Swift detection of the blazar OJ 287 in a deep UV-optical low-state in the course of the MOMO program

ATel #15145; S. Komossa (MPIfR), D. Grupe (Morehead State University), M. Valtonen (University of Turku)
on 26 Dec 2021; 20:16 UT
Credential Certification: St. Komossa (stefanie.komossa@gmx.de)

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We have monitored the blazar OJ 287 densely during the last six years with the Neil Gehrels Swift observatory in the course of the project MOMO (Multi-wavelength Observations and Modelling of OJ 287; e.g. Komossa et al. 2017, 2020, 2021a,b,c). Based on our most recent observations, we report the detection of a deep UV-optical low-state in all six UVOT filters. The low-state has so far lasted for at least three weeks, between 2021 December 2-25.

On 2021 December 20, we measured Swift UVOT magnitudes in the VEGA system of UVW2: 15.75+/-0.06 (15.55), UVM2: 15.63+/-0.05 (15.39), UVW1: 15.56+/-0.05 (15.39), U: 15.54+/-0.04 (15.41), B: 16.37+/-0.04 (16.26), and V: 15.83+/-0.06 (15.75), where values in brackets are corrected for Galactic extinction. A similar optical-UV faintness was last seen during the 2017 October-December deep fade and in 2020 September (ATel #14052).

While the UV and optical emission is in low-state, the X-rays do not follow the deep fade but remain relatively constant within the errors. On 2021 December 20, we measured an absorption-corrected X-ray flux of f_x = 5.0+/-0.4 10^-12 ergs/s/cm^2 and a photon index Gamma_x = 2.12+/-0.20 in the 0.3-10 keV band (using a single powerlaw fit with Galactic absorption). This pattern is similar to the remarkable UV-optical deep fade seen in 2017 October-December (Komossa et al. 2020, 2021c), where the flux in the UVOT bands dropped by a factor 3 and the X-rays did not follow. (During outbursts, the UV-optical emission is closely correlated with the X-rays.)

UV-optical deep fades are well suited to obtain imaging observations of the host galaxy of OJ 287 that are much more difficult to perform when the blazar emission is brighter.

The blazar OJ 287 is one of the best candidates to date for hosting a binary supermassive black hole (Valtonen et al. 2016). The Swift results presented here are part of our dedicated multi-year, multi-frequency (radio--X-ray) project MOMO (e.g. Komossa et al. 2017, 2020, 2021a,b,c).

We would like to thank the Swift team for carrying out the observations we proposed.