Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: J1721$+$8842: a gravitationally lensed binary quasar with a proximate
  damped Lyman-$\alpha$ absorber   High-redshift binary quasars provide key insights into mergers and quasar
activity, and are useful tools for probing the spatial kinematics and chemistry
of galaxies along the line-of-sight. However, only three sub-10-kpc binaries
have been confirmed above $z=1$. Gravitational lensing would provide a way to
easily resolve such binaries, study them in higher resolution, and provide more
sightlines, though the required alignment with a massive foreground galaxy is
rare. Through image deconvolution of StanCam Nordic Optical Telescope (NOT)
monitoring data, we reveal two further point sources in the known, $z \approx
2.38$, quadruply lensed quasar (quad), J1721+8842. An ALFOSC/NOT long-slit
spectrum shows that the brighter of these two sources is a quasar with $z =
2.369 \pm 0.007$ based on the C III] line, while the C III] redshift of the
quad is $z = 2.364 \pm 0.003$. Lens modelling using point source positions
rules out a single source model, favouring an isothermal lens mass profile with
two quasar sources separated by $\sim6.0$ kpc (0.73$^{\prime \prime}$) in
projection. Given the resolving ability from lensing and current lensed quasar
statistics, this discovery suggests a large population of undiscovered,
unlensed sub-10-kpc binaries. We also analyse spectra of two images of the
quad, showing narrow Ly$\alpha$ emission within the trough of a proximate
damped Ly$\alpha$ absorber (PDLA). An apparent mismatch between the continuum
and narrow line flux ratios provides a new potential tool for simultaneously
studying microlensing and the quasar host galaxy. Signs of the PDLA are also
seen in the second source, however a deeper spectrum is still required to
confirm this. Thanks to the multiple lines-of-sight from lensing and two quasar
sources, this system offers simultaneous sub-parsec and kpc-scale probes of a
PDLA.

15
The categories of this paper are: nucl-ex
22
The categories of this paper are: chao-dyn
29
The categories of this paper are: funct-an
2
The categories of this paper are: math
10
The categories of this paper are: q-bio
14
The categories of this paper are: math-ph
20
The categories of this paper are: acc-phys
16
The categories of this paper are: stat
19
The categories of this paper are: eess
32
The categories of this paper are: atom-ph
11
The categories of this paper are: hep-lat
17
The categories of this paper are: q-fin
23
The categories of this paper are: solv-int
26
The categories of this paper are: bayes-an
31
The categories of this paper are: ao-sci
12
The categories of this paper are: nucl-th