The most well known of these is the ROSAT All Sky Survey (RASS) carried out over 6 months between 1990 and 1991. This was the first survey taken with an X-ray telescope which could image. All previous surveys could only measure the total count rate from a given &quot;pointing&quot; and so point and extended sources were not distinguishable. It detected more than 60 000 X-ray sources over the whole sky. The image above shows the 50 000 sources detected in the first round of the data processing. The map is in galactic coordinates, so that the top and bottom parts show the `extragalactic&apos; X-ray sky, i.e., the regions we see when looking away from the plane of the Milky Way which runs horizontally through the centre of the above image. The colours from red to white represent the average energies of the photons emitted by the different sources: red stands for low energies corresponding to relatively cool temperatures of several 100 000 K, whereas the detection of `white sources&apos; indicates the presence of gas at temperatures in excess of 20 million K.
X-Ray Astronomy and
X-rays Can’t Penetrate the
• X-ray detectors should be placed above
• Chandra, XMM-Newton, Rosat, Uhuru,
Integral etc are some X-ray astronomy
X-rays are Hard to Focuse
• X-ray telescopes usually perform "pointings,"
where the telescope is pointed at some
astrophysical object of interest. This of course
means that only sources which already look
interesting for other reasons, or known to be so
from a previous observation are observed.
• All sky surveys are useful for discovering some
unexpected phenomena as they scan the entire
sky over a large range in energy.
The soft (low energy) X-ray
background as seen by the
ROSAT satellite in the
(Image courtesy ROSAT)
The colours from red to white represent the average energies of the photons
emitted by the different sources: red stands for low energies corresponding
to relatively cool temperatures of several 100 000 K, whereas the detection
of `white sources' indicates the presence of gas at temperatures in excess of
20 million K.
Stars in X-rays
• Normal stars, like our Sun,
produce some X-rays in their
outer atmosphere. The gas in this
regions, known as the
Chromosphere, is very hot and
tenuous. Flares and prominences
on the surface of the Sun also
produce X-rays as a result of
reconnection of magnetic fields.
• Although in the history of X-ray
astronomy" it was stated that X-
rays from other stars could not be
observed, this was true for the
1960's, and today stars are
observed with X-ray telescopes.
Their X-ray emission does vary
and this is a field of study.
However they do not emit many
X-rays in comparison with the
emission associated with
accreting black holes and clusters
• An X-ray image of the closest
star, Proxima Centauri. This
shows that X-ray images from
nearby stars on the whole tell us
little, spectra on the other hand
can tell us more. (Image courtesy
• These are early type stars - O and Wolf-Rayet types. They have
large mass loss rates in the form of a large wind, much stronger
than the solar wind. The shocks in the wind heat the plasma which
then emits X-rays. Observations spread out in time of these stars
has allows researchers to show that sometimes the wind is confined
to a plane by a magnetic field, as the X-ray characteristics are
different in the different observations.
• Some of these stars are in binary systems, and then one of the pair
will have a less strong wind. The collision of the two winds causes a
steady shock wave. The X-rays from this wind can irradiate the
other star. If the binary is eclipsing, then the variation of the signal
as the stars orbit one another can determine the exact geometry of
• The matter ejected in a supernova explosion
compresses the tenuous gas in the interstellar
medium (ISM). This causes the emission of X-
• The newly formed neutron star is initially very
hot and this also emits X-rays.
• The X-rays that come from the central remnant
of the Supernova cause the elements in the
expanding gas shell to fluoresce. Different
elements show up at different energies, which
allows the composition of the gas shell and also
the star to be estimated.
Cas A SNR
Cassiopeia A Supernova remnant as
seen in X-rays.
The low, medium, and higher X-ray
energies of the
Chandra data are shown as red, green,
(Image courtesy CHANDRA)
Cassiopeia A Supernova remnant
as seen in visible light.
(Image courtesy CHANDRA)
image with X-
ray in blue,
radio in red.
• A binary star is a system of two stars that
rotate around a common center of mass.
• About half of all stars are in a group of at
least two stars. There may be triple
systems (though much rare).
Equipotential Surfaces in a Binary
At the Lagrange points
a test particle would be
stationary relative to the
• At the Lagrange
points a test
particle would be
to the stars.
of the two large
required to rotate
• The Roche lobe is the 8
shaped equipotential surface in
a binary system.
• Roche Lobe is the region of
space around a star in a binary
system within which orbiting
material is gravitationally bound
to that star.
• If a star expands past its Roche
lobe, then the material outside
of the lobe will be attracted to
the other star.
Roche Lobe Overflow
• Roche-lobe overflow
occurs in a binary
system when a star fills
its Roche-lobe by
expanding during a
stage in its stellar
• Matter streams over Lagrange
point L1 from donor onto
• Preservation of angular
momentum leads to the
formation of a disk rather than
• Matter streams over
Lagrange point L1 from
donor onto compact object.
• Preservation of angular
momentum leads to the
formation of a disk rather
than direct accretion.
• Matter coming
can not fall
directly on the
• It misses the
hits with itself
and diffuses to
form a disk.
• There are binaries in which one of the members
is a compact object (WD, NS or BH).
• If matter from the companion is accreted onto
the compact object X-rays are emitted and such
systems are called X-ray binaries.
• If the accreting compact object is a white dwarf
then the system is called a cataclymic variable.
These sytems emit UV instead of X-rays
because they are less compact than NS or BHs.
Two Types of XRB:
• Low Mass X-ray Binaries (LMXB)
• High Mass X-ray Binary (HMXB)
• Low & High labels the mass of the
companion star (the mass donor) and not
• Accretes via Roche Lobe overflow
• Donor star has late spectral type (A and later),
i.e. M = 1.2M.
• The origin of LMXBs is not very well understood. The most likely explanation is that
they form by capture: the lone compact object, has a close interaction in a cluster
and picks up a companion.
• The mass transfer on to the compact object is much slower and more controlled.
• This mass transfer can spin up a neutron star so that it is a millisecond pulsar,
spinning thousands of times a second.
• LMXBs tend to emit X-rays in bursts and transients and there could be many more
present in our galaxy than we see, but which are currently switched off.
• They also tend to have softer spectra (they emit lower energy X-rays), whereas the
HMXB's have harder spectra (more energetic X-rays).
• Accretion is via the wind of the mass donor
Stellar Wind Accretion
• Early type stars (spectral type O, B,
mass M & 10M) have strong winds,
driven by radiation pressure in
• Typical Mass loss rates: _10-7-10-5M
• Only a fraction of the wind (10-3-10-4)
can accrete onto compact object:
• HMXB form from two stars of different mass which are in orbit around each other.
• The more massive one evolves faster and reaches the end of its life first, after a few
million years or so. It becomes a giant and the outer layers are lost to its companion.
Then it explodes in a supernova leaving behind either a neutron star or a black hole.
• This can disrupt the binary system, but if the star that exploded was less massive
than its companion when it exploded they the systems will remain in tact, though the
orbits may be more eccentric.
• The companion star then comes to the end of its life and swells to form a giant. It then
looses its outer layers onto the neutron star or black hole. This is the HMXB phase.
• The material forms an accretion disc around the compact object, which heats up
because of friction. This heating, combined with jets that can be formed by the black
hole, cause the X-ray emission.
• Eventually the companion star comes to the end of its life, leaving a neutron
star/black hole - white dwarf/neutron star/black hole binary, depending on the initial
masses of the stars.
• Cygnus X-1 is this type of X-ray Binary. They are bright in X-rays not only because of
the accretion disc, but also because there is a corona which is much more powerful
than the Sun's corona.
• Cygnus X-1 is 10,000 times more powerful than the Sun, and most of it is powered by
the gravity caused by the black hole.
• Some early type stars (O9–B2) have very
high rotation rates )Formation of disk-like
stellar wind around equator region. Line
emission from disk: Be phenomenon.
• Collision of compact object with disk results
in irregular X-ray outbursts.
• Exact physics not understood at all.
• Typical Objects: A0535+ 26.
X-ray bursts from EXO 2030+ 375 as seen with
Interpretation: Thermonuclear explosions on NS
Peak flux and total fluence of bursts are
correlated with distance to the next burst.
Explanation: Accretion of hydrogen onto
surface )hydrogen burns quietly into
helium (thickness of layer 1 m) = )
thermonuclear flash when critical mass
• The disk has a life of its own.
• It has its own luminosity and is very bright.
• The luminosity of the disk is because the
disk is hot due to friction between adjacent
layers which converts gravitational
potential energy of the accreting matter