Elevated and Variable Optical Polarization Following the TeV Flare in Blazar OP 313

ATel #17051; Sunil Chandra (Physical Research Laboratory, Ahmedabad, India), Goldy Ahuja (Physical Research Laboratory, Ahmedabad, India), Rishit Tiwari (SVNIT Surat, Gujarat), Himani Jeevan Janve (Chandigarh University, Punjab), Nandini S Menon (St. Albert's College, Ernakulam Kerala), Shashikiran Ganesh (Physical Research Laboratory, Ahmedabad, India), Inderraj Singh (PRL, Mt Abu Observatory), Shankar Mahendra (PRL, Mt. Abu Observatory), Vijendra Singh (PRL, Mt Abu Observatory)
on 25 Feb 2025; 18:11 UT
Credential Certification: Sunil Chandra (sunil.chandra355@gmail.com)

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We present a summary of our recent polarimetric observations of the blazar OP 313 (R.A. = 197.61943°, Dec. = +32.34549°, J2000.0, redshift z = 0.997, also known as B2 1308+32) conducted using the EMPOL instrument (DOI link) on the 1.2m telescope at PRL Mt. Abu Observatory, Rajasthan (PRL Mt. Abu Observatory). Previous reports (ATels #16951, #16964, #16991) documented a significant increase in optical/IR flux, with the R-band magnitude brightening from 14.63 to 12.83 between December 21 and 31, 2024. Additionally, ATels #16940 and #16970 reported enhanced GeV flux at different periods from December 2024 to January 2025. On January 11, 2025, MAGIC detected a 10σ TeV excess within one hour of observations (ATel #16977). Furthermore, the VERITAS experiment observed a flux of (7.8 ± 1.0) × 10⁻⁷ m⁻² s⁻¹ above 150 GeV with a 15σ significance during a 1.7-hour observation on January 24, 2025 (ATel #16993). Subsequently, MAGIC detected a 30σ excess in a three-hour observation on January 27, 2025, with the flux above 80 GeV reaching levels comparable to the Crab Nebula. Consistent with these findings, the MACE telescope reported TeV emission above 80 GeV with a statistical significance of 5.8σ from observations on January 26, 2025, corresponding to ~40% of the Crab Nebula flux. Our multi-epoch observations reveal significant variability in optical polarization: 29/01/2025
V (%P, PA): 18.60 ± 1.26, 137.5 ± 4.4
R (%P, PA): 19.68 ± 1.33, 134.8 ± 3.6
I (%P, PA): 18.16 ± 1.07, 137.7 ± 3.7
30/01/2025
B (%P, PA): 12.73 ± 1.38, 130.5 ± 4.2
V (%P, PA): 14.66 ± 1.06, 134.8 ± 4.4
R (%P, PA): 14.65 ± 1.32, 144.8 ± 6.2
I (%P, PA): 14.20 ± 1.01, 134.7 ± 5.4
01/02/2025
B (%P, PA): 8.62 ± 1.96, 61.2 ± 6.8
V (%P, PA): 10.63 ± 1.13, 60.4 ± 4.7
R (%P, PA): 11.84 ± 1.68, 74.5 ± 6.9
I (%P, PA): 10.60 ± 1.05, 63.6 ± 5.6
22/02/2025
B (%P, PA): 20.69 ± 1.59, 64.73 ± 3.8
V (%P, PA): 21.62 ± 1.38, 67.8 ± 4.4
R (%P, PA): 22.35 ± 1.29, 74.9 ± 6.1
I (%P, PA): 21.31 ± 1.4, 63.7 ± 5.4
These multi-epoch observations reveal significant variability in both the polarization degree and the electric vector position angle (EVPA). Notably, our measurements indicate an elevated fractional polarization at several epochs compared to the values reported for January 10–13, 2025 (ATel #16979). PRL Mt. Abu Observatory will continue monitoring this blazar, and we strongly encourage further multi-wavelength follow-up observations.