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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.

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X-Ray Astronomy and Accretion Phenomena

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  1. X-Ray Astronomy and Accretion Phenomena

  2. 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.

  3. 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.

  4. 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.

  5. 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)

  6. 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.

  7. Supernovae • Thematterejected in a supernovaexplosioncompressesthetenuousgas in theinterstellarmedium (ISM). Thiscausestheemission of X-rays. • Thenewlyformedneutron star is initiallyvery hot andthisalsoemits X-rays. • The X-raysthatcomefromthecentralremnant of theSupernovacausetheelements in theexpandinggasshelltofluoresce. Differentelementsshowup at differentenergies, whichallowsthecomposition of thegasshellandalsothe star to be estimated.

  8. 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)

  9. Crab SNR • CrabSupernovaremnant - threecolourimagewith X-ray in blue, optical in green, andradio in red. (Image courtesyCHANDRA)

  10. Crab Nebula

  11. 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

  12. 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

  13. Roche Potential

  14. 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

  15. Roche Lobe • TheRochelobe is the figure-8 shapedequipotentialsurface in a binarysystem. • RocheLobe is theregion of spacearound a star in a binarysystemwithinwhichorbitingmaterial is gravitationallyboundtothat star. • If a star expandspastitsRochelobe, thenthematerialoutside of theRochelobewill be attractedtotheother star.

  16. RocheLobeOverflow • 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 overLagrange point L1 from donor onto compactobject. • Preservation of angular momentum leads to the formation of a disk rather than direct accretion.

  17. Roche Lobe Overflow • Matter streams overLagrange point L1 from donor onto compactobject. • Preservation of angular momentumleadstotheformation of a disk ratherthandirectaccretion.

  18. Accretion Disk • Mattercomingfromthesecondary has angular momentum and can not falldirectly on thethecompactobject. • Itmissesthecompactobject, hitswithitselfanddiffusesto form a disk.

  19. X-ray Binaries • Therearebinaries in whichone of themembers is a compactobject (WD, NS or BH). • Ifmatterfromthecompanion is accretedontothecompactobject X-raysareemittedandsuchsystemsarecalledX-ray binaries. • Iftheaccretingcompactobject is a whitedwarfthenthesystem is called a cataclymicvariable. Thesesytemsemit UV instead of X-raysbecausetheyarelesscompactthan NS orBHsandthustheaccretiontemperaturesarelower.

  20. Cygnus X-1 • Cyg X-1 is the most famous X-ray binary and thought to be a massive star sending material to a large black hole

  21. GRS 1915+105

  22. GRO J1655-40

  23. Cen X-3

  24. Naming XRB

  25. 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.

  26. LMXB • Accretes via Roche Lobe overflow • Donor star has late spectraltype (A and later), i.e. M = 1.2M.

  27. LMXB • Theorigin of LMXBs is not verywellunderstood. Themostlikelyexplanation is thatthey form bycapture: thelonecompactobject, has a closeinteraction in a clusterandpicksup a companion. • Themass transfer on tothecompactobject is muchslowerandmorecontrolled. • Thismass transfer can spinup a neutron star sothat it is a millisecondpulsar, spinningthousands of times a second. • LMXBstendtoemit X-rays in burstsandtransientsandtherecould be manymorepresent in ourgalaxythanwesee, but whicharecurrentlyswitchedoff. • Theyalsotendtohavesofterspectra (theyemitlowerenergy X-rays), whereastheHMXB'shaveharderspectra (moreenergetic X-rays).

  28. HMXB • Accretion is via the wind of the mass donor

  29. Stellar Wind Accretion • Early type stars (spectral type O, B, massM & 10M) have strong winds, driven byradiation pressure in absorption lines. • TypicalMass loss rates: 10-7-10-5Mperyear • Only a fraction of the wind (10-3-10-4) can accreteonto compact object: Bondi-Hoyle accretion.

  30. 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.

  31. Be Accretion

  32. Be Accretion • Some early type stars (O9–B2) have very highrotation rates ) Formation of disk-like stellarwind around equator region. Line emission fromdisk: Be phenomenon. • Collision of compact object with disk results inirregular X-ray outbursts. • Exact physics not understood at all. • Typical Objects: A0535+26.

  33. X-Ray Pulsar

  34. ThermonuclearBurst X-ray bursts from EXO 2030+375 as seen with EXOSAT. Interpretation: Thermonuclear explosions on NS surface.

  35. Thermonuclear Bursts Peak flux and total fluence of bursts arecorrelated with distance to the next burst. Explanation: Accretion of hydrogen ontosurface) hydrogen burns quietly into helium (thicknessof layer 1 m) -> thermonuclear flash whencritical mass reached.

  36. Compact Object ObservedMasses

  37. BHC=Black Hole candidates

  38. 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.

  39. Accretion Disk =

  40. Links • 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

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