Globular Cluster X-ray Sources

Transcription

1 Globular Cluster X-ray Sources David Pooley University of Wisconsin Chandra s First Decade of Discovery 2009 Sep 24

2 September 24, 1759

3 with thanks to friends, colleagues, and collaborators: Walter Lewin Frank Verbunt Cees Bassa Lee Homer Scott Anderson Bruce Margon Vicky Kaspi Brian Gaensler Derek Fox Rudy Wijnands Ed Cackett Jeroen Homan Josh Grindlay Craig Heinke Peter Edmonds Steve Murray Haldan Cohn Phyllis Luger Adrienne Cool Daryl Haggard Albert Kong Ting-Ni Lu Shih Hao Lan Didier Barret Bruce Gendre Natalie Webb Mathieu Servillat Piet Hut Simon Portegies Zwart John Fregeau Natalia Ivanova Harvey Tananbaum and the entire CXC staff

4 X-ray astronomy & globular clusters Luminous X-ray sources (L X > erg s 1 ) Discovered by Uhuru and OSO-7; argued to be formed via cluster dynamics Stimulated flurry of theoretical work Gursky 1973, Clark 1975, Katz 1975 Fabian, Pringle, & Rees 1975, Sutantyo 1975, Hills 1975, 1976, Heggie 1975, Verbunt & Hut globular clusters thought to contain one each All but one show Type-I X-ray bursts NS-LMXBs e.g., Kuulkers et al. 1996

5 Bright X-ray Sources: New Chandra discoveries M15 NGC 6440 Heinke et al DP et al White & Angelini 2001 in t Zand et al. 2001

6 X-ray astronomy & globular clusters Luminous X-ray sources (L X > erg s 1 ) Discovered by Uhuru and OSO-7; argued to be formed via cluster dynamics Stimulated flurry of theoretical work Gursky 1973, Clark 1975, Katz 1975 Fabian, Pringle, & Rees 1975, Sutantyo 1975, Hills 1975, 1976, Heggie 1975, Verbunt & Hut globular clusters thought to contain one each All but one show Type-I X-ray bursts NS-LMXBs e.g., Kuulkers et al Low Luminosity X-ray sources (L X < erg s 1 ) Discovered by Einstein Hertz & Grindlay 1983 More found with ROSAT e.g., Verbunt 2001 No secure identifications Suggested to be CVs (Hertz & Grindlay 1983), qlmxbs (Hertz & Grindlay 1983, Verbunt et al. 1984), radio MSPs (Saito 1997), magnetically active binaries (Bailyn et al. 1990) Extragalactic globular cluster X-ray sources (T. Maccarone) Detailed MSP studies (S. Bogdanov) Intermediate Mass Black Holes

7 Globular Cluster NGC R core = 0.74 parsec = 2.4 lightyears = cm ADH-6829 core = star pc 3 A us = 1 star pc 3 Illi. scan 30 CGB1 UP-6817 SB-1721 Illi. ann. Cheb. poly. corr log (r/arcsec) NGC 2808 credit: S. Juchnowski Trager, King, & Djorgovski 1995 adapted from Servillat et al. (2008) see poster by M. Servillat

8 Surface Brightness Profiles core collapsed 20% of globular clusters CGB1 ADH-6829 A UP-6817 SB-1721 Illi. scan Illi. ann. Cheb. poly. corr log (r/arcsec) NGC 2808 normal 80% of globular clusters Trager, King, & Djorgovski 1995

9 Simulating Globular Clusters Fregeau et al. 2003

10 Globular Cluster Life Stages Core contraction Binary burning 80% Binary burning 20% Core collapse Deep core collapse Gravothermal oscillations Fregeau et al. 2003

11 X-ray Sources in Globular Clusters Chandra image of 47 Tuc Low-mass X-ray Binaries Millisecond pulsars Cataclysmic Variables Active main-sequence binaries adapted from Grindlay et al. 2001

12 NGC 6397 M4 47 Tuc

13 47 Tuc in X-rays Einstein (8 ksec) ROSAT (77 ksec) Chandra (240 ksec)

14 Identifying the X-ray Sources 47 Tuc CX1 NGC 6752 CX2 CX3 N E CX4 CX5 CX6 CX7 CX10 CX11 Edmonds et al. (2003) DP et al. (2002)

15 Identifying the X-ray Sources NGC 6397 NGC 6752 Grindlay et al. (2001) DP et al. (2002)

16 Source Identification via X-rays Cen qlmxb kt = 67 ± 2 ev R = 12.6 ± 3 km Cen CV kt = /-10 kev Webb & Barret 2004 (from Gendre et al. 2003)

17 A Link to Stellar Dynamics DP et al Heinke et al Gendre et al. 2003

18 X-ray CMD To Date: 77 GCs 114 ACIS Obs. 3 Msec >1500 sources ~250 background L kev (erg/s) log 10 (C kev / C 2-8 kev )

19 X-ray CMD Uniform: L x > erg s GCs ~200 sources ~15 background qlmxb Active Binary Field Burster Field Pulsar Pulsar CV L X [0.5-6 kev] (erg s -1 ) L kev (erg s -1 ) adapted from DP & Hut 2006 NS Atmosphere log 10 (Flux [0.5-2 kev] / Flux [2-6 kev]) log 10 (F kev /F 2-6 kev ) + 50% PL + 20% PL

20 Cluster LMXBs Can rule out constant n at 4 n = a s + c s = 1.8 ± 0.4 c = 0.4 ± 0.5 n LMXB DP & Hut 2006 Specific units: n = N/M = /M M in units of 10 6 M building on Gendre et al. 2003, Heinke et al. 2003, DP et al 2003

21 Are Globular Cluster CVs overabundant? They should be: Hut & Verbunt 1983 Di Stefano & Rappaport 1994

22 Are Globular Cluster CVs overabundant? They re not: Shara 1996, elliptical galaxies. Likewise, we predict that there should be CVs for every 10 6 L, K in an old stellar population. The population of X-ray identified CVs in the globular cluster 47 Tuc is similar to this number, showing no overabundance relative to the field. The observed CN P orb distribution also contains evidence for a CV population Townsley & Bildsten 2005 Maybe a little? With our simulations, we predict that the formation rates of CVs and AM CVn systems in GCs are not very different from those in the field population. The numbers of CVs and AM CVn systems per mass unit are comparable to numbers in the field if the whole cluster population is considered, and they are only two to three times larger in the core than in the field. Dynamical formation is responsible only for per cent of CVs in the core. This fraction decreases Ivanova et al. 2006

23 X-ray CMD Uniform: L x > erg s GCs ~200 sources ~15 background qlmxb Active Binary Field Burster Field Pulsar Pulsar CV L X [0.5-6 kev] (erg s -1 ) L kev (erg s -1 ) NS Atmosphere log 10 (Flux [0.5-2 kev] / Flux [2-6 kev]) log 10 (F kev /F 2-6 kev ) + 50% PL + 20% PL

24 Bright, hard sources (mostly CVs) Can rule out constant n at 6 n = a s + c s = 0.8 ± 0.2 c = 0.3 ± 1.4 n CV DP & Hut 2006 Specific units: n = N/M = /M M in units of 10 6 M

25 Dynamical Formation All sources with L x > erg s -1 n x Core collapsed n = a s + c s = 0.45 ± 0.17 c = 5.3 ± Cen 47 Tuc DP & Hut 2006 Specific units: n x = N x /M = /M M in units of 10 6 M

26 Globular Cluster Life Stages Revisited Core contraction Binary burning 80% Binary burning 20% Core collapse Deep core collapse Gravothermal oscillations Paradigm shift? Fregeau et al Fregeau (2008) pointed out: Better simulations reveal r c was overestimated by 10 in binary-burning phase Production of X-ray sources in binary burning phase is 2 20 higher than in core contraction phase Chandra reveals that core collapsed clusters have many more binaries than the N- relation predicts 80% Binary burning + 20% Core collapsed 80% Core contraction + 20% Binary burning

27 Dynamical Formation All sources with L x > erg s "core-collapsed" GCs 48 "normal" GCs PRELIMINARY N X / M / M 6 DP et al. in prep.

28 Future Work Individual identifications Subpopulation dynamics Investigate importance of other parameters (e.g., metallicity) Fast (~3 day) transients Heinke et al. Low density clusters (primordial binaries) Kong, Lu, Lan et al. Deep exposures of M4 (DP et al.) and NGC 6397 (Grindlay et al.) Extending into rich open clusters: see poster by N. Gosnell Fermi survey to determine overall MSP population

29 Summary It s Guinness s 250 th birthday! X-ray sources are dynamically formed Great tracers for large scale simulations (especially LMXBs) CVs are finally being found in large numbers in globular clusters and are overabundant Chandra is the most efficient and effective means of finding the important close binaries in a globular cluster Possible revolution in our understanding of the current dynamically states of globular clusters

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31 Understanding Globular Clusters (is tough) No Thermodynamic Limit M R 3 E kin M but E pot M 2 /R M 5/3 Infrared Divergence Negative Heat Capacity Mass segregation stratified system dense core Encounters extract energy from core, increasing temperature Nearly Unlimited Reservoir of Binary Binding Energy 3-body encounters tap into binary energy Ultraviolet Divergence Feedback between Stellar Dynamics and Stellar Evolution No Easy Way to Compare Theory to Observation Input theory to simulations Compare simulations to observations

32 Bright X-ray Sources: Neutron Star LMXBs Discovered by Uhuru and OSO-7; argued to be formed via cluster dynamics Gursky 1973, Clark 1975, Katz 1975 Stimulated flurry of theoretical work Fabian, Pringle, & Rees 1975 Sutantyo, 1975 Hills, 1975, 1976 Heggie 1975 Verbunt & Hut globular clusters thought to contain one each All but one show Type-I X-ray bursts NS-LMXBs e.g., Kuulkers et al Good evidence that many have ultrashort periods: P orb < 80 min e.g., Deutsch et al Ultrashort Period Normal Period Unknown Period X=0 X=0.73 Kuulkers et al courtesy L. Homer

33 GLAST Observations of Globular Cluster MSPs BPS: R 6 =0.8, I 45 =0.6 FP: R 6 =1.0, I 45 =1.0 PS: R 6 =1.6, I 45 =2.2 PSR J B 0 = 5.8 x 10 8 G P = 5.8 ms d = 0.18 kpc F CR (1 GeV) = cnts cm 2 s 1 MeV 1 GeV/(cm 2 s) CR EGRET GLAST MAGIC ICS H.E.S.S. Glob kpc BUT with 100s of MSPs! Energy (GeV) Harding, Usov, & Muslimov 2005 ( ) 2 F clus = F = F

34 Most Promising GLAST Clusters Cluster N LMXB d (kpc) MSP Flux Fig. of Merit Terzan NGC Tuc NGC NGC Liller NGC NGC

35 Cluster LMXBs Can rule out constant n at 4 n = a s + c s = 1.8 ± 0.4 c = 0.4 ± 0.5 n LMXB DP & Hut 2006 Specific units: n = N/M = /M M in units of 10 6 M

36 Kitchen Sink simulations coming soon... GRAPE-6 special purpose supercomputer: 64 Tflops See for more information Simulation by S. Portegies Zwart

37 Kitchen Sink simulations coming soon... M67 Simulation by J. Hurley

38 Brief History of Binaries in Globular Clusters 1960s, 70s: Theory predicted the inevitable collapse of cluster of single stars e.g., Hénon 1961, s: Computer simulations confirmed this and gave rich understanding of collapse e.g., work by Goodman, Heggie, Hut, Spitzer s, 80s: Observers found no binaries e.g., Gunn & Griffin s: Observers found some binaries see Hut et al s: Simulations showed how binaries postpone core collapse e.g., Goodman & Hut 1989

39 Low Mass X-ray Binary (LMXB) low mass star + neutron star

40 Cataclysmic Variable (CV) low mass star + white dwarf

41 Active Main Sequence Binary two stars with strong magnetic fields

42 What is a close binary? We can divide globular cluster binaries into two broad groups, based on binding energy: Gm 1 m 2 > 1 2 mv 2 Gm 1 m 2 < 1 2 mv 2 a a Hard or Close binaries Soft binaries Heggie (1975)

43 ( Encounter Frequency ) R = n 1 n 2 v rel σ σ = πd 2 ( 1+ 2G(m 1 + m 2 ) v 2 rel d ) πd 2G(m 1 + m 2 ) v 2 rel R ρ 2 /v Γ= rh 0 RdV Γ= rc 0 RdV ρ 2 0 r3 c /v = ρ1.5 0 r2 c

44 Heggie s Law in action (numerically): Perform scattering experiments for different initial parameters. Hut & Bahcall (1983) from A. Gualandris (

45 Heggie s Law in action (numerically): Hut (1983)

46 Key Role of Binaries in GCs 2% Binaries 20% Binaries Fregeau et al. 2003

47 ROSAT grayscale + Chandra point sources Verbunt 2004

48 The Revolutionary Chandra X-ray Observatory

49 Related Work: Young Massive Clusters The Search for Black Holes Westerlund 1 Muno et al. 2005

50 Cataclysmic Variable (CV) Intermediate Polar low mass star + magnetic white dwarf

51 10 34 Very low L x erg s -1 3 GCs 243 sources ~35 background L X [0.5-6 kev] (erg s -1 ) log 10 (Flux [0.5-2 kev] / Flux [2-6 kev])

52 (Mostly Active Main-sequence Binaries) Cluster / 6121 L V /L V,6121 N srcs N srcs /N srcs, Tuc NGC NGC Heinke et al. 2005, Bassa et al. 2004, Grindlay et al. 2001

53 Cluster Obs. N min N max <N> ± N N unique 47 Tuc ± NGC ± NGC ±

54 X-ray Sources in Globular Clusters

55 X-ray CMD Uniform: L 31 x > erg s GCs ~500 ~200 sources ~100 ~15 background L X L [0.5-6 kev] (erg s -1 ) ) log 10 (Flux [0.5-2 kev] / Flux [2-6 kev]) log 10 (F kev /F 2-6 kev )

56 X-ray CMD Uniform: L x > erg s GCs ~500 sources ~100 background L X [0.5-6 kev] (erg s -1 ) log 10 (Flux [0.5-2 kev] / Flux [2-6 kev])

57 Primordial vs. Dynamical Formation Compare number of sources (N) per cluster to Primordial quantity: mass of cluster (M) Dynamical quantity: encounter frequency of cluster ( ) Problem M and are correlated Solution use specific units n N/M /M Method primordial: n = c dynamical: n( ) = a s + c