Prosto Science
OJ 287 Analysis Summary
What is the mass of the black hole in OJ 287?
When people ask “what is the mass of the black hole in OJ 287”, our independent Swift/XRT X-ray spectral analysis – based on flux-selected averaged spectra and a flux–index saturation scaling approach – suggests a mass of about 200 million solar masses (≈ 2 × 108 M⊙) for the primary black hole in the system.
Core data-analysis problem
The Swift/XRT observations of OJ 287 consist of many short exposures, typically around one kilosecond each. Individually these snapshots contain too few counts for statistically meaningful spectral modeling, which makes it difficult to study how the X-ray spectral shape changes with flux using standard per-exposure fits.
The central task of this work was therefore to reconstruct usable spectra by aggregating and averaging the data in flux-selected groups. Multiple low-count exposures were combined into statistically robust XRT spectra while preserving calibration and intrinsic spectral shape—providing a basis for estimating the black hole mass in OJ 287.
Background
OJ 287 is a blazar and a long-standing candidate for hosting a binary system of supermassive black holes. Its X-ray emission offers a direct view of the accretion flow close to the central engine. Swift/XRT provides extensive monitoring of OJ 287, but the data come in many short, fragmented pointings, each with limited counts.
Motivation
This analysis was initiated in response to a request from a theoretical astrophysics group working on accretion-flow models that predict a characteristic behaviour of the X-ray spectral index as a function of flux: a correlation at low fluxes followed by saturation at higher fluxes.
To test this prediction in OJ 287, a reliable flux–index relation was required. Because individual Swift/XRT exposures were too short to model on their own, they had to be regrouped by flux and averaged into high-quality spectra—answering, in practice, the question of how to average XRT spectra without losing the underlying physics.
Analysis method
All Swift/XRT observations of OJ 287 were reprocessed in a uniform way and then sorted according to source flux. Within each flux range, multiple short exposures were averaged into a single spectrum with sufficient statistical quality for modeling.
This flux-based averaging made it possible to track how the X-ray spectral index changes with flux and to identify the transition from a correlated regime to a saturated regime in the flux–index diagram.
Results and interpretation
The reconstructed flux–index relation shows a clear correlation at low fluxes and saturation of the spectral index at higher fluxes, consistent with theoretical expectations for accreting black hole systems.
Using the saturation level and the location of this transition within the adopted scaling framework, the analysis yields an independent estimate of the black hole mass in OJ 287 of approximately 2 × 108 M⊙ (≈ 200 million solar masses).
Later studies of OJ 287 by Valtonen et al. (2023) and Komossa et al. (2023) have also argued for a black hole of comparable mass, though based on different physical models and observing strategies. Despite the differing approaches, these works converge on a consistent mass scale of a few ×108 M⊙ for OJ 287.
References
Kuznetsov, S. 2024, MNRAS, 535, 3732 —
article
Valtonen, M. J., et al. 2023, MNRAS, 525, 1153 —
article
Komossa, S., et al. 2023, MNRAS Letters, 522, L84 —
article