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\author{}
\title {\vspace{-8ex} \LARGE\bf 
        Probing jets at tight gravitational lense candidates
       }
\title {\vspace{-10ex}\hphantom{a}}
\date{}

\begin{document}

\maketitle

% \vspace{-21ex} \hfill \framebox{\Blb{\LARGE\bf Draft of \isodate}} \vspace{16ex}

\hA\vspace{-22ex}\hA\par
\section{\large Science case}

  The dark matter is one of the most poromient open questions in physics.
Wbhile existing searches are restricting conventional models, there is 
increasing interest in alternative models and lensing is a way to test 
such models.

  The proposed MERLIN observations are a part of the larger project
with the overall goal to search for rare lensing events in the lens 
mass range of $10^{6}$M$_\odot$ to $10^{10}$M$_\odot$. Such lensing
effects will results in separations 3--300~mas. The abundance of
lensing events depends on the model of the dark matter contribution to 
the gravitational field. Therefore, detecting gravitational lenses
at such separations will allow us to constrains models of dark models.


\begin{figure}[h]
   \centerline{\includegraphics[width=0.35\textwidth]{dm_fig.pdf}}
   \caption{Image separation distribution for various dark matter
            models. The right hand part (cyan background) shows
            the resolution range of existing large-scale lens surveys
            that probe halos with mass $10^{11} M_\odot$. Currently,
            viable dark matter models agree on such large scale and 
            high halo masses. The left hand region (yellow background) 
            is the VLBI regime where the models may be distinguished.
           }
   \label{f:dm}
\end{figure}

  Dissipative dark matter (DDM) models posit that DM is composed of 
dark-sector particles that follow dissipative dynamics similar to baryons.
While the supernova feedback from the postulated kinetic coupling of DM to 
photons is sufficient to maintain extended halos in galaxies more massive
than 1011 M , at lower masses DM is expected to collapse into an 
exponential disk much in the same way baryons form galaxies 
(Foot and Vagnozzi, 2016). We simulated this collapse, maintaining
mass and angular momentum conservation (see e.g. Mo et al., 1998, sec 2.3), and 
find that if 10\% of matter is dissipative 100 mas scale lenses will
be found by this survey with rates comparable to existing arcsecond-scale 
surveys (see Fig.~\ref{f:dm}). 

  The previous searches of gravitation lenses were based on surveys
with VLA and MERLIN. Such searches could not help finding lenses with 
separations shorter than VLA and MERLIN resolutions. We used the 
Radio Fundamental Catalogue of 16,845 sources detected with VLBI\footnote{
Available at \web{http://astrogeo.org/rfc}}
and identified 741 visually binary objects, i.e. objects with two 
or more distinct components in their VLBI images at 3--300~mas separations. 
Further, we stroke out the objects that had multiple components with 
obviously different surface brightness, or spectral indices between 
components, or whose morphology was not consistent with lensing.
Among remaining objects, we selected those that are considered
the most promising for being classified as gravitational lenses.
We observed 12 visually biniary objects with VLBA in 2018 at 15 and 
23~GHz. We have detected two compact components in six of them.
We have been awarded 140 hours at the EVN in 2019 for observing 
43 gravitational lenses at 5 and 22 GHz  in order to get high fidelity 
images, estimates of the spectral index and determine the frequency 
dependent position shift between components. 

\section{\large Proposed observations}

  We propose complimentary L-band MERLIN observations of eight
candidate gravitational lenses sources with declinations $ > -20^\circ$.
Of them, six show several components at 15 and 23~GHz and the 7th, J0118+3810,
shows extended structure at VLASS image, and the 8th, J0348+3353, is tripple.
We do not see a jet in neither VLBA observations because a jet is resolved 
out, not in VLASS images because of their insufficient resolution. Proposed 
Merlin observations will help us to detect jets. Jet detection and 
determination  of its direction will help us to classify the sources 
understand their nature.

  The goal of the proposed observations is to get images of kiloparsec
structure of these objects with a resolution of 150~mas. These images
will help us to understand the nature of visually binary sources.
Among visually binary objects there are sources with core/hot-spot
morphologies, compact symmetric objects (CSO), binary black holes,
and gravitational lenses. We expect the most common class is 
core/hot-spot. For such objects we expect jet directions from the 
kiloparsec structure be in general aligned with the direction of 
their parsec-scale structure, i.e. the direction between components 
at their VLBI images. Assuming compact symmetric objects are young,
no kiloparsec scale structure is expected for these objects. Detection
of an X-shape jets will be a strong argument in favour of true binary.

  MERLIN provides resolution intermediate between VLA and VLBI.
Compact features within 150~mas of the central part will be captured 
with VLBI, although extended features will be resolved out. MERLIN
images will show extended emission at scales 0.15--100 arcsec.

  Our major concern in the search of gravitational lenses is that
we may fail to weed out sources with core/hot-spot morphology. 
Resolution of VLASS is insufficient to trace features smaller 
than 4--$5''$. A curved jet may explain a misalignment of kiloparsec
scale features and parsec-scale features. We select the low frequency
in order to better trace jets. A jet along the symmetry axis of VLBI
images will indicate that the second component at the VLBI image
is likely be a hot spot at jet. A jet with significant misalignment
will support the hypothesis that a given object is a gravitational 
lens or a binary black hole.


\begin{figure}
   \caption{Target source J0118+3810. {\it Left} and {\it center}
            images are made with VLBA. The position angle of the
            symmetry axis is $312^\circ$. {\it Right} image is 
            made at VLA at 2--4 GHz under VLASS program. White
            cross denotes the source core detected with VLBI.
            }
   \includegraphics[width=0.306\textwidth]{J0118+3810_X_map.pdf}
   \includegraphics[width=0.306\textwidth]{J0118+3810_C_map.pdf}
   \includegraphics[width=0.380\textwidth]{J0118+3810_S_vla.pdf}
   \label{f:J0118+3810}
\end{figure}

\newpage

{\bf VLBA images of proposed targets}

\begin{figure}[h]
   \caption{VLBA images of seven visually binary targets at X, U and K bands}

   \begin{tabular}{p{0.33\textwidth} p{0.33\textwidth} p{0.33\textwidth} }
     \vspace{0pt} \includegraphics[width=0.33\textwidth]{J0031+5401_U_map.pdf} &
     \vspace{0pt} \includegraphics[width=0.33\textwidth]{J0200+2157_U_map.pdf} &
     \vspace{0pt} \includegraphics[width=0.33\textwidth]{J0348+3353_X_map.pdf} 
   \end{tabular}
   
   \begin{tabular}{p{0.25\textwidth} p{0.25\textwidth} p{0.25\textwidth} p{0.25\textwidth} }
     \vspace{0pt} \includegraphics[width=0.245\textwidth]{J0620+2102_K_map.pdf} &
     \vspace{0pt} \includegraphics[width=0.245\textwidth]{J0649-1839_K_map.pdf} &
     \vspace{0pt} \includegraphics[width=0.245\textwidth]{J1438+6211_X_map.pdf} &
     \vspace{0pt} \includegraphics[width=0.245\textwidth]{J2347+1135_K_map.pdf}
   \end{tabular}
   \label{f:vlba_images}
\end{figure}

\section{Bibliography}

\noindent\hspace{1em}
   Foot R and Vagnozzi S 2016 J. Cosm. \& Astroparticle Phys. 7, 013

\noindent\hspace{1em}
   Mo H J, Mao S and White S D M, 1998 MNRAS 295, 319--336.

\end{document}
