Transcript Part B
Who are the usual suspects? Type I Supernovae All very similar light curves (maximum light and shape) suggesting common process. Favoured model is the detonation of a white dwarf star in a binary system. No fusion in white dwarf, star is supported only by electron degeneracy pressure. This sets max mass for a white dwarf, Chandrasekhar limit, at approximately 1.4 solar masses. Beyond this it cannot be supported by degeneracy pressure alone. Type I supernova occurs when white dwarf acquires additional mass from a companion star, usually a red giant. At critical mass (> 1.4 times mass of Sun) the heat and pressure in the centre of the star are sufficient to initiate runaway nuclear fusion, and the white dwarf explodes. Who are the usual suspects? X-rays and gamma rays are produced during nucleosynthesis and also during radioactive decay of the various products. They also originate from the shockwave which heats interstellar gas creating thermal X-ray emission. Type I supernovae has led to their use as standard candles in extragalactic astronomy. Who are the usual suspects? Neutron star : a supernova remnant. Mass about 1 solar mass, with radius of about 15 to 30 km. Neutron stars have overall densities of 1017 kg/m3. During neutron star formation angular momentum and magnetic flux are conserved. Both are inversely proportional to the square of the radius, giving rotation periods between about 1.4 ms to 30 seconds and magnetic fields of up to 108 T. Energy is lost as neutrinos emitted carry away so much energy that the temperature falls within a few years to around 106 kelvin. Who are the usual suspects? X-ray pulsars Strong magnetic field of neutron star forces charged particles to follow magnetic field lines. http://www.youtube.com/watch?v=33Ldqkd0Fa4&feature=related They emit synchrotron radiation in a beam along the N-S magnetic field axis. Just as on Earth, the magnetic poles don't coincide with the rotational poles. Thus beams of radiation are at an angle to the rotational axis of the neutron star; as the neutron star rotates, the beams swing around in a cone. Mistake in notes Who are the usual suspects? X-ray pulsars Crab Nebula formed in 1054 A.D. supernova with neutron star (X-ray pulsar at its centre) Who are the usual suspects? X-ray bursters Neutron stars in close proximity to another star may be able to capture material, mostly hydrogen. Accretion of hydrogen on the neutron star’s surface produces a layer where hydrogen fusion takes place. Normally this would raise temperature and pressure and cause the material to expand and cool (-ve feedback). However ideal gas laws don’t apply on neutron stars and temperature increase just makes fusion go faster leading to a thermonuclear flash. X-rays are emitted via black body radiation in a process lasting a minute. This is an irregular process. Who are the usual suspects? X-ray bursters Who are the usual suspects? Gamma-ray bursters • Flashes of gamma rays associated with extremely energetic processes in distant galaxies ( 1044 joules). • Extremely rare (a few per galaxy per million years). They are the most luminous electromagnetic events known to occur in the Universe. • Short GRB lasts few seconds. • Long GRB lasts several minutes. • Unlike X-ray bursters GRB sources only emit once in their history. • Distributed isotropically across night sky suggesting extragalactic origin. • New GRBs discovered at rate of about one per day. Who are the usual suspects? Gamma-ray bursters • Flashes of gamma rays associated with extremely energetic processes in distant galaxies ( 1044 joules). • Extremely rare (a few per galaxy per million years). They are the most luminous electromagnetic events known to occur in the Universe. • First detected in 1967 by the Vela satellites, • Redshifts clarified their distance and combined with their luminosity connected them to the deaths of massive stars. • GRBs are now thought to be highly focused explosions, with most of the energy collimated into a narrow jet from between 2 and 20 degrees in angular spread. Who are the usual suspects? Gamma-ray bursters Short and long GRBs recorded by Vela. Who are the usual suspects? Long Gamma-ray bursters - origin • Hypernovae - collapse of stars of greater than 30 solar masses which are spinning rapidly. • The black hole forms before star outer layers contract very much. • An accretion disc from the surrounding stellar material forms. • Some of the infalling material does not fall into the black hole but is ejected in powerful back to back jets along the axis of rotation of the accretion disc. • If one of these jets is directed towards Earth we would see a powerful GRB formed as the relativistic particles slow down and convert their kinetic energy to gamma rays. • GRBs have a short existence as it doesn’t take long for the black hole to eat the accretion disc! http://www.youtube.com/watch?v=Xb1mkE4WCtQ&feature=related Who are the usual suspects? Short Gamma-ray bursters - origin • Merger of a binary system consisting of two neutron stars. • They rip each other apart and collapse into a single black hole. The infall of matter into the new black hole in an accretion disk then powers the GRB. http://www.youtube.com/watch?v =g8s81MzzJ5c Because GRBs are visible to massive distances encompassing billions of galaxies, they must be exceedingly rare events per galaxy. Expect rate of long GRBs in Milky Way to be about one burst every 1,000,000 years. Who are the usual suspects? Quasars Discovered in mid 1960s Doppler shift indicating massive source. 1 quasar emits over 100 times as much energy as our own galaxy with its 200 billion stars!!!!! Ultra luminous objects located at centre of very distance galaxies. At the centre of every quasar is a supermassive black hole. As matter is pulled into the black hole it releases vast amounts of energy. Strong magnetic field in the accretion disc, produces beams of electrons and plasma in the jets which generate http://www.youtube.com/watch?v=RkBak1aETYE&feature=related radio emission. Who are the usual suspects? Active Galactic Nuclei Active galaxies are galaxies which have a small core of emission embedded in an otherwise typical galaxy. This core is typically a quasar. The total energy of normal galaxies like the Milky Way is just the sum of the emission from each of the stars. For the "active" galaxies, this is not true and the core dominates light emission. This is a photo of Circinus. Who are the usual suspects? Other X-ray sources Lots of energetic interactions of cosmic bodies emit black body radiation peaked in X-rays. e.g. Galactic clusters formed by the merger of individual galaxies (infalling material on collision heated to 108 K). Bullet cluster, the colour representing X-ray emission. Who are the usual suspects? Other X-ray sources All main sequence stars are likely to have hot enough coronae to emit Xrays (right) and SN remnants (left) also emit in the X-ray band. Planetary nebula around a white dwarf is also a weak source of X-rays. Problems associated with Earth based observation Attenuation of light by dust and gas in the interstellar medium, and by the Earth’s atmosphere. Only the visual, infrared, and radio regions of the electromagnetic spectrum are directly observable from the Earth’s surface.