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Acta Polytechnica CTU Proceedings 1(1): 123–126, 2014

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doi: 10.14311/APP.2014.01.0123

A Multi-Wavelength View of the XMM-Newton Galactic Plane

Ada Nebot Gómez-Morán1, Christian Motch1, on behalf of the XMM-Newton Survey Science
Centre

1Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l’Université, 67000
Strasbourg, France.

Corresponding author: ada.nebot@astro.unistra.fr

Abstract

We present an X-ray survey of the Galactic Plane conducted by the Survey Science Centre of the XMM-Newton satellite.
The survey contains more than 1300 X-ray detections at low and intermediate Galactic latitudes and covering 4 deg2 well
spread in Galactic longitude. From a multi-wavelength analysis, using optical spectra and helped by optical and infrared
photometry we identify and classify about a fourth of the sources. The observed surface density of soft X-ray (< 2 keV)
sources decreases with Galactic latitude and although compatible with model predictions at first glance, presents an excess
of stars, likely due to giants in binary systems. In the hard band (> 2 keV) the surface density of sources presents an
excess with respect to the expected extragalactic contribution. This excess highly concentrates towards the direction of
the Galactic Centre and is compatible with previous results from Chandra observations around the Galactic Centre. The
nature of these sources is still unknown.

Keywords: stars - binaries - spectroscopy - photometry - IR - optical - X-rays.

1 Introduction

Galactic X-ray surveys can help us to learn about
the Galaxy’s structure, stellar formation and evolu-
tion. Since long we know that the soft X-ray emis-
sion (< 2 keV) of the Galaxy is dominated by stars.
But the X-ray luminosity function of hard X-ray emit-
ting sources (>2 keV) is still not well constrained, in
particular at low to intermediate X-ray luminosities
(10 <LX< 1034erg s−1). Cataclysmic variables and
active binaries contribute at X-ray luminosities up to
∼ 1033 erg s−1(Sazonov et al. 2006), but recent re-
sults show that other sources, such as massive OB stars,
Wolf-Rayet and wind colliding binaries also contribute
to this LX range (Mauerhann et al. 2010, Anderson et
al. 2011). A census of these populations is thus needed.

Discrepancies between observations and model pre-
dictions exist since the first X-ray surveys of the Galaxy
were carried out by the Einstein satellite. Stellar pop-
ulation models predict less stars than we observe, in
particular, there is an excess of stars with spectral type
K, this is the so called “yellow excess”. The origin of
this excess is highly debated and there are two possible
explanations: either these K stars are young stars or old
evolved giants, which can be either single or in binary
systems (Favata et al. 1988, Sciortino et al. 1995).

X-ray surveys can reveal concentrations of specific
populations in certain regions of the Galaxy: high-mass
X-ray binaries (HMXB) seem to be concentrated in the

Norma Arm (Lutovinov et al. 2005); the Gould Belt
contains a young population of late type stars (Guill-
out et al. 1998). Last but not least, different popula-
tions of theoretically predicted X-ray binaries are still
to be discovered. Binary population models predict a
population of X-ray binaries at low luminosities, pre-
HMXBs and pre-low-mass X-ray binaries that would be
accreting matter coming from the wind of the compan-
ion star at low rates rather than from Roche lobe over-
flow (Willems & Kolb 2003, Pfahl et al 2002). Other
predicted objects are HMXBs with white dwarf as com-
pact object instead of a classical HMXB where the com-
pact object is a neutron star (Motch et al 2007). So far,
none of these objects is confirmed observationally. To
learn about all these questions we need to study the
X-ray content of the Galaxy.

2 XMM-Newton Survey Science
Centre

The XMM-Newton satellite can help us to give an-
swer to these questions. Thanks to it’s large field
of view and it’s high sensitivity, each XMM-Newton
pointed observation discovers from 30 to 100 serendipi-
tous sources. The XMM-Newton Survey Science Centre
(SSC) generates catalogues of serendipitous sources and
publishes them on a regular basis. The latest available
catalogue, the 2XMMi-DR3 catalogue, contains about
250 000 unique sources, and the 3XMM catalogue will

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http://dx.doi.org/10.14311/APP.2014.01.0123


Ada Nebot Gómez-Morán, Christian Motch

soon be released containing more than half a million de-
tections corresponding to about 370 000 unique sources.
One of the goals of the SSC is to characterise and iden-
tify these sources in an statistical manner. For that,
specific surveys have been studied in great detail, such
as a bright flux sample by Della Ceca et al. 2004, a
medium flux sample by Barcons et al. 2002 and the
Galactic survey from Hands et al. 2004, with spectro-

scopic identification carried out by Motch et al 2010.
These surveys serve as learning samples for an statis-
tical identification. In this paper we briefly describe
the main results of a survey carried out by the XMM-
Newton SSC in the Galactic Plane. For details on the
survey we refer the reader to Nebot Gómez-Morán et
al. 2013.

EPIC-pn

WR110

DSS 2MASS

Figure 1: Example field studied in this survey. From left to right, X-ray (EPIC-pn), optical (DSS) and infrared
(2MASS) images around the WR 110.

3 The Galactic Plane Survey

The XMM-Newton SSC survey of the Galactic Plane
contains 26 pointings, covering about 4◦ at low and
intermediate Galactic latitudes (|b| < 20◦) and well
spread in Galactic longitude. It contains about
1300 detections up to limiting fluxes of ∼ 2 ×
10−15 erg cm−2s−1in the soft (kT< 2 keV) band and ∼
10−14 erg cm−2s−1in the hard band (2< kT< 12 keV).

3.1 Multi-wavelength identification

In order to identify these sources in the optical and the
infrared we cross-correlated the X-ray positions with
large optical and infrared published catalogues, such as
the SDSS, USNO and 2MASS. We show an example of
one of the fields studied in this survey in Figure 1. The
XMM-Newton detected about 50 sources around the
pointed observation of WR 110. If we look in the optical
(middle panel) the field clearly contains a large number
of sources, while in the infrared (right panel) the field
becomes very crowded. This example reflects the diffi-
culty of multi-wavelength identification of X-ray sources
in the Galactic Plane. Due to the relatively large X-ray
positional errors, and the high surface density of sources
at faint fluxes, only not too distant and relatively bright
sources can be identified in the optical and the infrared.

Among the ∼ 1300 X-ray detected sources we could find
counterparts in the optical to about 60%, number which
drops to about 40% in the infrared.

3.2 Source classification

A spectroscopic campaign was carried out to classify the
sources. About one forth of the detected X-ray sources
are classified. Most of them are active coronae, but we
also found other interesting objects, such as young T
Tauri and Herbig-Ae stars, and old cataclysmic vari-
ables. We identify four Be stars with X-ray luminosi-
ties around 1032 erg s−1 a value too high for normal Be
stars. These systems thus belong to the class of γ-Cas
analogues, which means that we have almost doubled
the number of γ-Cas objects known until the date (see
talk by C. Motch in this congress).

We cross-matched our X-ray sources with other pub-
lished catalogues making use of Vizier and the XCat-DB
databases, and produced spectral energy distributions
which helped us to discern the type of source. Infrared
data from the WISE catalogue revealed one of the T
Tauri has a transition disk, which highlights the rele-
vance of having a multi-wavelength view of high energy
sources.

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A Multi-Wavelength View of the XMM-Newton Galactic Plane

‣

‣

‣

10-15 10-14 10-13 10-12

S (0.5-2 kev) [erg cm-2s-1]

10-1

100

101

102

N
(>

S
) 

[s
o
u
rc

e
s
/d

e
g

2
]

|b| = 0o

|b| = 15o

|b| = 50o

|b| = 60o

b = 50º

b = 60º

b = 15º

b = 0º

0.5-2.0 keV

Observed

Figure 2: logN – logS curves in the soft band
(< 2 keV) and as a function of Galactic latitude. We
included the data from Lopez-Santiago et al. 2007 and
Barcons et al. 2007 at high Galactic latitudes.

Using infrared colour-colour diagrams for the stel-
lar content of our survey we could distinguish main se-
quence stars from evolved giants. On the one hand, due
to magnetic braking, stellar rotation decreases with the
age of the stars. On the other hand the X-ray lumi-
nosity is strongly related to rotation. This implies that
normal old giant stars are not expected to be strong
X-ray emitters. A possible explanation for the origin of
the X-ray emission of these evolved stars could be if they
are in synchronised binary systems, where the rotation
period is equal to the orbital period, breaking thus the
relation between X-ray emission and age. To test this
hypothesis we calculated the X-ray colours expected for
stars of different age and for binaries, in particular for
RS CVn and BY Draconis. We found that observed X-
ray and infrared colours for dwarfs are compatible with
young to intermediate age stars, while giant stars have
colours compatible with RS CVn binaries, i.e. binaries
where at least one of the stars is a giant star. The giant
stars we find in our survey mostly have K spectral type,
i.e. they are “yellow stars”.

4 Results

We computed the logN – logS curves in the soft band,
i.e. the surface density of sources as a function of the
sensitivity of our survey, for different bins of Galactic
latitude. We find that the number of stars per square
degree decreases with Galactic latitude. Although this
result is not a surprise, it’s the first time that is shown in
X-rays. We also see that the X-ray luminosity function
has a varying slope with Galactic latitude. We com-
pared our results with the expected curves from a mod-
ified version of the Galactic X-ray model from Guillout
et al. 1996. At first glance we obtained compatible

results, a varying slope of the X-ray luminosity func-
tion and a decreasing number of sources towards higher
Galactic latitudes, reflecting the different scale-height
of stars and the relative contribution of different pop-
ulations. But for a given flux, the number of observed
stars is higher than the predicted value. This result is
not surprising since binaries have not been taken into
account in our model. In other words, the excess of
observed sources with respect to model predictions is
due to the yellow giant stars that we have found in our
survey, and are likely in binary systems.

10-14 10-13 10-12 10-11

S (2-12 keV) [erg cm-2s-1]

10-1

100

101

102

103

N
(>

S
) 

[s
o
u
rc

e
s
/d

e
g

2
]

Galactic Center 
Motch et al 2010

1032 1033 1034
Luminosity (2-12 keV) [erg s-1]

2-12 keV GC Region
(l,b) ~ (1º,0º)

‣

‣

‣

Hands+2004, Motch+2010 
(l,b) =(20º,0º)

b ~ 0º

Figure 3: logN – logS curves of hard sources (> 2 keV)
in the direction of the Galactic Center. The expected
extragalactic contribution from Mateos et al. 2008 has
been subtracted.

We carried out the same exercise for hard sources,
we constructed logN – logS curves for sources harder
than 2 keV. Since stars are not expected to contribute
at this high energies, we subtracted the expected extra-
galactic contribution. The logN – logS curves present
an excess of sources with respect to the expected extra-
galactic contribution close to the Galactic Center region
studied in this survey (l ∼ 0.9◦, b ∼ 0◦). We com-
pared this result with that obtained by Motch et al.
2010 at l ∼ 20◦, b ∼ 0◦ and obtained that the num-
ber of sources per square degree observed at a given
flux decreases steeply with Galactic longitude in the di-
rection of the Galactic Center. We obtained consistent
results with Hong et al. 2009 based on Chandra ob-
servations of seven fields around the Galactic Center.
This sources contribute to the Galactic Ridge emission,
initially thought to be of diffuse origin. The nature of
these sources is unknown, but if this population would
be associated to the Galactic Center itself, the X-ray lu-
minosities obtained are in the range 1033−1034 erg s−1,
i.e. values which are comparable with pre-HMXBs and
pre-LMXBs.

125



Ada Nebot Gómez-Morán, Christian Motch

Acknowledgement

We would like to thank the organisers of this conference
for their work and for giving us the opportunity to show
our results.

References

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DISCUSSION

DAVID BUCKLEY: Can you give the percentages
of the different optical counterpart types of the ∼ 300
sources identified?

ADA NEBOT: Among the 316 classified sources there
are three T Tauri stars, one Herbig Ae star, two cat-
aclysmic variables, and four γ-Cas analogues. A few
sources are extragalactic (in the highest Galactic lati-
tude fields), and the remaining sources are stars, where
about 10% are giants. Classification of giants was only
done for stars with the best 2MASS photometry, mean-
ing that the number of giants in our survey could be
higher.

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http://dx.doi.org/10.1088/0067-0049/181/1/255
http://dx.doi.org/10.1086/165957
http://dx.doi.org/10.1111/j.1365-2966.2004.07777.x
http://dx.doi.org/10.1088/0004-637X/706/1/223
http://dx.doi.org/10.1086/341197

	Introduction
	XMM-Newton Survey Science Centre
	The Galactic Plane Survey
	Multi-wavelength identification
	Source classification

	Results