Spectral States of OJ 287 blazar from Multiwavelength 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.

Long-term Multiband Near-infrared Variability of the Blazar OJ 287 during 2007–2021

“…2021; Singh et al 2022). Both these bright X-ray phases came after the claimed double-peaked outbursts: the 2015 (Valtonen et al 2016) and 2019 (Laine et al 2020) flares of the ∼12 yr optical QPOs.…”

“…In short, although blazars are known for dynamic flux variability, they rarely show significant spectral departures in the broadband SEDs. OJ 287, on the other hand, is quite unique with spectral changes persisting for much longer time (e.g., Brien & VERITAS Collaboration 2017;Kushwaha et al 2018aKushwaha et al , 2018bKushwaha et al , 2021Prince et al 2021;Singh et al 2022) and thus, a potential source for fresh inputs not only on relativistic jets above what is generally known about blazars but also on aspects related to accretion as well (e.g., Kushwaha 2020Kushwaha , 2021.…”

“…Significant spectral changes at NIR energies as well as a hysteresis between NIR and optical color variation have also been reported (Bonning et al 2012). In terms of strong all-around changes in observational behavior, the period around the latest double-peaked outburst has been remarkable (Gupta et al 2017(Gupta et al , 2019Kushwaha et al 2018aKushwaha et al , 2021Singh et al 2022). However, the data used in these studies are mostly biased toward high-activity states.…”

“…Alternative interpretations of these recurrent outbursts invoke simple jet precession scenarios (e.g., Britzen et al 2018;Butuzova & Pushkarev 2020, and references therein). The jet precession models, however, are not favored by the spectral changes reported in the near-infrared (NIR) to γrays during and after the most recent outbursts of 2015 (Komossa et al 2017(Komossa et al , 2020O'Brien 2017;Kushwaha et al 2018aKushwaha et al , 2018bPal et al 2020) and 2019 (Komossa et al 2020;Kushwaha et al 2021;Singh et al 2022). The timing of the most recent outbursts (2015 and 2019) considered within the binary disk-impact model indicates the significant effect of gravitational-wave (GW) energy loss.…”

“…Under the project, here we present the first densely sampled multiband long-term NIR temporal and spectral variability study of the blazar OJ 287 from 2007 December 18-2021 November 13. A multiwavelength study reporting the spectral and temporal behavior will be presented in follow-up work (P. Kushwaha et al 2022, in preparation) that will deal with the vastly different sampling and data integration times in different portions of the EM spectrum.…”