Search for QPOs in the Light Curve of the Blazar OJ 287 : Preliminary Results from 2015 / 16 Observing Campaign

We analyse the light curve in the R-band gathered during the 2015/16 observing season of OJ287. We did a search for QPOs using several methods in wide time domain. No statistically significant periods were found in the high frequency domain both in the ground data and also in K2 observations. In the longer periods domain the Lomb-Scargle periodogram revealed several peaks above the 99% significance level. The longest one, about 95-day, corresponds to the ISCO period of the more massive SMBH. The 43-day period could be an alias or can be attributed to accretion in the form of two armed spiral wave.


Introduction
OJ287 is the only blazar known to exhibit certain quasi-periodic variability in its light curve with a rough period of 12 years.A model that successfully explains this observational feature requires the blazar central engine to contain a binary, consisting of two SMBHs (Valtonen et al. 2008 [6] and references therein).The less massive BH (150 million solar mass) orbits the more massive one (18 billion solar mass) and pierces the accretion disk surrounding the latter BH twice per orbit.The general relativistic orbital precession naturally explains the quasi-periodic light curve variability of OJ287.Since 2006, OJ287 has been regularly monitored in optical at the Mt.Suhora observatory with supporting observations at Krakow and Athens.In the 2015/16 season, we started observations in September, soon after the blazar became visible after the summer conjunction with the Sun.In anticipation of the outburst predicted for this season by the binary model, a multi-site campaign was organized.Polarimetric observations were also scheduled to reveal the nature of the expected brightening.The predicted outburst started at the end of November 2015 with an initial slow rise in brightness followed by a very rapid brightening.On our alert, almost two dozen telescopes in 4 continents joined photometric observations providing a very good coverage of the event shown in upper panel of Fig. 1.Polarimetric observations were taken at Canary Islands, Hawaii, Mt.Suhora and in India.The season LC of OJ287 taken until mid May 2016 is presented in bottom panel of Fig. 1, symbols in green denote dates when low polarisation (PD lower than 11%) was measured.UV and X-ray data were also obtained with the SWIFT satellite.Timing of this and previous outbursts allowed to revise masses of the SMBHs and to measure the spin of the more massive BH to be 0.31±0.01(Valtonen et al. 2016 [8]).

Ground-based data
Variability at all wavelengths is commonly observed in blazars.Amplitude of flux change in optical band can reach a few magnitudes.These variations can be fast, often the intra day varability can be seen.There are physical processes in blazars that could lead to periodic or quasi-periodic behaviour (eg.these arising at the innermost stable circular orbit).Detection of such QPO variations could give a better understanding of the underlying physical processes in blazars.There were numerous periodicity analyses and discussions of the physical significance of the various frequencies in OJ287.Results covering the previous outburst in 2005 were published in Valtonen et al. (2012) [7] and in Pihajoki et al. (2013) [3].
The intensive multi-site monitoring of OJ287 in the 2015/16 season resulted in best coverage ever obtained from ground: between mid November 2015 and mid May 2016 OJ287 was observed a few times per day.Our first goal was to search for any periodic signal present in the data around the December flare.We analyzed the residuals left after the trend plotted as the model line (Fig. 1 top panel) have been subtracted.Three methods were applied: regular Fourier Transform (FT), wavelet and the running FT (rFT).We found no significant (above 4 σ level) peaks with FT.A period of about 3 hours can be recognized, however, at about 2 σ level only.Both wavelet and rFT techniques revealed a presence of a statistically significant, short living period of about 3 days at the outburst maximum.The period of its visibility was centered at the maximum of brightness (Fig. 2) -it showed up near JD 2457360 and disappeared after about 4 days.We also performed a thorough search using the entire season dataset covering the period from mid September 2015 to mid May 2016.Several statistical tools have been used and we show the Lomb-Scargle periodogram (LSP; Lomb 1976 [2], Scargle 1982 [4]) in the left panel of Fig. 3.The red-noise (β=1.5)light curves were simulated by the randomization of both phase and amplitude as described in Timmer & Koenig 1995 [5].The light curves were then re-sampled according to the sampling of the real light curve, and their LSP was computed.The mean LSP of 1000 simulated light curves is shown in green in Figure 3a.No significant peaks corresponding to short periods were found.In the longer periods domain there seems to be statistically significant peaks in the range between 0.01 and 0.1 c/d.However, the WWZ analysis (Foster 1996 [1]) indicate they are not stable.As seen in Fig. 3 (right panel), the length of the longest, about 95 days period, has been increasing since it started to be visible at about JD 2457330.

K2 observations
OJ287 had been observed by the Kepler spacecraft during K2 Campaign 5.This run resulted in almost continuous 75 days coverage (2015, April 27 to July 10) with about 1 minute cadence.We used both short and long cadences target pixel files.We employed our custom IRAF tasks to pull out fluxes, applying three-pixel circular apertures.We computed PSD functions for the resulting light curve and also 2015/16 ground based data.Neither show any statistically significant periodicities that could be attributed to QPOs.

Conclusions
We found no stable periods in the OJ287 photometric data over the entire 2015/16 season.However, the 95-day peak in the power spectrum is close to the period for the more massive BH ISCO while the 43-day peak half to it.Accretion in the form of one armed stationary spiral density wave should show up as the full ISCO period, while two armed stationary wave will feed the central black hole at one-half of the ISCO period.Both types of density waves are observed e.g. in galactic disks under perturbation.These phenomena are not expected to produce stable periodicities as interactions between the exact ISCO period and wave frequencies may occur.The 95-day period started to be visible somewhat before the December outburst and its best visibility continued after the outburst.

Figure 1 .
Figure 1.R filter light curve of OJ287 gathered during the 2015/2016 season.(a) The light curve of the December 2015 outburst is shown in the top panel.(b) The 2015/16 season light curve is presented in the bottom panel.The December 2015 high amplitude flare turned out to be unpolarized.

Figure 2 .
Figure 2. Running FT of the OJ287 data gathered during the 2015/2016 outburst.

Figure 3 .
Figure 3. (a) left panel: Lomb-Scargle periodogram of 2015/16 data (blue line).The 99% confidence level is shown as the red contour.(b) right panel: the resulting graph from the wavelet Z-transform analysis.