Published by the Association Pro ISSI No. 37, May 2016

INTERNATIONAL
SPACE
SCIENCE
INSTITUTE
SPATIUM
Published by the Association Pro ISSI
No. 37, May 2016
Editorial
A quiet glance at the starry night
sky may give you the impression
that nothing is happening out
there: the stars are still at the very
same spot you saw them in your
early youth. Their light is f lickering a bit, yes, but you know, it is
the turbulent atmosphere that
causes the illusion. So, business as
usual in the heavens!
Beware! Your conclusion might be
a bit over-hasty. Rather read first
this Spatium to learn why. You are
blessed with living in an amazingly
clement corner of the Universe,
where things remain more or less
stable over periods that exceed by
far an ephemeral human’s life span.
The Universe has other measures
of time. Our Sun, for instance,
started shining 4.6 billion years
ago, and it will do so for a further
appeasing 5 billion years. But suddenly then, your calm corner will
turn into an apocalyptic scene: our
merciful daytime star will explode,
shed parts of its matter out to space
to form a beautiful planetary nebula similar to those you may know
from the skyrockets on Swiss National Holiday. That is what physics requires stars to do at the end
of their lives.
Yet, physics has much more in
stock. Take for example a black
hole. It is a kind of a handy star,
perhaps a mere 20 km across. Yet,
it is so densely packed and hence
possesses such a tremendous gravity that nothing can escape, not
even a f limsy beam of light. The
hole is there, but you cannot see it.
And every now and then, two such
monsters collide and produce a ripple in the fabric of space-time that
SPATIUM 37
2
rushes right through the entire
Universe.
Curious to learn more? Okay, let
us enter the store of astrophysics to
rummage around in what they
have on offer! Follow Professor
Thierry Courvoisier from the University of Geneva and Head of the
INTEGRAL Science Data Centre
in Versoix: he will present you a
rich assortment of quasars, gamma
ray bursts and the like, and when
you feel familiar with all that violent stuff, you will gladly lean back,
happy to live in your quiet corner
of the Universe!
We are thankful to Professor Courvoisier for the kind support he
granted in publishing the current
issue of Spatium, which summarizes his lecture for the Pro ISSI association in March 2014.
Hansjörg Schlaepfer
Brissago, May 2016
Impressum
ISSN 2297-5888 (Print)
ISSN 2297-590X (Online)
SPATIUM
Published by the
Association Pro ISSI
Association Pro ISSI
Hallerstrasse 6, CH-3012 Bern
Phone +41 (0)31 631 48 96
see
www.issibern.ch/pro-issi.html
for the whole Spatium series
President
Prof. Adrian Jäggi,
University of Bern
Layout and Publisher
Dr. Hansjörg Schlaepfer
CH-6614 Brissago
Printing
Stämpf li AG
CH-3001 Bern
Front Cover
This is an artist’s rendering of Cygnus X-1, a black hole 10,000 light years
away from Earth. Its tremendous gravitational field pulls matter away
from its companion star. As the gas spirals towards the black hole, it heats
up and gives off x-rays and gamma rays. (Based on an ESA image)
The Violent Universe1
by Professor Thierry Courvoisier, University of Geneva and INTEGRAL Science Data Centre,
Versoix, Switzerland.
Prologue
The space age was just dawning
when a few scientists at the AS&E2
conducted an experiment that was
­going to change our view of the
Universe radically. On 12 June
1962, they launched an experimental sensor on a sounding rocket to
probe the Moon’s x-ray radiation.
Even though no lunar signal was
observed, incidental findings
turned out so revolutionary that
they earned the programme manager Riccardo Giacconi3 (Fig. 1) the
Nobel Prize in Physics forty years
later.
Traditional Earth-bound telescopes
portray the sky in the waveband of
visible light, for which the atmosphere is transparent. This radiation
comes mainly from objects in the
temperature range of a few thousand K showing galaxies, stars and
planets in their eternal quietness.
In contrast, x-rays and the even
more energetic gamma rays4 stem
from objects heated up to several
million K and therefore reveal
them in a phase of fervid turmoil.
This is why high energy astrophysics is so important for understanding the most powerful processes occurring in the Universe and other
events that are at the origin of intense non-thermal phenomena.
country, the INTEGRAL Science
Data Centre (ISDC) at Versoix
plays a pivotal role when it comes
to exploiting the science data collected by INTEGRAL. The signals gathered by the spacecraft arrive at Versoix a mere six seconds
later, where they are then processed, archived and distributed for
the benefit of the international science community.
Unfortunately for astrophysicists,
fortunately, however, for all other
beings, the Earth’s atmosphere absorbs x-rays. Therefore, to probe
the x-ray sky, sensors must be placed
high enough beyond the atmosphere. This was known to Giacconi. Yet, he could not know that
his instrument’s sensitivity was insufficient for registering the Moon’s
weak x-ray radiation he was heading for. It was, however, sensitive
enough to stumble across a bright
x-ray source in the constellation of
Scorpius and mysterious diffuse
background ­radiation. These discoveries opened an entirely new
window to the Universe to
astronomy.
Unsurprisingly,
astrophysicists
grasped the potential of this new
window quickly by commissioning an entire f lotilla of spacecraft
of ever-increasing sophistication.
Among the most successful missions stands the European Space
Agency’s International GammaRay Astrophysics Laboratory (INTEGRAL). Launched in 2002, it
continues to gather the most energetic radiation from deep space in
2016 telling us stories far beyond
any imagination. It offers scientists
insights in spectacular cosmic
events from within our Milky Way
out to the very edge of the observable Universe. On the other hand,
inside the borders of our small
Fig. 1: Riccardo Giacconi won the
Altogether, high-energy astrophysics has evolved to become a
staggering and prolific research
field to which the current issue of
Spatium intends to guide our
readers.
Nobel Prize in Physics in 2002 for pioneering contributions to astrophysics,
which have led to the discovery of cosmic x-ray sources.
This text is based on a lecture by Prof. Thierry Courvoisier for the Pro ISSI audience in March 2014 as well as on several of his publications. It was prepared by Dr. Hansjörg Schlaepfer and reviewed by Prof. Courvoisier.
2
American Science and Engineering, Inc., Billerica, Massachusetts, USA, a US manufacturer of x-ray equipment and
­related technologies.
3
Riccardo Giacconi, 1931, Genova, Italy, Italian astrophysicist, Nobel Prize laureate in physics in 2002.
4
Gamma rays represent the most energetic part of the electromagnetic spectrum.
1
SPATIUM 37
3
Introduction
Virtually all types of mass-rich
compact cosmic objects are significant sources of high-energy emission because of the enormous
strength of their gravitational
fields. Here, gravity can accelerate
particles to extreme velocities,
which then emit x-rays and gamma
rays. In order to familiarize ourselves with those compact objects,
we are now going to sketch the
processes that make them come
into being.
Stellar Evolution
Stars are born; stars live, stars die,
much like everything else in
nature.
Stars are born during the collapse
of giant nebulae that are large interstellar clouds of dust, hydrogen,
helium and other ionized gases,
(Fig. 2) . These clouds are really
huge, they may measure several
light years across. They are not stable in the long run; rather, internal turbulences cause knots to form
which then collapse under their
own gravitational attraction. As
the knots condense, the material at
their core heats up giving rise to a
protostar. The cloud as a whole
does not collapse into just one single protostar, but each different
knot produces an individual protostar. This is why these nebulae
are often referred to as stellar nurseries, the places where myriads of
stars are born.
SPATIUM 37
4
The protostar attracts matter from
its environment thereby gaining
further mass. Its core gets increasingly hotter and denser, due to the
growing strength of its gravity. At
some point, it is hot enough and
dense enough for hydrogen to start
fusing into helium. This nuclear
reaction releases huge amounts of
energy heating the core further up
to several million K. This makes
the protostar become a veritable
shining star.
Of the various chapters of a star’s
biography, hydrogen fusion is by
far the longest. Its duration depends on the star’s size: the larger
the mass, the faster it consumes the
hydrogen supplies. Our Sun for example is a relatively small star; its
initial hydrogen stock grants it a
lifetime of 10 billion years (of
which 4.6 billion are gone). In
contrast, a more massive star with
several times the mass of the Sun
has a life expectancy in the order
of some hundred million years
only. Anyway, when the hydrogen
is consumed, the star’s core can no
more balance gravitational attraction and radiation pressure: its inner layers collapse thereby squeezing the core, increasing its pressure
and temperature even more. While
the core collapses, the star’s outer
layers expand outward to a size
never reached before. This is a Red
Giant. At this point, our Sun will
become sufficiently large to engulf
the current orbits of Mercury, Venus, and possibly Earth as well.
What happens next depends on the
mass of the star.
Small Stars
A small star in the range of 0.05 …
1 solar mass equivalents reaches the
density and temperature required
to fuse hydrogen to helium. Models suggest that such a dwarf star’s
hydrogen stock may last for some
1012 years, which is considerably
longer than the actual age of the
Universe. This is why the evolution of such small stars is beyond
any observational reach.
Medium-sized Stars
A medium-size star holds between
one to seven solar mass equivalents.
Similar to a small star, its core starts
with hydrogen fusing; yet, in contrast, when the hydrogen stock is
gone, its larger core still has enough
heat and pressure to advance to a
second mode of nuclear fusion.
The helium produced in the first
fusion step now starts to fuse into
carbon. This gives the core a short
reprieve from further collapse.
Once the helium inventory is
spent, the star contracts, leaving
behind a small, hot and dense ball
called a White Dwarf. The shock
Fig. 2: Star nursery in the Great Nebula in Orion. Also known as M42, it is
one of the most famous nebulae in the sky, about 1,500 light-years away. In the star
forming region’s glowing gas clouds many hot young stars can be seen. Within
this well-studied stellar nursery, astronomers have identified what appear to be
numerous infant planetary systems. (Credit: Terry Hancock, Down Under
­
Observatory)
waves from the collapse shed the
star’s outer layers out to space,
forming a short, but showy cloud
of ionized gas called a ­Planetary
Nebula5. This will be the ultimate
fate of our Sun five to seven billion years ahead from now.
Massive Stars
A massive star with more than
seven times the mass of the Sun is
fated to end more spectacularly.
Such a high-mass star goes initially
through the same two steps as medium-sized stars: upon consumption of its hydrogen supply, the
star’s outer layers swell out into a
giant star, but even bigger, forming a Red Supergiant 6 while the
core will shrink becoming very
hot and dense. Then, fusion of helium into carbon begins just as in
the case of medium-sized stars.
When the supply of helium runs
out, the core contracts again, but
now in contrast to medium size
stars, it becomes hot and dense
enough to ignite a further series of
nuclear fusion processes gradually
reaching a state where its core consists mostly of iron nuclei.
Once again, the star’s future evolution depends on its initial mass: a
star with seven to twenty solar mass
equivalents ends up as a N
­ eutron7
Star, while stars even more massive
collapse into a Black Hole 8. A Neutron Star consists of an extremely
dense package of neutrons: it may
compress an entire star’s mass in a
volume measuring a mere 10 km
across. Even more striking is the
fact that Neutron Stars may rotate
very rapidly with periods of seconds or even fractions thereof. Such
a speedy Neutron Star may become
a Pulsar emitting radiation observable as short pulses of electromagnetic waves.
Every now and then two Neutron
Stars collide or a new Black Hole
forms. Such crashes produce short,
yet extremely energetic f lashes,
which are probably at the origin of
Gamma Ray Bursts.
If a protostar emerges in the stellar
nursery close enough to another
protostar, the pair may commence
circling one another becoming a
system of Binary Stars. In fact,
more than 50% of the stars in the
Universe may have stellar companions. In this respect, our lonely Sun
is a notable exception9.
Most stars, irrespective of their
size, belong to a galaxy. The Milky
Way, our home galaxy for example, may contain 400 billion stars.
This fantastic number leads us to a
further group of high-energy radiation sources. There is strong
­evidence that Black Holes exist at
the centres of all, even small galaxies; some of these Black Holes
are embedded in material that is
attracted violently by their enormous gravity. These central regions are called Active Galactic
­Nuclei (AGN). Powered by the accretion of mass by supermassive
Black Holes at the centre of the
host galaxy, these AGN emit electromagnetic radiation in many if
not all wavebands. Of those Active
Galactic Nuclei, Quasars are the
most energetic representatives: one
single Quasar may be as bright as
100 Milky Ways.
Stellar evolution started early in
the emerging Universe. Nevertheless, the great era of star formation
is now over. Galaxies evolved and
eventually merged to very large
galaxies with huge star formation
clouds producing numerous stars.
That was some 4 billion years after the Big Bang. Smaller galaxies
produced star nurseries as well, but
William Herschel (1738–1822) coined the term planetary nebula. When he saw these beautiful objects in his telescopes,
they resembled the rounded shapes of planets. Yet, planetary nebulae are not related to planets, rather, they are expanding, glowing shells of ionized gas ejected from old Red Giants.
6
Red Supergiants are large stars typically several hundred to over a thousand times the radius of the Sun with relatively
cool surfaces (below 4,100 K as compared to the Sun’s surface of 5,780 K).
7
Neutrons are subatomic particles with a specific mass but no electric charge. Together with protons, neutrons constitute
the nuclei of atoms.
8
See Spatium no. 28: How Black are Black Holes? By Maurizio Falanga, December 2011.
9
If, during solar system formation, planet Jupiter had managed to get much more mass, say fifty times more, its core
would have started fusing hydrogen like the Sun did and hence would have entered the league of stars and become a
stellar companion to our Sun.
5
SPATIUM 37
6
generally later and smaller, peaking at about 7 billion years after the
Big Bang. Now, at an age of 13.8
billion years, star formation goes
on at a relatively slow pace10. In the
distant future, star formation will
cease completely and the only survivors will be the longest-lived
stars, the Red Dwarfs.
This is the menagerie of compact
cosmic objects, which we are going now to examine in some more
detail.
Fig. 3: The lifecycle of a star. The life
matter from the environment. The result is a star with its planetary system.
Mass then determines the star’s future
fate: if it is below a certain threshold, it
will become a Sun-like star, which after some 10 billion years will turn into
a Red Giant. Upon consumption of its
helium inventory, it will shed part of its
matter out to space forming a planetary
nebula while the rest contracts to a
White Dwarf (cycle to the left). A more
massive star’s life expectancy is much
shorter; after some hundred million
years, it will become a Red Supergiant
and eventually explode as a supernova
ending up as a Neutron Star or even a
Black Hole (cycle to the right). (Credit:
NASA)
of all stars begins in large star-forming
nebulae, shown here in the centre. The
nebula’s dust and gas eventually collapse
under their own gravitational force
forming knots of higher matter density.
This initiates a runaway process whereby
the increasing mass leads to higher gravitational forces that in turn collect more
See Spatium no. 2: Birth, Age and the Future of the Universe by G. A. Tammann, May 1999.
10
SPATIUM 37
7
Cosmic Sources
of High-Energy
Radiation
Neutron Stars
Early astrophysicists such as Walter Baade11 and Fritz Zwicky12 suggested the existence of Neutron
Stars as early as 1934. In seeking an
explanation for the phenomenon
of supernovae, they proposed the
theory that ordinary stars would
evolve into compact objects during supernova explosions consist-
Sun. Neutron Stars are supported
against further collapse by the phenomenon described by the Pauli13
Exclusion Principle. This concept
stipulates that no two neutrons can
occupy the same place and quantum, posing the lower limit for the
packaging density. Such a densely
packed object generates of course
an unimaginably strong gravity
field: a hypothetical object released
one meter above a Neutron Star’s
surface would accelerate so fast that
it would hit the surface at a speed
of some 2,000 km/s! This compares with the snail’s pace reached
under the same conditions on
Earth of about 4.5 m/s.
Neutron Stars are the remains of
collapsed massive stars.
ing of extremely closely packed
neutrons. They called these hypothetical stars Neutron Stars.
Today, we know that Neutron Stars
evolve from the collapse of massive
stars following a supernova explosion. Neutron Stars are the densest
and smallest stars known to exist
in the Universe: with a radius in
the order of 10 km, they can have
a mass of about twice that of the
the source of a strong f lux of
­neutrinos14 and radiation in the
gamma- and x-ray range. This loss
of energy, however, causes the
Neutron Star to cool down rapidly
in the order of a few years.
Some Neutron Stars rotate very
rapidly, up to several hundred
times per second. This leads to
an incredible circumferential velocity of about 10 % of the speed of
light. Some Neutron Stars also
emit beams of electromagnetic radiation as Pulsars. Neutron Stars
can only be easily detected in certain instances, such as if they are
a Pulsar or a part of a binary system. Non-rotating and non-accreting Neutron Stars are virtually
undetectable.
Neutron Stars are very hot: models suggest initial temperatures inside a newly formed Neutron Star
of 1012 K. At this stage, it will be
Fig. 4: Chinese astronomers observed an outstandingly bright object in the con-
stellation of Taurus in 1054, which was visible even during daytime. They called
the newcomer a guest star. After two years, the guest disappeared. Today, we know
that it was a supernova leaving us the beautiful Crab nebula at a distance of 6,500
light-years. In its centre lies the Crab Pulsar, a Neutron Star 30 km across with the
incredible spin rate of 30 times per second. It emits pulses of radiation spanning a
spectrum from gamma rays to radio waves. (Credit: NASA, ESA)
Wilhelm Heinrich Walter Baade, 1893, Schröttinghausen, Germany – 1960, Göttingen, German astronomer and
astrophysicist.
12
Fritz Zwicky, 1898, Varna, Bulgaria – 1974, Pasadena, USA, Swiss astronomer.
13
Wolfgang Ernst Pauli, 1900, Vienna – 1958, Zurich, Austrian scientist and Nobel Prize Laureate in Physics 1945.
14
Neutrinos are fundamental particles which are similar to electrons but without an electric charge and an ­extremely small
mass.
11
SPATIUM 37
8
Pulsars
The first pulsating radio star (Pulsar) was found by chance by Jocelyn Bell Burnell15 and Antony
Hewish16 in 1967 when they observed sequences of strange regular radio pulses separated by 1.3 s.
No one would have expected such
signals to come from deep space:
the pulses’ short period eliminated
most astrophysical sources of radiation, such as stars. Further, since
the pulses followed sidereal time17,
it could not be a man-made radio
frequency interference. Later observations with other telescopes
confirmed the emission from the
same spot in the sky, which eliminated any sort of instrumental effects. Another hypothesis interpreted the signal as coming from a
distant civilization prompting Bell
and Hewish to call it LGM-1, for
“Little Green Men”. Yet, when in
a different part of the sky a second
pulsating source made its appearance, they quickly abandoned the
LGM hypothesis and continued
looking for further explanations …
radio waves. Their signals have the
form of short radio waves with
very precise intervals in between
that may last from a few milliseconds to seconds. Appropriate electronics can convert the radio emissions of Pulsars into audible signals
producing a thrilling sign of life of
an entity billions of light-years
away. Our readers are warmly encouraged to enjoy some of the latest Pulsar hits by downloading
from http://www.radiosky.com/
rspplsr.html.
Today, Pulsars are interpreted as a
rare subcategory of Neutron Stars,
as most Neutron Stars do not emit
Pulsars are rapidly spinning
­Neutron Stars.
Fig. 5: The principle of a Pulsar. A
lighthouse models the principle of a pulsar: the lighthouse to the left emits a narrow beam of light in a horizontal plane.
This light can be seen from afar in the
moments when the beam is exactly directed ­towards the observer. This gives
rise to seemingly short light pulses. The
Pulsar to the right emits a radio beam
along its magnetic axis, which rotates
around its spin axis. From Earth, this
signal can be observed as short radio
pulses during the m
­ oments that the radio beam directs exactly towards Earth.
Susan Jocelyn Bell Burnell, 1943, Lurgan, Northern Ireland, Northern Irish astrophysicist.
Antony Hewish, 1924, Fowey, Cornwall, British radio astronomer, Nobel Prize Laureate in Physics, 1974.
17
Sidereal time is a time scale based on the Earth’s rate of rotation measured relative to the fixed stars.
15
16
SPATIUM 37
10
Binary Stars
The term binary (star) was first
coined by Sir William Herschel18
in 1802. He discovered hundreds
of Binary Stars and even multiple
systems. His outstanding theoretical and observational work provides the foundation of modern Binary Star astronomy.
When two stars emerge in a star
nursery not too far apart, they can
form a binary system whereby each
star orbits a common barycentre
(Fig. 6) . Systems of two, three, four,
or even more stars (multiple star
systems) have been found. The
stars in such multiple systems share
a common fate. If they are in the
appropriate mass range, their life
will end in a supernova explosion.
As the more massive star reaches its
end earlier, the greater star will
blast first, producing a Neutron
Star (or a Black Hole). If the ex-
plosion does not kick the second
star away, the binary survives. The
Neutron Star will emit electromagnetic radiation powered by its
rotational speed, which thereby
decreases. On the other hand, it
may attract and accrete matter
from the second star by its extremely strong gravity field. If this
matter falls onto the Neutron Star,
it can spin it up again. This is an
interesting case of recycling as it
returns the Neutron Star to a fast
spinning mode again. The matter
falling on the Neutron Star piles
up on its surface, and when the
coating reaches a height of some
10 metres, it ignites a thermo­
nuclear explosion releasing huge
f lashes of x-rays lasting from s­ everal
seconds up to several minutes.
Binary Stars are pairs of stars
­orbiting each other.
Fig. 6: A supernova in the southerly
constellation of Lupus. The expand-
ing cloud originates from a stellar explosion in the year 1006 AD some 7,000
light-years away producing a cosmic
light show across the entire electromagnetic spectrum. This image combines
x-ray data in blue, optical data in yellowish hues, and radio image data in
red. The cloud is now 60 light-years
across and constitutes the remains of a
White Dwarf. Part of a Binary Star system, the dwarf gradually captured material from its companion star. The
build-up in mass finally triggered a
thermonuclear explosion that destroyed
the dwarf star. (Credit: ESA, NASA,
Zolt Levay, STScI)
Sir Frederick William Herschel, 1738, Hannover, Germany – 1822, Slough, Berkshire, Britain, German-born British
­a stronomer and composer.
18
SPATIUM 37
11
Black Holes
The concept of Black Holes is astonishingly old: it was in 1783,
when John Michell19, one of the
most brilliant and original scientists of his time and virtually forgotten today, stipulated the existence of cosmic objects so massive
that even light could not escape. Still,
science ignored Mitchell’s dark stars
for a long time, since it was not
clear how gravity could inf luence
Fig. 7: An artist’s interpretation of
Black Hole Cygnus X-1 feeding on the
Blue Giant companion star’s ­
m atter.
Gravity accelerates the gas to tremen-
a massless wave such as light. This
changed only when Albert Einstein developed his theory of General Relativity in 1915. That gave
the Black Holes a theoretical justification even though the same Albert Einstein rated them a theoretical oddity rather than a serious
reality some 25 years later.
Black Holes are certainly among
the most alluring entities of which
the Universe is so incredibly rich.
Indeed, the reality of Black Holes
exceeds any imagination; this induced US-American physicist Kip
S. Thorne to hallmark them the
brightest objects in the Universe that
emit no light. This lack of light
makes them invisible, and they are
not just black, but also tiny, some
tens of kilometres across holding
many solar mass equivalents. Earth,
for instance, squeezed enough to
become a Black Hole, would feature the size of a grape.
dous velocities causing it to emit x-rays
and gamma rays that reveal the presence
of the Black Hole at left. In addition, this
Black Hole emits jets along its rotation
axis with nearly the speed of light. The
underlying mechanism remains one of
the great mysteries of ­modern physics.
(Credit: NASA/CXC/M. Weiss)
John Michell, 1724, Eakring, Nottinghamshire, UK – 1793, Thornhill, Yorkshire, English clergyman and natural
philosopher.
19
SPATIUM 37
12
Nevertheless, astronomers would
like to get hold of them. This inspired the Russian physicist Yakov
Zel’dovich 20 to suggest an indirect
way of observation: if a Black Hole
would orbit a visible companion
star in a binary system, the visible
star could betray the Black Hole’s
presence by the Doppler effect on
its radiation due to the periodic
variation of its speed relative to
Earth. Making a further step, he
argued that a Black Hole might
heat up the surrounding in-falling
matter to extremely high temperatures, which in turn would emit
x-rays, see Fig. 7. Armed with this
insight, astronomers started looking for a binary star with the required properties and found it indeed with Cygnus X-1. It consists
of a Blue Giant along with an invisible companion that is bright in
the x-rays. That was in 1978, and
in the meantime, after many more
observations, astronomers rate the
probability that the dark companion is really a Black Hole, at a convincing 95%.
The key feature of Black Holes is
their mass. They come in a great
variety of sizes ranging from a few
times the solar mass (stellar mass
Black Holes) up to millions of solar masses (supermassive Black
Holes). The latter are found in the
centre of galaxies, where they can
be very bright (Active Galactic
Nuclei) or dormant as is the case
with the Black Hole in the centre
of our galaxy. Indeed, most Black
Holes in the recent Universe are
quiescent, because there is simply
no longer enough matter falling
into them. It is assumed that some
unknown mechanism links the
formation of the galaxy to that of
its Black Hole and vice versa regulating each other’s growth.
The lower size limit of Black Holes
is the object of ongoing scientific
debates. While there is consensus
fraction of a second before fully
fusing together, they emit powerful gravitational waves. The merger
product is again a massive Black
Hole, whose mass, however, is less
than the sum of the masses of the
individual Black Holes as a significant part of their mass is converted
into energy in the form of gravitational waves according to Einstein’s famous law E=mc2. It was
also Albert Einstein who predicted
Black Holes are the remains of
­collapsed extremely massive stars.
regarding the minimal mass required for a star to collapse into a
Black Hole, some hypotheses predict that micro Black Holes could
exist as well and could form at energy levels available in modern
particle accelerators. This prompted
concerns about the Large Hadron
Collider (LHC) at CERN21 because of fears that it could generate micro Black Holes with unknown consequences for Earth.
The LHC has been operational
since 2008 and no micro Black
Hole has made an appearance so
far.
Like any celestial body, two Black
Holes may constitute a binary system orbiting each other. After billions of years, these Black Holes
may eventually merge into one single Black Hole. During the final
the existence of gravitational waves
in 1915 in the frame of General
Relativity. This miraculous vibration of the space-time fabric had to
wait a hundred years for its observational confirmation: in August
2015, scientists at the LIGO Observatory22 witnessed the first record of gravitational waves passing
through our planet. They originated from the merger of two
Black Holes some 1.3 billion years
ago. One of them had 29 times the
solar mass while the other was even
more massive with 36 solar mass
equivalents. The resulting Black
Hole now has a mass of 62 solar
mass units while, within a mere
0.25 s, three solar masses were converted into the energy carried away
by gravitational waves.
Yakov Borisovich Zel’dovich, 1914, Minsk – 1987, Moscow, Russian physicist.
European Organization for Nuclear Research, Meyrin, Switzerland.
22
LIGO stands for the Laser Interferometry Gravitational Wave Observatory. It is a twin system operating in Hanford,
Washington and in the 3,000 km distant Livingston, Louisiana, USA.
20
21
SPATIUM 37
13
Quasars
Quasars are the most distant and
most energetic members of a class
of objects called Active Galactic
Nuclei (AGN), Fig. 8. Hypothetically, they formed approximately
12 billion years ago when the first
galaxies collided and their central
Black Holes merged to form either
a supermassive Black Hole or a binary Black Hole system.
The first Quasar was discovered in
the late 1950s as a radio source
lacking any corresponding visible
counterpart. It was only in 1963
when such a radio source could be
collocated with an object discernible in the optical waveband: astronomers detected a faint blue star
at the very location of the radio
source. Yet, analysing the blue
star’s emission spectrum brought
up a great surprise: it contained
many so-far unknown broad emission lines. This weird finding defied interpretation for many years
Fig. 8: Portraits of Quasar 3C 273.
Its light has taken some 2.5 billion years
to reach us. Despite this great distance,
it is still one of the closest Quasars. Discovered in the early 1960s, it was the first
Quasar ever identified (top table, Credit:
ESA/NASA.). Quasars are the enormously violent centres of distant, active
galaxies, powered by a huge disc of particles surrounding a supermassive Black
Hole. As material from this disc spirals
inwards, this Quasar fires off super-fast
jets of matter into surrounding space.
One of these jets appears as a cloudy
streak, measuring a staggering 200,000
light-years in length (bottom table,
Credit: R.C. Thomson, IoA, Cambridge, UK; C.D. Mackay, IoA, Cambridge, UK; A.E. Wright, ATNF,
Parkes, Australia)
SPATIUM 37
14
and the initial claim of an extremely large redshift was not generally accepted. Later, it could be
shown by Maarten Schmidt23 that
this spectrum originates indeed
from hydrogen redshifted by a
breath-taking receding speed of
47,000 km/s, which, due to the expansion of the Universe, betrays an
incredibly great distance. Other
Quasars came up with even higher
speed making them objects at the
edge of the observable Universe.
Because of their enormous distance
and the finite velocity of light, we
see them today as they existed in
the very early Universe.
The luminosity of Quasars is variable with time scales ranging from
hours to months. This in turn
means that Quasars generate and
emit their energy from an amazingly small region, more specifically a Quasar varying on a time
scale of a few weeks cannot be
larger than a few light-weeks across.
The emission of enormous amounts
Fig. 9: Final greetings from a dead
Quasar. This image shows green fila-
the filaments glows brightly at green
wavelengths. These structures are so far
away from the galaxy’s centre that light
travelled tens of thousands of years to
reach the filaments and light them up.
Even though the Quasar has turned off
ments around the galaxy 2MASX
J22014163+1151237 illuminated by a final blast of radiation from the Quasar in
the galaxy’s centre. Ionized oxygen in
of power from a relatively small region requires a power source far
more efficient than the nuclear fusion process that powers ordinary
stars. The release of gravitational
energy by matter falling towards a
massive Black Hole is the only process known to produce such high
power levels continuously.
Quasars are compact regions in
the centre of galaxies surrounding
a supermassive Black Hole.
in the meantime, the green clouds will
continue to glow for much longer before they too will fade away. (Credit:
NASA, ESA, W. Keel, University of Alabama, USA)
Maarten Schmidt, 1929, Groningen, Dutch astronomer.
23
SPATIUM 37
15
Gamma Ray Bursts
Gamma Ray Bursts (GRBs) are
extremely energetic short f lashes
of gamma rays from galaxies billions of light-years away (Fig. 10) .
Such a f lash of a few seconds may
contain the same amount of energy
as the Sun releases over its entire
10-billion-year lifetime. GRBs are
hence the brightest electromagnetic events in the Universe. While
the shortest bursts are thought to
come from colliding Neutron
Stars, the longer bursts (up to 100s
seconds) may result from supernova explosions of the very first
generation of supermassive stars in
the Universe.
Gamma Ray Bursts were detected
incidentally in 1967 by US VELA
intelligence satellites that aimed at
revealing secret nuclear weapons
tests. During the following years,
the science community was busy
with putting forward theoretical
models to explain GRBs. Yet, they
kept their secrets until 1997, when
a GRB could be collocated with an
object in the x-ray band and then in
the optical waveband. This allowed
an estimation of their redshift to be
made and hence their distance and
energy outputs. These observations
placed them among the most distant observable galaxies.
On the other hand, such f lashes are
extremely rare: scientists estimate
their frequency at a few f lashes per
galaxy per million years. ESA’s INTEGRAL satellite observing the
entire sky comes across GRBs at a
rate of about one per day. Fortunately, all GRBs observed so far
originated beyond the Milky Way:
SPATIUM 37
16
the energy of a Gamma Ray Burst
produced within our home galaxy
could probably cause a mass extinction event on our planet.
Gamma Ray Bursts are flashes
of gamma rays from very distant
galaxies.
Fig. 10: Farther than any known galaxy, the Gamma Ray Burst GRB 090423
recorded in April 2009 had a redshift of 8.2 indicating its occurrence at a time,
when the Universe had a mere 4% of its present age. A few minutes after dis­covery,
large ground telescopes registered its faint infrared afterglow (within the circle).
An exciting possibility is that this GRB happened in one of the very first generations of stars announcing the birth of an early Black Hole. (Credit: Gemini Observatory/NSF/AURA, D. Fox & A. Cucchiara (Penn State U.) and E. Berger, Harvard Univ.)
Cosmic Background
We owe the father of x-ray astronomy, Riccardo Giacconi, not only
the discovery of a bright x-ray
source, but also the finding of a
diffuse x-ray component coming
from all parts of the sky. This adds
to other types of background radiation known in different regions of
the electromagnetic spectrum,
each disclosing the tale of an important part of the history of the
Universe, Fig. 11.
X-ray background radiation: The xray background results from a
combination of many yet unresolved x-ray sources outside the
Milky Way. With increasing resolution of x-ray space telescopes, scientists can also collocate the sources
with objects discernible in the visible range, which are mostly Active Galactic Nuclei. This radiation
is therefore generated by matter
falling into supermassive Black
Holes at the edge of the observable Universe. The diffuse extragalactic x-ray background is hence
the sum of individual faint sources.
time. The background light is the
relict of the hot Big Bang; it carries the signature of the early history of the Universe. Due to the
expansion of the Universe, we see
it strongly redshifted appearing
now in the microwave region. This
radiation is considered to be one of
the major confirmations of the Big
Bang theory.
Distorted microwave background radiation: Photons emanating from the
cosmic microwave background
may interact with energetic electrons distributed between distant
Cosmic Background radiation is
electromagnetic radiation from no
optically discernible source.
Fig. 11: Cosmic Background Radiation spans over the entire electromagnetic
spectrum. Starting with the green line in the radio waveband, the red line continues in the infrared, visible and ultraviolet regions to reach the x-ray part of the spectrum (blue line) discovered by R. Giacconi in 1962. The graph ends with the grey
line in the gamma ray waveband. (Credit: R. Gilli, Bologna Astronomical
Observatory)
Microwave background radiation.
Discovered unintentionally in
1965 by Arnold Penzias23 and Robert Wilson 24 this type of radiation
comes from photons produced by
the Big Bang 14 billion years ago,
that have streamed from an epoch
when the Universe became transparent to radiation 25 for the first
Arnold Allan Penzias, 1933, Munich, German physicist and astronomer, Nobel Prize Laureate in Physics, 1978.
Robert Woodrow Wilson, 1936, Houston, Texas, US-American physicist, Nobel Prize Laureate in Physics.
25
See Spatium no. 1: Entstehung des Universums by Johannes Geiss, April 1998.
23
24
SPATIUM 37
17
clusters of galaxies. Thereby, they
receive an energetic boost resulting in a slight shift towards shorter
wavelengths (blue shift). This is
called the Sunyaev26 -Zel’dovich
effect, which is currently used to
detect extremely distant clusters of
galaxies.
Infrared/optical background radiation:
In the infrared and optical domains, the background radiation is
the superposition of the light of
faint galaxies. It summarizes the
cosmic history of starlight and its
components reprocessed by dust.
This light therefore emanates from
nuclear fusion reactions taking
place as stars evolve. Another
source of infrared background radiation is Quasars. Their x-ray
emission is partly absorbed by the
dust in the accretion disk and then
re-emitted in the infrared.
Fig. 12: The Moon in x-rays. The Ger-
background, which can be seen outside
the Moon’s disk. Some residual radiation stems from the Earth’s extended atmosphere, which surrounds the orbiting ROSAT observatory. The bright
hemisphere to the right shines in x-rays
because of interactions of the solar wind
with the Moon’s surface. (Credit: MaxPlanck-Institut für extraterrestrische
Physik, Garching, Germany)
man ROSAT Observatory gathered this
image of the Moon. Interestingly, its
dark side is darker than the background,
yet not completely dark. Obviously, the
Moon shields radiation coming from the
Rashid Alievich Sunyaev, 1943, Tashkent, Russia, Russian astrophysicist.
26
SPATIUM 37
18
Outlook
More than half a century ago, Riccardo Giacconi opened the window to the x-ray universe. Highenergy astrophysics has made
tremendous progress since providing surprising insights into the
emergence and the evolution of the
Universe. Yet, we are only at the
beginning. Many mysteries are still
unexplained and many questions
remain unanswered: How did the
Universe originate and what is it made
of? These topics establish one of the
four major strategic directions defined by the European Space Agency’s Cosmic Vision programme for
the 2015–2025 period. This longterm programme aims at defining
the basic programmatic objectives
to which the science community
is invited to respond and to allow
the industrial partners to prepare
the required technological skills.
Space technologies will continue
to evolve further and provide novel
means to build new space probes
with ever-increasing capabilities.
Based thereon, scientists can address new frontiers as for example,
observing the earliest structures in
the Universe. From here onwards,
the subsequent co-evolution of
galaxies and super-massive Black
Holes, and the accretion process of
matter falling into Black Holes can
be observed, the powerful source
of the most energetic radiation
reaching us from space. These objectives will require new missions
such as for instance a new gammaray imaging observatory with an
incredible focal length calling for
two separate spacecraft f lying in
tandem to accommodate the building blocks of the observatory: one
will hold the telescope system
while some 500 m behind a second
spacecraft will carry the detector
system.
Such complex missions require
new technologies, not least the expertise for precise formation-f lying spacecraft. And above all, a
new generation of scientists and
engineers will take over the helm
to advance our knowledge towards
the mesmerizing realm of the
unknown.
Recommended further reading:
T. Courvoisier: High Energy Astrophysics, Springer Verlag, Berlin
Heidelberg.
Fig. 13: Beauty meets Violence. Zeta
Ophiuchi, a star about 20 times more
massive than the Sun some 460 lightyears away, produces an arcing interstellar bow shock seen in this infrared portrait. The star rushes at a velocity of 24
km/s to the right. The interstellar medium compresses and heats its stellar
wind causing it to glow in a variety of
hues. Zeta Ophiuchi was likely once a
member of a binary star system, its companion star was more massive and hence
shorter lived. When the companion exploded as a supernova, it ejected Zeta
Ophiuchi out of the system. (Credit:
NASA, JPL-Caltech)
SPATIUM 37
19
SPATIUM
The Author
Thierry J.-L. Courvoisier con­
cluded his studies in theoretical
physics with a thesis on General
Relativity in 1977 at the Swiss
­Federal Institute of Technology in
Zurich (ETHZ) and a Ph. D. thesis thesis at the University of Zurich on the transport of neutrinos
in ­supernovae. Then, he moved to
the European Space Operations
Centre in Darm­stadt as an EXOSAT Duty Scientist and ­later to the
European Southern Observatory
in Garching, Germany. From 1988
onwards, he held several positions
at the University of Geneva, where
he became full professor in astrophysics in 1999. During that period, he served also as Principal
­Investigator and Director of the
INTEGRAL Science Data Centre
in Versoix, Switzerland. From July
2009 to June 2010 he was skipper
on his sailing boat Cérès for a
­cruise around the Atlantic Ocean
together with his wife Barbara.
T. Courvoisier is the author or
co-author of more than 400 papers.
The advisory career in numerous
international scientific boards culminated with his Presidency of the
Swiss Academy of Sciences from
2012 to 2015 giving him an excellent opportunity to foster scientific reasoning in the political decision-making process.