WHAT IS IT THAT ACTUALLY HAPPENS WHEN YOU DO HAVE A BLACK HOLE?
DIFFERENT
PHENOMENA THAT ACCOMPANY A BLACK HOLE
ACCRETION DISCS
Stars that are
candidates for a black hole generally have an “atmosphere” consisting of dust
and gas before they collapse. When the star collapses into itself and forms a
black hole it does not use up all of this matter and therefore after the
formation of the black hole there is a lot of dust and gas (actually this dust
and gas is in the plasma state) still floating around it.
This dust doesn’t
directly fall into the black hole but gets disturbed by the presence of the
black hole.
This dust starts
revolving around the black hole with very large speeds.
The black hole has a very powerful gravitational field around it. If we consider dust a little farther away from a black hole it has tremendous gravitational potential and thus very high gravitational energy. This is because the higher up a particle is in a gravitational field, the greater energy it has.
This dust then starts
falling into the black hole thus its gravitational potential decreases and this
decrease in potential energy is accompanied by an increase in kinetic and heat
energy.
With an increase in
kinetic energy the dust follows a spiraling elliptical orbit into the black
hole. And because of the increase in heat energy it starts to radiate heat
energy and gradually starts visibly glowing and then after a period of time
starts to emit x-rays. So such black holes can be detected by the use of X-Ray
telescopes.
WHY DOES THIS DUST FALLING INTO A BLACK HOLE FORM A DISC AND NOT A
SPHERE OR ANY OTHER SHAPE AROUND IT?
The
ergosphere (see picture) connects with the black hole’s outer event
horizon and the poles of its axis of rotation (It's a lot like a squished
orange on a stick: the peel billows out and connects with the pulp where the
stick punctured the orange. Something like that, you see). . The ergosphere is
the fullest in the area where the accretion disc forms. At the two poles of the
black hole there is no ergosphere or it is too small and there is the outer
event horizon. The dust in the ergosphere starts moving with the rotation of
the black hole and thus the dust in the ergosphere moves faster than that at
the poles. So, the dust at the poles crosses the event horizon faster than that
at the equator. And that is why a disc forms.
A short
summary:
BLACK
HOLE HAS DUST AND GRAVITATIONAL FIELD AROUND IT→DUST DISTURBED→DUST
STARTS FALLING IN→ERGOSPHERE FULLEST WHERE ACCRETION DISK
FORMED→DUST IN ERGOSPHERE FASTER THAN AT THE POLES→THUS DUST AT
POLES CROSSES EVENT HORIZON FASTER THAN AT EQUATOR→WE HAVE A DISK!
ACCRETION DISC IN A BINARY:
JETS
False-color
images of plasmas in a spheromak taken with a high-speed digital camera show
the development of jet-like structure (a) and helical instability (b) in the
jet.
A
spheromak is a device originally created to study nuclear fusion but it was
found to mimic black hole jets.
Scientists
have caught a supermassive black hole in a distant galaxy in the act of
spurting energy into a jet of electrons and magnetic fields four distinct times
in the past three years, a celestial take on a Yellowstone geyser. (Artist's
rendering courtesy Dr. Wolfgang Steffen, Project Cosmovision; University of
Guadalajara, Mexico)
JETS
The jets form when the presence of the rotating
accretion
disc twists the magnetic field around the black
hole. The matter that is trapped inside the black hole approaches the speed of
light on its way inside and gets converted to plasma.
But how does this actually happen?
As we know now, there is a huge amount of dust or
plasma revolving around the black hole. There is a build up of static
electricity with all the plasma revolving around it. As the electric field is
in motion, a magnetic field builds up around it. This is a very powerful field
and the electrons that were going to fall into the black hole get caught up in
his field. Moreover the direction of the generated magnetic field is
perpendicular to the direction of the electric field (by the right hand thumb
rule: curve the fingers of your right hand in the direction of
the electric field, and the thumb of your right hand points in the direction of
the magnetic field).
The
electrons that move in the magnetic field streak out along the axis of rotation
of the black hole and attain very large speeds and energy by the action of the
magnetic field. i.e. Magnetic forces squeeze the plasma and its magnetic energy
is embedded into narrow jets that shoot out along the axis of rotation. These hypervelocity streams of superheated
gas, or plasma, are concentrated into two narrow paths that travel in opposite
directions, along the black hole’s axis of rotation. These streams of electrons
pick up a huge mount of energy and become x-rays and sometimes even gamma rays.
The jets are like huge fountains of light shooting out from the accretion disc
away from the black hole.
SO
WHICH BLACK HOLES DO WE ALREADY KNOW ABOUT?
Although
the existence of black holes is theoretically proven, no one has “seen” a black
hole yet. But there are a lot of suspects or candidates that we think are black
holes. We can predict this as black holes, with all their overflow of energy
are major sources of x-rays.
THE
BLACK HOLE CANDIDATE LMC X-3:
LMC X-3
is associated with a binary system and is an x-ray source with a period of 1.7
days. It is located in the Large Magellanic Cloud. The visible component, a
main sequence B3 star has a severely distorted shape due to the gravitational
field of its companion. The compact object is estimated to be at least 3 time
the solar mass and is probably higher.
THE
BLACK HOLE CANDIDATE CYGNUS X-1:
Cygnus
X-1, when detected in 1962 was one of the first X-ray sources to be discovered.
The visible object is HDE226868, a 9th magnitude supergiant star
with an orbital period of 5.6 days as estimated from its radial velocity curve.
The companion might be a black hole, which can be inferred from the fact that
the object is a strong x-ray emitter, and also the optical and x-ray emission
vary on very short timescales (like 1/1000th of a second). Assuming
that the primary star is a normal star, analysis of the radial velocity
variation suggests that the mass of the companion is about 6 solar masses.
THE
BLACK HOLE CANDIDATE NOVA MUSCAE 1991:
Nova
Muscae detected in 1991 is an x-ray binary believed to be composed of a
low-mass late type companion that is orbiting a massive object that is possibly
a black hole. It was detected in 1991 both as an x-ray transient source and
also as an increase in brightness by about eight magnitudes. Optometric and
photometric observations conducted recently show an orbital period of 10.4
hours. The low mass K0-K4 orbits around a companion with a maximum observed
velocity of 409 km/s. the inclination of the orbit to the plane of the sky is
unknown and therefore the mass of the companion cannot be determined
accurately. But it is likely that the
invisible star is about 3 solar masses and might be a black hole.
THE
BLACK HOLE CANDIDATE V616 MON (A0620-00):
An
x-ray source known as A0620-00 in the constellation Monoceros brightened more
than 100,000 times in thee winter of 1975. This x-ray nova event is associated
with a main sequence K star known to show optical brightness variations and
designated V616Mon. The K star orbits an unseen companion once every 7.75 hours
with a maximum velocity if 457 km/s. the light of the K star is believed to
vary due to the gravitational pull of its unseen but massive companion. The
mass of the compact star must be greater than 3.2 solar masses and may be
greater than 7.3 solar masses which makes it an excellent candidate for a black
hole.
RELATIVISTIC
JETS IN SS443
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The
bizarre object SS433 is a binary system in which material for one component
is being accreted onto a compact, massive companion. The accretion disk is so
hot that matter and radiation are being expelled from its surface in powerful
beams at velocities close to the speed of light. SS433 serves as a nearby
example of the beaming of radiation by the strong gravitational field around
a massive object also believed to power the distant quasars. |
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REFERENCES:
http://physics.syr.edu/courses/PHY312.98Spring/projects/jebornak/html/outside.html
http://www.space.com/scienceastronomy/blackhole_antics_021111.html
http://astrosun.tn.cornell.edu/courses/astro201/bh_candidates.htm