Long-term meter wavelength variability study of Blazar J1415+1320 using the Ooty Radio Telescope

TL;DR

This study analyzes long-term radio variability of blazar J1415+1320 at 327 MHz over nearly three decades, detecting major flares and exploring the nature of symmetric achromatic variability possibly caused by gravitational lensing.

Contribution

It provides the first long-term 327 MHz variability data for J1415+1320 and investigates the connection between low-frequency variability and high-frequency SAV events, proposing models for observed phenomena.

Findings

Detected two major flares at 2007.6 and 2008.6.

Flares are correlated with higher frequency variability with a lead time.

No variability associated with SAV events at 327 MHz.

Abstract

J1415+1320 is a well-studied blazar that exhibits strong flux density variability at a wide range of radio frequencies (2.4 $-$ 230~GHz). In this letter, we present a variability study of this source at 327~MHz using the data obtained with the Ooty Radio Telescope taken during the period 1989 to 2018. Two significant flares are detected at epochs 2007.6 and 2008.6. These flares are also seen in the publicly available 15 and 37~GHz light curves but with a lead time of a few months. The fractional changes in the flux densities are larger at frequencies $>$ 15~GHz compared to those at 327~MHz and the spectral indices of the increased flux densities are flatter compared to the quiescent spectrum during these flares. These observed features are consistent with a model of uniformly expanding cloud of relativistic electrons or shock-in-jet model. Our 327~MHz dataset also overlaps with a rare form of variability $-$ symmetric achromatic variability (SAV) $-$ seen at higher frequencies ($>$ 15~GHz) toward the source. SAV is possibly due to gravitational milli-lensing of the core emission by an intervening massive object and is expected to be detected at all frequencies \cite{Vedantham2017}. No variability in association with the SAV events is seen in the 327~MHz dataset; however, if SAV is due to the lensing of core emission alone then the expected variability is less than 3$\sigma$ uncertainty in our measurements.