A 6.65-day pseudo-periodicity in OJ 287’s flaring in 2023 Mark Kidger

2024 February 8

Mark Kidger

Since they were discovered in the early 1960s, quasars have posed many problems for astrophysics. It is now generally accepted they are galactic nuclei in which activity is triggered by a supermassive black hole accreting material. The many different manifestations of activity are linked to the orientation of the relativistic jet emitted from the poles of the accretion disc. Here, we examine the light curve of one of the extreme class of quasars known as blazars, OJ 287, which shows large, high-amplitude and often very rapid variations, and is an object long suspected of showing epochs of quasi-periodic variations (often termed ‘Quasi-Periodic Oscillations‘, or QPOs). A strong periodicity shows up in analysis of intensive amateur CCD monitoring of OJ 287 over the 2022–’23 observing campaign, although further analysis reveals that this period, which is clearly visible in the light curve, is limited to data from the first quarter of 2023. The QPO manifests in rapid flares in the light curve with a characteristic interval of 6.65±0.34 days and is probably due to a shock spiralling in the magnetic field in the blazar’s relativistic jet.

Introduction

The Active Galactic Nucleus bestiary

It is generally accepted that quasars are galactic nuclei in which activity is triggered by an accreting supermassive black hole (Lynden-Bell, 1969).¹ However, the discovery of quasars was a direct consequence of efforts in the late 1950s to identify and measure the diameter of radio sources. As of the mid-1950s, just eight radio sources had been correlated with known celestial objects (the Sun, Jupiter, the Galactic Centre, the Andromeda Galaxy, the Crab Nebula, Cygnus A, Cassiopeia A and Puppis A). The Crab, Cassiopeia A and Puppis A were all linked with supernova remnants; in contrast, Cygnus A was identified with a peculiar galaxy. At the time, an animated cosmological debate was continuing between the Steady State and Big Bang theories. It was suggested that, if Cygnus A were typical of radio sources, measurements of their diameters would allow the predictions of the two models to be compared. The Steady State theory predicted the minimum observed diameter of radio sources would be eight arcseconds; if smaller, it would instead support the Big Bang model.

A race started to measure as many diameters as possible using interferometry. The story of this fascinating race is retold by Sir Bernard Lovell in his book Out of the Zenith.² Lovell recounts how the scientists at Jodrell Bank first made measurements between two radio telescopes on the Jodrell Bank site, but several objects stubbornly refused to resolve themselves. They used progressively longer baselines, taking advantage of other telescopes around the British Isles, each time convinced the next step in separation between telescopes would resolve the remaining targets. Even though it became obvious that a stubborn minority were far smaller than cosmological theories would predict, there was great interest in discovering what these anomalous sources were. As we know now, the problem was that many of the objects were not like Cygnus A, so the basic assumption that all would have the same physical diameter was incorrect. Combining telescopes on different continents and using shorter wavelengths allowed even greater resolution to be achieved, but a few sources still could not be resolved. In the most extreme case, one object, 3C 345 (later identified as a strongly variable quasar at redshift 0.595), was found to emit a significant fraction of its energy from a region less than 0.0004 arcseconds in diameter, making it a tiny target, even at a cosmological distance. The recognition that some radio sources appeared to have diameters no greater than stars made it imperative to identify what type of celestial object they might be.

As later pointed out by Maarten Schmidt (2011),³ the discovery of quasars was a gradual, incremental process between 1960 and 1963. The eureka moment occurred on 1963 February 6, when Schmidt realised the strange emission lines in 3C 273 and 3C 48 were those of highly redshifted hydrogen and magnesium (Schmidt, 1963).⁴ The phenomenon of the quasar was born, later to be grouped within the umbrella of Active Galactic Nuclei (AGN). Initially, they were split into radio-loud objects (Quasi-Stellar Sources, QSSs) and radio-quiet objects (Quasi-Stellar Objects, QSOs – that looked like QSSs, but had no detectable radio emission).

Over time, many types of objects have been included within the spectrum of those classed as AGN. These include radio-quiet quasars, radio-loud quasars, Seyfert galaxies, BL Lac objects, blazars and N galaxies. The common factor among them is that there is an underlying galaxy, which may or may not be detectable, with a brilliant, active nucleus. In this nucleus there is a supermassive central black hole accreting material from the host galaxy, liberating huge quantities of energy as it does so. Depending on the state of rotation of the singularity, between six and 43 per cent of the rest mass of the accreted material is liberated as energy as it is accreted (Hills, 1975).⁵

A relativistic jet is emitted from the poles of the accretion disc. The jet is like a lighthouse beam, concentrating the light because it is approaching us at relativistic velocity and thus beaming the signal strongly in the direction of movement. Depending on how well the jet is aligned relative to our line of sight, we can be deeper inside the core of the beam and see increasingly extreme properties. Broadly speaking, objects are radio quiet if the jet is not aligned in our direction and radio loud if it is. Blazars (sometimes denominated ‘Optically Violent Variables’, or OVVs) are the small fraction of objects in which we see into the throat of the jet itself. They show particularly violent variability, plus other properties such as high and variable polarisation. At redshift 0.306 and 1.9 degrees to our line-of-sight, OJ 287’s relativistic jet is positioned such that we can see further down its throat than in the case of any other object. Some blazars have a jet at a smaller angle but are more distant, so we do not see as far down the throat. For example, quasar 0642+449 has its jet aligned at just 0.8 degrees, but it is at redshift 3.396 (Savolainen et al., 2010).⁶

Periodicity vs pseudo-periodicity

Periodicity analysis is a powerful tool to investigate processes in a wide range of astrophysical objects. Many types of periodicity exist in the world of variable stars: pulsational, vibrational, rotational, and orbital. Periodicity is not a significant element of quasar light curves, although a long and sometimes bitter debate raged about it from the 1960s to 1980s. However, there are many cases of what appear to be short-term periodic signals in quasar light curves. Such pseudo-periodicities, often referred to as Quasi-Periodic Oscillations (QPOs), have been largely ignored until quite recently but carry potentially valuable information about the mechanisms that generate them in the quasar.
What do we mean by pseudo-periodicity? Periodicity is a predictable, repeated signal that appears or reproduces at an exactly defined interval. Examples are Cepheid variables, pulsars, Algol-type variables and RS CVn stars. In many cases, such as Cepheids and Algol types, the light curve repeats exactly every period. In other cases, such as pulsars, even though the period is exactly repeating, the amplitude may vary. In all cases, we see a regular period because there is an underlying astrophysical process repeating in a precise pattern. This may be the oscillation of a star, the orbit of two stars about their common centre of gravity, or the rotation of a star that reveals and hides massive starspots.

In contrast, pseudo-periodicity is an apparently periodic variation, often of highly variable amplitude, which appears and disappears at random moments and, usually, does not repeat. It is a temporary, unstable period with little or no predictive power other than in the short term because it may disappear at any moment and not reappear. As such, it is frustrating for the observer because just when you think that you are seeing what appears to be a cyclical variation, it disappears. You can never be certain whether it is a genuine signal, or just a result of random variations that seem to stabilise briefly, mimicking periodicity. For this reason, unless there are multiple cycles of the variation, it is not reasonable to claim that there is even a temporary periodic signal in the light curve. A further complication in the detection and analysis of QPOs is that they are superimposed on other variations in the blazar, which may mask them in the light curve.

Periodicity vs random behaviour in quasar light curves
Between the 1960s and 1980s, many studies undertook periodicity analysis of quasar light curves, resulting in many claims of periodic and possible periodic behaviour. Often the claimed ‘period’ was based on a data sample little longer than the light curve itself, limiting its credibility. However, there were some cases of more credible possible periodicities that led to considerable debate. Two of the earliest and best-documented cases concerned 3C 273 and 3C 345.
Immediately after the identification of 3C 273, it became apparent that this bright (magnitude 13) source exists in hundreds of pre-discovery Harvard plate collection images. This allowed a light curve to be constructed from 1885 showing slow variations with a maximum amplitude of ~1.5 magnitudes. Attention focused rapidly on a particularly well-covered epoch between the 1920s and 1940s that showed what appeared to be an almost perfect sinusoidal variation with a period of 13 years (Smith, 1968).⁷ A frequently heated debate grew between groups who found this period throughout the full light curve (e.g. Ozernoy, Gudzenko & Chertoprud, 1977) and those who believed it to be a single, random event in a light curve dominated by random processes (e.g. Fahlman & Ulrich, 1976).8,9 It is now accepted that the apparently sinusoidal variation was, indeed, a random event that has not repeated since.

3C 345 was recognised in 1965 and was one of the first quasars to be identified showing rapid, violent variations in its visible light curve. A three-year light curve study by Tom Kinman and colleagues at Lick Observatory (Kinman et al., 1968) showed regular flares occurred every 80 days.10 Over the course of their study, nine were observed; what was particularly interesting was the amplitude remained remarkably constant even though the base level of the light curve from which they occurred varied considerably. Kinman et al. waited until they were certain of the reality of the periodicity before publishing but, even so, the 80-day cycle disappeared even before the paper appeared in print!

(Log in to view the full illustrated article in PDF format)