Decade-long periodicity study of 2FHL blazars with historical optical data

TL;DR

This study investigates decade-long periodicity in blazars using archival optical data, finding initial signals that are ultimately not statistically significant, emphasizing the importance of accounting for sampling artefacts in long-term variability analysis.

Contribution

The paper extends previous work by analyzing multiple blazars for long-term periodicity and highlights the challenges of false positives due to observational limitations.

Findings

No statistically significant decade-long periodicity detected in the studied blazars.

Sampling artefacts can mimic periodic signals, leading to false detections.

Robust methodologies are essential for reliable long-term periodicity searches.

Abstract

In our recent investigation, we utilized a century's worth of archival optical data to search for a decade-long periodicity from the blazar PG 1553+113, finding a hint of a 22-yr period. Building on this foundation, the current study extends our analysis to include 10 blazars from the Fermi-Large Area Telescope Second Catalog of Hard Sources (2FHL) catalogue to uncover similar long-term periodic behaviour. To ensure the reliability of our findings, we consider the impact of observational limitations, such as temporal gaps and uneven sampling, which could potentially introduce artefacts or false periodic signals. Our analysis initially identifies decade-scale periodicity in four of these blazars (AP Librae, MKN 421, MKN 501, PG 1246+586). However, further investigation reveals that three of these are likely influenced by noise and poor sampling. The most promising candidate, approximately 51 $\pm$ 9 yr signal in MKN 421, corresponds to fewer than three full cycles and cannot be considered significant. Furthermore, global significance suggests none of the candidate periodicities meet the threshold for statistical significance. These results underscore the importance of accounting for sampling artefacts and highlight the need for robust methodologies in long-term periodicity searches.