Dynamical formation of LMXBs

in the inner bulge of M31

by

Rasmus Vosswith

Marat Gilfanov

X-rays from nearby galaxies ESAC September 2007

OverviewOverview

Introduction: LMXBsWhat are they, how are they formed

Observations: LMXBs in M31The spatial distribution

Theory: Dynamical formation of LMXBs

Conclusions: Dynamical formation of LMXBs is important in the bulge of M31

LMXBsLMXBs

NS or BH accreting from a companion star < 1 Msol

Mass transfer driven by angular momentum lossfrom magnetic braking and gravitational radiation

Formation of (primordial) LMXBsFormation of (primordial) LMXBs

Only weaklydependent onthe stellarenvironment.

LMXBs trace the mass in galaxies

ObservationsObservationsGilfanov (2004):

LMXBs do follow the stellar mass in nearby galaxies

ObservationsObservationsGilfanov (2004):

LMXBs do follow the stellar mass in nearby galaxies

Centaurus A Voss & Gilfanov (2006)with Chandra

LMXBs in globular clustersLMXBs in globular clusters

Out of ~150 LMXBs inthe Milky Way13 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 formationof LMXBs)

M31M31

M31M31

ObservationsObservations

M31 with Chandra

Something wrong!

Dynamical formation?

Dynamical LMXB formation channelsDynamical LMXB formation channelsTidal capture Exchange reactionCollision

A few equationsA few equations

encounter rate per unit volume:

Two body encounters

Important: Gravitational focusing

A few equationsA few equations

Two body encounters

Kroupa IMF

Padova stellar isochrones (Girardi et al. 2002)

M31 versus globular clustersM31 versus globular clusters

Density: 102 -5·105 Msol

/pc3 103-104 Msol

/pc3

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 <107 Msol

>1010Msol

Dynamical LMXB formation channelsDynamical LMXB formation channelsTidal capture Exchange reactionCollision

Predictions for M31Predictions for M31

•24 BH LMXBs• 5 NS LMXBs• 3 UCXBs

Conclusions: M31Conclusions: 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 clustersPredictions 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 evidenceAn extra piece of evidenceThe luminosity function of the LMXBs:The luminosity function of the LMXBs:

MW globularclusters

M31 globularclusters

M31 inner1 arcmin