OJ 287 in a deep X-ray-UV-optical low-state, following the April-June 2020 "super-outburst"

ATel #14052; S. Komossa (MPIfR), D. Grupe (Morehead State University), A. Kraus (MPIfR), J. L. Gomez (Instituto de Astrofisica de Andalucia), A. Gopakumar (TIFR), M. Valtonen (University of Turku), L. Dey (TIFR), M. L. Parker (XMM-Newton Science Operations Centre)
on 30 Sep 2020; 09:56 UT
Credential Certification: St. Komossa (stefanie.komossa@gmx.de)

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We have monitored OJ 287 densely with the Neil Gehrels Swift observatory and the Effelsberg radio telescope since its December 2015 maximum. Before OJ 287 became unobservable due to its solar proximity, it was in outburst reaching the second-brightest X-ray state with Swift (ATel #13658, #13702, #13785; Komossa et al. 2020), characterized by a very soft spectral component and interpreted as "after-flare" in the context of the binary black hole model for OJ 287.

Here, we report detection of OJ 287 in a low-state in all bands, when we re-observed it with Swift UVOT and XRT after emerging from sun-constraint on 2020 September 20. On that date, we measured an (0.3-10) keV X-ray countrate of 0.13 cts/s, and UVOT magnitudes in the VEGA system of: UVW2: 15.12+-0.07 (15.07), UVM2: 15.15+/-0.08 (14.91), UVW1: 15.14+/-0.08 (14.97), U: 15.40+/-0.07 (15.27), B: 16.27+/-0.08 (16.16), V: 15.85+/-0.10 (15.77), where values in brackets are corrected for Galactic extinction. This is a factor of ~8 fainter in X-rays than at its peak value at outburst, and ~2 magnitudes fainter than its peak UVW2 magnitude at outburst.

The X-ray photon index Gamma_x follows the "softer-when-brighter" variability pattern we reported previously with Swift: During the ongoing low-state, we measure very flat powerlaw indices with Gamma_x ~ 2.1 (assuming a single powerlaw with Galactic absorption), while at high-state, Gamma_x was much steeper up to ~3 and the spectrum was best described by a two-component model consisting of a log-parabola emission model at low X-ray energies, and a flatter NuSTAR powerlaw component up to ~70 keV(Komossa et al. 2020).

While the X-rays show no variability in observations taken between 2020 September 20 and September 28, the optical-UV has been on rise, most systematically seen in the V and B band, with B magnitudes increasing from 16.27 on September 20 to 16.04 on September 28.

In the radio regime, the flux density of OJ 287 is still high with 6.2+-0.1 Jy at 14.25 GHz during the last Effelsberg observation on 2020 September 27.

The blazar OJ 287 is one of the best candidates to date for hosting a binary supermassive black hole, with past "impact flares" reported in 2015 and 2019 (Valtonen et al. 2016, Laine et al. 2020). The Swift and Effelsberg results presented here are part of our dedicated multi-year, multi-frequency monitoring of OJ 287 (e.g., ATel #8411, #9629, #10043, #12086, #13785; Komossa et al. 2017, 2020).
We would like to thank the Swift and Effelsberg teams for carrying out the observations.