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 (firstname.lastname@example.org)
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).