The search and study of close pairs of supermassive black holes ( SMBHs ) is important in the study of galaxy mergers which can possibly trigger active galactic nucleus ( AGN ) activity , and in the context of their evolution into the gravitational wave emitting regime .
The quasar SDSS J1048+0055 was identified as a SMBH binary candidate based on the observed double-peaked [ O iii ] \lambda \lambda 4959,5007 emission lines and two distinct radio components separated by \sim 20 pc \citep 2004ApJ…604L..33Z .
To ascertain the binary nature of this source , we analyzed multi-frequency , multi-epoch very long baseline interferometry ( VLBI ) data to investigate its pc-scale radio properties .
The source shows double components with the western feature being brighter than the eastern one .
This brighter component has a brightness temperature of \sim 10 ^ { 10 } K , spectral index of \alpha = -0.09 \pm 0.09 ( flat ) and is indicative of mildly relativistic beaming .
In contrast , the faint component has a lower brightness temperature of \sim 10 ^ { 8 - 9 } K and steep spectrum .
These clues are consistent with a core–jet structure , moreover , the apparent separation speed between the two components is much higher than the expected orbital motion in a binary SMBH .
Thus the present study excludes the association of the two VLBI components with the cores of a SMBH binary , although the SMBH binary possibility ( e.g. , a pair of radio-loud and radio-quiet AGNs ) is not fully ruled out .
In the single active galactic nucleus ( AGN ) scenario , the double-peaked optical emission lines can originate from the jet interacting with the narrow-line region as indicated by a change in the jet direction at \sim 140 pc .