Quasi-simultaneous Optical Flux and Polarization Variability of the Binary Super Massive Black Hole Blazar OJ 287 from 2015 – 2023: Detection of an Anti-correlation in Flux and Polarization Variability

OJ 287 is an archetypal blazar of the BL Lacertae class at a redshift of 0.306 (Sitko & Junkkarinen, 1985). In its optical spectra, the lines appear only weakly above the non-thermal continuum, but are visible at most times (for spectral surveys, see Nilsson et al., 2010; Valtonen et al., 2021). Very high energy $\gamma-$ray emission $>$100 GeV was detected from OJ 287 with VERITAS (Very Energetic Radiation Imaging Telescope Array System) (Mukherjee & VERITAS Collaboration, 2017) and so it is listed as a TeV emitting blazar in the TeV catalogue¹¹1http://tevcat.uchicago.edu/. Historically, it is one of the most variable extragalactic sources at optical and radio bands, exhibiting variations in both flux and polarization (Valtaoja et al., 1987). OJ 287 is among a few active galactic nuclei (AGN), which is understood to host an in-spiraling supermassive black hole (SMBH) binary system and hence is a special candidate to emit nano-hertz gravitational waves (e.g., Valtonen et al., 2021, 2023a, and references therein).

Being one of the most optically bright AGNs, its favorable location close to the ecliptic has led to optical data having been collected over a very extended period – more than a century, since 1888. Using the available early subset of this data, Sillanpaa et al. (1988) discerned for the first time that the blazar shows a period of $\sim$ 12 yrs and using the similarity of flare profile with simulations, hypothesized it to be a binary SMBH. The model successfully predicted the next outburst as well as the expected tidally-induced, second peak, $\sim$ 1.2 yr later (Sillanpaa et al., 1996a, b). The denser monitoring cadence during this period, however, revealed much sharper flare profiles, leading to a drastically refined model with flares arising out of the impact of secondary on the primary’s accretion disk (Lehto & Valtonen, 1996). Alternative hypotheses argue the recurrent flares arise from jet precession or a combination of jet and accretion disk emission (e.g. Katz, 1997; Britzen et al., 2023). Comparison of observational features explored to date (timing, spectral, and polarization) to those expected in these models, largely favor the disk impact scenario (Kushwaha, 2020; Kushwaha et al., 2018) yet the case is not ironclad. Given our incomplete understanding of accretion physics and compact sources, a persistent effort across the EM spectrum is crucial to eventual elimination of any lingering doubts (Valtonen et al., 2023b, and references therein).

Along with BL Lacertae itself, OJ 287 was a prototype BL Lac object used for the observational characterization of BL Lac source properties. Thus its optical and near infra-red (NIR) flux and polarization properties have been extensively explored on diverse timescales (e.g., Holmes et al., 1984; Kikuchi et al., 1988; Sillanpaa et al., 1991, 1996a, 1996b; Gupta et al., 2017, 2019, 2022, and references therein). Optical and NIR polarization studies have reported frequency-dependent polarization (Holmes et al., 1984; Kikuchi et al., 1988) and led to the proposal that two-emission regions were important (see also Villforth et al., 2010). In optical bands and on intra-night timescales frequency-dependent polarization behavior has been reported with an anti-correlation between flux and polarization degree (PD) (Sillanpaa et al., 1991). Blinov et al. (2011) analyzed four-color optical photometric data and R-band polarimetric data of the blazar OJ 287 over 5 yr (2005 – 2009) and employed a model with one constant, and several variable, sources of polarized radiation. They concluded that the observed variability of the polarization and colors in the optical range could be explained with a model consisting of the constant source with a degree of polarization $\approx$10% and position angle of polarization PA$\sim$162^∘, and 4$\pm$2 variable sources with similar degrees of polarization and random PAs.

The theory that a binary SMBH system is present in OJ 287 proposed that the $\sim 12$-yr recurrent optical flares are of thermal bremsstrahlung origin (Lehto & Valtonen, 1996). Hence polarization is one of the key observables for discerning the nature of the flare, as it is expected to decrease during the impact flares because thermal emission has no polarization. The first evidence for this already was available from 1983, where the expected trend was seen (Smith et al., 1985). During the rising part of the flares in 1990s polarization observations were not available and this was again the case in 2005. In 2005 the first polarization measurement was carried out after the peak flux (Villforth et al., 2010). Therefore the next real test, since 1983, of the unpolarized nature of the impact flares came in 2007 when it was shown that the flare arose only from the unpolarized component (Valtonen et al., 2008; Valtonen & Sillanpää, 2011). The next opportunity to study the polarization of an impact flare came in 2015, when, in addition to demonstrating the unpolarized nature of the optical emission, it was shown that there was no X-ray counterpart to the flare (Valtonen et al., 2016) further supporting its thermal nature.

The spectral slope over a wide range of frequencies was measured during the 2005 flare and the expected thermal bremmstrahlung spectral index of $\alpha_{\nu}\sim-0.2$ was found (Valtonen et al., 2012). It is quite different from the usual spectral index of $\alpha_{\nu}\sim-1.3$ over the same spectral range (Kidger et al., 2018) which is typical for non-thermal synchrotron emission. The spectral index $\alpha_{\nu}\sim-0.2$ was confirmed during the 2015 and 2019 flares (Laine et al., 2020; Valtonen et al., 2021). The complete change in the nature of emission in the impact flare as compared with the usual out-of-flare emission excludes the possibility that the flares are somehow related to Doppler boosting variations in a turning jet (Villata et al., 1998; Rieger, 2004).

Even though the nature of radiation during impact flares has now been well studied, there still remain questions. The second component after the bremsstrahlung peak is highly polarized (Smith et al., 1985; Villforth et al., 2010; Valtonen et al., 2016) which indicates that here we receive non-thermal emission of the impact flare in OJ 287 (Valtonen et al., 2019). Further flares associated with tidal increase of accretion flow were expected in 2016/2017 and 2020 (Sundelius et al., 1997; Pihajoki et al., 2013; Valtonen et al., 2021). A so-called precursor flare also falls into the time period studied here (Pihajoki et al., 2013). In addition, the long range evolution of the polarization behaviour has been described by models which associate the polarization variability to the wobble of the jet, caused by the companion, and the resulting changes in the jet viewing angle (Valtonen et al., 2012; Valtonen & Pihajoki, 2013; Dey et al., 2021; Valtonen et al., 2021).

Here we report our dense quasi-simultaneous monitoring of the optical flux and polarization behavior of the source between 2015 – 2023. These observations are part of our ongoing optical and multi-wavelength project to densely monitor the behavior of OJ 287 (Gupta et al., 2017, 2019; Kushwaha et al., 2018, 2021). This interval corresponds to much of one cycle of the observationally-favored binary SMBH model of Dey et al. (2018, see ()). The next section details the optical monitoring and reduction procedure, followed by analysis and results in section §3. A discussion and a summary are provided in sections §4 and §5, respectively.

Optical R-band photopolarimetric observations of the blazar OJ 287 were performed during 2017 March 7 to 2022 March 26 with the Hiroshima Optical and Near-InfraRed camera (HONIR) (Akitaya et al., 2014) which is installed on the “Kanata” 1.5m telescope, Hiroshima, Japan. Calculation of the PD, PA, and their errors were done from Stokes parameters obtained from four exposures at 0.0^∘, 45^∘.0, 22^∘.5, and 67^∘.5 positions of the half-wave plate in each exposure (Kawabata et al., 1999). We obtained the offset angle from observation of strongly polarized stars (BD+64d106, BD+59d389) and depolarization correction was performed from the wire grid star observation. We confirmed the instrumental polarization was negligible ($<$ 0.2%) from observation of an unpolarized star (HD14069). For the photometric observations, standard reduction procedures were adopted and we calculated the magnitude based on the Pan-STARRS1 catalog after conversion of the filter system. The photometry of the blazar was done using standard CCD image reduction procedures. Detailed descriptions of observations, photometric and polarimetric data analyses made with this telescope and instrumentation are described in Ikejiri et al. (2011). The data from the Kanata telescope are plotted in blue symbols in the different panels of Figure 1, which give the photometric magnitude, degree of polarization, and polarization angle from bottom to top.

Optical photometric and polarimetric data of OJ 287 were obtained from 2015 September 25 to 2022 December 24 using the St. Petersburg University, Russia 40-cm LX-200 telescope and the Crimean observatory 70-cm AZT-8 telescope under regular monitoring of a sample of blazars within the framework of a GASP/WEBT (GLAST-AGILE Support Program/Whole Earth Blazar Telescope) project. We have used differential aperture photometry to obtain photometric measurements with standard stars 1, 2, and 3 in VRI from Fiorucci & Tosti (1996) and in B from Smith et al. (1985). Detailed description of our data analyses are provided in Larionov et al. (2008). The data from the St. Petersburg and Crimea telescopes are plotted in green in the different panels of Figure 1.

Optical flux and polarimetric observations of OJ 287 were carried out from 2015 September 18 to 2023 February 12 at the Perkins telescope of the Perkins Telescope Observatory (Flagstaff, AZ, USA) (Jorstad et al., 2010). They were obtained with the PRISM camera²²2https://www.bu.edu/prism/ which possesses a polarimeter with a rotating half-waveplate. A polarization observation consists of a series of 3–5 Stokes Q and U measurements for a given object. Each series consists of four measurements at instrumental position angles 0^∘, 45^∘, 90^∘, and 135^∘ of the waveplate. We use field stars to perform both interstellar and instrumental polarization corrections. We use unpolarized calibration stars from Schmidt et al. (1992) to check the instrumental polarization, which is usually within 0.3%–0.5%, and polarized stars from the same paper to calibrate the polarization position angle. These data are plotted in black in Figure 1.

Additional optical R band photometric and polarimetric data of OJ 287 from 2022 January 21 to 2023 February 17 were taken using the 0.6-m telescope of the Belogradchik Observatory, Bulgaria. A double-barrel filter set from FLI was installed in the Belogradchik 0.6-m telescope, enabling the combination of polarimetric and UBVRI filters to perform a linear polarimetry of the blazar in the chosen optical band. The PD and PA were obtained using three polarimetric filters that were orientated at 0-180, 60-240, and 120-300 degrees, respectively. Details about this data analysis are provided in Bachev et al. (2023). The data from Belogradchik Observatory telescope are plotted in violet in Figure 1.

In addition to the above data that we collected, we also employed optical R band photometric and polarimetric data obtained from the spectro-polarimeter at Steward Observatory, University of Arizona, USA, on OJ 287 from 2015 September 18 to 2018 June 22 taken from the public archive³³3http://james.as.arizona.edu/~psmith/Fermi/DATA/Objects/. The details of this observational program and analysis procedures are explained in Smith et al. (2009). The Steward Observatory photometric magnitude, degree of polarization, and polarization angle of OJ 287 are plotted in red in Figure 1.

We note that during periods of overlap for measurements taken at different observatories the magnitude and fractional polarization measurements agree very well. We noticed occasional ambiguity in the measurements of the polarization angles and corrected them by $\pm\rm{n}\pi$, where n = 1, 2, …..

The simultaneous optical photometric and polarimetric behavior of the blazar OJ 287 is displayed in Figure 1. On visual inspection, we noticed that the flux and polarization degree are generally correlated. But on one occasion an anti-correlation between optical flux and PD is seen and is marked by a continuous line in Figure 1.

In the present study of OJ 287, PD and flux are positively correlated (refer to Figure 3) in general with almost every flux change being accompanied by a change in PD and often PA too. This is consistent with studies reporting flux and optical polarization variations in BL Lac objects which show them to be very dynamic sources in the polarization domain (e.g. Otero-Santos et al., 2023). The optical flux/brightness variation of OJ 287 in terms of amplitude is similar to that seen at NIR bands over a decade duration ($\sim$ 2.8 mag; Gupta et al., 2022) if the long-term base evolution is taken into account, as pointed out in Gupta et al. (2022). The fluctuation of PA values around $\sim$ 140^∘ in the present study indicate a preferential direction and is also consistent with the result of Otero-Santos et al. (2023) using almost a decade of optical polarization study of blazars from the Steward data, except for the ambiguity of 180^∘ in PA. The authors have argued such behavior can be attributed to the presence of large-scale magnetic field and turbulence, as has been argued from RoboPol studies of blazars (Angelakis et al., 2016).

Flux observations at different wavelengths, as well as polarization measurements at radio and optical bands, provide valuable information for understanding the behavior of blazars and for modeling the physics of their jets. It is generally believed that the emission mechanism in blazars in the radio to optical bands is predominantly synchrotron radiation emitted by relativistic jets, enhanced by Doppler boosting. Polarimetric observations are needed to study the structure of the magnetic fields in the jets that is required to produce synchrotron radiation.

In the present work, we observed varied combinations of flux, PD, and PA fluctuation in the various data segments presented in Figure 2. Below we mention some interesting aspects of the polarization behavior with their implications for the binary black hole model of OJ 287.

The anti-correlation between the optical flux and PD is most clear in Segment 3B in early 2018, following a strong fade in the optical flux. In the SMBH binary model this is the time when the secondary impacts the jet from its side. During this period the secondary moves from (-5000 AU, -8400 AU) to (-3100 AU, -8000 AU), in the coordinates specified in Figure 1 of Valtonen et al. (2023c). In the same figure one can see that the outline of the jet may go through the same coordinate interval. In principle this could lead to magnetic field compression which is orthogonal to the usual compression when shock waves propagate along the jet. This could be one of the explanations of why at this time the PD behaves differently from usual positive correlation with the optical flux. It is also possible that the fade in the optical flux is related to jet bending, which produces a temporarily reduced Doppler boosting just before the impact on the jet (Valtonen et al., 2022).

Another interesting moment in the light curve is at the end of 2020, in Segment 6. The optical flux rises monotonically between JD 2459175 (2020 Nov 21) and JD 2459195 (2020 Dec 11), while the PD initially drops from about 20$\%$ to 5$\%$ and then rises again rapidly to about 25$\%$. Thus we see both the correlation and anti-correlation in the same flare. This is classed as a precursor of the 2022 disk impact in the binary black hole model. It is thought to arise from a superposition of emission from both the primary and the secondary jets (Valtonen et al., 2023b), which should definitely show up as exceptional polarization behavior.

We have carried out an intensive and extensive quasi-simultaneous monitoring of the optical flux and polarization of the binary super massive black hole blazar OJ 287. Our data were collected between 2015 and 2023 using optical telescopes in the USA, Japan, Crimea, Russia, and Bulgaria. We have also included public archival data from Steward Observatory, USA. The entire duration of these observations covers eight observing seasons, and each observing season’s data is treated as a separate segment. A summary of our results are as follows:

1. 1.

   Over the span of these observations, OJ 287 shows a big optical R-band flux variation of $\sim$3.0 mag, a large change in PD of $\sim$37%, and a substantial range in PA of $\sim$215^∘.

2. 2.

   In general, magnitude and PD are correlated in the sense that PD rises with OJ 287 gets brighter, and vice-versa. But in the Segment 3B, the magnitude and PD are anti-correlated.

3. 3.

   During the course of these observations, different temporal segments show various features in PA, e.g., smooth rotation, non variable, and random variation.

4. 4.

   The detection of anti-correlation in optical flux and PD in Segment 3B is a rare finding in OJ 287. In this segment a smooth rotation of PA of $\sim$ 30^∘ (from 125^∘ to 155^∘) is followed by an opposite rotation of $\sim$ 40^∘ (from 155^∘ to 115^∘), from JD 2458156 to JD 2458210 and JD 2458210 to JD 2458270, respectively.

5. 5.

   Another exceptional finding is at the end of 2020 when both correlated and anti-correlated behavior occurs in the same flare. This is the time when in the binary model the secondary jet was expected to show up and overpower the primary jet for a short period of time (Valtonen et al., 2023b). This may be the first time (through polarization at least) that we actually see the emission of the secondary black hole in the binary black hole system of OJ 287. Overall, the PA of polarization follows the binary model during the low brightness states of OJ 287 throughout the entire period of these observations.

Data from the Steward Observatory spectropolarimetric monitoring project were used. This program is supported by Fermi Guest Investigator grants NNX08AW56G, NNX09AU10G, NNX12AO93G and NNX15AU81G. This study was based in part on observations conducted using the Perkins Telescope Observatory (PTO) in Arizona, USA, which is owned and operated by Boston University.

We thank the anonymous reviewer for useful comments. ACG is partially supported by Chinese Academy of Sciences (CAS) President’s International Fellowship Initiative (PIFI) (grant no. 2016VMB073). PK acknowledges support from the Department of Science and Technology (DST), government of India, through the DST-INSPIRE Faculty grant (DST/INSPIRE/04/2020/002586). RI’s work is supported by JST, the establishment of university fellowships towards the creation of science technology innovation, Grant Number JPMJFS2129. MFG is supported by the National Science Foundation of China (grant 11873073), Shanghai Pilot Program for Basic Research-Chinese Academy of Science, Shanghai Branch (JCYJ-SHFY-2021-013), the National SKA Program of China (Grant No. 2022SKA0120102), the science research grants from the China Manned Space Project with No. CMSCSST-2021-A06, and the Original Innovation Program of the Chinese Academy of Sciences (E085021002). The research at Boston University was supported in part by National Science Foundation grant AST-2108622, and a number of NASA Fermi Guest Investigator grants; the latest is 80NSSC22K1571. ZZ is thankful for support from the National Natural Science Foundation of China (grant no. 12233005). RB and AS are partially supported by the Bulgarian National Science Fund of the Ministry of Education and Science under grants KP-06-H38/4 (2019), KP-06-KITAJ/2 (2020), and KP-06-H68/4 (2022). HG acknowledges the financial support from the DST, GoI, through DST-INSPIRE faculty award IFA17-PH197 at ARIES, Nainital, India.