Figure 1: The artist's rendering shows the changing look quasar at full brightness (Credit: Michael Helfenbein/Yale University)
Quasars are the most luminous active galactic nuclei (AGN), and are thus very important to probe the distant Universe. Their huge luminosities come from the gravitational energy of accreting matters around the supermassive black holes. According to their spectral properties, AGN can be usually divided into two classes, Type 1 AGN with both broad and narrow emission lines and Type 2 AGN with only narrow emission lines.
Recently, in the spectra of some AGN, the broad emission lines are found to disappear in a few years, and may even re-appear in later years. These rare objects with obvious changes in the spectra within a few years are called “changing-look AGN”. Although only about 20 changing-look AGN have been found, they present serious challenges to the widely accepted unification scheme of AGN, which attributes the difference of Type 1 and Type 2 AGN to the different orientations to the dust torus. According to this scheme, Type 1 and Type 2 AGN can not evolve into each other in a short time because the orientations are hard to change.
A team led by Prof. Xue-Bing Wu, the Associate Director of KIAA, has conducted a survey of changing-look quasars in the last two years. They have discovered 21 changing-look quasars at redshift from 0.08 to 0.58, including 5 from repeat spectra in SDSS archive, 10 from repeat spectra in SDSS and LAMOST, and 6 from new spectroscopic observations of photometric variability selected candidates with the 2.16m Xinglong telescope at NAOC. These discoveries almost double the number of changing-look AGNs. The result was announced in a paper published in Astrophysical Journal on August 1st, 2018, with Qian Yang, a PhD student recently graduated from KIAA, being the first author.
The spectral transition time scale is from 1 to 13 years for these 21 new changing-look quasars, which confirms the dramatic changes in very short time. When the quasars “turn on (turn off)”, the optical and mid-infrared radiation become stronger (weaker). The colors of these quasars become “bluer-when-brighter” in optical, but “redder-when-brighter” in mid-infrared. This may be due to the stronger influences by the hot dust on the longer-wavelength mid-infrared radiation than the shorter-wavelength mid-infrared radiation when the AGN activity becomes stronger. The results are not consistent with the ideas of the varying obscurations and tidal disruption events proposed for changing-look AGN, and support the idea of the rapid change of accretion rates in the inner part of accretion disks.
Figure 2: Left: The optical and mid-infrared light curves of a newly discovered changing-look quasar J1115+0544. Right: The spectra change over 14 years. The black and blue curves are SDSS and LAMOST spectra respectively. The lower part shows the difference between two spectra (Figure is taken from Yang et al. (2018))
The physics mechanisms for changing-look AGN are crucial to understand the evolution of AGN. Now Prof. Xue-Bing Wu’s team is doing more spectroscopic observations on many changing-look quasars with the domestic and international telescopes. More and more changing-look quasars are expected to be found, which will be absolutely helpful to probe the nature of these rare objects in the Universe.
The research was supported by the Chinese MOST key R&D project and NSFC grant.