Peeking into the crust of a neutron star

Nathalie DegenaarUniversity of Michigan

Neutron stars: heating and cooling provide a window into their dense interior

This talk

X-ray observations Interior propertiesThermal evolution

Neutron starsEndpoints of stellar

evolution

Mass: 1.4 MsunRadius: ~10 km

Extremely dense objects!

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

What we know

Atmosphere: ~cm

Crust: ~kmIons, electrons,neutrons

Core: ~10 kmProtons, electrons, neutrons

What we want to know

Crust: ~kmStructure?Gravitational waves

Core: ~10 kmExotic particles?Behavior of ultra-dense matter

Neutron stars in X-ray binaries

Neutron star accreting matter from a companion

X-ray binaries

Neutron star

Neutron stars in transient X-ray

binaries

Quiescence:No/little accretion

Faint X-ray emission

Accretion outburst:Rapid accretion

Bright X-ray emission

X-ray bright Detectable by

many satellites

X-rays fromAccretion disk

Transient outbursts

Outburst

Quiescence

Terzan 5

Duration of weeks-months Recur every few

years-decades

Transients in quiescence

Outburst

Quiescence

Terzan 5 X-ray faint Detectable by

sensitive satellites

X-rays fromNeutron star

Examine the X-ray spectrum

X-ray energy spectrum

Quiescent X-ray spectra

X-ray image

EXO 0748-676 Components:1) Thermal- < 2 keV - Neutron star

surface- Atmosphere

model

Temperature

1) Thermal emission

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

Neutron star thermal emission

Origin thermal emissionAccretion induces nuclear reactions in the crust

1 km

10 m

cm

10 km

Image courtesy of Ed Brown

Origin thermal emissionAccretion sets the temperature of the neutron

star

1 km

10 m

cm

10 km

~1.5 MeV/par

ticle

Image courtesy of Ed Brown

Neutron star coolingGained heat is re-radiated via the surface and core

Surface: Thermal photons

Core: Neutrino emissions

Temperature set by heating/cooling

balance

Neutron star interior isothermal

X-ray emission tracks core temperature

Prior to an accretion outburst

Neutron star crust heated

Surface not observableX-ray emission dominated

by accretion disk

During an accretion outburst

Neutron star crust hotter than core

X-ray emission track crust temperature rather than

core

Just after an accretion outburst

Can we detect cooling of the heated crust?

Time since 1996 January 1 (days)

RX

TE

AS

M c

ou

nt

rate

(co

un

ts/s

)

Good candidates to try

12.5 yr accretion

ended 2001

2.5 yr accretionended 2001

Long outbursts severely heated crust good targets!

Outburst:Monitoring satellites

Time since 1996 January 1 (days)

RX

TE

AS

M c

ou

nt

rate

(co

un

ts/s

)

Good candidates to try

12.5 yr accretion

ended 2001

2.5 yr accretionended 2001

Long outbursts severely heated crust good targets!

Quiescence:Sensitive satellites

Neu

tron

sta

r te

mp

era

ture

(eV

)

Time since accretion stopped (days)

t ~ 4 yr

Wijnands+ ‘01, ‘02, ‘03, ‘04 Cackett+ ‘06, ‘08, ‘10

Quiescent monitoring

Neu

tron

sta

r te

mp

era

ture

(eV

)

Time since accretion stopped (days)

t ~ 4 yr

Crust cooling!

Neu

tron

sta

r te

mp

era

ture

(eV

)

Time since accretion stopped (days)

t ~ 4 yr

Crust cooling!

Temperaturecore

Neu

tron

sta

r te

mp

era

ture

(eV

)

Time since accretion stopped (days)

t ~ 4 yr

Temperature crustCooling

Crust cooling!

Temperaturecore

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

observationsthat require a high crust temperature

Is there extra heating in the crust that we missed?

Task for observers:

More sources +more observations

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)

Neu

tron

sta

r te

mp

era

ture

(eV

)

Crust cooling: 4 sources

Time since accretion stopped (days)

Neu

tron

sta

r te

mp

era

ture

(eV

)

Similarities: Crust cooling

observable Decay requires

conductive crust

Differences: Cooling time

Can we explain differences?

Observe and model more

sources

Practical issue: Rare opportunities

Crust cooling: 4 sources

Time since accretion stopped (days)

Neu

tron

sta

r te

mp

era

ture

(eV

)

Observable for more common neutron

stars?

10-week accretion outburst2010 October-December

Time since 2009 July 1 (days)

MA

XI

inte

nsi

ty (

cou

nts

/s/c

m2

)

Globular cluster Terzan 5

Quiescence:Chandra

Quiescence:Chandra

OutburstIGR J17480-2446

Test case!

Statistics not great (2 photons / hour)

But: looks thermal

IGR J17480–2446

X-ray spectra before and after

(Outburst: 2010 Oct-Dec)

Clear difference before and after

2 months after4 months after1 year before

IGR J17480–2446

X-ray spectra before and after

Crust cooling?

Neu

tron

sta

r te

mp

era

ture

(eV

)

Time since accretion stopped (days)

(Outburst: 2010 Oct-Dec)

- Initially enhanced, but decreasing

IGR J17480–2446

Thermal evolution: crust cooling?

Neu

tron

sta

r te

mp

era

ture

(eV

)

Time since accretion stopped (days)

(Outburst: 2010 Oct-Dec)

- Initially enhanced, but decreasing

- Standard heating no match!

Thermal evolution: crust cooling?

Neu

tron

sta

r te

mp

era

ture

(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!

Neu

tron

sta

r te

mp

era

ture

(eV

)

Time since accretion stopped (days)

Quite high: Current models2 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!

Neu

tron

sta

r te

mp

era

ture

(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…

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

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