Feature selection for classification of blazars

Uemura, M., Abe, T., Yamada, Y., and Ikeda, S., “Feature selection for classification of blazars based on optical photometric and polarimetric time-series data”, PASJ, 72, 74, 2020

We investigated the differences of two sub-types of blazars, FSRQs, and BL Lac objects, in the variability. We characterize the variability using the Ornstein–Uhlenbeck (OU) process, and search for the features that are discriminative for the two subtypes. We used optical photometric and polarimetric data obtained with the 1.5 m Kanata telescope for 2008–2014. We found that four features, namely the variation amplitude, characteristic timescale, and non-stationarity of the variability obtained from the light curves and the median of the degree of polarization (PD), are essential for distinguishing between FSRQs and BL Lac objects. The characteristics of the variability imply that the nature of the variation in the jets is common in FSRQs and BL Lac objects. We found that BL Lac objects tend to have high PD medians, which suggests that they have a stable polarization component. FSRQs have no such component, possibly because of a strong Compton cooling effect in sub-parsec-scale jets.

Degeneracy of the SED Model of the blazar Mrk 421

Yamada, Y.,  Uemura, M.,  Itoh, R.,  Fukazawa, Y.,  Ohno, M., and Imazato, F., “Variations of the physical parameters of the blazar Mrk 421 based on analysis of the spectral energy distributions”, PASJ, 72, 42, 2020

The MCMC code for the SSC model optimization to the SED data is available at the project page of yysscfit

We report on the variations of the physical parameters of the jet observed in the blazar Mrk 421, and discuss the origin of X-ray flares in the jet, based on analysis of several spectral energy distributions (SEDs). The SEDs are modeled using the one-zone synchrotron self-Compton model, its parameters determined using a Markov chain Monte Carlo method. The lack of data at TeV energies means many of the parameters cannot be uniquely determined and are correlated. These are studied in detail. We find that the optimal solution can be uniquely determined only when we apply a constraint to one of four parameters: the magnetic field, the Doppler factor, the size of the emitting region, and the normalization factor of the electron energy distribution. The result of our SED analysis suggests that, in the X-ray faint state, the emission occurs in a relatively small area with a relatively strong magnetic field. The X-ray bright state shows a tendency opposite to that of the faint state. The presence of two kinds of emitting areas implies that the one-zone model is unsuitable for reproducing at least part of the observed SEDs.

Optical polarization variations in the blazar PKS 1749+096

Uemura, M., Itoh, R., Liodakis, I., Blinov, D., Nakayama, M., Xu, L., Sawada, N., Wu, H.-Y., and Fujishiro, I., “Optical polarization variations in the blazar PKS 1749+096”, PASJ, 69, 96, 2017

We report on the variation in the optical polarization of the blazar PKS 1749+096. The object favors a polarization angle (PA) of 40°-50° at the flare maxima, which is close to the position angle of the jet (20°-40°). Three clear polarization rotations were detected in the negative PA direction associated with flares. The light-curve maxima of the flares possibly tended to lag behind the PD maxima and color-index minima. We propose a scenario to explain these observational features, where transverse shocks propagate along curved trajectories. The favored PA at the flare maxima suggests that the observed variations were governed by the variations in the Doppler factor, δ. Based on this scenario, the minimum viewing angle of the source, θ _min = 4.8°-6.6°, and the location of the source, Δr ≳ 0.1 pc, from the central black hole were estimated. The combined effect of the variation in δ and acceleration/cooling of electrons is probably responsible for the observed diversity of the polarization variations in the flares.

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