New Tests of Millilensing in the Blazar PKS 1413+135

TL;DR

This paper investigates symmetric achromatic variability in the blazar PKS 1413+135, attributing it to gravitational millilensing, and explores potential millilens candidates, with implications for studying active galactic nuclei and dark matter structures.

Contribution

It presents the first consistent gravitational lens model fitting four SAV events in PKS 1413+135 and discusses potential millilens candidates, advancing understanding of microarcsecond-scale phenomena.

Findings

All four SAVs fit a single lens model.

Possible millilens candidates include a molecular cloud or dwarf galaxy.

Tentative periodicity of 989 days in SAVs was identified.

Abstract

Symmetric Achromatic Variability (SAV) is a rare form of radio variability in blazars that has been attributed to gravitational millilensing by a ~$10^2 - 10^5$ $M_\odot$ mass condensate. Four SAVs have been identified between 1980 and 2020 in the long-term radio monitoring data of the blazar PKS 1413+135. We show that all four can be fitted with the same, unchanging, gravitational lens model. If SAV is due to gravitational millilensing, PKS 1413+135 provides a unique system for studying active galactic nuclei with unprecedented microarcsecond resolution, as well as for studying the nature of the millilens itself. We discuss two possible candidates for the putative millilens: a giant molecular cloud hosted in the intervening edge-on spiral galaxy, and an undetected dwarf galaxy with a massive black hole. We find a significant dependence of SAV crossing time on frequency, which could indicate a fast shock moving in a slower underlying flow. We also find tentative evidence for a 989-day periodicity in the SAVs, which, if real, makes possible the prediction of future SAVs: the next three windows for possible SAVs begin in August 2022, May 2025, and February 2028.