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Peeking into the crust of a neutron star Nathalie Degenaar University of Michigan

Peeking into the crust of a neutron star Nathalie Degenaar University of Michigan. Neutron stars: heating and cooling provide a window into their dense interior. X-ray observations. Interior properties. Thermal evolution. This talk. Endpoints of stellar evolution Mass: 1.4 Msun

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Peeking into the crust of a neutron star Nathalie Degenaar University of Michigan

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  1. Peeking into the crust of a neutron star Nathalie Degenaar University of Michigan

  2. Neutron stars: heating and cooling provide a window into their dense interior X-ray observations Interior properties Thermal evolution This talk

  3. Endpoints of stellar evolution Mass: 1.4 Msun Radius: ~10 km Extremely dense objects! Neutron stars

  4. Neutron stars are the densest, directly observable objects in the universe Gateway to understand the fundamental behavior of matter Outstanding probes of strong gravity Motivation

  5. Atmosphere: ~cm Crust: ~km Ions, electrons, neutrons Core: ~10 km Protons, electrons, neutrons What we know

  6. Crust: ~km Structure? Gravitational waves Core: ~10 km Exotic particles? Behavior of ultra-dense matter What we want to know

  7. Neutron stars in X-ray binaries

  8. Neutron star accreting matter from a companion Neutron star X-ray binaries

  9. Quiescence: No/little accretion Faint X-ray emission Neutron stars in transient X-ray binaries Accretion outburst: Rapid accretion Bright X-ray emission

  10. X-ray bright • Detectable by many satellites X-rays from Accretion disk Terzan 5 Outburst Quiescence • Duration of weeks-months • Recur every few years-decades Transient outbursts

  11. Terzan 5 • X-ray faint • Detectable by sensitive satellites X-rays from Neutron star Outburst Quiescence Examine the X-ray spectrum Transients in quiescence

  12. Quiescent X-ray spectra X-ray energy spectrum X-ray image

  13. EXO 0748-676 • Components: • Thermal • < 2 keV • Neutron star surface • Atmosphere model • Temperature 1) Thermal emission

  14. 2) Non-thermal emission EXO 0748-676 • Components: • Thermal • < 2 keV • Neutron star surface • Atmosphere model  temperature • 2) Non-thermal • > 2-3 keV • Not understood

  15. Neutron star thermal emission

  16. Accretion induces nuclear reactions in the crust cm 10 m Origin thermal emission 10 km 1 km Image courtesy of Ed Brown

  17. Accretion sets the temperature of the neutron star cm 10 m Origin thermal emission ~1.5 MeV/particle 10 km 1 km Image courtesy of Ed Brown

  18. Surface: Thermal photons • Core: Neutrino emissions Temperature set by heating/cooling balance Gained heat is re-radiated via the surface and core Neutron star cooling

  19. Prior to an accretion outburst Neutron star interior isothermal X-ray emission tracks core temperature

  20. During an accretion outburst Neutron star crust heated Surface not observable X-ray emission dominated by accretion disk

  21. Just after an accretion outburst Neutron star crust hotter than core X-ray emission track crust temperature rather than core

  22. Can we detect cooling of the heated crust?

  23. Good candidates to try Long outbursts  severely heated crust  good targets! Outburst: Monitoring satellites 12.5 yr accretion ended 2001 RXTE ASM count rate (counts/s) 2.5 yr accretion ended 2001 Time since 1996 January 1 (days)

  24. Good candidates to try Long outbursts  severely heated crust  good targets! 12.5 yr accretion ended 2001 RXTE ASM count rate (counts/s) 2.5 yr accretion ended 2001 Quiescence: Sensitive satellites Time since 1996 January 1 (days)

  25. Quiescent monitoring Wijnands+ ‘01, ‘02, ‘03, ‘04 Cackett+‘06, ‘08, ‘10 Neutron star temperature (eV) Time since accretion stopped (days) t ~ 4 yr

  26. Crust cooling! Neutron star temperature (eV) Time since accretion stopped (days) t ~ 4 yr

  27. Crust cooling! Temperature core Neutron star temperature (eV) Time since accretion stopped (days) t ~ 4 yr

  28. Crust cooling! Temperature crust Cooling Temperature core Neutron star temperature (eV) Time since accretion stopped (days) t ~ 4 yr

  29. What have we learned? • Crust cooling is observable! • Cooling timescale requires conductive crust  Crust has a very organized ion structure New challenges: • Conductive crust problem for other observations that require a high crust temperature Is there extra heating in the crust that we missed?

  30. Task for observers: More sources+ more observations

  31. Crust cooling: 2 more sources Better sampling! XTE J1701-462: Active 1.5 yr Quiescent 2007 EXO 0748-676: Active 24-28 yr Quiescent 2008 Neutron star temperature (eV) Time since accretion stopped (days)

  32. Crust cooling: 4 sources Similarities: • Crust cooling observable • Decay requires conductive crust Differences: • Cooling time Neutron star temperature (eV) Time since accretion stopped (days)

  33. Crust cooling: 4 sources Can we explain differences? Observe and model more sources Practical issue: Rare opportunities Neutron star temperature (eV) Time since accretion stopped (days)

  34. Observable for more common neutron stars?

  35. Test case! Outburst IGR J17480-2446 Quiescence: Chandra Quiescence: Chandra Globular cluster Terzan5 MAXI intensity (counts/s/cm2) 10-week accretion outburst 2010 October-December Time since 2009 July 1 (days)

  36. X-ray spectra before and after IGR J17480–2446 Statistics not great (2 photons / hour) But: looks thermal

  37. X-ray spectra before and after IGR J17480–2446 (Outburst: 2010 Oct-Dec) Clear difference before and after 2 months after 4 months after 1 year before Crust cooling?

  38. Thermal evolution: crust cooling? • (Outburst: 2010 Oct-Dec) • Initially enhanced, but decreasing IGR J17480–2446 Neutron star temperature (eV) Time since accretion stopped (days)

  39. Thermal evolution: crust cooling? • (Outburst: 2010 Oct-Dec) • Initially enhanced, but decreasing • Standard heating •  no match! Neutron star temperature (eV) Time since accretion stopped (days)

  40. Thermal evolution: crust cooling! • (Outburst: 2010 Oct-Dec) • Initially enhanced, but decreasing • Standard heating •  no match! • Extra heating •  match! Neutron star temperature (eV) Time since accretion stopped (days)

  41. Thermal evolution: crust cooling • (Outburst: 2010 Oct-Dec) • Initially enhanced, but decreasing • Standard heating •  no match! • Extra heating •  match! Quite high: Current models 2 MeV/nucleon Neutron star temperature (eV) More source available for study! Time since accretion stopped (days)

  42. Work in progress… Initial calculations not `fits’ to the data Observations are ongoing How much heat do we really need? What causes it? Neutron star temperature (eV) Time since accretion stopped (days)

  43. Theoreticians: • Observations of three new sources  match with models, can we explain differences? • What could be the source of the extra heat release?  nuclear experimentalists? Observers: • Continue monitoring current cooling neutron stars • Stay on the watch for new potential targets Work to be done

  44. Neutron stars: • Matter under extreme conditions • Strong gravity probes • Try to understand their interior Neutron stars in X-ray binaries: • Crust temporarily heated during accretion • Crust cooling observable in quiescence • Probe the interior properties of the neutron star To take away

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