1. INAF, Osservatorio Astronomico di Roma XI Advanced School of Astrophysics, Brazil, 1-6 September 2002

2. * Low mass X ray binaries and millisecond pulsars GCs * The case of PSR J1740-5340 and the helium remnant WDs * Low mass X ray binaries and millisecond pulsars GCs * The case of PSR J1740-5340 and the helium remnant WDs Summary

3. King, Cool & Piotto HST data The global HR diagram of NGC6397

4. 1)White dwarfs are today’s stellar remnants in GC (M evolving ~ 0.8Mo 2)What about the remnants of the more massive stars? We have already seen that inermediate masses influence the chemistry of the GCs through selfpollution from their low v winds: these stars too (M<6Mo) evolve into (more massive) WDs 3) More massive stars have evolved into supernovae in the first phases of life of the clusters: but we see their remnant neutron stars population through the huge millisecond pulsar population and the (few) low mass Xray binaries 1)White dwarfs are today’s stellar remnants in GC (M evolving ~ 0.8Mo 2)What about the remnants of the more massive stars? We have already seen that inermediate masses influence the chemistry of the GCs through selfpollution from their low v winds: these stars too (M<6Mo) evolve into (more massive) WDs 3) More massive stars have evolved into supernovae in the first phases of life of the clusters: but we see their remnant neutron stars population through the huge millisecond pulsar population and the (few) low mass Xray binaries The other stellar remnants in GCs

5. Binaries containing neutron stars Millisecond radio Pulsars Low mass companion B ~ 10 8 – 10 9 G Low mass X-ray binary Low mass companion B ~ 10 8 – 10 9 G

6. X-ray binaries with known P orb in GCs a Deutsch, Margon, & Anderson 2000. c Ilovaisky et al. 1993. d in’t Zand et al. 2000. e Sansom et al. 1993. f Homer et al. 1996. g van der Klis et al. 1993. h Stella et al. 1987. Three are ultracompact !

7. X-ray imaging of the core of 47 Tuc (Chandra) Grindlay et al. 2001 Science 70Ks exposure with resolution half with Lx<10 30.5 All the 15 MSPs are identified. The authors suggest: 50% are MSPs 30% accreting WDs 15% MS binaries in flare outbursts 2-3 quiescent LMXBs with NS

8. The Pulsar population from D.R. Lorimer 2001

9. The millisecond Pulsar population 1)~1400 radio pulsars known 2)~100 have at least one of the properties: * very short pulse period (~77 have P<12ms) * relatively weak magnetic field (~46 have B<10 10 G) * are found in binaries (~66) * are located in a globular cluster (~50) 3) One major class of binary radio-pulsar in the disk have low mass companions (0.1 –0.4 Mo) and nearly circular orbit. P orb goes from a fraction of day to 10 3 days 4) Most of the wide systems thus seem to have low mass helium white dwarfs companion, remnant of mass transfer starting when the secondary star is a subgiant or a giant with a helium core. 1)~1400 radio pulsars known 2)~100 have at least one of the properties: * very short pulse period (~77 have P<12ms) * relatively weak magnetic field (~46 have B<10 10 G) * are found in binaries (~66) * are located in a globular cluster (~50) 3) One major class of binary radio-pulsar in the disk have low mass companions (0.1 –0.4 Mo) and nearly circular orbit. P orb goes from a fraction of day to 10 3 days 4) Most of the wide systems thus seem to have low mass helium white dwarfs companion, remnant of mass transfer starting when the secondary star is a subgiant or a giant with a helium core.

10. List of MSPs in GCs

11. MSPs in Globular Clusters Total Number53 (Sept. 2001) 47 Tuc20 NGC 67525 M158 Terzan 22 Not all in binaries Spin periodsFrom 2.1ms to 1s Orbital periodsFrom 0.07 to 256 days

12. Why so many MSPs in Globular Clusters? In the field of the Galaxy, it is necessary that : 1)the primordial binary in which the NS is formed survives the SN explosion; 2) The binary parameters allow, later on, a phase of mass transfer which spins the pulsar up to ms periods In the field of the Galaxy, it is necessary that : 1)the primordial binary in which the NS is formed survives the SN explosion; 2) The binary parameters allow, later on, a phase of mass transfer which spins the pulsar up to ms periods GCs are the best place to produce binary systems containing a NS even a long time after the NS independent formation

13. (Why so many neutron stars in Globular Clusters?) (In the following we will not discuss a key problem: the embarassing presence of so many NSs in GCs. In fact, in the Galactic disk, ~200Km/s (implying some “kick” velocity at the SN event). Being v esc ~25Km/s typically from Gcs, a low fraction of NS should be retained in the clusters. For this reasons some invoke an AIC scenario for the NS formation)

14. Mechanisms for binary formation Bound system of two stars formed from an unbound configuration: a sink of orbital energy is required: 1)Collision involving three stars; one takes up the excess energy and escapes (very small probability); 2)Exchange interaction of a NS with an existing binary: the NS replaces one of the components; 3)Tidal capture: deformation of a normal star by close passage of a compact star takes away kinetic energy of the orbit, then dissipated through oscillations and heating of the envelope. If  Ek exceeds the total positive energy of the initial orbit, a bound system is formed. 1)Collision involving three stars; one takes up the excess energy and escapes (very small probability); 2)Exchange interaction of a NS with an existing binary: the NS replaces one of the components; 3)Tidal capture: deformation of a normal star by close passage of a compact star takes away kinetic energy of the orbit, then dissipated through oscillations and heating of the envelope. If  Ek exceeds the total positive energy of the initial orbit, a bound system is formed.

15. Which mechanism dominates? Exchange encounters: i)Direct exchange ii)Resonance encounter in which a temporary triple is formed, followed by the ejection of the third component (more frequent than i). For equal masses M  bin ~ n NS n bin M a/v inf (v inf =vel. at large separation) Tidal captures: (require d<3R)   bin ~ n NS n (m+M) 3R/v inf  (cross section is favoured for exchange encounters - a versus 3R-, but n bin <<n) Exchange encounters: i)Direct exchange ii)Resonance encounter in which a temporary triple is formed, followed by the ejection of the third component (more frequent than i). For equal masses M  bin ~ n NS n bin M a/v inf (v inf =vel. at large separation) Tidal captures: (require d<3R)   bin ~ n NS n (m+M) 3R/v inf  (cross section is favoured for exchange encounters - a versus 3R-, but n bin <<n)

16. Tidal capture 1) MS of RG companion (relatively distant encounters) 2) Binaries with WD companions, from direct collision with RGs 1) MS of RG companion (relatively distant encounters) 2) Binaries with WD companions, from direct collision with RGs 1) Mass transfer will begin soon, leading either to larger or to shorter Porb, depending on the nuclear evolution of the acquired companion. The system becomes a LMXB 2) GR - AML can bring the system into contact at very short (minutes) Porb. Mass transfer increases P leading to a system like the 11m binary in NGC 6624 (But also another channel..) 1) Mass transfer will begin soon, leading either to larger or to shorter Porb, depending on the nuclear evolution of the acquired companion. The system becomes a LMXB 2) GR - AML can bring the system into contact at very short (minutes) Porb. Mass transfer increases P leading to a system like the 11m binary in NGC 6624 (But also another channel..)

17. Recycling model for MSPs Old Neutron stars spun up by accretion from a companion 1)LMXB phase preceding the MSP stage; 2) mass transfer stops 3) the radio MSP emerges Orbital evolution: * P initial >P bifurcation : nuclear evolution drives mass exchange: MS  RG  RLO  NS spun up  Porb increases  He WD + MSP ** P initial <P bifurcation : systemic AML drives mass exchange during MS  Porb decreases  when MSP appears, the strong MSP radiation evaporates the companion (!)  single MSP 1)LMXB phase preceding the MSP stage; 2) mass transfer stops 3) the radio MSP emerges Orbital evolution: * P initial >P bifurcation : nuclear evolution drives mass exchange: MS  RG  RLO  NS spun up  Porb increases  He WD + MSP ** P initial <P bifurcation : systemic AML drives mass exchange during MS  Porb decreases  when MSP appears, the strong MSP radiation evaporates the companion (!)  single MSP

18. Bifurcation period The evolutionary meaning of the bifurcation is: has the secondary enough time to grow a helium core? In that case, it will evolve towards large orbits

19. The bifurcation period from 100 binary sequences Podsiadlowski, Rappaport & Pfhal 2002

20. Again about the bifurcation period in GCs The known binary periods of LMXBs in GCs, ranging from 17hr to 11min …. seem all in favour of tidal capture which leads naturally to periods 13 –17hr. These values are below or very close to P bif, so that the systems will evolve towards shorter P orb In addition, partial hydrogen burning in the core of the secondary, may lead to very short P orb (with the secondary transformed into a degenerate white dwarf having a hydrogen-helium intermediate composition, see Ergma and Fedorova 1998, Podsiadlowski et al. 2002, and the following lesson)

21. The “normal” MSP companions Most binary MSPs have long orbital periods and mass function identifying the companions as low mass helium white dwarfs *Porb’s are compatible with the evolution with mass transfer from a subgiant or giant, which finally leaves a helium WD remnant of mass 0.45 >~ M/Msun>~0.2 *if we notice that the radius is determined mainly by the core mass Mc, and put R=R 2,R a relation (Porb,Mc,M 2 ) is obtained * Pfin is then obtained by assuming Mc=M2 (at which stage Mdot stops) *Porb’s are compatible with the evolution with mass transfer from a subgiant or giant, which finally leaves a helium WD remnant of mass 0.45 >~ M/Msun>~0.2 *if we notice that the radius is determined mainly by the core mass Mc, and put R=R 2,R a relation (Porb,Mc,M 2 ) is obtained * Pfin is then obtained by assuming Mc=M2 (at which stage Mdot stops)

22. Is the recycling model reasonable? As we have seen, the majority of binary MSP have long P orb and He-WD companions, as predicted by evolution at P initial >P bifurcation. (Caveat: in Gcs the LMXBs with known P orb are all at short P orb, and GC MSPs have P orb <2.6days -apart from two at hundreds of days- It is possible that in GCs the long period binaries have been broken by further encounters) The answer is YES! The recycling model makes sense As we have seen, the majority of binary MSP have long P orb and He-WD companions, as predicted by evolution at P initial >P bifurcation. (Caveat: in Gcs the LMXBs with known P orb are all at short P orb, and GC MSPs have P orb <2.6days -apart from two at hundreds of days- It is possible that in GCs the long period binaries have been broken by further encounters) The answer is YES! The recycling model makes sense

23. Proposed solutions: 1) Mass transfer very large in LMXBs; 2)Accretion and spin up inhibited by propeller Proposed solutions: 1) Mass transfer very large in LMXBs; 2)Accretion and spin up inhibited by propeller But: 1)Birthrates of LMXBs << MSPs; 2)why we do not find MSP modulation in many LMXBs? 3)Where are the submilllisecond pulsars? But: 1)Birthrates of LMXBs << MSPs; 2)why we do not find MSP modulation in many LMXBs? 3)Where are the submilllisecond pulsars? Open Problems

24. The problem of the birthrates Birthrate of LMXBs ~1/10 – 1/100 than birthrate of MSPs Birthrate of LMXBs ~1/10 – 1/100 than birthrate of MSPs *this is found both in the disk (Kulkarni and Narayan 1988) and in the GCs (Fruchter and Goss 1990); *observationally exhacerbated by the fact that the radio MSP population is not complete, the LMXBs are all known *this is found both in the disk (Kulkarni and Narayan 1988) and in the GCs (Fruchter and Goss 1990); *observationally exhacerbated by the fact that the radio MSP population is not complete, the LMXBs are all known The result depends mainly on taking  ~ 10 9 yr as lifetime of the LMXB phase IRRADIATION by the X ray emission enhances the mass transfer and may be at least part of the solution

25. The lack of X-ray MSPs There are now only 3 LMXBs (transients) which show X-ray millisecond modulation SAX J1808.4-3658: Ps=2.5ms Porb=2hr (Wijnands & van derl Klis 1998) XTE J1751-306: Ps=2.3ms Porb=42m (Markwardt et al.2002) XTE J0929-314 Ps=5.4ms, Porb=43.6m (Galloway et al. 2002) SAX J1808.4-3658: Ps=2.5ms Porb=2hr (Wijnands & van derl Klis 1998) XTE J1751-306: Ps=2.3ms Porb=42m (Markwardt et al.2002) XTE J0929-314 Ps=5.4ms, Porb=43.6m (Galloway et al. 2002) The “standard” LMXB phase is, after all, NOT the main acceleration phase of the MSP? Again this may be linked to the evolutionary timescale of LMXBs

26. The lack of sub-ms pulsars In the standard LMXBs evolution, enough mass may be accreted to lead to Ps<1ms Either conservative mass transfer leads often to accretion induced collapse of the NS to Black Hole (for mass M NS ~3M sun for the stiffest EOS) (Cook Shapiro Teukolski ); Or –in short P orb systems- the detection of the radio pulsar is hampered by the strong orbital Doppler modulation of the radio signal Or not enough matter is accreted (see later) Either conservative mass transfer leads often to accretion induced collapse of the NS to Black Hole (for mass M NS ~3M sun for the stiffest EOS) (Cook Shapiro Teukolski ); Or –in short P orb systems- the detection of the radio pulsar is hampered by the strong orbital Doppler modulation of the radio signal Or not enough matter is accreted (see later)

27. The propeller does not work! When R m >R corotation, the matter can not be accreted (Illarionov and Sunayev 1975) Consequence: We need to expel matter far away from the NS surface! Consequence: We need to expel matter far away from the NS surface! 1)But in these systems R m ~R NS, so there is a huge energy requirements to eject matter 2) Even with ad hoc tuning, the maximum efficiency of a propeller is <50% 1)But in these systems R m ~R NS, so there is a huge energy requirements to eject matter 2) Even with ad hoc tuning, the maximum efficiency of a propeller is <50%

28. The radio- ejection When the radiation pressure of the rotating magnetic dipole becomes large enough, it prevents accretion directly at the inner Lagrangian point! Requiremnt: 1)Mass transfer must stop (or be very much reduced) to allow the radio pulsar switch on 2) P s short enough that P psr > P matter Requiremnt: 1)Mass transfer must stop (or be very much reduced) to allow the radio pulsar switch on 2) P s short enough that P psr > P matter (Burderi et al. 2001)

29. The first MSP in an interacting binary: J1740-5340 and in a globular cluster! is observed during the radio-ejection phase? (Burderi D’Antona & Burgay 2002) is observed during the radio-ejection phase? (Burderi D’Antona & Burgay 2002)