An artist’s impression of an accreting Low Mass X-ray Binary. The donor star
fills its Roche lobe and its material overflows the inner Lagrangian points and
accretes on the relativistic star (in this case a black hole). Due to the large
angular momentum of the infalling material an accretion disk is formed around
the compact object. Credit: ESA 2002/medialab
A sketch of the innermost part (~1000 gravitational radii) in a low mass X-ray
binary in the so called hard state. The inner part of the accretion flow is
filled with hot and tenuous, optically thin plasma. Comptonization of the low
frequency radiation in the plasma cloud is the main mechanism of the spectral
formation in this state. Some fraction of this radiation illuminates the surface
of the accretion disk and of the donor star. It is reprocessed by the material
of the accretion disk and of the donor star giving rise to the so called
‘reflected component’, depicted in Fig.3.
Credit: Gilfanov M., 2010, in Belloni T., ed., Lecture Notes in Physics, Vol.
794, The Jet Paradigm. Springer-Verlag, Berlin, p. 17
The spectrum of the reflected component for an accretion disk of solar
abundance. Superposed on top of the reflected continuum produced by Compton
scatterings on electrons, are absorption edges and fluorescence lines of various
elements. Also shown is the Comptonized continuum produced by the hot plasma
cloud in the vicinity of the compact object (see Fig.2). An observer near the
Earth will observe the sum of the two components.
Low mass X-ray binaries (LMXBs) are stellar systems consisting of two stars, one
of which is a relativistic object - a neutron star or a black hole - and the
other is a normal low-mass star, like our Sun, for example (Fig.1). If the
separation between the two objects is comparable to the size of the normal star
(which is hundreds thousands to millions of times larger than its relativistic
companion), it may overfill it’s Roche lobe - the region of space where dynamics
of matter are dominated by the gravitational attraction of the star.
Consequently, it will start losing its outer layers under the gravitational pull
of the second star. Material is predominantly lost through the so called inner
Lagrangian point - the point on the line connecting the two stars where the
forces of gravity and the centrifugal force balance each other out. The material
of the donor star will flow through this point and will fall into the
gravitation potential well of the relativistic star, initiating the process
which is called accretion. Due to its large angular momentum, the infalling
matter will form an accretion disk around the relativistic object (Fig.1). The
classical theory of accretion disks around black holes and neutron stars was
developed by Nikolai Shakura and Rashid Sunyaev in 1972. Due to the small size
of the relativistic object (~15 km for a neutron star) the gravitational energy
released during accretion constitutes a significant fraction of the rest mass
energy of the accreting material, typically about 5-20%. This makes these
systems very luminous sources of X-ray emission.
There is a small but fascinating subclass of low-mass X-ray binaries, called
Ultra-compact X-ray binaries (UCXBs) in which the donor star is a white dwarf -
a remnant of a moderately massive normal star. These systems are extremely
compact (hence their name) and have orbital periods shorter than 40 minutes, the
fastest one having a period as short as 11 minutes.
An interesting feature of these systems is that the chemical composition of the
donor star is dramatically different from the composition of the donor star in
‘normal’ low-mass X-ray binaries. While donor stars in normal LMXBs have
chemical composition similar to our Sun, i.e. are made of mostly hydrogen and
helium with small admixture of metals, UCXBs feature donors that are depleted of
hydrogen. They can be made of the ashes of nuclear burning of hydrogen (mostly
helium and nitrogen), of helium (mostly carbon and oxygen) or carbon (mostly
oxygen and neon).
Depending on the particular evolutionary path through which UCXBs form, they may
have a variety of donors ranging from non-degenerate helium stars to white
dwarfs. It is critically important to distinguish between these possibilities,
in order to understand the processes that lead to UCXB formation and control
their evolution. So far this task has been performed using methods of optical
astronomy, with various degrees of success.
MPA scientists have recently proposed and tested a principally new method of
diagnostics of the nature of the donor star in UCXBs by the means of X-ray
The method is using the phenomenon called X-ray reflection. A fraction of the
emission produced near the compact objects illuminates the surface of the
accretion disk and the donor star (Fig.2) and is reprocessed by this material.
In the jargon of high energy astrophysics this reprocessed emission is called
“reflected component”. An example of its spectrum is shown in Fig.3.
On top of the continuum produced by the Compton scatterings off electrons in the
accretion disk, the reflected component also contains a number of characteristic
lines. These lines (called emission lines) are due to the different chemical
elements present in the accreting material. They are produced by the process
called fluorescence and have well known energies, unique for each chemical
element. Their shape and relative strength carry information about the geometry
of the accretion flow and chemical composition of the accreting material.
The reflected component is heavily diluted by the primary emission, therefore
the fluorescent lines of most of the elements are very weak and difficult to
detect. Except for the fluorescent line of iron, which in the case of neutral
iron is located at 6.4 keV. Thanks to the high fluorescent yield and abundance
of iron, this is the brightest spectral feature in an otherwise relatively
smooth continuum. All normal LMXBs have this line easily observable in their
While the reprocessing of X-ray radiation by the accretion disc and particularly
the shape and strength of the iron line has been thoroughly investigated since
1970s, all prior work concentrated on accretion disks of nearly solar abundance
of elements, with only moderate variations of the element abundances considered
in a few papers. MPA scientists have now taken the first step in modeling X-ray
reflection off hydrogen poor material with anomalous abundances, as expected in
the accretion disks in Ultra-compact X-ray binaries. The model developed using
the Monte Carlo technique is the first simulation of reflection spectra of C/O,
O/Ne/Mg or helium rich disks.
Using these simulations, MPA scientists came to a paradoxical conclusion: The
strongest and most easily observable effect of the hydrogen poor, C/O rich
material is not an appearance of strong fluorescent lines of carbon and oxygen -
as one might expect - but nearly complete disappearance of the fluorescent line
of iron! This is caused by the screening of iron by the much more abundant
carbon and oxygen.
In a neutral material of solar abundance, the most likely process for a photon
with energy exceeding 7.1 keV - the photoionisation threshold of K-shell
electrons in iron (so called K-edge) - is absorption by iron due to the
photoionisation of its atoms. Photoionisation of iron is followed in about
one-third of the cases by the emission of a 6.4 keV fluorescent photon.
Consequently, the majority of photons with energies above this threshold will be
absorbed by iron and will, therefore, contribute to its fluorescent line.
In the case of a C/O (or O/Ne) white dwarf though, the overwhelming
overabundance of oxygen makes it the dominant absorbing agent even at energies
far beyond its own K-edge, leaving only a few photons to fuel the iron
fluorescent line. Although the fluorescent line of oxygen produced in the
process is significantly boosted, it is still strongly diluted by the primary
continuum and therefore is difficult to detect. A much more visible effect is
the significant attenuation or complete disappearance of the iron line.
Helium, on the other hand, is not capable of screening iron, due to its smaller
charge and, correspondingly smaller absorption cross-section at the iron K-edge.
Therefore in the case of a helium-rich donor reflection proceeds ‘as usual’ and
the iron line has its nominal strength.
This opens an exciting possibility to discriminate between helium and oxygen
rich donors by means of X-ray spectroscopy. MPA scientists calibrated the method
using extensive Monte-Carlo simulations, investigated its luminosity dependence
and proposed observational tests of the picture. They used the data of
XMM-Newton satellite to verify results of theoretical calculations using
observations of UCXB systems with a donor star of known composition.
Furthermore, they provided tentative identifications of the donor star in
several ultra-compact binaries, where its nature remained so far unknown.
Filippos Koliopanos and Marat Gilfanov
1. Koliopanos F., Gilfanov M., Bildsten L., 2013, MNRAS, 432, 1264
2. Koliopanos F., Gilfanov M., Bildsten L., M.Diaz Trigo, 2014 MNRAS