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

SPATIUMINTERNATIONALSPACESCIENCEINSTITUTE

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Editorial

Front CoverThis 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)

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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 licker-ing 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 sud-denly 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 neb-ula similar to those you may know from the skyrockets on Swiss Na-tional Holiday. That is what phys-ics 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 grav-ity 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 rip-ple in the fabric of space-time that

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 Uni-versity 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 vio-lent stuff, you will gladly lean back, happy to live in your quiet corner of the Universe!

We are thankful to Professor Cour-voisier for the kind support he granted in publishing the current issue of Spatium, which summa-rizes his lecture for the Pro ISSI as-sociation in March 2014.

Hansjörg SchlaepferBrissago, May 2016

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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 experimen-tal 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 man-ager Riccardo Giacconi3 (Fig. 1) the Nobel Prize in Physics forty years later.

Unfortunately for astrophysicists, fortunately, however, for all other beings, the Earth’s atmosphere ab-sorbs x-rays. Therefore, to probe the x-ray sky, sensors must be placed high enough beyond the atmos-phere. This was known to Giac-coni. Yet, he could not know that his instrument’s sensitivity was in-sufficient for registering the Moon’s weak x-ray radiation he was head-ing 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 dis-coveries opened an entirely new window to the Universe to astronomy.

Traditional Earth-bound telescopes portray the sky in the waveband of visible light, for which the atmos-phere is transparent. This radiation comes mainly from objects in the temperature range of a few thou-sand 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 astrophys-ics is so important for understand-ing the most powerful processes oc-curring in the Universe and other events that are at the origin of in-tense non-thermal phenomena.

Unsurprisingly, astrophysicists grasped the potential of this new window quickly by commission-ing an entire f lotilla of spacecraft of ever-increasing sophistication. Among the most successful mis-sions stands the European Space Agency’s International Gamma-Ray Astrophysics Laboratory (IN-TEGRAL). Launched in 2002, it continues to gather the most ener-getic 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 observ-able Universe. On the other hand, inside the borders of our small

country, the INTEGRAL Science Data Centre (ISDC) at Versoix plays a pivotal role when it comes to exploiting the science data col-lected by INTEGRAL. The sig-nals gathered by the spacecraft ar-rive at Versoix a mere six seconds later, where they are then pro-cessed, archived and distributed for the benefit of the international sci-ence community.

Altogether, high-energy astro-physics has evolved to become a staggering and prolific research field to which the current issue of Spatium intends to guide our readers.

The Violent Universe1

by Professor Thierry Courvoisier, University of Geneva and INTEGRAL Science Data Centre, Versoix, Switzerland.

Fig. 1: Riccardo Giacconi won the Nobel Prize in Physics in 2002 for pio-neering contributions to astrophysics, which have led to the discovery of cos-mic x-ray sources.

1 This text is based on a lecture by Prof. Thierry Courvoisier for the Pro ISSI audience in March 2014 as well as on sev-eral 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.

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Introduction

Virtually all types of mass-rich compact cosmic objects are signif-icant sources of high-energy emis-sion 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 our-selves 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 in-terstellar 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 sta-ble in the long run; rather, inter-nal 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 sin-gle protostar, but each different knot produces an individual pro-tostar. This is why these nebulae are often referred to as stellar nurs-eries, the places where myriads of stars are born.

The protostar attracts matter from its environment thereby gaining further mass. Its core gets increas-ingly 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 de-pends on the star’s size: the larger the mass, the faster it consumes the hydrogen supplies. Our Sun for ex-ample 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 attrac-tion and radiation pressure: its in-ner layers collapse thereby squeez-ing 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, Ve-nus, 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. Mod-els 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 evolu-tion 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 con-trast, 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)

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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 bil-lion 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 me-dium-sized stars: upon consump-tion of its hydrogen supply, the star’s outer layers swell out into a giant star, but even bigger, form-ing a Red Supergiant6 while the core will shrink becoming very hot and dense. Then, fusion of he-lium 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 con-sists mostly of iron nuclei.

Once again, the star’s future evo-lution depends on its initial mass: a star with seven to twenty solar mass equivalents ends up as a Neutron7 Star, while stars even more massive collapse into a Black Hole8. A Neu-tron 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 sec-onds or even fractions thereof. Such a speedy Neutron Star may become a Pulsar emitting radiation observ-able as short pulses of electromag-netic 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 compan-ions. 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 exam-ple, may contain 400 billion stars. This fantastic number leads us to a further group of high-energy ra-diation sources. There is strong evidence that Black Holes exist at the centres of all, even small gal-axies; some of these Black Holes are embedded in material that is attracted violently by their enor-mous gravity. These central re-gions are called Active Galactic Nuclei (AGN). Powered by the ac-cretion of mass by supermassive Black Holes at the centre of the host galaxy, these AGN emit elec-tromagnetic 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. Neverthe-less, 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 af-ter the Big Bang. Smaller galaxies produced star nurseries as well, but

5 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 expand-ing, 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.

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10 See Spatium no. 2: Birth, Age and the Future of the Universe by G. A. Tammann, May 1999.

Fig. 3: The lifecycle of a star. The life 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 grav-itational forces that in turn collect more

matter from the environment. The re-sult 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 af-ter 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)

generally later and smaller, peak-ing 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 sur-vivors will be the longest-lived stars, the Red Dwarfs.

This is the menagerie of compact cosmic objects, which we are go-ing now to examine in some more detail.

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Cosmic Sources of High-Energy Radiation

Neutron Stars

Early astrophysicists such as Wal-ter Baade11 and Fritz Zwicky12 sug-gested 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 dur-ing supernova explosions consist-

ing of extremely closely packed neutrons. They called these hypo-thetical stars Neutron Stars. Today, we know that Neutron Stars evolve from the collapse of massive stars following a supernova explo-sion. 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

Sun. Neutron Stars are supported against further collapse by the phe-nomenon described by the Pauli13 Exclusion Principle. This concept stipulates that no two neutrons can occupy the same place and quan-tum, 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 com-pares with the snail’s pace reached under the same conditions on Earth of about 4.5 m/s.

Neutron Stars are very hot: mod-els suggest initial temperatures in-side a newly formed Neutron Star of 1012 K. At this stage, it will be

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 ve-locity of about 10 % of the speed of light. Some Neutron Stars also emit beams of electromagnetic ra-diation as Pulsars. Neutron Stars can only be easily detected in cer-tain instances, such as if they are a Pulsar or a part of a binary sys-tem. Non-rotating and non-ac-creting Neutron Stars are virtually undetectable.

11 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.

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)

Neutron Stars are the remains of collapsed massive stars.

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Pulsars

The first pulsating radio star (Pul-sar) was found by chance by Joce-lyn Bell Burnell15 and Antony Hewish16 in 1967 when they ob-served sequences of strange regu-lar 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 radi-ation, such as stars. Further, since the pulses followed sidereal time17, it could not be a man-made radio frequency interference. Later ob-servations with other telescopes

confirmed the emission from the same spot in the sky, which elim-inated any sort of instrumental ef-fects. Another hypothesis inter-preted 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 appear-ance, they quickly abandoned the LGM hypothesis and continued looking for further explanations …

Today, Pulsars are interpreted as a rare subcategory of Neutron Stars, as most Neutron Stars do not emit

radio waves. Their signals have the form of short radio waves with very precise intervals in between that may last from a few millisec-onds to seconds. Appropriate elec-tronics can convert the radio emis-sions of Pulsars into audible signals producing a thrilling sign of life of an entity billions of light-years away. Our readers are warmly en-couraged to enjoy some of the lat-est Pulsar hits by downloading from http://www.radiosky.com/rspplsr.html.

15 Susan Jocelyn Bell Burnell, 1943, Lurgan, Northern Ireland, Northern Irish astrophysicist.16 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.

Pulsars are rapidly spinning Neutron Stars.

Fig. 5: The principle of a Pulsar. A lighthouse models the principle of a pul-sar: the lighthouse to the left emits a nar-row beam of light in a horizontal plane. This light can be seen from afar in the moments when the beam is exactly di-rected 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 moments that the ra-dio beam directs exactly towards Earth.

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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 theoreti-cal and observational work pro-vides the foundation of modern Bi-nary 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 electro-magnetic 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 ex-tremely 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 several seconds up to several minutes.

18 Sir Frederick William Herschel, 1738, Hannover, Germany – 1822, Slough, Berkshire, Britain, German-born British astronomer and composer.

Fig. 6: A supernova in the southerly constellation of Lupus. The expand-ing cloud originates from a stellar ex-plosion in the year 1006 AD some 7,000 light-years away producing a cosmic light show across the entire electromag-netic spectrum. This image combines x-ray data in blue, optical data in yel-lowish 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 sys-tem, the dwarf gradually captured ma-terial 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)

Binary Stars are pairs of stars orbiting each other.

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Black Holes

The concept of Black Holes is as-tonishingly old: it was in 1783, when John Michell19, one of the most brilliant and original scien-tists of his time and virtually for-gotten today, stipulated the exist-ence 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

a massless wave such as light. This changed only when Albert Ein-stein developed his theory of Gen-eral Relativity in 1915. That gave the Black Holes a theoretical justi-fication even though the same Al-bert Einstein rated them a theoret-ical 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 in-duced 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 fea-ture the size of a grape.

19 John Michell, 1724, Eakring, Nottinghamshire, UK – 1793, Thornhill, Yorkshire, English clergyman and natural philosopher.

Fig. 7: An artist’s interpretation of Black Hole Cygnus X-1 feeding on the Blue Giant companion star’s matter. Gravity accelerates the gas to tremen-

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)

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Nevertheless, astronomers would like to get hold of them. This in-spired the Russian physicist Yakov Zel’dovich20 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 temper-atures, which in turn would emit x-rays, see Fig. 7. Armed with this insight, astronomers started look-ing for a binary star with the re-quired properties and found it in-deed with Cygnus X-1. It consists of a Blue Giant along with an in-visible 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 compan-ion is really a Black Hole, at a con-vincing 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 so-lar 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 reg-ulating each other’s growth.The lower size limit of Black Holes is the object of ongoing scientific debates. While there is consensus

regarding the minimal mass re-quired for a star to collapse into a Black Hole, some hypotheses pre-dict that micro Black Holes could exist as well and could form at en-ergy levels available in modern particle accelerators. This prompted concerns about the Large Hadron Collider (LHC) at CERN21 be-cause of fears that it could gener-ate micro Black Holes with un-known 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 sys-tem orbiting each other. After bil-lions of years, these Black Holes may eventually merge into one sin-gle Black Hole. During the final

fraction of a second before fully fusing together, they emit power-ful 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 signif-icant part of their mass is converted into energy in the form of gravita-tional waves according to Ein-stein’s famous law E=mc2. It was also Albert Einstein who predicted

the existence of gravitational waves in 1915 in the frame of General Relativity. This miraculous vibra-tion of the space-time fabric had to wait a hundred years for its obser-vational confirmation: in August 2015, scientists at the LIGO Ob-servatory22 witnessed the first re-cord of gravitational waves passing through our planet. They origi-nated 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 con-verted into the energy carried away by gravitational waves.

Black Holes are the remains of collapsed extremely massive stars.

20 Yakov Borisovich Zel’dovich, 1914, Minsk – 1987, Moscow, Russian physicist.21 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.

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Quasars

Quasars are the most distant and most energetic members of a class of objects called Active Galactic Nuclei (AGN), Fig. 8. Hypotheti-cally, 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 bi-nary 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 discern-ible in the optical waveband: as-tronomers 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 emis-sion lines. This weird finding de-fied 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. Dis-covered in the early 1960s, it was the first Quasar ever identified (top table, Credit: ESA/NASA.). Quasars are the enor-mously violent centres of distant, active galaxies, powered by a huge disc of par-ticles 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, Cam-bridge, UK; C.D. Mackay, IoA, Cam-bridge, UK; A.E. Wright, ATNF, Parkes, Australia)

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and the initial claim of an ex-tremely large redshift was not gen-erally 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 ex-pansion 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 vari-able with time scales ranging from hours to months. This in turn means that Quasars generate and emit their energy from an amaz-ingly small region, more specifi-cally 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

of power from a relatively small re-gion requires a power source far more efficient than the nuclear fu-sion process that powers ordinary stars. The release of gravitational energy by matter falling towards a massive Black Hole is the only pro-cess known to produce such high power levels continuously.

Fig. 9: Final greetings from a dead Quasar. This image shows green fila-ments around the galaxy 2MASX J22014163+1151237 illuminated by a fi-nal blast of radiation from the Quasar in the galaxy’s centre. Ionized oxygen in

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

in the meantime, the green clouds will continue to glow for much longer be-fore they too will fade away. (Credit: NASA, ESA, W. Keel, University of Al-abama, USA)

Quasars are compact regions in the centre of galaxies surrounding a supermassive Black Hole.

23 Maarten Schmidt, 1929, Groningen, Dutch astronomer.

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SPATIUM 37 16

Gamma Ray Bursts

Gamma Ray Bursts (GRBs) are extremely energetic short f lashes of gamma rays from galaxies bil-lions 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 electromag-netic 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 super-nova 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 dis-tant 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 IN-TEGRAL satellite observing the entire sky comes across GRBs at a rate of about one per day. Fortu-nately, all GRBs observed so far originated beyond the Milky Way:

the energy of a Gamma Ray Burst produced within our home galaxy

could probably cause a mass ex-tinction 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 dis covery, 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 genera-tions of stars announcing the birth of an early Black Hole. (Credit: Gemini Obser-vatory/NSF/AURA, D. Fox & A. Cucchiara (Penn State U.) and E. Berger, Har-vard Univ.)

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Cosmic Background

We owe the father of x-ray astron-omy, 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 radi-ation known in different regions of the electromagnetic spectrum, each disclosing the tale of an im-portant part of the history of the Universe, Fig. 11.

X-ray background radiation: The x-ray background results from a combination of many yet unre-solved x-ray sources outside the Milky Way. With increasing reso-lution of x-ray space telescopes, sci-entists can also collocate the sources with objects discernible in the vis-ible range, which are mostly Ac-tive Galactic Nuclei. This radiation is therefore generated by matter falling into supermassive Black Holes at the edge of the observa-ble Universe. The diffuse extraga-lactic x-ray background is hence the sum of individual faint sources.

Microwave background radiation. Discovered unintentionally in 1965 by Arnold Penzias23 and Rob-ert Wilson24 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 trans-parent to radiation25 for the first

time. The background light is the relict of the hot Big Bang; it car-ries the signature of the early his-tory 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 radi-ation: Photons emanating from the cosmic microwave background may interact with energetic elec-trons distributed between distant

Fig. 11: Cosmic Background Radiation spans over the entire electromagnetic spectrum. Starting with the green line in the radio waveband, the red line contin-ues in the infrared, visible and ultraviolet regions to reach the x-ray part of the spec-trum (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)

Cosmic Background radiation is electromagnetic radiation from no optically discernible source.

23 Arnold Allan Penzias, 1933, Munich, German physicist and astronomer, Nobel Prize Laureate in Physics, 1978.24 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.

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clusters of galaxies. Thereby, they receive an energetic boost result-ing 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 do-mains, 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 ra-diation 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-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

background, which can be seen outside the Moon’s disk. Some residual radia-tion stems from the Earth’s extended at-mosphere, which surrounds the orbit-ing 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: Max-Planck-Institut für extraterrestrische Physik, Garching, Germany)

26 Rashid Alievich Sunyaev, 1943, Tashkent, Russia, Russian astrophysicist.

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Outlook

More than half a century ago, Ric-cardo Giacconi opened the win-dow to the x-ray universe. High-energy astrophysics has made tremendous progress since provid-ing 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 de-fined by the European Space Agen-cy’s Cosmic Vision programme for the 2015–2025 period. This long-term 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 ad-dress 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 ob-jectives will require new missions such as for instance a new gamma-ray imaging observatory with an incredible focal length calling for

two separate spacecraft f lying in tandem to accommodate the build-ing 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 ex-pertise for precise formation-f ly-ing 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 Astro-physics, Springer Verlag, Berlin Heidelberg.

Fig. 13: Beauty meets Violence. Zeta Ophiuchi, a star about 20 times more massive than the Sun some 460 light-years away, produces an arcing interstel-lar bow shock seen in this infrared por-trait. The star rushes at a velocity of 24 km/s to the right. The interstellar me-dium 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 com-panion star was more massive and hence shorter lived. When the companion ex-ploded as a supernova, it ejected Zeta Ophiuchi out of the system. (Credit: NASA, JPL-Caltech)

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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. the-sis thesis at the University of Zu-rich on the transport of neutrinos in supernovae. Then, he moved to the European Space Operations Centre in Darm stadt as an EXO-SAT 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 astro-physics in 1999. During that pe-riod, 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 cul-minated with his Presidency of the Swiss Academy of Sciences from 2012 to 2015 giving him an excel-lent opportunity to foster scienti-fic reasoning in the political deci-sion-making process.

The Author

SPATIUM

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