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

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

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

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

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

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

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)

M31

M31

Observations M31 with Chandra Something wrong! Dynamical formation?

Dynamical LMXB formation channels Tidal capture Exchange reactionCollision

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

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

M31 versus globular clusters Density: ·10 5 M sol /pc M sol /pc 3 Velocity dispersion: 1-20 km/s 150 km/s Metallicity: solar 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

Dynamical LMXB formation channels Tidal capture Exchange reactionCollision

Predictions for M31 24 BH LMXBs 5 NS LMXBs 3 UCXBs

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

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)

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