\documentstyle[twoside, epsf,hyperref]{article}

\input ibvs3.sty

\begin{document}

\IBVSheadDOI{63}{6240}{05 April 2018}

\IBVStitle{2MASS J06422218-0226285 - a new Outburst Source\IBVSfm}
\IBVSauth{Blex, Susanne$^1$; Hackstein, Moritz$^1$; Haas, Martin$^1$; Kimeswenger, Stefan$^{2,3}$; Chini, Rolf$^{1,2}$; Hodapp, Klaus$^4$}
\IBVSinst{Astronomisches Institut, Ruhr-Universit{\"a}t Bochum, Germany; e-mail: sublex@astro.rub.de}
\IBVSinst{Instituto de Astronom{\'i}a, Universidad Cat{\'o}lica del Norte, Chile}
\IBVSinst{Institute for Astro- and Particle Physics, University of Innsbruck, Austria}
\IBVSinst{Institute for Astronomy, University of Hawaii, USA}

\SIMBADobjJ2000{2MASS J06422218-0226285}{06 42 22.188 -02 26 28.55}
\IBVStyp{ S, FU:, NL: }
\IBVSkey{photometry: BVRIJHK, infrared, stars: variables: Cataclysmic Variable, FUor, EXor, binaries: symbiotic, stars: individual: 2MASS J06422218-0226285}
\IBVSabs{We discovered the outburst of 2MASS J06422218--0226285. Between end 2012 and early 2014, this object brightened by 3 mag in \textit{r} and \textit{i}, and by 3.7 mag in \textit{J}. Since then, it has stayed at high brightness of about 16 mag in \textit{r} and 15 mag in \textit{i}. Possible explanations for this kind of light curve might be a Catalysmic Variable, a Symbiotic Binary or a FUor or EXor type Young Stellar Object. The color properties favor an outbursting Young Stellar Object.}

\begintext

\IBVSfoot{Based on data collected under the ESO/RUB --- USB agreement at the Paranal Observatory}

2MASS J06422218--0226285 brightened between the end of 2012 and early 2014 by
about 3 mag in \textit{r} and \textit{i}, and by 3.7 mag in \textit{J}, and has stayed at high brightness since then.

This object has been photometrically surveyed by several missions at optical and infrared wavelengths.
Among these surveys are GSC~II  in 1991 (Lasker et al.\ 2008), 2MASS in 1998 (Skrutskie et al.\ 2006),
DENIS in 2000 (Epchtein et al.\ 1994), IPHAS in 2006 (Barentsen et al.\ 2014), and WISE in 2010 (Wright et al.\ 2010).
Viironen et al.\ (2009) described J06422218--0226285 as a planetary nebula (PN)
and also have found that H$\alpha$ probably has been in emission before the outburst.
There is no prominent star forming region close to the object.

While analyzing exceedingly red and variable objects among the Galactic Disc Survey (GDS, Haas et al.\ 2012, Hackstein et al.\ 2015), Blex (2017)
discovered the brightening of 2MASS J06422218--0226285 (or GDS J064221-022628). The discovery of the outburst motivated further measurements at the Universit{\"a}tssternwarte Bochum (USB) near Cerro Armazones, Chile.
%Between November 23\textsuperscript{\textrm{\footnotesize{rd}}} and December 12\textsuperscript{\textrm{\footnotesize{th}}} in 2017, the latest optical data in the 
Between November 23$^{\rm rd}$ and December 12$^{\rm th}$ in 2017, the latest optical data in the 
\textit{b}, \textit{V}, \textit{r}, and \textit{i} filters have been collected. During three nights in February to March 2018, we were able to obtain narrow-band spectro-photometry of
HeI, H$_{\rm 2}$ (1-0) S1, Br$\gamma$, CO, and $K_c$, as wll as $JHK_s$ broadband photometry.
Our search for H$\alpha$ emission after the outburst using narrow bands at 6450, 6563 and 6721 \AA{} has failed due to a too low object brightness and poor S/N at these wavelengths.

\IBVSfig{9cm}{6240-f1.eps}{GDS light curve in \textit{r} and \textit{i} with additional IPHAS, DENIS, and GSC II data points; the IPHAS error is smaller than the symbol size.}

Figure 1 shows the \textit{r}- and \textit{i}-band light curves from the GDS together with previous photometry of GSC~II, DENIS, and IPHAS (Barentsen et al.\ 2014) data points. The light curve values are listed in Tables 2 and~3 (at the end of the paper).
The latest measurement in December 2017 yielded an \textit{r} magnitude of $16.170 \pm 0.139$ and an \textit{i} magnitude of $15.173 \pm 0.140$.
A check of the single segments of the GDS light curve showed no short-term periodicity.
The GSC~II, IPHAS, and GDS measurements suggest a constant brightness of about $r = 19$~mag between 1991 and 2012.
A constant faint state lasting back from 1991 to 1955 is further supported by the sequence of
past DSS1, DSS2, and present GDS image cutouts (Fig.~2).

\IBVSfig{6cm}{6240-f2.eps}{Comparison of the cutouts from the red filter of the DSS and the Sloan \textit{r} filter of the GDS; angular size: approximately 100 $\times$ 100\arcs{}.}

The optical to mid-infrared spectral energy distribution (SED) is depicted in Fig.~3, separated for both faint and bright states.
Already before 2012, J06422218--0226285 has shown an infrared excess in the 2MASS and WISE color-color diagrams,
consistent with a classical T\,Tauri star surrounded by circumstellar dust.
After the outburst, the star has become much redder, suggesting dispersed dust.
Although H$\alpha$ does not appear in emission after the outburst,
a strong P~Cygni-type absorption could balance out potential emission.

\IBVSfig{12cm}{6240-f3.eps}{Spectral energy distribution; depicted GDS filters: \textit{B}, \textit{V}, \textit{r}, \textit{i}, \textit{J}, \textit{H}, $K_s$;
error bars (if not seen) are smaller than the symbol size.}

Our near-infrared $JHK_s$ and narrow-band spectro-photometry reveals a potential 
Brackett--gamma (Br$\gamma$) emission (Fig.~4, Tab.~1). 
The resulting Br$\gamma$ flux would be about $5.1 \cdot 10^{-17}$ W/m\textsuperscript{2}, comparable 
to the range found by Carr et al.\ (1990) for Young Stellar Objects (YSOs).
The large Br$\gamma$ equivalent width of about 19 \AA{} would place 
J06422218--0226285 among strongly accreting YSOs.
In this scenario, the increase in brightness can be explained as a FUor- or EXor-type outburst.

\IBVSfig{9.5cm}{6240-f4.eps}{Average near infrared photometry of 2MASS J06422218--0226285
(left large panel) and
two nearby stars of similar brightness (right, two small panels).
The photometry was obtained in three nights in Feb--Mar 2018
with the IRIS telescope at USB in the broadband filters $JHK_s$
and five narrow bands (FWHM = 275 \AA{}) centered at
2.05,  2.121, 2.167, 2.29 and 2.314 $\mu$m
(HeI, H$_{\rm 2}$ (1-0) S1, Br$\gamma$, CO, $K_c$, black filled circles
connected with a red line).
The horizontal dashed lines indicate the bandwidth and
error range of the broadband \textit{J}, \textit{H}, $K_s$.
For 2MASS J06422218-0226285 the error range in all bands
is $\sim$2\%, thus significantly
larger than for other nearby stars of similar brightness ($<0.5$\%);
this indicates a remaining small variability of 2MASS J06422218--0226285.
For 2MASS J06422218--0226285
the flux in the Br$\gamma$ filter lies above both the
$K_s$ broadband flux and the continuum as interpolated between
HeI and CO and $K_c$ (blue dotted line).
While HeI and CO absorption cannot be ruled out yet,
for an outbursting object it appears more likely that Br$_\gamma$ and hydrogen S1 are in emission.}

\begin{table}
\begin{center}
\caption{Near-infrared photometry obtained in three nights in Feb--Mar.~2018 with IRIS.}
\begin{tabular}{cccccc}\hline
\textbf{Filter} & \textbf{$\lambda$} & \textbf{$f_{\nu}$} & \textbf{$f_{\nu}$} error & flux & flux error\\ 
 & $\mu$m & mJy & mJy & $10^{-15}$ erg/s/cm\textsuperscript{2}/\AA{} & $10^{-15}$ erg/s/cm\textsuperscript{2}/\AA{} \\ \hline
\textit{J} & 1.235 & 22.3641 & 0.235612 & 4.3988384 & 0.0463431 \\
\textit{H} & 1.662 & 36.5833 & 0.438685 & 3.9732172 & 0.0476444 \\
$K_s$ & 2.159 & 43.2917 & 0.527097 & 2.7862547 & 0.0339240 \\
\hline
HeI & 2.052 & 38.1709 & 0.437991 & 2.7232539 & 0.0312480 \\
H$_{\rm 2}$ (1-0) S1 & 2.121 & 42.2975 & 0.928504 & 2.8223840 & 0.0619564 \\
Br$\gamma$ & 2.166 & 44.9465 & 0.899277 & 2.8779934 & 0.0575820 \\
CO & 2.295 & 47.3339 & 0.944012 & 2.6997119 & 0.0538422 \\
$K_c$ & 2.314 & 46.3109 & 1.20148 & 2.5981670 & 0.0674066 \\
\hline
\end{tabular}
\end{center}
\end{table}

Furthermore, matching with 2MASS allowed for searching the environment of \linebreak
J06422218--0226285 for $K$-excess objects in the $JHK_s$ color-color 
diagram, which lie at least 2$\sigma$ right-hand of the slope 
$(J-H) = 1.7 \, (H-K) - 0.12$.
We considered only precisely measured stars with 2MASS quality flag
A or B in all three filters.
We searched a 1200\arcs{} box around the target to maintain a balance
between the consideration of only the close environment of the star
and sufficient statistics.
This yields a rate of 1.49\,\% (16 out of 1077) $K_s$ excess stars
near J06422218--0226285.
The resulting rate needs to be compared with the expected frequency
of $K_s$ excess stars near the galactic plane.
For this purpose, we used the center coordinates of 15 randomly
selected GDS fields with 6h $<$ RA $<$ 11h and investigated the
2MASS stars in a 1200\arcs{} box around these coordinates.
In total, 26588 2MASS stars (with flag A, B) are covered by these boxes with
151 of them (0.57\,\%) being $K$-excess stars.
To estimate the field-to-field fluctuation, the fraction of $K$-excess stars
is calculated individually for each field and then averaged,
resulting in a mean of 0.62\,\%  and standard deviation of 0.31\,\%.
Thus, the rate of $K_s$ excess stars
near J06422218--0226285 is almost 3$\sigma$ above that of the mean.
Note that only in one of the 15 boxes the rate of $K$-excess stars is as high as in the case of J06422218--0226285.
Hence, one might speculate that J06422218--0226285 is located in a region
of thin star formation or a star forming region at the end of its lifespan.
Additionally, IRAS-IRIS and AKARI images indicate a nebulous surrounding.
These findings, H$\alpha$ emission before and Br$\gamma$ emission after the outburst, and the present and past infrared excess support the claim of a YSO;
albeit it is not close to a known star-forming region and there are no emission or reflection nebulae
nor a high number of H$\alpha$ objects near J06422218--0226285 and the amplitude is rather low for a FUor.
Accordingly, these indications require further confirmation by spectroscopy.

Alternatively to a YSO, J06422218--0226285 could be a cataclysmic variable (CV).
As already noted in Warner (1995a),
some subclasses of CVs show stable high states after an outburst for several years up to decades
(see, e.g., MV Lyr in Warner, 1995b, and RX~And and TZ~Per in  Simonsen et al., 2014).
It is believed that this is caused by a mass transfer feedback heating the secondary star.
In this case, the H$\alpha$ and Br$\gamma$ emission can be explained by the surrounding accretion disk.
Also, the irregular $r - i$ color variations of up to 0.8~mag fit this scenario.

Several features of J06422218--0226285 in the light curve and the SED are reminiscent of a symbiotic binary.
Among them are the signs of circumstellar gas and dust and different variability effects on the time scales of days to months.
These could explain the shape of the SED and the minor variations of the light curve after the outburst.
Since the novae of symbiotic binaries rise up to 3~mag in the optical in a couple of years at most and last for up to a century (see Skopal, 2015 and Munari, 2012),
the characteristics of the outburst of J06422218--0226285 fit this scenario as well.

\IBVSfig{14cm}{6240-f5.eps}{$IJK_s$ color-color diagram: blue curve -- main sequence stars; blue crosses -- position of B0, A0, F0, G0, K0, M0 stars;
red dashed-dotted lines -- reddening paths for $A_V = 3.5$; cyan area -- expected colors for planetary nebulae; green and purple cross -- 2MASS J06422218--0226285.}

Viironen et al.\ (2009) identified J06422218-0226285 as a planetary nebula candidate due to its position in IPHAS and 2MASS color-color diagrams.
However, in a DENIS $IJK_s$ color-color diagram (Fig.~5), the object lies outside of the area of PNs;
instead it exhibits symbiotic Mira colors (see Schmeja \& Kimeswenger, 2001 and Schmeja \& Kimeswenger, 2003).
Furthermore, the light curve does not fit a pulsating star,
and the increase in brightness certainly is too vast and rapid for Post-AGB evolution.
After the outburst, J06422218--0226285 still resides outside the area of PNs. Here, the \textit{I} magnitude has been estimated from a black-body fit to the SED (Fig.~3).

To summarize, based on the Bochum Galactic Disk Survey, 
we detected a remarkable 3--4~mag outburst
of J06422218--0226285 in 2013. 
The nature of the star is still puzzling.
The multi-band photometry is consistent with a FUor- or EXor-type YSO, 
albeit the star is located in a thin star forming region.
Also, the alternatives of a cataclysmic variable 
or a symbiotic binary or a PN/post-AGB are possible. 
In any case, the system shows exceptionally rare features, 
worth to clarify with future observations (e.\,g. spectroscopic or X-ray or radio).

\vskip 5mm

\textit{Acknowledgements:} We thank the referee for the instructive comments.


\references

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Warner, B., 1995a,
Cataclysmic Variable Stars,
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\endreferences

\begin{table}
\begin{center}
\caption{GDS \textit{r} magnitudes; the first line gives the magnitude of co-added images between 2010 and 2012.}
\begin{tabular}{l c c p{0.05cm} l c c p{0.05cm} l c c} \hline
\textbf{MJD} & \textbf{mag} & \textbf{err} && \textbf{MJD} & \textbf{mag} & \textbf{err} && \textbf{MJD} & \textbf{mag} & \textbf{err} \\ \hline
55197-55927 & 18.840 & 0.260 && 56949.270 & 16.364 & 0.145 && 57791.030 & 16.306 & 0.140 \\
56246.235 & 17.538 & 0.377 && 56953.250 & 16.338 & 0.142 && 57800.043 & 16.278 & 0.137 \\
56377.999 & 17.347 & 0.324 && 56963.259 & 16.333 & 0.141 && 57804.022 & 16.151 & 0.124 \\
56541.379 & 17.173 & 0.281 && 56964.242 & 16.375 & 0.146 && 57806.020 & 16.337 & 0.144 \\
56547.376 & 16.997 & 0.244 && 56965.229 & 16.439 & 0.154 && 58008.386 & 16.230 & 0.132 \\
56551.386 & 17.184 & 0.284 && 56966.229 & 16.414 & 0.151 && 58011.375 & 16.327 & 0.143 \\
56558.372 & 17.176 & 0.282 && 56967.218 & 16.487 & 0.160 && 58012.381 & 15.992 & 0.108 \\
56561.326 & 17.241 & 0.297 && 56968.217 & 16.063 & 0.113 && 58014.379 & 16.146 & 0.123 \\
56571.313 & 16.958 & 0.236 && 56969.216 & 16.501 & 0.162 && 58015.372 & 16.175 & 0.126 \\
56572.368 & 16.929 & 0.231 && 56978.220 & 16.349 & 0.143 && 58016.374 & 16.047 & 0.113 \\
56576.293 & 17.529 & 0.374 && 56979.217 & 16.260 & 0.133 && 58018.364 & 16.195 & 0.128 \\
56577.290 & 17.170 & 0.281 && 56980.215 & 16.214 & 0.128 && 58019.363 & 16.559 & 0.173 \\
56586.329 & 16.629 & 0.180 && 56981.211 & 16.203 & 0.127 && 58021.356 & 16.350 & 0.146 \\
56588.334 & 17.092 & 0.263 && 56982.209 & 16.363 & 0.145 && 58022.353 & 16.385 & 0.150 \\
56591.360 & 16.809 & 0.209 && 56983.205 & 16.121 & 0.118 && 58023.350 & 16.176 & 0.126 \\
56615.275 & 16.854 & 0.217 && 56984.202 & 16.324 & 0.140 && 58024.349 & 15.931 & 0.103 \\
56616.287 & 16.473 & 0.159 && 57308.297 & 16.342 & 0.142 && 58025.345 & 16.132 & 0.122 \\
56617.226 & 16.589 & 0.174 && 57311.297 & 15.980 & 0.105 && 58027.347 & 16.244 & 0.133 \\
56619.207 & 16.703 & 0.192 && 57317.256 & 16.236 & 0.130 && 58028.337 & 16.270 & 0.136 \\
56620.208 & 16.616 & 0.178 && 57318.256 & 16.134 & 0.120 && 58030.336 & 16.081 & 0.117 \\
56622.199 & 16.484 & 0.160 && 57320.256 & 16.173 & 0.124 && 58032.327 & 16.298 & 0.140 \\
56623.176 & 16.904 & 0.226 && 57321.256 & 16.232 & 0.130 && 58033.324 & 16.404 & 0.152 \\
56624.176 & 16.785 & 0.205 && 57322.256 & 16.438 & 0.154 && 58034.322 & 16.297 & 0.140 \\
56625.176 & 16.455 & 0.156 && 57323.256 & 16.334 & 0.141 && 58035.309 & 16.196 & 0.128 \\
56626.177 & 16.571 & 0.172 && 57324.312 & 16.246 & 0.131 && 58036.316 & 16.283 & 0.138 \\
56627.179 & 16.743 & 0.198 && 57325.299 & 16.154 & 0.122 && 58037.312 & 16.023 & 0.111 \\
56641.126 & 16.565 & 0.171 && 57328.256 & 16.116 & 0.118 && 58038.314 & 16.365 & 0.148 \\
56642.129 & 16.546 & 0.168 && 57330.266 & 16.071 & 0.114 && 58039.338 & 16.252 & 0.134 \\
56646.116 & 16.308 & 0.138 && 57331.224 & 16.072 & 0.114 && 58040.298 & 16.188 & 0.127 \\
56647.117 & 16.787 & 0.205 && 57332.224 & 16.248 & 0.132 && 58041.295 & 16.037 & 0.112 \\
56648.119 & 16.544 & 0.168 && 57333.224 & 16.268 & 0.134 && 58042.292 & 16.231 & 0.132 \\
56649.105 & 16.676 & 0.187 && 57334.224 & 16.325 & 0.140 && 58043.289 & 16.271 & 0.137 \\
56653.106 & 16.653 & 0.184 && 57338.224 & 16.164 & 0.123 && 58044.331 & 16.037 & 0.112 \\
56654.107 & 16.232 & 0.130 && 57655.340 & 15.934 & 0.101 && 58045.352 & 15.971 & 0.107 \\
56660.305 & 16.313 & 0.139 && 57657.335 & 16.130 & 0.119 && 58046.341 & 16.155 & 0.124 \\
56661.304 & 16.410 & 0.150 && 57658.336 & 16.147 & 0.121 && 58047.363 & 16.135 & 0.122 \\
56665.267 & 16.146 & 0.121 && 57659.367 & 16.094 & 0.116 && 58049.310 & 16.166 & 0.125 \\
56667.275 & 16.279 & 0.135 && 57660.322 & 15.997 & 0.107 && 58050.356 & 15.967 & 0.106 \\
56668.266 & 16.322 & 0.140 && 57784.034 & 16.498 & 0.165 && 58051.342 & 16.032 & 0.112 \\
56669.263 & 16.217 & 0.128 && 57785.034 & 16.201 & 0.129 && 58052.364 & 16.102 & 0.119 \\
56937.286 & 16.458 & 0.157 && 57786.033 & 16.340 & 0.145 && 58053.359 & 16.141 & 0.123 \\
56938.348 & 16.203 & 0.127 && 57787.033 & 16.351 & 0.146 && 58056.293 & 16.151 & 0.124 \\
56941.291 & 16.275 & 0.135 && 57788.032 & 16.152 & 0.124 && 58057.350 & 16.212 & 0.130 \\
56942.291 & 16.411 & 0.151 && 57789.032 & 16.051 & 0.114 && 58058.360 & 16.311 & 0.141 \\
56943.285 & 16.298 & 0.137 && 57790.032 & 16.202 & 0.129 && 58097.696 & 16.170 & 0.139 \\
\hline
\end{tabular}
\end{center}
\end{table}

\begin{table}
\begin{center}
\caption{GDS \textit{i} magnitudes; the first line gives the magnitude of co-added images between 2010 and 2012.}
\begin{tabular}{l c c p{0.05cm} l c c}\hline
\textbf{MJD} & \textbf{mag} & \textbf{err} && \textbf{MJD} & \textbf{mag} & \textbf{err} \\ \hline
55197-55927 & 17.780 & 0.300 && 56967.218 & 15.441 & 0.167 \\
56362.121 & 16.376 & 0.354 && 56968.217 & 15.369 & 0.157 \\
56541.379 & 16.037 & 0.271 && 56969.216 & 15.187 & 0.135 \\
56547.376 & 15.989 & 0.261 && 56978.220 & 15.198 & 0.136 \\
56548.378 & 15.890 & 0.241 && 56979.217 & 15.217 & 0.138 \\
56551.386 & 15.979 & 0.259 && 56980.215 & 15.262 & 0.143 \\
56558.372 & 16.114 & 0.288 && 56981.211 & 15.244 & 0.141 \\
56560.341 & 16.081 & 0.281 && 56982.209 & 15.085 & 0.124 \\
56561.326 & 16.053 & 0.275 && 56983.205 & 15.005 & 0.116 \\
56576.293 & 16.315 & 0.337 && 56984.202 & 15.256 & 0.143 \\
56585.271 & 16.109 & 0.287 && 57308.297 & 15.164 & 0.132 \\
56623.176 & 15.472 & 0.171 && 57311.297 & 15.109 & 0.126 \\
56625.176 & 15.320 & 0.151 && 57317.256 & 15.175 & 0.133 \\
56626.177 & 15.509 & 0.176 && 57318.256 & 15.132 & 0.129 \\
56646.116 & 15.733 & 0.212 && 57320.256 & 15.232 & 0.140 \\
56649.105 & 15.458 & 0.169 && 57321.256 & 15.244 & 0.141 \\
56653.106 & 15.487 & 0.173 && 57322.256 & 15.285 & 0.146 \\
56660.305 & 15.443 & 0.167 && 57323.256 & 15.212 & 0.138 \\
56661.304 & 15.593 & 0.189 && 57324.312 & 15.311 & 0.150 \\
56665.267 & 15.589 & 0.188 && 57325.299 & 15.296 & 0.148 \\
56667.275 & 15.248 & 0.142 && 57328.256 & 15.126 & 0.128 \\
56668.266 & 15.199 & 0.136 && 57330.266 & 15.428 & 0.165 \\
56669.263 & 15.431 & 0.165 && 57331.224 & 15.229 & 0.140 \\
56937.286 & 15.155 & 0.131 && 57332.224 & 15.228 & 0.140 \\
56938.348 & 15.332 & 0.152 && 57333.224 & 15.303 & 0.149 \\
56941.291 & 15.326 & 0.151 && 57334.224 & 15.253 & 0.142 \\
56942.291 & 15.261 & 0.143 && 57338.224 & 15.190 & 0.135 \\
56943.285 & 15.288 & 0.147 && 57655.340 & 14.994 & 0.115 \\
56949.270 & 15.318 & 0.150 && 57657.335 & 15.046 & 0.120 \\
56953.250 & 15.354 & 0.155 && 57658.336 & 15.127 & 0.128 \\
56963.259 & 15.336 & 0.153 && 57659.367 & 15.058 & 0.121 \\
56964.242 & 15.165 & 0.132 && 57660.322 & 15.172 & 0.133 \\
56965.229 & 15.396 & 0.161 && 58097.696 & 15.173 & 0.140 \\
56966.229 & 15.294 & 0.147 && & & \\
\hline
\end{tabular}
\end{center}
\end{table}


\IBVSedata{6240-t2.txt}
\IBVSedata{6240-t3.txt}

\end{document}
