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An Overabundance of X-ray Binaries in the Central Parsec of the Galaxy

An Overabundance of X-ray Binaries in the Central Parsec of the Galaxy. Michael Muno (UCLA) with Fred Baganoff (MIT), Eric Pfahl (UVa), Mark Morris, Andrea Ghez, Jessica Lu (UCLA), Geoff Bower (Berkeley), Farhad Yusef-Zadeh, and Doug Roberts (Northestern). We Study the Galactic Center.

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An Overabundance of X-ray Binaries in the Central Parsec of the Galaxy

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  1. An Overabundance of X-ray Binaries in the Central Parsec of the Galaxy Michael Muno (UCLA) with Fred Baganoff (MIT), Eric Pfahl (UVa), Mark Morris, Andrea Ghez, Jessica Lu (UCLA), Geoff Bower (Berkeley), Farhad Yusef-Zadeh, and Doug Roberts (Northestern)

  2. We Study the Galactic Center • To search for rare objects, such as accreting black holes and neutron stars. • How does accretion occur at low rates? • To understand how the central 4 million solar-mass black hole grows. • Does stellar dynamics feed stars into the central black hole? • How did dozens of young, massive stars come to lie within 1 pc of the central black hole?

  3. Sgr A* 1 light-year Black Holes Settle into the Galactic Center • In 1993, M. Morris (UCLA) predicted that massive objects -- such as black holes -- should settle toward the super-massive black hole. • 1% of the mass in the central light-years could be black holes. (see also Miralda-Escudé & Gould 2000) Infrared laser guide-star image courtesy W.M. Keck Observatory.

  4. Black Holes are Found byTheir X-ray Emission Image creates with binsim, by Rob Hynes Location of Black Hole One Solar Radius Black holes can be identified when they are accreting matter, often from a companion star. The matter falling into the black hole produces X-rays.

  5. Searching for Stellar-Mass Black Holes • Chandra images of the central 25 pc of the Galaxy reveal 2000 X-ray sources. • The X-ray sources are white dwarfs, neutron stars, and black holes in binary star systems. Sgr A* 5 pc

  6. Searching for Stellar-Mass Black Holes • We identified those X-ray sources most likely to be black holes and neutron stars by looking for sources that produce sudden, bright outbursts. • We found 7 such transients within 75 light-years of the Galactic center. New Transient Sgr A* 10 light-years

  7. Sgr A* 1light-year An Overabundance of X-ray Binaries in the Central 3 Light-years • 4 of 7 transients are within 3 light-years of the Galactic Center. • The chance of this happening randomly is less than 1 in 5000. • The best explanation for the overabundance is it results from a concentration of black holes near Sgr A*.

  8. A Low-Mass X-ray Binary One transient has an 8 hour orbital period and no infrared companion with K<15.

  9. X-ray Binaries Form When Black Holes Capture Stars • In ten billion years, 1 in 100 black holes will encounter a binary star system, and steal one of the stars. • A fraction of the resulting black-hole binaries will become X-ray sources. • These X-ray binaries would be concentrated near Sgr A*, as we see. simulation by E. Pfahl (Uva)

  10. 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.

  11. Summary • Chandra observations reveal that transient X-ray binaries are overabundant in the central 3 light-years of the Galaxy (compared to the number between 3 and 75 light-years). • These transients are most likely accreting black holes or neutron stars. • The overabundance can be explained if black holes and neutron stars have settled into the central light-years over the last 10 billion years.

  12. Future Work: Infrared Observations • Measure the ages of the mass-donor stars, to confirm they are billions of years old. • Over the 10 years, A. Ghez will determine the number of black holes at the Galactic Center by observing the orbits of individual stars around Sgr A*.

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