Pronounced linear polarization variability of the flaring FSRQ OP 313 at mm and cm wavelengths

ATel #16517; Ioannis Myserlis (IRAM, MPIfR) and Ramprasad Rao (CfA) on behalf of the SMAPOL and QUIVER collaborations
on 9 Mar 2024; 08:16 UT
Credential Certification: Ioannis Myserlis (imyserlis@iram.es)

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Following up on the recent announcements of the currently increased activity state of the FSRQ OP 313 in gamma-rays (ATel #16497), X-rays (ATel #16504) and optical bands (ATel #16515), we hereby report on the latest activity of the source in total flux and linear polarization over a wide range of radio frequencies, from 2.6 GHz to 225 GHz. The observations were performed in the framework of our multi-year SMAPOL and QUIVER polarization monitoring programs using the Submillimeter Array (SMA) and the Effelsberg 100m radio telescope, respectively.

The observations reported here correspond to the period from December 4, 2023 (MJD 60282) to March 1, 2024 (MJD 60370), over which we monitored the target with a mean sampling of ~ 2 weeks. Its total flux density and linear polarization were recovered at 225 GHz (1.3 mm) using SMA as well as for 7 bands between 2.6 GHz to 41.3 GHz (11 cm to 7 mm) using the Effelsberg 100m radio telescope.

Concerning the total flux density, up to 41.3 GHz it remains fairly stable throughout the given period and proportional to the frequency, ranging from ~ 1.8 Jy at 2.64 GHz to ~ 3.2 Jy around 40 GHz. This corresponds to an inverted radio spectral index of α ~ + 0.3 (defined as S ∝ ν^α). At higher frequencies, at 225 GHz, the total flux density showed a recent increase from ~ 1.78 Jy in the end on January 2024 to ~ 2.16 Jy in the beginning of March 2024.

In addition, the source showed pronounced linear polarization variability over the last three months. Our dense, SMAPOL observations (at 225 GHz) showed a sharp decline in linear polarization degree from 3.1 % to 0.8 % within one month (December 4, 2023 to January 2, 2024), followed by a slight increase to ~ 1.4 % until the end of February 2024. In the last observing session on March 1, 2024 we recorded a new sharp increase in the linear polarization degree to ~ 2.5 %. Within this period, OP 313 showed also a pronounced polarization angle (electric vector position angle or EVPA) variability at 225 GHz (1.3 mm), which can be described by a counter-clockwise (CCW) rotation from 149° to 246° until the end of January 2024 followed by a clockwise (CW) rotation back to 179° by March 1, 2024.

At lower frequencies, from 2.6 GHz to 41.3 GHz, the polarization degree shows a slow decline throughout this period, from 2.3% to 1.2% and the polarization angle shows a more stable behavior that at 225 GHz, remaining constrained between ~ 150° and 200° with the exception of a slow CCW rotation from 145° to 176° at intermediate frequencies (~ 10 - 15 GHz). Using our multi-wavelength polarization observations at the longer wavelengths (11 cm to 7 mm), we can also detect a significant variation in the Faraday rotation measure (RM) from -200 rad/m² in early December 2023 to ~ 0 rad/m² in late January 2024, indicating recent changes of the plasma density and/or magnetic field strength and topology along the line of sight.

We will continue monitoring the FSRQ OP 313 with the SMAPOL and QUIVER monitoring programs at high-cadence and report any changes of its activity in total flux density or linear polarization. Further multi-wavelength observations are also encouraged.

More information about SMAPOL can be found here: SMAPOL Newsletter - January 2024