Spectral States of OJ 287 blazar from Multi-wavelength Observations with $AstroSat$

Abstract

We present AstroSat soft X-ray, near-UV (NUV) and far-UV (FUV) observations of a blazar, OJ 287, carried out in 2017, 2018, and 2020. The simultaneous observations with NuSTAR in 2017 provide a broad-band look encompassing NUV, FUV, soft and hard X-rays. Captured in three different broadband spectral states in three observations, the X-ray spectrum is found to be the hardest during 2018, while the high-energy-end of the simultaneous optical-FUV spectrum shows a steepening that is modeled with a broken power-law spectrum. The spectral energy distribution (SED) in 2017 shows a relatively flatter optical-FUV and soft X-ray spectra, implying an additional emission component. The 2020 optical-FUV spectrum is harder than in 2017 and 2018, with an extremely soft X-ray spectrum and a hardening above ∼1 GeV, similar to the SEDs of High-energy-peaked BL Lac objects (HBL), thereby establishing that this additional emission component has HBL-like properties. The AstroSat multi-wavelength observations trace the spectral evolution from the end-phase of the HBL component in 2017 to its disappearance in 2018 followed by its revival in 2020. A single zone leptonic model reproduces the 2018 broadband spectrum while the 2017 and 2020 SEDs require an additional HBL-like emitting zone. The spectral evolution of the high-energy-end of optical-UV spectrum, revealed by the FUV observations in 2017 and 2018, strongly suggests that X-ray spectral changes in the normal broadband spectral state of OJ 287 are primarily due to the evolution of the optical-UV synchrotron spectrum.

MOMO V. Effelsberg, Swift and Fermi study of the blazar and supermassive binary black hole candidate OJ 287 in a period of high activity

“…This non-thermal outburst was reported and discussed in detail by Komossa et al (2020) including the initial Swift discovery along with dedicated deep follow-up observations with XMM-Newton and NuSTAR and their modelling and interpretation, and by Komossa et al (2021d) in context with the other OJ 287 flux and spectral states of interest since 2005. [Our initial report about the detection of this outburst (ATel #13658) also triggered an additional X-ray observation (Prince et al 2021;Singh et al 2021) that still found OJ 287 in the supersoft X-ray state that characterized this outburst.] Here, we only point out that the detection of radio flaring activity independently confirms the previous conclusions about the nonthermal nature of the outburst.…”

MOMO – V. Effelsberg, Swift, and Fermi study of the blazar and supermassive binary black hole candidate OJ 287 in a period of high activity

“…This non-thermal outburst was reported and discussed in detail by Komossa et al ( 2020 ) including the initial Swift disco v ery along with dedicated deep follo w-up observ ations with XMM-Ne wton and NuSTAR and their modelling and interpretation, and by Komossa et al ( 2021d ) in context with the other OJ 287 flux and spectral states of interest since 2005. [Our initial report about the detection of this outburst (ATel #13658) also triggered an additional X-ray observation (Prince et al 2021 ;Singh et al 2021 ) that still found OJ 287 in the supersoft X-ray state that characterized this outburst.] Here, we only point out that the detection of radio flaring activity independently confirms the previous conclusions about the nonthermal nature of the outburst.…”

“…These campaigns were essential in identifying the sharp double-peaks o v er decades and in developing the binary model (e.g. Smith et al 1987 ;Sillanp ä ä et al 1988 ;Pursimo et al 2000 ;Villforth et al 2010 ;Valtonen & Sillanp ä ä 2011 ;Pihajoki, Valtonen & Ciprini 2013a ;Zola et al 2016 ;Dey et al 2018 ;Valtonen et al 2022 , and references therein), and in tracing the short-time variability characteristics with the Kepler mission (Wehrle, Carini & Wiita 2019 ). However, these important campaigns still lacked the broad-band co v erage that is necessary to distinguish between v arious dif ferent thermal and nonthermal emission processes from the jet, the accretion disc and the binary at all times.…”