In this talk Deep crustal heating on accreting neutron stars The fate of accreted matter, and deep crustal heating. NEW: heating is sensitive to composition Superbursts Probes of the neutron star interior Ignition mass set by heating in the crust NEW: crust heating sets the critical ignition mass Edward Brown Credit: NASA Supported by the National Science Foundation, AST and PHY (JINA)

Deep crustal heating

cf. talk by Hendrik Schatz Schatz et al. 2001, PRL Woosley et al. 2004, ApJ

Deep crustal heating

Coupled thermal structure code with reaction network Include lg(ft) for excited states (Möller) Analytical approximation to phase space integration (Gupta) Starts from distribution of rp-process nuclei (Schatz et al., PRL) Gupta, Brown, Schatz, Möller, & Kratz (2006) See poster 267, A. Becerril et al.

Coupled thermal structure code with reaction network Include lg(ft) for excited states (Möller) Analytical approximation to phase space integration (Gupta) Starts from distribution of rp-process nuclei (Schatz et al., PRL) Gupta, Brown, Schatz, Möller, & Kratz (2006)

Coupled thermal structure code with reaction network Include lg(ft) for excited states (Möller) Analytical approximation to phase space integration (Gupta) Starts from distribution of rp-process nuclei (Schatz et al., PRL) Gupta, Brown, Schatz, Möller, & Kratz (2006)

Coupled thermal structure code with reaction network Include lg(ft) for excited states (Möller) Analytical approximation to phase space integration (Gupta) Starts from distribution of rp-process nuclei (Schatz et al., PRL) Gupta, Brown, Schatz, Möller, & Kratz (2006) from Haensel & Zdunik 2003

Superbursts First detected in 2000 (Cornelisse et al. 2000) 13 detected from 10 sources Duration: 2–12 hrs (Kuulkers et al. 2002) Energetics: –10 42 ergs Likely cause: unstable ignition of 12 C (Cumming & Bildsten 2001, Strohmayer & Brown 2002, Cooper & Narayan 2005) time (hrs) from in’t Zand (2004)

Determination of ignition mass (plots from Cumming et al. 2006)

=P/g=P/g Superbursts probe the deep NS interior calculations confirmed by Cooper & Narayan 2005, Cumming et al mUrca dUrca factor of 20 increase in ignition depth from Brown (2004) Requires low thermal conductivity and (unrealistically?) weak neutrino emissivity

With 1 S 0 Cooper pairing in crust mUrca×10 –3 mUrca mUrca×10 3 cf. Cumming et al. 2006

Detection of crust thermal relaxation Cackett et al KS 1731–260 Lightcurve courtesy of ASM team KS also had a superburst! For sources with long accretion outbursts, crust is not in thermal equilibrium with core Can observe the crust cooling! Consistent with Tcore ≈ 0.1 GK

Buildup of He layer at low dM/dt Either pure He accretion (Cumming et al. 2006) or repeated weak H flashes (Peng et al. 2006) can build up amassive He layer Ignition depth sensitive to heat flux from reactions in crust from Peng, Brown, & Truran (2006) 1.0 MeV/u 0.2 MeV/u 0.1 MeV/u

Inferred core temperature Range of core temperatures depending on envelope composition 4 He rp-process; A ≈ 100

A = 104 A = Exc. energy (MeV)

HZ03 HZ90 New estimates: heating in outer crust Gupta, Brown, Schatz, Möller, & Kratz (2006)

Effect of composition on ignition depth Gupta, Brown, Schatz, Möller, & Kratz (2006)

Summary Deep crustal heating of accreting neutron stars Sets ignition column of unstable thermonuclear flashes Superbursts He flashes at low dM/dt Determines lightcurve of transient accretors (now observed!) Heating in the outer crust Depends on nuclides made during rp-process and previous superbursts Might resolve problem of shallow 12 C (superburst) ignition Open question: what happens at neutron drip?

Equations Hydrostatic balance Area correction Thermal evolution Luminosity Continuity Metric

Thermal structure Simplified core model Analytical EOS (Tolman 1939) Both mUrca, dUrca in core with arbitrary n t (Yakovlev, Levinfish, & Haensel 2003) Equation of state relativistic, degenerate electrons Coulomb interactions degenerate neutrons Thermal conductivity phonon (Yakovlev & Urpin 1980) impurity (Itoh & Kohyama 1992) amorphous crust (Cf. estimate by Jones 2004)—estimate by setting structure factor to unity

1 S 0 Critical temperatures for n-matter Compilation from Page et al. 2004

Effect of Ocean Composition Brown 2004 L c = 1 (MeV/m u ) (dM/dt)

An Amorphous Crust Crust unlikely to be a pure lattice Different phases of nuclear matter may coexist in inner crust (Magierski & Heenan 2002) Fluctuations in composition during cooling from birth (Jones 2004) Distribution of isotopes from burning of H, He Estimate relaxation time by setting structure factor to unity (as for a liquid) Cf. estimate of Jones (2004, PRL) Neglects phonon transport: May be important in the inner crust