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As matter falls onto a black hole, it isionizedand accelerated to speeds close to that of light, and the matter radiates light as it accelerates.

The faster the infalling matter goes, the higher the energy of the photons.

The surface of planets or stars would stop an in falling particle before it approached the speed of light, but such speeds are possible when falling into a black hole.

Below is an illustration of what sort of radiation is produced by matter falling onto increasingly dense objects.

X-rays and Gamma rays can only be produced when matter is falling onto a blackhole.

How much energy does accretion release? The infalling matter releases gravitational potential energy up to several tenths of its rest mass in form of X-rays. About 10% of the rest mass of in falling particles can be turned into energy in the form of light in this manner. (The other 90% is added to the mass of the black hole.) For reference, Hydrogen fusion releases only about 0.7 percent of rest mass. So gravity around a black hole can actually produce much more energy than nuclear fusion reactions in stars. What is the name of the firstBH candidate discovered? Cygnus X-1 (this means that it is a star in the constellation of Cygnus, the Swan).

The brightest star in this image is orbiting around a black hole every 5.6 days. The black hole itself cannot be seen.

Cygnus X-1 is one of the first X-ray sources discovered. Its optical counterpart is a massive star (visible in the image bellow). The star itself is unable to account for observed X-ray radiation. However, through Doppler shift of spectral lines, we inferred that this star has a binary companion. The companion is dark in optical but bright in X-rays.

SDSS image; Cyg X-1 is located near large active regions of star formation in the Milky Way, as seen in this image that spans some 700 light years across.

Artist illustration: The black hole pulls material from a massive, blue companion star toward it. This material forms a disk (shown in red and orange) that rotates around the black hole before falling into it or being redirected away from the black hole in the form of powerful jets.

The mass of Cygnus X-1 Mass: 14.8 Msun(from optical observations of the companion star’s motion).

How is it possible for a blue main sequence star (fusing H --> He in the core) to be more massive that its binary companion that is a red giant star? Look up the Algol paradox example in the book on page 61. What happens to a binary star system if one star explodes as a supernova?
  1. If the exploding star is the more massive one, then the less massive star will be flung into space.
  2. If the exploding star is the less massive one, the more massive star in the binary will stay intact.

Can we distinguish a black hole from a neutron star?

Accretion signatures from a BH vs. a neutron star should be different since a neutron starhas a surface–would see a ‘splash’. A BH doesn’t have a surface so the material fromthe last stable orbit would not make a splash as it crosses the event horizon and we wouldn’t see it.We have not yet observed such difference in accretion.

The only way to tell black holes and neutron stars apart in X-ray binaries is to confirm that the mass of the unseen star is larger than 3 Msun. If the star is less massive, then it must be a neutron star.

What happens if a White Dwarf accretes so much matter that it exceeds the Chandrasekhar limit of 1.4 Msun? The white dwarf will blow up in a supernova explosion. No star remains afterwards.
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