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 $\gtrsim 10^3\,M_\odot$ 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 $\sim$weekly-cadence flux-monitoring survey of $10^3-10^4$ 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 ($\nu\sim 10^2\,{\rm MHz}$) 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 ($\nu\sim 10^2\,{\rm MHz}$) 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 $\gtrsim \,$hour-long with $\gtrsim 50\%$ circular polarised fraction at brightness temperatures of $\gtrsim 10^{12}\,$K at $\sim 100\,{\rm MHz}$ 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 $\gamma<5$) electrons. Furthermore we independently constrain the magnetic field in the cold confining medium, which provides the Faraday rotation, to be between $10$ and $30\,$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

On associating Fast Radio Bursts with afterglows

A radio source that faded over six days, with a redshift of $z\approx0.5$ 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, $m=2|S_1-S_2|/(S_1+S_2)\geq0.7$, and 0.07 for $m\geq1$, 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

Radio wave scattering by circumgalactic cool gas clumps

We consider the effects of radio-wave scattering by cool ionized clumps ($T\sim 10^4\,$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 ($T\gtrsim 10^6\,$K) in pressure equilibrium with numerous $\lesssim 1\,$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 $\gtrsim 10^{11.5} M_\odot$ haloes, and the ensuing radio-wave scattering is akin to that caused by turbulence in the Galactic warm ionized medium (WIM). If $30\,$per-cent of cosmic baryons are in the CGM, we show that for a cool-gas volume fraction of $f_{\rm v}\sim 10^{-3}$, sources at $z_{\rm s}\sim 1$ suffer angular broadening by $\sim 15\,\mu$as and temporal broadening by $\sim 1\,$ms at $\lambda = 30\,$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 $<10\,\mu$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

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$\sim10$ deg) by computing the statistics of Fresnel diffraction from a stochastic plasma, and provide an intuitive understanding. For typical ionospheric conditions (diffractive scale $\sim 5-20$ km at $150$ MHz), we show that the resulting ionospheric `scintillation noise' can be a dominant source of uncertainty at low frequencies ($\nu \lesssim 200$ 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