LMXB in Globular LMXB in Globular Clusters: Optical Clusters: Optical PropertiesPropertiesSivakoff et al. 2007Sivakoff et al. 2007

David RiebelDavid Riebel

&&

Justice BruursemaJustice Bruursema

The Cast of CharactersThe Cast of Characters

LMXBs (low-mass X-ray binary)LMXBs (low-mass X-ray binary)– Compact stellar remnant (BH or NS)Compact stellar remnant (BH or NS)– Low-mass companion generally Low-mass companion generally

transferring mass through Roche-lobe transferring mass through Roche-lobe overflowoverflow

– Globular Clusters contribute ~.1% of the Globular Clusters contribute ~.1% of the light, but ~10% of the active LMXBslight, but ~10% of the active LMXBs

Globular clusters in early-type galaxiesGlobular clusters in early-type galaxies– Taken from ACSVCS + NGC 4697 Taken from ACSVCS + NGC 4697

observationsobservations

The SampleThe Sample

Includes 10 brightest ACSVCS galaxies Includes 10 brightest ACSVCS galaxies along with NGC 4697along with NGC 4697

6,758 GC’s, of which 270 have detectable 6,758 GC’s, of which 270 have detectable X-ray emissionX-ray emission

Used HST-Used HST-ACS/WFC for ACS/WFC for GCsGCs

Used CXO for Used CXO for LMXBsLMXBs

Sample GroupsSample Groups

Detected Sample: All sources Detected Sample: All sources with a positive net luminosity with a positive net luminosity (N=270)(N=270)

SNR Sample: All sources SNR Sample: All sources detected at the ≥3detected at the ≥3σσ level level (N=160)(N=160)

Complete Sample: All sources Complete Sample: All sources with L≥3.2*10with L≥3.2*103838 erg/s (N=61) erg/s (N=61)

Luminosity and MassLuminosity and Mass

GC mass was determined by using z band GC mass was determined by using z band magnitudes as tracers where magnitudes as tracers where ψψz z = 1.45 M= 1.45 Moo/L/Loo

o)M0.4(M

z M10ΨM oz,z

Findings confirm that LMXBs Findings confirm that LMXBs are found more often in are found more often in brighter GCsbrighter GCs

Median MMedian Mzz= -8.5 = -8.5 withoutwithout LMXBsLMXBs

Median MMedian Mzz= -9.9 = -9.9 withwith LMXBs LMXBs Possible power-law Possible power-law

dependence of probability of dependence of probability of GC containing a LMXB on GC containing a LMXB on massmass

Size and MetallicitySize and Metallicity

rrhh correlates with (g-z) where “red” correlates with (g-z) where “red” GC’s are ~17% smaller than “blue” GC’s are ~17% smaller than “blue” GCsGCs

So rSo rh,Mh,M is more relevant: is more relevant:

– Follows from Jordan et al. (2005)Follows from Jordan et al. (2005) This takes care of color dependence This takes care of color dependence

of half-light radii (confirmed by of half-light radii (confirmed by data)data)

corhzg

hh,M rrr ,]2.1)[(17.010

ColorColor

Findings confirm previous ideas that Findings confirm previous ideas that redder GC’s preferentially host LMXBsredder GC’s preferentially host LMXBs– Found to be 3.15 ± 0.54 times more likely Found to be 3.15 ± 0.54 times more likely

(however there is considerable scatter (however there is considerable scatter between galaxies and the real relation is between galaxies and the real relation is more continuous)more continuous)

Possible exponential dependence of Possible exponential dependence of probability of GC having LMXB based probability of GC having LMXB based on color.on color.– Means it is more dependent on Means it is more dependent on

environment within cluster rather than environment within cluster rather than formation history of clusterformation history of cluster

Size does matterSize does matter

First First directdirect evidence that LMXBs are evidence that LMXBs are found more often in GCs that have found more often in GCs that have smaller half-light radiismaller half-light radii

Since rSince rhh is not correlated to mass, this is not correlated to mass, this means LMXBs are found in denser GCsmeans LMXBs are found in denser GCs

Median rMedian rhh= 2.6pc = 2.6pc withoutwithout LMXBs LMXBs

Median rMedian rhh= 2.2-2.3pc = 2.2-2.3pc withwith LMXBs LMXBs Probability that a GC has a LMXB decreases Probability that a GC has a LMXB decreases

roughly as a power law.roughly as a power law. There is a similar, but slightly flatter There is a similar, but slightly flatter

dependence on half-mass radiusdependence on half-mass radius

Relaxation TimescaleRelaxation Timescale

Found using half-mass radiusFound using half-mass radius

Previously thought that you needed Previously thought that you needed more than 5 relaxation timescales to more than 5 relaxation timescales to produce a LMXB (tproduce a LMXB (trelaxrelax > 2.5 Gyr) > 2.5 Gyr)

But larger sample in this paper shows But larger sample in this paper shows ~15% have t~15% have trelaxrelax > 2.5 Gyr and in-fact > 2.5 Gyr and in-fact show an show an oppositeopposite trend: trend:

Median tMedian th,relax,corh,relax,cor= 1.0 Gyr = 1.0 Gyr withoutwithout LMXBs LMXBs Median tMedian th,relax,corh,relax,cor= 1.3-1.5 Gyr = 1.3-1.5 Gyr withwith LMXBs LMXBs

yr

2/3

1,

2/1

00

*)*4.0ln(

610055.2,,

1

pcMhr

M

M

M

m

Ncorrelaxht

Dynamical RatesDynamical Rates

Two dynamical rates that could Two dynamical rates that could affect formation of LMXBs:affect formation of LMXBs:

Stellar crossing rate: S Stellar crossing rate: S M M1/21/2rr-3/2-3/2

Encounter rate: Encounter rate: ΓΓhh M M3/23/2rr-5/2-5/2

Both of these rates are found to Both of these rates are found to be higher in LMXB containing GCs be higher in LMXB containing GCs (this is consistent with LMXB GCs (this is consistent with LMXB GCs being more massive and smaller)being more massive and smaller)

ConclusionsConclusions

Multiple parameters were fit individually Multiple parameters were fit individually and in concertand in concert

The linear dependence of LMXB The linear dependence of LMXB formation can be ruled out with 99.89% formation can be ruled out with 99.89% confidenceconfidence

24.025.0

076.0075.0 22.2

,)(90.0237.1 10

corh

zgt rM

Open QuestionsOpen Questions

Several models have been Several models have been proposed to explain the influence proposed to explain the influence of metallicityof metallicity– Ivonova (2006) suggests that metal Ivonova (2006) suggests that metal

rich stars’ convective zones increase rich stars’ convective zones increase magnetic brakingmagnetic braking

– Maccarone (2004) suggests weaker Maccarone (2004) suggests weaker winds from metal rich stars will impact winds from metal rich stars will impact LMXB formationLMXB formation

Undercount?Undercount?

ReferencesReferences

Bregman et al. (2006) ApJ, 640, Bregman et al. (2006) ApJ, 640, 282282

Ivanova (2006) ApJ, 636, 979Ivanova (2006) ApJ, 636, 979 Jordan et al. (2005) ApJ, 634, 279Jordan et al. (2005) ApJ, 634, 279 Maccarone et al. (2004) ApJ, 606, Maccarone et al. (2004) ApJ, 606,

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