1,202 research outputs found
Science with the ngVLA: Extreme Scattering Events and Symmetric Achromatic Variations
Radio variability in quasars has been seen on timescales ranging from days to
years due to both intrinsic and propagation induced effects. Although
separating the two is not always straight-forward, observations of singular
`events' in radio light curves have led to two compelling, and thus far
unresolved mysteries in propagation induced variability--- extreme scattering
events (ESE) that are a result of plasma lensing by AU-scale ionized structures
in the interstellar medium, and symmetric achromatic variability (SAV) that is
likely caused by gravitational lensing by objects.
Nearly all theoretical explanations describing these putative lenses have
remarkable astrophysical implications. In this chapter we introduce these
phenomena, state the unanswered questions and discuss avenues to answer them
with a weekly-cadence flux-monitoring survey of
flat-spectrum radio quasars with the ngVLA.Comment: To be published in the ASP Monograph Series, "Science with a
Next-Generation VLA", ed. E. J. Murphy (ASP, San Francisco, CA
On the mechanism of polarised metrewave stellar emission
Two coherent radio emission mechanisms operate in stellar coronae: plasma
emission and cyclotron emission. They directly probe the electron density and
magnetic field strength respectively. Most stellar radio detections have been
made at cm-wavelengths where it is often not possible to uniquely identify the
emission mechanism, hindering the utility of radio observations in probing
coronal conditions. In anticipation of stellar observations from a suite of
sensitive low-frequency () radio telescopes, here I
apply the general theory of coherent emission in non-relativistic plasma to the
low-frequency case. I consider the recently reported low-frequency emission
from dMe flare stars AD Leo and UV Ceti and the quiescent star GJ 1151 as test
cases. My main conclusion is that unlike the cm-wave regime, for reasonable
turbulence saturation regimes, the emission mechanism in metre-wave
observations () can often be identified based on the
observed brightness temperature, emission duration and polarisation fraction. I
arrive at the following heuristic: M-dwarf emission that is hour-long with circular polarised fraction at brightness
temperatures of K at in canonical
M-dwarfs strongly favours a cyclotron maser interpretation.Comment: Revised version (under review MNRAS
Upper Limits on the 21 cm Epoch of Reionization Power Spectrum from One Night with LOFAR
We present the first limits on the Epoch of Reionization 21 cm H I power spectra, in the redshift range z = 7.9–10.6, using the Low-Frequency Array (LOFAR) High-Band Antenna (HBA). In total, 13.0 hr of data were used from observations centered on the North Celestial Pole. After subtraction of the sky model and the noise bias, we detect a non-zero Δ^2_I = (56 ± 13 mK)^2 (1-σ) excess variance and a best 2-σ upper limit of Δ^2_(21) < (79.6 mK)^2 at k = 0.053 h cMpc^(−1) in the range z = 9.6–10.6. The excess variance decreases when optimizing the smoothness of the direction- and frequency-dependent gain calibration, and with increasing the completeness of the sky model. It is likely caused by (i) residual side-lobe noise on calibration baselines, (ii) leverage due to nonlinear effects, (iii) noise and ionosphere-induced gain errors, or a combination thereof. Further analyses of the excess variance will be discussed in forthcoming publications
Faraday conversion and magneto-ionic variations in Fast Radio Bursts
The extreme, time-variable Faraday rotation observed in the repeating fast
radio burst (FRB) 121102 and its associated persistent synchrotron source
demonstrates that some FRBs originate in dense, dynamic and possibly
relativistic magneto-ionic environments. Here we show that besides rotation of
the linear-polarisation vector (Faraday rotation), such media can generally
convert linear to circular polarisation (Faraday conversion). We use
non-detection of Faraday conversion, and the temporal variation in Faraday
rotation and dispersion in bursts from FRB\,121102 to constrain models where
the progenitor inflates a relativistic nebula (persistent source) confined by a
cold dense medium (e.g. supernova ejecta). We find that the persistent
synchrotron source, if composed of an electron-proton plasma, must be an
admixture of relativistic and non-relativistic (Lorentz factor )
electrons. Furthermore we independently constrain the magnetic field in the
cold confining medium, which provides the Faraday rotation, to be between
and mG. This value is close to the equipartition magnetic field of the
confined persistent source implying a self-consistent and over-constrained
model that can explain the observations.Comment: Submitted to MNRAS; An error in arguments of sec 2.2 of the previous
version has been correcte
Radio wave scattering by circumgalactic cool gas clumps
We consider the effects of radio wave scattering by cool ionized clumps (T ∼ 10^4 K) in circumgalactic media (CGMs). The existence of such clumps is inferred from intervening quasar absorption systems, but has long been something of a theoretical mystery. We consider the implications for compact radio sources of the ‘fog-like’ two-phase model of the CGM recently proposed by McCourt et al. In this model, the CGM consists of a diffuse coronal gas (T ≳ 10^6 K) in pressure equilibrium with numerous ≲1 pc scale cool clumps or ‘cloudlets’ formed by shattering in a cooling instability. The areal filling factor of the cloudlets is expected to exceed unity in ≳10^(11.5) M⊙ haloes, and the ensuing radio wave scattering is akin to that caused by turbulence in the Galactic warm ionized medium. If 30 per cent of cosmic baryons are in the CGM, we show that for a cool-gas volume fraction of fv ∼ 10^(−3), sources at z_s ∼ 1 suffer angular broadening by ∼15μ as and temporal broadening by ∼1 ms at λ = 30 cm, due to scattering by the clumps in intervening CGM. The former prediction will be difficult to test (the angular broadening will suppress Galactic scintillation only for <10μ Jy compact synchrotron sources). However the latter prediction, of temporal broadening of localized fast radio bursts, can constrain the size and mass fraction of cool ionized gas clumps as a function of halo mass and redshift, and thus provides a test of the model proposed by McCourt et al
Radio wave scattering by circumgalactic cool gas clumps
We consider the effects of radio-wave scattering by cool ionized clumps
(K) in circumgalactic media (CGM). The existence of such clumps
are inferred from intervening quasar absorption systems, but have long been
something of a theoretical mystery. We consider the implications for compact
radio sources of the `fog-like' two-phase model of the circumgalactic medium
recently proposed by McCourt et al.(2018). In this model, the CGM consists of a
diffuse coronal gas (K) in pressure equilibrium with numerous
pc scale cool clumps or `cloudlets' formed by shattering in a
cooling instability. The areal filling factor of the cloudlets is expected to
exceed unity in haloes, and the ensuing radio-wave
scattering is akin to that caused by turbulence in the Galactic warm ionized
medium (WIM). If per-cent of cosmic baryons are in the CGM, we show that
for a cool-gas volume fraction of , sources at suffer angular broadening by as and temporal broadening
by ms at cm, due to scattering by the clumps in
intervening CGM. The former prediction will be difficult to test (the angular
broadening will suppress Galactic scintillation only for Jy compact
synchrotron sources). However the latter prediction, of temporal broadening of
localized fast radio bursts, can constrain the size and mass fraction of cool
ionized gas clumps as function of halo mass and redshift, and thus provides a
test of the model proposed by McCourt et al.(2018).Comment: In press MNRA
On associating Fast Radio Bursts with afterglows
A radio source that faded over six days, with a redshift of
host, has been identified by Keane et al. (2016) as the transient afterglow to
a fast radio burst (FRB 150418). We report follow-up radio and optical
observations of the afterglow candidate and find a source that is consistent
with an active galactic nucleus. If the afterglow candidate is nonetheless a
prototypical FRB afterglow, existing slow-transient surveys limit the fraction
of FRBs that produce afterglows to 0.25 for afterglows with fractional
variation, , and 0.07 for , at 95%
confidence. In anticipation of a barrage of bursts expected from future FRB
surveys, we provide a simple framework for statistical association of FRBs with
afterglows. Our framework properly accounts for statistical uncertainties, and
ensures consistency with limits set by slow-transient surveys.Comment: Accepted version (ApJL
Scintillation noise in widefield radio interferometry
In this paper, we consider random phase fluctuations imposed during wave
propagation through a turbulent plasma (e.g. ionosphere) as a source of
additional noise in interferometric visibilities. We derive expressions for
visibility variance for the wide field of view case (FOV deg) by
computing the statistics of Fresnel diffraction from a stochastic plasma, and
provide an intuitive understanding. For typical ionospheric conditions
(diffractive scale km at MHz), we show that the resulting
ionospheric `scintillation noise' can be a dominant source of uncertainty at
low frequencies ( MHz). Consequently, low frequency widefield
radio interferometers must take this source of uncertainty into account in
their sensitivity analysis. We also discuss the spatial, temporal, and spectral
coherence properties of scintillation noise that determine its magnitude in
deep integrations, and influence prospects for its mitigation via calibration
or filtering.Comment: Accepted versio
- …