Transient X-ray Binaries in the Galactic Center

1. Transient X-ray Binaries in the Galactic Center Michael Muno (Caltech)

2. A Brief Introduction to X-ray Binaries RXTE ASM

3. Monitoring X-ray Binaries

4. Are disk-fed. Accretion is unstable at low mass transfer rates. Neutron stars produce thermonuclear bursts. Are wind-fed. Only form disks at high accretion rates. Neutron stars are pulsars. X-ray Binaries High-Mass X-ray Binary Low-Mass X-ray Binary solar radius Made with binsim, by R. Hynes

5. The Need to Go Fainter • Wide-field X-ray surveys are only sensitive to ~1036 erg s-1 at the Galactic Center (8 kpc). • The Galactic population may be dominated (90%) by fainter X-ray binaries with low accretion rates. • The number of fainter systems could answer: • What the structure of the accretion flow is at low mass transfer rates. • Questions about population synthesis: • How three-body interactions form LMXBs in dense star systems.

6. N E 30 pc A Sample of Faint X-ray Binaries The central 1o by 2o contains 1% of the Galactic stellar mass, but requires high angular resolution to observe. Wide field surveys: 5’ angular resolution, 1036 erg s-1 sensitivity

7. N E 30 pc A Sample of Faint X-ray Binaries ASCA/ BeppoSAX: 1’ angular resolution, 40 mCrab sensitivity

8. N E 30 pc A Sample of Faint X-ray Binaries Chandra: 0.5” angular resolution, 1032 erg s-1 sensitivity (Wang, Gotthelf, & Lang 2002; NASA/UMass)

9. A Sample of Faint X-ray Binaries Most of the sources are cataclysmic variables(Muno et al. 2006). However, there are several known X-ray binaries, most of which are transient (Porquet et al. 2005; Muno et al. 2005; Wijnands et al. 2006).

10. N E 30 pc Finding the Faintest Transients Field: Surveys at two epochs (Wang et al. 2002; Muno et al.) Orange: Bi-monthly XMM and Chandra monitoring (Wijnands et al.) Blue: Chandra monitoring on time scales of weeks (Baganoff et al.)

11. Sgr A* 5 pc Finding the Faintest Transients • 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 transients in 5 years of data. Muno et al. (2005)

12. Sgr A* 5 pc Finding the Faintest Transients Neutron Star LMXB GRS 1741.9-2858 • 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 transients in 5 years of data. Muno et al. (2005)

13. A LMXB 0.1 pc from Sgr A* with Periodic Eclipses To Sgr A* (0.1 pc) The X-ray light curve displays partial eclipses at the 8 hour orbital period. Infrared images reveal no companion with K<15, ruling out a high-mass star. Muno et al. (2005b); Porquet et al. (2005)

14. The Low Luminosities Need Explanation • Based on Lx, the accretion rates are ~5 10-12 M yr-1. • Assuming the orbits shrink through gravitational radiation, the low rates imply donors with M<0.1 M, • The disks have R~1010 cm. • Outbursts occur when the disk has M~1022 g. • The viscous timescale is a few days. • We expect outbursts with Lx~5x1036 erg s-1, much larger than we see. King 2000, King & Wijnands 2005 Time (Days since 1999 Jan 1)

15. An Overabundance of Transients 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

16. 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.

17. 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 gravitational potential.

18. 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.

19. 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

20. 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 1 Gyr, 103 LMXBs could form. schematic by E. Pfahl; Muno et al. (2005)

21. Persistent X-ray Sources Trace the Stellar Distribution Sgr A* 5 pc The number of X-ray sources is consistent with their specific density in the Galactic plane (Sazonov et al. 2006) and in globular clusters with low collision rates (Pooley and Hut 2006).

22. Conclusions • To date, 19 transient X-ray binaries have been detected in the Galactic center. This is ~1% of the Galactic population. • Outbursts from the X-ray binaries in the central 20 pc are fainter than would be expected from disk instability models. • LMXBs in the central parsec of the Galaxy formed through three-body interactions from a population of ~10,000 black holes.