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Register a new userOptical Emission and Particle Acceleration in a Quasi-Stationary Component in the Jet of OJ~287
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We analyze the linear polarization of the relativistic jet in BL Lacertae object OJ~287 as revealed by multi-epoch Very Long Baseline Array (VLBA) images at 43 GHz and monitoring observations at optical bands. The electric-vector position angle (EVPA) of the optical polarization matches that at 43 GHz at locations that are often in the compact millimeter-wave core or, at other epochs, coincident with a bright, quasi-stationary emission feature $sim0.2$~milliarcsec ($sim$0.9~pc projected on the sky) downstream from the core. This implies that electrons with high enough energies to emit optical synchrotron and $gamma$-ray inverse Compton radiation are accelerated both in the core and at the downstream feature, the latter of which lies $geq10$~pc from the central engine. The polarization vector in the stationary feature is nearly parallel to the jet axis, as expected for a conical standing shock capable of accelerating electrons to GeV energies.
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We present a multi-wavelength spectral and temporal investigation of OJ 287 emission during its strong optical-to-X-ray activity between July 2016 - July 2017. The daily $gamma$-ray fluxes from emph{Fermi}-LAT are consistent with no variability. The strong optical-to-X-ray variability is accompanied by a change in power-law spectral index of the X-ray spectrum from $< 2$ to $>2$, with variations often associated with changes in optical polarization properties. Cross-correlations between optical-to-X-ray emission during four continuous segments show simultaneous optical-ultraviolet (UV) variations while the X-ray and UV/optical are simultaneous only during the middle two segments. In the first segment, the results suggest X-rays lag the optical/UV, while in the last segment X-rays lead by $sim$ 5-6 days. The last segment also shows a systematic trend with variations appearing first at higher energies followed by lower energy ones. The LAT spectrum before the VHE activity is similar to preceding quiescent state spectrum while it hardens during VHE activity period and is consistent with the extrapolated VHE spectrum during the latter. Overall, the broadband spectral energy distributions (SEDs) during high activity periods are a combination of a typical OJ 287 SED and an HBL SED, and can be explained in a two-zone leptonic model, with the second zone located at parsec scales, beyond the broad line region, being responsible for the HBL-like spectrum. The change of polarization properties from systematic to chaotic and back to systematic, before, during and after the VHE activity, suggest dynamic roles for magnetic fields and turbulence.
We report the detection of a probable $gamma$-ray quasi-periodic oscillation (QPO) of around 314 days in the monthly binned 0.1 -- 300 GeV $gamma$-ray {it Fermi}-LAT light curve of the well known BL Lac blazar OJ 287. To identify and quantify the QPO nature of the $gamma$-ray light curve of OJ 287, we used the Lomb-Scargle periodogram (LSP), REDFIT, and weighted wavelet z-transform (WWZ) analyses. We briefly discuss possible emission models for radio-loud active galactic nuclei (AGN) that can explain a $gamma$-ray QPO of such a period in a blazar. Reports of changes in the position of quasi-stationary radio knots over a yearly timescale as well as a strong correlation between gamma-ray and mm-radio emission in previous studies indicate that the signal is probably associated with these knots.
The latest flare of the regular $sim$ 12 years quasi-periodic optical outbursts in the binary SMBH candidate system OJ 287 occurred in December 2015. Following this, the source has exhibited enhanced multi-wavelength (MW) variability in spectral, temporal and polarization domains with new features never seen before. Our MW investigation show that the overall MW variability can be divided into two-phase, (i) November 2015 -- May 2016 with variability from near-infrared (NIR) to Fermi-LAT $rm gamma$-ray energies (0.1 -- 300 GeV), and (ii) September 2016 -- July 2017 with intense NIR to X-ray variability but without any activity in the Fermi-LAT band, and the very first detection at very high energies (VHEs, E $>$ 100 GeV) by VERITAS. The broadband SEDs during the first phase show a thermal bump in the NIR-optical region and a hardening in the $rm gamma$-ray spectra with a shift in its peak. The thermal bump like feature is consistent with the description of the standard accretion-disk associated with the primary SMBH of mass $sim 1.8times10^{10} M_odot$ while the $rm gamma$-ray emission can be naturally reproduced by inverse Compton scattering of photons from the broad line region which has been seen during the close encounter duration of the binary SMBHs, thereby suggesting a sub-parsec scale origin. The SEDs during the second phase (VHE detection) is a mixture of typical OJ 287 SED with hardened $rm gamma$-ray spectra and an HBL SED and can be explained in a two-zone model, one located at sub-parsec scales and other at parsec scales. During both the phases, the MW variability is simultaneous and almost always accompanied by changes in the polarization properties, exhibiting random and systematic variations, suggesting a strong role of magnetic field and turbulence.
We report the re-emergence of a new broadband emission through a detailed and systematic study of the multi-wavelength spectral and temporal behavior of OJ 287 after its first-ever reported VHE activity in 2017 to date, which includes the second-highest X-ray flux of the source. The source shows high optical to X-ray flux variations, accompanied mainly by strong spectral changes. The optical to X-ray flux variations are correlated and simultaneous except for two durations when they are anti-correlated. The flux variations, however, are anti-correlated with the X-ray spectral state while correlated with optical-UV (ultraviolet). Weekly binned {it Fermi}-LAT data around the duration of the highest X-ray activity show a few detections with a log-parabola model but none with a power-law; yet the extracted LAT spectral energy distribution (SED) of the high activity duration for both the models is similar and show a hardening above 1 GeV. Further, near-infrared (NIR) data indicate strong spectral change, resembling a thermal component. Overall, the combined optical to gamma-ray broadband spectrum establishes the observed variations to a new high-energy-peaked (HBL) broadband emission component, similar to the one seen during the highest reported X-ray flux state of the source in 2017. The observed activities indicate some peculiar features that seem to be characteristic of this emission component while its appearance, a few years around the claimed (sim 12)-year optical outbursts strongly indicate a connection between the two.
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.
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