近赤外線星数密度分布との比較による銀河系中心部拡散X線の放射源

近赤外線星数密度分布との比較による
銀河系中心部拡散X線の放射源の制限
長友竣、長田哲也（京都大）、西山正吾（宮城教育大）、永山貴宏（鹿児島大）
銀河系中心で観測される広がったように見えるX線の放射源は点源説と加熱された星間物質説の間で議論が続いてい
Map of the Galaxy in the 6.7-keV emission line
L15
る。本研究では、その鉄輝線強度分布と近赤外線観測から得た星数密度分布を比較することで放射源に制限をつけ
REFERENCES
た。Nishiyama+ 2013と異なる銀経8度で規格化を行ったところ、同じくFe
XXV輝線強度の中心部での超過を確認した。また、
Bleach R. D., Boldt E. A., Holt
S. S., Schwartz D. A., Serlemitsos P. J., 1972,
ApJ, 174, L101
Cooke B. A., Griffiths
R. E., Pounds K. XXVIの輝線比の増加が見られた。これらのこ
A., 1970, in Gratton L., ed., Proc.
中心部でFe XXVI輝線強度の超過、鉄輝線と星数密度の分布の差異、Fe
XXVとFe
IAU Symp. 37, Non-Solar X- and Gamma-Ray Astronomy. Reidel, Dordrecht, p. 280
とは、放射源が加熱された星間物質であることを支持している。 Ebisawa
K. et al., 2005, ApJ, 635, 214
概要
銀河系内の「広がった」X線
結果
条件： SN > 10
色指数で切り分け
減光補正後のKs等級
S230
規格化
星数密度(左軸)
AC K N OW L E D G M E N T S
2
-7
[ arcmin ]
10
→星と鉄輝線の分布比較により
10
S. Yamauchi
al.been supported by the DFG-SchwerpunktThis work ethas
programme
(SPP
1177).
This
放射源に制限をつける。
2
1
0 research-1made use-2of data obtained
-3
from the High Energy Astrophysics Science Archive Research Cen-
8
Fe XXV (1.17 ± 0.13) ⇥ 10
(1.09 ± 0.2) ⇥ 10 8 )
(先行研究約3倍の超過
5.0
[Vol.
10 61,
●
●
5.00
20
Nishiyama et al.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
H−Ks [mag]
-6
10
10.0
Fe XXVI intensity
The number of stars
200
-7
5.0
べきの値 0.44
100
2.0
50
1.0
20
0.5
10
0.2
5
9
●
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0.1
0.2
0.5
1.0
2.0
5.0
10.0
2
●
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2.00
(1.09×10 photons s cm arcmin /arcmin )
●
●
●●
●
1.00
●
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0.50
●
●
●
●
●
●
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●
●
●
●
●●
●
0.20
エラー大
4.2.
The 6.4 keV Line
2
0.10
l=8
輝線比がの
値に対して何倍か
1.5
●
●
●
●
●
0.05
-2
●
-2
Fe XXVI 輝線強度
●
●
Fe XXV Intensity/Number of Stars
-30°
Fe XXVI Intensity [10^−7 erg s^−1 cm^−2 arcmin^−2]
0°
-10°
10°
Longitude [deg]
Fe XXV Intensity/Number of Stars
30°
-1
-2
(1.09×10-8 photons s cm-2 arcmin /arcmin-2)
10.00
40
Fe XXVI (3.43 ± 0.65) ⇥ 10
10
20
2.0
1
0.5
0
-0.5
-1
in a collisional ionization
equilibrium (CIE) plasma (6.68
keV),
先行研究と
l* (deg)
ter Online Service provided by
the NASA/Goddard Space Flight
10
statistical 2011
errors and the calibration
uncer(deg)
This paper has been typeset from a TEX/LATEbX* file
prepared by the author. 1.0
先行研究には規格化する領域の Center. taking account of theUchiyama+
一致
Yamauchi+
2009
tainty of the energy scale (0.2% at 6 keV: Koyama et 5al. 2007a).
0.5
問題があった。
Furthermore, the flux ratios of the 6.97 keV=6.7 keV lines
are
0.1
0.2
0.5
1.0
2.0
5.0
10.0
Yamauchi+ 2009
in good agreement with those expected from a CIE plasma with Galactic Longitude |l*| [deg]
a temperature of 2–8 keV. Thus, our results support that the
l=-0.17
3.5
GCXEの領域?
GRXE is likely to be thermal emission from a CIE plasma,
2.5as
3
shown
for GCDX by Koyama et al. (2007b) and 領域
for one of the
XXVI
GRXEFe
fields
(R8) by Ebisawa et al. (2008).
2
2.5
]
Number of Stars [/arcmin^2]
放射源は不明。
点源か？加熱された星間物質か？
2
cm
Fe XXV Kα (photons/s cm2 arcmin2)
-6
100of unit-stellar-mass X-ray emissivity measured in the solar
value
10
neighbourhood (Sazonov et al. 2006) provides further evidence that
the bulk of the GRXE is made up by faint Galactic X-ray point
sources.
60
12
14
Fe XXV Ka [10^(−7) photons /(s cm^2 arcmin^2)]
2
Number of Stars (/arcmin )
Galactic Center X-ray Emission Fe XXV
[erg s cm arcmin ]
(GCXE)
(iv) The aforementioned observational facts along with the
鉄輝線／星数密度 [erg s
2
Number of Stars (/arcmin )
Galaxy (see middle panel) Middle: near-infrared surface brightness map of
the Galaxy (COBE/DIRBE 4.9-µmNishiyama+
data, corrected for 2013
reddening) Bottom:
Map of the surface brightness of the inner Galaxy in the 6.7-keV emission
line. The white contours are iso-brightness contours (右軸)
of NIR emission.
1
2
2
抽出したデータ
B: 3041個
A: 3029個
Fe XXV Kα (photons/s cm2 arcmin2)
→放射源は激変星 (CV)や
コロナの活発な星
(ASB)Journal
The Astrophysical
Letters,
769:L28
1 band 3–
Figure
4. Top: time-averaged
map (5pp),
of the inner2013
Galaxy June
in the energy
20 keV obtained with RXTE/PCA. The domination of bright point sources is
などの点源。
evident. The contours are iso-brightness contours of the NIR emission of the
80
10
Fe XXVI Ka [10^(−7) photons /(s cm^2 arcmin^2)]
/
X線強度近赤外線面輝度分布
100
8
Ks [mag]
50°
Number of Stars [/arcmin^2]
Galactic Ridge X-ray Emission
Revnivtsev+ 2006
(GRXE)
20°
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50
Galactic Longitude |l*| [deg]
|l|  0 .7
|l| > 0 .7
1.26 ± 0.25
1.04 ± 0.13
0.2
0.5
1.0
2.0
5.0
10.0
abs(Galactic Longitude) [deg]
星数密度分布の取得方法
C
考察
C
-8
→銀経8度で規格化を行う
0.1
-1
addition 0°.5
to the 6.72°.0
keV and10°.0
6.97 keV lines, the 6.4 keV
0°.1
1.5In
emission line
was found
1
Longitude
[deg]in various galactic plane regions,
1
which suggests omnipresence of the 6.4 keV line in the galactic
plane
regions. The EW of the 6.4 keV line from GRXE
0.5is
0.5
! 2006 The
Authors. Journal compilation ! 2006 RAS, MNRAS 373, L11–L15
< 400 eV. The GC region has been known to exhibit a strong
2
1
0
-1
-2
-3
0.5
0
-0.5
-1
6.4 keV line; in particular, a strong enhancement of the 6.4 keV1
Fig. 3. (a) 6.4 keV, (b) 6.7 keV, and (c) 6.97 keV line fluxes as
l* (deg)
a function of the galactic longitude. The unit of the line flux is
b* (deg)
line flux at the Sgr B2
cloud was found with ASCA (Koyama
! 10"8 count s"1 cm"2 arcmin"2 . The error shows the 90% confidence
et al. 1996). The Sgr B2 X-ray spectrum exhibited a strong
level.
仮定: X線放射点源
Figure 3.比較的低質量の星
Top: longitudinal (left) and
(right)
profiles
density
after a completeness correction (red crosses). Overplotted are the 6.7 keV
6.4latitudinal
keV emission
line
with of
an the
EWstellar
of >number
1 keV and
an absorpWainscoatetmodel
emission profiles (blue “×”; Koyama
al. 2007;
2011).
region
outside
the NB, 1.◦ 5 ! | l∗ | !2.◦ 8, is used to scale the 6.7 keV emission profile
tionetedge
of theUchiyama
neutral iron al.
at 7.1
keV,The
which
is well
explained
(Wainscoat+
1992)
・輝線強度の超過
to
have
the
same
value
as
the
stellar
number
density.
The
number
density
is
calculated
in
the
same rectangles as those used・輝線比の増加
in Uchiyama et al. (2011), with a size of
by the reflection
of X-rays
from an external bright X-ray
source
低質量の星の分布を知る
1.0
◦
◦
◦
◦
◦
0. 1(l) × 0. 2(b) for the longitudinal
profile,Reflection
and 0. 2(l) ×Nebula,
0. 1(b) atXRN),
the position
l = A*
−0.is
17 presumfor the latitudinal profile. The same scaling factor is used for the latitudinal
8.5kpcに
(X-ray
whereofSgr
・分布のべきの違い
(*はNishiyama+ 2013より)
ためには測光された星の
profile. Bottom: longitudinal (left)ably
and latitudinal
(right)
profiles
of the ratios
the 6.7
keV emission
the irradiating
source
(Sunyaev
et al. of
1993;
Koyama
et al. to the stellar number density, scaled to be unity at the position
l=8 deg
集中
for normalization, 1.◦ 5 ! | l0.8∗ | !
2.◦ 8. These profiles represent
All Stars a contribution of point sources, traced by our NIR observations, to the GCDX in the assumption that
Fe
XXVI�
Fe
XXV *�
星数密度 *
1996). Recently, similar
M III XRN objects have been found in
リストから、以下を満たす
the contribution of truly diffuse hot plasma is negligible at the
K III position for normalization (i.e., the Galactic ridge region).
the GC region with ASCA,
Chandra, and Suzaku (Murakami
0.44
±
0.03�
0.44
±
0.02�
0.30 ± 0.03�
星の抽出が必要。
(A color version of this figure is available
in
the
online
journal.)
et al. 2000, 2001a, 2001b; Park et al. 2004; Koyama et al.
0.6
観測できる
2007c, 2008; Nobukawa et al. 2008). On the other hand, the
• 低質量星の分布を M型巨星
further from the GC, and これらの説明を考えてみる。
thus the same
The NB and NSC have a different formation history from
values at 1.◦ 5 < | l∗0.4 | <GRXE
2.◦ 8, fields
i.e., are
in much
a region
outside of
XRN scenario as that in the GC region cannot be expected.
トレース
モデルの
theemission
Galactic bulge (GB), and have formed stars over their entire
the NB (l∗ and b∗ denote Therefore,
the angular
distance
from
Sgr
A*
another scenario to produce the 6.4 keV
• 十分遠くでも観測できる
lifetime,
indicating more bright stars in the NB and NSC. Here,
along the Galactic longitude
and
latitude,
respectively,
星分布
line is
required.
Interactions
between and
the interstellar
medium
0.2
◦
◦
sources and the GRXE 77
ASBのプラズマ変化
use Low-L
the Xsynthetic
CMD computation以下のようなプラズマがあ
(Aparicio & Gallart
046)).
and
high-energy electrons or X-ray photons in we
the
galactic
8.5 kpc遠方(l∗ , b∗ ) = (l + 0. 056, b + 0.観測できる
the spectral assumptions underlying
the calculations. Clearly, these
◦
plane fields
can range
produce
the 6.4 keV lines. Assuming
2004) totheestimate the fraction of the total
number of stars
Fitting the longitudinal K型巨星
profile
in the
assumptions
need to be −0.
justified. 7 ! l∗ !
れば説明できる。
Warwick
2014
0.0
We
first
consider
the
X-ray
spectra
of
CVs.
In
magnetic
CVs,
◦
−α
originan accretion
of
Valinia
et toal.high(2000)
proposed
formedthat
in the computation, Nall , to the number of stars with
−0. 1 with a power law of ∝diffuse
θ , where
θ isGRXE,
angular
offset
shock heats accreting
material from
temperature
を抽出したい
(kT
>
15
keV).
The
resulting
highly
ionized
plasma
cools
in
the
interactions
between
the
interstellar
medium
and
cosmic-ray
10
15
20
. ,0represents
.7 に存在
K < 10.5, NK<10.5 . This ratio, R ≡ Nall /N|l|
Sgr A*, gives αstar = 0.300 ± 0.035 for the
stellar
number
post-shock
flow and eventually
settles ondensity.
to the white dwarf surface
K<10.5
electronsDistance
areviaresponsible
for the 6.4 keV line. In this case,
an
theaccretion
sun [kpc]column. The resulting X-ray spectrum comprises
Fig. 4. Flux ratios of 6.4 keV=6.7 keV
(upper)
and 6.97 keV=6.7
the ratio
of the theoretically expected total number of stars to
This
is different
bothkeVfrom 0.44
± 0.02from
6.7produced
keVatline
in densities
afor
blend ofthe
aprofile
range temperatures,
the distribution
of components
the
intensity
isinexpected
to be
and optical
depths 6.4
with akeV
‘characteristic’ temperature
typically
中心ほど高密度
(lower) lines as a function of the galactic longitude. The error shows
+0.02
3.01998; Cropper
(Wainscoat
model)
= 8−0.03
range 10–20
keV (e.g. Hellier, Mukai
& Osborne
the number
the same range, and from l0.60
forthe the
integrated
色—等級図で切り分け
spatially
correlated
with
that ofemission
the
molecular
cloud,
which of stars detected in our observations. The SFHs
the 90%
confidence level.
et al. 1999; Ezuka & Ishida 1999; Yuasa et al. 2010).
Our (albeit
sample
of CVs is in fact comprised of both
used here are: a burst star formation from 10領域内で等温
to 13 Gyr ago for
of Fe 6.7 and 6.9 keV lines may
(Heard
& Warwick
be observable
insmall)
the2013).
future.
magnetic and non-magnetic systems in roughly2.5
equal measure.
→前景星を除去
30 which
−1 the majorityfaint
of the local
CV
If GRXE
isNon-magnetic
composed
ofcomprise
X-ray
sources,
the
GB the
(i.e., outside the NB; Zoccali et al. 2003), a constant
The majority of faint X-ray (L
< 10CVs,
erg
snumerous
) sources
2−10 keV
population, are also well-established sources of X-ray emission
2.0 exhibited
Figure 4 shows the flux ratios
of have
the 6.4
keV=6.7
composite
spectra
exhibit
a similar
to formation
that of
at the lowermust
endto
of the
range GCDX
of X-ray
luminosity
by
star
rate for 13 Gyr for the NSD (Figer et al. 2004),
which
not
been keV
resolved
but contribute
the
arespectrum
magnetic systems. In the current X-ray selected sample, the non減光補正後のKs等級で選別
超新星爆発
and 6.97
keV=6.7 keV lines. The
6.4 likely
keV=6.7
ratios
for systems
GRXE, particularly
the
narrow
Fe ofemission
atthe
enermagnetic systems
on average,
roughly aUsing
factor
10 times less lines
and
history derived by Pfuhl et al. 加熱源
(2011) for 
the
NSC.
most
tokeV
be old
binary
(Sazonov
etare,three
al.
2006).
Figure 14. The EW of the 6.9 keV H-like Fe Kα line plotted versus the EW
1.5 In most nonluminous than the confirmed magnetic systems.
GCDX are significantly larger than those for GRXE and the Nishiyama+ gies of 6.4, X-ray
6.7,
and
6.97
keV.
Cataclysmic
variables
(CVs)
of
the
6.7
keV
He-like
Fe
Kα
line.
The
measurement (black point plus error
magnetic CVs, the X-ray flux is produced as thermal emission from
 
Sgr
A*のフレア
When
we
scale
the
ratio
to
be
unity
for
the
GB,
we
obtain
2006
the
synthetic
CMD
computation
(Aparicio
&
Gallart
2004),
and
bars)GCXEの等価幅
is from Suzaku
(Ebisawa
et al. 2008).
The grid
(red lines)R
indicates
→8.5kpc先の the boundarystars
layer where
the accretionsuch
flow slows
down
to match
6.97 keV=6.7 keV ratios also show a similar tendency.
and active binary
(ABs),
as
RS
CVn-type
stars,
the EWs expected
for a 3:1 mix of ASB and CV spectra on the basis of
the rotation of the white dwarf surface (Patterson &1.0
1985).
assumptions
for the ASB
The grid
10.5 history
Rdifferent
: Rtemperatures
: Rspectra.
≈points11 keVcorrespond
:20eV
0.8to : 0.6. This result means
that
the
a constant star formation
(SFH)
during
13 Gyr
for Raymond
the
GB
NSD
NSC
Theto
X-raybe
emission
will againcandidate
be comprised of a blend
of compoare proposed
prime
populations
ofFe
GRXE
 
磁
気リコネクション
coronal
plasma
ranging from510
3 to 7 keV in±
steps with
M型巨星を抽出
XXV
nents with temperatures ranging from the shock temperature at the
iron abundances ranging from 0.2 to 0.5 Z# in 0.1 Z# steps (as labelled).
densities
NSD (Figer et al. 2004), we(Revnivtsev
have confirmed
that
about
4. Discussion
et
The
spectrum
of CVs
exhibitsof the old stellar population in the NSD and NSC are
outeral.
edge2006).
of the
boundary
layerX-ray
to the75%
temperature
of the white
0.5of
+20
dwarf photosphere. Observationally, the characteristic temperature
thermal
with
the
6.4,
6.7,
and
keV
overpredicted
stars. Taking into
account this ratio,
240bright
the stars with KS,0 < 10.5a are
olderemission
Gyr.
Sofrom
the
NIRCVs
Felines
XXVI by the
プラズマの拘束→磁場？
ofthan
the X-ray 1
continuum
emanating
non-magnetic
is typ-6.97
10 eV
4.1. The 6.7 and 6.97 keV Lines
ically in the range 5–20 keV (Patterson & Raymond 1985; Baskill,
Given the above properties of stellar coronae, our assumed spec(e.g.,trace
Hellier
et
al.
1998;
Ezuka
&
Ishida
1999;
Ishida
et
al.
we
have
found
that
the
contributions
of the old stellar population Nishiyama+ 2013
map and profiles shown here
the
distribution
of
the
old
Wheatley & Osborne 2005; Rana et al. 2006; Byckling et al. 2010;
tral model for the ASBs, namely a T = 35 MK (kT
= 3 keV) apec
は典型的なASBでは
Reis
et
al. 2013).
with a metal abundance Z = 0.4 Z# , would again seem to
Thanks to the excellent energy
resolution
of the
2007;
Mukai
etIn1.5this
al.
2007),
while
that
ofkeV
ABs exhibits
athe
thermal
0.0 0.2
0.5
1.0
2.0
2.5
3.0
toplasma
GCDX
stars,
and they
are Suzaku
clearly
different
from
those
the
6.7spectra
paper, we haveof
modelled
the X-ray
of the CVs (both
be representative
of this object are
class. (1/1.5) × 0.8 ∼ 0.5 and (1/3) × 0.6 ∼ 0.2
magnetic
as a 10but
keV thermal
bremsstrahlung
Although
have demonstrated that the integrated emission of
XIS, the Fe line complex was clearly resolved into three emission with
the
6.7non-magnetic)
keV line,
without
the strong
6.4wekeV
H−Ks
[mag]and
説明できない。
for
the
NSD
and
NSC,
in the assumption that the
emission.
continuum, which given the above would seem to be representative
unresolved
ASBs
and CVs may
well produce
the bulkrespectively,
of the GRXE
narrow emission lines at # 6.4, # 6.7, and # 6.97 keV, and line (e.g., G¨udel
1999). Consequently, the Fe line
features
of this et
classal.
of object.
intensity
in both the broad 2–10 keV band and the more restricted 6–
We
next consider theto
X-ray spectra
of GCDX
the ASBs. In active stars,
10 keV range,
a further consideration is
whether a mix of ASB
X-ray
luminosity
function
is and
universal, and that the contribution
contribution
discrete
sources
the6.4
their spatial distribution along the Agalactic
plane wasfrom
mani-faint
of GRXE,
omnipresence
of
the
keV line
in addition
the distribution of emission
measure
with temperature
often shows
CV spectrato
can the
explain the observed Fe-line features in the GRXE
a double peak,
with the hotter
extending up to 30–
spectrum.
Ebisawa
et al. (2008) measure
equivalent is 100%. The contribution at the
of連星形成率の増加
point
sources
to emission-line
the GRXE
hasfound
been
claimed
(Wang 6.7
et keV
al. and
2002;
Muno
etcomponent
al. often2004;
fested. The 6.7 keV line is clearly
in all
of the galactic
6.97 keV
lines,
implies
that CVs are
the
major
40 MK, if not higher (G¨udel 2004). Stellar X-ray surveys have also
widths (EWs) for the He-like Fe Kα line at 6.67 keV and the H-like
that
there
is
tight correlation
between
the charFe Kα line
of 350 ± 40
and 70 ± 30 eV, respectively.
NSC
isat 6.96
inkeVgood
agreement
with ∼1/6 derived by Koyama et al.
et al.hot2007).
investigate
the
contribution
the
ridge and GC regions, indicatingRevnivtsev
that a thin thermal
plasma Tocontributor
toshown
the
flux
ofa relatively
GRXE,
if itofhas
a point-source
origin.
acteristic coronal temperature (as determined from low-resolution
An Fe K輝線比の増加が説明
fluorescent line arising in neutral atoms or lightly ionized
spectralpopulation
data) of the ‘hotter’ component
and the normalized coronal
ions was also observed near 6.4 keV with an EW of 80 ± 20 eV.
is located
along the galactic
plane.
equivalent
widthsof the old stellar
(2009).
point The
sources,
especially
detectable
IRSF/SIRIUS
H・Ksバンド
4.3. Fe Lines
fromL /LCVs
(e.g. G¨udel, Guinan & Skinner 1997; Schmitt
luminosity
For CVs, the line EWs are found to be, typically, ∼170 eV for the
(EWs) of the 6.7 keV and 6.97 keV
# 300–980 eVwe
and construct longitudinal
1997). This correlationand
also appears
to extend into the regime of
6.7 A
keVできない。
line
and ∼100 eV
for the two other
Fe lines, although with
larger
X-ray
emissivity
per unit stellar mass for the GCDX
in lines
our are
observations,
latitudinal
stellar flares, in the sense that the larger flares are generally hota very considerable source-to-source scatter (Hellier et al. 1998;
# 20–240
eV, =
respectively.
The observed
center energies
the
CVs
(polars
andAschwanden,
a subclass
(l, b)
A (8 .04,
0 .05),
B ratio
(8of.44,
ter (e.g. Telleschi
et al. 2005;
Stern & G¨udelpolars),
2008).
Ezuka
& Ishida 1999; Hellier & Mukai 2004; Bernardini et al.
than
the GRXE has been claimed to explain the different
profiles
for the
of the 0Magnetic
6.7.05)
keVの2領域
emission
to intermediate
the stellar
Large flares have durations typically ranging from hours to days 点源説での説明は厳しい。
2012; Warwick et al. 2014). In the case of the ASBs, the assumed
6.7 keV line are consistent with the theoretical value of Fe XXV of CVs, arewithefficient
X-ray emitters with luminosities
of
total X-ray luminosities and temperatures reaching up to 10
3 keV spectral model implies an He-like 6.7 keV Fe Kα line with
|l| . 0 .7 での
Normalized number density
/
点源説
加熱された星間物質説
5
0
→M型巨星
10
Ks [mag]
Downloaded from http://mnras.oxfordjournals.org/ at Kyoto University Library on October 7, 2014
15
The Astrophysical Journal Letters, 769:L28 (5pp), 2013 June 1
観測
X
bol
Nishiyama et al.
distributions
of stars and 6.7 keV emission (Revnivtsev et al.
number density (Figure 3, bottom panels).
When the profiles
erg s and 100 MK, respectively (e.g. Pan et al. 1997; Tsuboi et al.
800 eV EW. However at this plasma temperature, there is negligible
& Schmitt 1999; Pandey & Singh 2008; Pandey &
H-like 6.9 keV Fe-line photon flux. In order to enhance the H-like
2007;
Heard & Warwick 2013). To change the emissivity, at
are scaled to be unity at 1.◦ 5 < | l∗ | <1998;
2.◦Favata
8, the
Singh
2012).
Clearly, ratios
the occurrenceare
of giant∼1.5
flares in very active
line, the temperature needs to rise by roughly a factor of 2; for
systems, for example RS CVn binaries, will enhance the detection
example for a Z = 0.4 Z# plasma with kT in the range 5–7 keV,
least
of Feinitial
mass
function
(IMF), binary fraction (BF),
and ∼3 in the NSD and NSC, respectively.
rate of such sources in hard-band selected X-ray samples (e.g. Pye
the EW ofone
the 6.9 keV
line is between
90 and 140
eV. The
33
Figure 4. Polarimetry results covering 3.◦ 0 × 2.◦ 0 in the Galactic coordinate, together with an intensity map of 6.7 keV line emission (Nobukawa et al. 2012). The cyan
vectors show the inferred magnetic field direction, and the lengths are proportional to polarization percentage. The vectors are averaged in a circle of 2.# 4 radius with a
3.# 0 grid, and plotted with thick bars (detected with more than 3σ ) and thin bars (detected with 2σ –3σ ).
(A color version of this figure is available in the online journal.)
& McHardy 1983; Matsuoka et al. 2012). The fraction of the time
that active systems spend in a flaring state is also relatively high.
For example, Pandey & Singh (2012) quote a flare occurrence rate
of ∼20 per cent for RS CVn binaries, whereas Pandey & Singh
(2008) obtain a similar rate for flares on G-K dwarf stars. Finally,
we note that recent high-resolution X-ray grating spectroscopy has
revealed that highly active stars show the presence of an inverse
‘first ionization potential’ effect, namely stellar spectra with de-
3
impact of such a temperature enhancement is illustrated in Fig. 14,
which shows the 6.7 and 6.9 keV Fe-line EWs expected for a 3:1
mix of ASB and CV components, as the temperature characterizing
the ASB spectrum varies from 3 keV up to 7 keV. It would seem
entirely plausible that the ASBs present in our XSS sample are in
flare states characterized by this range of temperature.
Although a suitable blend of hard X-ray emission from flaring
ASBs together with a more modest contribution from CVs has the
or SFH is required to be different in the NB from the GB. In
the preceding paragraphs, we have shown that different SFHs
cannot explain the different spatial distributions of the stellar
number density and 6.7 keV emission. Considering a universal
IMF (Bastian et al. 2010) and a possible top-heavy IMF in
the GC (Figer et al. 1999), the number of old, low-mass stars
(i.e., CVs and ABs) per unit stellar mass never increases, it
only decreases. Also, a higher stellar density tends to destroy
binaries rather than to form them via a capture process, which
seems to play a small role in binary formation (Tohline 2002).
These imply a smaller X-ray emissivity per unit stellar mass by
120
100
ber of Fields
−1
80
60
all
o
|b|< 0.4
o
| b | > 0.4

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