We report the discovery of a quasar, SDSS J125809.31+351943.0 (J1258), which brightened in optical wavelengths for 4 mag from 1983 to 2015: one of the largest quasar brightening events so far. The history of optical photometry data of this quasar from the Catalina Real-time Transient Survey and All Sky Automated Survey for Super Novae (ASAS-SN), mid-infrared photometry data from the WISE satellite, and the broad emission line (BEL) flux obtained by spectroscopy of the Sloan Digital Sky Survey shows significant increases between 2003 and 2015. Investigating the Canada–France–Hawaii Telescope photometric observations in 1983 and the United States Naval Observatory B catalogue, which contains data from 1975 and 1969, we found that the source was 4 mag fainter before than at the peak of the recent ASAS-SN photometry. From the history of these data, we identified J1258 as a new changing-state quasar (CSQ). We also performed follow-up spectroscopic observations in 2018 December and 2019 May using the 2 m telescope at the Nishi-Harima Astronomical Observatory. The results show that the continuum flux and the BEL flux decreased to about 50% of their peak. This indicates that J1258 present two changing states for the BEL flux and continuum flux. We argue that J1258’s variability, especially its brightening event, can be explained by the propagation of the heating front and the accretion disk state transitions based on the timescale and Eddington ratio variations. The estimated mass of the black hole of J1258 is about an order of magnitude larger than the CSQs found so far. Since both the changing timescale and the size of the accretion disk depend on the black hole mass, the J1258 brightening event can be interpreted as a scaled version of the variability in other CSQs. This suggests that samples of distant quasars with larger black hole masses may contain objects with longer and more severe variations.

Recently, a new class of quasars, called changing-look quasars (CLQs), has been reported. CLQs exhibit significant changes in optical and mid-infrared luminosity, accompanied by the appearance or disappearance of broad emission lines. We performed spectroscopic observations at the Nishi-Harima Astronomical Observatory in 2018 May to identify objects that show long-term (∼10 yr) continuous optical luminosity variations as candidates for CLQs. We discovered significant fluctuations in the Hβ emission line of 3C 332. This source has previously been reported to have a characteristic double-peaked Hα emission line. To explain the shape of the emission lines, a ring disk model has been proposed, and the possibility of a binary black hole has also been suggested. To further investigate the characteristics of 3C 332, we performed nine spectroscopic observations at Okayama Observatory (Kyoto University) from 2020 February to June. Based on the change in the velocity offset of the emission lines and the negative correlation between the line intensity and the velocity difference between the two peaks, we conclude that the double-peaked origin of this source is consistent with the ring disk model. In addition, the timescale of the changing look (of the optical and mid-infrared luminosities) is consistent with the thermal timescale or the propagation timescale of the heating/cooling front.

Brightness variation is an essential feature of quasars, but its mechanism and relationship to other physical quantities are not understood well. We aimed to find the relationship between the optical variability and spectral features to reveal the regularity behind the random variation. It is known that a quasar’s Fe ii/Hβ flux ratio and equivalent width of [O iii]5007 are negatively correlated; this is called Eigenvector 1. In this work, we visualized the relationship between the position on this Eigenvector 1 (EV1) plane and how the brightness of the quasars had changed after ∼10 yr. We conducted three analyses, using a different quasar sample in each. The first analysis showed the relation between the quasars’ distributions on the EV1 plane and how much they had changed brightness, using 13438 Sloan Digital Sky Survey quasars. This result shows how brightness changes later are clearly related to the position on the EV1 plane. In the second analysis, we plotted the sources reported as “changing-look quasars” (or “changing-state quasars”) on the EV1 plane. This result shows that the position on the EV1 plane corresponds to the activity level of each source, and the bright or dim states of them are distributed on the opposite sides divided by the typical quasar distribution. In the third analysis, we examined the transition vectors on the EV1 plane using sources with multiple-epoch spectra. This result shows that the brightening and dimming sources move on a similar path and they reach a position corresponding to the opposite activity level. We also found this trend is opposite to the empirical rule that $R_{\rm {Fe\, \small {II}}}$ positively correlated with the Eddington ratio, which has been proposed based on the trends of a large number of quasars. From all these analyses, it is indicated that quasars tend to oscillate between both sides of the distribution ridge on the EV1 plane; each of them corresponds to a dim state and a bright state. This trend in optical variation suggests that significant brightness changes, such as changing-look quasars, are expected to repeat.

We present spectroscopic and photometric observations of 17 dwarf-nova superoutbursts obtained by KOOLS-IFU mounted on the 3.8 m telescope Seimei at the Okayama Observatory of Kyoto University and through the Variable Star Network collaboration (VSNET). Our spectroscopic observations for six outbursts were performed within 1 d of their optical peak. 11 objects (TCP J00590972+3438357, ASASSN-19ado, TCP J06073081−0101501, ZTF20aavnpug, ASASSN-19ady, MASTER OT J061642.05+435617.9, TCP J20034647+1335125, ASASSN-20kv, ASASSN-20kw, MASTER OT J213908.79+161240.2, and ASASSN-20mf) were previously unknown systems, and our observations enabled quick classification of their transient type. These results illustrate that the Seimei telescope has the capability to conduct quick follow-up observations of unknown transients. Our photometric observations yielded that 11 of the objects are WZ Sge-type dwarf novae and their candidates, and the other six are SU UMa-type dwarf novae and their candidates. The He ii 4686 Å emission line was clearly detected among ASASSN-19ado, TCP J06073081−0101501 and MASTER OT J213908.79+161240.2, the association of which with a spiral arm structure in an accretion disk has been suggested in previous studies. Our result suggests that a higher-inclination system shows a stronger emission line of He ii 4686 Å, as well as larger-amplitude early superhumps.

Brightness variation is an essential feature of quasars, but its mechanism and relationship to other physical quantities are not understood well. We aimed to find the relationship between the optical variability and spectral features to reveal the regularity behind the random variation. It is known that quasar's FeII/Hβ flux ratio and equivalent width of [OIII]5007 are negatively correlated, called Eigenvector 1. In this work, we visualized the relationship between the position on this Eigenvector 1 (EV1) plane and how they had changed their brightness after ∼ 10 years. We conducted three analyses using different quasar sample each. The first analysis showed the relation between their distributions on the EV1 plane and how much they had changed brightness, using 13,438 Sloan Digital Sky Survey quasars. This result shows how brightness changes later are clearly related to the position on the EV1 plane. In the second analysis, we plotted the sources reported as Changing-Look Quasars (or Changing-State Quasars) on the EV1 plane. This result shows that the position on the EV1 plane corresponds activity level of each source, the bright or dim state of them are distributed on the opposite sides divided by the typical quasar distribution. In the third analysis, we examined the transition vectors on the EV1 plane using sources with multiple-epoch spectra. This result shows that the brightening and dimming sources move on the similar path and they turn into the position corresponding to the opposite activity level. We also found this trend is opposite to the empirical rule that R FeII positively correlated with the Eddington ratio, which has been proposed based on the trends of a large number of quasars. From all these analyses, it is indicated that quasars tend to oscillate between both sides of the distribution ridge on the EV1 plane, each of them corresponds to a dim state and a bright state. This trend in optical variation suggests that significant brightness changes, such as Changing-Look quasars, are expected to repeat.