The December 2015 optical outburst of OJ 287: X-ray and UV time-domain monitor by Swift

ATel #8401; S. Ciprini (ASDC Rome & INFN Perugia, Italy), M. Perri (ASDC Rome & INAF OAR Rome, Italy), F. Verrecchia (ASDC Rome & INAF OAR Rome, Italy), M. Valtonen (Turku Univ., Finland)
on 12 Dec 2015; 00:39 UT
Credential Certification: Stefano Ciprini (stefano.ciprini@asdc.asi.it)

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The Swift satellite is monitoring from 2015 Nov. 28 the BL Lac object OJ 287 (z=0.306). Our Swift X-ray and UV time-domain monitor program has been proposed and granted in response to a predicted, and then observed, phase of increased optical activity from OJ 287 (see, ATel#8372 and ATel#8378). A regular ground-based optical photometric monitoring is also performed by several observatories since the beginning of this season, with the aim of catching the outburst foreseen by a model of in-spiralling spinning binary super massive black holes (Valtonen et al. 2008, Nature, 452, 851; Pihajoki et al. 2013, ApJ, 764, 5; Byrd et al. 2015, in Edited by C. Rovelli, General Relativity: The most beautiful of theories, De Gruyter Publ., 67).

Swift XRT data are being taken in Photon Counting mode for a total exposure of about 20 ksec divided in daily observations. The first 8 observations, from Nov. 28 to Dec. 8, 2015 are reported here (part of XRT results are anticipated in ATel#8395). Each single X-ray spectrum (0.3-10 keV) can be fit by an absorbed single (or broken) power-law model, with an HI column density consistent with the Galactic one in the direction of the source (n_H = 2.56 X 10^20 cm^-2, Kalberla et al. 2005, A&A, 440, 775). The X-ray spectra have photon indexes between about 1.4 and 1.9. The preliminary corresponding unabsorbed (0.3-2.0 keV) integral daily fluxes are reported in the table below, joined with simultaneous de-reddened UVOT flux density values obtained with the 3 UV filters:

Mean MJD	X-ray (0.3-2.0 keV) flux [-err/+err]	Mean MJD	UV-W1_flux+/-err	UV-M2_flux+/-err	UV-W2_flux+/-err
[day]	[X 10^-12 erg cm^-2 s^-1]	[day]	[mJy]	[mJy]	[mJy]
57354.00307	3.50 [-0.30/+0.30]	57354.00346	2.11+/-0.082	1.92+/-0.088	1.52+/-0.059
57358.97296	4.25 [-0.27/+0.27]	57358.98149	3.54+/-0.117	3.05+/-0.119	2.47+/-0.082
57359.58098	4.96 [-0.31/+0.34]*	57359.58725	3.64+/-0.121	3.19+/-0.125	2.54+/-0.084
57360.64844	5.26 [-0.28/+0.31]	57360.65550	6.26+/-0.208	5.55+/-0.184	4.50+/-0.149
57361.43231	6.03 [-0.31/+0.34]	57361.43870	5.35+/-0.178	4.61+/-0.153	3.74+/-0.124
57362.16991	4.39 [-0.29/+0.37]	57362.17362	---	2.66+/-0.104	2.13+/-0.071
57363.17519	5.11 [-0.29/+0.28]	57363.18084	3.67+/-0.122	3.05+/-0.119	2.45+/-0.081
57364.30174	4.79 [-0.22/+0.26]	57364.30984	3.88+/-0.129	3.44+/-0.134	2.81+/-0.093

*broken power law model.

These X-ray flux values are approximately from 1 to 4 times the 0.3-2.0 keV fluxes observed during the XRT monitor performed in June 2015. These data preliminary suggests a limited range of X-ray and UV variability consistent, on average, with previous observations. Even if the XRT data do not show a prominent peak as observed in optical bands between Dec. 4 and Dec. 6, they may suggest a simultaneous and slightly enhanced X-ray brightness. Our Swift time domain monitor program for this target is continuing.

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 D. Malesani as the Swift Observatory Duty Scientist.