Drink Your OJ 287

Title: Absence of the predicted 2022 October outburst of OJ 287 and implications for binary SMBH scenarios

Authors: S. Komossa, D. Grupe, A. Kraus, et al.

First Author’s Institution: Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Status: Published in MNRAS [open access]

Title: On the need of an ultramassive black hole in OJ 287

Authors: M. Valtonen, S. Zola, A. Gopakumar, et al.

First Author’s Institute: FINCA, University of Turku, FI-20014 Turku, Finland

Status: Published in MNRAS [open access]

OJ 287 is the brightest BL Lacertae (BL Lac) blazars in the sky, clocking in at an apparent magnitude of 15.43 in the V-band. It’s been observed regularly since the late 1880s, and given its position right on the ecliptic in the past two decades it has been incidentally well observed by surveys like TESS. As of May 2024, there are over 250 refereed publications with OJ 287 in the title, and it has been studied in many more.

A plot with time in years on the x-axis and V-band flux / mJy on the y-axis. It shows the brightness of OJ 287 over time from 1888 to 2024 in purple dots. There are a number of flaring events with an 11-12 year period.
Fig 1: Valtonen+23 Fig.1 Left Panel. V-band time series data for OJ 287 dating all the way back to 1888.

BL Lac sources are a subtype of active galactic nuclei (AGN), named after the prototype of the class, BL Lac. These sources are characterized by their large and fast changes in brightness and significant polarization at the optical wavelengths. In the unified scheme of AGN classifications, which theorizes that the different “classes” of AGN are due to differences in their relative orientations to us, BL Lac objects are a subclass of blazars, and are thought to have a relativistic jet pointed directly at the observer. 

It’s debated what exactly is going on in the central galaxy of OJ 287. There is evidence of a flaring event with a period of 11-12 years in its light curve, which suggests it contains a supermassive black hole binary (SMBHB) with two co-orbiting black holes in the center of the accretion disk from which the jets are launched. There are two main models for the flares: one where they are due to interactions between the secondary BH and the accretion disk, and one where they’re due to relativistic beaming in the jets. 

One version of the former model has been in development since the 90’s by M. Valtonen and collaborators.  It predicts a precessing binary (PB) system, with one extremely massive 1010 solar mass primary black hole, and a smaller 108 solar mass secondary in a highly elliptical (e=0.66), highly precessing orbit. That is a really big primary black hole – others have been observed in that mass range but the theoretical upper limit for an SMBH is ~1011 solar masses. The PB model does a good job predicting the flare behavior of OJ 287, with a number of verified predictions from the mid-90s through the 2010s, and claims an accuracy on the order of days. 

The premise behind the PB model is that the 11-12 year periodic flares come from the secondary black hole’s impact onto the accretion disk. This impact leads to thermal emission due to interaction with the disk and increased jet outflows from the secondary BH as its Roche Lobe fills with disk gas. It may even be possible that the jets from the secondary black hole outshine the jet from the primary black hole under these circumstances.

A 2D diagram showing a side cross-section (x-y plane) of the hypothesized system. There is a red accretion disk surrounded by a purple corona stretching horizontally, in which sits the primary black hole and its two orange jets in the y direction. The smaller black hole's precessing orbit is shown, along with the positions of its impacts with the accretion disk.
Fig 2: Valtonen+23 Fig. 1 right panel. This shows the binary black hole setup, with the smaller black hole in precessing orbit around the primary shown in blue lines. The accretion disk is in red, the corona is in purple, and the orange/yellow regions show the jets from the black holes. Impacts of the secondary black hole are indicated with their crossing dates.

Since the PB model predicts flares extremely accurately in time (down to 4 hours in 2019), multi-wavelength observation campaigns have been performed around the predicted dates to observe the flares. If these flares can be well predicted and observed, they will provide valuable information about blazar disk-jet interactions and act as tests of general relativity, like the black hole no-hair theorem and SMBHB orbit shrinking due to gravitational wave emissions. 

One of those observing campaigns was the Multiwavelength Observations and Modelling of OJ 287 (MOMO). The PB model predicted there would be a flare in July 2022, when it would be unobservable due to its proximity to the Sun, but this was later updated to October 2022. The MOMO team performed observations using the Swift observatory in UV-optical, x-ray, and radio from September-December 2022. 

The published MOMO results (Komossa+23b)  did not find evidence for a flare during that period. They also used their observations to test the PB model, which requires the very large primary BH mass and a very high Eddington ratio Ldisk/LEdd = 0.08. This ratio compares the observed luminosity of the disk to the maximum theoretical accretion luminosity of the given black hole (the Eddington Luminosity, LEdd). A ratio of 0.08 indicates that the AGN is emitting at 8% of its possible maximum. 

Komossa+23b estimated, based on broad line scaling relations of the H_\beta line, a total SMBH mass of just 1.3×108  solar masses, 100 times less than the mass predicted by PB. They also calculated the Eddington ratio from both the bolometric disk luminosity and the disk’s H \alpha line, and found estimates that were a factor of ~10 and ~100 smaller than the PB model. If this were the case, then the disk physics used in the PB model would not be accurate, and the flares could not be caused by impacts of the secondary black hole. However, when the Eddington luminosity is calculated using the 108 solar mass black hole the Eddington ratio is ~0.13. This is consistent with other BL Lac objects, and suggests that this smaller primary mass might be more accurate. 

Later in 2023, Valtonen+23 and the group behind the PB model responded to Komossa+23b’s work, suggesting that they overestimated the luminosity contributed by the accretion disk. Valtonen+23 showed accretion disk models that suggest that most of the power in the disk is taken up by the hot corona, the jets, and the disk wind, so the V-band of the disk is not likely to be brighter than the host galaxy. The Komossa+23b paper places all of the power and radiation into the V-band, giving their estimate of the disk as 6 orders of magnitude brighter than the host galaxy. This overestimate would propagate through their calculations, leading to inaccurate estimates of the primary BH mass and accretion rate. 

No model for such a complex and dynamic system is going to be perfect. Even if the Komossa luminosity estimation is off, there is plenty of literature presenting variants on the PB model and even entirely different methods of explaining OJ 287. While some questions remain about when the 2022 flare was expected (a preprint updated the date to October 2022, but later papers by the group still use the original July 2022), no other model has had such success in predicting the optical flaring long-term. 

What does this mean for OJ 287? More models, more observations, and more debate via the arXiv. The next major flare predicted by the PB model is in the 2030s, but work on this fascinating system will certainly continue in the meantime. 

The papers covered in this bite mostly focus on the optical band, but for more on the radio observations and possibility of EHT coverage, check out this astrobite. 

Astrobite edited by Magnus L’Argent

Featured image credit: USDA, NASA

About Lindsey Gordon

Lindsey Gordon is a fourth year Ph.D. candidate at the University of Minnesota. She works on AGN jets, radio relics, MHD simulations, and how to use AI to study all those things better.

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