Peeking into the crust of a neutron star

Nathalie Degenaar University of Michigan

X-ray observations Interior propertiesThermal evolution

QuiescenceNo/little accretion

Neutron star visible

Accretion outburstRapid 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/nucleon

Compression nuclear reactions crust heated

Haensel & Zdunik 2008

After accretion stops

10 km

1 km

10 m

cm

~2 MeV/nucleon

Thermal conduction crust cools down

Rutledge+ 2001

Can we detect cooling of a heated crust?

Monitor temperature afterlong accretion outbursts

Crust cooling: observations

XTE J1701-462 1.5 yr, ~1038 erg/s

EXO 0748-676 25 yr, ~1036 erg/s

KS 1731-26012.5 yr, ~1037 erg/s

MXB 1659-292.5 yr, ~5x1036 erg/s

Time since accretion stopped (days)

Neu

tron

sta

r te

mpe

ratu

re (e

V)

Wijnands+01;Cackett+06;Degenaar+10;Fridriksson+11

Crust cooling: what we can learn

Time since accretion stopped (days)

Neu

tron

sta

r te

mpe

ratu

re (e

V)

Magnitude+distributionof heat sources

Nuclear reactions + last outburst

Crust cooling: what we can learn

Time since accretion stopped (days)

Neu

tron

sta

r te

mpe

ratu

re (e

V)

Magnitude+distributionof heat sources

Thermal conductivity

Nuclear reactions + last outburst

Properties of the crust

Crust cooling: what we can learn

Time since accretion stopped (days)

Neu

tron

sta

r te

mpe

ratu

re (e

V)

Magnitude+distributionof 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)

Neu

tron

sta

r te

mpe

ratu

re (e

V)

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)

MAX

I int

ensi

ty (c

ount

s/s/

cm2 )

Globular cluster Terzan 5

Quiescence:Before outburst

Quiescence:After outburst

OutburstIGR J17480-2446

Test case10-week accretion outburst 2010 October-December

11-Hz pulsar: relatively strong magnetic field

Terzan 5

Thermal evolution: crust cooling?

Outburst:2010 Oct – Dec

Degenaar+2011Degenaar+ in prep.

Thermal evolution: crust cooling?

Outburst:2010 Oct – Dec

Cooling curve with standard

heat:no match

Degenaar+2011Degenaar+ 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+2011Degenaar+ in prep.

Thermal evolution: crust cooling!

Quite high: Current models2 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