We present optical and near-infrared (NIR) photometry of a classical nova, V2362 Cyg (= Nova Cygni 2006). V2362 Cyg experienced a peculiar rebrightening with a long duration from 100 to 240 d after the maximum of the nova. Our multicolor observation
indicates an emergence of a pseudophotosphere with an effective temperature of 9000 K at the rebrightening maximum. After the rebrightening maximum, the object showed a slow fading homogeneously in all of the used bands for one week. This implies that the fading just after the rebrightening maximum ( less or equal 1 week ) was caused by a slowly shrinking pseudophotosphere. Then, the NIR flux drastically increased, while the optical flux steeply declined. The optical and NIR flux was consistent with blackbody radiation with a temperature of 1500 K during this NIR rising phase. These facts are likely to be explained by dust formation in the nova ejecta. Assuming an optically thin case, we estimate the dust mass of 10^(-8) -- 10^(-10) M_solar, which is less than those in typical dust-forming novae. These results support the senario that a second, long-lasting outflow, which caused the rebrightening, interacted with a fraction of the initial outflow and formed dust grains.
We report on optical-near-infrared photopolarimetric observations of a blazar 3C 454.3 over 200 d. The object experienced an optical outburst in July 2007. This outburst was followed by a short state fainter than $V=15.2$ mag lasting $sim 25$ d. The
object, then, entered an active state during which we observed short flares having a timescale of 3-10 d. The object showed two types of features in the color-magnitude relationship. One is a bluer-when-brighter trend in the outburst state, and the other is a redder-when-brighter trend in the faint state. These two types of features suggest a contribution of a thermal emission to the observed flux, as suspected in previous studies. Our polarimetric observation detected two episodes of the rotation of the polarization vector. The first one was a counterclockwise rotation in the $QU$ plane during the outburst state. After this rotation event of the polarization vector, the object entered a rapidly fading stage. The second one was seen in a series of flares during the active state. Each flare had a specific position angle of polarization, and it apparently rotated clockwise from the first to the last flares. Thus, the object exhibited rotations of the polarization vector in opposite directions. We estimated a decay timescale of the short flares during the active state, and then calculated an upper limit of the strength of the magnetic field, $B$=0.2 G, assuming a typical beaming factor of blazars, $delta=20$. This upper limit of $B$ is smaller than those previously estimated from spectral analysis.
We report detailed, long term near-infrared (NIR) light curves of GRS 1915+105 in 2007-2008, covering its long soft state for the first time. From our NIR monitoring and the X-ray data of the All Sky Monitor (ASM) onboard Rossi X-ray Timing Explorer
(RXTE), we discovered that the NIR flux dropped by > 1 mag during short X-ray flares with a time-scale of days. With the termination of the soft state, the H-Ks color reddened and the anti-correlation pattern was broken. The observed H-Ks color variation suggests that the dominant NIR source was an accretion disk during the soft state. The short X-ray flares during the soft state were associated with spectral hardening in X-rays and increasing radio emission indicating jet ejection. The temporal NIR fading during the X-ray flares, hence, implies a sudden decrease of the contribution of the accretion disk when the jet is ejected.
We present the result of near-infrared and optical observations of the BL Lac object S5 0716$ + $714 carried out by the KANATA telescope. S5 0716$ + $714 has both a long term high-amplitude variability and a short-term variability within a night. The
shortest variability (microvariability) time-scale is important for understanding the geometry of jets and magnetic field, because it provides a possible minimum size of variation sources. Here, we report the detection of 15-min variability in S5 0716$ + $714, which is one of the shortest time-scales in optical and near-infrared variations observed in blazars. The detected microvariation had an amplitude of $0.061{pm}0.005$ mag in $V$ band and a blue color of $Delta(V-J)=-0.025{pm}0.011$. Furthermore, we successfully detected an unprecedented, short time-scale polarimetric variation which correlated with the brightness change. We revealed that the microvariation had a specific polarization component. The polarization degree of the variation component was higher than that of the total flux. These results suggest that the microvariability originated from a small and local region where the magnetic field is aligned.
