MNRAS 000, 1–12 (2015) Preprint 25 November 2021 Compiled using MNRAS L A T E X style file v3.0 A hadronic emission model for black hole-disc impacts in the blazar OJ 287 J. C. Rodr´ ıguez-Ram´ ırez, 1 ⋆ P. Kushwaha, 1 ,2 † E. M. de Gouveia Dal Pino 1 ‡ and R. Santos-Lima 1 § 1 Instituto de Astronomia, Geof´ ısica e Ciˆ encias Atmosf´ ericas (IAG-USP), Universidade de S˜ao Paulo. S˜ ao Paulo-SP 05508-090, Brasil. 2 Aryabhatta Research Institute of Observational Sciences (ARIES). Nainital 263002, India. Accepted XXX. Received YYY; in original form ZZZ ABSTRACT A super-massive black hole (SMBH) binary in the core of the blazar OJ 287 has been invoked in previous works to explain its observed optical flare quasi-periodicity. Follow- ing this picture, we investigate a hadronic origin for the X-ray and γ-ray counterparts of the December 2015 major optical flare of this source. An impact outflow must re- sult after the lighter SMBH (the secondary), crosses the accretion disc of the heavier one (the primary). We then consider acceleration of cosmic-ray (CR) protons in the shock driven by the impact outflow as it expands and collides with the active galactic nucleus (AGN) wind of the primary SMBH. We show that the emission of these CRs can reproduce the X-ray and γ-ray flare data self-consistently with the the optical component of the December 2015 major flare. We derive different emission profiles explaining the observed flare data having magnetic fields in the range B = 0.3 − 3 G in the emission region and power-law indices of q = 2.2 − 2.4 for the energy distribution of the emitting CRs. The mechanical luminosity of the AGN wind represents  10% of the mass accretion power of the primary SMBH in all the derived emission profiles. Key words: accretion – shock waves – astroparticle physics – radiation mechanisms: non-thermal 1 INTRODUCTION Theoretical arguments as well as indirect observational ev- idences suggest the presence of super-massive black hole (SMBH) pairs coalescing in the core of certain galaxies. Galaxy mergers (Springel et al. 2005), for instance, might be a natural process leading to the formation of such SMBH binaries. Compelling examples of active galactic nu- clei (AGNs) approaching each other can be found in the recent works by, e. g., Pfeifle et al. (2019) and Deane et al. (2014), where the SMBHs of approaching AGNs are localised at distances from tens to hundreds of parsecs between each other. When the distance among two SMBHs shrinks to sub- parsec scales, the system is theoretically expected to enter its gravitational wave (GW)-driven regime for orbital decay. In such a stage, SMBH binaries are thought to be the most prominent sources of GWs in the cosmos (Begelman et al. 1980; Mingarelli et al. 2017). Current instruments however, ⋆ E-mail: juan.rodriguez@iag.usp.br † E-mail:pankaj.tifr@gmail.com ‡ E-mail: dalpino@iag.usp.br § E-mail: lima.reinaldo.santos.de@gmail.com are not able to detect either GWs from SMBHs systems (expected in the nHz- µHz domain), or resolve SMBHs bina- ries at sub-parsec scales. Alternatively, indirect signatures as double line emission (Popovi´ c 2012) and quasi-periodical flares in certain AGNs (Komossa & Zensus 2016) are em- ployed to trace the presence of compact, orbiting SMBH pairs. Due to a persistent quasi-periodical feature in opti- cal, the blazar OJ 287 is perhaps the strongest candidate for hosting a sub-parsec SMBH pair (Dey et al. 2018). OJ 287 (at a red-shift z = 0.304) is categorised as a BL Lac object and is known for its regular ∼12 year, double peaked optical variations registered for over 130 years (Sil- lanpaa et al. 1988; Hudec et al. 2013). These periodic fea- tures have motivated a number of possible explanations (e.g., Lehto & Valtonen 1996; Katz 1997; Tanaka 2013; Britzen et al. 2018). Particularly, the SMBH binary scenario pro- posed by Lehto & Valtonen (1996) (see also Valtonen et al. 2008) appears to predict naturally the timing of the dou- ble peaked observed outbursts. Additionally, this model is consistent with the sharp rise of the flare emission and its low polarisation degree, being these aspects not satisfacto- rily explained by other models (see Dey et al. 2019; Kush- waha 2020, for more details). © 2015 The Authors arXiv:2005.01276v1 [astro-ph.HE] 4 May 2020