Stellar Life-Cycles in Chandra Observations of the Galactic Center

1. Stellar Life-Cycles in Chandra Observations of the Galactic Center Michael Muno (UCLA/Hubble Fellow)

3. N E Chandra Observations of our Galaxy Wang, Gotthelf, & Lang 2002; NASA/UMass 30 pc

4. N E Chandra Observations of our Galaxy We see thousands of X-ray sources in a sea of diffuse emission. Our goal is to identify the natures of the point sources in order to: • Provide a census of accreting compact objects and young stars. • Address the nature of the diffuse X-ray emission. • (Eventually) study the physics of X-ray production. Wang, Gotthelf, & Lang 2002; NASA/UMass 30 pc

5. N E Chandra Observations of our Galaxy Wang, Gotthelf, & Lang 2002; NASA/UMass 30 pc

6. N E Sgr A* 5 pc Baganoff et al. (2003); Muno et al. (2003a, ApJ, 589, 225)

7. X-ray Point Sources • 2287 sources have been resolved. • About 200 are in the foreground of the Galactic center. • About 40 are background AGN • Sources have LX=1030-1033 erg s-1 =10-9 - 10-6LEdd. Muno et al. (2003a, ApJ, 589, 225)

8. X-ray Sources Trace the Stellar Population The surface density of X-ray sources falls off as R-1, just like the stellar population in the infrared.

9. Predictions for the Population • Pfahl, Rappaport, & Podsiadlowski (2002) predict ~100 HXMBs in our field. • Belczynski & Taam (2004) predict a similar number of LMXBs. see also Iben et al. 1997, Howell et al. 2001, Bleach 2002, Willems & Kolb 2003, Cheng et al. 2004 Tauris & van den Heuvel (2004)

10. Predictions for the Population Questions: • The efficiency of the common envelope phase. • Whether accretion-induced collapse can form neutron stars. • What compact objects accreting at low rates look like. • The duty cycle of transient outbursts from accretion disks. Tauris & van den Heuvel (2004)

11. Galactic X-ray Point Sources See also A. Bykov’s talk on supernova fragments.

12. Spectra of the Point Sources keV Color = (H-L/H+L). L = 3.3-4.7 keV H = 4.7-8.0 keV Muno et al. (2004b, ApJ, 613, 1179)

13. Spectra of the Point Sources mCV HMXB pulsar WR/O, LMXB YSO, RS CVn CV Muno et al. (2004b, ApJ, 613, 1179)

14. Spectra of the Point Sources Muno et al. (2004b) • Lines are observed from He-like and H-like Fe, which could result from a 8 keV thermal plasma or a photo-ionized plasma. CVs exhibit similar spectra.

15. Periodic X-ray Modulations • Eight sources show modulations. • Periods between 300s and 4.5h • Amplitudes > 45% rms • Seven have hard spectra (HR > 0.1) • Consistent either with magnetically accreting WD or NS. Muno et al. (2003c, ApJ, 599, 465)

16. Point Sources and Diffuse X-ray Emission Muno et al. (2004a, ApJ, 613, 326)

17. Spectrum of Diffuse Emission Muno et al. (2004a) • Lines from Si, S, and Ar are consistent with a 107 K plasma. • Lines from Fe could be from a 108 K plasma, but they also resemble those from the point sources.

18. Number-Flux Distribution • The log(N)-log(S) distribution is very steep: slope a=-1.7 • >105 undetected sources would be required to produce the observed diffuse emission. • Only ~104 CVs should be present in the field. Muno et al. (2004a)

19. Point Sources and Diffuse X-ray Emission • No known class of point source can account for the (apparently) diffuse X-ray emission. • The intensity of the diffuse emission varies spatially. • See talk by R. Belmont to find out what we think the diffuse emission really is. Muno et al. (2004a, ApJ, 613, 326)

20. Reasons to Do Better • Although bright, hard CVs have been observed in the Galactic plane, observations of globular clusters do not reveal similar sources. • Age effect? • There should be a significant population of younger objects, such as HMXBs and WR/O stars.

21. Finding Young, Massive Stars Multi-wavelength Observations: • 2MASS: J,H,K to a depth of K<11 • SIRIUS/IRSF (South Africa): J,H,K to a depth of K<13 (Nagata and Nishiyama) • NICMOS: J,H,K, Pa-alpha to K<17 (Archival) • KECK: J, H, K, Br-gamma to K<19 (Morris/Muno) • VLA: 8.4 GHz to a depth of 1 mJy (Bowers) For comparison, CVs should have K<21, and HMXBs and WR/O stars should have K<17.

22. Diffraction-limited Keck Observations 104 total foreground sources 1”

23. Diffraction-limited Keck Observations 104 total foreground sources 1” Confusion makes IR surveys inefficient for identifying countperparts.

24. Sources with Radio Counterparts G. Bower; Muno et al. (2005) • Found two X-ray sources with radio and IR matches. • One is a known massive, young star in an HII region (H2). 8.4 GHz

25. Million-Solar Luminosity Stars Thanks to Adam Burgasser • IR spectra reveal lines from H and He characteristic of winds from stars with T >15,000 K, often classified as B[e], Of, and LBV stars. • Steve Eikenberry has found another object that may be an HMXB. • The fact that these are isolated has important implications for star formation.

26. Sgr A* 5 pc Searching for X-ray Binaries • We identified accreting black holes and neutron stars by looking for sources that: • varied by at least a factor of 10, and • had peak luminosities >1034 erg s-1. • We found 7 such systems.

27. Sgr A* 5 pc Searching for X-ray Binaries • We identified accreting black holes and neutron stars by looking for sources that: • varied by at least a factor of 10, and • had peak luminosities >1034 erg s-1. • We found 7 such systems. Neutron Star LMXB GRS 1741.9-2858 Muno et al. (2005, astro-ph/0412492)

28. An Overabundance in the Central Parsec • Four lie within 1 pc of Sgr A*. The enclosed stellar mass is 2 106 Mo. • Three lie between 1-25 pc of Sgr A*. The enclosed stellar mass is >3 107 Mo. • Transients are over-abundant by >20x in the inner parsec! Muno et al. (2005) 1 pc

29. An Overabundance in the Central Parsec D. Porquet will explain how we know this one is an LMXB. Muno et al. (2005) 1 pc

30. 1 pc 47 Tuc LMXBs Are Also Concentrated in Globular Clusters Keel et al. Grindlay et al. 2001; Pooley et al. 2003 Optical: 1.5 m telescope in Chile X-ray: Chandra In globular clusters, LMXBs are over-abundant by a factor of 100 per unit stellar mass.

31. Dynamical Friction • Lighter objects tend to collect in the wakes of heavier ones. • As a result, the heavier object is slowed down. • The heavier object loses energy, and falls deeper into the gravitatational potential.

32. 1 pc Dynamically Forming LMXBs Grindlay et al. 2001; Pooley et al. 2003 47 Tuc simulation by E. Pfahl In globular clusters, LMXBs are over-abundant by a factor of 100 per unit stellar mass.

33. 1 pc 47 Tuc Dynamically Forming LMXBs • rc = 6 104 Mo pc-3 • s = 12 km s-1 • rc = 7 106 Mo pc-3 • s = 70 km s-1

34. Dynamically Forming LMXBs • 104 black holes have dynamically settled into the central pc (Morris 1993, Miralda-Escudé & Gould 2000). • Pfahl & Loeb (in prep.) estimate that these form LMXBs via binary-single interactions at a rate of 10-6 yr -1. • Over the dynamical time scale of 10 Gyr, 103 LMXBs could form. simulation by E. Pfahl; Muno et al. (2005)

35. Sgr A* Young HMXBs • Several dozen massive stars formed among 104 stars 7 Myr ago. • Up to 300 black holes may have already formed. • At most 10% of these could be in HMXBs, or on order 30 systems. 1 pc Muno et al. (2005) Infrared laser guide-star image courtesy W.M. Keck Observatory.

36. Conclusions • The vast majority are CVs that trace the old stellar population. • Known stellar populations cannot account for the diffuse emission, unless they are overabundant by a factor of 10 near the GC. • Radio and IR observations have identified two young, million-solar luminosity stars. • An overabundance of X-ray transients in the central parsec provides evidence for a concentration of stellar- mass black holes in the central parsec .