Multi-band optical variability of the blazar OP 313 in the outburst state during 2024-2025

TL;DR

This study analyzes optical flux and spectral variability of the blazar OP 313 during its 2024-2025 outburst, revealing intraday variations, spectral index behavior, and short-term color trends using multi-band photometry.

Contribution

It provides the first detailed multi-band optical variability analysis of OP 313 during an outburst, highlighting its unique spectral and temporal behavior.

Findings

Detected intraday variability with a 34% duty cycle.

Observed over two magnitude variations across all bands.

Found a bluer-when-brighter trend contrary to typical FSRQ behavior.

Abstract

We present the analysis results of flux and spectral variability of the blazar OP 313 across intra-night to short-term timescales using BVRI photometric data, gathered over 25 nights from Nov 2024 to May 2025, using two optical telescopes in ARIES, India. The source was in an outburst state during this period. We searched for intraday variations (IDV), using two powerful statistical tests: the Power Enhanced F-test and the Nested ANOVA test. The source displayed IDV in the R band for five of the ten nights, yielding a duty cycle of 34$\%$. During the entire monitoring of the source, it showed variations of over two mag in all B, V, R, and I data bands. We obtained a variability timescale for a variable light curve, giving us an upper limit for the size of the emission region. We generated optical SEDs of the blazar for these 25 nights, fitted a power law of form $(F_\nu \propto \nu^{-\alpha_{o}})$ and found the weighted mean spectral index to be 1.471$\pm$0.004. An analysis of the color-magnitude diagram shows that, contrary to the redder-when-brighter (RWB) trend typically observed in FSRQs, this source exhibits a bluer-when-brighter (BWB) trend on short-term variability (STV) timescales - a behavior more commonly associated with BL Lac object. We explore potential physical mechanisms responsible for the observed spectral variability.