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INTERNATIONAL SPACE SCIENCE INSTITUTE SPATIUM Published by the Association Pro ISSI No. 28, December 2011 Editorial

Do you, dear reader, want to taste no hair as theoretical physicists love the exciting smell of theoretical to say. Though remain cautious: Impressum physics? Go ahead! This Spatium your expert instructs you that black ­offers you an opulent cocktail of holes are not quite black. Hence its ­exotic flavours. colour might be just as grey as that of your intimidated mouse. In any To begin with, imagine a . case, do not give up in your efforts SPATIUM It’s about one millionth of a mil- to search for black holes: there is Published by the lionth of a millimetre across. With one right in your front yard, as your Association Pro ISSI the small size of our proton, a baby expert says, right in the centre of brings some millions of our . That one is a bit fatter tons of on the balance. Now, than yours: it accounts for some if you find such an object say in the 10 million times the mass of our refrigerator of your kitchen, for . It helps our daytime find Association Pro ISSI heaven’s sake, do not touch it, don’t the right path around the galactic Hallerstrasse 6, CH-3012 Bern call the police either. Better call the centre, and to return to where it is Phone +41 (0)31 631 48 96 nearest theoretical physicist. He today within a short time, more pre- see will explain to you with tears in his cisely in a mere 230 million years www.issibern.ch/pro-issi.html eyes that you are the first to discover from now. for the whole Spatium series such a lovely baby black hole, some- thing once thought to have been If you are still - or now even more - President extinct for billions of years. Gener- keen on scenting the fragrances of Prof. Nicolas Thomas, ations of theoretical physics students theoretical physics turn the page University of Bern will remember your name linked to and start reading quickly. You can your seminal discovery. be sure: there is no baby black Layout and Publisher hole hidden there. Even better: Dr. Hansjörg Schlaepfer You then might want to inspect it you will find yourself an excellent CH-6614 Brissago closer. To this end, your theoretical escort: ­ISSI’s black hole expert, physics expert recommends that Dr. Maurizio Falanga, will accom- Printing you count the number of hairs on pany you, and take care of your Stämpfli Publikationen AG its otherwise bald head. If this count safety. CH-3001 Bern yields a bare zero: look out! Beyond any doubt, this is a veritable baby Now, it is up to us to thank him for black hole. Yet, as charming as it may making the present summary of his look, its hunger is insatiable: it de- lecture for the PRO ISSI Associa- vours everything, from your furni- tion on 11 November 2010 availa- ture to the flowers in your garden. ble, and to wish you a stimulating Not even your old-fashioned TV journey to the realm of theoretical set will be spared. Your wife will be physics. delighted: the moment has come to buy her a new model. Hansjörg Schlaepfer Brissago If, however, your inspection yields November 2011 a number of hairs greater than zero, it might simply be about an ordi- nary grey mouse seeking shelter from your cat since black holes have

SPATIUM 28 2 How Black Are Black Holes?1 Dr. Maurizio Falanga, International Space Science Institute, Bern

Introduction Gravitation ing to the story, prompted him to conceive the universal law of grav- itation (see Fig. 1). Whether or not Sir Isaac Newton actually sat under How black are black holes? This is What causes objects to fall down an apple tree while pondering the the question that the present issue to Earth? Why do the of is not known. The of SPATIUM would like to address the Sun? What holds to- fact, however, that all objects fall to- and answer. One of the most excit- gether? All these questions relate to ward Earth was empirically under- ing predictions of Einstein’s relativ- one fundamental aspect of physics: stood long before Newton. Galileo ity theory is the existence of black gravity. Galilei3 had demonstrated earlier holes. These are very compact ob- that all objects drop down to Earth jects, which are so dense and whose Sir Isaac Newton2 was first to pro- with the same acceleration, and this gravity is so strong that not even pose a mathematical model describ- acceleration is independent of the light can escape. They can, therefore, ing the gravitational attraction be- mass of the falling object. Sir Isaac only be detected indirectly through tween objects. A popular story tells Newton was familiar with this con- gravitational effects on their envi- how Sir Isaac Newton was sitting cept, of course, when he formulated ronment. After a historical review, under an apple tree, when an apple a broader and far-reaching new the- the theoretical predictions and the fell on his head. This event, accord- ory of gravitation. His universal law latest astronomical observations of black holes are discussed. Currently, hundreds of black holes are being observed; they are also strongly ­suspected to exist in the centre of galaxies, including the centre of our own .

Fig. 1: Sir Isaac Newton, the apple and gravity. (Credit: www.templatelite.com)

1 The present issue of Spatium reports on a lecture for the PRO ISSI Association given by the author on 11 November 2010. 2 Sir Isaac Newton, 1642, Woolsthorpe-by-Colsterworth, Lincolnshire, United Kingdom – 1727, Kensington, United King- dom, founder of classical mechanics. 3 Galileo Galilei, 1564, Pisa, Italy – 1642, Arcetri, Florence, Italian philosopher, mathematician, physicist and astronomer.

SPATIUM 28 3 of gravitation encompasses not only escape Earth’s gravity. In contrast, life. This, however, changes dramat- the behaviour of an apple near the from the Moon’s surface 2.3 km/s ically in the neighbourhood of large Earth’s surface, but also the motions (8,300 km/h) would be required, as such as the Sun or huge , of much larger bodies far away. The its gravity is far lower than the and even more so in the vicinity of essential feature of his theory is that Earth’s, and from the Sun 600 km/s black holes where gravity becomes the force of gravity between two (2,160,000 km/h) would be needed. so strong that nothing, not even objects is inversely proportional to light, can escape. For centuries, sci- the square of the distance between entists have speculated about such them. It is universal because all ob- strange worlds, but only the last say jects in the universe are attracted to Black Holes forty years have brought clear evi- all other objects according to this dence about their existence. relationship. Admittedly, it is a long way from Sir Gravity is a universal force ruling Isaac Newton’s apple falling on his Conceptual Origins the motion of all objects in the uni- head to objects fast enough to leave verse. It commands planets to orbit the Earth’s gravity. Our Earth has rel- The existence of “dark stars” or around their central star, it makes us atively low mass, therefore the forces “black holes” can be traced back to stick to the Earth’s surface. Now, we of gravity remain gentle in our daily John Michell4 in the 18th century, ask ourselves, how fast we would and later to Pierre-Simon Laplace5, have to accelerate Sir Isaac New- who speculated that, if a or a ton’s apple upwardly in order to star were massive enough, the escape make it leave the Earth’s gravity velocity would equal the speed of field? The escape velocity is defined light. Light particles () leav- as the minimum velocity an object ing the surface of such a world, must have to escape the gravita- would rise, stop, and then fall back tional field of a celestial body, e.g., down like the projectiles in Fig. 2 the Earth, without ever falling back preventing it to be seen from the again (see Fig. 2). Newtonian me- outside. This is a Newtonian view of chanics provide us the answer: black holes, which, despite being a nice picture, is an inaccurate descrip- tion of what really happens to light Fig. 2: A Treatise of the System of near a real massive body. According the World, London, 1728. A virtual can- to Albert Einstein6, the non shoots projectiles at increasing ve- where G is the universal gravita- locity. Below the escape velocity, the bul- is a universal constant. So we must tional constant, M the mass of the lets fall down to Earth at increasing describe the process near that mas- celestial body, and r the distance distances. When escape velocity is sive body in a different way. from its centre of gravity. So, our ap- reached, the bullets will enter an orbit around Earth, and never fall back again. ple requires a velocity of approxi- (Credit: Isaac Newton, Philosophiæ Na­ In the early 20th century, he devel- mately 11.2 km/s (40,320 km/h) to turalis Principia Mathematica, 1687). oped the theories that revolution-

4 John Michell, 1724, Thornhill, Yorkshire, United Kingdom – 1793, same, English philosopher and geologist. 5 Pierre-Simon (Marquis de) Laplace, 1749, Beaumont-en-Auge, Normandy, France – 1827, Paris, French mathematician and astronomer. 6 Albert Einstein, 1879, Ulm, Germany – 1955, Princeton, New Jersey, USA, theoretical physicist, Nobel prize laureate in physics, 1921. 7 See Spatium no. 18: Einstein in Bern: The Great Legacy, by Rudolf von Steiger, February 2007.

SPATIUM 28 4 ized our view of space and time7. is the theoretical foundation of our ing prediction comes from Ein- The first is the Special Theory of understanding of black holes. stein’s interpretation of planets Relativity, essentially dealing with orbiting a star. Those circular or el- the question of whether rest and Einstein’s theories of relativity have liptical are not due to a cen- motion are relative or absolute, and been confirmed to be accurate to a tral gravitational force, but rather with the consequences of Einstein’s very high degree. Moreover, they the planets are travelling on straight conjecture that they are relative. It predict many unexpected processes lines through curved space. Einstein describes the motion of particles of which most have been corrobo- interpreted gravity as a geometric moving close to the speed of light. rated experimentally since. The property of space and time, causing In fact, it gives the correct laws of most famous such experiment was the space-time to be curved around motion for any particle at any speed on the occasion of the solar eclipse massive objects. including the cases where New­ in 1919, when scientists were able tonian mechanics are valid. The to show for the very first time that ­second is the General Theory of the light of distant stars is indeed Theoretical Predictions Relativity, which primarily applies deflected by the Sun’s gravity as the of Black Holes to particles as they accelerate, partic- light passes near the Sun on its way ularly due to gravitation. It constitutes­ to Earth. The total solar eclipse al- Einstein introduced the new con- a radical revision of Newtonian me- lowed astronomers to see the faint cept of space-time in 1915. Only chanics predicting important new near the edge of the Sun about a month later, Karl Schwarz­ results for fast-moving and/or mas- which is normally inaccessible due schild8 found the first exact solution sive bodies such as black holes. to the Sun’s own intense brightness. for the special case of a single The results fully corroborated Ein- spherical non-rotating mass. This Einstein’s basic concept was to drop stein’s predictions. Another surpris- Schwarz­schild solution leads to the Newton’s idea of a mysterious force so-called , des- (called gravitation) attracting masses, ignating the size of the event hori- and to generalize special relativity zon9 of a non-rotating body, later and Newton’s law of universal grav- called non-. itation, providing a unified descrip- tion of gravity as a geometric Schwarzschild had little time to property of space and time, or think about his solution: he died space-time. shortly after his work was published as a result of a disease he contracted When Einstein applied his theory while serving in the German army to gravitational fields, he derived at the Russian front during World the curved space-time continuum, Fig. 3: Gravity causes space-time to War I. Its interpretation as a region which depicts space and time as a curve around massive objects. of space, from which nothing can four-dimensional surface where Space-time is a dynamic entity, it is dis- escape, was not fully appreciated for torted by matter and it tells matter how massive objects create valleys and to move. (Credit: Time Travel Research another four decades. Long consid- dips in the surface (see Fig. 3). This Center) ered a mathematical curiosity, it was

8 Karl Schwarzschild, 1873, Frankfurt am Main, Germany – 1916, Potsdam, German astronomer and physicist. 9 The is the boundary around a massive body at which the gravitational pull of the body becomes so great as to make escape of matter and light impossible.

SPATIUM 28 5 during the 1960s that theoretical The theory of black hole, or is falling into it, dis- work showed that black holes were predicts that a sufficiently compact appears behind the event horizon, a generic prediction of Einstein’s mass will deform space-time to and remains there permanently in- general relativity. form a black hole. The term black accessible to external observers (see comes from the fact that from such Fig. 4). The discovery of neutron stars10 a body nothing, not even light, can sparked interest in gravitationally escape. The term hole comes from Long before space technology collapsed compact objects as a the singularity, i.e. a space-time that reached the required sophistication, ­possible astrophysical reality. In possesses infinite density. John A. black holes were predicted by 1963, a young mathematician, Roy Wheeler12 introduced not only the physicists, but were perceived as Patrick Kerr11, found an exact so- term black hole, but also the “No fantastic by-products of theory at lution to Einstein’s field equations Hair Theorem” postulating that all that time. Today, however, a wealth of general relativity. His solution black holes can be characterized of observational data provides models the gravitational field out- completely by only three externally strong evidence of their existence side an uncharged rotating massive observable classical parameters: even though they are not directly object (later the so-called rotating mass, , and angular visible, and there is growing con- black hole). This solution is a gen- . All other information sensus that super-massive black eralization of the Schwarzschild­ (for which “hair” is a metaphor) holes exist in the centres of most if solution. about the matter which formed the not all galaxies. In particular, there

Fig. 4: According to J. A. Wheeler, all information regard- Fig. 5: Linear x-ray versus radio emission from super-mas- ing the matter that formed the black hole is lost: “black sive black holes (top right) down to stellar black holes (bottom holes have no hair”. (Credit: C. W. Misner, K. S. Thorne, left). This is an indication that mass is the only fundamental J. A. Wheeler, Gravitation, W.H. Freeman & Co.). para­meter characterizing black holes. (Credit: A. Merloni, S. Heinz, T. di Matteo, 2003, MNRAS, 345, 1057).

10 A is a type of stellar remnant that can result from the of a massive star. Such stars are com- posed almost entirely of neutrons. 11 Roy Patrick Kerr, 1934, Kurow, New Zealand, New Zealand mathematician. 12 , 1911, Jacksonville, Florida – 2008, Hightstown, New Jersey, USA, US American theoretical phycisist.

SPATIUM 28 6 are strong signs of a black hole of Up to now, no member of the Before black holes were discovered, more than 4 million solar masses fourth category, the miniature black it was known that the collision of right at the centre of our Milky holes, has been seen. It is thought two photons can cause pair produc- Way. that they formed in the early uni- tion14. This is an example of con- verse, but disappeared in the mean- verting into mass, in contrast time. S. Hawking13 was one of the to fission or fusion, which turn mass Types of Black Holes first to consider the details of a black into energy. is one hole whose Schwarzschild radius of the primary methods of forming According to observation and the- was the size of an atom. Such black matter in the early universe. Hawk- ory, there might be four types of holes are not necessarily low mass: ing showed that the strong gravita- black holes: for example, it requires 1 billion tional gradients near black holes 1. Super-massive black holes counting tons of matter to make a black hole could also lead to pair production. millions to billions of solar masses. the size of a proton. Rather, the In this case, the gravitational energy It is not exactly known how they small size means that their behav- of the black hole is converted into form, but it is likely that they are iour is a mix of quantum mechan- particles. If the matter/anti-matter a by-product of galaxy formation. ics and General Relativity. particle pair is produced below the Because of their location close to event horizon, then the particles re- many tightly packed stars and gas main trapped within the black hole. clouds, they continue to grow on In contrast, when the pair is pro- a steady diet of matter. duced above the event horizon, it is 2. Intermediate black holes featuring a possible for one member to escape few thousand to a few tens of to space, while the other falls back thousands of solar masses. They into the black hole. Thus, the black are thought to emerge from the hole can lose mass by a quantum agglomeration of stellar masses. mechanical process of pair produc- 3. Stellar black holes having a mass of tion to the outside of the event at least 3 solar masses. These form ­horizon (see Fig. 6). This process is when a massive star collapses. called . 4. Miniature (or micro) black holes with masses much smaller than Hawking also showed that the rate that of our Sun. of pair production is stronger when Fig. 6: Evaporation of a black hole: the curvature of space-time is high. Using simultaneous x-ray and radio - pairs, gener- Small black holes have high curva- observational data of black holes, ated near the event horizon by quantum ture, so the rate of pair production their x-ray can be re- uncertainty, might separate: one particle is inversely proportional to the size of a pair might fall into the event hori- lated linearly versus radio emission zon, while the other one can gain enough and mass of the black hole, mean- intensity (see Fig. 5). This strong energy to escape the mighty gravitational ing that it is faster for smaller black correlation suggests that mass and field of the black hole and become a holes. Thus, Hawking expects mini radius are the only fundamental pa- real particle: energy of the “black hole” or primordial black holes, formed is thus lost by the emission of particles. rameters of black holes as predicted (Credit: ORACLE ThinkQuest Educa- in the early universe, to have disap- by the General Relativity Theory. tion Foundation) peared since, thereby resolving the

13 Stephen William Hawking, 1942, Oxford, United Kingdom, British theoretical physicist and astrophysicist. 14 The term pair production refers to the creation of an elementary particle and its antiparticle, for example an and its antiparticle, the , may be created.

SPATIUM 28 7 dilemma of where all those unob- holes directly in the near future thanks Hawking radiation, ­either totally or served mini black holes are today. to the particles escaping in the form leaving only a very short-lived weakly of Hawking radiation. Indeed, some interacting residue. The resulting Hawking radiation theories predict that micro black has two fundamental implications: holes could be formed at low energy levels such as are ­attainable now in Evolution of Stellar-Mass 1. It steadily reduces the mass of the particle accelerators. This prompted Black Holes black hole which hence eventu- recently popular concerns to be raised ally will disappear, and fearing that at the Large Hadron Col- Now that we have the mathemati- 2. It causes black holes to be not lider at the CERN in Geneva black cal theory, i.e., the description of completely black. holes could be generated with un- gravity and space-time around a known further consequences. Such massive body, we come to the ques- The second effect will eventually al- low ­energy quantum black holes, tion of how black holes form, and low astrophysicists to observe black however, would ­instantly evaporate as where they come from.

Fig. 7: . Stars are formed within dense clouds of dust and gases. Depending on their initial mass, indicated here as multiples of solar masses, they end as brown dwarfs, white dwarfs, neutron stars or black holes. (Credit: www.ogonek.net)

SPATIUM 28 8 During his sea journey from Ma- with masses greater than the Chan- Observational Evidence dras to Southampton in 1930, drasekhar limit undergo a further of Black Holes S. Chandrasekhar15 developed the gravitational collapse, evolving into ­theory of white dwarfs16. Specifi- a different type of stellar remnant, The mathematical theory of black cally, he derived a mass limit for a such as a neutron star or a black holes was firmly established, and , and a universal rela- hole. their existence as a consequence of tionship between the mass and the star core collapse was also predicted, radius of a star. During most of a Fig. 7 depicts the processes whereby but no black hole candidate was star’s lifetime, nuclear fusion in the a star undergoes a sequence of rad- ever observed up to 1972. This is core generates electromagnetic ra- ical changes during its lifetime. De- due to the fact that the footprints diation. This makes our Sun shine. pending on the star’s initial mass, this of black holes are absorbed by the The radiation exerts an outward lifetime ranges from a few million Earth’s atmosphere. pressure that exactly balances the years only (for the most massive) to inward pull of gravity caused by the trillions of years (for the least mas- star’s own mass. When the nuclear sive, which is considerably more The Evolution of X-Ray fuel is exhausted, the outward than the age of the universe). Now, Astronomy forces of radiation diminish, letting depending on the initial mass of the gravitation compress the star in- star, it will end up in a different type As shown above, a black hole may ward. The contraction of the core of collapsed object. For instance, a grow by incorporating matter from causes its temperature to rise, allow- star with the mass of our Sun will its vicinity. Before entering the ing the remaining material to be evolve to a , and then col- black hole, this matter is heated up used as fuel. At the end phase of this lapse to a white dwarf. If, however, to very high temperatures, say to a evolution, a massive star can no the initial mass is at least 30 solar million degrees to hundreds of mil- longer produce energy in its core, masses, a black hole is expected to lions of degrees. At such tempera- and therefore the radiation from its form when the heavy star collapses tures matter is expected to emit nuclear reactions can no longer into a at the end of its life x-rays. It was, hence, up to x-ray keep the star “puffed up”. Gravity cycle. So, a black hole forms when ­astronomy to explore the sky for then causes the core to collapse. a sufficiently massive object reaches black hole candidates. Since x-rays The star’s outer layers may blast a certain critical density, and its are absorbed by the Earth’s atmos- away into space, and the core may gravity causes it to collapse to an al- phere, such instruments must be fall into a collapsed compact mas- most infinitely small point. After a taken to high altitude by balloons, sive object. black hole has formed, it can con- sounding rockets17, or satellites. tinue to grow by absorbing matter S. Chandrasekhar predicted that a from its surroundings. It may even The era of x-ray astronomy began massive star could collapse into absorb other stars and merge with with a sounding rocket flight car- something denser depending on the other black holes thereby forming rying a simple Geiger counter star’s initial mass. R. Oppenheimer super-massive black holes of mil- aboard in 1962. Its purpose was to and H. Snyder showed in 1939 that lions of solar masses. investigate x-rays from the Moon, massive stars can collapse into black instead it discovered the first x-rays holes. Consequently, white dwarfs from an outside

15 Subrahmanyan Chandrasekhar, 1910, Lahore, India – 1995, Chicago, US American astrophysicist with Indian roots, Nobel prize laureate in physics, 1983. 16 A white dwarf is a small, very dense star whose mass is comparable to the Sun while its volume is comparable to the Earth. 17 Sounding rockets are sub-orbital rockets that carry a payload above the Earth’s atmosphere for a limited period of up to 15 minutes, but which do not place the payload into orbit around the Earth.

SPATIUM 28 9 the . The Sun was difficult than with light or radio object was an x-ray binary system18; known to be a x-ray source, but be- waves. Therefore, the optical coun- Cyg X-1 was estimated to have cause it is so much closer than other terpart of those x-ray sources could around 10 solar masses orbiting a stars, no other x-ray source was ex- not be determined at that time. companion star that was previously pected to be found. It came as a catalogued as a blue great surprise that it discovered In December 1970, NASA launched with 30 times the mass of the Sun. both Scorpius X-1, the brightest x- the first satellite specifically de- ray source in the sky, and a com- signed for x-ray astronomy, So now, what is Cyg X-1? Astro- pletely unexpected diffuse glow of ­UHURU. In 1971, it found that physical logics exclude the possibil- x-rays known as the cosmic x-ray Cygnus X-1 exhibited a rapid var- ity that it could be a red giant since background radiation. During fur- iability of its x-ray flux with a pe- these stars would be easily seen in the ther sounding rocket flights, Cyg- riod of 5.6 days, see Fig. 10. These optical wavelength band. Further, it nus X-1 (Cyg X-1) in the constel- variations allowed for gathering of cannot be a white dwarf since the lation Cygnus was discovered data to assess its accurate position: Chandrasekhar mass limit is around ranking amongst the strongest x-ray two independent teams of radio as- 1.4 solar masses, therefore a 10 solar sources (Fig. 9). tronomers discovered variable radio mass white dwarf cannot exist in a emissions from within the possible stable form. The same applies­ to the Now, x-ray astronomy was penalized positions of Cygnus X-1. The meas- neutron star; the mass limit is around by the fact that the angular resolu- ured changes in the radio bright- 3 solar masses to be stable. By elim- tion of such observatories were in- ness occurred at the same time as ination, we are left with a black hole. herently far lower than that of op- the changes in x-ray brightness. Cyg X-1 was therefore the first ce- tical telescopes as a consequence of With the more accurate radio po- lestial body widely accepted to be a the difference in wavelengths. This sitions, astronomers could finally candidate, and it re- means that measuring the direction pin down the exact location of mains among the most studied as- to an x-ray source is much more Cygnus X-1. It turned out that the tronomical objects in its class.

Fig. 8: X-ray emission observed Fig. 9: The 30 orbits every 5.6 days around an from the source Cyg X-1 using the optically unseen, but very bright x-ray object, i.e. Cyg X-1. (Credit: NASA images EXOSAT observatory. (Credit: ESA/ archive) EXOSAT)

18 The term refers to two objects in space, usually stars, but also planets, galaxies or asteroids, which are so close that their gravitational interaction causes them to orbit about a common centre of mass.

SPATIUM 28 10 Cyg X-1 is about five million years Black Holes in X-Ray wise invisible black hole gaining in- old, and formed from a progenitor Binaries sights about the physics of . star that had more than 40 solar Most importantly, black holes in x- masses. Hence, the majority of the We know today that the bright x- ray binaries provide a laboratory to star’s mass was shed, most likely as ray source Cyg X-1 is a compact test the behaviour of matter under . If this star had then ex- star, i.e., a black hole in a binary physical conditions that are by far ploded as a supernova, the resulting ­system orbiting around an optical unattainable on Earth. force would most likely have ejected bright supergiant star. Despite its in- the remnant from the binary sys- visibility, the presence of the black The matter attracted by the black tem. Hence, the progenitor star may hole is inferred through its interac- hole cannot fall directly into it, have collapsed directly into a black tion with the matter coming from since it has first to lose its angular hole instead. the companion star (see Fig. 10). momentum. Rather, it forms a This in-falling matter is heated by ­rotating around the the strong gravitational field of black hole as predicted by N. I. the black hole. It is this process by Shakura and R. A. Sunyaev (1973). which astronomers can detect and The rotation of the disk is differen- study the environment of an other- tial, with the inner portions com-

Fig. 10: An artist’s impression of the flow from the star providing the material for an accretion disk around the x-ray source, black hole. Matter in the inner disk is heated to millions of degrees, generating the observed x-rays. (Credit: M. Kornmesser, L. L. Christensen, ESA & Hubble European Space Agency Information Centre)

SPATIUM 28 11 pleting an orbit faster than the outer stellar radio sources,” which by 1964 3C 273 could be understood portions. The basic idea behind the was shortened to quasar. as coming from hydrogen atoms accretion disk is that viscosity in the with a red shift of 16%. This implies gas disk converts the free energy of The optical spectra of pre- that their emission lines were shifted into thermal sented a new mystery: their emis- toward longer, redder wavelengths energy, which is in turn is expended sion lines were at odds with all ce- by the expansion of the universe. by radiation. As the energy is re- lestial sources then familiar to With this red shift, 3C 273 is placed leased, the gas spirals inward, com- astronomers. The puzzle was solved at a distance of slightly more than pleting many revolutions before by Maarten Schmidt, who in 1963 two billion light-years. This was a significantly changing its distance recognized that the pattern of emis- large, though not unprecedented, from the central source. The amount sion lines of the brightest known distance. Bright clusters of galaxies of energy released by the gas in the disk increases as it draws closer to the centre. This means that most of the energy released by an accre- tion disk comes from the disk’s in- ner edge. In this accretion process energy is lost mainly as x-ray radi- ation making such binary systems some of the brightest x-ray sources in the sky.

Black Holes in the Centres of Galaxies

So far, we have discussed some as- pects of stellar black holes with a few ­solar masses. Yet, there is grow- ing consensus that black holes exist also in the centres of galaxies with masses millions of times that of the Sun.

The earliest radio surveys of the sky were executed in the 1950s. Out of the plane of our Milky Way, most galaxies, identified otherwise as normal-looking galaxies, were found to emit radiation in the ra- dio energy band. However, some of those radio sources coincided with objects that appeared to be unusu- ally blue stars embedded in faint, fuzzy halos in the galactic centres. Fig. 11: The heart of our galaxy is a veritable soup of stars, gas, and dust. In partic- ular, there is strong evidence of a black hole of more than 4 million solar masses at the Because of their almost star-like ap- centre of our Milky Way called Sagittarius A*. (Credit: Kassim, LaRosa, Lazio, Hyman, pearance, they were called “quasi- 1999, NRAO Very Large Array)

SPATIUM 28 12 for instance had been identified at gas spiralling at high velocity into tion of the then speculative black similar distances. However, 3C 273 an extremely large black hole. The hole model for quasars led Lynden- is about 100 times more luminous brightest quasars can easily outshine Bell and Rees in 1971 to anticipate than the brightest individual galaxy all the stars in the host galaxy, which that our galactic centre would also in those clusters, and nothing so makes them visible even at distances contain a super-massive black hole. bright had ever been seen so far away. of billions of light-years. Quasars are Subsequently, Balick and Brown therefore amongst the most distant (1974) found a compact radio­ It came as an even bigger surprise and luminous objects known so far. source indeed which was named when it was seen that the bright- Sgr A* (to distinguish it from the ness of quasars can vary significantly more extended emission of the on timescales as short as a few days. The Black Hole in Sgr A, and to emphasize its unique- This in turn implies that the total the Milky Way ness) eight years after its discovery. size of those quasars cannot be more More precise high-resolution ob- than a few light-days across. Later Quasar 3C 273 was the first extra- servations in 1981 with the Very research revealed that they reside in galactic black hole detected in 1963, Large Array (VLA) at the European­ the centres of their host galaxies. and it stimulated researchers to look Southern Observatory (ESO) in So, quasars are objects of very high right into the centre of our own Chile revealed that it is located near luminosity located in the very cen- galaxy to search for a strong radio the dynamical centre of the galac- tres of galaxies, and are powered by source there. The prescient applica- tic nucleus. All these observational signatures make it clear that Sgr A* is a very unusual object, rendering it a prime suspect for the location of a putative super-massive black hole (see Fig. 11).

Over 16 years of observation cam- paigns of the region right in the centre of our galaxy have confirmed the existence of a super massive black hole there. 28 individual stars have been tracked orbiting a com- mon, invisible point (Fig. 12). Usu- ally these stars would be obscured by gas and dust, though ESO’s in- frared telescopes were able to peer deep into the black hole’s lair. Judg- ing by the orbital trajectories of these stars, astronomers have not only been able to pinpoint the black hole’s exact location, they have also deduced its mass which amounts to 4 million solar masses. At exactly this location is the compact radio source Sgr A*. All the stars there are moving extremely rapidly: one of Fig. 12: Yearly location of stars near Sagittarius A* orbiting the common, in­ them even completes a full orbit visible compact radio source. (Credit: A. Ghez, UCLA Galactic Center Group) within 16 years.

SPATIUM 28 13 Flares From Our Galactic declining to pre-flare levels a few Centre Black Hole hours later. These events have pro- duced compelling evidence of a sig- The fast motion of stars and gas nificant modulation in the x-ray around the galactic centre suggested light-curve with a quasi-period of that something very massive must around 17-22 minutes (see Fig. 13). be hidden there. Indeed, Sagittarius These periods are rather intriguing A* is believed to be powered by a because simple considerations place super-massive black hole. Long- the regions where this emission is term monitoring has led to the dis- generated at roughly 3 Schwarz­ covery of several near-infrared and schild radii above the event horizon x-ray flares from this object. This is for a black hole mass of 4  106 so- extremely exciting because it’s the lar masses. The energy released in first time that the super-massive the flare corresponds to a sudden black hole right in our front yard in-fall of matter with about as much could be observed devouring mass as a comet or an asteroid. Sev- Fig. 13: Near-infrared flare observed chunks of material. During time eral models have been invoked to from our own galaxy super-massive black spans of just a few minutes, x-ray explain these quasi-periodic mod- hole, Sgr A*. Quasi-periodic modulations are indicated with arrows. (Credit: F. Me- emissions from Sgr A* became 45 ulations. Recent magnetohydrody- lia “The Galactic Supermassive Black times brighter than normal, before namics simulations of Sgr A*’s disk Hole”, Princeton University Press)

SPATIUM 28 14 have demonstrated disk instabilities Outlook So, accreting black holes are ideal that enhance the accretion rate for laboratories for studying both phys- several hours, possibly accounting ical properties of accretion onto for the observed flares. Falanga et al. compact objects and effects of gen- (2007) carried out ray-tracing cal- Since the beginning of x-ray astron- eral relativity in the strong gravita- culations in a omy in the 1960s, the steady in- tional field regime. New x-ray and to determine the light-curve pro- crease in the capability of space ob- black holes phenomena are fre- duced by general relativistic effects servatories has led to the detection quently discovered, which have no during such a disruption (see Fig. 14). of high-energy radiation from ob- explanation in terms of established This figure also shows how an ob- jects of all scales in the universe, theory. New missions exploiting all server from Earth would observe from compact sources such as black electro-magnetic wavelengths are space-time around the black hole holes to the diffuse hot plasma per- still needed to resolve new myster- located in our galactic centre. vading galaxies and clusters of gal- ies from massive black holes. At the axies. Thanks to this advancement, same time, experiments like those Fig. 14: The view of matter orbiting in we now know that the basic phys- executed at CERN in Geneva will an accretion disk at the edge of the ical processes behind the emission resolve the mysteries of micro black event horizon for a non-rotating black of energetic radiation in most cos- holes, e.g., the enigmatic evapora- hole. The first figure is for an observed in- mic sources pertain to two main tion from mini black holes. clination angle of 30 º, the second and third for 60 º and 80 º, respectively. (Credit: Fa- categories: accretion physics and langa et al, ApJ, 2007) particle acceleration mechanisms.

SPATIUM 28 15 SPATIUM

The Author

Service d’Astrophysique (High En- ber of the Editorial Board for Ad- ergy Division), Paris until 2006. vances in Astronomy Journal, and Thereafter, Maurizio Falanga was a for Astronomy Studies Develop- Research Scientist, at the Unité ment Journal. Mixte de Recherche, University of Since 2009, he has been Science Paris. Program Manager at the Interna- tional Space Science Institute (ISSI) His research interests are focused on in Bern, Switzerland. Apart from accretion and emission in neutron the programmatic responsibilities at stars, white dwarfs and black holes, ISSI, he continues active research in physics of the magnetosphere, high-energy astrophysics. x-ray polarization, accretion wind models, radiative transfer, star atmos­ pheres, magneto hydrodynamics, plasma instability, type I x-ray bursts, numerical simulations, mapping ­super-massive black holes. Maurizio Falanga has published over 100 pa- pers in his research fields. At the Maurizio Falanga grew up in Basel. same time, he was ­supervising un- After his electronics apprenticeship dergraduate and Ph.D. students. in 1990 under Edi Blatter, a Basel, ­Falanga has given numerous lectures Switzerland, based radio and tele­ for example at the ­Vatican High vision marketing company, he re- School in Rome, or for a variety of ceived the “Eidgenössische Matura associations in Switzerland, Italy, and Typus C” in Zürich, 1993. He then France. enrolled at the University of Basel in theoretical physics and astron- He is a member of the INTEGRAL omy. He concluded his studies with Users Group of ESA, and a mem- the diploma in 1998. In 2002, he ber of the Large Observatory For received his Ph.D. degree in astro- X-ray Timing (LOFT) Science physics from the University of Working Group (Dense Matter, Rome La Sapienza. His Ph.D. ­thesis Strong Gravity). LOFT is a me- work included theoretical general dium-class mission selected for the relativistic ray-tracing calculations assessment phase of the ESA M3 to reproduce the light curve emit- Cosmic Vision call. He has also ted by matter orbiting in the strong- served on numerous Time Alloca- field regime around a black hole, or tion Committees for orbiting mis- from a neutron star surface. He then sions like INTEGRAL, Chandra or was a Postdoctoral Fellow, at the XMM-Newton. He is also a mem-