  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;
or Changing-Look Quasar). 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.
