Discovery of a new extreme changing-state quasar with 4 mag variation,
  SDSS J125809.31+351943.0
  We report the discovery of a quasar, SDSS J125809.31+351943.0 (J1258), which
brightened in optical for 4 mag from 1983 to 2015, which is 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
Sloan Digital Sky Survey shows their significant increases between 2003 and
2015. Investigating its CFHT photometric observations in 1983 and the USNO-B
catalog, which contains data in 1975 and 1969, we found that the source was 4
mag fainter before than 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 December 2018
and May 2019, using the 2-meter telescope in Nishi-Harima Astronomical
Observatory. The results show that the continuum flux and the BEL flux
decreased to about 50 % of its peak. This indicates that J1258 is causing 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 severer variations.
