B2 1308+326: A Changing-look Blazar or Not?

Abstract

In our previous study, we identified a shift in the synchrotron peak frequency of the blazar B2 1308+326 from 10 ^12.9 to 10 ^14.8 Hz during a flare, suggesting it could be a changing-look blazar (CLB). In this work, we investigate the changing-look behaviour of B2 1308+326 by analysing a newly acquired optical spectrum and comparing it with an archival spectrum. We find that between the two epochs, the continuum flux increased by a factor of ~4.4, while the Mg ii emission line flux decreased by a factor of 1.4 ± 0.2. Additionally, the equivalent width of the Mg ii line reduced from ~20 to ~3 Å, indicating an apparent shift from a flat-spectrum radio quasar (FSRQ) class to a BL Lacertae (BL Lac) class. Despite this apparent change, the ratio of accretion disk luminosity to Eddington luminosity remains >10 ^−2 during both epochs, indicating efficient accretion persists in B2 1308+326. The measured black hole mass remains consistent with an average $\mathrm{log}{M}_{{\rm{BH}}}=8.44$ M _⊙ . Our findings suggest that B2 1308+326 is not a genuine CLB but rather an intrinsic FSRQ that emerges as a BL Lac during high-flux states due to enhanced nonthermal emission.

Keywords

• Flat-spectrum radio quasars
• Blazars
• Active galactic nuclei
• Radio loud quasars
• Relativistic jets