Discovery of a probable binary quasar

Abstract

We report discovery of a pair of quasars at a redshift of 1.345, separated by 4”.2 in projection, apparently associated with the radio source PKS 1145-071. The optical intensity ratio of the two images is approximately 2.5. The spectra are very similar, and all emission lines except for the C IV λI549 have the same equivalent widths, within the error bars. The redshift difference based on the C IV line alone, which is the strongest feature, and on the cross-correlation of the two spectra including the C IV line, is approximately 250 ± 100 km s^(-1) in the rest frame. If this discrepant emission line is excluded, the redshift difference drops to approximately 100 ± 100 km s^(-1), and is consistent with zero. Thus, on the basis of the optical data alone, we would have concluded that this double QSO is another example of gravitational lensing. However, the radio source is single and unresolved at all VLA frequencies, and positionally coincident with the brighter of the two optical images. The fainter of the two optical components is at least a factor of 500 fainter in the radio. This dramatic difference in L_(opt)/L_(radio) between the two components suggests that the system is a genuine binary quasar, and the first such well-documented case. Assuming that the two QSO’s are gravitationally bound, we derive a lower limit to the total mass of the system of ~ 10^(11)h^(-1)M_⊙ this is the first such direct determination of a quasar mass. The close projected separation of the pair (~ 20h^(-1) kpc) and the small velocity difference are suggestive of an interaction, which may be causally related to the QSO activity, and of the existence of a parent group or cluster, which would be a very interesting find in itself. In the view of this discovery, it is plausible that some other, previously reported gravitational lenses may be really pairs of physically distinct quasars.