A possible wave-optical effect in lensed FRBs

Abstract

Context: Fast Radio Bursts (FRBs) are enigmatic extragalactic bursts whose properties are still largely unknown, but based on their extremely small time duration, they are proposed to have a compact structure, making them candidates for wave-optical effects if gravitational lensed. If an FRB is lensed into multiple-images bursts at different times by a galaxy or cluster, a likely scenario is that only one image is detected, because the others fall outside the survey area and time frame. Aims: In this work we explore the FRB analog of quasar microlensing, namely the collective microlensing by stars in the lensing galaxy, now with wave optics included. The eikonal regime is applicable here. Methods. We study the voltage (rather than the intensity) in a simple simulation consisting of (a) microlensing stars, and (b) plasma scattering by a turbulent interstellar medium. Results: The auto-correlation of the voltage shows peaks (at order-microsecond separations) corresponding to wave-optical interference between lensed micro-images. The peaks are frequency dependent if plasma-scattering is significant. While qualitative and still in need of more realistic simulations, the results suggest that a strongly-lensed FRB could be identified from a single image. Conclusions: Microlensing could sniff out macro-lensed FRBs

Figures (4)

• Figure 1: Illustration of time delay due to turbulent ISM, normalized according to Eq. \ref{['eq:turbulnorm']}. The spatial scale is the same as the microlensing fields in left panels in Figs. \ref{['fig:highfreq']}--\ref{['fig:lowfreq']} below.
• Figure 2: Left. Micro-images in the lensing-dominated regime. Black dots mark stellar masses. Gray curves show arrival time contours. Ellipses are micro-images (blue for minima, red for saddle points). Right. Auto-correlation signals: blue for the real part, magenta for the imaginary part. A tanh activation function has been applied to reduce the contrast between the lines.
• Figure 3: Arrival time contours (left) and auto-correlation signals (right) for intermediate-frequency signals at $\nu/\nu_0$ of 13 (top row), 10 (middle row), 8 (bottom row)
• Figure 4: Arrival time contours (left) and auto-correlation signals (right) for low-frequency signals, $\nu/\nu_0$ being 7 (top row), 6 (middle row), 5 (bottom row). The slight wiggliness of the curves in the bottom left panel are due to small-scale fluctuations from plasma scattering.