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Effelsberg radio follow-up observations of the optical outburst of the blazar OJ 287: flux density and polarization

ATel #8411; S. Komossa (MPIfR), I. Myserlis (MPIfR), E. Angelakis (MPIfR), U. Bach (MPIfR), T. P. Krichbaum (MPIfR), D. Grupe (Morehead State University), W. Max-Moerbeck (MPIfR), A. Kraus (MPIfR), J. A. Zensus (MPIfR), M. Kramer (MPIfR)
on 14 Dec 2015; 23:12 UT
Credential Certification: St. Komossa (stefanie.komossa@gmx.de)

Subjects: Radio, X-ray, AGN, Black Hole, Blazar

We report radio observations with the Effelsberg 100m telescope of the blazar OJ 287, which is undergoing strong optical flaring activity (ATEL #8372, #8374, #8378, #8382). OJ 287 is one of the most massive blazars known, and a candidate for hosting a compact supermassive binary black hole (e.g., Valtonen et al. 2008, Nature 452, 851). The recent optical outburst may be related to the expected upcoming pericenter approach of the inspiralling secondary black hole (ATEL #8378).

OJ 287 was observed with the 100m Effelsberg telescope on 2015 December 13 at the frequencies of 4.85, 8.35, 10.45, 23.05 and 32.00 GHz. Data were taken using the cross-scan observing mode with a total observing time of 51 min, and with 3C48 and 3C295 as calibration sources. We measure flux densities of 2.98 +/- 0.05, 3.20 +/- 0.04, 3.48 +/- 0.31, 3.47 +/- 0.21, and 3.82 +/- 0.32 Jy, respectively. These values imply a mildly inverted spectrum (with index a = 0.12 +/- 0.02, where S ~ nu^a), and are comparable to the lower states of OJ 287 observed in the course of the F-GAMMA program between 2007 and 2014. The values of December 13 are similar to flux densities of OJ 287 during the last 6 months of Effelsberg observations (e.g., 2.59 +/-0.01 Jy at 4.85 GHz on November 10).

The latest flux density measurements with ALMA, based on the publicly available calibrator catalogue, indicate a possible slow rise at high frequencies, with flux densities of 3.96+/-0.1 Jy and 4.66+/-0.22 Jy at 91.5 GHz on November 25 and December 6, respectively.

In addition, polarimetry was performed with the Effelsberg telescope at 4.85 GHz and 8.35 GHz. We measure a degree of linear polarization of 2.0 +/- 0.2 % and 2.0 +/- 0.1 %, respectively, while the circular polarization degree is 0.6 +/- 0.2 and -0.3 +/- 0.1, respectively. The EVPA changes from -24.5 +/- 2.4 degrees at 4.85 GHz to -12.8 +/- 0.3 degrees at 8.35 GHz. The degree of linear polarization is slightly smaller than the median value obtained with the F-GAMMA monitoring program between July 2010 and January 2015 (2.2 and 2.8 % at 4.85 and 8.35 GHz, respectively; Myserlis 2015). This fact, together with the different EVPA and the possible change in the sign of circular polarization, may be indicating an upcoming flare. We will continue to monitor OJ 287 with the Effelsberg telescope.

A Swift observation was carried out on December 13, quasi-simultaneous with the Effelsberg observations, in the course of the recent OJ 287 monitoring campaign (ATEL #8401). We have fit the Swift XRT data with a powerlaw model with Galactic absorption of N_HI=2.49 10^20 cm^-2. The X-ray photon index of Gamma_x = 1.90+/-0.1 is consistent with the earlier value, Gamma_x=1.70+/-0.08, observed on December 5, when Swift detected an enhanced X-ray flux (ATEL #8395).