Fig. 1: 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
Fig. 2:
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
Fig. 3:
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.
Ultra-compact X-ray binaries are a small but fascinating subclass of
low-mass X-ray binaries, in which the donor is a white dwarf - a remnant
of a moderately massive normal star. In order to understand the
formation and evolution of these systems, it is critically important to
identify the nature of the donor, which can be made of either helium or
carbon and oxygen. MPA scientists have recently proposed and tested
using - XMM-Newton observations - a principally new method to answer
this question by the means of X-ray spectroscopy.
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 spectroscopy.
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 X-ray spectra.
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
Filippos Koliopanos and Marat Gilfanov
References:
1. Koliopanos F., Gilfanov M., Bildsten L., 2013, MNRAS, 432, 1264
2. Koliopanos F., Gilfanov M., Bildsten L., M.Diaz Trigo, 2014 MNRAS
2. Koliopanos F., Gilfanov M., Bildsten L., M.Diaz Trigo, 2014 MNRAS
