Simulating multiple X-ray bursts with the aim for superbursts

Friedrich-Karl ThielemannDepartment of Physics

University of BaselSwitzerland

Sofie Fehlmann, Julia Reichert, Ruben Cabezon and

Explosions caused by accretion in binary stellar systems

binary systems with accretion onto one compact object can lead (depending on accretion rate) to explosive events with thermonuclear runaway (under electron-degenerate conditions)- white dwarfs (novae, type Ia supernovae)- neutron stars (type I X-ray bursts, superbursts?)

Explosive H/He-burning and the onset of the rp-process

at temperatures around 1-2 108 K only hot CNO-type cycles exist

except for 19Ne

increasing temperatures up to 3-4 108 K break up cycles via proton captures

bridging by alpha reactions beyond 6 108 K

Fisker et al. 2005, Cooper & Narayan 2006, Tan et al. 2007, Heger et al. 2007, Fisker et al. 2007, 2008, Parikh et al. 2008, Galaviz et al. 2010, Cybert et al. 2010, José et al. 2010, Haettner et al. 2011, Tu et al. 2011 ..................................

Woosley & Taam 1976,Joss 1977, 1978,Taam 1980, Wallace & Woosley 81,Fujimoto et al. 1981,Ayasli & Joss 1982,Woosley & Waever 84,Wiescher et al. 86,Fushiki & Lamb 1987,van Wormer et al. 94,Lewin et al. 1995,Rembges et al. 97,Bildsten 1998, 2000Schatz et al. 98, 01,Wiescher et al. 1999,Brown et al. 2002,Cumming 2003,Narayan & Heyl 2003,Strohmayer&Bildsten 2003,Woosley et al. 2003,.

.

X-ray burst observations

(1735-444)

15 s

97-98

2000

typical X-ray burst not identical, and slow but continuous evolution in burst features, physics behind this ?

years

NASA RXTE: GS 1826-24 burst shape changes ! (Galloway 2003 astro/ph 0308122)

From type I X-ray bursts to superbursts

Energy released is about 1000 times higher than in regular X-ray bursts!When observed in bursting systems, period between superbursts is

characterized by about 1000 regular bursts.

Surface of an accreting neutron star

Neutron star surface

ocean

Innercrust

outercrust

H,He

gas

ashes

ashes

~ 25 – 70 m ρ=109-13 g/cm3

~ 4m, ρ=106 g/cm3 X-ray burst

graph from H. Schatz

according to P. Haensel

approx. 10-12Msol

at ignition, with an accretion rate of 1017g/s this corresponds to a repetition rate of about 4 hours

Site of ?

X-ray bursts

Superbursts

URCA Cooling(Schatz et al. 2014)

Crustal heatingKaminker+ 2014Degenaar+ 2015Waterhouse+ 2016<1.4-3.2MeV/nucleon

Break-out Reactions and the Stability of Burning

Change in stable burst conditions due to new 15O reaction cross section (Fisker et al. 2007)‏.Continuous improvements in reactions (e.g. Parikh et al 2009, Cybert et al. .and test of metallcity effects (e.g ‏(2010Jose et al. 2012, Parikh et al. 2013)‏

Determines also the critical accretion rate between constant and unstable (bursts) burning

bursts

constant burning

Tan et al. (2007)

Explosive H/He-burning and rp-process in realistic hydro models for X-ray Bursts

energy generation

burning cycles in adjacent

layers

⇧⇧⇧Ejection?

Accretion Accretion

Compression

Fisker, Schatz, FKT (2008)‏ - AGILE+Heger, Woosley, Keek .. (2013)‏ – KEPLER

Fehlmann et al. (2015)‏ - AGILE+

also MESA

How do we get to superburts?

Following ideas by Cumming & Bildsten 2001, Strohmayer & Brown 2002,and Weinberg & Bildsten 2007, superbursts can occur if 12C is ignited in the deep ashes of former X-ray bursts

This requires a minimum massfraction of 12C in these ashes(Cumming et al., 2006, Hashimoto et al., 2014)

The burst ashes must containa 12C mass fraction >0.1!!

Simulation code by Sofie Fehlmann (2015)

based onGeneral relativistic hydrodynamics: AGILE (M. Liebendörfer, 2002)Nuclear Network (W. R. Hix & F.-K. Thielemann, 1999, with Cyburt et al. 2010, 2015 rates)Convection, Heat transport (Conduction and Radiation) as in Fisker, Schatz, Thielemann (2008) with updated opacities from Schatz et al. (2013) with standard conditions: Neutron star: R = 11.2km, M = 1.41Msol EOS by K. Nomoto in computational domain of 129 adaptive grid zones above neutron star crust (of the order 5 1022g and 10m height, with logarithmic mass distribution) accreted matter with solar composition constant accretion rate: dM/dt=1.88 1017 g s−1 =0.1edd (high rate) or 4.72 1016 g s−1 =0.025 edd (low rate) fixed crustal heat source Qheat = initially 0.5MeV/nuc

averaged values for 50 bursts

Reduced and extended networks

Networks utilized in simulations

Burn rate and temperature as function of column density and time

Hot CNO-cycle andand break-out duringthe burst

Mass fraction before ignition,at maximum luminosity, andafter the burst

The composition as a function of density, i.e. position, between bursts in burned/unburned

layers

Variations in crustal heating(Shternin et al. 2011, Schatz et al. 2013, Turlione et al. 2013

showed that crustal heating is required)

12C in ashes dependent on crustal heating

Dependence of Recurrence Time Scales on Crustal Heating

Duration of bursts, peak and quiescence luminosity

Crustal heating changes the duration of bursts but less the maximum luminosity

Composition after bursts dependent on crustal heating

Increased 12C fraction and smaller amount of heavy nuclei!

Leftover 12C after bursts as a function of crustal heating

In order to have a 12C mass fraction >0.1 left after bursts, crustal heating of 1MeV/nuc would be required!! permitted by observations up to 1.4(Degenaar et al. 2015) or even 3.2 (Waterhouse et al. 2016)

Crustal Heating reduces the recurrence time of X-ray bursts,Which other parameters can counterbalance this effect?

Change in accretion rates

How does this effect light curve shapes?

On the way to superbursts triggered at a column density of about 1012 g cm−2 (Hashimoto et al. 2014, Keek and Heger 2011, Zamfir et al. 2014).

Fast accretion rate 2 1017 g/s

Standard accretion rate 1017 g/s

With a larger computational grid of 218 radial zones and a more optimized code - presently utilizing one month for one (physical) year of X-ray burst simulations

- We updated and optimized the former code of J. Fisker whichenables us to study self-consistent ignition of a superburstwithin reasonable time.

- The survival of carbon after the X-ray burst is enhanced inmodels with high crustal heating (Qheat > 1.0 MeV/nuc) buttoo high crustal heating leads to stable burning during the accretion and inhibits the ignition of X-ray bursts.

- Simulations of X-ray bursts with additional crustal heatingand an accretion rate of 1017 g/s do produce sufficient 12C in order to fulfill the conditions for superburst ignition.

Further simulations are running to produce superbursts with the help of Julia Reichert and Ruben Cabezon.

Projects (within the ISSI Team: Cummings, Heger, Keek, Brown, Galloway, Degenaar, Schatz, Reichert/Thielemann)

These are the objectives of the project identified during the first meeting.Please ensure to list the person (or people) responsible for carrying out (or at least reporting) on each task.1. Burst observations

- Assemble the sample of reference bursts for model comparisons; currently available via a bitbucket repository (contact Duncan for access). Suggestion to augment with examples of a super burst and intermediate duration burst. (Could also/alternatively create a wiki page for each "standard" burster, with details/refs about the sources, observed lightcurves, model results etc.?) [DG]- Explore analysis possibilities for bursters that are not regular. Want to try to reconstruct the burst recurrence time distribution despite data gaps, and/or correlate against spectral properties [DG & JC]- Complete MINBAR web interface to include burst light curves & share with team [DG]- Analysis of hard-to-soft transitions in systems other than 1826-24; does this produce a reliable change in the burst properties, and after what (if any) delay? This might have a bearing on the timescale for the heating process "switched on" by the change in accretion geometry [DG & LK]- NICER burst observations if we can manage in time [HS & LK]

2. Model runs

- Running Kepler with finer zoning to see what is going on with the burst-to-burst variations and strange burst shapes in the models that Andrew showed for GS 1826. This may be solved now with the revelation that Michael's light curves are from the outermost zone, which is subject to numerical issues [AH]- Incorporate the Kepler runs contributing to Hendrik's sensitivity studies into the tables of burst results [HS+DG]- Incorporate the Kepler runs contributing to Laurens' study of Mdot vs. Qb to the burst results table [LK+DG]- Write Python "generator" to simplify the process of running Kepler. Perhaps also a Python interface, making Kepler publicly available? :) [AH]- Run Kepler code on test cases and share light curves [AH]- Run Kepler code with suggested low-metallicity, enhanced-He composition for GS 1826-24 [AH+AC]- Run AGILE code on test cases and share light curves [JR]- Run AGILE code with a range of high Qb values to try to confirm irregular bursting [JR]- Build table of AGILE code results to date (similar to what's been done with Kepler) and publish/share with the team [JR]- Share standard cases with other code keepers, and encourage tests [DG &c]- Code convergence/sensitivity studies for standard bursters [HS &c]- Survey of mass uncertainty influence on burst light curves, following the approach of Cyburt et al. (2010) [HS+?]- Include predictions of equivalent widths in PRE bursts into nuclear sensitivity study (focus on composition when convection zone is maximal and that produces largest EWs) - focus on 4U1722-30 and 4U1820-30[HS]- Model expected absorption features in preparation of future observations [DG]

3. Ash composition and influence on the crust

- Survey the possible ashes from different types of burning: stable and unstable, rp or pure helium, different base fluxes, superburst ashes and intermediate duration burst ashes (for standard cases composition should be part of repository)- Add KEPLER final composition as a function of depth to KEPLER run database (use Richard's scripts?) [AH,HS]- Create a repository of predictions for crust compositions from various models (KEPLER, AGILE, steady state, one-zone) [AH,HS] - Full crust composition models and how the cooling curves are affected by the composition [EB,HS]

4. Model-observation comparisons

- Complete Cumming et al. paper on peak flux variation in 1826-24 [AC+DG+AH]- Develop code to do robust model-observation comparisons, incorporating redshift etc. - Incorporate this into a global likelihood calculation also incorporating recurrence times and perhaps even quiescent light curve models/observations [DG & AH]- Develop/share code to predict quiescent decay curves and Qb values for incorporation into model-observation comparison code [EB]

5. Irregular bursts

- Further work mapping out stable burning regime and sensitivity to nuclear physics [LK, JR & AH] - Investigate change in Qb at a constant accretion rate (Laurens may have some models of this already?)- Compare to irregular bursts in GS 1826-24 (and other sources?)

6. Nuclear Physics Work

Series of experiments targeted at key uncertainties identified (though JINA-CEE) Planned Experiments: Mass measurements: 65As, maybe 66Se, 24Si (few failed attempts, keep trying) 23Al(p,g) measurement at NSCL various (a,p) rate measurements at NSCL, ANL Ideas: 57Zn beta delayed proton emission Masses: 56Cu Update REACLIB [HS] Shell model calculations beyond fp-shell for weak interactions [YS]

Other things we've talked about

- Firestarter superburst [JC] - Opacities of mixtures [EB+AC] - Intermediate duration burst nuclear physics (Laurens has run some) - Three timescale bursts in GS 1826 seen in KEPLER - analyze nuclear origin of this - and then look for it observationally, need KEPLER run [HS,AH] - Predict crust composition for transients based on X-ray burst models ? Is this possible? - Different burning regimes and dependence on 15O(a,g) rate