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 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 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 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).

Equipotential Surfaces in a Binary System At the Lagrange points a test particle would be stationary relative to the stars.

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.

Roche Lobe 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 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.

Cygnus X-1

GRS

GRO J

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: _ M  per year Only a fraction of the wind ( ) can accrete onto compact object: Bondi- Hoyle accretion.

HMXB 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

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: A

X-Ray Pulsar

Thermonuclear Burst X-ray bursts from EXO 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=BH 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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