Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts” The complex and puzzling phenomenology of thermonuclear X- ray bursts

Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts”

The complex and puzzling phenomenology of thermonuclear X-ray bursts

APS April 2008, St. Louis

Duncan GallowayMonash University

Andrew Cumming McGill

Jean in ‘t Zand SRON

Deepto Chakrabarty & Jake Hartman MIT

Mike Muno Caltech

Dimitrios Psaltis Arizona


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

EOSOf course, there are also the details of the

thermonuclear burning, how it spreads over the star, the balance between stability and instability…


Burst theory: 3 ignition regimes

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 ignitioncurves

stableburning

accretionrate


In general, the behavior of bursts from individual sources do

NOT

match our expectations from theory


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


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

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

• 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


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 (8.8104 g cm-2 s-1 or 1.310-8 M 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!)


Examples of X-ray bursts from RXTE


Superbursts: carbon burning?

• 1000x more energetic than typical thermonuclear bursts (1042 erg)

• 1000x less frequent (recurrence times of months, instead of hours)

104 s

• Thought to arise from unstable ignition of carbon produced as a by-product of burning during “normal” thermonuclear bursts

4U 1636-536


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


… 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


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)


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)


What about all the other

burst sources?


No shortage of data

QuickTime™ and aTIFF (LZW) decompressor



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)


• 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

• 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)

Studies of large burst samples

H-ignition

He-ignitionH-poor fuel

He-ignitionH-rich fuel

Note that these are carefully-selected subsamples!


X-ray colors accretion rate?

1

23

This is about where we observe mHz oscillations; transition to stable He-burning? (Heger et al. 2007, ApJ 665, 1311)


Diverse bursts at moderate M-dot

The bursts in the accretion-rate range where the burst rate is decreasing are highly inhomogeneous


Burst rate vs. X-ray colors

shortlong


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 productionAll 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!


The (a) next step: MINBAR

QuickTime™ and aTIFF (LZW) decompressor


The Multi-INstrument Burst Archive (MINBAR) project


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


LMXB neutron star spins

0

2

4

6

8

10

12

14

16

18

20

22

24

26

0 50 100150200250300350400450500550600650700

Frequency (Hz)

Number

Burst oscillations

Millisecond pulsars

Radio pulsars

• 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


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


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


… 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?


Observed ignitioncolumn

Carbon Ignitioncurve

Superbursts and the core composition• C ignition is a plausible explanation for the

superburst propertiesBUT

• 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


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


… can similarly study energetics…

H-ignition

He-ignitionH-poor fuel

He-ignitionH-rich fuel

Documents

Galloway, “The complex and puzzling phenomenology of thermonuclear X-ray bursts” The complex and puzzling phenomenology of thermonuclear X- ray bursts