Showing papers by "Phil Arras published in 2023"

Bayesian radio interferometric imaging with direction-dependent calibration

Abstract: Context: Radio interferometers measure frequency components of the sky brightness, modulated by the gains of the individual radio antennas. Due to atmospheric turbulence and variations in the operational conditions of the antennas these gains fluctuate. Thereby the gains do not only depend on time but also on the spatial direction on the sky. To recover high quality radio maps an accurate reconstruction of the direction and time-dependent individual antenna gains is required. Aims: This paper aims to improve the reconstruction of radio images, by introducing a novel joint imaging and calibration algorithm including direction-dependent antenna gains. Methods: Building on the \texttt{resolve} framework, we designed a Bayesian imaging and calibration algorithm utilizing the image domain gridding method for numerically efficient application of direction-dependent antenna gains. Furthermore by approximating the posterior probability distribution with variational inference, our algorithm can provide reliable uncertainty maps. Results: We demonstrate the ability of the algorithm to recover high resolution high dynamic range radio maps from VLA data of the radio galaxy Cygnus A. We compare the quality of the recovered images with previous work relying on classically calibrated data. Furthermore we compare with a compressed sensing algorithm also incorporating direction-dependent gains. Conclusions: Including direction-dependent effects in the calibration model significantly improves the dynamic range of the reconstructed images compared to reconstructions from classically calibrated data. Compared to the compressed sensing reconstruction, the resulting sky images have a higher resolution and show fewer artifacts. For utilizing the full potential of radio interferometric data, it is essential to consider the direction dependence of the antenna gains.

Large Dynamical Tide Amplitudes from Small Kicks at Pericenter

Abstract: The effect of dynamical tide ``kicks"on eccentric binary orbits is considered using the orbital mapping method. It is demonstrated that when mode damping is negligible the mode amplitude will generically grow in time for all values of orbital eccentricity and semi-major axis, even for small kicks outside the regime exhibiting diffusive growth. The origin of the small-kick growth is the change in kick size from orbit to orbit, an effect quadratic in the mode amplitude. When damping of the mode is included, the growth is shut off when the damping time is shorter than the growth time. Hence, in practice, kicks of sufficient size and long mode damping times are required for interesting levels of growth to occur. Application to the circularization of hot Jupiters is discussed. Previous investigations found that diffusive growth of the planetary f-mode in the large-kick regime would lead to rapid orbital shrinkage, but upon exiting the diffusive regime at $e \sim 0.9$ the theory would predict a large population of highly eccentric orbits. Simulations presented here show that subsequent orbital evolution relying on the small-kick regime may further decrease the eccentricity to $e \sim 0.2$ on timescales much less than the Gyrs ages of these systems.

Astrophysical parameter inference on accreting white dwarf binaries using gravitational waves

Abstract: Accreting binary white dwarf systems are among the sources expected to emanate gravitational waves that will be detectable by the Laser Interferometer Space Antenna (LISA). We investigate how well we will be able to determine astrophysical parameters of accreting binary white dwarf systems from LISA’s measurements of the gravitational waves emanated by these binaries. We present expressions for the gravitational wave amplitude, frequency, and frequency derivative in terms of white dwarf parameters (masses, donor radius, etc.), which we derive using knowledge of mass accreting mechanisms for binaries containing low-mass donors. We then perform a Fisher analysis to reveal the accuracy of our measurements of these parameters, relying on models from Modules for Experiments in Stellar Astrophysics (MESA) to obtain realistic mass-radius relations. We find that with an independent measurement of the luminosity distance, we are likely to be able to determine the individual masses, donor radius, and a parameter describing the response of the donor to mass loss. Without an independent measurement of the luminosity distance, we can still use LISA’s measurements to determine the latter two parameters, but we are no longer able to constrain the individual masses.

Orbital Decay of Hot Jupiters due to Weakly Nonlinear Tidal Dissipation

Abstract: We study tidal dissipation in hot Jupiter host stars due to the nonlinear damping of tidally driven $g$-modes, extending the calculations of Essick&Weinberg (2016) to a wide variety of non-solar type hosts. This process causes the planet's orbit to decay and has potentially important consequences for the evolution and fate of hot Jupiters. Previous studies either only accounted for linear dissipation processes or assumed that the resonantly excited primary mode becomes strongly nonlinear and breaks as it approaches the stellar center. However, the great majority of hot Jupiter systems are in the weakly nonlinear regime in which the primary mode does not break but instead excites a sea of secondary modes via three-mode interactions. We simulate these nonlinear interactions and calculate the net mode dissipation for stars that range in mass from $0.5 M_\odot \le M_\star \le 2.0 M_\odot$ and in age from the early main sequence to the subgiant phase. For stars with $M_\star \lesssim 1.0 M_\odot$ of nearly any age, we find that the orbital decay time is $\lesssim 100 \textrm{ Myr}$ for orbital periods $P_{\rm orb} \lesssim 1 \textrm{ day}$. For $M_\star \gtrsim 1.2 M_\odot$, the orbital decay time only becomes short on the subgiant branch, where it can be $\lesssim 10 \textrm{ Myr}$ for $P_{\rm orb} \lesssim 2 \textrm{ days}$ and result in significant transit time shifts. We discuss these results in the context of known hot Jupiter systems and examine the prospects for detecting their orbital decay with transit timing measurements.