Transcript X-Ray Astronomy and Accretion Phenomena

X-Ray Astronomy and
Accretion Phenomena
X-rays Can’t Penetrate the
Atmosphere, so…
• X-ray detectors should be placed above
the atmosphere
• Chandra, XMM-Newton, Rosat, Uhuru,
Integral etc are some X-ray astronomy
missions.
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
1990s.
(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
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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 Xrays 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
of galaxies.
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
CHANDRA)
Active Stars
• These are early type stars - O and Wolf-Rayet types. They
have large mass loss rates in the form of a large stellar 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 system.
Supernovae
• The matter ejected in a supernova explosion
compresses the tenuous gas in the interstellar
medium (ISM). This causes the emission of X-rays.
• 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,
and blue
(Image courtesy CHANDRA)
Cassiopeia A Supernova remnant
as seen in visible light.
(Image courtesy CHANDRA)
Crab SNR
• Crab Supernova
remnant - three
colour image with
X-ray in blue,
optical in green,
and radio in red.
(Image courtesy
CHANDRA)
Crab Nebula
Binary Stars
• 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).
http://en.wikipedia.org/wiki/Binary_star
Equipotential Surfaces in a Binary
System
At the Lagrange points
a test particle would be
stationary relative to the
stars.
http://en.wikipedia.org/wiki/Roche_lobe
Roche
Potential
Lagrange Points
• At the Lagrange
points a test
particle would be
stationary relative
to the stars.
• Combined
gravitational pull
of the two large
masses provides
precisely the
centripetal force
required to rotate
with them.
http://en.wikipedia.org/wiki/Lagrangian_point
Roche Lobe
• The Roche lobe is the figure-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 Roche 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
evolution.
• 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.
Roche Lobe
Overflow
• 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.
Accretion Disk
• Matter coming from
the secondary has
angular momentum
and can not fall
directly on the the
compact object.
• It misses the
compact object,
hits with itself and
diffuses to form a
disk.
X-ray Binaries
• 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
and thus the accretion temperatures are lower.
Cygnus X-1
• Cyg X-1 is the
most famous Xray binary and
thought to be a
massive star
sending material
to a large black
hole
GRS 1915+105
GRO J1655-40
Cen X-3
Naming XRB
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
the accretor.
LMXB
• Accretes via Roche Lobe overflow
• Donor star has late spectral type (A and later),
i.e. M = 1.2M.
LMXB
• 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).
HMXB
• 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 absorption lines.
• Typical Mass loss rates: 10-7-10-5M per year
• Only a fraction of the wind (10-3-10-4) can
accrete onto compact object: Bondi-Hoyle
accretion.
HMXB
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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.
Be Accretion
Be Accretion
• 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 Pulsar
Thermonuclear Burst
X-ray bursts from EXO 2030+375 as seen with EXOSAT.
Interpretation: Thermonuclear explosions on NS surface.
Thermonuclear Bursts
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 reached.
Compact Object Observed Masses
BHC=Black Hole candidates
Accretion Disk
• 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
into radiation.
Accretion Disk
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Links
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http://www.oulu.fi/astronomy/astrophysics/pr/head.html
http://spiff.rit.edu/classes/phys240/lectures/future/future.html
http://cns.uni.edu/~morgan/astro/course/Notes/section2/xraybin.html
http://www.shokabo.co.jp/sp_e/optical/labo/opt_cont/opt_cont.htm
http://www-xray.ast.cam.ac.uk/xray_introduction/Blackholebinary.html