Lecture 35: Farewell Special - High-energy Astrophysics

**[[https://www.surveymonkey.co.uk/r/6DMBC3Q|>>PLEASE TAKE A QUICK SURVEY<<]]** **1. X-ray Astronomy from Balloon Flights:** Professor Lewin takes us back to 1966 when Professor George Clark and he pioneered X-ray observations from balloon-borne telescopes at altitudes of 145,000 ft. **2. Slides of High Altitude Ballooning Expeditions:** A series of slides are shown of the construction of an X-ray telescope, the manufacturing of the balloons, and balloon launches in both Alice Springs, Australia and Palestine, Texas. The risks that arise during launch and during flight are shown, as are some interesting encounters during payload recovery. **3. X-ray Observations:** The science gained from the balloon-borne telescopes is described, such as the first ever flaring event and the 2.3 minute periodicity observed from a previously unknown source (GX 1+4). We now know of hundreds of binary star systems where gas from a "donor" swirls onto a neutron star (the accretor). The gas reaches the neutron star's surface with about 1/3 the speed of light. It heats up its surface to a few million degrees Kelvin which is why the neutron star emits large amounts of X-rays. **4. Binary Stars and X-ray Bursts: ** Professor Lewin reviews the Doppler shift of the neutron star's pulsar period and of the donor star's spectral lines in X-ray binaries. He then talks about X-ray bursts. These are thermonuclear flashes (nuclear bomb explosions) on the surface of neutron stars. The X-rays from these flashes temporarily excite the matter in the accretion disk, resulting in delayed optical flashes. This delay provided the first measurement of the size of an accretion disk.