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\author{}
\title {\vspace{-5ex} \LARGE\bf 
        Source list adjustment for the population study of steep-spectrum radio sources
       }
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\date{}



\begin{document}

\maketitle

% Abstract
%
% We propose to adjust the source list in BP242 project and request to allot
% 6 more hours in priority B and 24 hours in priority C beyond that what
% already been allotted in addition to 19 thours that were allotted but not
% yet observed. We request this adjustments because 1) a part of BP242 
% project was scheduled in the filler mode and we could not predict which sources will be
% obsered; 2) VLASS images, unavailable when we started observations, were published,
% which offered a significant help in reaching the scientific goal; 3) we tried to
% address TAC comments.
%


\vspace{-28ex} \hfill \framebox{\Blb{\LARGE\bf Draft of \isodate}} \vspace{25ex}

\hA\vspace{-18ex}\hA\par
\section{\large Introduction}

  Our project ``Study of the population of the steep-spectrum compact radio sources''
BP242 was accepted for 2019B semester. The project aimed observation with VLBA
of 1350 target sources from two samples drawn from AT20G and GB6 catalogues to reach 
completeness at given flux density limits without a filtering according to
spectral index. As a result, we expected to have three samples drawn from NVSS,
AT20G, and GB6 that are observed with VLBI in prior projects and BP242. Statistics
of detection and non-detection will be used for achieving scientific goals:
to investigate the relationship between compactness, spectral index at kiloparsec 
scale (angular size at aresecond level), spectral index at parsec scale (angular 
size at milliarcsecond level), source sizes, and their morphology at parsec scales
from VLBA images and kiloparsec scales using VLA images from NVSS and VLASS. 

  This project was suitable to the filler mode: we developed software for scheduling, 
and the array operator generates observing blocks using the back-end via web interface. 
However, according to our previous experience, there are zones that have low chances 
to be observed in the filler mode. Therefore, we requested to run 70\% of the project 
in the filler mode and 30\% in a non-filer mode. The TAC allocated 60 hours in the 
priority~C and 24 hours in priority~B. 

\section{\large Source list adjustment of BP242}
  
  We have processed all 14 segments, 65~hours, in priority C by 2019.12.12.
The current project status and results are available at \web{http://astrogeo.org/vcs10}.
We realized that we observed 99\% target sources. Two factors contributed 
to higher than anticipated number of observed sources: 1)~filler time blocks
were allocated more favorably; 2)~we have managed to improve efficiency of
the scheduling procedure.

   We have 19 hours that were allotted in priority B but not used. We proposed
to make adjustment in the source list keeping the scientific goals intact.
We request 24 hours in priority B (as it was allotted, but not yet used) and ask 
for 24~hour extra time in priority C, thus changing original request of the 
filler time from 60 hours to 84~hours.

\subsection{\large Extension of the source list in the ecliptic band}

   We have identified 920 sources in right ascensions 14.0 -- 22.0 hours within
$7.5^\circ$ of the ecliptic plane that were observed at a three station CVN network
and not detected, but were not previously observed with VLBA. We would like to observe 
them within 24 hour of priority B. This list is a subset of the bigger list
of 5756 objects that were observed with CVN in the framework of the VLBI Ecliptic 
Plane Survey (VEPS). BP242 observed 616 objects that VEPS did not observed, but
we did not re-observe those VEPS sources the CVN observed. There are several factors 
that prompted us to re-observe {\it some} VEPS source:

\begin{enumerate}
   \item We assume the detection limit of VEPS is around 20~mJy (10 mJy in BP242). 
         We would like to make a robust assessment of the VEPS detection limit 
         using VLBA observations as the ground truth.

   \item VEPS observed each target in two scans at a 3-element network. A failure 
         of two antennas will result in a false negative. We would like to evaluate
         the rate of false negative.

   \item We used positions from our parent catalogue GB6 as a~priori. In September 
         2019, after we started BP242 observations, VLASS images that cover 
         over 95\% of our survey area have been released. Many sources in BP242 were 
         detected far away from the a~priori position: $20''$--$200''$. Although short 
         accumulation periods and high spectral resolution reduced losses due to smearing 
         to the minimum, the failure in detection because the target was on the edge of
         the main beam need to be assessed. We will use VLASS a~priori positions
         for the remaining observations.

   \item We would like to make an independent check of flux density estimates from
         the CVN.

   \item The CVN observations were made only in X-band. BP242 observations are made
         in 4.1/7.4 GHz. We would like to use spectral index of the core in our
         analysis.

   \item It will be beneficial for the goals of our project to have more sources
         with images in order calibrate CVN detection statistics. The CVN provided 
         from 1 to 2 scans of a 3-element array, which is not sufficient for making 
         images.
\end{enumerate}

\subsection{\large Extension of the source list in the polar cap}

  Our scientific analysis will be based on processing three samples drawn
from the NVSS, GB6, and AT20G. We have observed with VLBA the NVSS sample 
of 504 sources brighter 250~mJy at $\delta > +75^\circ$ in 2006 (project BK130)
at 256~Mbps. Comparison of positions of detected sources with VLASS showed that
many of them were found at large distance from a~priori positions. We do not 
know how many sources were not detected not because their correlated flux density
is below the limit that we assume in our analysis, but because of beam attenuation,
time and bandwidth smearing. The limitations of the hardware correlator caused 
noticeable bandwidth and time smearing. Therefore, we propose to observe 429
sources that were not detected at X-band in BK130 using a~priori from VLASS. 

  The purpose of these observations:
\begin{enumerate}
   \item Assess robustness of detection statistics from previous narrow-band 
         observations.

   \item To re-assess sources detected in S-band only in BK130: we lost
         detection at X-band for some sources because of poor a~priori, not
         because their X-band correlated flux density is below the detection
         limit, and we want to know how many sources were affected.

   \item To homogenize detection limit for all three samples: NVSS, GB6, and AT20G.
         At the moment, detection limit of the GB6 and AT20G samples from CVN+VLBA
         observations is at least 20~mJy, but it is 35~mJy from the NVSS sample.
\end{enumerate}

   Since these sources are at declination above 75~degrees, they are well suited
for filler time.

\subsection{\large Why director discretion time}

  We take great care to use the precious VLBA resource in the most efficient way.
That is why we routed 70\% requested time into the filler mode. We anticipated
that after finishing the filler part, we will need re-evaluate scheduling because
it is not possible to predict which sources will be observed in the filler
time and which will not. And that is just happened. Emergence of VLASS catalogue
which impact on the goals of our project exceeded our expectations prompted
us to increase the source list in order to maximize the scientific output. 

The TAC commented on BP242: ``Proposal weaknesses: There was some concern about the 
limited additional number of sources to be studied compared to the sizes of existing 
catalogues.'' We do our best to address TAC comments by spending as little as 
possible additional resources.

  We asked 6 hours more in priority B because an additional session in
the filler mode was scheduled than it was needed. It was already too late to 
cancel it when we learned it.

  We wrote in BP242 proposal ``{\sf Since we do not know beforehand which area 
will not be covered with the filler time, we are going to schedule six 4-hour 
blocks after we use of 60 hours of filler time. Therefore, we ask to extend 
the project over two semesters.}'' Since it remained only one month before the
end of semester, we ask not to cut off the project at 01 February 2020 and
extend it in 2020A.
  
  Our total request for this project, 49 hours, includes 19 hours that was 
allotted but not yet used for BP242.

  We waive proprietary period. Results will be put on the project web page 
immediately.

\end{document}
