1. 1 The faintest accretors A.R. King & R. Wijnands astro-ph/0511486

2. 2 1. Introduction Discovery of very faint X-ray transients (VFXTs) (Sakano et al. 2005, Muno et al. 2005, Hands et al. 2004) Nature: NS/BH Peak: 10^34-10^36, quiescent: 10^31-10^33 Mean: <=10^33, correspond to <=10^-13Ms/yr A brighter transient SAX J1808.4-3658 Porb=2.01hr, X-ray pulsation=401Hz, distant=2.5kpc degenerate low-mass donor <=0.1Ms King 2000, Bildsten & Chakrabarty 2001

3. 3 2. Models for very faint X-ray transients Heterogeneous! Be X-ray binaries? No optical/IR counterparts detected Several have shown type I X-ray bursts – NS LMXB LMXB: the reason to abandon the orthodox model 1.Unfavorable view angles? Deficiency of brighter sources 2.Deviation from the mean transfer rate? (H/R_2)t_M =10^9m_2 yr << lifetime of LMXB Deficiency of brighter sources 3.Accreting the wind of low-mass stars? Deficiency of brighter sources 4.Strong absorption? Unsuitable for LMXBs

4. 4 3. Accretion from very low-mass companions For a simple picture the companion is modeled as an n=1.5 polytrope

5. 5 Paczynski 1971

6. 6

7. 7 Normal primary masses (m1<10Ms) hydrogen-rich companions are ruled out. Even … Timescale is too long! Then: 4. Discussion

8. 8 (a) Hydrogen-poor companions X=0, t_10=1.37 and m1>5 VFXTs should be extremely old BH LMXBs with hydrogen-poor companions. Weakness: Not significantly lower than 10^-13Ms/yr Not distinct from other accreting systems

9. 9 (b) Low initial companion masses HETE J1900.1-2455 (Kaaret et al. 2005), 0.016Ms Caveats: SN disruption Common envelope ejection Roche lobe filling Pulsar B1257+12, two earth mass planets Wolszczan & Frail 1992 Fallback disk origin?

10. 10 (c) Larger mass accretor IMBHs: Collision runaway in dense clusters, Portegies Zwart et al. 2004 Capture of the nuclei of small satellite galaxies,King & Dehnen 2005 Primordial (Population III), Z<10^-4, Madau & Rees 2001 Galactic center welcomes black hole immigrants, Portegies Zwart et al. astro-ph/0511397

11. 11 Observing programmes to monitor the VFXTs in Galactic center region are continuing… The fainter the transients in quiescence are the tighter constrains can we put on the physical model. Thanks for your patience!