Serendipitous discovery of a projected pair of QSOs separated by 4.5 arcsec on the sky

TL;DR

This paper reports the serendipitous discovery of a close projected pair of QSOs with a 4.5 arcsecond separation, providing insights into their properties and rarity compared to similar known pairs.

Contribution

The discovery of a new close projected QSO pair with detailed spectral analysis and comparison to known systems, highlighting its uniqueness.

Findings

Two QSOs with different redshifts and spectral features

Angular separation of 4.5 arcseconds, one of the smallest known

Comparison shows only about a dozen similar pairs exist

Abstract

We present the serendipitous discovery of a projected pair of quasi-stellar objects (QSOs) with an angular separation of $\Delta\theta =4.50$ arcsec. The redshifts of the two QSOs are widely different: one, our programme target, is a QSO with a spectrum consistent with being a narrow line Seyfert 1 AGN at $z=2.05$. For this target we detect Lyman-$\alpha$, \ion{C}{4}, and \ion{C}{3]}. The other QSO, which by chance was included on the spectroscopic slit, is a Type 1 QSO at a redshift of $z=1.68$, for which we detect \ion{C}{4}, \ion{C}{3]} and \ion{Mg}{2}. We compare this system to previously detected projected QSO pairs and find that only about a dozen previously known pairs have smaller angular separation.