1974: the discovery of the first binary pulsar
aa r X i v : . [ g r- q c ] F e b Thibault
Damour E-mail: [email protected] Institut des Hautes Etudes Scientifiques, 35, route de Chartres,91440 Bures-sur-Yvette, France
Abstract
The 1974 discovery, by Russell A. Hulse and Joseph H. Taylor, of thefirst binary pulsar PSR B1913+16, opened up new possibilities for thestudy of relativistic gravity. PSR B1913+16, as well as several other bi-nary pulsars, provided direct observational proofs that gravity propagatesat the velocity of light and has a quadrupolar structure. Binary pulsarsalso provided accurate tests of the strong-field regime of relativistic grav-ity. General Relativity has passed all the binary pulsar tests with flyingcolors. The discovery of binary pulsars had also very important conse-quences for astrophysics: accurate measurement of neutron star masses,improved understanding of the possible evolution scenarios for the co-evolution of binary stars, proof of the existence of binary neutron starsemitting gravitational waves for hundreds of millions of years, before coa-lescing in catastrophic events radiating intense gravitational-wave signals,and probably leading also to important emissions of electromagnetic ra-diation and neutrinos. This article reviews the history of the discovery ofthe first binary pulsar, and describes both its immediate impact, and itslonger-term effect on theoretical and experimental studies of relativisticgravity.
PACS numbers: 04.20.-q, 04.25.Nx, 04.30.-w, 04.80.Cc, 97.60.Gb1
Prelude
The first binary pulsar, PSR 1913+16 (henceforth referred to by its currentlyrecommended name, PSR B1913+16), was discovered, at the Arecibo radio tele-scope, on July 2, 1974 in the middle of a systematic pulsar search carried out byRussell A. Hulse as a basis for his PhD thesis under the guidance of Joseph H.Taylor [1]. The research proposal behind this collaborative work of Hulse andTaylor had been submitted by Joe Taylor to the US National Science Foundationin September 1972 and had mentionned, among the motivations for planning anextensive pulsar survey, that it would be highly desirable “ . . . to find even one example of a pulsar in a binary system, for measurement of its parameters couldyield the pulsar mass, an extremely important number” [2]. The discovery of abinary pulsar was not, however, the main motivation for the new pulsar searchundertaken by Joe Taylor and Russell Hulse. At the time of Joe’s proposal(September 1972), sixty seven pulsars had been discovered since the discoveryof the first pulsar by Jocelyn Bell and Antony Hewish at Cambridge University(UK) in 1967. However, Joe Taylor had realized that there was room for asystematic, high-sensitivity pulsar survey using the largest existing radio tele-scope, and, most importantly, using a computerized search, based on an efficientdispersion-compensating algorithm, in the three-dimensional space spanned bydispersion measure, period and pulse width. This computerized search algo-rithm, combined with the large Arecibo telescope, achieved a pulsar detectionsensitivity over ten times better than that reached by any previous pulsar search[3]. This high-sensitivity was crucial for the discovery of the first binary pul-sar. Indeed, PSR B1913+16 is a rather weak pulsar, and its initial detectionoccurred at 7 . σ , i.e. just above the search detection threshold of 7 . σ . [Sucha seemingly high detection threshold was necessary to keep the false detectionrate reasonably low in spite of the large parameter space being searched.] Inother words, the initial discovery of the first binary pulsar was a very close call,but the success was due to all the effort put by Russell Hulse and Joe Tayloron getting every last bit of possible sensitivity out of their computerized pulsarsearch algorithm.When Russell Hulse discovered PSR B1913+16 on July 2, 1974, he inscribedthe comment “fantastic!” on the discovery form because its 59 msec period madeit the second fastest pulsar known at the time (after the famous 33 msec Crabnebula pulsar). The binary nature of PSR B1913+16 (which made it a reallyfantastic pulsar) was discovered by Hulse only about two months later, in earlySeptember through a careful and dedicated study of its (pulsing) period. Indeed,the first attempt (on August 25, 1974) to obtain a more accurate value of thepulsar period was frustrated by a perplexing result: after correcting for the effectof the Doppler shift due to the Earth’ motion, the pulsar periods determinedduring separate, short observations differed by up to 80 microseconds from dayto day, i.e. by an enormous amount compared to normal (isolated) pulsars.After several weeks of detective work (using a special computerized de-dispersinganalysis) Russell Hulse contacted on September 18, from Arecibo, Joe Taylor atthe University of Massachusetts in Amherst to tell him the amazing news behind2hese large apparent variations in pulsar period, namely that the pulsar was ina high-velocity binary orbit with about an 8 hour period. Soon Joe Taylor wason a plane to Arecibo. He arrived with a special hardware de-disperser whichallowed a more efficient study of the apparent period variations, soon leading toan accurate determination of the complete “velocity curve” of PSR B1913+16,i.e. of the variation with orbital phase of the radial velocity of the pulsar onits orbit. Within a month they were ready to report the discovery of the firstpulsar in a binary system. Their discovery paper [4] (which was received on October 18, 1974) featuresan accurate determination of the velocity curve of PSR B1913+16, from which(using a least-squares fit) they inferred the orbital period , P b = 27908(7) s ,the projected semi-major axis of the pulsar orbit, a sin i = 1 . R ⊙ , itseccentricity e = 0 . , ω = 179(1) degree,the radial velocity semiamplitude K = 199(5) kms − and the mass function, f ( m , m ) ≡ ( m sin i ) / ( m + m ) = 0 . M ⊙ . As they did not know (at the time) the inclination angle i , their measurementof the mass function was compatible with a wide range of values for the mass, m , of the pulsar and the mass, m , of its companion. However, by restrictingtheir attention to values of m “thought to be reasonable for neutron stars”(namely 0 . ≤ m /M ⊙ ≤ . . . . will allow a number of interesting gravitational and rela-tivistic phenomena to be studied. The binary configuration providesa nearly ideal relativity laboratory including an accurate clock in ahigh-speed, eccentric orbit, and a strong gravitational field. We note,for example, that the changes of both v /c and GM/c r during the Here, and below, the parentheses indicate the error on the corresponding last digit(s). As was noted at the time, the discovery values of both e and ω are close to some specialnumbers: e to the golden ratio ( √ − ≃ .
618 and ω to 180 degrees = π . . Therefore, both the relativistic Doppler shift and thegravitational redshift will be easily measurable. Furthermore, thegeneral-relativistic advance of periastron should amount to about4 o per year, which will be detectable in a short time. The mea-surements of these effects, not usually observable in spectroscopicbinaries, would allow the orbit inclination and the individual massesto be obtained.”As last good news, they remarked that PSR B1913+16 promised to be a clean system, indeed: “No changes in dispersion measure exceeding ±
20 cm − pc [on a total DM = 167(5) cm − pc] have been observed over the binaryperiod, so it is clear that at most a small fraction of the dispersion can arisefrom electrons within the binary orbit.” The announcement , in October 1974, of the discovery created a lot of excite-ment worldwide. The author (who had recently arrived at Princeton Universityas a 23-year old postdoc) vividly remembers hearing about the discovery at oneof the weekly (Tuesday) lunches organized by John Bahcall at the Institute forAdvanced Study. These lunches, whose role was to circulate news and exchangeinformation about recent advances or discoveries in astrophysics, were well at-tended by the whole Princeton physics and astrophysics community (to whichnotably belonged Remo Ruffini, who had taken the author to this memorablelunch). Excitement filled the air when the discovery was announced at thislunch.In the following months, a flurry of papers discussed various potential physi-cal or astrophysical consequences of the Hulse-Taylor discovery, notably Refs. [5,6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]. Let us only highlight here some of themost significant suggestions made in these papers. Ref. [5] pointed out thatthe observation of the secular decrease of the orbital period of PSR B1913+16,˙ P b ≡ dP b /dt , would constitute a “test for the existence of gravitational radi-ation”. Ref. [10] pointed out that the orbital period decay ˙ P b was a sensitivetest of alternative relativistic theories of gravity, and notably of tensor-scalartheories (such as the Jordan-Brans-Dicke theory) because the emission of dipolegravitation radiation in theories containing scalar excitations would generallybe much larger than the usual quadrupolar emission. Ref. [14] (see also [9]) Before submitting, and sending around in preprint form, their October 18 discovery paper,Joe Taylor and Russell Hulse announced their discovery in the October 4, 1974 IAU circularn o Note that this paper is the only one which was published in 1974, i.e. even before the m ≪ m ; themore general, comparable-mass expression was provided a few months later, seeRefs. [15, 18, 19, 20].] A detailed analysis of the effect of spin-orbit precessionon the observed pulse width and polarisation sweep was presented in [21]. Fromthe astrophysical point of view, the study of possible consistent evolutionaryscenarios leading to the formation of binary pulsars was pioneered in Ref. [12]. After the initial flurry of papers spurred by the announcement of the discovery ofthe first binary pulsar, the rate of production of papers dealing with theoreticalaspects of the dynamics and timing of binary systems soon dropped down.However, a small sequence of papers dealing with the relativistic theory of thetiming of binary pulsars had a significant impact on the later data analysis ofbinary pulsars. This line of work was pioneered by a work of R. Blandford andS.A. Teukolsky [22] which derived the first version of the “timing formula” ofa binary pulsar, i.e. a mathematical expression giving the functional relationbetween the integer N labelling the N th pulse and its time of arrival (TOA) atthe Earth as measured by an observer on the Earth. The use of such timingformulas to analyze the observed sequence of TOA’s proved to be instrumentalfor extracting accurate physical information from binary pulsars. The timingformula derived in [22] included only some of the relativistic effects in binarypulsars, namely: (i) the combined second-order Doppler shift and gravitationalredshift (measured by a parameter called γ ); (ii) the secular advance of theperiastron ( ˙ ω ); (iii) the secular change of the orbital period ( ˙ P b ); and (iv) therelativistic time delay linked to the propagation of the pulsar signal in thegravitational field of the companion ( m ; sin i ). A subsequent work [23] includedthe aberration effect linked to the orbital velocity of the pulsar.The timing effects linked to the (periodic) first post-Newtonian (1PN) con-tributions to the two-body dynamics (i.e. the O ( v /c ) Einsteinian correctionsto Newton’s 1 /r law) were tackled in later papers [24], [25], [26]. The post-Newtonian-accurate timing formulas of Refs. [24] and [25] had several unsatis-factory features that were corrected in the “DD” timing formula [26], which wasbased on a new, simplified “quasi-Keplerian” representation of the 1PN-accuratemotion [27]. See [28] for an early use of the Epstein-Haugan timing model, and[29] for a comparison of the performances of the various timing models.The DD timing formula later led to defining the “parametrized post-Keplerian” Hulse-Taylor discovery paper which was published in January 1975. This was due to thecelerity of publication in the comptes rendus of the French academy of sciences.
One of the first, and most spectacular, legacies of the discovery of binary pulsarshas been to provide direct experimental evidence for the reality of gravitationalradiation damping in binary systems. This was first publicly announced byJoe Taylor in December 1978 at the 9 th Texas Symposium (Munich, Germany)and was soon published [33]. The experimental evidence consists of observinga secular decrease of the orbital period (i.e. a negative value of ˙ P b ). Theobserved value ˙ P obs b is then compared to the value of ˙ P b predicted by Einstein’stheory of General Relativity as a function of two other pulsar-timing observables,namely the secular rate of periastron advance ˙ ω and the combined second-order-Doppler-gravitational-redshift timing parameter γ . In this first announcementthe ratio between the observed and the GR-predicted value of ˙ P b was ñ ˙ P obs b ˙ P GR b [ ˙ ω obs , γ obs ] ô = 1 . ± . . (5.1)This remarkable observational discovery spurred a lot of theoretical work ongravitational-radiation-related effects in binary systems. Even before the an-nouncement some authors had pointed out some of the shortcomings of thederivation of both gravitational radiation damping and gravitational radiationenergy loss in binary systems [34], [35]. After the announcement, many au-thors worked towards improving the theoretical understanding of gravitationalradiation effects in binary systems [36], [37], [38], [39], [40], [41], [42], [43],[44]. Note that the last two references provided a direct computation of the ob-served orbital-period derivative ˙ P b from general relativistic two-body equationsof motion valid for strongly self-gravitating bodies and complete up to fractionalcorrections of order ( v/c ) . This computation (see also [45], [46]) shows thatthe observation of ˙ P b in the TOAs of a binary pulsar is a direct effect of theretarded propagation (at the speed of light, and with a quadrupolar structure)of the gravitational interaction between the companion and the pulsar. In thatsense, the Hulse-Taylor pulsar provides a direct observational proof that gravitypropagates at the speed of light, and has a quadrupolar structure.The latter point is confirmed by the theoretical computation of ˙ P b in alterna-tive theories of gravity where the non purely quadrupolar (i.e. non purely spin2) structure of the gravitational interaction generically induces drastic changesin the theoretically predicted function ˙ P theory b [ ˙ ω, γ ] [10], [47], [48], [49]. Suchdrastic changes are generally incompatible with the observed ˙ P b [50].6n the observational front, the initial result, Eq. (5.1), was later confirmedand refined [51, 29]. When, the observational precision on ˙ P obs b got better than1%, it became necessary to take into account non-GR contributions to ˙ P obs b coming from the galactic accelerations of the pulsar and the Sun, and from theproper motion of the pulsar [52]. After taking these contributions into accountthe ratio ˙ P obs − gal b / ˙ P GR b yielded a 0.8% confirmation of General Relativity. Thecurrent timing data [53] yield. ñ ˙ P obs b − ˙ P gal b ˙ P GR b [ ˙ ω obs , γ obs ] ô = 0 . ± . , (5.2)i.e. an experimental evidence for the reality of gravitational radiation dampingforces at the ( − ± × − level. Several other binary pulsars have givensimilar confirmations of the reality of gravitational radiation (for reviews ofpulsar tests see [32], and chapter 21 in the Review of Particle Physics [54]).Let us note in particular the stringent tests of the quadrupolar (ratherthan dipolar) structure of gravitational radiation damping obtained from themeasurement of the orbital period decay ˙ P b of several dissymetric pulsar-whitedwarf binary systems. The most stringent constraint is obtained from the low-eccentricity 8.5 hour pulsar-white dwarf system PSR J1738+0333 [55]. Another very important legacy of the 1974 discovery has been to provide cleanand accurate tests of the strong-field regime of relativistic gravity. Indeed,contrary to the solar system (which is made up of weakly self-gravitating bodies,so that the curved spacetime metric g µν ( x λ ) is everywhere a small deformationof a flat metric η µν ) a binary pulsar contains at least one strongly self-gravitatingbody, namely the rotating neutron star emitting the observed radio pulses,so that the metric g µν ( x λ ) near it significantly differs from η µν . Indeed, thesurface value of, say, h ( R ) ≡ η − g ≃ Gm/c R is of order 0 . ∼ than the surface potential of theEarth, and a mere factor 2 . h ( R ) = 1). Thelarge deviation of the spacetime metric away from its Minkowski value opensthe possibility of testing the strong-field regime of relativistic gravity, separatelyfrom its radiative aspects. [The ˙ P b − ˙ ω − γ test discussed in the previous sectionmixes radiative effects (in ˙ P b ) with static strong-field effects (in ˙ ω and γ ).] Ageneral phenomenological formalism for using binary pulsar data to test strong-field gravity was presented in [31]. It is called the “parametrized post-Keplerian”(PPK) formalism. It was shown in Ref. [31] that, in principle, as many as fifteentests of relativistic gravity can be extracted from the observational data of asingle binary pulsar. The data used for this purpose are both timing data andpulse-structure data. The phenomenological analysis of pulsar timing data isdone by using the general, theory-independent version of the DD timing formulamentioned in Section 4. 7he first experimental constraints on strong-field relativistic gravity wereobtained in Ref. [56]. They used 10 years of timing data of PSR B1913+16,and one year of timing data of the second discovered (double-neutron-star) bi-nary pulsar PSR B1534+12 [57]. In particular, the PSR B1534+12 timing dataallowed one to separately measure 4 phenomenological parameters: ˙ ω obs , γ obs , r obs and s obs , where r measures the “range” and s the “shape” of the Shapirotime delay caused by the companion. In any given theory of relativistic gravitythe 4 phenomenological parameters ˙ ω obs , γ obs , r obs , s obs can be expressed as(theory-dependent) functions of the mass m of the pulsar, the mass m of itscompanion, and of the Keplerian parameters. One can then use (for a given the-ory) the two equations ˙ ω obs = ˙ ω thy [ m , m ], γ obs = γ thy [ m , m ] to determine m and m in terms of ˙ ω obs and γ obs . This leads to two theory-dependent predic-tions for r = r thy [ ˙ ω, γ ] and s = s thy [ ˙ ω, γ ]. Comparing the two theory-dependentpredictions r thy (cid:2) ˙ ω obs , γ obs (cid:3) and s thy (cid:2) ˙ ω obs , γ obs (cid:3) to the two corresponding obser-vational values r obs , s obs then leads to two tests of strong-field gravity. Ref. [56]found that General Relativity passes these two strong-field tests. For instance,one had already at the time (i.e. when using only one year of PSR B1534+12timing data) ñ s obs s GR [ ˙ ω obs , γ obs ] ô = 0 . .
982 = 1 . . (6.1)Theoretical investigations of the strong-field predictions of alternative rel-ativistic theories of gravity [10, 47, 58, 49, 59, 60, 61, 62, 63, 64] have explic-itly shown that while strong self-gravity effects are “effaced” in GR (in thesense that they can be renormalized away by including them in the definitionof the two masses m , m ), this is not so in most alternative theories of gravity.It is generically found that the theoretical predictions relating PPK observ-ables to the two masses, ˙ ω thy [ m , m ], γ thy [ m , m ] , . . . , are significantly mod-ified by self-gravity effects in alternative theories of gravity. As a consequencetheoretically-predicted relations between PPK observables, such as the func-tion s thy [ ˙ ω, γ ], are modified by self-gravity effects. This explicitly shows thata binary-pulsar test of the type of Eq. (6.1) does indeed constrain the strong-field regime of relativistic gravity. Early examples of such effects [47, 49] werederived within ill-motivated theories, containing negative-energy excitations. Itwas later found that non-perturbative strong-field effects can however developwithin certain tensor-scalar theories, even in cases where the weak-field limit ofthese theories is arbitrarily close to GR [59].Accurate non radiative strong-field tests have been obtained in three binarypulsar systems: PSR B1534+12 [65] (two tests), PSR J1141-6545 [66] using sin i from [67] (one s − ˙ ω − γ test) and PSR J0737-3039 [68] (four tests). [For a reviewand more references see [54].] The last cited binary pulsar is the remarkable double pulsar [69, 70], made of two neutron stars which are both observableas pulsars. This system has given five independent tests of relativistic gravity[68, 71, 72]. 8eneral Relativity passes all the current strong-field tests with flying colors.The most accurate strong-field confirmation of GR is at the 5 × − level. Itwas obtained from double binary pulsar data combining the observables s, ˙ ω and the ratio R = x B /x A between the projected semi-major axes of the twopulsars [68]: ñ s obs s GR [ ˙ ω obs , R obs ] ô − = 1 . . (6.2)The (radiative or nonradiative) constraints on tensor-scalar theories providedby the various binary pulsar “experiments” (involving double neutron-star orneutron-star white dwarf systems) have been analyzed [60, 61, 73, 55] and shownto exclude a large portion of the parameter space allowed by solar-system tests.Finally, measurements over several years of the pulse profiles of various pul-sars have detected secular profile changes compatible with the prediction [14]that the general relativistic spin-orbit coupling should cause a secular changein the orientation of the pulsar beam with respect to the line of sight (“geode-tic precession”). Such confirmations of general-relativistic spin-orbit effects wereobtained in PSR B1913+16 [74], [75], [76], PSR B1534+12 [77], [65], PSR J1141-6545 [78], PSR J0737-3039 [71], and PSR J1906+0746 [79]. Note that while onehas actual measurements of geodetic precession rates for PSR B1534+12 andPSR J0737-3039, the evidence in the other systems is purely qualitative. In thisrespect, it is interesting to note (as Joe Taylor did in some talks) that (accordingto the precession model of [75]) PSR B1913+16 became visible from the Eartharound 1941 (i.e. around the birth of Joe himself) and will become invisiblearound 2025. Then it will disappear for about 200 years before reappearingagain as a pulsar visible from the Earth. It seems that other binary pulsarshave similarly appeared or disappeared over a relatively small number of years.Let us finally mention the remarkable recent discovery of a millisecond pul-sar in a triple system [80]: PSR J0337+1715 is a 2.73 millisecond pulsar, withvery good timing precision, around which two white dwarfs orbit in hierarchical,coplanar, nearly circular orbits. The gravitational field of the outer white dwarf(with orbital period 327.26 days) accelerates the two members (pulsar and whitedwarf) of the inner binary (with orbital period 1.629 days). This system hasthe potential to test the strong equivalence principle (SEP) for strongly self-gravitating bodies (i.e. the universality of free fall of self-gravitating bodies) toan unprecedented level of accuracy. Data from several nearly circular binary sys-tems (made of a neutron star and a white dwarf) have already led to strong-fieldconfirmations of the SEP (at the 4 . × − level), by checking that a stronglyself-gravitating neutron star and a relatively weakly self-gravitating white dwarffall with the same acceleration in the gravitational field of the Galaxy [81], [82].In the triple system PSR J0337+1715 the perturbing acceleration field (due tothe outer white dwarf) is at least 6 orders of magnitude larger than the Galacticacceleration used in previous pulsar tests of the SEP. This opens the promise ofan extremely interesting new test of strong-field gravity.9 Binary pulsars and astrophysics
Up to here we focused on the legacies of the 1974 discovery for relativistic gravity.However, this discovery had also very important consequences for astrophysics.Let us indicate some of them.First, it led to the first accurate measurement of neutron star masses. Forinstance, the masses of the pulsar and its companion in PSR B1913+16 werefound (when assuming a GR-based timing model [29]) to be [53] m = 1 . M ⊙ ; m = 1 . M ⊙ . (7.1)For a compilation of observed neutron star masses (obtained either throughpulsar timing or other methods) and references see [83]. Note that, recently,pulsar masses close to 2 M ⊙ have been discovered [84, 85]. These large neutronstar masses put strong constraints on the equation of state of nuclear matter[86].The discovery of binary pulsars, as well as the later discovery of fast-spinning“millisecond” isolated pulsars [87], has given an enormous impetus to the de-velopment of astrophysical scenarios for the co-evolution of binary stars. Thesestudies had been initiated in 1972 with the discovery, by the UHURU satellite,of pulsating X-ray sources in close binaries around main-sequence stars [88]. InApril 1973 (and therefore before the discovery of the first binary pulsar) G.S.Bisnovatyi-Koyan and B.V. Komberg had already suggested that some pulsatingX-ray sources in binaries may later in their life, after their massive companionstars have exploded as a supernova, become observable as binary radio pulsars[89]. For an authorative review of the formation and evolution of relativisticbinaries see the contribution of E. P. J. van den Heuvel to the book [90].Another legacy of the 1974 discovery concerns the later life of binary neutronstars. Radio pulsars are thought of having a limited lifetime (as active pulsars).But the discovery of binary pulsars shows the existence of a population of bi-nary neutron stars that will emit gravitational waves for hundreds of millions ofyears. For instance, PSR B1913+16 will coalesce, because of gravitational radi-ation damping, in a few hundred million years. In the last few minutes beforecoalescence, such a binary neutron star will be a strong emitter of gravitationalradiation (as was first pointed out by Freeman Dyson [91]). These sources aresome of the prime targets for the upcoming network of advanced ground-basedinterferometric gravitational wave detectors. Binary pulsar observations are cru-cially used to estimate the number of gravitational-wave signals expected in suchdetectors (see [92] and references therein). One expects to extract interestingscientific information from the future observation of inspiralling and coalescingbinary neutron stars. In particular, the gravitational wave signal from the lateinspiral will give useful constraints on the equation of state of nuclear matter[93, 94, 95]. In addition, it is hoped that the final coalescence process will giverise to catastrophic events leading to an important emission of electromagneticradiation and neutrinos. These events might be related to the so-called short gamma-ray bursts [96]. 10 Concluding remarks
The 1974 discovery of the first binary pulsar has given us a cornucopia of im-portant scientific benefits. The most spectacular ones concern the first experi-mental evidence that Einstein’s theory of General Relativity is valid beyond theusually tested quasi-stationary, weak-field regime. Indeed, binary pulsar datahave probed, for the first time, relativistic gravity in regimes involving (eithertogether or separately) radiative effects and strong-field effects. The citationaccompanying the award, in October 1993, of the Nobel Prize in Physics toRussell A. Hulse and Joseph H. Taylor read: “for their discovery of a new typeof pulsar, a discovery that has opened up new possibilities for the study of grav-itation”. As we have discussed, these new possibilities for studying gravitationhave been even more sucessful than what was envisaged in the months followingthe discovery.Even more importantly, the class of systems discovered by Hulse and Taylorpromises to bring new discoveries in the near future, through the physics ofthe late stages of evolution of compact binaries: gravitational waves, probes ofnuclear-matter equation of state, possible connection with gamma-ray bursts, . . .
Let us finally mention the hope that radio pulsars in orbit around a black holewill soon be discovered. The black hole companion could be either a ∼ M ⊙ black hole, or, possibly, a much more massive black hole. Recently, a magnetarwas discovered near the massive ( ∼ × M ⊙ ) black hole at the center of ourGalaxy [97]. Searches are underway for discovering pulsars having better timingstability, and closer to the galactic center. Such a discovery would be a fantasticnew milestone for General Relativity. Acknowledgments
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