0746+2000AB is a close stellar binary comprised of an L0 and L1.5 dwarf that was previously identified as a source of 5 GHz radio emission. We used Very Long Baseline Interferometry (VLBI) to precisely track the radio emission over seven epochs in 2010–2017, and found both components to be radio emitters—the first such system identified—with the secondary component as the dominant source of emission in all epochs. The previously identified 2.07 hr periodic bursts were confirmed to originate from the secondary component, although an isolated burst was also identified from the primary component. We additionally fitted the VLBI absolute astrometric positions jointly with existing relative orbital astrometry derived from optical/infrared observations with Markov Chain Monte Carlo methods to determine the orbital parameters of the two components. We found the masses of the primary and secondary optical components to be 0.0795 ± 0.0003 M⊙, and 0.0756 ± 0.0003 M⊙, respectively, representing the most precise mass estimates of any UCDs to date. Finally, we place a 3σ upper limit of 0.9 M_(jup) au on the mass and separation of planets orbiting either of the two components
The - Diagram: Transforming pulsar scintillation spectra to coordinates on highly anisotropic interstellar scattering screens
We introduce a novel analysis technique for pulsar secondary spectra. The
power spectrum of pulsar scintillation (referred to as the "secondary
spectrum") shows differential delays and Doppler shifts due to interference
from multi-path propagation through the interstellar medium. We develop a
transformation which maps these observables to angular coordinates on a single
thin screen of phase-changing material. This transformation is possible without
degeneracies in the case of a one-dimensional distribution of images on this
screen, which is often a successful description of the phenomenon. The double
parabolic features of secondary spectra are transformed into parallel linear
features, whose properties we describe in detail. Furthermore, we introduce
methods to measure the curvature parameter and the field amplitude distribution
of images by applying them to observations of PSR B0834+06. Finally, we extend
this formalism to two-dimensional distributions of images on the interstellar
screen.Comment: 11 pages, 14 figures, 1 table, v2: matches accepted versio
Multi-Epoch VLBI of L Dwarf Binary 2MASS J0746+2000AB: Precise Mass Measurements and Confirmation of Radio Emission from Both Components
Surveys have shown that up to one tenth of all ultracool dwarfs (UCDs) are
appreciable radio emitters, with their emission attributed to a combination of
gyrosynchrotron radiation and the electron cyclotron maser instability (ECMI).
2M J0746+2000AB is a close stellar binary comprised of an L0 and L1.5 dwarf
that was previously identified as a source of 5 GHz radio emission. We used
very-long-baseline interferometry (VLBI) to precisely track the radio emission
over seven epochs in 2010-2017, and found both components to be radio emitters
-- the first such system identified -- with the secondary component as the
dominant source of emission in all epochs. The previously identified 2.07 h
periodic bursts were confirmed to originate from the secondary component,
although an isolated burst was also identified from the primary component. We
additionally fitted the VLBI absolute astrometric positions jointly with
existing relative orbital astrometry derived from optical/IR observations with
Markov-chain Monte Carlo (MCMC) methods to determine the orbital parameters of
the two components. We found the masses of the primary and secondary optical
components to be 0.0795 +/- 0.0003 Msun and 0.0756 +/- 0.0003 Msun,
respectively, representing the most precise mass estimates of any UCDs to date.
Finally, we place a 3-sigma upper limit of 0.9 Mjup au on the mass and
separation of planets orbiting either of the two components.Comment: 14 pages, 7 figures, 5 tables; ApJ, accepte
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