2025ApJ...990..214D

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 2024 November to 2025 May, using two optical telescopes in Aryabhatta Research Institute of Observational Sciences, India. The source was in an outburst state during this period. We searched for intraday variability (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 10 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 spectral energy distributions 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 ± 0.004. An analysis of the color-magnitude diagram shows that, contrary to the redder-when-brighter trend typically observed in FSRQs, this source exhibits a bluer-when-brighter trend on short-term variability timescales-a behavior more commonly associated with BL Lac objects. We explore potential physical mechanisms responsible for the observed spectral variability.