1. Dynamical formation of LMXBs in the inner bulge of M31 by Rasmus Voss with Marat Gilfanov X-rays from nearby galaxies ESAC September 2007

2. Overview Introduction: LMXBs What are they, how are they formed Observations:LMXBs in M31 The spatial distribution Theory: Dynamical formation of LMXBs Conclusions: Dynamical formation of LMXBs is important in the bulge of M31

3. LMXBs NS or BH accreting from a companion star < 1 Msol Mass transfer driven by angular momentum loss from magnetic braking and gravitational radiation

4. Formation of (primordial) LMXBs Only weakly dependent on the stellar environment. LMXBs trace the mass in galaxies

5. Observations Gilfanov (2004): LMXBs do follow the stellar mass in nearby galaxies

6. Observations Gilfanov (2004): LMXBs do follow the stellar mass in nearby galaxies Centaurus A Voss & Gilfanov (2006) with Chandra

7. LMXBs in globular clusters Out of ~150 LMXBs in the Milky Way 13 LMXBs reside in globular clusters Globular clusters contain <0.1% of the mass of the Milky Way Improbable that they can be primordial LMXBs Due to stellar encounters (dynamical formation of LMXBs)

8. M31

9. M31

10. Observations M31 with Chandra Something wrong! Dynamical formation?

11. Dynamical LMXB formation channels Tidal capture Exchange reactionCollision

12. A few equations encounter rate per unit volume: Two body encounters Important: Gravitational focusing

13. A few equations Two body encounters Kroupa IMF Padova stellar isochrones (Girardi et al. 2002)

14. M31 versus globular clusters Density: 10 2 -5·10 5 M sol /pc 3 10 3 -10 4 M sol /pc 3 Velocity dispersion: 1-20 km/s 150 km/s Metallicity: 0.01-1.0 solar 1.5-2.0 solar Globular clusters M31 inner bulge Mass segregation: strong not effective Black holes: No Yes Neutron stars: 10% retention 100% retention Mass <10 7 M sol >10 10 M sol

15. Dynamical LMXB formation channels Tidal capture Exchange reactionCollision

16. Predictions for M31 24 BH LMXBs 5 NS LMXBs 3 UCXBs

17. Conclusions: M31 First evidence of dynamical formation of LMXBs in M31! First theoretical investigation of DF in this environment! Theory works Mainly short period systems Mainly BH accretors

18. Predictions for globular clusters Metal-rich With lifetimes: 2 “normal” LMXB 3 LMXB with ~0.1 M sol comp. 1.5 UCXB Observed: 2 LMXB P>5hr 4 with P unknown 2 UCXB (P<1hr)

19. An extra piece of evidence The luminosity function of the LMXBs: MW globular clusters M31 globular clusters M31 inner 1 arcmin