Galloway, Nuclear burning on the surface of accreting neutron stars Nuclear burning on the surface of accreting neutron stars Duncan Galloway University of Melbourne SINS Summer School, Jan ‘07

Galloway, Nuclear burning on the surface of accreting neutron stars Low-mass X-ray binaries LMXBs consist of compact objects (in most cases neutron stars) accreting material from a low-mass stellar companion. Approx. 100 are known within our Galaxy, with orbital periods between 10 min and 16.6 d In some accreting neutron stars the magnetic field is strong enough to channel the flow, producing X-ray pulsations at the neutron star spin LMXBs are thought to be “old” systems where the neutron star magnetic field has decayed and the accreting material can spread evenly over much of the surface

Galloway, Nuclear burning on the surface of accreting neutron stars Discovery of thermonuclear bursts Netherlands/USA satellite X-ray experiment ANS first observed bright X-ray flashes from around the Galactic center and elsewhere in the early ‘70s Clearly originating from discrete sources Two characteristic types identified: –“slow” or type-I bursts, with typical recurrence times of a few hours, from multiple sources –“rapid” or type-II bursts, recurring as fast as every few seconds; but only in one source (the “Rapid Burster”)

Galloway, Nuclear burning on the surface of accreting neutron stars Examples of X-ray bursts from RXTE

Galloway, Nuclear burning on the surface of accreting neutron stars Bursts are interesting because Burst oscillations observed only during bursts trace the neutron-star spin (e.g. Chakrabarty et al. 2003, Nature 424, 42) Eddington-limited bursts allow us to constrain the distance to bursting sources (e.g. Kuulkers et al. 2003, A&A 399, 663) The energy spectrum in the burst tail allows us (in principle) to constrain the blackbody radius (e.g. Özel et al. 2006, Nature 421, 1115) Observation of discrete emission/absorption lines during bursts allow measurement of the surface gravitational redshift (e.g. Cottam et al. 2004, Nature 420, 51) The nuclear processes are quite complex!

Galloway, Nuclear burning on the surface of accreting neutron stars Type-I bursts: nuclear H/He burning Ratio of integrated burst flux (fluence) to integrated persistent X-ray emission (arising from accretion) constrains the burst energetics - the  -value Compactness of the neutron star means that accretion liberates roughly 50% of the rest-mass energy of the accreted material Nuclear burning is much less efficient, at around 1%; expected ratio is then ~50 or more, in agreement with measurements Rapid (type-II) bursts, on the other hand, are thought to be transient accretion events, confused the issue early on, and remain rather poorly understood

Galloway, Nuclear burning on the surface of accreting neutron stars Superbursts: carbon burning? Recently-discovered phenomenon 1000x more energetic than typical thermonuclear bursts (10 42 ergs instead of ) 1000x less frequent (recurrence times of months, instead of hours) 10 4 s Thought to arise from unstable ignition of carbon produced as a by-product of burning during “normal” thermonuclear bursts 4U

Galloway, Nuclear burning on the surface of accreting neutron stars The accreting layer The temperature and density structure of the accreting layer is set primarily by the accretion rate, which we infer from the persistent X-ray flux The accretion rate cannot (much) exceed the Eddington limit, at which point the radiation pressure balances gravitational attraction, preventing further accretion Many LMXBs are transients, that is they undergo outbursts of ~weeks separated by months-years During these outbursts, the X-ray flux (and hence the accretion rate) varies by many orders of magnitude, up to L Edd -> wide range of conditions in the fuel layer

Galloway, Nuclear burning on the surface of accreting neutron stars Nuclear burning processes Accreted fuel is composed of H, He, and metals (primarily C, N, O) He burns rapidly via the 3  reaction, and subsequently via  p processes H burns more slowly by CNO burning initially, and subsequently via rp-process burning At ~1% of the Eddington accretion rate, CNO burning becomes saturated due to the  -decay wait time, and proceeds stably between bursts Heating from “hot” CNO burning leads to earlier burst ignition in this regime; cf. with thermal & compositional “inertia” He-burning is also thought to stabilise at around the Eddington accretion rate (although this is not consistent with observations)

Galloway, Nuclear burning on the surface of accreting neutron stars Endpoint of the rp-process Early studies with limited reaction networks could not identify the end-point of rp-process burning, but it is now thought to terminate in a closed Sn-Sb-Te cycle (Schatz et al. 2001) Modeling suggests burning may not reach this cycle unless the temperature and density become sufficiently high

Galloway, Nuclear burning on the surface of accreting neutron stars Three regimes of ignition 3 cases, in order of increasing accretion rate (e.g. Fujimoto et al. 1981): 3) H-burning is unstable, ignition is from H in mixed H/He fuel; 2) H-burning stable, H is exhausted prior to unstable He-ignition, pure He burst; 1) H is not exhausted prior to He-ignition, mixed burst; Case 1 Case 2 Case 3 ignition curves stable burning accretion rate

Galloway, Nuclear burning on the surface of accreting neutron stars Burst ignition: case 1 Ignition by unstable He burning in a mixed H/He environment First burst in the simulation, and the most intense Subsequent bursts illustrate the “inertia” characteristic of this case low Z 0 ) For 2nd and later bursts, ignition takes place in the layer of ashes produced from the previous burst Lower temperatures and densities reached (simulations from Woosley et al. ‘04)

Galloway, Nuclear burning on the surface of accreting neutron stars Resulting composition Lack of convection after the start of the burst prevents most of the ashes being mixed back up into the burning layer for further burning Don’t reach the endpoint of the rp- process (cf. with Schatz et al. 2001) Very little 12 C - not enough to power a superburst

Galloway, Nuclear burning on the surface of accreting neutron stars Burst ignition: case 2 Lower accretion rate, so that enough time passes between bursts to exhaust H at the base Ignition of He in a pure He-layer, with freshly accreted H on top Burst is much shorter and intense, and convection plays a bigger role

Galloway, Nuclear burning on the surface of accreting neutron stars Lighter products, & more C In case 2 ignition, more carbon is left which may accumulate and subsequently power a superburst Otherwise the ashes are much lighter than in case 1 burning Radial composition gradient results in additional density- driven (“thermohaline”) convection Ashes are thus well-mixed

Galloway, Nuclear burning on the surface of accreting neutron stars Burst ignition: case 3 At lowest accretion rate, unstable ignition of H in a mixed H/He environment; perhaps the least well understood case Short-recurrence time bursts (doublets and even triplets) characteristic of this bursting regime Possibly arising from ignition of unburnt or partially burnt fuel

Galloway, Nuclear burning on the surface of accreting neutron stars The textbook burster, GS This source, discovered in the late 80s by the Ginga satellite, is unique in that it consistently exhibits highly regular bursts Lightcurves are extremely consistent, and recurrence times exhibit very little scatter within an observation epoch We infer “ideal” case 1 burst conditions: steady accretion, complete coverage of fuel, complete burning etc. -> unique opportunity to test theoretical models

Galloway, Nuclear burning on the surface of accreting neutron stars Recurrence time vs. flux In observations spanning several years, the persistent X-ray flux increased by almost a factor of two The burst recurrence time decreased by a similar factor, exactly as expected for low- metallicity ignition models However, the  -values decreased by 10%, indicating a slight change in fuel composition, which requires approximately solar metallicity

Galloway, Nuclear burning on the surface of accreting neutron stars Lightcurve comparison However, using the time-dependent model of Woosley et al reveals that the inconsistency is a result of the effects of thermal- and compositional inertia The bursts are consistent with ignition of H/He in fuel with approximately solar metallicity In addition, we obtained stunning agreement between the observed and predicted lightcurves Except for a “bump” during the burst rise, which may be a propagation effect, or something arising from a particular nuclear reaction

Galloway, Nuclear burning on the surface of accreting neutron stars Propagation of the flame Almost certainly affects the observational properties, but not really clear how Rise time in He bursts is extremely rapid (<1s) so this limits propagation effects (Spitkovsky et al. 2002)

Galloway, Nuclear burning on the surface of accreting neutron stars Observed ignition column Carbon Ignition curve Unanswered questions: C ignition C ignition is a plausible explanation for the superburst properties BUT Models don’t produce enough C to power them Ignition occurs at too low a column A possible explanation is that the crust is not cooling like we expect Such “premature” ignition also occurs in normal thermonuclear bursts

Galloway, Nuclear burning on the surface of accreting neutron stars Early ignition/double bursts Already mentioned early ignition bursts, these appear to occur preferentially in case 3 (unstable H) ignition We don’t understand how these events occur Future modeling might help

Galloway, Nuclear burning on the surface of accreting neutron stars “Late” ignition As the accretion rate increases, we expect that the burst rate also increases, up to the point when He burning stabilises Instead we observe that bursts become less frequent, and exhibit profiles more consistent with He-rich bursts One explanation is that the area over which accretion is occuring is changing in a sufficiently diabolical way to give exactly the opposite trend we want… Alternatively some have proposed yet another phase of bursting, “delayed mixed bursts”, which burn much of their fuel stably prior to ignition (Narayan & Heyl 2003, ApJ 599, 419)

Galloway, Nuclear burning on the surface of accreting neutron stars Onset of stable burning Drop in burst rate well below the Eddington accretion rate is not expected From modelling the onset of steady H-burning is expected at around 0.92 of that level Around the transition models exhibit erratic switching between bursting and oscillatory behaviour “flickering burning” This may be observable as quasi-periodic X-ray oscillations (e.g. Revnitsev et al. 2001)