Quasars or, as the scientific terminology defines them, quasi-stellar sources of radio and visible light waves belong to a larger class of objects called active galactic nuclei (AGN). They are extremely luminous although very distant objects that appear to be similar to stars. But differently from ‘regular’ stars these sources contain very broad emission lines, and their luminosity can be 100 times greater (or even more) compared to our Milky Way.
There are two principal classes of AGNs differing in their radio-loudness and optical spectra characteristics. Type 1 objects have broad emission lines, and Type 2 objects have only narrow emission lines. Intermediate types of AGN also exist and they contain weak or unusual line emission. “But what happens when multiple AGN types are seen in the same source at different epochs?”, say the authors of the new study published on arXiv.org.
In their paper, the authors note that such quasar systems are rare but not unheard of – to the scientific community they are known under the title “changing-look” AGN. It is assumed that such transitions may happen via variable absorption resulting from gas clouds moving in and out of the line-of-sight. Some theories also suggest that changes in type could be expected with variations in AGN accretion rate or transient events, like the tidal disruption of a star by a black hole.
In the current study, the team re-observed AGN initially reported in 2001 Sloan Digital Sky Survey (SDSS) Data Release 1 (full identificator J015957.64+003310.5, or J0159+003 in an abbreviated form). This source was classified as a quasar in SDSS data releases DR1, DR3, DR5 and DR7. However, in 2010 it was determined that the broad Hβ emission component completely disappeared from this source and only weak, broad Hα component remained visible. Because of this reason this AGN was not included in subsequent SDSS-III quasar catalogs.
During investigation, the team found that since 2010 the object J0159+003 has remained in the same emission state, so the overall transition happened in an incredibly short time, unlike in any other “changing-look” quasar observed till now. J0159+003 is also “much more distant and luminous than previously reported ‘changing-look AGNs and is the first true quasar discovered to have undergone such a dramatic fading”, the authors write.
The analysis of optical spectra revealed that the AGN flux decreased by a factor of ~8.5 between 2000 and 2010, and the broad H-alpha emission faded and broadened, i.e. the quasar transitioned from a Type 1 quasar to a Type 1.9 AGN.
Based on the results of spectral analysis the authors argue that the variable absorption that could be expected according to the AGN unification paradigm is not the cause of this transition and it should have happened relatively near or even inside the AGN. Such ‘dimming’ could be caused by a reduced supply of photons available to excite gas in the immediate vicinity around the black hole inside AGN, the scientists say.
The authors of the study suggest that J0159+003 and other similar objects could provide valuable insights into the dynamics of black hole growth in quasars, as well as new opportunities to study quasar physics (in the bright state) and the host galaxy (in the dim state), which has been impossible to do by observing single sources until now.
Written by Alius Noreika