LMXB in Globular Clusters: Optical Properties Sivakoff et al David Riebel & Justice Bruursema

The Cast of Characters LMXBs (low-mass X-ray binary) LMXBs (low-mass X-ray binary) –Compact stellar remnant (BH or NS) –Low-mass companion generally transferring mass through Roche-lobe overflow –Globular Clusters contribute ~.1% of the light, but ~10% of the active LMXBs Globular clusters in early-type galaxies Globular clusters in early-type galaxies –Taken from ACSVCS + NGC 4697 observations

The Sample Includes 10 brightest ACSVCS galaxies along with NGC 4697 Includes 10 brightest ACSVCS galaxies along with NGC ,758 GC’s, of which 270 have detectable X-ray emission 6,758 GC’s, of which 270 have detectable X-ray emission Used HST- ACS/WFC for GCs Used HST- ACS/WFC for GCs Used CXO for LMXBs Used CXO for LMXBs

Sample Groups Detected Sample: All sources with a positive net luminosity (N=270) Detected Sample: All sources with a positive net luminosity (N=270) SNR Sample: All sources detected at the ≥3σ level (N=160) SNR Sample: All sources detected at the ≥3σ level (N=160) Complete Sample: All sources with L≥3.2*10 38 erg/s (N=61) Complete Sample: All sources with L≥3.2*10 38 erg/s (N=61)

Luminosity and Mass GC mass was determined by using z band magnitudes as tracers where ψ z = 1.45 M o /L o GC mass was determined by using z band magnitudes as tracers where ψ z = 1.45 M o /L o Findings confirm that LMXBs are found more often in brighter GCs Findings confirm that LMXBs are found more often in brighter GCs Median M z = -8.5 without LMXBs Median M z = -8.5 without LMXBs Median M z = -9.9 with LMXBs Median M z = -9.9 with LMXBs Possible power-law dependence of probability of GC containing a LMXB on mass Possible power-law dependence of probability of GC containing a LMXB on mass

Size and Metallicity r h correlates with (g-z) where “red” GC’s are ~17% smaller than “blue” GCs r h correlates with (g-z) where “red” GC’s are ~17% smaller than “blue” GCs So r h,M is more relevant: So r h,M is more relevant: –Follows from Jordan et al. (2005) This takes care of color dependence of half-light radii (confirmed by data) This takes care of color dependence of half-light radii (confirmed by data)

Color Findings confirm previous ideas that redder GC’s preferentially host LMXBs Findings confirm previous ideas that redder GC’s preferentially host LMXBs –Found to be 3.15 ± 0.54 times more likely (however there is considerable scatter between galaxies and the real relation is more continuous) Possible exponential dependence of probability of GC having LMXB based on color. Possible exponential dependence of probability of GC having LMXB based on color. –Means it is more dependent on environment within cluster rather than formation history of cluster

Size does matter First direct evidence that LMXBs are found more often in GCs that have smaller half- light radii First direct evidence that LMXBs are found more often in GCs that have smaller half- light radii Since r h is not correlated to mass, this means LMXBs are found in denser GCs Since r h is not correlated to mass, this means LMXBs are found in denser GCs Median r h = 2.6pc without LMXBs Median r h = 2.6pc without LMXBs Median r h = pc with LMXBs Median r h = pc with LMXBs Probability that a GC has a LMXB decreases roughly as a power law. Probability that a GC has a LMXB decreases roughly as a power law. There is a similar, but slightly flatter dependence on half-mass radius There is a similar, but slightly flatter dependence on half-mass radius

Relaxation Timescale Found using half-mass radius Found using half-mass radius Previously thought that you needed more than 5 relaxation timescales to produce a LMXB (t relax > 2.5 Gyr) Previously thought that you needed more than 5 relaxation timescales to produce a LMXB (t relax > 2.5 Gyr) But larger sample in this paper shows ~15% have t relax > 2.5 Gyr and in-fact show an opposite trend: But larger sample in this paper shows ~15% have t relax > 2.5 Gyr and in-fact show an opposite trend: Median t h,relax,cor = 1.0 Gyr without LMXBs Median t h,relax,cor = 1.0 Gyr without LMXBs Median t h,relax,cor = Gyr with LMXBs Median t h,relax,cor = Gyr with LMXBs

Dynamical Rates Two dynamical rates that could affect formation of LMXBs: Two dynamical rates that could affect formation of LMXBs: Stellar crossing rate: S  M 1/2 r -3/2 Stellar crossing rate: S  M 1/2 r -3/2 Encounter rate: Γ h  M 3/2 r -5/2 Encounter rate: Γ h  M 3/2 r -5/2 Both of these rates are found to be higher in LMXB containing GCs (this is consistent with LMXB GCs being more massive and smaller) Both of these rates are found to be higher in LMXB containing GCs (this is consistent with LMXB GCs being more massive and smaller)

Conclusions Multiple parameters were fit individually and in concert Multiple parameters were fit individually and in concert The linear dependence of LMXB formation can be ruled out with 99.89% confidence The linear dependence of LMXB formation can be ruled out with 99.89% confidence

Open Questions Several models have been proposed to explain the influence of metallicity Several models have been proposed to explain the influence of metallicity –Ivonova (2006) suggests that metal rich stars’ convective zones increase magnetic braking –Maccarone (2004) suggests weaker winds from metal rich stars will impact LMXB formation Undercount? Undercount?

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