PoS(P2GC)017 http://pos.sissa.it/ ∗ [email protected] Speaker. Classical black holes are solutions ofcal the observations field suggest equations that of black General holes Relativity.for really Many their exist astronomi- existence in is nature. still However, lacking.been an observed unambiguous Neither by proof event means horizon of astronomical nor techniques. This intrinsic paper curvature introduces singularity to have particular featuresastronomical of techniques black to holes. detect Then, blackimportant we holes. in give a the synopsis Further near on methods future.completely current are presented outlined and For classified that the into will kinematical, firstscurative, spectro–relativistic, become time, accretive, aberrative, eruptive, temporal, the ob- and zoo gravitational–wave ofand induced black technical verification hole obstacles methods. avoid detection Principal undoubtfully techniquesalternatives proving to is black the black hole hole. existence. However,retical classical We model rotating critically to Kerr explain black discuss astronomical holes observations. are still the best theo- ∗ Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. c

School on Particle Physics, Gravity and21 Cosmology August - 2 September 2006 Dubrovnik, Croatia Andreas Müller Max–Planck–Institut für extraterrestrische Physik, p.o. boxE-mail: 1312, D–85741 Garching, Germany Experimental Evidence of Black Holes PoS(P2GC)017 ]. M 8 . 133 0 6 [ ∼ a ∼ supermas- z Andreas Müller ] describes the 120 ]. Recently, it has been 31 , 43 was coined significantly later, in ]. The extreme quasar luminosities of 81 , black hole 80 , ]. The same physics is at work. ] pointed to BHs in the centers of galaxies that 88 38 2 116 , 137 ]. The result of these developments is the first BH mass family: 105 could be explained by BHs that swallow matter. Accretion turned out to 1 erg/s

L . (SMBHs). Paradoxically, the darkest object in the Universe produces the most 33 14 10 × 10 85 ∼ . 3 '

erg/s L The giant SMBHs are supposed to inhabit almost any center of galaxiesCurrently, there causing is an an active active debate on the cosmological role of BHs. In hierarchical galaxy In the Sixties the understanding of active galactic nuclei (AGN) like quasars and radio galaxies Black holes (BHs) are the most compact objects known in the Universe. They are the most Astonishingly, at the advent of GR in 1916 the first solution of Einstein’s complicated field It soon became clear that nature may allow for the existence of these mysterious objects. In ]. 1 47 134 quasars–like phase in an earlier statedormant of SMBHs. the galaxy evolution. Local galaxies contain non–active or formation scenarios is it possible to explain luminous quasars at cosmological redshift sive black holes shown that the timescale of X–raycally variations linked from if stellar one corrects and for supermassive BHs the accretion are rate in [ fact physi- SMBHs are formed on cosmological timescalesProbably, from these seed BHs seeds with originated several hundred fromgrew solar the by masses. first merging population and of accretionto very to explain massive form the stars. the spin–up nearby The ofby initial SMBHs. BHs rapidly BHs spinning in In BHs this the – context same essentiallythin of disk by scenarios accretion BH gas it but accretion. growth. is similarly Maximally for possible The spinning thick spin BHs disk are accretion distribution about the is 4/5 result dominated have of very high spins, powerful source of luminosity.marks Of the course, ultimate blackness. this Nowadays, mechanism astrophysiciststhe are works convinced key outside that role stellar–mass the BHs in event play black(GRBs), horizon and hole that maybe X–ray also binaries in (BHXBs) ultra–luminous such X–ray sources as (ULXs) microquasars, [ in gamma–ray bursts are significantly more massive than the first family. This second mass family is called stellar–mass BHs 10 be the most efficient process toHowever, estimates produce with radiation the in Eddington the relation whole [ range of electromagnetic emission. [ delivered another piece to the BH puzzle [ efficient gravitational lens, a(GR) lens is that a powerful captures theory even tocurved light. describe space–time. BHs In mathematically. Albert a It Einstein’s sense, iscontinuum. General a Gravity According theory Relativity is to that this not describes picture, a Gravity BHscurvature force are as singularity but an that a extreme swallows geometric form matter of feature and curved light. of space–time a containing deformable a 4D equation was found in the same year. This (external) Schwarzschild solution [ the classical GR collapseforms. of massive This stars picture nothing wasand can Hartland essentially stop Snyder pushed in gravitational forward 1939 pressure by [ so the that work a of BH Julius Robert Oppenheimer Experimental Evidence of Black Holes 1. Introduction simplest BH type mathematically. However, the notion 1967 by the relativist John Archibald Wheeler. PoS(P2GC)017 M g, the 998 . develop 33 0 ]. At first 10 M with New- = 2 ± 32 × c Andreas Müller max / = | c [ a / a 989 M | . 1 G M G = = g =

r M M / = J but the cases M = M a + ]. We measure BH rotation in terms and 97 [ M M − ) that holds 3 ] and references therein. Another review can be ]. There is still no rotating BH alternative available 16 ]. Hence, the restriction to classical GR BH solutions 93 99 , ]. Accretion theory favours a limit of 92 3km. In theory, it is convenient to use geometrized units ]. Essentially, this is justified by two reasons: first, space– 108 ' 68 Kerr parameter S R ]. Based on these data it could be shown that the number density of 62 ] and GRBs [ 17 ´ c at this school, see [ 1000 AGN [ http://www.mpe.mpg.de/˜ amueller/downloads/PhD/PhD_AMueller.pdf 2 ∼ ] ] and the Kerr solution [ 100 120 1 so that length is measured in units of mass = c Download at To prepare to detection techniques for BHs, we first arrange the properties of these objects in The natural length scale of GR is the gravitational radius defined by This contribution is devoted to experimental techniques in astronomy that are used to detect In the present paper, the treatment is restricted to classical BHs i.e. the (external) Schwarzschild Another interesting finding is the so–called luminosity—dependent density evolution. Data Currently, astronomers use multi–wavelength surveys to collect a huge amount of observa- A thorough introduction to BH physics and BH phenomenology has been given by Jorge ]. 2 ]. = 127 91 a naked singularity that is forbidden [ the next two subsections. time rotation is a crucial ingredient forjets BH launching astrophysics e.g. in in AGN the [ view of paradigms for relativistic ton’s constant G and vacuum speedSchwarzschild of radius light amounts c. to ForG one solar mass, of the specific angular momentumglance, the ( Kerr parameter can take any value between is sufficient. to date that could replace a classicala Kerr good BH. choice Second, in the some regime of distancegravity classical to effects GR a are is BH supposed expected candidate. to to be radius Deviations emerge from or at GR even length solutions smaller. due scales Thisity to that is quantum has are currently already comparable ’out clear to ofapproaching astronomical the signatures the range’. Schwarzschild starting hole. The at regime As someamounts of suggested distance to strong by several to grav- relativistic tens gravitational the emission radii line horizon [ diagnostics and this getting critical stronger distance as [ cosmic BHs. Afterof an detection introduction into methods BH iscritically features given. discuss the a question what complete We we observe: have presentationalternatives What a to and are the the closer classification classical astronomical facts? BH look What solutions? at are possible BH spin measurements.2. Finally, Black hole we features solution [ found in [ from ROSAT, XMM–Newton, and Chandra surveyssamples have with been collected together to provide huge tional data. The analysis reveals interestingseems connections to between be galaxy a and cosmological central link SMBH. between There the two that pointsZanelli towards co–evolution and scenarios. Neven Bili Experimental Evidence of Black Holes high–luminosity AGN (’quasars’) peaks at significantlyluminosity lower AGN cosmological (’Seyfert redshift galaxies’). than This for anti–hierarchical[ low– growth can be explained theoretically PoS(P2GC)017 3 4 ' (2.2) (2.3) (2.4) (2.1) WD C Andreas Müller ], a Kerr black hole is the dimensionless ∗ 23 , r 2 37, Schwarzschild BHs θ . –factor for short hereafter d 0 g 2 ] for alternatives. ρ ' + 121 2 QS , and radius 8 C dr ∗ ∆ M / 2 , ρ 1. Therefore, BHs rotating at their limit , ) ∗ . + = M r r . is generally a function of the velocity field of 2 2 2 M ) ∆ c g ], see however [ ( − Σ dt √ KBH / 1 ∗ ρ 4 ω C 127 [ ]. In Boyer–Lindquist form [ r − = M GM 11 φ = [ α d ≡ S hereafter. 998 a ( . 2 α 0 C ˜ ω throughout the paper. ' . In a good approximation, any slowly rotating mass may be 16, quark stars may have a + . 0 2 dt ' + ++) 2 ]. One important metric coefficient is the redshift or lapse function α − NS ( 26 − C 5 = 2 ) can be generalized to be the lapse for rotating BHs ds 2.2 compactness parameter Schwarzschild factor 2 and extreme Kerr BHs satisfy / 1 ] –factor is a measure for both, energy shift of emission and suppression or enhancement = is assumed here for simplicity but the exact value of the maximum angular momentum of BHs is still under g 100 M , and specific angular momentum varies between zero and unity where a value close to unity signals an extremely compact SBH = M C The signature of the metric is In contrast, Kerr BHs belong to a two–parameter family described by two physical quantities, Here, the event horizon radius isa assumed to be the ’surface’ of a BH. The blackness of BHs is a result of strongly curved space–time forcing light rays (null geodesics) In the framework of classical GR a point mass is described by the (external) Schwarzschild GR tells us that mass causes gravitational redshift effects i.e. the strong pull of gravity shifts C 5 3 4 ]. The that measures the amount of gravitational redshift of photons and time dilation. It holds for static 0004, neutron stars hold . 99 mass described by this factor. and is known as where expression ( debate. Usually, one assumes Thorne’s limit the emitter (as measured inZAMO), a suitable the observer’s curved frame space–time e.g. (represented thethe zero photon by as angular metric discovered momentum coefficients), in observer, and [ constants of motion of of intensity as compared to the emitter’s rest frame. α BHs is given by the line element object. Let us look at some examples: White dwarfs at the Chandrasekhar limit may satisfy to bend back to the gravitational source.rameter This to is measure a compactness consequence of of a their stellar compactness. body A with suitable pa- mass solution. Schwarzschild BHs representspace–time a is one–parameter fully family described by in the GR. mass The parameter static Schwarzschild Experimental Evidence of Black Holes 2.1 Blackness quantity [ which is denoted 0 hold represent the most compact objects in the Universe. [ radiation emitted near masses towards theis red mathematically spectral range described and by the a intensity general is relativistic reduced, Doppler too. factor This or PoS(P2GC)017 and (2.5) (2.6) (2.7) (2.8) (2.9) (2.11) (2.10) 0. Since = darkness α Andreas Müller = S α 0, there is only one horizon . = as a µ  : , d θ d 2 of the next SMBH in the centre of the , 1kpc 0 and consequently 2 sin a  BH . It is sufficient to consider only the first = ∆ 6 θ − , 2 as) scale! We provide a useful equation to × , ∆ 2 2 . For rotating Kerr BHs there are in fact two a µ θ a  so that spectral flux is strongly reduced as the M 2 M −

+ , . 2 3 1 for cosmic BH candidates. 2 ) 2 p ) 5 g ρ d M cos Σ 2 = / M / 6  Mr 2 ± / a ∝ S θ S 2 a of electromagnetic radiation in the vicinity of a BH. d and BH distance 10 R + M / ( − + obs 2 the inner horizon. The inner or Cauchy horizon can only sin S ν  aMr M 2 2 r = F R r r 2 Σ − d H × r ± H / r 4 S = = = ( = = . R 2 2 ˜ ∆ 2 39 2 arctan ω ω cm Σ ρ 18 ' ' = any emission 10 BH × θ 1 . 3 ' at the event horizon. For static Schwarzschild BHs, 26 lightyears . 3 denotes the outer horizon and = + H r , the redshift influences blackness Kerr BHs reveal a speciality that lacks in case of Schwarzschild BHs: rotation of space– 1 pc We conclude that the essential BH indicator is darkness as a result from gravitational redshift. Per definition, any classical BH horizon satifies ]. α 6 ∝ 108 compute this immediately from BH mass time. The spin of Kerr BHs forces anything to co–rotate: matter, light, close observers, because order term of the arctan expansion since 2.2 Rotating space–time Milky Way can be found on the micro–arcsecond ( The values are adapted to SMBHsmasses that and have typical typical masses distances that from range from several million kpc to to billion Mpc solar be intersected once i.e. itcensorship is conjecture by a Roger one–way Penrose ticket the[ to intrinsic ’true’ the singularity curvature is singularity. hidden by Following these the horizons cosmic Hence, astronomers seek for ’blackdidates spots’ are (BS) supposed in to the bepossibly sky e.g. in especially in centres at of X–ray positions globular binaries wherelenge clusters (XRBs), BH is and gamma–ray can- to confidently burst in resolve remnants these centres (GRBRs), of tiny galaxies. spots. The The technical apparent chal- size where Experimental Evidence of Black Holes Other metric functions satisfy g even Observed intensity of radiation satisfies emitter approaches to the event horizon. The consequence is the main BH feature: horizons that hold that is located at the Schwarzschild radius, R PoS(P2GC)017 ). . In anti– erg (2.12) ]. The right r 12 < r an invariant ] – however, Andreas Müller 64 not . It is important to note 3 − ) with rotating BHs ( and positive for r erg ∝ left r . The frame–dragging frequency ω . > r ϑ 2 cos 0, a feature that may be entitled as 2 a = − ω 2 M frame–dragging 6 p + to rotate, gravitational radii. The accretion disk whobbles around M 4 not ) = ϑ to 10 ), parameterizes the rotation of space as viewed from infinity. ( 2 erg 2.8 r 0 and we find = 2, the ergosphere lies at the Schwarzschild radius – independent of BH tt / g π in Eq. ( 0. = ω = ϑ ϑ gives a comparison on BHs and compares static ( ]. From this view, a Kerr BH interacts by gravitoelectric and gravitomagnetic 1 . 128 ]. ]. Schwarzschild BHs are not sufficient to explain the observed relativistic jets of AGN 130 89 , Kerr BHs are endowed with a zone where the rotation of space–time becomes extraordinarily Finally, Fig. BH rotation is important: Rapidly rotating space–time plus magnetic fields are the main in- We summarize this section that blackness and rotating space–time are the two observable How can one detect a cosmic BH? In principle, one can distinguish direct and indirect meth- Frame–dragging can be nicely interpreted by an analogue to electromagnetism, called grav- 77 , 70 frame–dragging strong. It called the ergoregion thatbe is computed oblate. according The to outer edge of the ergoregion, the ergosphere, can that Schwarzschild BHs force anything We stress that this is just aview ’structural view’ because showing it the main is BH based features.onto on But BHs it the is is radii possible by of using a curvature specific invariants such coordinate as system. the Kretschmann A scalar better [ and invariant view the BH. Hence, hot diskquasi–periodic radiation features that in is the detected power density inquency spectra the [ that X–ray are range dominated might by be the Lense–Thirring variable fre- and show gredients to drive relativistic jetsgeneral relativistic magnetohydrodynamically. magnetohydrodynamics (GRMHD) Recently, simulations numericaltating have non–radiative near shown their that limit Kerr are BHs efficient rotators[ ro- to spin–up magnetic field lines andand drive GRBs. Poynting fluxes properties of classical black holes. 3. Black hole detection methods forces with its surroundings. Gravitoelectric forcestest are bodies. just ordinary But gravitational forces thephenomenon attracting interpretation associated with of gravitomagnetic gravitomagnetic forces forces isthat is the two Lense–Thirring more gyroscopes effect. interact complicated: by Itscopes One simply exchanging is states relevant gravitomagnetic a forces. rotating One BHthe realisation surrounded rotating of by accretion two a disk gyro- rotating into accretion thetransition region disk. equatorial can plane be In by found this means at 10 case of the gravitomagnetic forces Kerr [ BH pushes this is not that intuitive. Approaching the Kerr BH, the ’spin of space’ steeply increases, itomagnetism [ Experimental Evidence of Black Holes space itself rotates. The forcing propertyalready is introduced called as the equatorial plane, spin. However, for lower poloidal angleswith the it ergosphere at approaches the the poles, BH horizon and coincides The metric coefficient flips sign at this radius: it is negative for PoS(P2GC)017 method Andreas Müller indirect ). We assume the same right 0, due to zero redshift (or = ]. Based on these arguments, g 63 ] BH intrinsic singularities are hidden by an 108 ]. 7 ) and rotating Kerr BHs ( 14 , left 1 0–statement could be made at best. On the other hand, ≈ g impossible to prove direct evidence for cosmic BHs by means of electromag- as pointed out recently [ 0. On one hand, this is certainly difficult because any astronomical brightness measure- Architecture of static Schwarzschild ( principally = α Hence, the diversity of current methods could be summarized as indirect methods. Astronomers In the following, we present different methods that are in use by astronomers. But we also Kinematical methods to detect black holes are widely–used and very successful. The simple because the observer does not care about the black hole itself, but about the tracer. In practise measure the amount of massat and the the plausible associated conclusion volume. thatthouroughly From nothing if density else alternative arguments can scenarios one fit may may but fitpact arrive a such objects, as BH. e.g. compact Of stellar neutron course, clusters stars, astronomersdetected. or have boson other to stars, types test of or com- fermion balls etc. If not,go a beyond good practice BH and candidate showmethods is further are methods classified by that a might suitable be label. applied in the near3.1 future. Kinematical All methods these idea is that movingtracers objects to that derive feel properties of the the deep black gravitational hole potential i.e. of mass the and black spin. hole Hence, serve it as is an ods. Direct evidences for cosmica BHs classical black means hole that by ancosmic observations: astronomer censorship the has conjecture event to horizon by prove and Roger the the Penrose key curvature [ features singularity. of Due to the netic radiation. We feel thatgravitational the waves only possibility for direct methods consists in clear signatures of mass. One immediately notices that Kerr BHs are more compact than Schwarzschild BHs. it seems Figure 1: event horizon. Therefore, a proofof of an the event curvature horizon singularity meanshorizons seems to to the show be general strong impossible. relativistic evidence Doppler The for proof factor a vanishes zero–emission exactly, region because at event Hawking has demonstrated that inon a a semi–classical GR quantum background gravity description metric) (quantized black fields hole event horizons radiate [ Experimental Evidence of Black Holes lapse) ments involve error bars so that only a PoS(P2GC)017 , the (3.1) . This shows σ S 2 1600 R Andreas Müller . Fig. ≈ S2 ]. Previously, another 9997. Strong gravity . 0 50 ]. Stars orbiting SMBHs ' 3 [ . ) 0 117 S , ) However, in theoretical units ± 51 3.6 , 75 . ) 0 3 1600 R σ = / ]. This value undershoots significantly = ]. Both correlations hint for a physical σ β ( r 83 ( 129 S , and the stellar velocity dispersion, log α M 3 [ β . 0 + 8 ± 200 km/s this correlation can be fitted to galaxy 32 kpc. From Kepler’s third law a central mass of α . 0 . 0 = 4 ]. Essentially, these results are confirmed by another ]. ) = ± 0 =

37 σ . The orbital period amounts to 5.6 days. This is the first 62 , 106 M . β

, / 7 36 40 M = ( 0 18M R log ∼ ] and overshoots another one ]. Plotted as log–linear relation 47 50 is deduced [ 4 [ Another example is the compact radio source Sgr A* that is associated .

0 The first BH was detected by the Canadian astronomer Tom Bolton in 1971 M ± ]. 6 27 99 relation [ But not only single stars are suited. Ensembles of stars that follow their orbits in . 10 5 σ ]. Proper motion and radial velocity measurements reveal Keplerian orbits of the × = ) 55 is still far away from the BH because the periastron distance amounts β , 32 and HDE 226868 has . 0 S2 54

relation ± This compact and dark mass is very likely a SMBH because any other alternatives such as σ ]. He studied the X–ray binary system Cygnus X–1. Bolton was able to measure the radial 61 10M . – 3 22 are thereby suitable tracersapproach to the BH determine in BH our Galaxy features. andthe to star probe Astronomers GR effects plan (see follow–up Sec. is projects rather the to asymptotical flateffects region become of important space–time, at aline diagnostics few [ tens Schwarzschild radii as proposed by relativistic emission with a suited referencesamples. value Recent chosen results to fit thefirst fits slope of to relation was found that links BH mass and bulge mass [ compact star clusters, boson stars, or fermion balls are ruled out [ M S–stars allowing to determine a periastron distance of 16 light hours for the star groups can also be usedsituation e.g. is if typical the for resolution distantanalyse is galaxies. stellar not sufficient velocity Slit enough dispersion spectroscopy tostrong profiles. of detect correlation galactic single between The nuclei stars. mass study is of This a the of central valuable galaxy BH, tool samples to shows evidence for a group [ prominent M– velocities of the giantkinematical O–star method. The HDE compact 226868. companion wassuch favoured as Therefore, to a be he a white stellar dwarf determined BH or∼ the because a alternatives mass neutron star function were byhistorical ruled BH out. a verification. Today, the BH in Cygnus X–1Single amounts to stellar orbits with the SMBH inwaveband the so Galactic that astronomers Centre have (GC). tolook Unfortunately, use into radio the waves, the view near–infrared centre (NIR) is of radiation obscuredmotions the or Milky of in X–rays Way. to stars the The around optical first the infrared GC group started BH in [ 1992 to monitor gas and ( Experimental Evidence of Black Holes suitable tracers are luminous stars, gasbe or detected flaring on objects. Earth Any can test body inBH that principle on is serve Keplerian luminous as orbits. enough dynamical to In tracer.scale that at Typically the case, given radius. tracer the surrounds tracking the time scale isPioneering determined work by the Keplerian time nicely the Kepler ellipses ofGalactic this Centre innermost is updated star to and be other S–stars. From this the distance of the [ PoS(P2GC)017 . This r Andreas Müller relation was probed by ]. , breaks down. Recently, 4 σ σ 135 – . Another essential parameter ∝ M σ M ] or not [ ]. However, very recent results from 2 90 relation has significant intrinsic scatter , σ 59 – M line widths serve as tracers to determine the ] ]. ]. relation [ 40 9 136 relation is still complied. However, at very high OIII [ bulge σ M – – M M relation [ σ 3 the ]. The redshift evolution of the ' 13 lines. Narrow z β Another kinematical verification method of black holes involves the ]. Whereas faint normal galaxies drop out of observability, these luminous AGN relation is also theoretically understood [ 122 σ Observed orbital shapes of six S–stars orbiting the Galactic Centre SMBH as projected to the sky. The M– – at least at low BH masses [ M31 (including Milky Way and M32) state that the is determined by comparing primarysecondary emission emission from from the the BLRs galactic (emission core lines) which (continuum is radiation) nothing with else than the response of the emission of gas. It issupposed called to reverberation mapping be technique. luminouscore. The matter broad clouds Due line almost regions to consisting (BLRs) theirTherefore, are of fast the line hydrogen motion width that emission is a surrounds lines measure a are for the galactic significantly velocity broadened of the by BLRs, the Doppler effect. Figure 2: The image is based on NIR observations taken from [ can still be detectedluminosities at and high width cosmological of redshifts. H velocity dispersions. BH masses Out areredshifts, to there measured are from observational hints continuum that the strong correlation, Reverberation mapping Experimental Evidence of Black Holes connection between stars from the galactic bulgeif and the Seyferts central and BH. quasars There follow is a the controversial same debate using quasars [ it was suggested to use a curved M– involved in reverberation mapping is the distance of the BLRs to the centre of that galaxy, PoS(P2GC)017

M (3.2) 7 10 × 6 . Andreas Müller enclosed within 0.4 pc

M 6 10 × 100 s, 219 s, 700 s, 1150 s, and 2250 s. ∼ . G / 2 σ r 10 ≈ 17 min that is interpreted as relativistically modulated M ]. The nature of the flare emitters – possibly linked to ∼ 7 It is challenging to determine whether or not a BH rotates [ ]. The power density spectra of the X–ray flares observed at M 52 close to the GC BH has been observed in NIR and in X–rays: The 9939 . 26 [ . 0 0 = ± a 52 . 0 and ] and in case NGC 3079 astronomers found 2 ' The next kinematical method that is based on gas motion involves maser line

a M 111 6 Let us assume a very sharp line profile in the rest frame of the emitter e.g. an accretion 10 × behaviour, rotation of space–time is extraordinarily strong only at a few gravitational radii 72 3 . − 2 r In general, the notion spectro–relativistic refers to relativistic effects in observed spectra that ]. = ∝ 73 can be used to determineemitter BH has properties. to be The sufficiently condition close for to this the BH. ideaFe to K lines work is of course that the the GC BH revealed distinctA peaks comparison yielding with periods characteristic of frequenciesprecession, associated Keplerian with orbital, accretion vertical, disksM i.e. and Lense–Thirring radial epicyclic frequency,accretion match flow to or orbiting a stars BH – is satisfying still unknown. 3.2 Spectro–relativistic methods disk. Further, this disk rotatestional around a radii BH away and from extends the downwould event to observe horizon. the a ISCO line An i.e. profile astronomical only thattons observer a is are in few very subject a gravita- different to distant from a laboratory that numberDoppler frame of in effect produces physical the a effects rest Doppler that frame redshift happen because atpart on the the of their part line the that way pho- is disk to receding that the and is observer. a approaching First, Doppler to blueshift the at the the observer. The result is a broadened line maybe with two emission of circulating gas. Assuming thatorbit (= the innermost flare stable emitter circular moves orbit, atthe ISCO) last GC the stable observed BH Keplerian period to circular matches be to a Kerr parameter for Kinematical BH spin measurements by using kinematical techniques.ω In most cases indicatorsdistance are to the to BH. far away from theQuasi–periodic BH. flare emission Due to NIR flare exhibits a quasi–periodicity of Experimental Evidence of Black Holes core radiation. From the Virial theorem the central mass is deduced to be [ Today, astronomers are convinced that in nearlyi.e. any center accreting of SMBH a galaxy thereby thereSMBHs causes is in a the SMBH. galaxies An enormous are active AGNorbiting dormant clouds luminosity. and available such inactive In as e.g. BLRsto the (especially Sgr look local for into A* AGN the Universe type–1 core), or the i.e. then M31*. AGN there that is allow good ButMaser observers chance if emission to measure there the are BH mass. emission. luminous Water masers canThis be coherent found microwave emission in from the Keplerianmeasure molecular rotating the torus gas central orbiting mass. located the In at SMBH case thewithin of can 0.13 NGC pc be 4258, pc used scale the [ to of BH mass a could galaxy. be determined to 3 PoS(P2GC)017 : the 2.1 . The line 3 − r ∝ Andreas Müller ) r ( ε ]. It was found that even a Kerr 126 ] for a recent review). 95 . X–ray astronomers often observe such kind of 11 3 line that has a rest frame line energy of 6.4 keV. In α ]. They are part of the so–called ’reflection bump’ in X–ray 44 998. The disk extends from 1.24 (ISCO) to 100 gravitational radii and is . 0 = ] and stellar–mass BHs (see [ M / a 112 , 96 (typical value for AGN type–1). The radial disk emissivity profile obeys ◦ Prototype of an observed relativistic emission line profile that is emitted from a thin accretion disk Meanwhile, many examples are observed that exhibit this kind of relativistic iron lines. This In principle, gravitational redshift is a long–ranging effect that extends to infinity. However, includes quasars [ BH would be consistent with these observations. the effect decreases steeplyBH. with In distance a and recent is theoreticalout study hard to it to several was 10000 detect possible gravitational in to radii show some distance that to distance gravitational the to redshift BH the can in be mass AGN if probed or sufficient spectral resolution profiles as iron K fluorescent linesspectra [ that occurs around several tensdisk when of hit keV. by This hard bump X–ray is radiationnent the e.g. iron from response fluorescent the of primary line the Compton is continuum. ionized the The accretion neutral most Fe promi- K observed photon energy is redshiftedlower relative in to the the observer’s rest frame. frame Alltic energy these emission and three the effects line produce spectral profile an line asymmetric that flux and is is skewed shown relativis- in Fig. 1995, the first relativistically broadenedMCG–6–30–15 iron with K the profile Japanese has X–ray been observatory observed ASCA in [ the Seyfert–1 galaxy Figure 3: around a Kerr BH with inclined to 30 Doppler peaks if the disk is sufficientlyThis inclined already towards the happens observer (longitudinal in Doppler Newtonianis effect). physics. a special Second, relativistic beaming for effectthe intermediate at speed to work. of large light The inclinations Keplerian so there velocitiesintensity that are is radiation amplified indeed is as comparable bent compared to towards to thespace–time the observer. curvature. rest Therefore, frame. This the Third, observed gravitational the radiation BH redshift captures effect line is photons two–fold by strong as described in Sec. Experimental Evidence of Black Holes profile is subject of threerelativistic beaming effects: intensifies The the blue Doppler line effect wing,and due and produces gravitational to an redshift extended rotation smears red out of line the the whole wing. line disk profile causes two horns; special PoS(P2GC)017 ]. and ) = 101 M obs ν = a ( . A Gaussian ◦ Andreas Müller ISCO ]. This offers a r ]. The ISCO de- 21 extend down to the –factor is just the full 123 g not is based on ray tracing simulation ] and in X–rays [ 4 . As a consequence the line emission 71 g ] for details). The r 6 99 ) = 0 = a 12 ( with radiation frequency in the observer’s frame em ISCO ν r / ]. Indeed, this is feasible today and was demonstrated for obs ν 101 = g is the ring radius with maxium emission and the only variable quantity in this where a profile from a Kerr BH is compared to that of a Schwarzschild BH peak 5 R . em ]. In any case, an observed broad profile strongly favours a Kerr BH. But the often ν 99 Relativistic emission line profiles from rings in Keplerian rotation around a Kerr BH obeying 998. The rings are approximately one gravitational radius narrow and inclined to 75 . 0 = Relativistic emission line profiles also indicate BH spin. The basic idea is the following: It is However, this idea is plagued by the fact that real accretion disks may , as compared to Schwarzschild BHs, g M r / ISCO. Radiative cooling might cause diskBH truncation and rotates produce [ a narrow profile even though the known from accretion theory that standard disks extend down to the ISCO [ the Narrow–Line Seyfert–1 galaxy Mrknew 110 detection in method the for optical BHs [ redshifted to features derive should masses point with towards the multi–wavelength same data. mass. All Fig. gravitationally Essentially, the BH space–time induces avicinity relativistic of line the broadening BH. and a line suppression in the pends on BH spin and is closer1 to the event horizon for rapidly spinning Kerr BHs, imposed coupling of ISCO and Kerr parameter should be handled with care. and the inner disk edgeAs couples to expected, the the ISCO line in shapewhereas each is the case line broader (all shape and other around line exhibits thethe Schwarzschild parameters an inner BH are extended disk is the broad edge narrow. same). The and red red (due wing wing to for serves the as the imposed a coupling) Kerr tracer for BH for BH spin. Figure 4: a emissivity profile was chosen toGR Doppler mimic factor luminous that rings is defined (see by [ in the Kerr geometry and shows how the line profile develops when approaching the BH [ comes from regions that areline deeper profiles in the from gravitational the potential vicinitydocumented if of in the Kerr Fig. BH BHs rotates. are Therefore, more the influenced by gravitational redshift. This is with modern telescopes is imposed [ Experimental Evidence of Black Holes emitter’s frame study. The emission line profiles become broader and fainter as the rings move towards the BH. PoS(P2GC)017 . 3 − r 998, . 0 ∝ ] may ) = r ( 103 ε M / a Andreas Müller gives a sketch overview: At the sub– 6 and single power law emissivity g r ]. These authors also compared this model ]. Let us divide the disk into rings each with 0 . 78 123 100 = 13 out r The rest frame spectrum that is emitted near the BH ]. 88 0, green). The inner disk edge is identical to the ISCO in each case (Schwarzschild). All other parameters are the same in both cases such = g r M 0 . / 6 a , disk outer edge ◦ = 30 in r = i 7. . 0 First, we turn to the AGN paradigm. Fig. & (Kerr) and M / g r a Direct comparison of observed relativistic emission line profiles around a Kerr ( 24 . 1 = Accretive methods involve active BHs i.e. BHs that accrete matter thereby causing luminous in r effects. As mentioned incases, the stellar introduction and there supermassive BHs is [ in principle the same physics involved in both 3.3 Accretive methods to observations of BHXBsparameters, and were able to measure BH spins. They found rather high Kerr parsec scale there is aflows due huge to rotating instabilities donut–like into configuration,thick the the accretion center cold flow of develops: dusty the torus.a the AGN. geometrically standard Thereby, This thick a accretion matter and geometrically disk. opticallyform flat thin only Depending and advection–dominated several mainly optically accretion tens on flow ofSMBH accretion (ADAF) gravitational magnetic rate [ radii effects are away favoured from togalaxies the and drive central (radio–loud) strong quasars. SMBH. outflows, Therefore, the In any Jets, the AGN that discovery vicinity are proves of an observed the active in SMBH. radio a specific temperature. Then, theof resulting each disk ring. blackbody spectrum This isenough spectrum built–up to is by the called the BH blackbody multitemperaturethe GR blackbody case effects spectrum. and are a If expected ray the tracing to model disk influence has is the been close proposed blackbody [ radiation. In fact, this is AGN paradigm Figure 5: blue) and a Schwarzschild BH ( i.e. could be of other kind,temperature maximum too. close to the Thin inner accretion disk edge disks [ are known to have a temperature profile and a Multitemperature blackbody spectrum Experimental Evidence of Black Holes as disk inclination angle The velocity field of the emitting disk is Keplerian. PoS(P2GC)017 ]. ]. 9 18 rotating Andreas Müller ]. mm–VLBI offers the best resolution 75 14 . The time evolution of different initial configurations is stud- 7 ] ]. The energy source is the rotational energy of the Kerr BH. 4 , 17 3 , 35 , The physics in the direct SMBH vicinity is currently explored in theory. 48 , 33 , 69 Sketch of the inner parts of an active galactic nucleus (AGN). Consider that this is a sectional view Radio astronomers hope two clarify this point because very long baseline interferometry with Here, the first papers of each group are given; there are many follow–up papers available. 7 . The accretion flow transports the field lines into the BH’s ergosphere. Frame–dragging spins millimetre radio waves (mm–VLBI)Schwarzschild already radii for has near AGN spatial such resolutions as M87 that in Virgo amount [ to only 15–20 Currently, the two mechanisms are under debate. ied in these supercomputer simulations e.g. plasmafield. tori or In accretion disks 1991 or it just an turnedthe electromagnetic out essential that input weak for magnetic anThe fields operating MRI in MHD drives a strong instability, differentially magnetic the rotating turbulence magnetorotationalport. configuration thereby instability are producing This (MRI) efficient is [ angular the momentumThen, precondition trans- two to mechanisms may start produce accretion. magneticallyflux the MHD jets. tubes turbulence The that pushes first pierce mechanism an the involvesmatter inflow magnetic accretion from into disk. the the disk Analoguously BH. causing a totion. magnetically the This driven solar disk extraction surface wind mechanism that the not may flux involving form a tubes the BH extract jets is after known collima- as Blandford–Payne scenario [ available to date. The structures at the jet base will give rise for the jet launching mechanism at Jet base investigations Figure 6: with logarithmic scale along thedisk, axes. hot advection–dominated The inner accretion main flow, structures central SMBH, in and an Jets. AGN are dusty torus, standard accretion The theoretical branch is calleda general number relativistic of magnetohydrodynamics (GRMHD). groups There world–wideflow, that are BH have and Jet GRMHD [ codes to simulate the interaction of accretion Here, the energy source is theBH magnetized accretion flow. The secondup mechanism the needs magnetic a field lines andenough to produces produce a leptonic strong pairs. magnetic An field.accelerates outward directed the At electromagnetic plasma. some energy point flux The (Poynting the associated flux) the field MHD is Blandford–Znajek waves strong drive process the [ plasma magnetically. This is called Experimental Evidence of Black Holes PoS(P2GC)017 ]. 24 (3.3) ]. The 124 and radius GRB there ∗ ]. Further, if m Andreas Müller 74 each feature [ α 4 even indicates BH spin. . 0 ... 1 . 0 3 ' / 1 ε 100. A gravitational collapse of a massive ] that links luminosity to accretion rate and ) ∗ ∼ 38 m / M ( 15 ∗ –ray emission that is followed by X–ray, optical, and R γ ≈ ] astrophysicists are convinced that in ]. From the observed luminosity it is possible to estimate T R 82 41 ]. Thereby, the different accretion and spectral states of BHXBs . In the tidal deformation phase (II) the star is strongly deformed 94 100 AGN in that field can be combined together to form a summed M ∼ The final method that could be termed as accretive verification method Another eruptive detection method for BHs involves stellar tidal disrup- ] for a review. Using Eddington’s relation [ The best examples are GRBs. Due to current paradigms for short–term 131 4 keV rest frame energy. This is interpreted as a stacked Fe K . ] suggest that illustrates this scenario: In the approaching phase (I) the star with mass 6 61 7 ∼ ] and for long–term GRBs [ 114 Eruptive verification methods are always associated with burst–like phenomena. Such events , ]. approaches a BH of mass ∗ 102 76 are quite common in astronomy andBH. the challenge is to select those that areGamma–ray truly associated bursts with a [ 3.4 Eruptive methods Tidal disruption events tions. Fig. can be described in a unified scheme [ best fitting line profile suggests rotating BHs. This finding is consistent with another analysis [ forms a stellar–mass BH. Short–termor GRBs a are neutron merging binaries starthe consisting and total of a energy two output BH. neutron of stars Long–term supernovae GRBs by a are factor also of called hypernovae because they exceed R by tidal forces of the BH. At the tidal radius that satisfies Eddington’s relation BH mass, accretive methods are of particularrate. interest for Accretion high onto accretion a rates SMBH close is toX–ray the a binaries Eddington widely that accepted paradigm contain thatstrong stellar drives AGN X–ray black luminosity. source. holes But also are ProminentGRO observed examples J1655-40 where it are could black Cyg be shown hole X–1 observationally that accretionthe and magnetic Blandford–Payne accretion Cyg drives scenario operates a X–3. as [ expected from Recently, for the BHXB the BH mass. A higher value of the radiative efficiency feature at star e.g. a Wolf–Rayet starultrarelativistic leads jets. to They a produce hypernova. a prompt Inradio any afterglows, see case [ a stellar–mass BH forms and drives Experimental Evidence of Black Holes work. If the jetthe base Blandford–Znajek is process very and small disfavourvolves in the an size Blandford–Payne extended e.g. scenario disk only because withastronomers a the at compare few latter least the Schwarzschild in- several observed radii tens structures thismass of with would and Schwarzschild GRMHD spin. favour radii simulations Mass in this is size mightby restricted [ constrain the by BH apparent the spatial apparent distance size[ of of the the BH jet at bases the if expected a position two–sided and jet also is visible e.g. in case of Cyg A Deep field observations involves deep field observationsThen, i.e. very astronomical faint observations and distantman with hole objects very [ can long be exposure studied. times. X–ray deep field observations of the Lock- PoS(P2GC)017 years per erg that is ]. The BH 4 53 51 ] with typical Andreas Müller 10 42 [ ' 3 / 5 − t ∝ X L .

M 8 ]. Practically, this Hawking emission is to dim for 16 10 63 × 2 ' because for larger BHs the tidal radius is smaller that the

M 8 totally black [ 10 ≈ Sketch of the tidal disruption scenario, see text for details. In the early Seventies, it was shown that even a semi–classical approach ]. ]. A huge drop of the X–ray luminosity in the post–flare spectrum by a are not 65 60 , Figure 7: 72 200 and other characteristics as mentioned before favour that indeed a stellar disruption ]. To exclude flares from AGN activity the X–ray observations have to be accompanied ' 84 The observation of a stellar tidal disruption of a solar–type star constrains immediatelyIn 2004, the it has been reported on a stellar tidal disruption that occured in the non–active galaxy This X–ray flare has typical signatures that hint for a tidal disruption e.g. it is thermal radiation, Recently, another stellar tidal disruption event has been reported also in the UV [ astrophysical BH candidates and estimates show that the horizon illuminated by Hawking radiation timescales of months and thecomparable integrated to supernovae. luminosity Stellar yields tidal an disruptionsgalaxy energy are [ rare output events of that occur once in 10 by simultaneous optical observations. BH mass to be smaller than RX J1242-1119 [ factor of has been detected. Thegives brightness an in estimate the for the blue SMBH band mass as to observed with the Hubble space telescope the X–ray luminosity decays with time according to a power law, the BH’s tidal forces dominate selfgravitytidal of disruption the star. phase As (III). a Theaccreted consequence stellar partly. the If debris star so, is spread a disrupted over characteristic in a X–ray the region flare develops close in to the the accretion and BH flare and phase might (IV). be Schwarzschild radius [ mass can be derived from blackbody spectralbe fits. greater Due than to ISCO the these fact fits that also the provide radius an of estimate emission of should Hawking BH evaporation spin. with quantized scalar fieldsthe on possibility the that background BHs of non–quantized 4D BH space–times results in Experimental Evidence of Black Holes PoS(P2GC)017 24 − (3.4) 10 Planck’s ∼ ¯ h Andreas Müller ] years exceeding GeV, 5 64 2 (as suggested by 19 & 10 n × 221 , from the number of spatial . D 1 , Pl = , M ) G n / m regime has been tested but spatial + 2 µ ( ¯ hc / n p  , is in the ADD scenario [ R = ¯ h Rc Pl  M ) n 17 + 2 ] – but still there are no observational clues that hint ( / . A solar–mass BH decays after 10 2 Pl 3 25 − M Newton’s constant. For all cases g [ M = G 15 ∝ D , τ Pl ]. M 34 . If there is something bright in the environment the BH emerges 1 TeV. However, the mm to ∼ EW m obscuratio , and the compactification Radius, n vacuum speed of light and denotes the classical Planck scale of ]. c Pl 29 , M ]. These authors also estimate that in lead–lead collisions at LHC the absorbtion rate for The lifetime of mini BHs is quite short e.g. a BH with mass of 3 TeVThe lives decay only rate suggests that Hawking evaporation of mini BHs is rather something like an These methods make use of the blackness of the BH. The method’s name is based on the Latin The general formula to compute the reduced Planck scale, For typical cosmic BHs the decay due to energy loss by Hawking radiation can be neglected We make an aside and concentrate onto a new branch of BH physics that will be of great 56 , 30 34 extra dimensions, in the foreground. In BHstreams physics like the a luminous swirl background into the source hole may or be it the may accretion be flow the that cosmic microwave background (CMB). explosion. If the scenarioentitle this outlined as here eruptive verification should method be of a in BH. fact3.5 verified Obscurative at methods LHC then it is justified to word for darkness: elementary particles is much smaller thantheir the surroundings. evaporation rate so that mini BHs can hardly absorb where s [ because the decay time scales with importance in the nearphysics future will that deliver is fruitful new tomensions insights say (more into BHs than BH three) in research: exist particle in Onlower nature accelerators. the and energy condition the scales, that classical Possibly, Planck it spatial high–energy scale extra[ may is di- reduced be to possible significantly to produce mini BHs in high–energy particle collisions Experimental Evidence of Black Holes deviates hardly from the classical zero–luminositydoes horizon. not On allow one to hand, detect the dim signal–to–noiseblurred Hawking ratio with emission other currently; kinds on of the radiation other that hand, are BH even surroundings more are intense. the Hubble time by far.that have However, canonical Hawking masses evaporation around might 10 for be these interesting small for BHs primordial in the BHs Early Universe. constant, M–theory, string theories and supergravity)theory a is modified gravity called occurs TeV atthe quantum the electroweak gravity sub–mm scale, because scale. the This extra Planck dimensions energy have is not been reducedLarge found and Hadron so Collider comparable far. (LHC) at to TeV CERN quantumpossible or gravity at to scenarios International verify Linear can Hawking Collider be emission (ILC). testedproduced Maybe, in at it in terrestrial will the in experiments be TeV because quantum evaporatingdifferent gravity. mini from It BHs classical is are GR important BHs: toPlanck note scale mini that i.e. BHs more these are complex mini higher–dimensional [ BHs and are "stringy" significantly at the reduced PoS(P2GC)017 , so

. The α M 9 ∝ 10 g for this black × Andreas Müller , (counter–clockwise ]. We think that this M 45 ' 0 sets both horizons of a a ] and can be confirmed by = 32 ∆ Black Spot (BS) –factor which satisfies g . M87 is with 18.7 Mpc more distant

). This is the dominant factor producing M 6 ]. a sequence is shown that proves the defor- 2.4 10 75 8 , × 74 , 18 45 ). 0 i.e. spectral flux vanishes at the horizons. denotes the horizon function. 2.11 7→ ∆ α . The study illustrates an extreme Kerr BH, 7→ ], we introduce here again the notion g , where has been presented in Eq. ( 100 α ∝ ∆ α The resulting dark feature has been called ’shadow’ by [ ); at high inclination this ratio deviates significantly to become 1:1.3 and even more Black Spots (BS) 0, then also ]. A detailed study of these models allows in principle for constraining inclination 7→ ) suggests that the intrinsic shape of the event horizon is spherically symmetric since ∆ upper left ). 125 A sequence of apparent shapes of the BH horizon surrounded by a flat accretion disk – in the 2.10 , 100 Krichbaum et al. also suggested good candidate objects with a realistic chance to observe the Eq. ( The lapse function lower right angle of the BH’s rotationcandidate axis is to known. the Interferometric observer, techniques BHthis especially kind spin, in of the and measurements radio BH in band mass the are near if supposed future the to [ distance allow ofBS the soon BH with mm–VLBI techniques,Sgr namely A* the are SMBH 8 at kpc Sgr distance A* and and a M87. SMBH The of parameters mass for 3 notion is somewhat misleading becausegravity. BHs It do is not strong cast space–time curvature a thatAs shadow warps previously geometrically light rays suggested as that [ in they Newtonian cannot escape from the BH. there is no angle dependence.ity the However, appearance this of radius theon is event rotational not horizon state depends an strongly of invariant. on the As inclination BH. angle observed This of from was the infin- first observermation shown and of in Kerr the horizons work in by thedeformations middle [ can of be an used accretion to disk determineies with BH show variable properties inclination that by angles. some observations. These whether uncertainties However, theoretical or are stud- not involved an because accretionthin the disk [ BS is shapes present are or model–dependent whether e.g. or not the accretion disk is geometrically the blackness. Spectral flux is reducedlapse by function higher obeys powers of the hole feature. modern relativistic ray tracing simulations. In Fig. than Sgr A* but thethat putative SMBH the BS is is also bigger by in a size, factor see of Eq. thousand ( more massive, 3 Figure 8: present work called Experimental Evidence of Black Holes rotation) as seen under differenttracing. inclination The angles. BS is The successivelycan shapes deformed be are with used increasing numerical to inclination results parametrizesymmetric angle. from the ( relativistic The deformation: ratio ray At of low the inclination crossing lines the ratio is 1:1 and the BS is more or less Kerr BH. If ( Black spot studies PoS(P2GC)017 Andreas Müller : This is ROSAT’s view of the half Moon taken 9 . The radiation may come from accretion flow – 19 10 . The colour–coded intensity grows from black over blue to white. The bright ◦ ]. The X–ray background emerges around the black half of the Moon’s disk on the Luminous counter–clockwisely rotating accretion disk around an extremal Kerr BH. The incli- X–ray image of the sunlit half Moon taken by the German X–ray observatory ROSAT in 1990 118 In the future, a detector of sufficient spatial resolution and sensitivity could image a com- Let us now turn to the second possibility i.e. that the CMB acts as ambient background source The aberrative verification technique is associated with the fact that BHs cause strong gravita- ]. ]. A nice analogy to this idea is shown in Fig. 27 118 parable snapshot of aphotograph BH of immersed a BH in and ambient should radiation. look like Fig. This technique will deliver the first left–hand side and outlines together with the bright right–hand side the Moon’s total silhouette. [ in 1990 [ beaming spot is close to the BH. tional lensing effects. This means thatdistorted. the visual appearance Many of examples an object can close be to a found BH in in generally the literature proving very strange simulated images nation angle amounts to 40 or even more spectacular from theBH, CMB. whether Using or the not CMB it this is opens the accreting. possibility Inactive, to dormant pinpoint and any 3.6 isolated BHs Aberrative could methods thereby be detected! Figure 10: Figure 9: [ Experimental Evidence of Black Holes PoS(P2GC)017 ◦ 001 . , 60 0 ◦ ]. The = 79 a , 10 Andreas Müller around a Schwarzschild black hole, g 6 r ' r 20 ). The apparent position of the BH is at the origin of the observers black 998 ( . 0 = a . From top to bottom the inclination angles of the orbital plane to the observer are 30 ) Apparent shapes of circular orbits located 0; 0 ( ). . At highest inclinations a clear discrepancy between a non–rotating and a rotating BH is visible. ◦ ), and a Kerr BH, 3.2 gray ( Figure 11: screen in e.g. the shape of an accretion disk around a BH. Pioneering work has been done by [ and 85 knowledge resulting from such simulationsmust can be be stated that used the to current constrainthe spatial BH resolution warped of parameters. appearance. telescopes is State However, still it ofthat not the sufficient are art enough distorted is to by image that a astronomers BHplification use candidate: of two background Either alternatives sources they to from use analyse lensing photometricspectroscopic BHs images methods methods in by i.e. the measuring the foreground, light or total am- astronomers emissionSec. benefit over relativistically from distorted images is measured (see Experimental Evidence of Black Holes PoS(P2GC)017 , orb θ 2 kpc . For stable . 2 11 ' d Andreas Müller and

as. This is feasible by 10M ). Hence the deviation µ S ) as viewed by a distant ' R 4 3 the Kerr parameter i.e. spe- ∼ M a black = , orb , of the Schwarzschild geometry. M r g the outer radius of the orbit in units 6 r 998 . out r 0 ' r = 8 kpc. These are the two spatially closest because . a 1 BH ' θ denotes BH mass, proves that orbit shapes become asymmetric in d 3 21 M ]. All renderings presented here are based on the 11 = and can be found to be only 99 08 in the undermost example shown in Fig. . orb ◦ , is the inner and

0 θ Under special circumstances i.e. background point source, ] resulting from 3D relativistic ray tracing including also 85 in where 46 r ∼ is the inclination angle of the orbital plane relative to the 7M } = i ), 106 i ) it is found that the apparent sizes of these BHs amount only ◦ ' out indicating stable orbits around a Kerr black hole. In contrast, r ) with g , M in 2.11 r 6 r , 2.11 Let us now turn to relativistically distorted images of tight orbits i , . M orb / r a . All results only show images of first order i.e. higher order images from { g r ]. The corresponding light curve is very symmetric and can be easily selected ) vs. a rapidly rotating BH (Kerr, ] and for heavy dark matter galaxy clusters that lens the background emission of as (cf. Eq. ( 107 from top to bottom. We conclude that at low inclinations i.e. nearly face–on situa- µ ]. This phenomenon has been observed for other lens objects such as microlenses – 49 gray ◦ , shows a comparison of apparent orbit shapes around a non–rotating (Schwarzschild, 39 as in both cases (nas–scale!). This is by far out of current astronomical detectability. 85 M µ , 11 3 − 60 001 , . 10 Fig. Let us consider also two BHXBs in contrast, Cyg X–1 satisfying We add here that as already shown in pioneering work strong gravitational lensing emerges Imaging of objects in the BH vicinity is characterised by relativistic blurring. A more so- 0 30 ' = = of gravitational radii, light bending and returning radiation are neglected. mm–VLBI techniques within the next years. as well as XTE J1118+480 with cific angular momentum ofdistant the observer BH, in units of degrees ( observer at infinity. EachThe orbit variable is parameter located in at this thei study ISCO, is the inclination angle of the orbitalKeplerian plane orbit that obeying cycles over at high inclinations i.e.orbit nearly shapes edge–on depending situations on the BHmaximum observer spin. relative clearly From spatial detects the offset different apparenta to apparent orbits 3.6–million–solar–mass be at BH the in sky 7.6amounts we kpc to crudely distance 56 estimate the the apparentbetween size Schwarzschild and of Kerr the at orbital diameter, to only at high inclination angles. Further, Fig. phisticated example is presented in [ tions the observer cannot distinguish a Schwarzschild from a Kerr BH, unless he observes a stellar BHs to Earth. With Eq. ( case of rotating BHs. This iswith the the signature space–time. of frame–dragging causing matter and light to co–rotate physical parameter set a BH in the foreground, and observer arerings collinear occur characteristic [ emission features known as Einstein distant galaxies [ in the zoo of observed light curves.BH However, there only is by to lensing. date But no in singlebeen principle, observation successfully that such has tested a proven in technique a is cosmology possible. for heavy The dark power of matter that lenses, technique seeLensing has e.g. of the Abell orbital catalogue. shapes around BHs. We render theseThe images code by solves relativistic the ray tracing equationequations techniques of by on motion using the for Kerr elliptical null geometry. integrals geodesics [ in Kerr space–time i.e. the geodesics so called MACHOs i.e. massive compact haloin objects, the most Galaxy probably [ brown dwarfs, even planets – Experimental Evidence of Black Holes Lensing of background point sources PoS(P2GC)017 ] τ 6 d ]. The 66 , have been only direct ◦ ]. It has been 115 Andreas Müller 1 with eigenzeit interval dt / τ d = α gravitational waves (GWs) provide the 22 3 ]. 32 . Together with the gravitational redshift effect an external observer ]. These turbulent accretion flows demonstrate that the aberrative dt ). Thereby, lapse is defined as 119 . They were able to demonstrate that the released energy flux is sharply 2.4 g r 6 ]. Flare position and orbital timescale have been used to estimate the SMBH 67 ) = 0 = a ). ( 3.4 ISCO r Hence in principle, such observations constrain the Kerr BH parameters. The computations As anticipated in the beginning of this Sec. The idea for the temporal verification method is that a suitable ’clock’, a periodical event in Giant and rapid X–ray variabilities have been reported for a narrow–line Seyfert–1Recently, it galaxy was shown that the modulation of a transient, redshifted iron K line in the Seyfert Temporal methods are based on the fact that gravity also causes time dilatation effects. This Further examples with ray traced accretion flows are based on pseudo–Newtonian MHD [ < ]. These observations are consistent with strong relativistic effects that have impact onto the orb 20 idea in both proposals isused that to the map the inspiral space–time. of However, a this appealing compact concept object is onto plagued a by the central two massive main body problems can be demonstrated that event horizons have a characteristic imprint onto GW waveforms [ mass. by Cunningham & Bardeencurve suggest and this that radius the wouldr the also constrain orbit BH radius mass can andpeaked spin be in e.g. time determined a if the from Kerr orbit BH the is again close light is to signaled the by event3.8 horizon. Gravitational wave–induced methods the vicinity of a BHoriginate serves from as orbiters a close to BH a indicator. BH.a First Time Kerr calculations dilation BH of were effects the performed observed e.g. light in occur curve the in of Seventies a light [ star curves orbiting that [ X–ray light curve. galaxy NGC 3516 can be explaineda by corotating an flare orbiting [ spot on the accretion disk that is illuminated by or GRMHD simulations [ and coordinate time interval states that "an infalling luminousbined with clock a becomes slow–down of red the and("freezing ticks". effect"). faint For However, when this classical is GR approaching principally BHs thefor not the external hole observable clock observers due com- stops at to ticking infinity. the at Additionally,tidal fact the tidal damage that horizon forces is it will extraordinary appears stretch strong black and for(see squeeze stellar Sec. BHs the rather clock. than The for their supermassive counterparts method for detecting BHs. Electromagnetic waves cannot serve a fiducial tracers [ has been verified by terrestrial experiments (Gravitysible Probe–A). for Strong gravity significant from BHs time isthe dilatation respon- Kerr effects geometry, as Eq. discussed ( at the formula for the lapse function on structure becomes more complicated. 3.7 Temporal methods Experimental Evidence of Black Holes multiple images of higher orders (returningto radiation). a The cardioid complete in orbital thisassumed. trajectory case ressembles where a Schwarzschild BH and moderate inclination of 45 PoS(P2GC)017 ]. 110 event , ]. The 87 109 , 86 Andreas Müller share the same , ]. However, this 85 not 104 does not prove . There is a controversial 2.1 ]. The ADAF argument can be 1 ] or not [ ]. Nevertheless, we feel that within the next 57 113 from a neutron star but it . At least in one case, alternatives to the SMBH a 23 ]. In other cases, there is few space for alternatives . As an astronomer that uses electromagnetic waves 117 , 51 and spin Recently, theoretical results were presented that go beyond M something different ], GW waveforms (once detected) offer the opportunity to count the vanishing luminosity as discussed in Sec. 14 Observations reconcile in many cases with classical BH solutions that are with the BH singularity. In simple words: BH event horizons are in our per se const in principle non–observable = t event horizons (see below) by this ADAF argument. . The interior obeys an equation of state with anisotropic pressure. This behaviour can 2 − without r We further stress here a temporal aspect of BHs that may be regarded as alternative interpre- Based on string theories, a BH alternative was proposed called fuzzball [ As proposed recently [ However, a convincing proof consists in verifying two features of a classical BH: the event ∝ horizons. We argue that it isboth not possible to rule out BH alternatives such as Gravastar or Holostar, tation of gravitational redshift: BH eventat horizons the are Penrose in diagrams the of future classicalinfinity. of BHs external Therefore, the observers! an intrinsic Looking singularity external has observerhypersurface the located character at of asymptotical a flatness future does null interior of this compact dark object consists of strings, too. future and hence we cannot be assuredsphere that of we a compact observe object an without event event horizon. horizon orAlternatives the to gravitational the redshifted classical andEinstein. BH dark Petri found a solution of Einstein’s field equation that is called holostar [ exploited to distinguish neutron starsargument from only BH proves candidates that in it X–ray is binaries [ This new compact object has an externalholostar has metric a that thin is matter identical shell to andρ the an Schwarzschild internal metric solution. that The satisfies a totalnicely energy density be distribution interpreted in theconsists framework of of string radial theories: strings. thesingularity. spherically Interestingly, symmetric holostars holostar are core lacking both, event horizon and curvature years GW astronomy will overcome these rather technical difficulties. debate if black hole event horizons are observable [ hairs and to rule out e.g. SMBH or boson star. 4. Discussion What do we observe? endowed with only two hairs: mass horizon and the intrinsic singularity.The Both problem is have that not the been event horizon observedclassical and by GR its BHs astronomical direct has methods, surroundings are yet! too faint. The event horizon of Experimental Evidence of Black Holes of GW astronomy: first, GWswaveforms have at not their been disposal; detected second,will directly there be so is that a hard source astronomers to confusion have pinpointtimes not problem the and these in non–Kerr GW space–times GW emitter. could astronomy be Another i.e. confused it more [ specific problem is that e.g. Kerr space– are ruled out i.e. in thebecause Galactic rapid Centre space–time [ rotation provesphysics. to From be this an perspective, essential there ingredient is in evidence BHXB, for Kerr GRB, BHs. and AGN PoS(P2GC)017 Andreas Müller ]. This solution exhibits an 98 ]. 15 , 132 24 Besides, classical BH physics is currently confronted ]. Meanwhile, it has been demonstrated that the internal de Sitter 28 ]. But is it was shown that gravastars become stable and more compact 132 ]. 58 , lines do only occur in a fraction of all BH candidates. 19 α Kinematical methods are very robust and deliver precise parameter measurements for both, Meanwhile, spectro–relativistic verification methods such as broad iron K fluorescence lines GRB detections as an eruptive method signal clearly the existence of a BH but only give Accretive methods give reliable estimates for BH mass and spin if one considers Eddington’s The properties and stability of these new BH–like solutions are matter of current research. If all Another alternative to the Schwarzschild BH is the gravastar [ BH mass and BH spin. Allproperties. kinematical We guess methods that are reverberation standard mapping in techniquesespecially astronomy will in to become get more X–rays and insights more because into important Constellation–X BH several are high–precision underway. instruments such as eROSITA, XEUS and represent a standard diagostic tool to studynent innermost Fe accretion flow K and BH. Unfortunately, promi- vague parameters for massevents and are too spin. rare. The most Thisradiation. promising is upcoming But eruptive better in method this for is case basedthe stellar on the underlying decay tidal BHs theory by have offers disruptions, Hawking many particle however variantsdimensions size. – these is The the questionable detecability so serious far. is point quite is that uncertain the because existence of spatial extra 5. Conclusions argument. Promising investigations involveSMBHs deep in surveys the that field. give For risesupposed the for to future, deliver average detailed valuable mappings properties input on of of the the BH structure properties around if BHs compared to with supercomputer VLBI simulations. are external Schwarzschild metric, too. Thean transition region ultrarelativistic consists plasma. of a Unlike thin holostars,Sitter matter space) gravastars shell as are filled internal endowed with metric. withpressure Unfortunately, a the is dark original unstable energy gravastar [ core model with (de isotropic internal metric can be replaced by other forms of dark energy [ with anisotropic pressure [ these solutions turn out to beout stable, one astrophysicists have of a these serious competing problem:only static how possibility solutions should one for is single a that given rotationis Schwarzschild of BH no the candidate? BH alternative To candidate to date, can theholostars, the be classical fuzzballs, measured and Kerr astronomically. gravastars BH. this Then, might But there lead if BH researchers astrophysics will into a find crisis. Developments rotating in generalizations loop of quantum gravity with new clues that followmodel from for the the gravitational framework collapse of of ain quantum scalar field, the geometry. effects course of Considering of loop quantum a the gravitythat simple collapse. cause drives a toy "Loopy bounce strong effects" mass in outflows. granularDetailed space–time research Finally, develop this is a prevents in negative from progress pressure singularity building–up but [ a it curvature is singularity. indicated that this effect might remove the classical BH Experimental Evidence of Black Holes PoS(P2GC)017 ] 40 by [ 2 Andreas Müller ´ c Institute in Zagreb ´ c, Rudjer Boškovi 25 ] to the MPE X–ray group. 118 by [ 9 .

The next important breakthrough is expected to come from gravitational wave astronomy and I thank the organizers of the Dubrovnik summer school for invitation and an excellent Very interesting ongoing research is presented in the discussions. However, strong astronomi- By means of obscurative methods the Black Spot (BS) can be measured. This becomes feasible From aberrative verification methods the rotational state of a BH canTemporal be BH determined verification from methods the are not very common in astronomy. One possible reason Gravitational wave astronomy is a very active research field. The existence of GWs have been Whatsoever the true nature of the compact dark objects may be, mass-energy curves space– M 9 [1] Abramowicz, M. A., Klu´zniak,W., & Lasota, J.-P., 2002,[2] A&A 396, Adams, L31 F. C., Graff, D. S., &[3] Richstone, D. Anninos, O., P., 2001, Fragile, ApJ P. C., 551, & L31 Salmonson,[4] J. D., Anton, 2005, L., ApJ Zanotti, 635, O., 723 Miralles, J.[5] A., et al., Arkani-Hamed, 2006, N., ApJ Dimopoulos, 637, S., 296 & Dvali, G., 1998, Phys. Lett. B 429, 263 to the MPE IR group and Fig. References (Croatia) for inspiring and fruitful discussions. I gratefully acknowledge the use of Fig. we will see if the the classical BH will succeed or fail. Acknowledgments organisation of the whole meeting. I thank Neven Bili cal clues that hint for these newto issues only are a still two–parameter lacking. familiy So proves far, very the powerful classical to Kerr solution explain belonging astronomical observations. with mm–VLBI observations within the nextdetermining few both, years. BH In mass principle, and the BHBS BS size angular and shape momentum, rotation allows because strongly also deforms a for the higherto BS have mass shape. a results More model–dependent theoretical in BS investigations a should gallery be larger that done can be compared to observations. shape of intrinsically circular orbits i.e. if data of a full revolutionfor of that a might close orbiter be are thatthis available. a is expensive. suitable time Mostly, astronomers sequencecurves. prefer needs other observations methods with that longer rather exposure involve times spectra – than light documented indirectly at the famousuntil GW Hulse–Taylor detectors pulsar. provide the BH first astrophysicists directallows source have us detection. to to Then, probe be this BH revolutionary patient space–times neware technique very supposed precisely. to As supply summarized firsthand here, informations characteristic on event waveforms horizons – the keytime. feature of Null BHs. geodesicsTherefore, follow the compact curved masses space–time capturecompact due light. dark to masses where Indeed, the BHs metric astronomers fitmass nicely interpretation found range in. of an – These Gravity. BH impressive from candidates10 number the have of been stellar detected up in a to broad the supermassive regime i.e. BHs with a few up to several Experimental Evidence of Black Holes PoS(P2GC)017 ] ] ] Andreas Müller hep-ph/0301239 gr-qc/0505137 astro-ph/0612491 ] ] ] 26 hep-ph/0210296 ] gr-qc/0604064 gr-qc/0605101 astro-ph/0610657 ´ ´ c, N., Tupper, G. B., & Viollier, R. D.,c, 2006, N., JCAP Lecture 0602, Notes 013 to School on Particle Physics, Gravity and Cosmology, Dubrovnik, 21 Aug - Oviedo, 2005 (ed. L. Mornas) [ 355, 526 & B. S. DeWitt, Gordon and Breach, New York, 241 2 Sep 2006 [ [6] Armitage, P. J., & Reynolds, C. S.,[7] 2003, MNRAS Aschenbach, 341, B., 1041 Grosso, N., Porquet, D.,[8] & Predehl, Aschenbach, P., 2004, B., A&A 2004, 417, A&A 71 425, 1075 [9] Balbus, S. A., & Hawley, J. F., 1991, ApJ 376, 214 [28] Cattoen, C., Faber, T., & Visser, M., 2005, Class. Quantum Grav. 22, 4189 [ [22] Bolton, C. T., 1972, Nature 235,[23] 271 Boyer, R. H. & Lindquist, R.[24] W., 1967, J. Brusa, Math. M., Phys. Gilli, 8, R., 265 & Comastri,[25] A., 2005, Carr, ApJ B. 621, J., L5 2003, Lect. Notes[26] Phy. 631, 301 Carter, B., 1968, Phys. Rev. 174, 1559 [27] Carter, B., 2006, Contrib. to Encuentros Relativistas Espanoles: A Century of Relativity Theory, [29] Cavaglia, M., 2003, IJMPA 18, 1843 [ [30] Chamblin, A., Cooper, F., & Nayak,[31] G. C., 2004, Colbert, Phys. E. Rev. J. D M., 69, & 065010 Mushotzky, [ [32] R. F., 1999, Cunningham, ApJ C. 519, T., 89 & Bardeen, J.[33] M., 1973, De ApJ Villiers, 183, J.-P., 237 & Hawley, J. F.,[34] 2002, ApJ 577, Dimopoulos, 866 S., & Landsberg, G., 2001,[35] Phys. Rev. Lett. Duez, 87, M. 161602 D., Liu, Y. T., Shapiro,[36] S. L., Eckart, & A., Stephens, Genzel, B. R., C., Krabbe, 2005, A., Phys. Hofmann, Rev. R., D van 72, der 024028 Werf, P. P., & Drapatz, S., 1992, Nature Experimental Evidence of Black Holes [10] Bardeen, J. M., 1973, "Rapidly rotating stars, disks, and black holes",[11] in Black Bardeen, Holes, J. ed. M., C. 1974, DeWitt IAU Symp.[12] 64: Gravitational Bardeen, Radiation J. and M., Gravitational & Collapse, Petterson, 132 J.[13] A., 1975, Bender, ApJ R., 195, Kormendy, L65 J., Bower, G., et[14] al., 2005, Berti, ApJ E. 631, & 280 Cardoso, V., 2006 [ [15] Bili [16] Bili [17] Blandford, R., & Znajek, R., 1977,[18] MNRAS 179, Blandford, 433 R. D., & Payne, D.[19] G., 1982, Bojowald, MNRAS M., 199, Goswami, 883 R., Maartens, R.,[20] & Singh, Boller, P., 2005, T., Brandt, Phys. W. Rev. N., Lett. Fabian, 95, A. 091302 [21] C., & Boller, Fink, T., H. Balestra, H., I., 1997, & MNRAS Kollatschny, W., 289, 2006, 393 accepted by A&A [ PoS(P2GC)017 ] Andreas Müller ] astro-ph/0402468 astro-ph/9912263 27 ´ oth, G., 2003, Astrophys. J. 589, 444 B., 2003, Nature 425, 934 G., 2005, ApJ 620, 744 [46] Fanton, C., Calvani, M., Felice, F. de[47] & Cadez, Ferrarese, A., L., 1997, & PASJ Merritt, 49, D., 159 2000,[48] ApJ 539, Gammie, L9 C. F., McKinney, J. C., &[49] T Gaudi, B. S., & Han, C.,[50] 2004, ApJ Gebhardt, 611, K., 528 Bender, R., Bower, G., et[51] al., 2000, Genzel, ApJ R., 539, Eckart, L13 A., Ott, T.,[52] Eisenhauer, F., 1997, Genzel, MNRAS R., 291, Schödel, 219 R., Ott, T., Eckart, A., Alexander, T., Lacombe, F., Rouan,[53] D., & Gezari, Aschenbach, S., Martin, D. C., Milliard,[54] B., et Ghez, al., A. 2006, M., Astrophys. Klein, J. B. 653, L., L25 [55] Morris, M., Ghez, & A. Becklin, M., E. Salim, E., S., 1998, Hornstein, ApJ S. 509, D., 678 Tanner, A., Lu, J.[56] R., Morris, Giddings, M., S. Becklin, E. B., E., & & Thomas, Duchene, S.,[57] 2002, Phys. Glampedakis, Rev. D K., 65, & 056010 Babak, S., 2006,[58] Class. Quantum Goswami, Grav. 23, R., 4167 Joshi, P. S., & Singh,[59] P., 2006, Hähnelt, Phys. M. Rev. Lett. G., 96, & 031302 Kauffmann, G.,[60] 2000, MNRAS Halpern, 318, J. L35 P., Gezari, S., & Komossa,[61] S., 2004, Hasinger, Astrophys. G., J. 2004, 604, Nucl. 572 Phys. [ B[62] (Proc. Suppl.) Hasinger, 132, G., 86 Miyaji, T., & Schmidt, M.,[63] 2005, A&A Hawking, 441, S. 417 W., 1975, Commun. Math. Phys.[64] 43, 199 Henry, R. C., 2000, ApJ 535,[65] 350 Hills, J. G., 1975, Nature 254,[66] 295 Hughes, S. A., 2001, Class. Quantum[67] Grav. 18, 4067 Iwasawa, K., Miniutti, G., & Fabian,[68] A. C., Kerr, 2004, R. MNRAS P., 355, 1963, 1073 Phys. Rev. Lett. 11, 237 Experimental Evidence of Black Holes [37] Eckart, A., & Genzel, R., 1996,[38] Nature 383, Eddington, 415 A. S., 1924, MNRAS 84,[39] 308 Einstein, A., 1936, Science 84, 506 [40] Eisenhauer, F., Genzel, R., Alexander, T., et[41] al., 2005, Esin, ApJ A. 628, A., 246 McClintock, J. E.,[42] & Narayan, Evans, R., C. 1997, R., ApJ & 489, Kochanek, 865 C.[43] S., 1989, ApJ Fabbiano, G., 346, 1989, L13 ARA&A 27, 87 [44] Fabian, A. C., Iwasawa, K., Reynolds,[45] C. S., & Falcke, H., Young, A. Melia, J., F., 2000, & PASP Agol, 112, E., 1145 2000, ApJ 528, L13 [ PoS(P2GC)017 Andreas Müller ] ] ] ] hep-th/0510180 hep-th/0401115 28 hep-th/0502050 astro-ph/0609447 ] ] astro-ph/0402468 astro-ph/0611288 J. 603, L17 [ of ’The 8th European VLBI NetworkTechnology’, Symposium ed. on A. New Marecki Developments et in al.,[ VLBI held Science in and Torun, Poland, on Sep 26-29, 2006 [86] Mathur, S. D., 2005, Fortsch. Phys.[87] 53, 793 Mathur, [ S. D., 2006, Class. Quantum Grav. 23, R115 [ [76] Krichbaum, T. P., priv. commun. [77] Krolik, J. H., Hawley, J. F., &[78] Hirose, S., Li, 2005, L.-X., ApJ Zimmerman, 622, E. 1008 R., Narayan,[79] R., & Luminet, McClintock, J.-P., J. 1979, E., A&A 2005, 75, ApJS 228 157,[80] 335 Lynden-Bell, D., 1969, Nature 223, 690 [81] Lynden-Bell, D. & Rees, M. J.,[82] 1971, MNRAS MacFadyen, 152, A. 461 I., & Woosley, S. E.,[83] 1999, ApJ Magorrian, 524, J., 262 Tremaine, S., Richstone, D.,[84] et al., Magorrian, 1998, J., Astrophys. & J. Tremaine, 115, S., 2285 1999,[85] MNRAS 309, Mathur, 447 S. D., 2005, OHSTPY-HEP-T-04-001 [ Experimental Evidence of Black Holes [69] Koide, S., Shibata, K., & Kudoh,[70] T., 1999, ApJ Koide, 522, S., 727 Shibata, K., Kudoh, T.,[71] & Meier, D. Kollatschny, W., L., 2003, 2002, A&A Science 412, 295, L61 1688 [72] Komossa, S., Halpern, J., Schartel, N., Hasinger, G., Santos-Lleo, M. & Predehl,[73] P., 2004, Kondratko, Astrophys. P. T., Greenhill, L. J., &[74] Moran, J. Krichbaum, M., T. 2004, P., Zensus, IAUS 2004, J. 325 A., &[75] Witzel, A., Krichbaum, 2005, T. AN P., Agudo, 326, I., 548 Bach, U., Witzel, A., & Zensus, J. A., 2006, to appear in the proceedings [88] McHardy, I. M., Koerding, E., Knigge,[89] C., Uttley, P., McKinney, & J. Fender, C., R. 2006, P., 2006, MNRAS Nature 368,[90] 444, 1561 730 McLure, R. J., & Dunlop, J.[91] S., 2001, Merloni, MNRAS A., 327, 2004, 199 MNRAS 353, 1035 [92] Meszaros, P., & Rees, M. J.,[93] 1997, ApJ 482, Meszaros, L29 P., 2002, ARA&A 40, 137 [94] Miller, J. M., Raymon, J., Fabian,[95] A. C., Miller, et J. al., M., 2006, 2006, Nature AN 441, 999, 953 [96] No. 88, Mineo, 789 T., [ Fiore, F., Laor, A., et[97] al., 2000, Misner, A&A W., Thorne, 359, K. 471 S., & Wheeler,[98] J. A., Mottola, 1973, E., Gravitation, & Freeman Mazur, San P. O., Francisco 2002,[99] APS Meeting Müller, Abstracts, A., 12011 & Camenzind, M., 2004, A&A 413, 861 PoS(P2GC)017 Andreas Müller 29 ] gr-qc/0405007 geometry, Landessternwarte Heidelberg, Germany Trümper, J., 1991, Nature 349, 583 A&A 432, 395 University Press, New Haven and London Experimental Evidence of Black Holes [100] Müller, A., 2004, Ph.D. Thesis: Black hole astrophysics: Magnetohydrodynamics on the[101] Kerr Müller, A., & Wold, M., 2006,[102] A&A 457, 485 Narayan, R., Paczynski, B., & Piran,[103] T., 1992, ApJ Narayan, 395, R., L83 & Yi, L. 1994,[104] ApJ 428, Narayan, L13 R., Garcia, M. R., &[105] McClintock, J. Oppenheimer, E., J. 1997, R. ApJ & 478, Snyder, L79 H.,[106] 1939, Phys. Rev. Paumard, 56, T., 455 Perrin, G., Eckart, A.,[107] et al., 2005, Pelló, AN R., 326, Schaerer, D., 568 Richard, J.,[108] Le Borgne, Penrose, J.-F., & R., Kneib, 1969, J.-P., Riv. 2004, Nuovo A&A Cim. 416,[109] 1, L35 252 Petri, M., 2004 [ [110] Petri, M., 2006, submitted to IJMPE [111] Pietsch, W., & Read, A. M.,[112] 2002, A&A 384, Porquet, 793 D., Reeves, J. N., O’Brien,[113] P., & Brinkmann, Remillard, W., R. 2004, A., A&A Lin, 422, D., 85 Cooper,[114] R. L., Ruffert, & M., Narayan, Janka, R., H.-Th., 2005, & AAS Schafer, Meeting[115] G., Abstracts 1995, 207 Ryan, Ap&SS F. 231, D., 423 1995, Phys. Rev. D[116] 52, 5707 Salpeter, E. E., 1964, ApJ 140,[117] 796 Schödel, R., Ott, T., Genzel, R.,[118] et al., Schmitt, 2002, J. Nature H. 419, M. 694 M., Snowden, S. L., Aschenbach, B., Hasinger, G.,[119] Pfeffermann, E., Schnittman, Predehl, J. P. D., & Krolik, J. H.,[120] & Hawley, J. Schwarzschild, F., K., 2006, 1916, ApJ Sitzber. 651, Deut. 1031 Akad.[121] Wiss. Berlin 189 Shapiro, S. L., 2005, ApJ 620,[122] 59 Shields, G. A., Gebhardt, K., Salviander,[123] S., et Shakura, al., N. 2003, L., ApJ & 583, Sunyaev, 124 R.[124] A., 1973, A&A Streblyanska, 24, A., 337 Hasinger, G., Finoguenov, A., Barcons, X., Mateos, S., & Fabian,[125] A. C., Takahashi, 2005, R., 2004, ApJ 611, 996 [126] Tanaka, Y., Nandra, K., Fabian, A.C., et[127] al., 1995, Thorne, Nature K. 375, S., 659 1974, ApJ 191,[128] 507 Thorne, K. S., Price, R. H., & MacDonald, D. A., 1986,[129] Black holes: Tremaine, The S., membrane Gebhardt, paradigm, K., Yale Bender, R.,[130] et al., van 2002, der ApJ Klis, 574, M., 740 2000, ARA&A 38, 717 PoS(P2GC)017 Andreas Müller 30 Heidelberg, Germany Experimental Evidence of Black Holes [131] van Paradijs, J., Kouveliotou, C., &[132] Wijers, R. A. Vigelius, M., M. 2004, J., diploma 2000, thesis: ARA&A 38, Structure 379 and stability of Gravastars, Landessternwarte [133] Volonteri, M., Haardt, F., & Madau, P.,[134] 2003, ApJ Volonteri, 582, M., 559 Madau, P., Quataert, E., &[135] Rees, M. Wandel, J., A., 2005, Peterson, ApJ B. 620, M., 69 &[136] Malkan, M. Wyithe, A., J. 1999, S. ApJ B., 526, 2006, 579 MNRAS[137] 365, 1082 Zel’dovich, Y. B. & Novikov, I. D., 1964, Sov. Phys. Dokl. 158, 811