aa r X i v : . [ a s t r o - ph ] S e p Microquasars: summary and outlook
I.F. Mirabel
Laboratoire AIM,Irfu/Service d’Astrophysique, Bat. 709, CEA-Saclay, 91191Gif-sur-Yvette Cedex, France and Instituto de Astronom´ıa y F´ısica del Espacio(IAFE), CC 67, Suc. 28, 1428 Buenos Aires, Argentina [email protected]
Summary.
Microquasars are compact objects (stellar-mass black holes and neutronstars) that mimic, on a smaller scale, many of the phenomena seen in quasars.Their discovery provided new insights into the physics of relativistic jets observedelsewhere in the universe, and in particular, the accretion–jet coupling in black holes.Microquasars are opening new horizons for the understanding of ultraluminous X-ray sources observed in external galaxies, gamma-ray bursts of long duration, andthe origin of stellar black holes and neutron stars. Microquasars are one of the bestlaboratories to probe General Relativity in the limit of the strongest gravitationalfields, and as such, have become an area of topical interest for both high energyphysics and astrophysics. At present, back hole astrophysics exhibits historical andepistemological similarities with the origins of stellar astrophysics in the last century.
Microquasars are binary stellar systems where the remnant of a star that has col-lapsed to form a dark and compact object (such as a neutron star or a black hole) isgravitationally linked to a star that still produces light, and around which it makesa closed orbital movement. In this cosmic dance of a dead star with a living one, thefirst sucks matter from the second, producing radiation and very high energetic par-ticles (Fig. 1). These binary star systems in our galaxy are known under the nameof “microquasars” because they are miniature versions of the quasars (‘quasi-stellar-radio-source’), that are the nuclei of distant galaxies harboring a super massive blackhole, and are able to produce in a region as compact as the solar system, the lu-minosity of 100 galaxies like the Milky Way. Nowadays the study of microquasarsis one of the main scientific motivations of the space observatories that probe theX-ray and γ -ray Universe.Despite of the differences in the involved masses and in the time and lengthscales, the physical processes in microquasars are similar to those found in quasars.That is why the study of microquasars in our galaxy has enabled a better under-standing of what happens in the distant quasars and AGN. Moreover, the study ofmicroquasars may provide clues for the understanding of the class of gamma-raybursts that are associated to the collapse of massive stars leading to the formation I.F. Mirabelof stellar black holes, which are the most energetic phenomena in the Universe afterthe Big-Bang. Fig. 1.
In our galaxy there exist binary stellar systems where an ordinary stargravitates around a black hole that sucks the outer layers of the star’s atmosphere.When falling out to the dense star, the matter warms and emits huge amounts ofenergy as X- and γ -rays. The accretion disk that emits this radiation also producesrelativistic plasma jets all along the axis of rotation of the black hole. The physicalmechanisms of accretion and ejection of matter are similar to those found in quasars,but in million times smaller scales. Those miniature versions of quasars are knownunder the name of ‘microquasars’. During the second half of the 18th century, John Michell and Pierre-Simon Laplacefirst imagined compact and dark objects in the context of the classical conceptof gravitation. In the 20th century in the context of Einstein’s General Relativitytheory of gravitation, those compact and dark objects were named black holes. Theywere then identified in the sky in the 1960s as X-ray binaries. Indeed, those compactobjects, when associated to other stars, are activated by the accretion of very hotgas that emits X and γ -rays. In 2002, Riccardo Giacconi was awarded the Nobelicroquasars: summary and outlook 3Prize for the development of the X-ray Space Astronomy that led to the discoveryof the first X-ray binaries [12]. Later, Margon et al. [18] found that a compact binaryknown as SS 433 was able to produce jets of matter. However, for a long time, peoplebelieved that SS 433 was a very rare object of the Milky Way and its relation withquasars was not clear since the jets of this object move only at 26% of the speed oflight, whereas the jets of quasars can move at speeds close to the speed of light.In the 1990s, after the launch of the Franco–Soviet satellite GRANAT, growingevidences of the relation between relativistic jets and X-ray binaries began to appear.The on-board telescope SIGMA was able to take X-ray and γ -ray images. It detectednumerous black holes in the Milky Way. Moreover, thanks to the coded-mask-optics,it became possible for the first time to determine the position of γ -ray sources witharcmin precision. This is not a very high precision for astronomers who are usedto dealing with other observing techniques. However, in high-energy astrophysics itrepresented a gain of at least one order of magnitude. It consequently made possiblethe systematic identification of compact γ -ray sources at radio, infrared and visiblewavelengths.With SIGMA/GRANAT it was possible to localize with an unprecedented pre-cision the hard X-ray and γ -ray sources. In order to determine the nature of thoseX-ray binaries, a precision of a few tens of arc-seconds was needed. Sources thatproduce high energy photons should also produce high energy particles, that shouldthen produce synchrotron radiation when accelerated in magnetic fields. Then, withLuis Felipe Rodr´ıguez, we performed a systematic search of synchrotron emissionsfrom X-ray binaries with the Very Large Array (VLA) of the National Radio As-tronomy Observatory of the USA.In 1992, using quasi-simultaneous observations from space with GRANAT andfrom the ground with the VLA, we determined the position of the radio counterpartof an X-ray source named 1E 1740.7-2942 with a precision of sub-arc-seconds. WithGRANAT this object was identified as the most luminous, persistent source of soft γ -rays in the Galactic center region. Moreover, its luminosity, variability and theX-ray spectrum were consistent with those of an accretion disk gravitating arounda stellar mass black hole, like in Cygnus X-1. The most surprising finding with theVLA was the existence of well collimated two-sided jets that seem to arise from thecompact radio counterpart of the X-ray source [22]. These jets of magnetized plasmahad the same morphology as the jets observed in quasars and radio galaxies. Whenwe published those results, we employed the term microquasar to define this new X-ray source with relativistic jets in our Galaxy. This term appeared on the front pageof the British journal Nature (see Fig. 2), which provoked multiple debates. Todaythe concept of microquasar is universally accepted and used widely in scientificpublications.Before the discovery of its radio counterpart, 1E 1740.7-2942 was suspected tobe a prominent source of 511 keV electron-positron annihilation radiation observedfrom the centre of our Galaxy [17], and for that reason it was nicknamed as the“Great Annihilator”. It is interesting that recently it was reported [40] that thedistribution in the Galactic disk of the 511 keV emission, due to positron-electronannihilation, exhibit similar asymmetric distribution as that of the hard low massX-ray binaries, where the compact objects are believed to be stellar black holes.This finding suggests that black hole binaries may be important sources of positronsthat would annihilate with electrons in the interstellar medium. Therefore, positron-electron pairs may be produced by γ – γ photon interactions in the inner accretion I.F. Mirabeldisks, and microquasar jets would contain positrons as well as electrons. If this recentreport is confirmed, 1E 1740.7-2942 would be the most prominent compact sourceof anti-matter in the Galactic Centre region. Fig. 2.
The British journal Nature announced the 16th of July, 1992 the discoveryof a microquasar in the galactic centre region [22]. The image shows the synchrotronemission at a radio wavelength of 6cm produced by relativistic particles jets ejectedfrom some tens of kilometers to light-years of distance from the black hole binarywhich is located inside the small white ellipse.
If the proposed analogy [25] between microquasars and quasars was correct, it shouldbe possible to observe superluminal apparent motions in Galactic sources. How-ever, superluminal apparent motions had been observed only in the neighborhoodof super-massive black holes in quasars. In 1E 1740.7-2942 we could not be ableto discern motions, as in that persistent source of γ -rays the flow of particles issemi-continuous. The only possibility of knowing if superluminal apparent move-ments exist in microquasars was through the observation of a discreet and veryintense ejection in an X-ray binary. This would allow us to follow the displacementin the firmament of discrete plasma clouds. Indeed, with the GRANAT satelliteicroquasars: summary and outlook 5was discovered [3] a new source of X-rays with such characteristics denominatedGRS 1915+105. Then with Rodr´ıguez we began with the VLA a systematic cam-paign of observations of that new object in the radio domain, and in collaborationwith Pierre-Alain Duc (CNRS-France) and Sylvain Chaty (Paris University) we per-formed the follow-up of this source in the infrared with telescopes of the SouthernEuropean Observatory, and telescopes at Mauna Kea, Hawaii.Since the beginning, GRS 1915+105 exhibited unusual properties. The observa-tions in the optical and the infrared showed that this X-ray binary was very absorbedby the interstellar dust along the line of sight in the Milky Way, and that the in-frared counterpart was varying rapidly as a function of time. Moreover, the radiocounterpart seemed to change its position in the sky, so that at the beginning wedid not know if those changes were due to radiation reflection or refraction in aninhomogeneous circumstellar medium (“Christmas tree effect”), or rather due to themovement at very high speeds of jets of matter. For two years we kept on watchingthis X-ray binary without exactly understanding its behavior. However, in March1994, GRS 1915+105 produced a violent eruption of X and γ -rays, followed by abipolar ejection of unusually bright plasma clouds, whose displacement in the skycould be followed during 2 months. From the amount of atomic hydrogen absorbedin the strong continuum radiation we could infer that the X-ray binary stands atabout 30000 light years from the Earth. This enabled us to know that the movementin the sky of the ejected clouds implies apparent speeds higher than the speed oflight.The discovery of these superluminal apparent movements in the Milky Way wasannounced in Nature [23] (Fig. 3). This constituted a full confirmation of the hypoth-esis, that we had proposed two years before, on the analogy between microquasarsand quasars. With Rodr´ıguez we formulated and solved the system of equations thatdescribe the observed phenomenon. The apparent asymmetries in the brightness andthe displacement of the two plasma clouds could naturally be explained in termsof the relativistic aberration in the radiation of twin plasma clouds ejected in anantisymmetric way at 98% of the speed of light [26]. The super-luminal motionsobserved in 1994 with the VLA [23] were a few years later re-observed with higherangular resolution using the MERLIN array [10].Using the Very Large Telescope of the European Southern Observatory, it waspossible to determine the orbital parameters of GRS 1915+105, concluding that itis a binary system constituted by a black hole of ∼
14 solar masses accompanied bya star of 1 solar mass [13]. The latter has become a red giant from which the blackhole sucks matter under the form of an accretion disk (see Fig. 1).
The association of bipolar jets and accretion disks seems to be a universal phe-nomenon in quasars and microquasars. The predominant idea is that matter jetsare driven by the enormous rotation energy of the compact object and accretiondisk that surrounds it. Through magneto-hydrodynamic mechanisms, the rotationenergy is evacuated through the poles by means of jets, as the rest can fall towardsthe gravitational attraction centre. In spite of the apparent universality of this re-lationship between accretion disks and bipolar, highly collimated jets, the temporalsequence of the phenomena had never been observed in real time. I.F. Mirabel
Fig. 3.
The journal Nature announces the 1st of September, 1994 the discoveryof the first Galactic source of superluminal apparent motions [23]. The sequence ofimages shows the temporal evolution in radio waves at a wavelength of 3.6 cm of apair of plasma clouds ejected from black hole surroundings at a velocity of 98% thespeed of the light.Since the scales of time of the phenomena around black holes are proportionalto their mass, the accretion-ejection coupling in stellar-mass black holes can beobserved in intervals of time that are millions of time smaller than in AGN andquasars. Because of the proximity, the frequency and the rapid variability of energeticeruptions, GRS 1915+105 became the most adequate object to study the connectionbetween instabilities in the accretion disks and the genesis of bipolar jets.After several attempts, finally in 1997 we could observe [24] on an interval oftime shorter than an hour, a sudden fall in the luminosity in X and soft γ -rays, fol-lowed by the ejection of jets, first observed in the infrared, then at radio frequencies(see Fig. 4). The abrupt fall in X-ray luminosity could be interpreted as the silentdisappearance of the warmer inner part of the accretion disk beyond the horizonof the black hole. A few minutes later, fresh matter coming from the companionstar come to feed again the accretion disk, which must evacuate part of its kineticenergy under the form of bipolar jets. When moving away, the plasma clouds expandadiabatically, becoming more transparent to its own radiation, first in the infraredand then in radio frequency. The observed interval of time between the infrared andradio peaks is consistent with that predicted by van der Laan [39] for extragalacticradio sources.icroquasars: summary and outlook 7Based on the observations of GRS 1915+105 and other X-ray binaries, it wasproposed [11] proposed a unified semiquantitative model for disk-jet coupling inblack hole X-ray binary systems that relate different X-ray sates with radio states,including the compact, steady jets associated to low-hard X-ray states, that had beenimaged [5] using the Very Long Baseline Array of the National Radio AstronomyObservatory.After three years of multi-wavelength monitoring an analogous sequence of X-ray emission dips followed by the ejection of bright super-luminal knots in radio jetswas reported [19] in the active galactic nucleus of the galaxy 3C 120. The meantime between X-ray dips was of the order of years, as expected from scaling withthe mass of the black hole. Fig. 4.
Temporal sequence of accretion disk – jet coupling observed for first timein real time simultaneously in the X-rays, the infrared and radio wavelengths in themicroquasar GRS 1915+105 [24]. The ejection of relativistic jets takes place after theevacuation and/or dissipation of matter and energy, at the time of the reconstructionof the inner side of the accretion disk, corona or base of the jet. A similar process hasbeen observed years later in quasars [19], but on time scales of years. As expected inthe context of the analogy between quasars and microquasars [25], the time scale ofphysical processes in the surroundings of black holes is proportional to their masses. I.F. Mirabel
Horizon is the basic concept that defines a black hole: a massive object that con-sequently produces a gravitational attraction in the surrounding environment, butthat has no material border. In fact, an invisible border in the space-time, which ispredicted by general relativity, surrounds it. This way, matter could go through thisborder without being rejected, and without losing a fraction of its kinetic energy ina thermonuclear explosion, as sometimes is observed as x-ray bursts of type I whenthe compact object is a neutron star instead of a black hole. In fact, as shown inFig. 4, the interval of time between the sudden drop of the flux and the spike in theX-ray light curve that marks the onset of the jet signaled by the starting rise of theinfrared synchrotron emission is of a few minutes, orders of magnitude larger thanthe dynamical time of the plasma in the inner accretion disk. Although the dropof the X-ray luminosity could be interpreted as dissipation of matter and energy,the most popular interpretation is that the hot gas that was producing the X-rayemission falls into the black hole, leaving the observable Universe.So, have we proved with such observations the existence of black holes? Indeed,we do not find any evidence of material borders around the compact object thatcreates gravitational attraction. However, the fact that we do not find any evidencefor the existence of a material surface does not imply that it does not exist. In fact,such type I x-ray bursts are only observed in certain range of neutron star massaccretion rates. That means that it is not possible to prove the existence of blackholes using the horizon definition. According to Saint Paul, “faith is the substance ofhope for: the evidence of the not seen” . That is why for some physicists, black holesare just objects of faith. Perhaps the intellectual attraction of these objects comesfrom the desire of discovering the limits of the Universe. In this context, studyingthe physical phenomena near the horizon of a black hole is a way of approachingthe ultimate frontiers of the observable Universe.
For an external observer, black holes are the simplest objects in physics since theycan be fully described by only three parameters: mass, rotation and charge. Althoughblack holes could be born with net electrical charge, it is believed that because ofinteraction with environmental matter, astrophysical black holes rapidly becomeelectrically neutral. The masses of black holes gravitating in binary systems can beestimated with Newtonian physics. However, the rotation is much more difficult toestimate despite it being probably the main driver in the production of relativisticjets.There is now the possibility of measuring the rotation of black holes by at leastthree different methods: a) X-ray continuum fitting [41, 20], b) asymmetry of thebroad component of the Fe K α line from the inner accretion disk [38], and c) quasi-periodic oscillations with a maximum fix frequency observed in the X-rays [35].The main source of errors in the estimates of the angular momentum resides in theuncertainties of the methods employed.The side of the accretion disk that is closer to the black hole is hotter andproduces huge amounts of thermal X and γ radiations and is also affected by theicroquasars: summary and outlook 9strange configuration of space-time. Indeed, next to the black hole, space-time iscurved by the black hole mass and dragged by its rotation. This produces vibrationsthat modulate the X-ray emission. Studies of those X-ray continuum and vibrationssuggest that the microquasars that produce the most powerful jets are indeed thosethat are rotating fastest. It has been proposed that these pseudo-periodic oscillationsin microquasars are, moreover, one of the best methods today to probe by meansof observations general relativity theory in the limit of the strongest gravitationalfields.Analogous oscillations in the infrared range, may have been observed in thesuper massive black hole at the centre of the Milky Way. The quasi-periods of theoscillations (a few milliseconds for the microquasars X-ray emission and a few tensof minutes for the galactic centre black hole infrared emission) are proportionallyrelated to the masses of the objects, as expected from the physical analogy betweenquasars and microquasars. Comparing the phenomenology observed in microquasarsto that in black holes of all mass scales, several correlations among observables suchas among the radiated fluxes in the low hard X-ray state, quasi-periodic oscillations,flickering frequencies, etc., are being found and used to derive the mass and angularmomentum, which are the fundamental parameters that describe astrophysical blackholes. Have microquasars been observed beyond the Milky Way galaxy? X-ray satellitesare detecting far away from the centers of external galaxies large numbers of com-pact sources called ‘ultraluminous X-ray sources’, because their luminosities seemto be greater than the Eddington limit for a stellar-mass black hole [9]. Althougha few of these sources could be black holes of intermediate masses of hundreds tothousands solar masses, it is believed that the large majority are stellar-mass blackhole binaries.Since the discovery of quasars in 1963, it was known that some quasars couldbe extremely bright and produce high energetic emissions in a short time. Theseparticular quasars are called blazars and it is thought that they are simply quasarswhose jets point close to the Earth’s direction. The Doppler effect produces thus anamplification of the signal and a shift into higher frequencies. With Rodr´ıguez weimagined in 1999 the existence of microblazars, that is to say X-ray binaries wherethe emission is also in the Earth’s direction [26]. Microblazars may have been alreadyobserved but the fast variations caused by the contraction of the time scale in therelativistic jets, make their study very difficult. In fact, one question at the timeof writing this chapter is whether microblazars could have been already detectedas “fast black hole X-ray novae” [14]. In fact, the so called “fast black hole X-raynovae” Swift J195509.6+261406 (which is the possible source of GRB 070610 [14]),and V4541 Sgr [31] are compact binaries that appeared as high energy sources withfast and intense variations of flux, as expected in microblazars [26].Although some fast variable ultraluminous X-ray sources could be microblazars,the vast majority do not exhibit the intense, fast variations of flux expected in rel-ativistic beaming. Therefore, it has been proposed [15] that the large majority are0 I.F. Mirabelstellar black hole binaries where the X-ray radiation is –as the particle outflows–anisotropic, but not necessarily relativistically boosted. In fact, the jets in the Galac-tic microquasar SS 433, which are directed close to the plane of the sky, have kineticluminosities of more than a few times 10 erg/sec, which would be super-Eddingtonfor a black hole of 10 solar masses.An alternative model is that ultraluminous X-ray sources may be compact bina-ries with black holes of more than 30 solar masses that emit largely isotropically withno beaming into the line of sight, either geometrically or relativistically [33]. Thisconclusion is based on the formation, evolution and overall energetics of the ionizednebulae of several 100 pc diameter in which some ultraluminous X-ray sources arefound embedded. The recent discoveries of high mass binaries with black holes of15.7 solar masses in M 33 [32] and 23-34 solar masses in IC 10 [34] support this idea.Apparently, black holes of several tens of solar masses could be formed in starburstgalaxies of relative low metal content. γ -ray emission from compact binaries Very energetic γ -rays with energies greater than 100 gigaelectron volts have recentlybeen detected with ground based telescopes from four high mass compact binaries[21]. These have been interpreted by models proposed in the contexts representedin Fig. 5. In two of the four sources the γ radiation seems to be correlated with theorbital phase of the binary, and therefore may be consistent with the idea that thevery high energy radiation is produced by the interaction of pulsar winds with themass outflow from the massive companion star [8, 6]. The detection of TeV emissionfrom the black hole binary Cygnus X-1 [2] and the TeV intraday variability in M 87[1] provided support to the jet models [37], which do not require relativistic Dopplerboosting as in blazars and microblazars . It remains an open question whether the γ -ray binaries LS 5039 and LS I +61 303 could be microquasars where the γ radiationis produced by the interaction of the outflow from the massive donor star with jets[37] or pulsar winds [8]. It is believed that gamma-ray bursts of long duration (t > Fig. 5.
Alternative contexts for very energetic γ -ray binaries [21]. Left: micro-quasars are powered by compact objects (neutron stars or stellar-mass black holes)via mass accretion from a companion star. The interaction of collimated jets with themassive outflow from the donor star can produce very energetic γ -rays by differentalternative physical mechanisms [37], depending on whether the jets are baryonic orpurely leptonic. Right: pulsar winds are powered by rotation of neutron stars; thewind flows away to large distances in a comet-shape tail. Interaction of this windwith the companion-star outflow may produce very energetic γ -rays [8].object. Knowing the distance, proper motion, and radial velocity of the centre ofmass of the binary, the space velocity and past trajectory can be determined. Usingmulti-wavelength data obtained with a diversity of observational techniques, thekinematics of eight microquasars have so far been determined.One interesting case is the black hole wandering in the Galactic halo, which ismoving at high speed, like globular clusters [27] (Fig. 6). It remains an open questionwhether this particular halo black hole was kick out from the Galactic plane by anatal explosion, or is the fossil of a star that was formed more than 7 billions of yearsago, before the spiral disk of stars, gas and dust of the Milky Way was formed. Inthis context, the study of these stellar fossils may represent the beginning of whatcould be called ‘Galactic Archaeology’. Like archaeologists, studying these stellarfossils, astrophysicists can infer what was the history of the Galactic halo.The microquasars LS 5039 [36] and GRO J1655-40 [29] which contain compactobjects with less that ∼ Fig. 6.
A wandering black hole in the Galactic halo [27]. The trajectory of theblack hole for the last 230 million years is represented in red. The bright dot on theleft represents the Sun.consistent with the recent finding of gamma-ray bursts of long duration in the nearuniverse without associated luminous supernovae [4].There are indications that the mass of the resulting black hole may be a functionof the metal content of the progenitor star. In fact, the black holes with 16 solarmasses in M 33 [32] and more than 23 solar masses in IC 10 [34], are in smallgalaxies of low metal content. This is consistent with the fact that the majority ofthe gamma-ray bursts of long duration take place in small starburst galaxies at highredshift, namely, in galactic hosts of low metal content [16]. Since the power andredshift of gamma-ray bursts seem to be correlated this would imply a correlationbetween the mass of the collapsing stellar core and the power of the γ -ray jets.Gamma-ray bursts of long duration are believed to be produced by ultra rela-tivistic jets generated in a massive star nucleus when it catastrophically collapses toform a black hole. Gamma-ray bursts are highly collimated jets and it has been pro-posed [28] that there may be a unique universal mechanism to produce relativisticjets in the Universe, suggesting that the analogy between microquasars and quasarscan be extended to the gamma-ray bursts sources, as illustrated in the diagram ofFig. 7.
10 Conclusions
Black-hole astrophysics is presently in an analogous situation as was stellar astro-physics in the first decades of the 20th century. At that time, well before the physicalunderstanding of the interior of stars and the way by which they produce and radiateenergy, empirical correlations such as the HR diagram were found and used to derivefundamental properties of the stars, such as the mass. Similarly, at present beforea comprehensive understanding of black hole physics, empirical correlationsbetweenX-ray and radio luminosities and characteristic time scales are being used to deriveicroquasars: summary and outlook 13
Fig. 7.
The same physical mechanism can be responsible for three different typesof objects: microquasars, quasars and massive stars that collapse (‘collapsars’) toform a black hole producing gamma-ray bursts. Each one of these objects contains ablack hole, an accretion disk and relativistic particles jets. Quasars and microquasarscan eject matter several times, whereas the collapsars form jets only once. Whenthe jets are aligned with the line of sight of the observer these objects appear asmicroblazars, blazars and gamma-ray bursts, respectively. Reproduced from [28].the mass and spin of black holes of all mass scales, which are the fundamental pa-rameters that describe astrophysical black holes. Therefore, there are historical andepistemological analogies between black hole astrophysics and stellar astrophysics.The research area on microquasars has become one of the most important areas inhigh energy astrophysics. In the last 14 years there have been seven internationalworkshops on microquasars: 4 in Europe, 1 in America and 2 in Asia. They are cur-rently attended by 100–200 young scientists who, with their work on microquasars,are contributing to open new horizons in the common ground of high energy physicsand modern astronomy.
Apologies:
This manuscript is based on short courses given at internationalschools for graduate students, intended to give an introduction to this area of re-search. It is biased by my own personal choice, and hence it is by no means acomprehensive review. Because references had to be minimized I apologize for in-completeness to colleagues working in the field. Part of this work was written whilethe author was staff member of the European Southern Observatory in Chile.4 I.F. Mirabel
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