The complex and puzzling phenomenology of thermonuclear X-ray bursts

1. The complex and puzzling phenomenology of thermonuclear X-ray bursts Duncan Galloway Monash University Andrew Cumming McGill Jean in ‘t Zand SRON Deepto Chakrabarty & Jake Hartman MIT Mike Muno Caltech Dimitrios Psaltis Arizona APS April 2008, St. Louis Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

2. Motivation • Thermonuclear bursts are a key observational phenomenon which uniquely allow us to derive information about the neutron stars upon which they occur • Burst oscillations -> NS spin • Peak flux of radius-expansion bursts -> distance • Spectrum in the burst tail -> NS radius… • … and hence (in principle) constrain the neutron star EOS Of course, there are also the details of the thermonuclear burning, how it spreads over the star, the balance between stability and instability… Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

3. Burst theory: 3 ignition regimes 3 cases, in order of increasing accretion rate (e.g. Fujimoto et al. 1981): stable burning 3) H-burning is unstable, ignition is from H in mixed H/He fuel; accretion rate Case 1 Case 2 2) H-burning stable, H is exhausted prior to unstable He-ignition, pure He burst; Case 3 ignition curves 1) H is not exhausted prior to He-ignition, mixed burst; Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

4. In general, the behavior of bursts from individual sources do NOT match our expectations from theory Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

5. The exceptions are the rule • Bursts tend to decrease in frequency at higher accretion rates, rather than increasing, as expected theoretically • Such bursts are often much fainter than might be expected (given the wait time and inferred accretion rate), suggesting an “energy leak” • A (possibly) related issue is that “normal” thermonuclear bursts don’t produce enough carbon to power “super” bursts • We also see bursts with inexplicably short recurrence times, down to a few minutes, sometimes in groups of three or four Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

6. Low-mass X-ray binaries • compact objects accreting material from a low-mass stellar companion • LMXBs are thought to be “old” systems with weak magnetic fields allowing the accreting material to fall directly onto the surface • Approx. 100 are known within our Galaxy, with orbital periods between 10 min and 16.6 d • Unstable nuclear burning of accumulated fuel on the neutron star surface leads to thermonuclear (type-I) bursts • The Rossi X-ray Timing Explorer (RXTE) has observed more than 1200 bursts from 40 of these systems over it’s 10-year mission lifetime, with high time resolution and signal-to-noise Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

7. Important scales • Neutron star mass ≈ 1.4 M, & radius ≈ 10 km • Distance to Galactic LMXB systems ≈ 8 kpc • Typical persistent intensity 10-9 erg cm-2 s-1 • … corresponding to an accretion rate of ≈10% of the Eddington rate (8.8104 g cm-2 s-1 or 1.310-8M yr-1) • Typical burst peak intensity 10-7 erg cm-2 s-1 • Characteristic burst fluence (integrated flux) of 1039 erg • Ratio  of integrated persistent flux to burst flux is 40 (i.e. accretion is much more efficient than thermonuclear burning!) Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

8. Examples of X-ray bursts from RXTE Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

9. Superbursts: carbon burning? • 1000x more energetic than typical thermonuclear bursts (1042 erg) • 1000x less frequent (recurrence times of months, instead of hours) 4U 1636-536 104 s • Thought to arise from unstable ignition of carbon produced as a by-product of burning during “normal” thermonuclear bursts Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

10. A well-behaved burster: GS 1826-24 • 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” burst conditions: steady accretion, complete coverage of fuel, complete burning etc. -> unique opportunity to test theoretical models 1997-8 2000 Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

11. … aka the textbook burster • In RXTE 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 constant accreted mass at ignition • A ~10% change in  over this time suggests solar fuel composition (Galloway et al. 2004, ApJ 601, 466; see also Thompson et al. 2007, ApJ accepted, arXiv:0712.3874) 1997-8 2000 Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

12. Data & model comparisons • We also compared lightcurves with predictions by the time-dependent model of Woosley et al. 2004 • Confirms that the bursts occur via ignition of H/He in fuel with approximately solar metallicity (i.e. CNO mass fraction) • In addition, we obtained stunning agreement between the observed and predicted lightcurves (this is not a fit!) • Except for a “bump” during the burst rise, which may be an artifact of the finite time for the burning to spread, or something arising from a particular nuclear reaction (Heger et al. 2007, ApJL 671, L141) Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

13. Simpler models are sometimes OK • The time-independent models are still OK where steady H-burning is not the dominant heating process • For example, at low accretion rates where the recurrence time is long enough to exhaust all the hydrogen prior to ignition • For the bursts observed during the 2002 outburst of the millisecond pulsar SAX J1808.4-3658, we obtained excellent agreement between model predictions and observations • Can derive the distance of 3.4-3.6 kpc (Galloway et al. 2006, ApJ 652, 559) Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

14. What about all the other burst sources? Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

15. No shortage of data Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

16. 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 (Boirin et al. 2007, A&A 465, 559) Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

17. Studies of large burst samples • Long-duration X-ray satellite missions have accumulated large samples of bursts from many sources • Analysis can identify global trends, as well as revealing important exceptions (e.g. Cornelisse et al. 2003, A&A 405, 1033) H-ignition He-ignition H-rich fuel • A comparable sample of almost 1200 bursts has been assembled from observations by the Rossi X-ray Timing Explorer(Galloway et al. 2007, astro-ph/0608259) • Although RXTE has a relatively narrow field of view, the large effective area allows us to make precision measurements of burst properties and the persistent emission He-ignition H-poor fuel Note that these are carefully-selected subsamples! Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

18. 1 3 2 This is about where we observe mHz oscillations; transition to stable He-burning? (Heger et al. 2007, ApJ 665, 1311) X-ray colors  accretion rate? Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

19. Diverse bursts at moderate M-dot The bursts in the accretion-rate range where the burst rate is decreasing are highly inhomogeneous Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

20. long short Burst rate vs. X-ray colors Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

21. Steady He-burning at 10% Eddington? Observations at 10-30% Eddington, including • Infrequent, He-like bursts with ~1000 • mHz oscillations • The occurrence of superbursts requiring efficent carbon production All suggest that steady He-burning occurs in this range of accretion rates HOWEVER It is far from clear how steady and unsteady (i.e. bursts) He-burning can happen simultaneously! Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

22. The Multi-INstrument Burst Archive (MINBAR) project The (a) next step: MINBAR Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

23. Summary and future work • Studies of thermonuclear bursts are experiencing a “renaissance” with much recent theoretical and observational activity • Wide-field observations by INTEGRAL and Swift are a promising new source of observations, as well as existing samples from BeppoSAX and the growing RXTE sample • Detailed comparisons with predictions by state-of-the-art models confirm our present understanding of the nuclear physics • Studies of larger burst samples can help to resolve some of the discrepancies with the observational properties Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

24. LMXB neutron star spins • Majority of NS don’t ever exhibit pulsations! • Can detect pulsations (and hence measure the spin) by • Burst oscillations • Persistent pulsations in 7 sources • Intermittent pulsations in up to three more • LMXBs are the evolutionary progenitors to rotation-powered MSPs • And so cluster at high spin frequencies (in the accreting aka “recycling” phase) • Despite this, the fastest-spinning NS known is a rotation powered pulsar at 716 Hz Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

25. A neutron star spinning at 1122 Hz? • A burst from XTE J1739-285 revealed evidence for oscillations at 1122 Hz (Kaaret et al. 2007; astro-ph/0611716) • Maximum Leahy power 42.82 • Significance from M-C simulations is 3.97; taking into account the number of trials, at most 3.5 • Signal was present at high significance only in a single time bin of a single burst -> needs confirmation Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

26. Discovery of thermonuclear bursts • Instruments like the Netherlands/USA satellite ANS first observed bright X-ray flashes from around the Galactic center and elsewhere in the early ‘70s • Multiple bursts from some sources • 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; nuclear burning is much less efficient, at around 1% • Expected -ratio is then ~50 or more, in agreement with measurements Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

27. … and probe the global burning physics • (i.e. the composition of the burst fuel) depends only upon the burst rate -> dominated by the steady (H) burning Possible role of steady He-burning? Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

28. Carbon Ignition curve Observed ignition column Superbursts and the core composition • C ignition is a plausible explanation for the superburst properties BUT • Models don’t produce enough C to power them • Furthermore, ignition occurs at too low a column • (Such “premature” ignition also occurs in H/He-burning thermonuclear bursts) • The crust is too hot; cooling mechanisms are inefficient Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

29. A possible solution: No crust! • The compact object is composed of “strange quark matter”, overlaid by a layer of normal matter supplied by accretion • The fuel layer can be electrostatically supported, and bursts may yet occur (Cumming et al. ApJ 646, 429; See also Page & Cumming 2005, ApJL 635, 157) • Superbursts are difficult to study because of their scarcity (only a few have been observed; estimated recurrence times are ~months) • H/He bursts on the other hand, also depend on the crust cooling, and are more frequent -> an alternative test for strange stars Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

30. … can similarly study energetics… He-ignition H-poor fuel He-ignition H-rich fuel H-ignition Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”