PoS(INTEGRAL 2012)009 http://pos.sissa.it/ ∗ [email protected] Speaker. INTEGRAL discovered two new typesscured of systems High-Mass and X-ray the Binaries Supergiantperiods (HMXB): Fast of X-ray the the Transients highly neutron (SFXT). ob- stars The inFurthermore, causes supergiant the for HMXBs formation the and rate, long of duration the spin phase outbursts of of of the SFXTs the X-ray are HMXBs phase discussed. Zytkow and are Objects. the discussed. The evolution fate after of Many such thesupergiant objects systems HMXBs X-ray is in will still the unclear, later Galaxy but implies the in that relatively their life high remnants formation merge must rate be to of all form around us. Thorne- ∗ Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. c

“An INTEGRAL view of the high-energy9th sky INTEGRAL (the Workshop first and 10 celebration years)” ofOctober the 15-19, 10th 2012 anniversary of the launch Bibliotheque Nationale de France, Paris, France University of Amsterdam, The Netherlands E-mail: E.P.J. van den Heuvel INTEGRAL and the Evolution of X-ray Binaries PoS(INTEGRAL 2012)009 , confirming J E.P.J. van den Heuvel 2 to the mass of its companion, indicating that this neutron star is moving around a J Forty years ago the discovery of the first-known X-ray binary Centaurus X-3 was published. By the mid-1970s also the B-emission X-ray binaries had been recognized as a separate new Soon after the discovery of the binary character of Cen X-3 it was understood that the HMXBs massive star. That same year twoUHURU: more Her pulsating X-1 and (Tananbaum et eclipsing al. X-raymassive binaries 1972a) eclipsing were system and discovered 4U1700-37 SMC by (Jones X-1 et (Schreierand al. et Murdin 1972). al. (1971,1972) 1972b), Also that as in Cyg226868, well 1972 X-1 was as the is suggestion the confirmed, associated by thanks with Webster toapril the the 1971 massive arc-second-precision appeared blue location in supergiant of binary the1971). the HD — radio Since then this source still radio which large source in spectrum — appeared of error at the exactly box source the of (Tananbaum samethat et the time of al. X-ray the as 1972b), source radio a and source, (Braes drastic the it andsystem change position was are Miley in of clear the the that same HD object. X-ray the 226868 radio The coincidedorbit large source, with radial then the velocity X-ray showed amplitude source that of and the the the blue compact supergiant massive object in binary in its 5,6-day this system has a mass larger than 5M Etan Schreier of thelow UHURU states team of discovered this indiscovered source the November with simultaneous 1971 a periodic the 2,087-day Doppler(Schreier recurrence et period, modulation of al. of indicating 1972a) the its that . X-ray 4,84-secondof it The about X-ray is derived 16 pulse Doppler M an period velocity eclipsing amplitude of binary, 415,1 and km/s sets a lower limit class of HMXBs (Maraschi, Treves andclasses van den of Heuvel X-ray 1976). binaries So, were byINTEGRAL, thought the which to mid-1970s all be discovered important known. two new However,HMXBs, in classes of 2003 which of this IGRJ HMXBs: all 16318-4848 is changedwith (1) the thanks a the most to column highly extreme density example obscured inside (e.g.extinction, supergiant the Filliatre and system and (2) is Chaty the orders 2004), Supergiant of Fastwhich magnitude X-ray show higher large Transients than short-lasting (SFXTs, that X-ray e.g. of flares, Sguerastars. the and et In interstellar turned al. this out 2005), paper to sources I havethe blue will supergiants restrict two as myself new companion to classes the of formationsystems. HMXBs and I might evolution particularly fit of focus the into also HMXBs on theof and (i) the over-all to the HMXBs, evolutionary how origin and picture of (ii) of the on massive very the binary long final pulse evolution of periods these observed systems. in2. many Evolutionary picture of HMXBs 2.1 Overview of the problem owe their existence to the large-scale mass transfer that occurs during the evolution of a close binary INTEGRAL and the Evolution of X-ray Binaries 1. Introduction: forty years of X-ray binaries Webster and Murdin’s suggestion that thisby object the is end a of black 1972 hole fourhole (Tananbaum High-Mass - et X-ray as al. Binaries well 1972b). as (HMXBs) one were Thus, more pulsating known years Low-Mass — before X-ray one also Binary of the (LMXB), them binary HerSco character a X-1. of black X-1, But non-pulsating whose it LMXBs would optical was take discovered, counterpart0,787day starting three was (Gottlieb, with E.W. found et to al. show 1975). a brightness modulation with a period of PoS(INTEGRAL 2012)009 J it is assumed 1 years) before the 6 E.P.J. van den Heuvel helium star (the helium J yrs after the mass transfer) leaves a 2M 6 depicts a rough outline of the evolution of a 1 3 O-type main-sequence star (the original secondary that J star) and a 22,66M J Outline of the evolution of a massive close binary into a supergiant High Mass X-ray Binary (van After the first phase of mass transfer the system consists of a 5,34M received the H-rich envelope of itsRayet(WR) companion). stars Massive helium (Paczynski stars 1967), are which identifiedthe in with companion many the star Wolf- cases is are an members O-type(van star. of den These close Heuvel WR binaries 1973; binaries in are Tutukov which the and direct Yungelson progenitors 1973). of the In HMXBs the scenario of figure core of the 20M that the supernova explosion of theneutron WR star. star Due (0,56 to x10 system the becomes sudden a explosive mass runaway loss, star.companion the star After leaves center the this of main it mass sequence takes gets and a accelerated becomes very a and blue long the supergiant time with a (3,65x10 strong stellar wind Figure 1: den Heuvel 1976). Numbers near the stars indicate masses in solar masses. massive binary with initialtaken components from of van den 20 Heuvel and (1976).is: 8 In this solar conservation picture masses of “conservative” binary up mass evolutioncourse, is to and a assumed, the orbital rather that crude X-ray angular assumption). binary momentum during phase, mass transfer phases (this is, of INTEGRAL and the Evolution of X-ray Binaries prior to the supernova explosion in1972; which the Tutukov compact and star Yungelson was 1973). formed— (van Thanks which den Heuvel to initially and this was Heise the massenvelope more transfer to massive the its one more companion, — evolvedless such will component massive later that component in by of the the the evolution system.ejection time lose from Simple it its the celestial explodes hydrogen-rich less mechanics massive as shows componentbut a that of imparts the supernova, a explosive it it binary mass does with has not become a disrupt the the runaway system velocity. (Blaauw 1961), Figure PoS(INTEGRAL 2012)009 E.P.J. van den Heuvel of the freely infalling 2 v ρ of this star. Ω / π 2 = P of the dipole magnetic field of the neutron star. 4 . This is the radius where the Keplerian period of π 3 8 / 1 / ) 2 2 B Ω / Ω / GM c = = ( c l co r , which is the radius where the ram pressure A r are the density and velocity of the infalling gas. (after Sidoli 2012). They are typically hundreds of seconds and can even range v 2 the infalling matter is forced by the magnetic field to rotate with the neutron star. A r and < Corbet Diagram of High Mass X-ray Binaries (courtesy L. Sidoli, 2012 and S. Drave, 2012). ρ r For Here matter equals the magnetic pressure matter orbiting the neutron star equals the spin period For a spinning and accreting magnetized neutron star one can distinguish the following char- 3. The co-rotation radius 2. The Alfven radius 1. The light cylinder radius Figure 2: acteristic radii (e.g. see Ghosh 2007): 2.2 Spindown of a neutron star born as a companion of a massive OB-type star INTEGRAL and the Evolution of X-ray Binaries which turns the neutron star intoX-ray a pulsating pulsars X-ray with source. blue The supergiant pulsediagram companions periods in of are figure the all wind-accreting very long,to as over can an be hour, seen asand from in 2S0114+65 the 4U2206+54 (P=2,73 Corbet (P=5560 hours). sec), Thelong which spindown time to has interval these an between long O9.5V the periods supernova main-sequenewhen must explosion companion, the have and companion taken the star place onset becomes in of ainto the the blue strong a supergiant stellar strong and wind X-ray accretion phase, from source.star its and wind We its now turns spin consider the during neutron in this long some time more interval. detail what happened to the neutron PoS(INTEGRAL 2012)009 2 w v / (1) (2) (3) . r equals GM 2 2 = a r (r).v(r) ρ E.P.J. van den Heuvel is the sound velocity in s on the magnetosphere of c F 3 w v / yrs. w where 5 ρ dt s 2 2 / c ) ) , matter will be able to penetrate into Ω dJ  A A GM r = r ( w ( v > · 2 π c ) 4 l Ω A ) r = A ( r : w ρ ( A v 2 A 5 r ρ · r · 5 A w π r F ρ 4 π 2 a 4 r = = π = N F = N of the dipole magnetic field of the NS at that distance π acc ˙ /8 M 2 ) r ( B . A , one can calculate the propellor spindown force r A r < c l , given by the condition that the ram pressure of the infalling matter A . A 303 (a 27-day orbit gamma- and radio-emitting Be system), LS 5039 (O6V companion, r o the wind), and the accretion rate will be Using this the NS: the light cylinder and thedepend relativistic on pulsar whether wind or will not stop.accretion the accreting disk. The matter further has In spindown sufficient the willhowever, angular latter now strong momentum case evidence to from also form hydrodynamic the an computationalin co-rotation HMXBs simulations radius (e.g. of Taam will and wind Fryxell come accretion 1988;accreted into Blondin by et play. the al. neutron 1991) There that star is, the from net the angularAssuming wind momentum is the practically net zero angular (see momentum alsozero, of the the the next section). inflowing spinning matter neutron accretedwind, star from and (NS) the its spindown wind with can to its be be calculated magnetic asOnly field wind follows. matter will that approaches act the NS as within a a Bondi-Hoyle propellor accretion radius in this McSwain and Gies 2002) and the FermiThe source 1FGL1018.6-5856 duration (O6V of companion). thefield gamma-emitting of phase the NS is (which determined determines2007), its by and spindown the also rate, cf. by strength Manchester the of and strengthof Taylor the of 1977; the Ghosh magnetic wind of its companionOne star expects which this influences phase the to value last up to at most a few times 10 will be gravitationally captured (we assume here Assuming the accreted matter toradius fall r freely towards the NS,the it magnetic will pressure be stopped at the Alfven Here infalling matter is unable tolike penetrate a into normal the radio light pulsar cylinder.inverse-Compton and process The produces the neutron a relativistic star particles highly behaves from relativistic thisof electron-positron wind the wind. convert photons star By of into the the light gammaWe know rays, several and gamma-ray the emitting system61 OB is close expected binaries to which be are a in strong this gamma-ray phase, source. e.g.: LSI which leads to a spindwon torque Once the neutron star has spun down such that B. Propellor phase A. Ejector: INTEGRAL and the Evolution of X-ray Binaries The neutron star willexpected be to born subsequently go as through a the following rapidly spindown spinning phases (Ghosh pulsar, 2007): similar to the Crab pulsar, and it is PoS(INTEGRAL 2012)009 . Ω (4) (5) (6) (7) (8) for (9) yrs, / 7 Ω is the π t 2 k 10 Ω = × 8 , . 100 s ns , where yrs. = R k and 1 5 3 Ω P E.P.J. van den Heuvel = . r 10 Ω × 8 , ) cm for a NS, one obtains for 2 6 that differed onlyfrom expres- 10 GM N 2 GM k ) = ) / ( 2 / dt ns Ω t / 3 Ω R 4 ns ) 4 ns Ω r 6 ns R R g, 2 0 / Ω d R 2 0 ( 2 0 B ns 2 ns 33 B ( 2 ns B ) R / ) ( 6 10 . 2 6 . 0 MR / A / 3 MR 2 B r 1 2 1 k k GM 6 = = 2 can be neglected, yielding a spindown time = = ( ) = k r = ( = M 1 r 1 J 6 yrs for spinning down to a period ( − 0 − 0 in this expression is replaced by dt 1, 5 B = , / Ω Ω 2 G, one obtains timescales of 1 G, they will in the winds of their companion stars spin 0 10 Ω Ω t − x dJ 11 13 = 1 8 given by: , 2 − 1 N = dJ/dt k ) Ω t ( to 10 = Ω Ω 12 t G and 10 13 is very large , 10 0 Ω = 0 B G a timescale is the strength of the magnetic field at the NS surface, where is the spin angular momentum of het NS. is the radius of gyration of the NS, one has 12 0 J k B 10 = 0 sion (5) by the fact that the factor where By combining equations (1)–(4) one then obtains which is completely independent ofshould the have properties no angular of momentum). the This stellartorque expression wind for differs only (except the slightly that case from that the thethis the wind propellor wind case does Illarionov have and angular Sunyaev momentum, (1975) such obtained that a a torque disk forms. For where We assume a dipole magnetic field keplerian angular velocity in the disk at Since where Combination of equations (5) and (7) and integration yields Since in general reaching an angular velocity With the characteristic values down to very long periods on a timescale of not more than a few times 10 B Similarly, for respectively. Since mosttypically X-ray in pulsars the in range HMXBs 10 have surface magnetic field strengths These simulations show that turbulent eddies form in the wake behind the compact star (Fryx- Transients (SFXTs) INTEGRAL and the Evolution of X-ray Binaries 3.1 Outcome of hydrodynamic simulations of wind accretion from a massive companion star ell and Taam 1988, Taam and Fryxell 1988, Blondin et al. 1991, Matsuda et al. 1992, Ruffert 3. Angular momentum of accreted wind matter, and Supergiant Fast X-ray PoS(INTEGRAL 2012)009 (e.g. 2 . To observe 1 − yr 4 − E.P.J. van den Heuvel 10 × 6 ∼ (Grebenev and Sunyaev 2007). If the NS spins 7 in the Galaxy (Helfand and Moran 2001). Of these, ) J 20 M > . The work of Ducci et al (2009) shows that this model can explain the years, one expects — assuming that due to supernova kicks half of the systems are 6 yrs, leading to a galactic formation rate of these systems of O-stars (stars with masses 10 6 4 × 10 Clumpy stellar winds shapes and magnitudes of the flares5952. of Also sources like Ducci the et 164,5-day al.’s (2009) orbitsystems simulation HMXB very of IGRJ well the 11215- represents accretion the in observed flaring the behaviour Vela X-1 of and theseEffects 4U1700-37 systems. of a magnetic centrifugal barrier faster than the keplerian angularBut velocity when at the the accretion Alfven rate radius,keplerian temporarily accretion angular increases cannot velocity the take at Alfven place. velocity, the radius and Alfven will the decrease, surface “gate” and may willshowed the become that temporarily faster also open, this than mechanism leading the provides to a spin plausible a angular way flare. to explain the Grebenev observed and flares. Sunyaev 10 Although the accretion spikes found in the simulations of Taam and Fryxell (1988) have a The discovery of the SFXTs and the highly obscured HMXBs by INTEGRAL have almost × × strong X-ray phase of these systems 2 (a) (b) , some 30% are in close binariesorder that 5 will produce HMXBs (Sanadisrupted et in al. the 2012). supernova Since explosions O-starsin — live 5 that of some 3100 O-stars with compact companions form similar timescale as the observedin flares the of spikes the are SFXTs,several only the orders about amplitudes of magnitude. of one the Therefore orderconsidered accretion the of (see following variations for magnitude, other example explanations Bozzo while of et the these al. luminosities flares 2008): have of been SFXTs vary by quadrupled the known numbervolume). of supergiant From HMXBs the (see factknew Sidoli, for that a this 4 long such volume; time systems and that are the Chaty, known total this within Galactic number 3 of kpc such from systems the should Sun, be we of order already 10 INTEGRAL and the Evolution of X-ray Binaries 1997), resulting in accretion ofbehaviour matter of with alternating the directions accretion of results1988). the in Due angular accretion to momentum. this spikes “flip-flop” This lasting behaviourcourse only of of the a accretion, time few the is hours net zero. angular (Taam Thisshow momentum and explains accreted a why Fryxell in continuous the the spin slow wind-accreting up, X-rayalternating but pulsars episodes keep in of hanging HMXBs spin do around up not and thepulsars spin same has down. long been The pulse precise studied period, physics in with ofet a wind irregularly al. number accretion (2012, of of 2013), these important Postnov slow recent et papers al., this by volume. Ikshanov (2007) and3.2 Shakura Origin of the flaring behaviour of the SFXTs The discussion about which of these mechanisms may be the dominant one is still4. going on. The total number of Supergiant HMXBs in the Galaxy and the duration of the van den Heuvel 1976), and these newly2 discovered systems support such a number. There are some PoS(INTEGRAL 2012)009 donor star and J E.P.J. van den Heuvel 1 year for a 16M > 8 years as blue supergiants that are very close to filling their Roche lobes. Both yrs, and then kill the X-ray source by a too high accretion rate. This problem of them as supergiant HMXBs, the duration of the supergiant HMXB phase yrs. 5 4 donor (Taam 1996). Only one of the supergiant HMXBs has an orbital period 2 5 10 J 10 × × 5 , 1 ∼ Such a long lifetime would not be possible if the supergiants in these systems were already be- The mass ratio of HMXBs with an accreting NS is extreme, and once the massive star begins to Here during the CE-phase the H-rich envelope of the massive star is ejected, and only its obscured HMXBs 2 yrs for a 24M INTEGRAL and the Evolution of X-ray Binaries at any time some 10 must be yond the main sequence (that is:the beyond envelopes core-hydrogen burning), of since the beyondtimescale the supergiants of main order expand sequence 10 very rapidly and would overflow the Roche lobe on a overflow its Roche lobe, deep spiral-in of1973). the systems Most is likely unavoidable (van the den systemsOstriker Heuvel will and 1978). DeLoore go Much into work a on CommonTaam and Envelope the his phase evolution collaborators (Taam, of (e.g. Bodenheimer systems see and the throughin review order this by to Taam CE survive and as phase Sandquist a has 2000). binary, the been The initial outcome done orbital is by period that, must be was pointed out already by Ziolkowskiare (1977), undermassive who for suggested their that luminosities the due OBbership to companions enhanced in of wind HMXBs a mass loss, close resultingmay, binary. from according their mem- to Such Ziolkowski stripped (1977, hydrogen-burningstill and stars evolving this on may volume) a resemble have nuclear blue a timescale,radius, their supergiants. bloated which radius radius could Since may explain they and for are their ait long rather was time long realized stay lifetimes close that as to with athought the the blue Roche-lobe before, new supergaint. and opacities Also, these the in massivephase main looking recent stars sequence like years may blue for supergiants, spend the even the without massiveume). having stars later Since lost during is part much the mass wider of core-hydrogen (see burning than their Ziolkowski, phasemay this spend stars core-hydrogen (1–2) vol- expand burning only on a nuclear timescale, they 5.1 Evolution towards a Common-Envelope phase these effects may explain the relativelythis long volume). duration This of reasoning, the however, holds supergiant only HMXBthan for phase 5 systems (Ziolkowski, days; with orbital beyond periods that, not bluehydrogen-shell much supergiants burning longer that phase, are and close their to envelopes will filling expand their rapidly. Roche lobes must be in5. the Final evolution and fate of the HMXBs; possible relation with the heavily that approaches such a valuebinaries (IRGJ fulfill 11215-5952, this P=164,5 condition. d), but quite a number of5.2 the Be/X-ray Fate of wide HMXB systems core, consisting of helium and heaviernarrow elements orbit. remains, together An with the example(suggested compact by of star, van in such den a Heuvel a very andand post-CE De a system Loore, compact 1973). star is (van It the Kerkwijk consists et 4,8-hour of al. X-ray a 1992). Wolf-Rayet binary Several star more Cygnus (helium of star) X-3 such helium star plus compact star > PoS(INTEGRAL 2012)009 ∼ Li, Mo, 7 E.P.J. van den Heuvel yrs), or (2) due to the 6 is accretion onto the NS, , their final remnants will 10 ) J ∼ J 8M < . The rp-process produces proton-rich J it is nuclear burning via the exotic rp-process. 1400R ) J ∼ 9 one year: complete spiral in and the formation of 14 M < yrs. In both cases a radiative zone develops between the > 6 10 ∼ =3200K and a radius of eff 35h). In the latter systems, with much longer orbital periods than Cyg X-3, the compact TZO has T /yr), which happens after ∼ J J M The drop in orbital binding energy during spiral in in close HMXBs is not sufficient to expell Thorne-Zytkow Objects 5 − INTEGRAL and the Evolution of X-ray Binaries close binaries are now known in otherperiods galaxies, e.g.: IC10 X-1 andstar NGC300 X-1( is both a with quite orbital massive blackWhen hole, the which helium may star be the inan reason such why eccentric a they close system did explodes not binary spiral asstars will in that a very result are supernova, deeply. presently and consisting known, the of andevolutionary system which two history all is (e.g. compact have not Flannery eccentric stars. disrupted, and orbits, clearly vanprogenitors are den The will the Heuvel nine in result 1975; of double most van such den neutron casesstars an Heuvel are have 2007, NSs, been 2009). and Be/X-ray since Their binaries, the Be since stars in are these mosly less systems massive the than compact 20 M also be NSs. The remants of systemsblack like hole IC10 X-1 binaries and (e.g. NGC 300 Voss X-1 and2009). will Tauris When most 2003, the probably be compact Tauris double and star is vanbe beginning den surrounded to Heuvel by enter 2006, the the van envelope first ofIGR16318-4848, den its which ejected Heuvel companion, has matter. the a system supergiant It will B[e]extinction, may spectrum produced then and by shows matter resemble very that the probably large highly isand circumstellar concentrated optical obscured Chaty in 2004); IGR a dense te source circumbinary only disk(references other (Filliatre in B[e] Filliatre X-ray and Chaty binary, 2004). which is very similar5.3 to Fate this of source HMXBs is with CI orbital Cam periods the H-rich envelope of theof massive He star before and the heavier compactin elements. object the enters center As the of a corea the result which Thorne-Zytkow massive consists the Object star. compact (TZO): object a Whenis massive spirals the very star in extended compact with giving completely object a it and is1977). NS the ends a in outside A up NS, its appearance major center; the of energyThorne a the resulting source red and envelope star of of supergiant Zytkow this is this (Thorne did star called and star notdone is Zytkow, independently solve accretion 1975, by the of Biehle problem matter (1991, from ofin 1994) the the massive and TZOs envelope Cannon nuclear occurs onto (1993) energy via the who source theby showed NS. in rapid Podsiadlowski, that proton TZOs. nuclear Cannon (rp) burning and process. This Rees(1996). was Further (1995), work The and on energy the TZOs source results was in carried were low-mass out TZOs summarized (envelope by mass Podsiadlowski while in TZOs with massive envelopes ( A 15M burning region and the outerto envelope, stop. and the This supply results of fresh in H accretion is heating shut of off, the causing layer the just burning above the neutron core, which boosts isotopes of which the abundancesBr, may Rb, be Y, enhanced Nb. by TZOs several may ordersthe be envelope of continuously recognized magnitude, brings by e.g. fresh the H overabundancesremarks into of the that these thin at elements. burning some zone Convection point around in (1) the a the NS. neutrino-runaway exhaustion Podsiadlowski becomes of unavoidable. theexhaustion seed of This elements the may envelope for mass be the due due rp-process to10 either the (this strong to takes stellar wind mass loss (typically for such stars PoS(INTEGRAL 2012)009 to ) is one 5 1 − < 10 yr ∼ 4 − 10 × ? The remnants of E.P.J. van den Heuvel where are they . With a TZO lifetime of 1 − , and yr 4 − 10 × 10 what are they as X-ray binaries: first as HMXBs, and later as BH- two lives /yr. One is left to speculate about what will happen further. It J M 6 − K, and accretion to be no longer Eddington-limited. At this point a neutrino runaway 4 . 9 10 The formation rate of TZOs is equal to the formation rate of HMXBs with orbital periods INTEGRAL has greatly contributed to a better understanding of the evolution of the High yrs, their galactic number is expected to be between 60 and 600. From the above it appears 6 > year, which — as mentioned above — is of order 6 5.4 Formation rate and galactic number of TZOs and their remnants is likely in the massivehad TZOs that the the angular NS momentum will offinal be the product converted original may into be binary, a a it black blackthat may hole. hole due then surrounded Since to collapse by the gravitational into a envelope instability massive asuch of disk. disk, that the Podsiadlowski possibly such disk, (1996) lateron that out speculates a of the lower-mass black-hole such envelopes, which a LMXB are disk could only low-mass form powered starshaving out by lost could of accretion, their condense, such might envelopes a by still stellar system. leave wind. a The neutron TZOs star, after with INTEGRAL and the Evolution of X-ray Binaries neutrino emission, causing neutrinoT cooling to become thebecomes dominant unavoidable, and energy the loss calculations mechanism byaccretion at Podsiadlowski rate et exceeded al. 10 (1994) were stopped when the 10 that their fate is still very unclear. However, the formation rate of their remnants (6 of order 20 per cent ofmust the be Type all Ia around supernova us. rate in The the big Galaxy, questions which are: implies that their remnants 5.5 Conclusions Mass X-ray Binaries.boost The of discovery new of interest in the aof SFXTs variety HMXBs, and of in long-standing the problems particular: highly concerningods, obscured (1) the in physics systems the the and winds spindown has evolution of of given onto their magnetized these massive neutron slowly companion rotating stars stars; magnetized toimportant (2) neutron probes very the of stars precise long the (Shakura process spin wind et of structure peri- the the al. of clumpiness wind the 2012, of blue accretion 2013). supergiants these and(Grebenev These winds may and NSs (Ducci provide Sunyaev are information et 2007); about also, al. precisepartially 2009), hydrodynamical obscured and simulations sources of on wind can the accretionvolume); even existence (3) in be The of the final used a evolution of to centrifugalnaries HMXBs, barrier determine consisting leading of for the two wide compact masses systems objects, to of andObjects, the for whose their formation close evolutionary of NSs systems products close to must (Walter, bi- the be this formationidentified. all of around Thorne-Zytkow us in the Galaxy, but so far have not been the lower-mass TZOs could be veryDim hot Isolated NSs. 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