Detection of a bright UV - X-ray flare of the blazar OJ 287 with Swift

ATel #13658; S. Komossa (MPIfR), D. Grupe (Morehead State University), J. L. Gomez (Instituto de Astrofisica de Andalucia)
on 23 Apr 2020; 21:13 UT
Credential Certification: St. Komossa (stefanie.komossa@gmx.de)

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We report the detection of an ongoing X-ray and UV-optical flare of the blazar OJ 287 during our monitoring with the Neil Gehrels Swift observatory. The flux levels are the highest values since several years. The X-rays are comparable in brightness to the December 2015 X-ray flare (interpreted as "impact flare" in the binary black hole model of OJ 287; Valtonen et al. 2016; ATel #8378, ATel #8401, ATel #8411) and to the February 2017 flare (which was likely jet-powered and coincident with a VHE detection; ATel #10043 and ATel #10051). On 2020 April 23, in X-rays, we measured a powerlaw of photon index Gamma_x = 2.42 +/- 0.08 (fixing absorption at the Galactic value). This implies an absorption-corrected (0.3-10 keV) X-ray flux of 2.02 e-11 erg/s/cm2. During this ongoing X-ray flare, the spectrum is softer than at quiescent levels (e.g., Gamma_x < 2 in late 2019). With Swift UVOT, we observed the following optical and UV magnitudes (Vega System; numbers in brackets are corrected for Galactic extinction): UVW2: 13.03+/-0.05 (12.83), UVM2: 13.01+/-0.06 (12.77), U: 13.38+/-0.05 (13.26), B: 14.30 +/-0.05 (14.19), V: 13.93+/-0.04 (13.85). In UVW2, OJ 287 is in its brightest state since the beginning of our Swift monitoring in 2015. A high-state in the optical waveband and a high degree of polarization was recently reported by Szola et al. (ATel #13637). Since then, the optical flux kept rising. Inspecting the public Fermi lightcurve of OJ 287 (https://fermi.gsfc.nasa.gov/), the weekly data reveal a detection during the last week. The Swift results reported here are part of our dedicated multi-year, multi-frequency monitoring of OJ 287 (Komossa et al. 2017, and 2020 in prep.). We will continue monitoring OJ 287 with Swift, and at radio frequencies with the Effelsberg telescope, and a GMVA observation of OJ 287 (PI: J.L. Gomez) is scheduled for April 26-27. Optical spectroscopy and polarimetry is encouraged.