Swift observation of strong optical-UV and X-ray flaring activity of OJ 287

ATel #9709; F. Verrecchia (ASDC Rome & INAF OAR Rome, Italy), S. Ciprini (ASDC Rome & INFN Perugia, Italy), M. Valtonen (Turku Univ., Finland), S. Zola (Jagiellonian Univ., Poland)
on 4 Nov 2016; 11:58 UT
Credential Certification: Francesco Verrecchia (francesco.verrecchia@asdc.asi.it)

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A new time-domain astronomy experiment performed with the Swift satellite is ongoing since mid Oct. 2016 on the peculiar BL Lac object OJ 287 (z=0.306) after the observation of a very-high optical flux (private communication and follow-up ATel#9650 and ATel#9675). The first brightening of OJ 287 was already reported on Sept. 12 2016 (ATel#9489).

The first four Swift observations are obtained through our renewed ToO program, after the program performed last winter (ATel#8401 and Valtonen et al. 2016, 819, 37). Some results about the first of our ToO, the Oct.12, 2016, have been published in ATel#9629, to trigger follow-up observations by ground-based radio/optical telescopes.

We report the results of a preliminary analysis of the October evolution of the OJ 287 UV and X-ray emission. We requested Swift XRT data in Photon Counting (PC) mode for a total exposure of about 8 ksec divided in four observations. After our first two observations performed on Oct.12 and Oct.15 another ToO program introduced an almost daily XRT monitoring of OJ 287 performed in Window Timing (WT) mode.

Each of the four single X-ray spectrum (0.3-10 keV) we report here, can be fit by an absorbed single power-law model, keeping fixed the Galactic HI column density value in the direction of the source (n_H = 2.5 X 10^20 cm^-2, Kalberla et al. 2005, A&A, 440, 775). The preliminary corresponding unabsorbed (0.3-2.0 keV) integral daily fluxes are (18.9+/-0.8), (12.2+/-0.9), (14.5+/-0.7) and (13.2+/-0.7) X 10^-12 erg cm^-2 s^-1 on Oct.12 (PC), 15 (PC), 18 (WT), and 21 (WT) respectively, which correspond to a factor 2 to 5.5 times higher than those reported with XRT in Dec. 2015 (ATel#8401) and 2 to about 13 times the fluxes observed in June 2015 in the same band. The corresponding power law photon spectral indexes are respectively 2.7+/-0.1, 2.5+/-0.1, 2.8+/-0.1, 2.7+/-0.1, which are steeper than the previously measured values for this blazar.

The de-reddened UVOT flux density values, obtained using the official calibration (Breeveld et al.2011) and a galactic E(B-V) value in the direction of the source (0.0244, Schlafly et al. 2011, and considering a mean galactic extinction law, Fitzpatrick et al.1999), for the 3 UV filters from Oct.12 to 18 are in the following ranges: W1=[4.5-6.6] mJy; M2=[5.0-5.7] mJy; W2=[4.1-4.9] mJy, are a factor 1 to 3 times higher than those reported in Dec. 2015 (ATel#8401) in the same filters.

Correlated X-ray and optical/UV flares are often observed in blazars and a rather smooth UV to X-ray spectrum produced by the synchrotron process is suggested for this active phase of OJ 287. In the frame of a binary supermassive black hole model, this current simultaneous optical-UV-X-ray flare is expected to be related to in-jet flaring activity which arise when the perturbation from the disk impact of Nov./Dec. 2015 by the secondary black hole (Valtonen et al. 2016, 819, 37) has progressed to its jet. The amplitude of the perturbation transferred in the jet as a function of time was predicted in Pihajoki et al. 2013, ApJ, 764, 5.

In consideration of the ongoing activity of OJ 287 we encourage multiwavelength observations. The Swift gamma-ray burst explorer mission is part of NASA's medium explorer (MIDEX) program, led by NASA with participation of Italy and the UK. We would like to thank the Swift Team for making these observations possible, in particular S. Sbarufatti as the Swift Observatory Duty Scientist.