Secular evolution of the reduced r-band magnitude (Panel a) and Afρ parameter (Panel b) as a function of heliocentric distance r_H. All points correspond to the photometric parameters measured from the aperture size of 10 000 km from the comet nucleus. The star symbol in Panel b denotes the time of our spectropolarimetric observation (UT 2021 June 26.2). The vertical dashed line indicates the perihelion distance at r_H = 2.055 au. An increase in the brightness of T2 accelerates around 2.4 au from the Sun. A linear least-square fit to the heliocentric evolution of the reduced r magnitudes gives different slopes (‘sl’ in panel a) before and after the acceleration.