10 research outputs found
Bayesian Approach to Find a Long-Term Trend in Erratic Polarization Variations Observed in Blazars
We developed a method to separate a long-term trend from observed temporal
variations of polarization in blazars using a Bayesian approach. The temporal
variation of the polarization vector is apparently erratic in most blazars,
while several objects occasionally exhibited systematic variations, for
example, an increase of the polarization degree associated with a flare of the
total flux. We assume that the observed polarization vector is a superposition
of distinct two components, a long-term trend and a short-term variation
component responsible for short flares. Our Bayesian model estimates the
long-term trend which satisfies the condition that the total flux correlates
with the polarized flux of the short-term component. We demonstrate that
assumed long-term polarization components are successfully separated by the
Bayesian model for artificial data. We applied this method to photopolarimetric
data of OJ 287, S5 0716+714, and S2 0109+224. Simple and systematic long-term
trends were obtained in OJ 287 and S2 0109+224, while no such a trend was
identified in S5 0716+714. We propose that the apparently erratic variations of
polarization in OJ 287 and S2 0109+224 are due to the presence of the long-term
polarization component. The behavior of polarization in S5 0716+714 during our
observation period implies the presence of a number of polarization components
having a quite short time-scale of variations.Comment: 12 pages, 7 figures, accepted for publication in PAS
Photopolarimetric Monitoring of Blazars in the Optical and Near-Infrared Bands with the Kanata Telescope. I. Correlations between Flux, Color, and Polarization
We report on the correlation between the flux, color and polarization
variations on time scales of days--months in blazars, and discuss their
universal aspects. We performed monitoring of 42 blazars in the optical and
near-infrared bands from 2008 to 2010 using TRISPEC attached to the "Kanata"
1.5-m telescope. We found that 28 blazars exhibited "bluer-when-brighter"
trends in their whole or a part of time-series data sets. This corresponds to
88% of objects that were observed for >10 days. Thus, our observation
unambiguously confirmed that the "bluer-when-brighter" trend is common in the
emission from blazar jets. This trend was apparently generated by a variation
component with a constant and relatively blue color and an underlying red
component. Prominent short-term flares on time scales of days--weeks tended to
exhibit a spectral hysteresis; their rising phases were bluer than their decay
phases around the flare maxima. In contrast to the strong flux--color
correlation, the correlation of the flux and polarization degree was relatively
weak; only 10 objects showed significant positive correlations. Rotations of
polarization were detected only in three objects: PKS 1510-089, 3C 454.3, and
PKS 1749+096, and possibly in S5 0716+714. We also investigated the dependence
of the degree of variability on the luminosity and the synchrotron peak
frequency, \nu_peak. As a result, we found that lower luminosity and higher
\nu_peak objects had smaller variations in their amplitudes both in the flux,
color, and polarization degree. Our observation suggests the presence of
several distinct emitting sources, which have different variation time-scales,
colors, and polarizations. We propose that the energy injection by, for
example, internal shocks in relativistic shells is a major factor for blazar
variations on time scales of both days and months.Comment: 39 pages, accepted for publication in PAS
Early Spectroscopy of the 2010 Outburst of U Scorpii
We present early spectroscopy of the recurrent nova U~Sco during the outburst
in 2010. We successfully obtained time-series spectra at 0.37--0.44~d, where denotes the time from the discovery of the
present outburst. This is the first time-resolved spectroscopy on the first
night of U Sco outbursts. At ~d the H line consists
of a blue-shifted ( km s) narrow absorption component and a wide
emission component having triple peaks, a blue ( km s), a
central ( km s) and a red ( km s) ones. The
blue and red peaks developed more rapidly than the central one during the first
night. This rapid variation would be caused by the growth of aspherical wind
produced during the earliest stage of the outburst. At ~d the
H line has a nearly flat-topped profile with weak blue and red peaks at
km s. This profile can be attributed to a nearly
spherical shell, while the asphericity growing on the first night still
remains. The wind asphericity is less significant after d.Comment: 5 pages, 3 figures, Accepted for publication of PASJ Letter