Prof Tom Marsh
(Warwick University)

Delving the depths of white dwarfs, neutron stars and black-holes

Spending the last 15 years doing observations in this field, The objects Tom studies are extremely dynamic and can change within minutes, seconds or even milliseconds. Tom and his team specialise in the high-speed data acquisition and analysis techniques needed to track them. His major project of the last few years has been the use of ULTRACAM, a high-speed CCD camera, but is now working on a successor instrument, bigger and better, with more channels called HIPERCAM.

Tom split the lecture into 4 topics:

  • The variable sky
  • Timescales
  • Observing at high speed
  • White Dwarfs and Black holes

The sky is full of variables, for instance the pulsations of variable stars can tell us a great deal about the interiors of stars. The Kepler satellite has carried out a great deal of work in this field; - observing the pulsations of stars. Then there are Gamma Ray Bursts - GRB’s which send out beams of particles.  Even cataclysmic variable stars of various types –SS Cygni, and dwarf nova have been monitored by Amateurs for over 100 years.

Tom then went to explain a little bit about Light crossing timescales. For instance, if the Sun switched off instantly it would take 2.3 seconds for it to go dark. The dynamical timescales of various types of stellar remnants, such as a typical White dwarf star, would be around 2 seconds. For a Neutron star of 1.4Msol and radius of 10km, the timescale would be 0.1sec with a similar value for a Black Hole of 10Msol and radius of 30km. So it can be seen that observing any events relating to such objects requires ultra fast observing techniques.

For much of the 1960s, 70s and 80s the choice of the detector to try to capture observations was the Photoelectric  Photometer. These were single pixel devices with fixed apertures of around 10 inch. They were extremely limited, picking up less than a quarter of responses.

Post 1985 the instrument of choice was the CCD device, offering a large catchment area;-anywhere from 100 million pixels -3.4 billion. These were very efficient, stable devices, good over all the optical range, but still quite slow when using single frame capture techniques. This led to the ultracam instrument at the end of the 90’s, high speed multi-channel CCD cameras. These employed wavelength dependant reflectors to split light into red green and blue. This was then fitted to the WHT telescope in Las Palmas and also on the VLT in Chile. ULTRACAM can produce lots of data, up to 3 MB per second. Over the course of the night, this can stack up to many gigabytes.

Prof Marsh then went on to talk about White Dwarfs, earth sized remnants of at least 95% of all stars. They are also the powder kegs underlying type 1a supernova.  Indeed most of the iron on earth and presumably on many planets is manufactured in this type of supernova.  With typical orbital speeds of 400km/s based on 12000km diameter, Prof Marsh have probed the structure of W.D’s using eclipses, often lasting around 30seconds. On this aspect the team are trying to figure out why mass in accreting and non accreting W.D systems is the opposite of expectations.  This 3.6MB animated gif shows an area around an eclipsing white dwarf/M dwarf binary called NN Ser during its eclipse. The right-hand panel shows the light-curve. The data were taken with 2 second exposures

Finally Prof Marsh touched on the work being carried out regarding black holes. By using Ultracam on the VLT in Chile, they have been able to measure one candidate is a binary object with a period of 1.8 days and 65 Solar masses.

A fascinating look at how research is undertaken in fields which require split second observations of often very remote objects, adding to our understanding of them.