Peeking into the crust of a neutron star Nathalie DegenaarUniversity of Michigan X-ray observations Interior properties Thermal evolution

Quiescence No/little accretion Neutron star visible Accretion outburst Rapid accretion Bright X-rays from disk Transient X-ray binaries

The neutron star crust 10 km Image courtesy of Ed Brown Composition: Atomic nuclei electrons, neutrons Structure? Thermal properties

The effect of accretion 10 km 1 km 10 m cm ~2 MeV/nucl eon Compression  nuclear reactions  crust heated Haensel & Zdunik 2008

After accretion stops 10 km 1 km 10 m cm ~2 MeV/nucl eon Thermal conduction  crust cools down Rutledge+ 2001

Can we detect cooling of a heated crust? Monitor temperature after long accretion outbursts

Crust cooling: observations XTE J yr, ~10 38 erg/s EXO yr, ~10 36 erg/s KS yr, ~10 37 erg/s MXB yr, ~5x10 36 erg/s Time since accretion stopped (days) Neutron star temperature (eV) Wijnands+01;Cackett+06;Degenaar+10;Fridriksson+11

Crust cooling: what we can learn Time since accretion stopped (days) Neutron star temperature (eV) Magnitude+distribution of heat sources Nuclear reactions + last outburst

Crust cooling: what we can learn Time since accretion stopped (days) Neutron star temperature (eV) Magnitude+distribution of heat sources Thermal conductivity Nuclear reactions + last outburst Properties of the crust

Crust cooling: what we can learn Time since accretion stopped (days) Neutron star temperature (eV) Magnitude+distribution of heat sources Thermal conductivity Core temperature Nuclear reactions + last outburst Properties of the crust Composition of the core

Crust cooling: modeling High thermal conductivity  Organized ion lattice Brown&Cumming’09 Different structure

Crust cooling: 4 sources Time since accretion stopped (days) Neutron star temperature (eV) Can we build a census of crust? Observe + model more sources Practical issue: Rare opportunity

Can we observe this for “normal” transients with shorter outbursts?

Time since 2009 July 1 (days) MAXI intensity (counts/s/cm 2 ) Globular cluster Terzan 5 Quiescence: Before outburst Quiescence: After outburst Outburst IGR J Test case 10-week accretion outburst 2010 October-December 11-Hz pulsar: relatively strong magnetic field

Terzan 5 Thermal evolution: crust cooling? Outburst: 2010 Oct – Dec Degenaar+2011 Degenaar+ in prep.

Thermal evolution: crust cooling? Outburst: 2010 Oct – Dec Cooling curve with standard heat: no match Degenaar+2011 Degenaar+ in prep.

Outburst: 2010 Oct – Dec Cooling curve with standard heat: no match Cooling curve with extra heat: much better! Thermal evolution: crust cooling? Degenaar+2011 Degenaar+ in prep.

Thermal evolution: crust cooling! Quite high: Current models 2 MeV/nucleon Can be crust cooling, but: substantial heating at shallow depth required Outburst: 2010 Oct – Dec Cooling curve with standard heat: no match Cooling curve with extra heat: much better! Degenaar+2011 Degenaar+ in prep.

Work in progress… Cooling is ongoing Continue observing Model full curve How much heat? Is it realistic?

Crust cooling observable after short outburst More source available for study Heating at shallow depth required Could be large, what can it be? Nuclear reactions, magnetic field, other?

Neutron stars in transient X-ray binaries:  Crust temporarily heated during accretion  Crust cooling observable in quiescence  Gives insight into crust properties Latest results:  Crust cooling after short accretion outbursts  Additional heating in outer layers of the crust To take away