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|%astro-ph/|0712.0280

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\SetRunningHead{Y.~Hyodo et al.}{X-ray Spectrum of a Peculiar Star}
\Received{2007/6/12}
\Accepted{2007/8/31}
\begin{document}
\title{Suzaku X-Ray Spectroscopy of a Peculiar Hot Star \\in the
Galactic Center Region}
\author{Yoshiaki \textsc{Hyodo},\altaffilmark{1}
Katsuji \textsc{Koyama},\altaffilmark{1}
Tetsuya \textsc{Nagata},\altaffilmark{5}
Hiroshi \textsc{Murakami},\altaffilmark{7}
}
Kyoto University, \\Kita-shirakawa Oiwake-cho, Sakyo, Kyoto 606-8502}
\altaffiltext{2}{Department of Astronomy \& Astrophysics, Pennsylvania
State University \\
525 Davey Laboratory, University Park, PA 16802, USA}
\altaffiltext{3}{Department of Physics, Rikkyo University, 3-34-1,
Nishi-Ikebukuro, Toshima, Tokyo 171-8501}
\altaffiltext{4}{National Astronomical Observatory of Japan, 2-21-1, Osawa,
Mitaka, Tokyo 181-8588}
\altaffiltext{5}{Department of Astronomy, Graduate School of Science,
Kyoto University, \\Kita-shirakawa Oiwake-cho, Sakyo, Kyoto 606-8502}
\altaffiltext{6}{Department of Astronomy, School of Science,
the University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo 113-0033}
\altaffiltext{7}{Institute of Space and Astronautical Science, 3-1-1,
Yoshinodai, Sagamihara, Kanagawa, 229-8510}
\email{hyodo@cr.scphys.kyoto-u.ac.jp}
\KeyWords{Galaxy: center --- stars: Wolf-Rayet ---
X-rays: individual (CXOGC\,J174645.3--281546, 2XMMp\,J174645.2--281547) ---
X-rays: stars --- X-rays: spectra}
\maketitle

\begin{abstract}
We present the results of a Suzaku study of a bright point-like source
in the 6.7~keV intensity map of the Galactic center region.
We detected an intense Fe\emissiontype{XXV} 6.7~keV line with an
equivalent width of $\sim$\,1~keV as well as emission lines of
highly ionized Ar and Ca from a spectrum obtained by the X-ray
Imaging Spectrometer.
The overall spectrum is described very well by a heavily absorbed
($\sim$\,2$\times$10$^{23}$~cm$^{-2}$) thin thermal plasma model
with a temperature of 3.8\,$\pm$\,0.6~keV and a luminosity
of $\sim$\,3$\times$10$^{34}$~erg~s$^{-1}$ (2.0--8.0~keV) at 8~kpc.
The absorption, temperature, luminosity, and the 6.7~keV line
intensity were confirmed with the archived XMM-Newton data.
The source has a very red ($J-K_{\rm s}=8.2$~mag) infrared
spectral energy distribution (SED), which was fitted by
a blackbody emission of $\sim$\,1000~K attenuated by a visual
extinction of $\sim$\,31~mag.
The high plasma temperature and the large X-ray luminosity
are consistent with a wind-wind colliding Wolf-Rayet binary.
The similarity of the SED to those of the eponymous Quintuplet
cluster members suggests that the source is a WC-type source.
\end{abstract}
\section{Introduction}
The distribution of the 6.7~keV X-ray emission line along the Galactic
plane is strongly peaked at the Galactic center \citep{koyama89}.
The line originates from a K$\alpha$ transition of highly
ionized iron. Together with its large penetrating power delving
through a $\approx$\,10$^{24}$~H~cm$^{-2}$ extinction, the emission is an
indispensable tool to trace the high-energy activity and to reveal the
X-ray source demographics in a heavily attenuated environment, such as
the Galactic center region, where the enormous
density of gas and dust, the strong magnetic fields, and the
existence of a super massive black hole influence every aspect of
the transmigration of energy and matter \citep{morris96}.

Both the extended emission and the ensemble of numerous discrete
sources contribute to the 6.7~keV emission.
The nature of the extended emission was identified as a hot plasma
having a temperature of $\sim$\,6.5~keV, distributed across $\sim$\,250~pc
(\cite{koyama89}, \yearcite{koyama07c}).
For discrete sources, several different X-ray populations with
thermal emission play dominant roles.
%However, it is unclear which population accounts for
%what fraction of the integrated 6.7~keV line emission.

Cataclysmic variables (CV), binaries of a white dwarf and
predominantly a late-type dwarf star,
are major contributors to the 6.7~keV line, considering its
large population and thermal plasma with emission lines
(\cite{ezuka99}; \cite{muno03}, \yearcite{muno04b}).
Low-mass young stellar objects (YSO) or pre--main-sequence late-type
sources are other types of candidates.
They have elevated X-ray activity compared to their main-sequence
phases, and show a hard
thermal X-ray emission with the 6.7~keV line both during the occasional
flare
and quiescent phases \citep{koyama96a,tsuboi98,imanishi01,ozawa05}.
They are expected to be numerous in the Galactic center region,
where $\sim$\,10\% of the star formation of the entire Galaxy takes
place \citep{figer04}.

In addition to these late-type populations, early-type sources are
also expected to contribute to the 6.7~keV emission.
Main-sequence field O-type stars do not show hard X-ray emission
with their plasma temperatures below $\sim$\,1~keV \citep{berghoefer97},
but recent observations reveal that some main-sequence
O-type stars in OB associations show hard thermal X-ray emissions of
$\gtsim$\,2~keV (\cite{tsujimoto07b} and references therein).
Intense 6.7~keV line emissions are confirmed from such sources
(\cite{albacete03}, \yearcite{albacete07}; \cite{broos07}; \cite{hyodo08}).
Early-type sources are extremely rare compared to
late-type sources in number, but their overwhelmingly large luminosity
can be comparable to the integrated luminosity of numerous late-type
sources.
In the Orion Nebula Cluster,
for instance, an O6V star ($\theta^{1}$ Ori\,C) alone accounts for
$\sim$\,34\% of the total hard-band X-ray emission integrated over the
entire Chandra field of view containing $>$\,1000 YSOs \citep{feigelson05}

Early-type stars become even brighter in X-rays when they evolve
off the main-sequence track.
Wolf-Rayet (WR) stars and Luminous Blue Variables
(LBVs) comprise the brightest X-ray population of thermal point-like
sources with a typical hard-band luminosity of
$\sim$\,10$^{34}$~erg~s$^{-1}$ and a plasma temperature of $\gtrsim$\,2~keV
\citep{koyama94,tsuboi97,portegies02b,wang06}.
In fact, the brightest stellar X-ray source not powered
by accretion reported to date in our Galaxy
is a WR star of a spectral type of WN7--8 in the Arches cluster
(A1\,S in \cite{yusef02, law04, wang06,tsujimoto07a}).
The source has the 0.2--10.0~keV luminosity exceeding
$\sim$\,10$^{35}$~erg~s$^{-1}$ with a conspicuous 6.7~keV
emission.
% \citep{yusef02,law04,wang06,tsujimoto07a}.
Moreover, the X-ray spectra of these stars commonly show strong metallic
emission lines against continuum including the 6.7~keV line, which is
a consequence of extreme hydrogen depletion caused by the mass loss
and hydrogen burning \citep{hucht1986}.

\smallskip

Although early-type stars known in the Galactic center region are
concentrated to the three massive young star clusters ---
the Arches \citep{nagata95, cotera96, serabyn98, figer99b},
the Quintuplet \citep{kobayashi83, okuda90, nagata90, figer99a},
and the Central cluster \citep{becklin68, krabbe95, ghez05} ---,
it is quite natural to expect that a much larger number of early-type
stars remain unidentified.
\citet{portegies01} claimed that the number of
young massive star clusters may exceed 50 in the Galactic center
region. The inconsistency with the observed value indicates that these
clusters are too obscured to be visible in the optical and infrared bands,
or that the cluster members are dissipated before
the earliest member reaches its end, or that that number is a
gross overestimate.
\citet{kim99} showed that the strong tidal disruption in
the Galactic center region makes massive star clusters become unbound on a
very short time scale comparable to the lifetime of an O star.
Isolated early-type stars may be distributed throughout the region, unlike
the other parts of the Galaxy where they are found in associations.

\begin{table*}[t]
\begin{center}
\caption{Observation log.}
\label{tabl:obslog}
\begin{tabular}{ccccccr}
\hline
Start Date&Observatory&ObsID&R.\,A. & Decl.&$t_{\rm exp}$\footnotemark[$*$]&
$\theta$\footnotemark[$\dagger$]\\
&&&\multicolumn{2}{c}{(J2000.0)}&(ks)&(\arcmin)\\
\hline
2000-09-23&XMM-Newton&0112970201&\timeform{17h47m21s}&--\timeform{28D09'02''}&11&11.7\\
2000-10-27& Chandra& 1036
&\timeform{17h47m22s}&--\timeform{28D11'36''}&35&10.5\\
2001-07-16& Chandra& 2271
&\timeform{17h47m28s}&--\timeform{28D16'29''}&10&9.5\\
2001-07-16& Chandra& 2274
&\timeform{17h46m42s}&--\timeform{28D10'23''}&10&5.7\\
2001-07-16& Chandra& 2285
&\timeform{17h46m18s}&--\timeform{28D20'23''}&10&7.9\\
2003-03-12&XMM-Newton&0144220101&\timeform{17h47m23s}&--\timeform{28D09'15''}&34&10.4\\
2006-09-21& Suzaku&501040010
&\timeform{17h46m46s}&--\timeform{28D22'51''}&70&7.4\\
2006-09-24& Suzaku&501040020
&\timeform{17h46m46s}&--\timeform{28D22'51''}&50&7.4\\
\hline
\multicolumn{4}{@{}l@{}}{\hbox to 0pt{\parbox{135mm}{\footnotesize
\par\noindent
\footnotemark[$*$] Exposure time after removing periods with
high background level.
The exposure times for the XMM-Newton observations refer to
those obtained with the EPIC-pn camera.
\par\noindent
\footnotemark[$\dagger$] Angular distance of the source
from the optical axis.
}\hss}}
\end{tabular}
\end{center}
\end{table*}

Some early attempts have been made to discover unidentified early-type
stars in the Galactic center region with the combination of X-ray and
infrared (IR) observations.
\citet{muno06a} made trailblazing observations of radio, IR, and X-ray
to reveal the population of young massive stars in the Galactic center
region.
They detected strong Br$\gamma$ and He\emissiontype{I} lines from two
sources, and classified them as either Of or candidate LBV stars.
\citet{mauerhan07} also conducted
\textit{K}-band spectroscopy of six bright hard X-ray sources with
very red IR colors.
Two of them, with IR colors of
\textit{J}--\textit{K}$=4$--5~mag, show broad hydrogen and helium
emission lines characteristic of evolved O-type stars.
They are classified as a Wolf-Rayet star of a spectral type of WN6
and an O-type Ia supergiant.
Likewise, \citet{mikles06} performed infrared spectroscopy of
CXO\,J174536.1--285638, which is one of new Chandra sources discovered
by \citet{muno03}.
They detected P Cygni profiles of He\emissiontype{II} lines of
a 170~km~s$^{-1}$ wind.
The spectral features in the X-ray and IR bands are most consistent
with the colliding wind binary system $\eta$ Car.
This source is of particular interest in terms of a strong
Fe\emissiontype{XXV} emission line,
which is also prominent in our source, presented below.
We see a high prospect of these methods to find similar sources en masse.

Near-IR spectroscopy may confront a challenge for even redder sources.
In addition to a $\sim$\,30~mag visual extinction ubiquitous
toward the Galactic center region \citep{catchpole90,schultheis99},
evolved early-type stars are behind
an additional local extinction by their own mass loss.
Photospheric emission is reprocessed into longer wavelength
radiation in the optically thick circumstellar matter.
The IR spectra turn out to be featureless by strong
veiling \citep{figer99a,crowther06}.
For such sources, the spectroscopy of the hard X-ray emission directly
from the vicinity of the star is the only practical tool.
These stars contribute to the 6.7~keV line emission,
and the diagnosis of the line emission helps to identify their nature.

\medskip

Here, we present the results of a Suzaku \citep{mitsuda07} study of an
X-ray
point-like source located at a $\sim$\,100~pc projected distance from
the Galactic center.
The source is exceptionally bright in the 6.7~keV map of the region,
and is redder (\textit{J}--\textit{K}\,$>$\,8~mag) than those
studied in previous studies. Aided by the large effective area of Suzaku's
telescope to produce spectra having high photon statistics,
we conducted a detailed hard-band spectroscopy of this source
to identify its nature.

\section{Observation and Data Reduction}

As a part of the Suzaku mapping campaign of the Galactic center region,
a field containing the Sgr\,B2 molecular cloud was observed twice on 2006
September 21--23 and 24--25 with an aim point at
(RA, Dec)\,$=$\,(\timeform{17h46m46s}, --\timeform{28D22'51''}).
The observation log is given in table~\ref{tabl:obslog}.
Suzaku observations produce two simultaneous data sets using
the X-ray Imaging Spectrometer (XIS: \cite{koyama07a}) and
the Hard X-ray Detector (HXD: \cite{kokubun07,takahashi07}).
We concentrate on the former in this paper.

The XIS is equipped with four X-ray charge coupled devices (CCDs)
at the foci of four X--Ray Telescopes (XRT: \cite{serlemitsos07}).
Each CCD chip has a format of 1024$\times$1024 pixels.
The four XRTs are aligned to observe the same field of
$\sim18\arcmin\times$18\arcmin\ with a half-power diameter (HPD) of
1\farcm9--2\farcm3 and a pointing accuracy of up to $\sim$\,50\arcsec.
%$\sim$40\arcsec.
Three of the four chips (XIS0, 2, and 3) are front-illuminated (FI) CCDs
superior in the hard band response covering the 0.4--12~keV energy range.
The remaining one is a back-illuminated (BI) CCD superior in the soft band
covering 0.2--12~keV.
The total effective area is $\sim$\,550~cm$^2$ at 8~keV.

The energy resolution, initially of $\sim$130 eV at 6~keV in the full width
at half maximum, is subject to degradation due to cosmic-ray
radiation in the orbit.
The XIS employs two techniques to calibrate and rejuvenate its spectral
capability.
One is standard radioactive sources.
Two \atom{Fe}{}{55} sources are installed to illuminate two corners of
each CCD to calibrate the absolute energy gain.
The other is the spaced-row charge injection (SCI).
Electrons are injected at one of every 54 rows, and are
transferred through columns to sacrificially fill in traps caused
by radiation.
This reduces the number of trapped charges by X-ray events throughout
the transfer, thus improving the energy resolution \citep{bautz04,
koyama07a}.

The observations were conducted using the normal clocking mode
with a frame time of 8~s. The SCI technique was used.
Data process version 1.2\footnote{See
http://www.astro.isas.jaxa.jp/suzaku/process/history/v1225.html.}
was retrieved and events were
removed during the South Atlantic Anomaly passages and at
earth elevation angles below 3~degrees.
After the filtering, the combined net integration time is $\sim$\,120~ks.

Because the calibration database for SCI observations is not yet released
as of this writing, we used data obtained during the ground calibration
with a charge-transfer efficiency of 1.
Using the Mn K$\alpha$ line from the calibration sources
$\left(\atom{Fe}{}{55}\right)$,
we measured the systematic gain offset as the deviation from
the theoretical value at 5.8951 keV \citep{bearden67, krause79}.
The offsets were found to be within $\sim$\,10~eV for the FI
and $\sim$\,30~eV for the BI chips.

\begin{table*}[!htb]
\begin{center}
\caption{Best-fit spectral parameters.\footnotemark[$*$]}
\label{tabl:apec}
\begin{tabular}{cccc}
\hline
Parameter&Unit&Suzaku&XMM-Newton\\
\hline
\nh&($10^{23}$~cm$^{-2}$)&2.4$^{+0.3}_{-0.2}$&2.3$^{+0.4}_{-0.3}$\\
\kt&(keV)&3.8$^{+0.5}_{-0.6}$&3.8$^{+0.7}_{-0.7}$\\
$Z_{\rm Ar}$& (solar)&8.7$^{+6.3}_{-4.9}$&2.9$^{+7.1}_{-2.9}$\\
$Z_{\rm Ca}$& (solar)&3.2$^{+2.3}_{-2.0}$&0.9$^{+2.8}_{-0.9}$\\
$Z_{\rm Fe}$& (solar)&0.8$^{+0.1}_{-0.1}$&0.9$^{+0.2}_{-0.2}$\\
\fx\footnotemark[$\dagger$]&($10^{-12}$~erg~s$^{-1}$~cm$^{-2}$)&
1.00$^{+0.03}_{-0.03}$&1.05$^{+0.07}_{-0.07}$\\
\lx\footnotemark[$\ddagger$]&($10^{34}$~erg~s$^{-1}$)&2.8&2.9\\
$\chi^2$/d.o.f.&&89.4/106&69.9/74\\
\hline
\multicolumn{2}{@{}l@{}}{\hbox to 0pt{\parbox{100mm}{\footnotesize
\par\noindent
\footnotemark[$*$] The uncertainty indicates the 90\% confidence
ranges of the fit.
\par\noindent
\footnotemark[$\dagger$] Energy flux in the 2.0--8.0~keV band.
\par\noindent
\footnotemark[$\ddagger$] Absorption-corrected luminosity
in the 2.0--8.0~keV band.
A distance of 8~kpc is assumed.
}\hss}}
\end{tabular}
\end{center}
\end{table*}

\section{Analysis}
\subsection{Images}
\begin{figure*}[!htb]
\begin{center}
\FigureFile(85mm,85mm){figure1a.eps}
\FigureFile(85mm,85mm){figure1b.eps}
\end{center}
\caption{XIS images in the (a) 0.7--2.0 and (b) 2.0--7.0~keV bands.
The narrow-band intensity of 6.57--6.77~keV is shown in (a)
by contours.
The source and background regions are respectively shown by
a polygon and a dashed ellipse in (b).
Both images were processed (1) to mask calibration sources,
(2) to subtract non--X-ray background signals constructed from
night earth observations \citep{tawa08},
(3) to correct for the vignetting and the non-uniformity caused
by the contamination material on the optical blocking filter,
(4) to resample adaptively to achieve a signal-to-noise ratio of
$>$\,8 at each bin using the weighted Voronoi tessellation algorithm
(\cite{diehl06}, \cite{cappellari03}), and (5) to register the
coordinate using the Chandra counterpart of the point-like source.
}\label{fg:image}
\end{figure*}
Figure \ref{fg:image} shows XIS images of the study field
in the (a) 0.7--2.0 and (b) 2.0--7.0~keV bands.
The overlaid contours in (a) are the narrow-band intensity in
6.75--6.77~keV.
All four XIS images at the two observations are combined.
Several distinctive features can be found in the hard band.
Two extended sources are
Sgr\,B2 (\cite{koyama96b}, \cite{murakami00}, \yearcite{murakami01};
\cite{koyama07b})
and G0.61--0.01 \citep{koyama07b}.
The brightest source, which is the main topic of this paper,
is found close to the northern edge of the hard-band image.
The source is point-like and an exceptionally intense 6.7~keV emitter
in the image, and also in the vicinity found in the Suzaku wide-field map.

\subsection{Spectra}
We extracted the source and background events from the polygonal and
elliptical regions, respectively (figure~\ref{fg:image}b).
The source was superimposed upon the intense diffuse emission ubiquitous in
the Galactic center region \citep{koyama96b}.
In order to maximize the signal-to-noise ratio against the
underlying emission,
we simulated point spread functions at the source position
with different enclosed energy fractions using a ray-tracing simulator
(\texttt{xissim}: \cite{ishisaki07}).
Consequently, a 60\% encircled energy polygon was chosen to
accumulate source photons.
The background region was selected from an ellipse adjacent to and
at a similar off-axis angle with the source region.

\begin{figure}[!htb]
\begin{center}
\FigureFile(85mm,85mm){figure2.eps}
\end{center}
\caption{Background-subtracted XIS spectra. BI spectrum is
shown in red, while the merged FI spectrum is in black. The
upper panel shows the data with crosses and the best-fit APEC
models with solid lines, while the lower panel shows the residuals
to the best-fit. Conspicuous emission lines of He-like ions
(gray dash lines) and H-like ions (gray solid lines) are labeled.
}
\label{fg:bi3fispec}
\end{figure}

Figure~\ref{fg:bi3fispec} shows the background-subtracted spectrum.
Here, we generated the telescope response files using
a ray-tracing simulator (\texttt{xissimarfgen}: \cite{ishisaki07}).
We used the detector response files as of 2005 August, which best describe
the spectral profiles of the calibration sources.

The spectrum is characterized by several emission lines as well as a hard
continuum extending to $\sim$\,10~keV with a strong soft-band cut-off.
The lines are identified as K$\alpha$ emission from highly
ionized (He-like and H-like) ions of Ar, Ca, and Fe.
The spectrum was first fitted by a bremsstrahlung model with
Gaussian lines attenuated by interstellar extinction \citep{morrison83}.
The strongest line is He-like Fe K$\alpha$ at $6.66\pm0.01$~keV
with an energy flux of $(2.5\pm0.3)\times10^{-13}$~erg~s$^{-1}$~cm$^{-2}$,
which corresponds to an equivalent width (EW) of $950\pm100$~eV.

In contrast, the line feature at 6.4~keV, if present, is very weak.
We added a narrow Gaussian line at the energy and derived
%By adding a narrow Gaussian line at 6.4~keV, we found that the line is
%very weak; our measurement gives
a 90\% upper limit of
1.5$\times10^{-14}$~erg~s$^{-1}$~cm$^{-2}$, which corresponds to an
EW of 50~eV.
The temperature derived from the continuum and those from the
intensity ratios of He-like and H-like K$\alpha$ lines are consistent
with $\sim$\,4~keV, indicating that the emission is thermal plasma
in collisional equilibrium.

We then fitted the spectrum with an optically-thin thermal plasma
model (APEC: \cite{smith01}) with interstellar
extinction \citep{morrison83}.
%The solar chemical composition is assumed \citep{anders89} in
%the model, and the abundance values of Ar, Ca, and Fe relative
%to solar were thawed.
%Other elements were fixed to 1~solar abundance.
The abundances of Ar, Ca, and Fe relative to solar were free parameters,
while the other elements were fixed to 1~solar abundance \citep{anders89}.
A single-temperature model yielded an acceptable fit with 3.8~keV
in plasma temperature ($k_{\rm{B}}T$),
$1.0\times10^{-12}$~erg~s$^{-1}$~cm$^{-2}$ in X-ray flux ($F_{{\rm X}}$)
in the 2.0--8.0~keV band
and 2.4$\times10^{23}$~cm$^{-2}$ in hydrogen column extinction
($N_{\rm{H}}$).
The best-fit model and values are shown in figure~\ref{fg:bi3fispec}
and table~\ref{tabl:apec}, respectively.
No systematic deviation was found in the single temperature fit,
requiring no extra components.

To examine how the results are influenced by our choice of the
background region,
we repeated the same procedures with several different regions.
All the best-fit parameters are consistent with each other, except for the
X-ray flux, which suffers $\lesssim$\,10\% systematic uncertainty.

\subsection{Chandra and XMM-Newton Counterparts}
We retrieved the archived Chandra \citep{weisskopf02} and
XMM-Newton \citep{jansen01} data to study the long-term behaviors of
the source, and to locate its position more precisely using telescopes
of smaller HPDs than that of Suzaku.
Four Chandra observations using the Advanced CCD Imaging Spectrometer
(ACIS: \cite{garmire03}) and two XMM-Newton observations using the
European Photon Imaging Camera (EPIC: \cite{turner01, strueder01}),
which is
comprised of two MOS and a PN, were found to cover
the Suzaku source (table~\ref{tabl:obslog}).

Within the Suzaku positional uncertainty of 50\arcsec,
we found only one Chandra source (CXOGC\,J174645.3--281546) in
\citet{muno06b}
and one XMM-Newton source (2XMMp\,J174645.2--281547) in the
Second XMM-Newton Serendipitous Source
Pre-release Catalogue, XMM-Newton Survey Science Centre (2006).
%Since the position accuracy of Chandra is the best, the astrometry of
%the Suzaku images in figure~\ref{fg:image} is reregistered using
%the Chandra position.
Also, hereafter we refer the Suzaku source as CXOGC\,J174645.3--281546.

\begin{figure}[!htb]
\begin{center}
\FigureFile(85mm,85mm){figure3.eps}
\end{center}
\caption{Background-subtracted EPIC spectra.
The pn spectrum is shown in red,
while the merged MOS spectrum is in black.
The symbols follow figure~\ref{fg:bi3fispec}.}
\label{fg:epicspec}
\end{figure}

Since no spectroscopic and temporal behaviors are presented for both of the
Chandra and XMM-Newton sources in the literature,
we analyzed the data and present the results here.
One of the two XMM-Newton observations showed a net exposure of
$\sim$\,34~ks
after removing data during high background.
Spectra with high statistics were obtained from MOS and PN, for which
we conducted spectral fits in a similar manner as with the Suzaku spectrum.
The best-fit model and parameters are shown in figure~\ref{fg:epicspec}
and table~\ref{tabl:apec}, respectively.
Although the emission lines are less conspicuous in the XMM-Newton data,
the Suzaku and XMM-Newton results are consistent with each other.

For the remaining one XMM-Newton and four Chandra observations,
the photon statistics were too poor for a detailed spectral analysis,
due to short exposures, chip gaps, and
large off-axis angles of the source position.
We therefore applied the best-fit Suzaku model to derive their flux.

\begin{figure}[h]
\begin{center}
\FigureFile(80mm,80mm){figure4.eps}
\end{center}
\caption{Long-term trend of X-ray flux in the 2.0--8.0~keV using
XMM-Newton (squares), Chandra (circles), and Suzaku (a diamond).
Error bars on the data points are plotted at the 90\% confidence level.
}
\label{fg:long}
\end{figure}

Figure~\ref{fg:long} shows a long-term flux variation,
which spans $\sim$\,6~years at seven different epochs.
Although no flux variation within each observation is found,
a long-term variation having a factor $\sim2$ is clearly found.

\subsection{Longer Wavelength Data}
We retrieved other databases to characterize the multi-wavelength features
of CXOGC\,J174645.3--281546.
Because of the extreme extinction, the source is inaccessible in the
optical bands.
%We examined the data in the near-infrared (NIR) bands using
In the near-infrared (NIR) bands, we examined
the Two Micron All-Sky Survey (2MASS: \cite{cutri03};
\cite{skrutskie06}) and
the NIR Galactic center survey \citep{nishiyama06} using
Simultaneous Infrared Imager for Unbiased Survey
(SIRIUS: \cite{nagashima99, nagayama03}) on the Infrared Survey Facility
(IRSF) telescope.
In the mid-infrared (MIR) bands, we
used the Midcourse Space Experiment (MSX: \cite{egan03}) and the Galactic
Legacy Infrared Mid-Plane Survey Extraordinaire program (GLIMPSE:
\cite{benjamin03}) using the Spitzer Space Telescope \citep{werner04}.

\begin{figure}[h]
\begin{center}
\includegraphics[width=8cm,height=5.55cm]{figure5.eps}
\end{center}
\caption{NIR pseudo-color image using 2MASS $J$- (blue), $H$- (green),
and $K_{\rm s}$- (red) band images.
The source positions with the uncertainty
of Chandra and MSX are shown with a small and large circle, respectively.
}
\label{fg:irimage}
\end{figure}

Figure~\ref{fg:irimage} shows an NIR pseudo-color image,
in which an isolated point-like source (2MASS J17464524--2815476 and
MSX C6 G000.7036$+$00.1375) is found in the 2MASS and MSX images
within the positional uncertainty range of the Chandra source.
The source is very bright and red with 2MASS magnitudes of
(\textit{J}, \textit{H}, \textit{K}$_{\rm{s}}$)\,$=$\,($>$\,15.4,
10.0, $>$\,7.2) mag.
The \textit{J}- and \textit{K}$_{\rm{s}}$-band magnitudes are
upper limits due to nearby source contamination.
In the SIRIUS data, while the $K_{\rm s}$-band is unavailable due to
saturation, the magnitudes of (\textit{J}, \textit{H})\,$=$\,(15.53, 10.05)
were derived with photometric errors of $\sim$\,0.01~mag.

In the MIR bands, the source is in the linearity range in the MSX
photometry,
but is too bright in Spitzer images,
from which no meaningful photometry was obtained.
Figure~\ref{fg:nirsed} shows the spectral energy distribution (SED) in the
1.2--30~$\mu$m band.
We fitted the data with a single-temperature blackbody and an
assumed extinction proportional to the inverse square of the wavelength.
We found a best-fit blackbody temperature ($T_{\rm{BB}}$) of
980\,$\pm$\,20~K, with a visual extinction ($A_{\rm V}$) of
31\,$\pm$\,1~mag,
and a bolometric luminosity
($L_{\rm{bol}}$) of (8.3\,$\pm$\,0.5)$\times10^4~\LO
\left(d/8.0~\rm{[kpc]}\right)^2$.

\begin{figure}[!htb]
\begin{center}
\FigureFile(85mm,85mm){figure6.eps}
\end{center}
\caption{SED constructed from SIRIUS (open squares) and MSX
(open triangles) photometry.
The solid curve shows the best-fit blackbody model with
$T_{\rm BB}\,=\,980$~K, $A_{\rm{V}}\,=\,$31~mag and
$L_{\rm{bol}}\,=\,8.3\times10^4$~\LO.
SEDs of the eponymous Quintuplet cluster members \citep{okuda90, figer99a}
are also shown with colors for comparison.}
\label{fg:nirsed}
\end{figure}

\section{Discussion}
\subsection{Location of Extinction Matter}
Adopting $N_{\rm H}/A_{\rm V}=1.79\times10^{21}$~cm$^{-2}$~mag$^{-1}$
\citep{predehl95}, $A_{\rm V}=31$~mag is converted to
$N_{\rm H}\sim$\,5.6$\times10^{22}$~cm$^{-2}$.
This is far smaller than that determined with the X-ray of
$N_{\rm H}\sim$\,2.4$\times10^{23}$~cm$^{-2}$.
Since the visual extinction of 31~mag is the typical value toward the
Galactic
center \citep{catchpole90}, the excess X-ray absorption is probably due
to local obscuring matter that radiates the detected IR emission.
If such an obscuring matter with solar abundance spherically surrounds
the X-ray source, the 6.4~keV line with an EW of $\sim200$~eV should be
detected \citep{inoue85}.
The lack of 6.4~keV line (EW\,$<$\,50~eV) requires either that the iron
abundance is $\lesssim0.25$~solar or that the local matter is concentrated
in front of the source along the line of sight.

\subsection{Nature of the Source}
\subsubsection{WR Binary Origin}
The SED of CXOGC\,J174645.3--281546 in the IR bands is very similar to
those of the eponymous Quintuplet cluster members (figure~\ref{fg:nirsed}).
The five stars have cool ($T\sim$\,1000~K) spectra and are bright
($\sim$\,7~mag in the $K$-band).
The deep absorption at $\sim\,$10~$\mu$m is due to interstellar silicate.
The lack of either emission lines or intrinsic absorption
features \citep{okuda89, figer99a} allows no spectral classification,
though their luminosity of $\sim$\,10$^5$~$\LO$
corresponds to those of supergiant stars.
These intriguing stars were recently spatially resolved \citep{tuthill06}.
Two (GCS\,3--2 and GCS\,4 of \cite{nagata90}, or Q2 and Q3 of
\cite{moneti01})
out of five showed beautiful pinwheel nebulae of dust plume, which are
seen in a few WC (carbon rich Wolf-Rayet) stars \citep{tuthill99,
monnier99}.
Circumstellar dust emission with temperatures of 700--1700~K is a
common character of WC stars \citep{williams87}.

Interestingly, the two sources that exhibit the pinwheel morphology
are the brightest hard X-ray sources \citep{law04, wang06} among
the Quintuplet members.
Since both the pinwheel dust plumes and the hard X-ray emissions indicate
fast wind-wind collision \citep{tuthill99, monnier99, oskinova03},
this coincidence is not probably accidental.
The summed spectrum of three X-ray emitting sources
(GCS\,3-2, GCS\,4, and source D of \cite{nagata90}) in the Quintuplet
cluster is very hard ($kT\sim$\,9~keV) and shows a hint of iron K emission
line, but the luminosity is smaller by $\sim$\,2 orders of magnitude
(7.6$\times10^{32}$~erg~s$^{-1}$ in the 0.3--8.0~keV band; \cite{wang06})
than CXOGC\,J174645.3--281546.
In the colliding wind binary scenario, the X-ray luminosity scales as
$\lx\propto D^{-1}$ \citep{stevens92}, where $D$ is
the binary orbital separation.
The large X-ray luminosity of CXOGC\,J174645.3--281546 indicates the close
separation,
and the moderate flux variation having a factor of $\sim$\,2 can be
interpreted as the variation of the separation due to the orbital motion.

Some of evolved massive~star+OB~star systems have very large X-ray
luminosity
up to $\sim$10$^{35}$~erg~s$^{-1}$ in the 0.5--10.0~keV band with thermal
spectra with a temperature of $\gtrsim$\,2~keV and an iron abundance of
1--2
($\eta$ Carinae: \cite{tsuboi97}, \cite{hamaguchi07}; WR\,140:
\cite{koyama94}, \cite{pollock05},
A1\,S, A1\,N, and A2 in the Arches cluster; \cite{wang06}).
The X-ray spectral property of CXOGC\,J174645.3--281546 coincides
with those of these massive star binaries, though near- and mid-IR
spectroscopy and high spatial resolution observation are essential for
further constraint.
Taking the discussion in \S4.1. into account, all of the observational
results
are consistent with the idea that CXOGC\,J174645.3--281546 is a
WC star+massive star
binary near the Galactic center, which has local obscuring
matter (figure~\ref{fg:cartoon}).

\begin{figure}[h]
\begin{center}
\FigureFile(85mm,85mm){figure7.eps}
\end{center}
\caption{Cartoon of the source system. Winds from the primary WC star and
the companion massive star collide to produce X-rays.
The obscuring material of $N_{\rm H}\sim$\,2$\times10^{23}$~cm$^{-2}$
around the binary system is heated to $T\sim$\,980~K by
photospheric emission to emit IR.
Whereas the IR emission suffers only interstellar medium (ISM) extinction,
the X-ray emission is subject to additional intrinsic absorption.
}
\label{fg:cartoon}
\end{figure}
Because the initial mass of a WR star exceeds $\sim\,$40~\MO\ and the
age is
less than $\sim$6~Myr \citep{maeder87},
we naturally expect that WR stars are found in
large young star clusters, assuming a usual initial mass function.
Our source, however, seems to be isolated (figure~\ref{fg:irimage}).
\citet{muno06a} and \citet{mauerhan07} also recently discovered
such isolated massive stars in the Galactic center region.
One possibility is that these stars were formed initially in a cluster,
and the cluster dissipated due to the strong tidal force of the Galactic
center \citep{portegies02a}.
Future IR and X-ray observations with a higher spatial resolution and
sensitivity may reveal that either this source is really a cluster member,
or truly an isolated star.

\subsubsection{Other Possible Origins}
The most popular class of hard X-ray sources that exhibits
iron K-shell emission lines is CV.
The X-ray luminosity of CXOGC\,J174645.3--281546 of
$3\times10^{34}$~erg~s$^{-1}$ is at the brightest end of CVs.
The bolometric luminosity of
$\sim10^5~\LO\left(d/8.0~\rm{[kpc]}\right)^2$ is, however far larger
than CVs, because most of the optical companions of CVs are late-type
main-sequence stars.
The spectra of CVs are characterized by hard continuum, with three
iron K-shell emission lines at 6.4~keV, 6.7~keV, and 6.97~keV.
The equivalent widths of both the 6.4~keV and 6.7~keV lines are, however,
around 100--200~eV, although in some cases that of the 6.7 keV line
shows an
exceptionally high value possibly due to resonance scattering
\citep{terada01}.
The 6.97~keV/6.7~keV flux ratio of CVs is larger than 0.1, indicating that
the ionization temperature is higher than $\sim\,$4~keV
\citep{ezuka99, hellier04, rana06}.
CXOGC\,J174645.3--281546 has a significantly larger equivalent width of
the 6.7~keV line ($\sim\,1\,$keV) and lower flux ratio of
the 6.97~keV/6.7~keV lines than those of CVs.
The upper limit of 50~eV for the 6.4~keV line, on the other hand,
can not give any constraint on the possibility of a CV.
Altogether, CXOGC\,J174645.3--281546 is very unlikely to be a CV.

The X-ray spectra of YSOs are characterized by a thin thermal plasma
with a temperature of 1--5~keV and a metal abundance of around 0.3~solar
in the quiescent phase \citep{feigelson02, imanishi03, ozawa05}.
The CXOGC\,J174645.3--281546 iron abundance of 0.8\,$\pm$\,0.1~solar is
higher
than the typical YSO value of 0.3~solar, but is not exceptional.
Although the ratio of the X-ray and bolometric luminosity
($L_{\rm X}/L_{\rm bol}$) of $10^{-5}$ is in the range of low-mass YSOs
\citep{imanishi01, feigelson02},
the absolute luminosities are too high for a YSO if
CXOGC\,J174645.3--281546
is located at 8~kpc.
The possibility still remains that CXOGC\,J174645.3--281546 is a
foreground YSO in a dense molecular cloud.
%a nearby YSO having dense molecular clouds
%in front is one remote possibility.

\subsection{Contribution to the Galactic Center 6.7~keV Line}
The 6.7~keV line flux from CXOGC\,J174645.3--281546 is
$2.5\times10^{-13}$~erg~s$^{-1}$~cm$^{-2}$.
The total flux of iron K-lines in the Galactic center region is
$\sim1.3\times10^{-10}$~erg~s$^{-1}$~cm$^{-2}$ \citep{yamauchi90}.
Since this value includes the 6.4~keV and 6.97~keV lines,
it overestimates the 6.7 keV flux by a factor of 2--3.
The detected point sources account for $\sim10\%$ of the total flux of
the 6.7~keV line \citep{wang02, muno04a}.
Therefore, the 6.7~keV line of CXOGC\,J174645.3--281546 alone accounts for
$\sim4$\% of that of the detected point sources and $\sim0.4$\% of that of
the total diffuse flux.
Such X-ray sources would comprise a substantial fraction of 6.7~keV
line-emitting point sources in the Galactic center.

\section{Summary}
\begin{enumerate}
\item We detected a strong 6.7~keV line with an equivalent width of
$\sim$\,1~keV from CXOGC\,J174645.3--281546.
The overall spectrum was very well fitted by a
heavily absorbed (\nh$\sim$\,2.4$\times10^{23}$~cm$^{-2}$)
3.8~keV thermal plasma with an iron abundance of $\sim$\,0.8~solar
and a luminosity of $\sim$\, 3$\times10^{34}$~erg~s$^{-1}$ in the
2.0--8.0keV band, assuming a distance of 8~kpc.
\item We also analyzed the archived data of Chandra and XMM-Newton, and
found that the X-ray flux spanning $\sim$\,6~years shows year-scale time
variability having a factor of $\sim$\,2.
\item The probable counterpart in the IR bands is very bright
($L_{\rm bol}\sim$\,$10^{4.9}$~\LO~at 8~kpc), and has a cool
($T_{\rm BB}\sim$\,1000~K) SED, which is similar to those of
the eponymous members of the Quintuplet cluster.
\item The X-ray spectral and temporal properties and the IR SED
are fully consistent with a WC~star+massive~star binary system.
\end{enumerate}
\bigskip

Y.\,H. and M.\,T. acknowledge financial support
from the Japan Society for the Promotion of Science.
The work is financially supported by the grants-in-aid for a
21st century center of excellence program
``Center for Diversity and Universality in Physics'' and No. 18204015
by the Ministry of Education, Culture, Sports, Science and Technology
of Japan.
This research has made use of data obtained from the Data ARchive and
Transmission System at ISAS/JAXA, the High Energy Astrophysics
Science Archive Research Center at the NASA/Goddard Space Flight Center,
and the Two Micron All Sky Survey, which is a joint project of
the University of
Massachusetts and the Infrared Processing and Analysis Center/California
Institute of Technology, funded by the National Aeronautics and
Space Administration and the National Science Foundation.
MSX was funded by the Ballistic
Missile Defense Organization with additional support from NASA
Office of Space Science. This research has also made use of the
NASA/ IPAC Infrared Science Archive, which is operated by the
Jet Propulsion Laboratory, California Institute of Technology,
under contract with the National Aeronautics and Space
Administration.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\end{document}