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 This talk X-ray observations Interior properties Thermal evolution

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

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

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

7. Neutron stars in X-ray binaries

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

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

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

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

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

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

14. EXO 0748-676 2) Non-thermal -> 2-3 keV -Not understood 2) Non-thermal emission Components: 1)Thermal -< 2 keV -Neutron star surface -Atmosphere model  temperature

15. Neutron star thermal emission

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

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

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

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

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

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

22. Can we detect cooling of the heated crust?

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

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

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

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

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

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

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! 1)XTE J1701-462: Active 1.5 yr Quiescent 2007 2)EXO 0748-676: Active 24-28 yr Quiescent 2008 Time since accretion stopped (days) Neutron star temperature (eV)

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

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

34. Observable for more common neutron stars?

35. 10-week accretion outburst 2010 October-December Time since 2009 July 1 (days) MAXI intensity (counts/s/cm2) Globular cluster Terzan 5 Quiescence: Chandra Quiescence: Chandra Outburst IGR J17480-2446 Test case!

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

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

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

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

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

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

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

43. Theoreticians:  Observations of three new sources  match with models, can we explain differences?  match with models, can we explain differences?  What could be the source of the extra heat release?  nuclear experimentalists?  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