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Register a new userFlux and polarization variability of OJ 287 during early 2016 outburst
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The gamma-ray blazar OJ 287 was in a high activity state during December 2015 - February 2016. Coinciding with this high brightness state, we observed this source for photometry on 40 nights in R-band and for polarimetry on 9 epochs in UBVRI bands. During the period of our observations, the source brightness varied between $13.20 pm 0.04$ to $14.98 pm 0.04$ mag and the degree of polarization (P ) fluctuated between $6.0 pm 0.3$% and $28.3 pm 0.8$% in R-band. Focusing on intra-night optical variability (INOV), we find a duty cycle of about 71% using $chi^2$-statistics, similar to that known for blazars. From INOV data, the shortest variability time scale is estimated to be $142 pm 38$ min yielding a lower limit of the observed Doppler factor $delta_0 = 1.17$, the magnetic field strength $B le 3.8$ G and the size of the emitting region Rs < $2.28 times 10^{14}$ cm. On inter-night timescales, a significant anti-correlation between R-band flux and P is found. The observed P at U-band is generally larger than that observed at longer wavelength bands suggesting a wavelength dependent polarization. Using V -band photometric and polarimetric data from Steward Observatory obtained during our monitoring period we find a varied correlation between P and V-band brightness. While an anticorrelation is seen between P and V -band mag at sometimes, no correlation is seen at other times, thereby, suggesting the presence of more than one short-lived shock components in the jet of OJ 287.
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We present recent optical photometric observations of the blazar OJ 287 taken during September 2015 -- May 2016. Our intense observations of the blazar started in November 2015 and continued until May 2016 and included detection of the large optical outburst in December 2016 that was predicted using the binary black hole model for OJ 287. For our observing campaign, we used a total of 9 ground based optical telescopes of which one is in Japan, one is in India, three are in Bulgaria, one is in Serbia, one is in Georgia, and two are in the USA. These observations were carried out in 102 nights with a total of ~ 1000 image frames in BVRI bands, though the majority were in the R band. We detected a second comparably strong flare in March 2016. In addition, we investigated multi-band flux variations, colour variations, and spectral changes in the blazar on diverse timescales as they are useful in understanding the emission mechanisms. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux, colour and spectral variability.
We report on a recent multi-band optical photometric and polarimetric observational campaign of the blazar OJ 287 which was carried out during September 2016 -- December 2017. We employed nine telescopes in Bulgaria, China, Georgia, Japan, Serbia, Spain and the United States. We collected over 1800 photometric image frames in BVRI bands and over 100 polarimetric measurements over ~175 nights. In 11 nights with many quasi-simultaneous multi-band (V, R, I) observations, we did not detect any genuine intraday variability in flux or color. On longer timescales, multiple flaring events were seen. Large changes in color with respect to time and in a color--magnitude diagram were seen, and while only a weak systematic variability trend was noticed in color with respect to time, the color--magnitude diagram shows a bluer-when-brighter trend. Large changes in the degree of polarization, and substantial swings in the polarization angle were detected. The fractional Stokes parameters of the polarization showed a systematic trend with time in the beginning of these observations, followed by chaotic changes and then an apparently systematic variation at the end. These polarization changes coincide with the detection and duration of the source at very high energies as seen by VERITAS. The spectral index shows a systematic variation with time and V-band magnitude. We briefly discuss possible physical mechanisms that could explain the observed flux, color, polarization, and spectral variability.
Suzaku observations of the blazar OJ 287 were performed in 2007 April 10--13 and November 7--9. They correspond to a quiescent and a flaring state, respectively. The X-ray spectra can be well described with single power-law models in both exposures. The derived X-ray photon index and the flux density at 1 keV were found to be Gamma = 1.65 +- 0.02 and S_{1 keV} = 215 +- 5 nJy, in the quiescent state. In the flaring state, the source exhibited a harder X-ray spectrum (Gamma = 1.50 +- 0.01) with a nearly doubled X-ray flux density S_{1 keV} = 404^{+6}_{-5} nJy. Moreover, significant hard X-ray signals were detected up to ~ 27 keV. In cooperation with the Suzaku, simultaneous radio, optical, and very-high-energy gamma-ray observations were performed with the Nobeyama Millimeter Array, the KANATA telescope, and the MAGIC telescope, respectively. The radio and optical fluxes in the flaring state (3.04 +- 0.46 Jy and 8.93 +- 0.05 mJy at 86.75 Hz and in the V-band, respectively) were found to be higher by a factor of 2--3 than those in the quiescent state (1.73 +- 0.26 Jy and 3.03 +- 0.01 mJy at 86.75 Hz and in the V-band, respectively). No notable gamma-ray events were detected in either observation. The spectral energy distribution indicated that the X-ray spectrum was dominated by inverse Compton radiation in both observations, while synchrotron radiation exhibited a spectral cutoff around the optical frequency. Furthermore, no significant difference in the synchrotron cutoff frequency was found between the quiescent and flaring states. According to a simple synchrotron self-Compton model, the change of the spectral energy distribution is due to an increase in the energy density of electrons with small changes of both the magnetic field strength and the maximum Lorentz factor of electrons.
We have obtained a smooth time series for the Electric Vector Position Angle (EVPA) of the blazar OJ 287 at centimeter wavelengths, by making $pm npi$ adjustments to archival values from 1974 to 2016. The data display rotation reversals in which the EVPA rotates counter-clockwise (CCW) for 180 deg and then rotates clockwise (CW) by a similar amount. The time scale of the rotations is a few weeks to a year, and the scale for a double rotation, including the reversal, is one to three years. We have seen four of these events in 40 years. A model consisting of two successive outbursts in polarized flux density, with EVPAs counter-rotating, superposed on a steady polarized jet, can explain many of the details of the observations. Polarization images support this interpretation. The model can also help to explain similar events seen at optical wavelengths. The outbursts needed for the model can be generated by the super-magnetosonic jet model of Nakamura et al. (2010) and Nakamura and Meier (2014), which requires a strong helical magnetic field. This model produces forward and reverse pairs of fast and slow MHD waves, and the plasma inside the two fast/slow pairs rotates around the jet axis, but in opposite directions.
We present the results of simultaneous multi-frequency imaging observations at 22, 43, 86, and 129,GHz of OJ,287. We used the Korean VLBI Network as part of the Interferometric MOnitoring of GAmma-ray Bright active galactic nuclei (iMOGABA). The iMOGABA observations were performed during 31 epochs from 2013 January 16 to 2016 December 28. We also used 15,GHz OVRO and 225,GHz SMA flux density data. We analyzed four flux enhancements in the light curves. The estimated time scales of three flux enhancements were similar with time scales of $sim$50 days at two frequencies. A fourth flux enhancement had a variability timescale approximately twice as long. We found that 225,GHz enhancements led the 15,GHz enhancements by a range of 7 to 30 days in the time delay analysis. We found the fractional variability did not change with frequency between 43 and 86,GHz. We could reliably measure the turnover frequency, $ u_{rm c}$, of the core of the source in three epochs. This was measured to be in a range from 27 to 50,GHz and a flux density at the turnover frequency, $S_{rm m}$, ranging from 3-6,Jy. The derived SSA magnetic fields, $B_{rm SSA}$, are in a range from $0.157pm0.104$ to $0.255pm0.146$ mG. We estimated the equipartition magnetic field strengths to be in a range from $0.95pm0.15$ to $1.93pm0.30$ mG. The equipartition magnetic field strengths are up to a factor of 10 higher than the values of $B_{rm SSA}$. We conclude that the downstream jet may be more particle energy dominated.
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