\documentclass[journal]{rmaa}


\title{On the Nature of the Non-Thermal Radio Source\\ at the Center of the Orion Streamers}

\author{Alfonso Trejo and Luis F. Rodr\'iguez\affil{Centro de Radioastronom\'ia y Astrof\'isica\\ 
Universidad Nacional Aut\'onoma de M\'exico, Morelia, Michoac\'an, Mexico}}


\fulladdresses{
\item Alfonso Trejo and Luis F. Rodr\'iguez: Centro de Radiostronom\'ia y Astrof\'isica, Universidad Nacional Aut\'onoma de M\'exico, 
Apdo. Postal 3-72, (Xangari), 58089 Morelia, Michoac\'an, Mexico (a.trejo,~l.rodriguez@crya.unam.mx).} 


\listofauthors{A.~Trejo \& L.~F.~Rodr\'iguez}

\indexauthor{Trejo, A.}
\indexauthor{Rodr\'iguez, L.~F.}


\shortauthor{Trejo \& Rodr\'iguez}

\shorttitle{The Non-Thermal Source in the Streamers}


\SetVolume{46} 
\SetYear{2010}
\ReceivedDate{2010 June 28} 
\AcceptedDate{2010 July 27} 


\resumen{Presentamos un an\'alisis de observaciones del VLA en el continuo a 20 y 6~cm hacia una fuente de radio no t\'ermica en la 
regi\'on de las Serpentinas ({\it Streamers}) en Ori\'on. Comparamos observaciones de archivo de 1991 con nuevas observaciones hechas 
por nosotros en 2008. Dicha fuente puede ser un objeto estelar joven o un objeto evolucionado, como un microcuasar, o incluso un jet 
de una radiogalaxia. Nuestro an\'alisis muestra que no hay evidencia de cambio en la densidad de flujo, entre ambas \'epocas, arriba 
del 10\%, y que la morfolog\'ia permanece constante dentro del ruido. Tampoco hay indicio de movimientos propios mayores que 23~msa~a\~no$^{-1}$, 
a un nivel de 3$\sigma$, que a la distancia de Ori\'on implican un l\'imite superior a la velocidad en el plano del cielo de 
$\sim 45$~km~s$^{-1}$. A 6~cm detectamos emisi\'on polarizada linealmente con un nivel de $\sim 5\%$, mientras que a 20~cm no se 
detecta esta emisi\'on por encima de $\sim 2\%$. Concluimos que esta fuente de radio no t\'ermica es muy probablemente una radiogalaxia vista por 
mera casualidad en la l\'inea de visi\'on hacia el centro del sistema de las Serpentinas de Ori\'on.}

\abstract{We present VLA continuum observations at 20 and 6~cm toward a non-thermal radio source located at the {\it Streamers} region 
in Orion. We compare 1991 archive observations with new ones taken by us in 2008. The radio source could be a young stellar source or 
an evolved object like a microquasar or even a jet from a radio galaxy. Our analysis shows that there is no evidence of changes in the 
flux densities between both epochs at a level higher than 10\%. Also, we set an upper limit at a 3$\sigma$ level of 23~mas~year$^{-1}$ 
for the proper motions. At the distance of Orion this implies an upper limit to the velocity in the plane of the sky of 45~km~s$^{-1}$. 
At 6~cm, we detect linearly polarized emission at a level of $\sim 5\%$, while at 20~cm we do not detect this emission above a level of 
$\sim 2\%$. We conclude that the non-thermal radio source most probably is a radio galaxy aligned by chance with the line of sight to 
the Streamers in Orion. }


\addkeyword{ISM: individual (Orion Streamers, HH 222)}
\addkeyword{radiation mechanisms: non-thermal} 
\addkeyword{radio continuum: ISM}



\begin{document}

\maketitle

\section{Introduction}

The system known as the Streamers in Orion is an elongated, arc-like optical structure in the L1641 cloud in Orion. It stretches in the 
sky for 5 arcmin in the NE-SW direction. The system has optical line emission (Cohen \& Schwartz 1983) and is very close to the HH-34 
system. There is a T Tauri star located at only 15 arcsec from the center of the system. On the basis of this association, the arc was 
suggested to be the result of an interaction from the wind of this star with the surrounding medium (Reipurth \& Sandell 1985).


%%%%% TABLA 1 %%%%%%%%%%%%%%%%%%%%%%%%

\begin{table*}[!t]
\vspace{0.1cm}
\centering
\caption{Parameters of the Observations}
%\small
\begin{tabular}{cccccc}
\toprule
& \multicolumn{2}{c}{Flux Density (Jy)} &  & \multicolumn{2}{c}{Image}\\
&\multicolumn{2}{c}{0541-056} &  & \multicolumn{2}{c}{rms noise ($\mu$Jy)} \\
\cmidrule{2-3} 
\cmidrule{5-6} 
Epoch &  20~cm & 6~cm &  & 20~cm & 6~cm \\
\midrule
1991.64 & $1.20 \pm 0.02$ & $1.24 \pm 0.03$ &  & 80.6 & 26.7 \\
2008.79 & $1.20 \pm 0.10$ & $1.27 \pm 0.07$ &  & 61.5 & 26.3 \\
\bottomrule
\end{tabular}
\label{flxs}
\vspace{0.25cm}
\end{table*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


In 1990, Yusef-Zadeh et al. (1990) reported a non-thermal radio source detected with the VLA in that region. The source is linearly polarized 
at a level of 5\% in the 6~cm emission and is located at the very center of the Streamers with its main axis approximately aligned with the 
axis of the system. These authors also found a 2-micron source at the position of the radio source and several low-mass young stars in 
the surroundings of it. They concluded that the non-thermal radio source could be a newly born star. 

A different interpretation of this association was given by Castets, Reipurth, \& Loinard (2004), who proposed that this radio source could 
be the result of a T Tauri star undergoing an energetic event or to emission from an X-ray binary system. However, using millimeter studies 
of the CO, $^{13}$CO, C$^{18}$O and CS molecules emission around a region centered in the Streamers, Castets et al. (2004) found no evidence 
to link the radio source with any molecular outflow. Since the probability of finding this radio source to be just a fortuitous alignment 
is only $7.5 \times 10^{-5}$ (Castets et al. 2004), the most probable interpretation was in terms of this source being an evolved galactic 
object.

In these cases, there always exists the possibility of a fortuitous alignment. We decided to test this hypothesis using new VLA observations. 
Since the source is at $\sim -20^{\circ}$ of Galactic latitude, relatively far from the galactic plane, we found that the existing HI absorption 
observations were not useful to test the distance to the source, given the lack of line-of-sight gas in that direction. Thus our new observations 
were planned to study any possible flux variability, morphology changes and proper motions. To acomplish this, we used 1991 archive 
observations plus new observations performed by us.

At the distance of the Streamers, proper motions over two decades would be of the order of 2 arcsec for a velocity of 200~km~s$^{-1}$, which 
is expected for Herbig-Haro objects. Typically, also flux variability would be present in these cases (Rodr\'iguez et al. 2000). For the extended 
radio lobes of an extragalactic source we would not see any proper motion, perhaps only flux changes between both epochs, arising in a possible 
compact core located between the lobes.

We present this work as a part of a series of studies of non-thermal radio sources apparently associated with galactic star forming regions or 
nebulae. We have previously shown that an association on the plane of the sky is not always a physical association (Trejo \& Rodr\'iguez 2006, 
2008, 2010).


%%%%% FIGURA 1

\begin{figure*}[!t]
\centerline{\includegraphics[width=5.5cm]{atrejo2fig1a.eps}%
\hspace{0.25cm}%
\includegraphics[width=5.5cm]{atrejo2fig1b.eps}%
\hspace{0.25cm}%
\includegraphics[width=5.5cm]{atrejo2fig1c.eps}}
\caption{20~cm continuum images for the non-thermal source VLA J05354198--0623018. Left: 2008 image with an rms noise 
of 61.5~$\mu$Jy. Center: 1991 image with an rms noise of 80.6~$\mu$Jy. Right: Residual image $0.92 \times 2008$ - 1991, 
the rms noise is 95.7~$\mu$Jy. The levels for all images are  $-$3, $-$4, 3, 4, 5, 6, 8, 10, 12, 15, 20, 30, and 40 
$\times$ the individual rms noise. The full-width half maximum of the restoring beam is 1$1\rlap.{''}39\times1\rlap.{''}39$ 
and is shown in the bottom left corner.
\label{hhl}}
\vspace{0.3cm}
\end{figure*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


\section{Observations}

We made our new observations with the VLA, an NRAO\footnote{The National Radio Astronomy Observatory is a facility of the National Science 
Foundation operated under cooperative agreement by Associated Universities, Inc.} telescope. The observations were planned for the A 
configuration and taken on October 16, 2008 (epoch 2008.79) under project AT 368. We matched the parameters of our observations to those of 
the August 20, 1991 (epoch 1991.64) archive observations made under project AY 037. Thus, the pointing center, the frequency, and the 
calibrators are the same. The rms noise is roughly the same in both data sets, as is the {\it uv} coverage, which allows a reliable comparison 
between both epochs. 

A total of 26 antennas in the A configuration were present at the time of the observations, both in 1991 and 2008. The flux calibrator was 
1331+305 at both epochs and for both frequencies. To calibrate the phase, the source 0541-056 was used, with bootstrapped flux densities 
at both epochs and wavelengths indicated in Table~\ref{flxs}. At 20~cm, a total of 1.25 hours was spent on the source in 2008, and 1.96 hours 
in 1991. At 6~cm, the times were 1.15 hours for 2008 and of 2.00 hours for 1991.

Since the source is weak at 6~cm, we made the images setting the robust parameter of the task IMAGR to 0, which produces images with the best 
compromise between angular resolution and noise level. The 20~cm observations show a brighter source, and a scheme with uniform weighing (robust 
parameter set to $-5$) is used, since it allows the highest angular resolution possible. This is important to look for the existence of morphology 
changes and proper motions. The data at both frequencies were calibrated and imaged following the standard procedures listed in the AIPS Cookbook. 


\section{Results and discussion}

We shifted the position of the phase calibrator in 1991 to that of 2008 to remove a small offset, which is necessary because of the continuous 
refinement of the calibrator positions made by NRAO personnel. Also, in order to reliably compare both data sets we made images convolved with 
a circular beam of $1\rlap.{''}39$ for the 20~cm data, and of $0\rlap.{''}47$ for the 6~cm data.


%%%%%% FIGURA 2

\begin{figure*}[!t]
\centerline{\includegraphics[width=5.6cm]{atrejo2fig2a.eps}%
\hspace{0.25cm}%
\includegraphics[width=5.6cm]{atrejo2fig2b.eps}%
\hspace{0.25cm}%
\includegraphics[width=5.6cm]{atrejo2fig2c.eps}}
\caption{6~cm continuum images for the non-thermal source VLA J05354198--0623018. Left: 2008 image, the rms noise 
is 26.3~$\mu$Jy. Center: 1991 image with an rms noise of 26.7~$\mu$Jy. Right: Residual image 2008 -- 1991, the 
rms noise is 36.8~$\mu$Jy. The levels for all the images are  $-$3, $-$4, 3, 4, 5, 6, 8, 10, 12, 15, 20, 30, and 
40 $\times$ the individual rms noise. The full-width half maximum of the restoring beam is $0\rlap.{''}47\times0\rlap.{''}47$ 
and is shown in the bottom left corner.}
\label{hhc}
\vspace{0.45cm}
\end{figure*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%



\subsection{20~cm}

After performing the normal imaging for the 20~cm observations, we still detected a small offset, of the order of $0\rlap.{''}03$ in 
declination and $0\rlap.{''}17$ in right ascension, between the 2008 and 1991 images. To test if the offset seen in the data was 
a real position change of the source or an observational problem, we measured the positions of four sources located inside the 
primary beam of the telescope. All of them showed roughly the same small offset between both epochs. We then conclude that the 
offset is most likely an observational problem, perhaps produced by the fact that the 1991 observations were made near solar maximum, 
which can affect the long wavelength observations and produce positional errors such as those observed (Kaplan et al. 1982). A phase 
cross-calibration was performed to the data to minimize the diferences in phase between both observations. We could then compare 
reliably the images and search for morphology changes and flux variations, if any. To subtract the images we used a scheme to 
minimize the rms noise of the difference image (Franco-Hern\'andez \& Rodr\'iguez 2004). This is done by selecting the appropriate 
weight to one data set while making the subtraction. In this case we made the subtraction as 2008 image -- $1.09 \times 1991$ image. 
This allows to correct any calibration error in the flux scale. We present in Figure~\ref{hhl} the maps for the 20~cm emission at 
both epochs, and also the difference image resulting from the subtraction, made as we just noted. The morphology is that of a 
well-defined double component source. We refer to the source as VLA J05354198--0623018, where the coordinates are those of the 
centroid of the source. The total flux density obtained is about 20~mJy. The right panel of Figure~\ref{hhl} shows there are no 
residuals above a 3$\sigma$ level of 0.3~mJy. The average total flux density of the source from both epochs is $17.9 \pm 0.6$~mJy.

We also looked for proper motions by measuring the positions of the two lobes of the source at the two epochs. The analysis showed that 
there were no proper motions larger than 10~mas~year$^{-1}$ at a 3$\sigma$ level. Taking the distance to Orion to be 414~pc (Menten et al. 
2007), that limit implies velocities below 20~km~s$^{-1}$. However, since the data at both epochs were aligned with the phase cross-calibration, 
we will use the more reliable upper limits obtained at 6~cm (see below).

To look for polarized emission at this frequency, several tests with different weighting schemes were applied to the data. We did not find 
any polarization in the data at a level larger than 2\%.


%%%%% FIGURA 3

\begin{figure*}[!t]
\centerline{\includegraphics[width=5.65cm]{atrejo2fig3a.eps}%
\hspace{0.25cm}%
\includegraphics[width=5.65cm]{atrejo2fig3b.eps}%
\hspace{0.25cm}%
\includegraphics[width=5.65cm]{atrejo2fig3c.eps}}
\caption{6~cm continuum images made with natural weighing showing polarized emission as vectors. A 0.5 
arcsec line represents a polarization of 5\%. Left: 2008 image with rms noise of 25.54~$\mu$Jy. Center: 
1991 image with rms noise of 23.30~$\mu$Jy. Right: 2008 - 1991 image with rms noise of 32.33~$\mu$Jy. 
Levels are as in previous figures. The full-width half maximum of the restoring beam is $0\rlap.{''}62\times0\rlap.{''}62$ 
and shown in the bottom left corner.}
\label{hhcp}
\vspace{0.4cm}
\end{figure*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


\subsection{6~cm}

At 6 cm, VLA J05354198--0623018 is weaker which makes a cross-calibration harder to perform. In addition, the data sets did not 
present the same small instrumental misalignment as did the 20~cm observations. Then, there was no need for a phase cross-calibration, 
apart from the shifting of the 1991 data for the refinement of the position of the phase calibrator. Neither was a scale correction in 
flux density needed. In Figure~\ref{hhc} we present both images for 2008 and 1991, as well as the subtracted image. At this resolution, 
the images clearly split into two main components. The average total flux density from both epochs is $7.7 \pm 0.5$~mJy. As can be 
seen in the right panel of Figure~\ref{hhc}, the residual image shows no evidence of emission at all, at least not above a 3$\sigma$ 
level of 0.1~mJy. The morphology remains essentially the same.

The spectral index from the 20 and 6~cm measurements is $\alpha=-0.67 \pm 0.06 (S_\nu \propto \nu^\alpha)$. This value is characteristic 
of extragalactic radio jets (Bridle \& Perley 1984). 

By measuring the peaks of the two main lobes of VLA J05354198--0623018 at the two epochs, we set an upper limit for proper motions 
of 3$\sigma$ of 23~mas~year$^{-1}$, equivalent to $\sim 45$~km~s$^{-1}$~ at the distance of Orion.

Contrary to the case of the 20~cm emission, we detected linearly polarized emission at 6~cm. In Figure~\ref{hhcp} we show maps of the 
two epochs and also the residual image. The maps were made with natural weighting in order to have the lowest rms noise possible. The 
electrical vectors are shown with their length proportional to the degree of polarization. Yusef-Zadeh et al. (1990) reported  a level of 
polarization of 5\%, and we confirmed this value both with the 1991 archive data and with our new 2008 observations. The position angles 
for the electrical vectors in Figure~\ref{hhcp} are consistent with those reported in their study at a lower angular resolution. The 
polarization level found in this source is within the range of 5\% to 40\% observed in most extragalactic radio jets (Bridle \& Perley 1984). 

The Faraday rotation measure in the direction of HH~222 is $\sim 20$ radians per square meter (Johnston-Hollitt, Hollitt, \& Ekers 2004). 
This implies rotations of order 0.07~radians ($\sim 4^{\circ}$) and 0.8~radians ($\sim 46^{\circ}$) at 6 and 20~cm, respectively. This difference 
in the expected rotations suggests that the non-detection of polarization at 20~cm could be due to Faraday depolarization.

Since the upper limit to the velocity implied by the proper motions at 6~cm is of the order of $\leq 45$~km~s$^{-1}$, we do not favor a 
galactic location for the non-thermal source. As we noted, HH objects have velocities of the order of 200~km~s$^{-1}$. Microquasars 
have ejecta that move at relativistic speeds and we can rule them out. Also, the absence of significant residual emission and the fact 
that no essential morphological changes at both frequencies were observed support an extragalactic location. We propose this source to be 
a radio galaxy. The double lobe morphology seen at both frequencies is typical of radio galaxies.

\section{Conclusions}

We present 20 and 6~cm observations of the non-thermal source VLA J05354198--0623018 at the very \adjustfinalcols center of the Streamers system in Orion. The 
probability of this source to be an alignment by chance is very low, as we noted. However, the upper limit to the velocity 
implied by the absence of detectable proper motions and the lack of significant morphology variation weigh against a physical association with
 the Orion region. The polarization  is consistent with that present in radio galaxies. We conclude that the radio source is most probably an 
extragalactic one, apparently a radio galaxy in the line of sight to the Streamers.


\vspace{0.25cm}

\acknowledgments

AT is supported by a Conacyt scholarship. LFR acknowledges the support of DGAPA, Universidad Nacional Aut\'onoma de M\'exico, and of 
Conacyt (Mexico). This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France.


\vspace{0.25cm}

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\end{document}

