Periodicity or Statistical Conspiracy? The Strange Case of OJ287

1. Periodicity or Statistical Conspiracy?The Strange Case of OJ287 Mark Kidger Herschel Science Centre, ESAC

2. What is OJ287? • OJ287 was first detected as a flat spectrum radio source in the Ohio surveys in 1968. • It was identified with a 15th magnitude compact blue object. • The optical spectrum was found to be featureless, but very weak variable emission lines have since been detected with a red shift of z=0.306. • OJ287 was classified as a BL Lac Object and, later, as a blazar.

3. What is OJ287? • OJ287 seems to be an exceptional object in that its relativistic jet is closer to our line of sight than any other AGN – we look down the throat of the jet. • There is a very high degree of relativistic beaming. • The host galaxy is barely detectable, even at 2 microns. DSS Near IR

4. Why is OJ287 interesting? • OJ287 is highly variable at all wavelengths from the centimetric to x-rays (and probably gamma rays). • The variability can be extremely violent – 50% or more in flux on time scales of less than an hour. • There is even a report of a 0.8 mag variation in the near-IR in 50 seconds!

5. Variation since identification • The optical light curve is characterised by occasional large outbursts and lower amplitude variability on all time scales. • There is a strong suggestion of long-term trends in the light curve.

6. Unlike many blazars, OJ287 shows little colour variation.

7. But not all outbursts are the same? • Slow, long-term variations in the colour index suggest that there are occasional large injections into the jet of relativistic electrons that then slowly decay radiatively. • There appears to be a long-term colour index change related to the large optical outburst in 1972. • But, not all outbursts affect the colour index – the 1983 outburst appears not to have affected the overall trend, nor have those of 1994 & 2005. • Expected if the injections occur in the jet, but the outbursts in the accretion disk.

8. Long-term variability • Light curve data for OJ287 exists since 20/01/1892 from archival photographic plates, although the level of sampling and data quality are much greater since identification. • Large outbursts are seen back to at least 1913.

9. A suggestion of periodicity • In 1988 Mauri Valtonen, Aimo Sillänpää at Tuorla Observatory in Finland noticed that the outbursts appeared to repeat at regular intervals. • They suggested that there was a period of 11.60.5 years and that the outbursts were due to the interaction of a binary supermassive singularity with an eccentric orbit.

10. Predictive power? • The 11.6 year period predicted correctly that a new outburst would occur in November 1994. • The maximum was also correctly predicted to be smaller than the 1983 outburst, according to the apparent 60-year modulation of amplitude. • This was a success for the model and widely regarded as confirming the periodicity.

11. Low frequency cut-off • Monitoring at radio frequencies shows that there was a pronounced low-frequency cut-off. • Only the second maximum, in November 1995, is seen in radio. • No outburst is seen at all at 4.8GHz. • The amplitude is very much greater at 22GHz than at 14GHz.

12. But is it that simple? • The optical-infrared outburst showed a double maximum with separation 13 months. • Only the second maximum is seen in the radio data. • Even at 860GHz there is no evidence of the first maximum. • OJ287 (apparently) was not in outburst in 1994 in ISO data, but the outburst is seen clearly at 2.2 microns.

13. Total indifference!  Outburst Quiescent

14. Amplitude v wavelength • Time averages in each frequency range show how the amplitude of variation changes with wavelength. • The width of the band gives the amplitude of variation. • The amplitude increases rapidly from the centimetric to the millimetric, but is highest in the visible/near-IR.

15. Time variation of the SED • Major changes are seen in the mean spectral energy distribution over the outburst. • On some occasions the spectrum becomes sharply peaked in the millimetric. • On others it is flat, or even increases to low frequencies.

16. But are the outbursts periodic? • Traditional methods of light curve periodic analysis (FFT, Deeming, etc.) do not work. • The light curve sampling has increased from <1 point per year in 1900 to 5600 points per year from 1993-95. • The error on each point has decreased by a factor of 10. • The Fourier Transform is totally dominated by data since 1972 (the 1993-95 data has 104-105 times more weight in the archive than the oldest data). • The method of data treatment affects the results obtained. • Although the outbursts have been “periodic” since 1972, only about three complete cycles have been observed – the historical data is critical in the light curve analysis.

17. The “exact” 11.6 year period seems to be a due to a statistical conspiracy. • There is no evidence for the Sillanpää period in the light curve before 1948.

18. The Lehto & Valtonen Model • Lehto & Valtonen (1996, ApJ, 460, 207) and Valtonen & Lehto (1997, ApJ, 481, L5), etc. propose • A binary singularity with masses 17x109 and 108M. • A rest frame orbital period of 8.9 years. • Eccentric orbit. • Pericentre advance 35º per orbit. • Superflares due to massive infall from the accretion disk caused by gravitational perturbations at pericentre. • Lifetime against decay by emission of gravitational radiation of 106 years. • Implies progressive decrease of period (1 week/orbit).

19. Open Questions in 2004 • How good is the predictive power of the model? • Is this true periodicity, or a pseudo-periodicity stable over a few cycles? • There were many predictions for the epoch of the next maximum (see Kidger, 2000, AJ, 119, 2053 for a summary). Most suggested that it will be between 2006.25 and 2006.75. • Much of this range is a period of poor visibility near conjunction, where radio observations would be crucial. • The secondary maximum in 2007 is also predicted to occur near conjunction. • Exact timing of the maxima should allow the model to be verified and refined. • Slightly different assumptions of initial parameters in the binary black hole model lead to very different predictions. • Consequence of the poor definition of the historical light curve.

20. Open Questions in 2004 • If the light curve is truly periodic, why is it that after >100 years there is such uncertainty in the period? • But, we have had only two well-observed outbursts (small number statistics!) • A lot of people have jumped on the periodicity bandwagon with ridiculously short data strings (20 years!!). • What is the relationship between the optical and radio outbursts? • Should alternatives to the binary black hole model such as a precessing jet perhaps be considered?

21. The 2006 outburst • The best-known prediction for the 2006 outburst (Pietilä, H. 1998, ApJ, 508, 669) and the periodic solution favour an outburst in summer 2006. • No coordinated monitoring programme has been organised to observe the outburst, unlike in 1993-1996. • At the 2005 AGM of the amateur “The Astronomer” Group in Basingstoke (England) a chance conversation with former British Astronomical Association Variable Star Section (BAA-VSS) Director, Gary Poyner, led us to set up an amateur monitoring campaign.

22. BAA-VSS Monitoring • The BAA-VSS has an archive of 343 observations of OJ287 from 1993 to mid-2005. • The majority are visual estimates made by the observer by eye at the telescope. • The first measure of the 2005-06 campaign was made on October 1st 2005. • 391 additional measures were made up to June 11th 2006 with telescopes from 20-50 cm. • 88 visual estimates • 186 unfiltered CCD measures • 55 CCD + V (8 in B, 25 in R, 29 in I)

23. Getting good quality data • The first step was to redefine the BAA-VSS sequence for OJ287. • One of the stars (#13) is now known to be a low amplitude variable. • There are 9 years of high precision BVRI photometry of the field stars from Teide Observatory that have been used. These tied down the faint end of the sequence. • Other CCD observations fixed the bright end.

24. Data quality checks • Quasisimultaneous data (t  1h) was used to check the zeropoint shift in the different V datasets. • As the colour variations of OJ287 are negligible the colour term can be ignored. • Mean colours in 2006 are (BV)=+0.62, (VR)=+0.44, similar to solar colours and slightly redder than historically. • Both unfiltered CCD and visual estimates had a significant zeropoint shift. • V = Visual + 0.14 • V = Unfiltered V + 0.09

25. Corrected amateur data • The corrected light curve shows an excellent agreement between filtered and unfiltered data. • The quality of the amateur data is excellent.

26. The campaign light curve • Initially we were disappointed to see a continuous fade rather than a slow rise to maximum. • OJ287 faded from V=13.6 on November 5th 2005 to V=16.1 on Feb. 16th 2006. • Several large flares are visible… • …but there was obviously no rise to a summer 2006 outburst.

27. Looking at the archive though, it was obvious that OJ287 was brighter in Nov. 2005 than at any time since March 1984.

28. The 2005 outburst • The peak of the 2005 outburst may have been missed, but even so the peak flux was higher than in 1994-95. • It is still a small outburst, at the minimum of the 60 year cycle. • It is advanced at least 10 months with respect to the 11.84 year period.

29. Bye bye stable period! • On a phase diagram the 1949, 1961, 1972, 1983 and 1994 maxima all line up for a stable 11.84 year period (red arrow). • Unless the 2005 maximum (blue arrow) is simply a precursor, there has been a large phase jump since 1994. • The 2005 maximum clearly does not fit a stable period.

30. The 2005 encounter • The Lehto & Valtonen model predicted that the 2005 encounter would be at low velocity and distant from the centre of the accretion disk. • A long delay from plane-crossing to optical flare. • A rather poorly defined maximum due to the encounter geometry. • A large advance in the date of the optical flare. 3295 test particles Path of secondary: 2005.19-2005.29

31. The 2005 encounter • Disk impact at 2005.09 • Primary accretion disk lifted 280AU out of plane. • Delay in optical flare 0.7yr. • Impact duration 0.23yr. • Secondary blast wave from impact reflection 0.2yr after impact. • Predicts that the 1994 maximum was at 1994.53! Impact Rebound Echo

32. Did we miss the 1994 outburst? • Superimposing the 2005 data on the 1994 light curve shows strong similarities, but the true 1994 maximum would have been hidden if it occurred when the revised model predicts  Perhaps we only saw the initial rise towards maximum and an aftershock in 1994.

33. The revised model • The 2005 maximum has allowed the model parameters to be revised. • The mass of the primary increases from 17 to 19 billion solar masses. • The precession rate increases from 33º to 39º per orbit. • The fit to the early data improves greatly. • A second, sharp maximum from a high-velocity ascending node encounter should occur in 2007.

34. Conclusions • The 2005 outburst and pre-1948 data show that a periodic light curve model is not viable. • OJ287 will show periodicity for 50 years at a time, but no longer. • The periodicity has just broken down. • But the binary singularity model can explain this behaviour. • The lifetime of the system against gravitational radiation is 106 years  There will be very few close AGN binaries. • The Lehto & Valtonen model is the only one that has accurately predicted the (unexpected) 2005 maximum.