Effelsberg and Swift observations of the blazar OJ 287 in outburst: detection of a radio flare

ATel #13702; S. Komossa (MPIfR), A. Kraus (MPIfR), D. Grupe (Morehead State University), L. Dey (TIFR), J. L. Gomez (Instituto de Astrofisica de Andalucia), A. Gopakumar (TIFR), M. Parker (XMM-Newton Science Operations Centre), M. Valtonen (University of Turku)
on 3 May 2020; 12:29 UT
Credential Certification: St. Komossa (stefanie.komossa@gmx.de)

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The blazar OJ 287 is experiencing a bright outburst in the UV, X-rays and optical (ATel # 13658, #13677, #13637). Here we report the detection of this outburst at radio frequencies. We observed OJ 287 with the 100m Effelsberg telescope on 2020 April 28 at frequencies of 2.6, 4.85, 10.45, 14.25 and 24.75 GHz. Data were taken using the cross-scan observing mode and we measured flux densities of 2.96 +/- 0.02, 3.86 +/- 0.01, 5.65 +/- 0.07, 6.27 +/- 0.07, 7.16 +/- 0.25 Jy, respectively. For comparison, on 2019 November 30 (at an epoch when OJ 287 was in an X-ray quiescent state) at frequencies of 2.6, 4.85, 10.45, 14.25, and 38.25 GHz, we obtained flux densities of 2.97 +/- 0.02, 3.70 +/- 0.03, 4.51 +/- 0.04, 4.72 +/- 0.09, 4.47 +/- 0.14 Jy, respectively. While the radio emission is enhanced at frequencies above ~10 GHz, the flare is not yet detected at lower frequencies as expected if opacity effects play a role. On the same day (2020 April 28), OJ 287 was also re-observed with the Neil Gehrels Swift observatory. Small systematic decreases in UV magnitudes (w.r.t our observation on April 23 reported in ATel #13658) by <0.1 mag indicate a plateau or decay of the outburst. X-rays were the brightest seen so far during this ongoing flare, with an absorption-corrected (0.3-10 keV) X-ray flux of 2.64 e-11 erg/s/cm2. The strong UV flux and the softness of the X-ray spectrum (Gamma_x = 2.63+/-0.06, assuming a single powerlaw and Galactic absorption) hint at a strong EUV bump possibly disk-related, while the radio flaring implies jet activity. 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 results reported here are part of our dedicated multi-year, multi-frequency monitoring of OJ 287 (Komossa et al. 2017, and 2020 in prep.).