Kind: captions
Language: en
on november 22 2014 a burst of x-rays
was detected by assassin that's the
all-sky automated survey for supernovae
but this was no supernova the signal
came from the center of a galaxy around
290 million light years away and what we
now believe happened was a star came too
close to a supermassive black hole with
a mass millions of times that of our sun
and it was eaten the black hole fed on
the star and yes this is the actual
terminology astrophysicists use to
describe it
events like these are thought to be rare
occurring maybe once every 10 to 100 000
years in a galaxy they're called tidal
disruption events or tidal disruption
flares
as the star approached the side closest
to the black hole experienced a much
stronger gravitational pull than the
other side ripping the star to shreds
matter spiraling into the black hole
formed an accretion disk an annular ring
of gas and dust that's accelerating and
heating up emitting visible light uv and
x-rays observable from earth
now what's remarkable about this event
is that it transformed a dormant or
quiescent black hole one that wasn't
really feeding into one that we can
observe thanks to the matter falling in
from that star and this is what it
looked like
okay if you're disappointed check out
these artist renditions of the same
event
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but if you're cynical you might say well
how do we know that's what really
happened what if the scientists are just
making this up to get more grant funding
or to inspire people to go into science
well i'll explain how we know this is
actually what went down but first things
got weirder scientists trained three
x-ray telescopes to observe this part of
the sky for years after the event and
what they found was a strong and regular
pulse of x-rays brightening and dimming
every 131 seconds and it shows up in the
data from all three telescopes they
observed periodically over 450 days but
the pulse maintained this rhythm and
didn't get weaker in fact as time went
on the relative strength of the pulse
got stronger modulating the x-ray signal
by around 40 percent
what was causing these periodic flashes
of x-rays and what could it tell us
about the black hole
well let's back up because black holes
are some of the simplest objects in the
universe by that i just mean that they
are characterized by only two attributes
mass and spin
okay there's also charge but since black
holes should essentially be neutral mass
and spin are the two that count
mass is relatively easy to determine far
away from a black hole you can even use
newtonian physics by measuring the
gravitational effects of the black hole
on other bodies you can estimate the
mass of the black hole this has been
done and black holes have been found
with masses ranging from just a few
times our sun stellar mass black holes
up to billions of solar masses
supermassive black holes it's generally
accepted that there is a supermassive
black hole at the centers of most
galaxies including our own
but what about spin
since black holes form from collapsing
stars and all known stars rotate all
black holes should also be rotating i
mean what are the chances that a bunch
of matter just collapses into a point
perfectly with no rotation it's just not
going to happen
and then additional matter falling into
the black hole contributes its angular
momentum so like a figure skater pulling
their arms into a point object you can
imagine black holes get spinning pretty
fast but spin is harder to measure
because unlike mass it only affects
objects relatively close to the black
hole but there is a way to do it
actually three ways to understand all of
them you have to understand isco
in newtonian physics around a compact
mass you can place an object in a
circular orbit at any radius and it will
be stable it doesn't matter how close
you get this is not the case according
to general relativity here there is an
innermost stable circular orbit with a
radius known as r isco
closer than this and no orbits are
stable they all fall into the black hole
so when you're looking at a black hole
that is feeding the innermost edge of
the accretion disk is at our isco
what's useful for our purposes is that
rsco depends on the spin of the black
hole the faster it's spinning the
smaller rsco becomes assuming it's
spinning in the same direction as the
matter in the accretion disk
the rotation enables particles to orbit
closer to the black hole than they'd be
able to for a non-spinning black hole so
you can kind of think of it as though
the spin is supporting the particles
against the relentless pull of gravity
now spin is normally discussed in terms
of a dimensionless parameter that ranges
from zero no spin to one maximum spin
though i guess you could also have spins
down to negative one if the black hole
is spinning in the opposite direction
from the accretion disk
now as spin increases our isco decreases
by a factor of 6 shrinking down to the
size of the event horizon and this sets
what many scientists think is the
maximum spin a black hole can have
because if the minimum stable orbit were
the size of the event horizon then light
could escape from the black hole
allowing us to see into the singularity
this is called a naked singularity and
it makes a lot of scientists
uncomfortable as yet there isn't a
strong theoretical reason why a black
hole can't exceed this maximum spin it's
just that we haven't seen one and the
thought of an exposed singularity just
kind of feels wrong
most suspect the maximum real-world spin
parameter is around 0.998
so how can you use rsco to measure the
spin of a black hole well first let's
think about how we measure the size of
anything far away from us in deep space
like the radius of a star most stars are
so far away that they're simply point
objects in our telescopes so how can you
figure out their radii well first look
at the spectrum of their light by seeing
how red shifted absorption lines are you
can determine how far away the star is
the spectrum also tells you the
temperature of the star because it
should approximate a black body curve
and now the power radiated per unit area
of the star is strongly dependent on its
temperature so if you know how bright
the star appears from earth how far away
it is and how much power it's radiating
per unit area well then you can work out
its area and hence its radius
you can actually do something very
similar for a black hole's accretion
disk just instead of estimating the
radius of a glowing sphere you're
estimating the radius of the dark circle
rsco in the middle of the glowing
accretion disk then you can use rsco to
find the spin parameter this has been
done for a number of black holes
revealing spin parameters from around
0.1 up to close to the maximum but this
method only works if the radiation from
the black hole is dominated by black
body radiation from the accretion disk
which often it's not another approach
involves looking at x-rays emitted by
iron around a black hole some black
holes show a distinct iron emission line
but instead of the single frequency
you'd expect the line is broadened by
factors like the doppler shift due to
the high velocity of the iron in the
accretion disk and gravitational
redshift due to the extreme
gravitational fields close to the black
hole by looking at the low energy limit
of the iron emission line you can
determine how close to the black hole it
was emitted and hence our isco
but what if there is no bright iron
emission line well luckily there is a
third way and that is to look for
periodic oscillations in the data like
the repeated x-rays observed every 131
seconds the thinking is these cycles
must be caused by clumps of matter
orbiting the black hole
and at frequencies that high they must
be orbiting very close in probably near
our isco even that close they'd be going
half the speed of light but what kind of
clumps or objects would these be well
the study's authors argue the best
candidate involves an unlikely scenario
years before the title disruption event
they proposed there was a white dwarf
star in orbit around this black hole now
it might be stable orbiting in this way
for perhaps one or two hundred years by
itself it wouldn't be visible from earth
but then the other star wandered by and
was ripped apart in the tidal disruption
event its mass fell in toward the black
hole forming an accretion disk with the
addition of this stellar debris the
white dwarf was cloaked in glowing
matter creating an x-ray hotspot
orbiting the black hole and its period
would directly relate to the spin of the
black hole
in this case the measured spin parameter
turned out to be at least 0.7 and
possibly as high as the theoretical
maximum of 0.998 meaning objects in the
accretion disk were going at least half
the speed of light this is the first
measurement of spin made possible by a
tidal disruption event the implication
is that this could provide a method for
determining the spin of black holes
particularly ones that have been dormant
which is about 95 percent of
supermassive black holes if they shred a
star we get insight into their spin
now why is this important well because
it helps us understand the origins of
black holes if supermassive black holes
grow in size mainly by feeding on a
steady stream of matter from within
their own galaxy you'd expect their
spins to be very large because the
angular momentum of that matter would be
more or less aligned so it would add up
over time but if instead supermassive
black holes grow predominantly by
merging with other large black holes you
might expect their spins to be lower
because the spins of two black holes are
likely to be randomly oriented rather
than aligned as we are able to measure
the spins of more black holes in
different ways farther out and therefore
further back in time we should be able
to better understand their growth and
since supermassive black holes lie at
the centers of most galaxies they also
lie at the center of an understanding of
how those galaxies have formed and
evolved over billions of years
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