Peeking into the crust of a neutron star

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

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

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

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

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

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

Crust cooling: observations Wijnands+01; Cackett+06; Degenaar+10; Fridriksson+11 Neutron star temperature (e. V) XTE J 1701 -462 1. 5 yr, ~1038 erg/s EXO 0748 -676 25 yr, ~1036 erg/s KS 1731 -260 12. 5 yr, ~1037 erg/s Time since accretion stopped (days) MXB 1659 -29 2. 5 yr, ~5 x 1036 erg/s

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

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

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

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

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

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

Test case 11 -Hz pulsar: relatively strong magnetic field Globular cluster Terzan 5 MAXI intensity (counts/s/cm 2) 10 -week accretion outburst 2010 October-December IGR J 17480 -2446 Quiescence: Before outburst Outburst Quiescence: After outburst Time since 2009 July 1 (days)

Thermal evolution: crust cooling? Terzan 5 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.

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

Thermal evolution: crust cooling! Outburst: 2010 Oct – Dec Quite high: Current models 2 Me. V/nucleon Can be crust cooling, but: substantial heating at shallow depth required 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?

To take away 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