A new sample of faint Blazar cAndidates with Strong opticAl variabiLiTy (BASALT)

¹ Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
² INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, I-10025 Pino Torinese, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 24 August 2025
Accepted: 9 March 2026

Abstract

We present a sample of blazar candidates from the Very Large Array Sky Survey (VLASS) catalog, with cross-referencing to optical and infrared data from the Panoramic Survey Telescope & Rapid Response System (Pan-STARRS) and the Wide-Field Infrared Survey Explorer (WISE), respectively. We focus on compact radio sources and employ a multistage selection process to minimize contamination from non-blazar sources. We considered constraints derived from the radio-optical spectral index and from the infrared and optical colors. We identified the variable objects through an optical variability analysis of Pan-STARRS light curves using two reference samples of stars. We further selected sources whose variability exceeds the level observed in quasars. This led to the selection of 3467 candidates with blazar-like properties and strong optical variability. Only 45% is available in other blazar candidate catalogs, with the remainder representing new identifications. In particular, we unveil a large population of faint sources, down to r ∼ 20.5. The blazar nature of a subsample of these sources is confirmed by available spectra. Our work offers new insights into the blazar population and serves as a foundation for future variability studies, especially in view of the upcoming Legacy Survey of Space and Time at the Vera C. Rubin Observatory, which will reach even fainter sources.