2009 March 31 2009 June 16 $\langle$publication date$\rangle$ T. Yuasa et
al.The origin of an extended X-ray emission of 47 Tuc

Galaxy: globular clusters: individual (47 Tuc) — X-rays: ISM

# The origin of an extended X-ray emission apparently associated with the
globular cluster 47 Tucanae

Takayuki Yuasa 11affiliation: Department of Physics, School of Science,
University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 Kazuhiro Nakazawa
11affiliationmark:  and Kazuo Makishima11affiliationmark:  22affiliation:
Cosmic Radiation Laboratory, The Institute of Physical and Chemical Research
(RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198 Last update:
<EMAIL_ADDRESS>
###### Abstract

Using the Suzaku X-ray Imaging Spectrometer, we performed a 130 ks observation
of an extended X-ray emission, which was shown by ROSAT and Chandra
observations to apparently associate with the globular cluster 47 Tucanae. The
obtained $0.5-6$ keV spectrum was successfully fitted with a redshifted thin
thermal plasma emission model whose temperature and redshift are
$2.2^{+0.2}_{-0.3}~{}$keV (at the rest frame) and $0.34\pm 0.02$,
respectively. Derived parameters, including the temperature, redshift, and
luminosity, indicate that the extended X-ray source is a background cluster of
galaxies, and its projected location falls, by chance, on the direction of the
proper motion of 47 Tucanae.

##  1 Introduction

Many globular clusters in the Galaxy move through the Galactic halo with a
typical velocity of $\sim 200$ km s-1, which exceeds the sound velocity (a few
tens of km s-1 to 150 km s-1) specified by roughly estimated halo plasma
temperatures ($T\sim 0.7-1.4\times 10^{5}$ K by [Savage & de Boer (1981)];
$T\sim 1.5-1.6\times 10^{6}$ K by [Pietz et al. (1998)]). Then, a bow shock is
expected to form between the halo plasma and any gas (intra-cluster gas) in a
moving globular cluster (Ruderman & Spiegel, 1971). Since the temperature of
the post-shock plasma should then become $\sim 10^{6}$ K (e.g., Okada et al.
(2007)), we expect to detect diffuse X-ray emission with a shape that traces
the bow shock. Several X-ray satellites have been observing globular clusters
in search for such bow-shock-heated X-ray emitting plasmas. Indeed, using the
Einstein satellite, Hartwick, Cowley, & Grindlay (1982) first detected such
diffuse emissions around 47 Tucanae (hereafter 47 Tuc), $\rm{\omega}$
Centauri, and M22, although a part of the emissions was resolved into point
sources by later observations (Koch-Miramond & Auriére (1987); Krockenberger &
Grindlay (1995); Gendre, Barret, & Webb (2003)). Subsequently, Krockenberger &
Grindlay (1995) newly reported diffuse emissions in 47 Tuc, followed by,
possible detections of such a diffuse emission in several globular clusters;
e.g., Hopwood et al. (2000) in NGC 6779, and Okada et al. (2007) in 47 Tuc,
NGC 6752, and M5.

Among the extended emissions so far detected, those in the globular clusters
47 Tuc and NGC 6752 are of particular interest. They spatially coincide with
the directions of projected proper motions of these globular clusters
(Krockenberger & Grindlay, 1995; Okada et al., 2007), and were hence
considered to have physical relationships with the globular clusters.
According to Okada (2005) and Okada et al. (2007), the 0.5-4.5 keV Chandra
spectra from these extended sources are so hard that they require power-law
models with photon indices of $\Gamma=2.1\pm 0.3$ (47 Tuc) and $\Gamma=2.0\pm
0.2$ (NGC 6752), or a thermal plasma emission model with temperatures of
$kT=3.7^{+2.7}_{-1.3}$ keV (47 Tuc) and $kT=2.9^{+1.0}_{-0.7}$ keV (NGC 6752)
which largely exceed values expected from the bow shock heating ($\sim 10^{6}$
K). While the two X-ray sources have no optical identifications (Krockenberger
& Grindlay, 1995; Okada et al., 2007), Okada et al. (2007) reported that both
have possible radio counterparts in the 843 MHz Sydney University Molongo Sky
Survey (SUMSS; Bock et al. (1999)).

From these properties, the extended emissions apparently associated with 47
Tuc and NGC 6752 were considered to arise via inverse Compton scattering
(Krockenberger & Grindlay, 1995) or non-thermal bremsstrahlung (Okada et al.,
2007) of high energy ($E\sim 20-100$ keV) electrons that are stochastically
accelerated in the bow shock. The interpretation is attractive because the
shock is expected to be a moderate one with a Mach value of $\sim 10$, and the
condition is much different from those in the more typical acceleration sites
such as supernova remnants and jets of active galactic nuclei.

As an alternative explanation to those extended X-ray sources, Krockenberger &
Grindlay (1995) and Okada et al. (2007) also considered a chance coincidence
with a background cluster of galaxies that is not related to the globular
clusters. This alternative must be kept in mind, even though its possibility
was estimated low ($<1\%$ by Krockenberger & Grindlay (1995) and Okada et al.
(2007)) in 47 Tuc.

In the previous spectral analysis of the Chandra data from the extended
emission in 47 Tuc and NGC 6752, Okada et al. (2007) were unable to
distinguish a power-law model from a thermal emission model because of rather
large statistical errors. In the present paper, we utilize the larger
effective area and lower background level of Suzaku, to perform detailed
spectral analysis of the extended emission of 47 Tuc. Based on the model
fitting result, we conclude that the emission is from a background galaxy
cluster with a redshift of 0.3, and the spatial coincidence between the
extended emission and the globular cluster is accidental.

Throughout the paper, cosmological parameters of $\Omega_{\rm{M}}=0.28$ and
$H_{0}=70$ km s-1 Mpc-1 are used in calculations.

##  2 Observation and Data Reduction

The globular cluster 47 Tucanae was observed with Suzaku (Mitsuda et al.,
2007) on 2007 June 10–12 (observation ID 502048010). Since the target of this
observation is the extended emission (EE) at the north east region of 47 Tuc,
we set the nominal pointing of the satellite at
$(\timeform{00h24m50s},\timeform{-71D59^{\prime}46^{\prime\prime}})$, which is
$\sim\timeform{6^{\prime}}$ off the cluster center.

In the present study, we focus on the data taken with the X-ray Imaging
Spectrometer (XIS; Koyama et al. (2007)), which comprises four X-ray charge
coupled device (CCD) sensors each placed on the focal plane of the X-ray
Telescope (XRT; Serlemitsos et al. (2007)). The four pairs of XIS and XRT are
co-aligned together, and have the same field of view (FOV) of
$\timeform{17.8^{\prime}}\times\timeform{17.8^{\prime}}$. Since one of three
front-side illuminated (FI) CCD chips, XIS2, was not operational since 2006
November, we utilized the data from the remaining two FI sensors (XIS0 and 3)
and a back-side illuminated (BI) one (XIS1). In the present observation, the
XIS was operated in the normal mode without any window or burst option, but
incorporating the spaced-row charge injection method (Nakajima et al., 2008)
to restore the energy resolution of the CCDs.

After removing periods of the Earth elevation angle less than $5^{\circ}$
(ELV$<5^{\circ}$), the day Earth elevation angle less than $20^{\circ}$
(DYE$\\_$ELV$<20^{\circ}$), and the South Atlantic Anomaly, we achieved a net
exposure of 132 ks. Flickering pixels were removed from the data by using
`cleansis` version 1.7. Then, cleaned event files were generated employing the
same event extraction criteria as in the Suzaku pipe line processing (version
2).

The present data reduction and analysis were performed using HEADAS package
version 6.4.1 and `XSPEC` version 11.3.2. In spectral fitting, redistribution
matrix files and ancillary response files (ARFs) for the XIS/XRT were
generated using `xisrmfgen` version 2007-05-14 and `xissimarfgen` (Ishisaki et
al., 2007) version 2008-04-05, respectively, with the calibration files which
are provided by the calibration database (CALDB) version 2008-04-01. In the
spectral fitting described below, we ignored data in the $1.8-1.9$ keV band so
as to avoid calibration uncertainties around the Si-K edge.

Events with energies above 10 keV, taken with the Hard X-ray Detector (HXD;
Takahashi et al. (2007)), were not utilized in the present analysis. This is
because the HXD lacks imaging capability, and hence we cannot exclude
contamination by X-rays from a number of point sources associated with 47 Tuc
(eg. Verbunt & Hasinger (1998); Grindlay et al. (2001); Heinke et al. (2005)).

Since the spatial resolution of the XIS/XRT is $\sim\timeform{2^{\prime}}$, we
cannot exclude, using the XIS data alone, X-ray point sources that overlap
with the EE. To determine their spectral shapes and fluxes, we also utilized
archived Chandra ACIS data of 47 Tuc acquired in 2000 March for a total
exposure of about 70 ks (obsid=953 and 955). We used `CIAO` (Chandra
Interactive Analysis of Observations) version 4.0.2 and CALDB version 3.4.5 to
extract point source spectra. Like in the Suzaku data analysis, we also used
`XSPEC` when performing model fitting to the ACIS spectra.

##  3 Image Analysis

###  3.1 Soft and hard band images

In Fgure 1, we present images obtained with the FI cameras (XIS0 and 3) in the
soft ($0.5-1.5$ keV) and hard ($1.5-6.0$ keV) bands, after subtracting the non
X-ray background (NXB) and correcting for vignetting and exposure. We
estimated the NXB of the XIS using dark (night) Earth data taken within $\pm
150$ days of our observation of 47 Tuc. The night Earth data were summed up,
with weights according to geomagnetic cut-off rigidity which the spacecraft
experienced at the data acquisition. This was performed by `xisnxbgen` (Tawa
et al., 2008). Then, we created NXB images in the soft and hard bands, and
subtracted them from the raw images. After subtracting the NXB, we smoothed
each image with a two-dimensional Gaussian of
$\sigma=\timeform{6^{\prime\prime}}$. The diffuse X-ray backgrounds, namely
the cosmic X-ray background (CXB) and Galactic diffuse emission, are still
included in the images.

In figure 1, we see several point sources, and the very bright 47 Tuc core
region which consists of numerous X-ray point sources (e.g., Heinke et al.
(2005)). At the center of the two images, we also observe a clear
concentration of X-ray events as Krockenberger & Grindlay (1995) and Okada et
al. (2007) reported. Thus, we reconfirm the EE phenomenon with the Suzaku
data. To extract photons from the EE region, we define a circular region
(white circle in figure 1) with a radius of $\timeform{150^{\prime\prime}}$,
centered on
$(\timeform{00h24m44.2s},\timeform{-71D59^{\prime}33.5^{\prime\prime}})$ where
Okada et al. (2007) found the maximal surface brightness.

As indicated with a black solid circle and a label “PS” in figure 1, a faint
point source is recognized at the north west rim of the event extracting
region. Although the EE is still apparent in the hard band image, the point
source is no longer visible therein. At a consistent position, we find a point
source also in the ACIS image. Therefore, we consider that the XIS source is
not a brightness enhancement associated with the EE, and hereafter exclude it
using a circular region with a radius of $\timeform{1^{\prime}}$. This region
is expected to enclose 50% of X-ray photons from the point source, while the
remaining half will fall out of the region; a half of those photons (25% of
the total flux from the source) are in turn estimated to fall inside the event
extracting region around the EE, and contaminate the EE spectrum. This effect
is considered later in section 4.

(170mm,70mm)figure1.eps

Figure 1: Soft ($0.5-1.5$ keV; left) and hard ($1.5-6.0$ keV; right) band
images of 47 Tuc taken with XIS FI (XIS0 plus XIS3), shown after removing the
two corner regions irradiated with the calibration source. The images are
scaled in units of $4\times 10^{-5}{\rm counts}\ {\rm s}^{-1}~{}{\rm
pixel}^{-1}$. The non X-ray background is subtracted using the night Earth
image (see text), followed by vignetting and exposure correction, although the
diffuse X-ray background is included. The white circle with a radius of
$\timeform{150^{\prime\prime}}$ is the event extracting region for the
extended emission (EE), and the black small one shows a soft point source (PS)
which is excluded from the spectral analysis.

###  3.2 Radial profile of the extended emission

In the previous studies by Krockenberger & Grindlay (1995) and Okada et al.
(2007), the EE was concluded to be extended over an angular radius of
$\sim\timeform{2^{\prime}}$ (2.7 pc assuming a 4.6 kpc distance to 47 Tuc). To
examine the spatial extent of the emission, we calculated its azimuthly
averaged radial profile using the NXB-subtracted and vignetting-corrected
$0.5-6.0$ keV XIS FI image. This was done utilizing a series of annular
extracting regions, each with $\timeform{30^{\prime\prime}}$ width, which are
concentric with the original event extraction region. The result is shown in
figure 2 after subtracting the CXB and Galactic diffuse background rate of
$9.4\times 10^{-7}~{}{\rm counts}\ {\rm s}^{-1}~{}{\rm pixel}^{-1}$, which we
estimated using another region of the CCDs with no evident point sources.

In the same figure, we also plot a radial profile of the point spread function
(PSF) of the XIS/XRT, calculated at the center of the EE, and averaged over
XIS0 and XIS3. Thus, the EE is clearly more extended than the PSF even though
the latter is much broader than those of ROSAT and Chandra.

(80mm,80mm)figure2.eps

Figure 2: Azimuthly averaged radial profiles of the EE (filled circles) shown
in units of counts s-1 pixel-1, after subtracting the CXB and Galactic diffuse
components (see text). The PSF of XIS FI (open squares) is also plotted, being
normalized to have the same maximum value as that of the EE at the innermost
annulus.

##  4 Spectral Analysis

###  4.1 Extraction of spectra

We extracted XIS FI and BI spectra of the EE using the event extracting region
shown in figure 1 (the white circular region but excluding the black circle).
The $0.5-6$ keV band count rates measured with XIS FI and BI are $28.3\pm
0.05\times 10^{-3}~{}{\rm counts}\ {\rm s}^{-1}$ and $22.7\pm 0.04\times
10^{-3}~{}{\rm counts}\ {\rm s}^{-1}$, respectively, with $1\sigma$
statistical errors.

In addition to the EE which is the subject of the present analysis, the
spectra also contain events from the NXB, the Galactic and extragalactic X-ray
backgrounds (altogether, diffuse X-ray background), and X-ray events from
several contaminating X-ray point sources that cannot be resolved with the
XIS/XRT spatial resolution. We assume the Galactic diffuse emission to have a
uniform brightness across the XIS FOV. Although its hard component (Worrall et
al., 1982; Koyama et al., 1986; Ebisawa et al., 2001; Revnivtsev et al., 2006;
Krivonos et al., 2007) has a strong concentration toward the Galactic plane
(with a scale height of $\timeform{1.5D}-\timeform{3D}$ ; e.g., Revnivtsev et
al. (2006), Krivonos et al. (2007)), and hence a steep brightness gradient, it
can be neglected at this high Galactic latitude of $\sim\timeform{45D}$ of 47
Tuc.

###  4.2 Background Spectra

In the following subsections, we estimate the spectral shapes and fluxes of
these individual background components, and create XIS spectral data for each
component. By summing all these components, we construct background spectra,
and then subtract them from the raw XIS spectra of the EE.

####  4.2.1 The non X-ray Background

We derived the NXB spectrum from the same stacked night-Earth data as
described in section 3.1. Since this component depends on the CCD location, we
examined spectral differences among several circular extracting regions on the
night-Earth image, with the radius ranging from
$\timeform{150^{\prime\prime}}$ to $\timeform{300^{\prime\prime}}$. Each
region is concentric with the EE extracting region (white circle in figure 1).
The derived $0.5-10$ keV NXB spectra were consistent with one another within
$\sim 3$%. Therefore, to minimize the statistical errors of the estimated NXB,
we adopted the largest extracting region ($\timeform{300^{\prime\prime}}$
radius) for both XIS FI and BI.

The constructed NXB spectrum is shown in figure 6 in green. The count rate has
been scaled to the ratio ($\sim 4.8$) of the NXB and signal extracting areas.
The $0.5-6$ keV band count rates with $1\sigma$ statistical errors are $4.4\pm
0.1\times 10^{-3}~{}{\rm counts}\ {\rm s}^{-1}$ (XIS FI) and $5.3\pm 0.1\times
10^{-3}~{}{\rm counts}\ {\rm s}^{-1}$ (XIS BI).

####  4.2.2 The diffuse X-ray background

The diffuse X-ray background consists of two components; the Galactic and the
extragalactic emissions. The former component is thought to originate from the
Galactic halo and the Local Hot Bubble (eg. Cox & Reynolds (1987)), and
expected to appear at energies below $\sim 2$ keV with its surface brightness
depending considerably on the sky direction. The latter, the extragalactic
component, has been understood as a superposition of numerous extragalactic
active Galactic nuclei. The spectrum is known to be expressed by a power-law
model with a photon index of $\Gamma=1.4$ (e.g. Parmar et al. (1999); Lumb et
al. (2002); Kushino et al. (2002)) at least over the $2-10$ keV band.

In order to determine the local diffuse X-ray background in the present XIS
FOV, we extracted another set of XIS FI and BI spectra from the same
observation data set of 47 Tuc, but applying a mask which excludes point
sources, the core region of 47 Tuc, and the EE itself. The masked image of XIS
FI is shown in figure 3, and the NXB-subtracted (as described in section
4.2.1) spectra of the diffuse X-ray background are plotted in figure 4. We
fitted these spectra jointly with a model which consists of three diffuse
X-ray background components; a thermal emission from the Local Hot Bubble
plasma (`mekal` model in `XSPEC`; Liedahl et al. (1995)); a thermal emission
from the Galactic halo plasma (`mekal`); and a power-law model with a fixed
photon index of 1.4 to account for the extragalactic component (`powerlaw`).
The latter two components were assumed to suffer the line-of-sight Galactic
absorption, with the absorbing column density fixed at $5\times 10^{20}$ atoms
cm-2 (Dickey & Lockman, 1990) which is a typical value toward the present
field. The photoelectric absorption coefficient by Morrison & McCammon (1983),
`wabs` model in XSPEC, was employed. We assumed that the diffuse background
has a uniform surface brightness over the XIS FOV, and utilized an ARF which
was calculated using `xissimarfgen` with the `UNIFORM` option and
`r_max`$=\timeform{20^{\prime}}$. We left free the temperatures, metal
abundances, and surface brightnesses of the two thermal models, as well as the
photon index and surface brightness of the power-law model. The model gave an
acceptable fit with $\chi^{2}_{\nu}=1.10~{}(\nu=175)$; the best fit parameters
are listed in table 1.

As to the power-law component, the best fit model gave the $2-10$ keV flux of
$4.4\times 10^{-8}~{}{\rm erg}\ {\rm cm}^{-2}\ {\rm s}^{-1}\ {\rm sr}^{-1}$,
which is $\sim 20$% lower than the previously reported values; $5.4\times
10^{-8}~{}{\rm erg}\ {\rm cm}^{-2}\ {\rm s}^{-1}\ {\rm sr}^{-1}$ by Lumb et
al. (2002), and $5.7\times 10^{-8}~{}{\rm erg}\ {\rm cm}^{-2}\ {\rm s}^{-1}\
{\rm sr}^{-1}$ by Kushino et al. (2002). The deviation can reasonably be
explained by the spatial fluctuation of the extragalactic emission which can
vary by about 20% (Kushino et al., 2002) depending on XIS pointings.

We might directly subtract the diffuse background spectrum of figure 4 from
that of the EE region. However, this introduces some systematic bias because
the energy-dependent vignetting effect of the XRT (figure 11 of Serlemitsos et
al. (2007)) will cause not only the observed background brightness but also
its spectral shape to differ between the two regions; in the present case, the
two ARFs, one for the masked region (figure 3) while the other for the EE,
differ by $10-20$% (due to energy dependent vignetting effect) in the $2-6$
keV range if we normalize them at 2 keV. Hence, to avoid this problem, we
simulated the expected contribution of the diffuse background to the EE
extracting region using the best fit model explained above and the
corresponding ARF. In producing the fake spectra, we assumed a sufficiently
long exposure ($10^{7}$ s), to suppress statistical errors. This is allowed
because the background components are understood from previous observations.
The $0.5-6$ keV band count rates of the faked spectra are $8.3\times
10^{-3}~{}{\rm counts}\ {\rm s}^{-1}$ (XIS FI) and $5.9\times 10^{-3}~{}{\rm
counts}\ {\rm s}^{-1}$ (XIS BI).

(65mm,65mm)figure3.eps

Figure 3: The XIS FI image after filtering out point sources, the 47 Tuc core
region, and the EE. The image is not corrected for the vignetting or exposure.
The events plotted in the image were used in the modeling of the diffuse X-ray
background in the field of 47 Tuc.

(80mm,80mm)figure4.eps

Figure 4: The NXB-subtracted diffuse X-ray background spectra of XIS FI (black) and BI (red), extracted from the image in figure 3. The solid lines represent the best fit model, while their components are individually plotted in dashed (thermal), dot-dashed (thermal with absorption), and dotted (power law with absorption) lines. Table 1: The best fit model parameters for the diffuse background spectra. Model | Parameter | Value
---|---|---
Thermal 1 | $kT$ | $0.17\pm^{+0.01}_{-0.02}~{}{\rm keV}$
|  $Z$**footnotemark: $*$ | $0.1\pm^{+0.9}_{-0.03}$
|  $\Sigma$\dagger\daggerfootnotemark: $\dagger$ | $1.07\pm^{+0.51}_{-0.98}$
Absorption |  $N_{\rm H}$\ddagger\ddaggerfootnotemark: $\ddagger$ | $5~{}({\rm fixed})$
Thermal 2 | $kT$ | $0.78^{+0.30}_{-0.39}~{}{\rm keV}$
|  $Z$**footnotemark: $*$ | $0.03^{+0.04}_{-0.03}$
|  $\Sigma$\dagger\daggerfootnotemark: $\dagger$ | $0.75^{+2.28}_{-0.35}$
Power law | $\Gamma$ | $1.4~{}({\rm fixed})$
|  $\Sigma$\dagger\daggerfootnotemark: $\dagger$ | $0.38^{+0.03}_{-0.03}$
| $\chi^{2}_{\nu}$ | 1.10 (175)
**footnotemark: $*$ Abundance in terms of the solar value (Anders & Grevesse,
1989).
\dagger\daggerfootnotemark: $\dagger$The $0.5-6$ keV band model surface
brightness in units of $10^{-8}~{}{\rm erg}\ {\rm cm}^{-2}\ {\rm s}^{-1}\ {\rm
sr}^{-1}$. Absorption is not corrected.
\ddagger\ddaggerfootnotemark: $\ddagger$Line-of-sight hydrogen column density
in units of $10^{20}~{}{\rm cm}^{-2}$.

####  4.2.3 Contamination from point sources

In the previous study using Chandra (Okada et al., 2007), six faint X-ray
point sources were found within $\timeform{2.5^{\prime}}$ of the EE region. In
table 2, we list their positions. Although they were successfully removed in
the Chandra case, we cannot do so from the present XIS data except for the
brightest one described in section 3, because of the broader PSF of the XRT
than that of Chandra. Therefore, we must model and subtract their
contributions, like the diffuse background. As explained below, we estimate
the contribution from the soft point source (Source 1; figure 1) using the
Suzaku XIS data themselves, and those of the remaining five point sources
(Source 2–5) using the Chandra ACIS data assuming that they are not variable.

Table 2: The coordinates of the contaminating point sources. Source # | Coordinate
---|---
1 | $(\timeform{00h24m14.51s},\timeform{-71D58^{\prime}50.4^{\prime\prime}})$
2 | $(\timeform{00h24m38.71s},\timeform{-72D00^{\prime}46.3^{\prime\prime}})$
3 | $(\timeform{00h24m34.83s},\timeform{-72D00^{\prime}40.2^{\prime\prime}})$
4 | $(\timeform{00h24m30.26s},\timeform{-72D00^{\prime}33.8^{\prime\prime}})$
5 | $(\timeform{00h25m00.70s},\timeform{-71D59^{\prime}59.9^{\prime\prime}})$
6 | $(\timeform{00h24m42.63s},\timeform{-71D59^{\prime}22.3^{\prime\prime}})$

Figure 5 shows XIS FI spectrum of Source 1, extracted from the black circle
(figure 1), shown after subtracting the NXB and the diffuse X-ray background.
The FI and BI spectra were fitted with an absorbed single power-law model in
the $0.5-5$ keV band. As listed in table 3, this gave an acceptable fit with
$\chi^{2}_{\nu}=1.10~{}(\nu=30)$. The summed spectrum of the remaining 5 point
sources was extracted from the ACIS data (section 2), using circular regions
each $\timeform{5^{\prime\prime}}$ in radius. The NXB was extracted from
another region of the same ACIS CCD with no evident point sources. Then, we
fitted the summed spectrum with a single power-law model in the $0.8-6$ keV
band. The best-fit ($\chi^{2}_{\nu}=2.54$ and $\nu=4$) model gave a null
hypothesis probability of $0.041$, and the parameters as listed in table 3.

To obtain a summed contribution of all the point sources to the EE, we then
faked the summed spectrum of the 5 point sources and the soft source
separately, by applying appropriate ARFs to the best fit models described
above. In calculating the ARF for the 5 point sources, we took an average of
individual ARFs weighted by their $0.5-5$ keV ACIS count rates. The ARF for
Source 1 was calculated referring to the XIS/XRT effective area for X-ray
photons which leak into the EE event extracting region; the source position
was set to be that of the soft source (the first row in table 2), whilst it is
located outside the EE region (white circle in figure 1)111A ratio of the
number of photons which leak into the EE region to that of photons falling
inside the Source 1 region (black circle in figure 1) is 23% in the 0.5-5 keV
band, which is close to with the rough estimation ($\sim 25$%) in section 3.1.
. Based on the faked spectrum, the implied $0.5-6$ keV band count rates are
$2.9\times 10^{-3}~{}{\rm counts}\ {\rm s}^{-1}$ (XIS FI) and $2.3\times
10^{-3}~{}{\rm counts}\ {\rm s}^{-1}$ (XIS BI).

(80mm,80mm)figure5.eps

Figure 5: The XIS FI spectrum (black crosses) and the best fit power-law model (solid line) of the soft point source at the north west of the EE. The NXB and diffuse X-ray background are subtracted. Data from XIS BI are excluded from the plot for clarity, although they were incorporated in the fitting. Table 3: The best fit model parameters for the contaminating soft point source and five faint sources. Model | Parameter | Source 1 | Source $2-5$
---|---|---|---
Absorption |  $N_{\rm H}$**footnotemark: $*$ | $2.2^{+2.3}_{-1.3}$ | 0 (fixed)
Power law | $\Gamma$ | $5.1^{+2.3}_{-1.1}$ | $1.7\pm 0.2$
|  ${\rm flux}$\dagger\daggerfootnotemark: $\dagger$ | $3.9^{+6.1}_{-1.6}$ | $3.2\pm 0.3$
| $\chi^{2}_{\nu}$ | 1.10 (30) | 2.54 (4)
**footnotemark: $*$ Hydrogen column density in units of $10^{21}~{}{\rm
cm}^{{}^{2}}$.
\dagger\daggerfootnotemark: $\dagger$The $0.5-6$ keV band model flux in units
of $10^{-14}~{}{\rm erg}\ {\rm cm}^{-2}\ {\rm s}^{-1}$. Not corrected for the
absorption.

(80mm,80mm)figure6a.eps (80mm,80mm)figure6b.eps

Figure 6: The raw (black) and the background-subtracted (cyan) spectra of the EE obtained with XIS FI (panel a) and BI (panel b). The long-accumulated non X-ray background and the faked diffuse X-ray background are plotted in green and red respectively. The blue line represents the simulated contamination from the six point sources.  Table 4: The $0.5-6$ keV count rates of individual spectral components. Component | Rate ($10^{-3}~{}{\rm counts}\ {\rm s}^{-1}$)
---|---
| XIS FI | XIS BI
Raw | 28.3 | 22.7
NXB | 4.4 | 5.3
DXB**footnotemark: $*$ | 8.3 | 5.9
PS\dagger\daggerfootnotemark: $\dagger$ | 2.9 | 2.3
BGD\ddagger\ddaggerfootnotemark: $\ddagger$ | 15.6 | 13.5
EE\S\Sfootnotemark: $\S$ | 12.7 (45%) | 9.2 (40%)
**footnotemark: $*$ Diffuse X-ray background.
\dagger\daggerfootnotemark: $\dagger$Six point sources.
\ddagger\ddaggerfootnotemark: $\ddagger$Sum of the NXB, diffuse X-ray
background, and six point sources.
\S\Sfootnotemark: $\S$Derived from Raw$-$BGD. Ratios to the Raw count rates
are also shown.

###  4.3 Model fitting to the Extended Emission Spectrum

Figure 6 shows the raw EE spectra, in comparison with the background
components estimated so far. Table 4 summerizes the estimated $0.5-6$ keV
count rate of each background component. As a whole, the background amounts to
about $50\%$ of the raw counts in each detector. In figure 6, cyan data points
indicate the EE spectra obtained after subtracting the three background
components. Below, we fit them with several models which give different
physical interpretations. An ARF for the EE was calculated assuming a uniform
circular emitting region with a radius of $\timeform{50^{\prime\prime}}$ based
on the Chandra ACIS imaging result (Okada, 2005; Okada et al., 2007). The FI
and BI spectra were fitted simultaneously, with the overall model
normalization fixed to be the same between them.

First, we fitted the spectra with a single power-law model and a single
temperature optically-thin thermal model (`apec` in `XSPEC`; Smith et al.
(2001)), each subjected to the interstellar absorption (`wabs`) as Okada et
al. (2007) did. The fitting results are shown in figure 7 and listed in table
5. However, neither the power-law nor optically-thin thermal model reproduced
the spectra well, with $\chi^{2}_{\nu}=1.31~{}(\nu=100)$ and $1.33~{}(\nu=99)$
respectively. The obtained photon index ($\Gamma=2.9\pm 0.2$) or the plasma
temperature ($kT=1.7\pm 0.3$ keV) implies a considerably softer spectral shape
than the previous report (Okada (2005); $\Gamma=2.1\pm 0.3$ or
$kT=3.7\pm^{2.7}_{1.3}$ keV). In section 5, we discuss this discrepancy.

(80mm,80mm)figure7a.eps (80mm,80mm)figure7b.eps

Figure 7: Spectral fitting to the XIS FI (black) and BI (red) spectra of the
EE, with (a) a power-law and (b) a single-temperature thermal models.

In figure 7, we notice some spectral structures around 0.85 keV, 1.5 keV, and
5 keV that cannot be explained by the employed models. Suspecting that these
structures originate from redshifted atomic emission lines, we next fitted the
spectra with a thermal model that has a free redshift $z$. The fit was
improved significantly to $\chi^{2}_{\nu}=1.10~{}(\nu=98)$, and yielded the
metal abundance and redshift of $0.38^{+0.25}_{-0.13}$ times solar and
$z=0.34\pm 0.02$, respectively. Especially the spectral features at $\sim
0.85$ keV, $\sim 1.5$ keV and $\sim 5$ keV have been reproduced successfully
by redshifted Fe-L, Si-K and Fe-K lines, respectively. Incorporating $z$ thus
determined, the observed flux can be converted to the intrinsic luminosity of
$L_{0.5-6~{}{\rm keV}}=5.5\times 10^{43}~{}{\rm erg}\ {\rm s}^{-1}$,
$L_{2-10~{}{\rm keV}}=2.8\times 10^{43}~{}{\rm erg}\ {\rm s}^{-1}$, and
$L_{0.1-200~{}{\rm keV}}=1.0\times 10^{44}~{}{\rm erg}\ {\rm s}^{-1}$ in the
$0.5-6~{}{\rm keV}$, $2-10~{}{\rm keV}$, and $0.1-200~{}{\rm keV}$ band
respectively.

Since the XIS background spectrum contains K$\alpha$ emission line from
aluminum used in, for example, the XIS housing and substrate of the CCD, the
line feature at 1.5 keV could be due to residual Al-K lines caused by a wrong
NXB subtraction. To examine this possibility, we also tried spectral fittings
with NXB spectra rescaled by $5-10\%$. However, the feature at $\sim 1.5$ keV
can be seen even after subtracting an NXB spectrum that is rescaled up by
$+10$%. Since the systematic error (or reproducibility) of the NXB estimation
is reported to be 5% (Tawa et al., 2008), we consider that the structure to be
real rather than instrumental. For reference, the fit results remain unchanged
within the errors even if we ignore the $1.4-1.6$ keV range in the fitting.

In figure 8, we notice fitting residuals both in the XIS FI and BI spectra at
3.5 keV. However, they have no corresponding background features (figure 6) or
redshifted major atomic lines. No such features are present in the Chandra
spectrum, either (Okada et al., 2007). They are hence considered as
statistical fluctuations.

(80mm,80mm)figure8.eps

Figure 8: The same EE spectra as presented in figure 7, fitted with a redshifted thermal emission model. Table 5: The best fit parameters of the EE spectra. Model |  $N_{\rm H}$**footnotemark: $*$ | $\Gamma$ |  $kT$\dagger\daggerfootnotemark: $\dagger$ |  $Z$\ddagger\ddaggerfootnotemark: $\ddagger$ |  $z$\S\Sfootnotemark: $\S$ |  $\Sigma$\|\|footnotemark: $\|$ | $\chi^{2}_{\nu}~{}(\nu)$
---|---|---|---|---|---|---|---
Power law | $20^{+6}_{-5}$ | $2.9\pm 0.2$ | $-$ | $-$ | $-$ | $7.5^{+1.2}_{-0.9}$ | 1.31 (100)
Theraml | $3.3^{+4.4}_{-3.3}$ | $-$ | $1.7\pm 0.3$ | $0.02^{+0.06}_{-0.02}$ | $-$ | $7.5^{+1.6}_{-1.3}$ | 1.33 (99)
Redshifted thermal | $6.9^{+5.7}_{-4.8}$ | $-$ | $2.2^{+0.2}_{-0.3}$ | $0.38^{+0.25}_{-0.13}$ | $0.34\pm 0.02$ | $7.6^{+1.3}_{-1.2}$ | 1.10 (98)
**footnotemark: $*$ Line-of-sight hydrogen column density in units of
$10^{20}~{}{\rm cm}^{-2}$.
\dagger\daggerfootnotemark: $\dagger$Thermal plasma temperature in units of
keV.
\ddagger\ddaggerfootnotemark: $\ddagger$Abundance in terms of the solar value.
\S\Sfootnotemark: $\S$Redshift.
\|\|footnotemark: $\|$The $0.5-6$ keV band model surface brightness in units
of $10^{-7}~{}{\rm erg}\ {\rm cm}^{-2}\ {\rm s}^{-1}\ {\rm sr}^{-1}$.

The above results strongly suggest that the EE is an extragalactic object,
rather than a source associated with 47 Tuc. Further considering the extended
nature and the thermal spectrum, it is most likely a background cluster of
galaxies at $z=0.34$. In the following section, we examine the galaxy cluster
interpretation of the EE based on the Suzaku results.

##  5 Discussion

###  5.1 $kT-L_{\rm{X}}$ relation and the counterpart in other wavelength

As shown so far, the spectra of the EE are well described by thermal plasma
emission with a rest-frame temperature of $kT=2.2$ keV and a redshift of
$z=0.34$. Furthermore, as plotted in figure 9, its luminosity and temperature
are consistent with the known temperature-luminosity relation ($kT-L_{\rm{X}}$
relation) of cluster of galaxies. Therefore, the EE is most naturally
interpreted as a background cluster of galaxies at a moderate redshift.

We find no counterpart in the optical (Digital Sky Survey; e.g. McLean et al.
(2000)) or near infrared (Two Micron All Sky Survey; Skrutskie et al. (2006))
surveys. Using Chandra deep survey data, Boschin (2002) however reported more
than 20 candidates of clusters of galaxies that have no optical counterpart.
The present background galaxy cluster is perhaps a member of those clusters. A
deeper optical imagery will reveal the expected galaxy clustering.

(80mm,80mm)figure9.eps

Figure 9: A $kT-L_{\rm{X}}$ relation of clusters of galaxies. Crosses
represent temperatures and bolometric luminosities of individual galaxy
clusters determined by ASCA observations (data taken from Fukazawa et al.
(2004)). Luminosities were obtained by integrating fluxes over the $0.1-200$
keV band. Filled triangle shows the EE of 47 Tuc.

###  5.2 Comparison with previous reports on the EE

The galaxy cluster interpretation has been examined as an origin of the EE in
previous papers as well. Using the $\log N-\log S$ relation of galaxy
clusters, Krockenberger & Grindlay (1995) and Okada et al. (2007) estimated
probabilities of a chance coincidence of the EE and a background galaxy
cluster emission to be less than 0.5% and 0.6%, respectively. Based on such
low probabilities, these authors argued that the EE cannot be a background
galaxy cluster.

In addition to the above probability estimation, Okada et al. (2007) used the
following argument to rule out the background cluster interpretation. First,
they determined the EE temperature as $kT=3.7~{}$keV from the Chandra ACIS
spectrum. They hence assigned a luminosity of $L_{\rm{X}}=1.1\times
10^{44}~{}{\rm erg}\ {\rm s}^{-1}$ to this putative cluster, using the $kT-
L_{\rm{X}}$ relation of clusters (e.g. Ikebe et al. (2002); Fukazawa et al.
(2004)). Comparing this $L_{\rm{X}}$ with the measured flux, the source
redshift was estimated as $z>0.5$, and hence the observed angular core radius
of the EE, $r_{\rm{c}}\sim\timeform{0.6^{\prime}}$, was converted to a
physical size of $r_{\rm{c}}>360~{}$kpc. Finally, they concluded this
$r_{\rm{c}}$ to be too large for a cluster.

In the present study, the use of the Suzaku XIS has enabled us to achieve two
major improvements (or revisions) over Okada et al. (2007). One is that we
clearly detected redshifted emission lines, which indicate $z=0.34\pm 0.02$;
the Chandra data gave no constraint on $z$. The other is that we measured a
significantly lower temperature, $kT=1.7~{}$keV if assuming $z=0$, or
$kT=2.2~{}$keV at the rest frame if adopting $z=0.34$; the latter now
satisfies the $kT-L_{\rm{X}}$ relation of clusters of galaxies (figure 9). In
addition, using the redshift, the physical core radius is now calculated to be
$\sim 160$ kpc, which is reasonable for galaxy clusters.

As reviewed so far, the difference of our conclusion from that of Okada et al.
(2007) comes mainly from the discrepant EE temperatures, $kT=1.7\pm 0.3~{}$keV
measured with Suzaku (without correction for the redshift) and
$kT=3.7^{+2.7}_{-1.3}~{}$keV with the Chandra ACIS. Possible causes of this
difference include an over estimation of the temperature with Chandra, or an
under estimation with Suzaku, or both. As the former possibility, the most
likely cause is systematic errors in the NXB subtraction. As the latter
possibility, we may presume that during the Suzaku observation, some soft
sources became brighter than in the Chandra observation.

Although the Suzaku data could thus be under-estimating the EE temperature,
significantly higher values of $kT$ would be still consistent with the $kT-
L_{\rm{X}}$ relation. Furthermore, the value of $z=0.34$ is not affected,
since it is determined by the redshifted atomic emission lines. We conclude
that the close spatial coincidence between the EE and 47 Tuc is accidental,
and they are not physically associated with each other.

T.Y. is financially supported by the Japan Society for the Promotion of
Science. This research has made use of data and softwares obtained from the
Data Archive and Transmission System at JAXA/ISAS and the High Energy
Astrophysics Science Archive Research Center, provided by NASA’s Goddard Space
Flight Center respectively. We obtained the Chandra data from the Chandra Data
Archive, and analyzed them with softwares provided by the Chandra X-ray
Center. The present research is supported in part by the Grant-in-Aid for
Scientific Research (S), No. 18104004.

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