The last cry of matter (Forwarded)

ESA News
http://www.esa.int

27 November 2003

The last cry of matter

'Black holes' are truly black. When an object gets within a certain distance 
from a black hole, it will get swallowed forever with no chance to escape. That 
includes light, which means that black holes do not shine.

How do astronomers detect black holes if they are unable to see them? Well, to 
be precise, astronomers do not detect black holes. But they do detect the 
phenomena that can only be explained by the existence nearby of objects that 
match the description of black holes!

The strong gravitational attraction of a black hole affects the motion of nearby 
objects. When astronomers see a star circling around something, but they cannot 
see what that something is, they may suspect it is a black hole, or a neutron 
star -- the ultra-dense 'corpse' of a star.

Astronomers can even infer the mass of a black hole by measuring the mass of the 
star and its speed. The same kind of calculation can be done whith supermassive 
black holes that lurk at the centre of many galaxies, including our own galaxy, 
the Milky Way.

In the Milky Way, observations have revealed the existence of stars and gas 
moving very fast near the centre, a behaviour that can only be explained if a 
mass of several million times that of the Sun is at the centre of the galaxy.

Such mass has to be concentrated within a radius of only 10 light-days -- 
roughly 40 times times the distance from the Sun to Pluto -- and is most likely 
to be a black hole.

In fact, at the very centre of our galaxy, radio and X-ray telescopes have 
detected a powerful source called 'Sagittarius A', identified as the candidate 
to be this massive black hole.

This idea has recently received strong support, with the measurement for the 
first time of the orbit of a star that approaches this mysterious object to 
within 17 light-hours -- only three times the distance between the Sun and Pluto 
-- while travelling at speeds of than 5000 kilometres per second!

Shining to death

Another piece of evidence in favour of the idea of supermassive black holes in 
the centre of galaxies is the existence of quasars, discovered in 1967. Quasars 
are very distant and very luminous at the same time -- the most luminous objects 
in the Universe.

To explain the incredible amount of energy they must release, astronomers also 
need black holes: just before disappearing into a black hole, the matter being 
swallowed heats up and emits great amounts of energy -- its 'last cry'. So 
quasars are believed to be caused by black holes with masses of one million to 
several billion times the mass of the Sun.

The 'last cry' of matter about to be swallowed is best detected with x-ray and 
gamma-ray telescopes, because the energy released is given off in the form of 
hard x-rays. In fact, ESA's orbiting observatories XMM-Newton and Integral have 
already shown their skills in studying black holes in several discoveries.

For example, XMM Newton has recently discovered a small black hole whirling in 
our galaxy, in the Ara constellation of the southern sky. Integral has detected 
what could be the first significant hard x-ray emission from the black hole in 
the centre of our galaxy.

More about ...

* Integral overview
  http://www.esa.int/esaSC/120374_index_0_m.html
* XMM-Newton overview
  http://www.esa.int/esaSC/120385_index_0_m.html

Related articles

* Black holes
  http://www.esa.int/esaSC/SEMDJ71P4HD_index_0.html
* Seeing the Universe in the gamma-ray wavelengths
  http://www.esa.int/esaSC/SEM3A2T1VED_index_0.html
* Seeing the Universe in X-ray wavelengths
  http://www.esa.int/esaSC/SEMTA2T1VED_index_0.html
* Integral -- tracking extreme radiation across the Universe
  http://www.esa.int/esaCP/ESAI0BTHN6D_Expanding_0.html
* A gamma-ray burst bonanza
  http://www.esa.int/esaCP/SEMIVX8YFDD_FeatureWeek_0.html
* Integral's first look at the gamma-ray Universe
  http://www.esa.int/esaCP/ESADW18708D_index_0.html

IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/export/esaCP/SEMPUMXLDMD_FeatureWeek_1.html]
Graphical depiction of gas swirling around a black hole before falling into it.

Credits: ESA

[Image 2:
http://www.esa.int/export/esaCP/SEMPUMXLDMD_FeatureWeek_1.html#subhead3]
GRO J1655-40 is the second so-called 'microquasar' discovered in our Galaxy. 
Microquasars are black holes of about the same mass as a star. They behave as 
scaled-down versions of much more massive black holes that are at the cores of 
extremely active galaxies, called quasars. Astronomers have known about the 
existence of stellar-mass black holes since the early 1970s. Their masses can 
range from 3.5 to approximately 15 times the mass of our Sun.

Using Hubble data, astronomers were able to describe the black-hole system. The 
companion star had apparently survived the original supernova explosion that 
created the black hole. It is an ageing star that completes an orbit around the 
black hole every 2.6 days. It is being slowly devoured by the black hole. 
Blowtorch-like jets (shown in blue) are streaming away from the black-hole 
system at 90% of the speed of light.

Credits: European Space Agency, NASA and Felix Mirabel (the French Atomic Energy 
Commission & the Institute for Astronomy and Space Physics/Conicet of Argentina)

[Image 3:
http://www.esa.int/export/esaCP/SEMPUMXLDMD_FeatureWeek_1.html#subhead7]
Artist's impression of XMM-Newton.

Credits: ESA

[Image 4:
http://www.esa.int/export/esaCP/SEMPUMXLDMD_FeatureWeek_1.html#subhead8]
The task of Integral, ESA's International Gamma-Ray Astrophysics Laboratory, is 
to gather the most energetic radiation that comes from space. The spacecraft was 
launched October 2002 and will help to solve some of the biggest mysteries in 
astronomy. Gamma rays are even more powerful than the X-rays used in medical 
examinations. Fortunately, the Earth's atmosphere acts as a shield to protect us 
from this dangerous cosmic radiation. However this means that gamma rays from 
space can only be detected by satellites. Integral is currently the most 
sensitive gamma-ray observatory ever launched. It detects radiation from the 
most violent events far away and from processes that made the Universe habitable.

Credits: ESA. Illustration by D. Ducros