Thermonuclear X-ray bursts as probes of nuclear physics€¦ · time resolution lightcurves for pulsation searches • Continuing to extend the catalog with additional newly public

Thermonuclear X-ray bursts as probes of nuclear physics

AwR VII meeting, Mar 2011, Philip Island

Duncan GallowayMonash [email protected] http://users.monash.edu.au/~dgallow

Neutron stars in low mass binaries

•  These are “old” (~109 yr) systems in which the magnetic field has decayed and the accreting material (usually?) directly impacts the surface

•  Orbital periods of typically minutes to hours, compared to as many as hundreds of days for neutron stars with high-mass companions

•  Distinct evolutionary paths to those systems, and are likely the precursors to millisecond (“recycled”) radio pulsars

Galloway, “Thermonuclear X-ray bursts as probes of nuclear physics”

Schematic of a LMXB system


Neutron stars in low mass binaries

•  Characteristically exhibit thermonuclear (or type-I) X-ray bursts every few hours or days, first observed in 1975


Unstable thermonuclear burning•  Material (expected to be mixed H/He) builds up on the surface of the star until a critical temperature/density is reached

•  “Thin-shell instability” – energy generation rate is more temperature sensitive than cooling -> “runaway” (Hansen & Van Horn 1975)

•  Burning rapidly engulfs the star, exhausting all the available fuel. Predicted efficiency ~5MeV/nucleon, as observed

•  Accretion continues, and the fuel buildup begins again


Burning processes

•  T>107K (set by the accretion rate) so prior to the burst, H burns via hot CNO cycle:

Rate is limited by the mass fraction of CNO nuclei

•  Steady burning heats the fuel layer, encouraging earlier ignition, and also affects the H-fraction at ignition -> observational consequences for the burst properties


12C(p, !) 13N(p, !) 14O("+) 14N(p, !) 15O("+) 15N(p,#) 12C


Burst theory: 3 ignition regimes3 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

Fuel composition & burst properties

•  H-burning is limited by β-decay lifetimes (relatively slow)

•  He-burning occurs via the much more rapid triple-α reaction

Bursts with H have longer durations than bursts burning pure He

•  Breakout from the CNO cycle leads to rp-process burning to very heavy ashes


42He +4

2 He!84 Be +4

2 He!126 C + e+ + e!

Examples – can you guess the fuel?


Mixed H/He – long duration from H-burning

Pure He – fast! Pure He –

but very thick fuel layer

More burning reactions

•  When both H & He are present, high temperatures permit “breakout” reactions from the CNO cycle

•  Subsequent burning via the rapid-proton (rp) process, a series of successive proton captures and β-decays


15O(!, ") 19Ne18Ne(!, p) 21Na

Endpoint of the rp-process


prot

on num

ber

no. of neutronsgrey=stable

Schatz et al. 2001 PRL 86, 3471

Our data•  A unique sample of ~1200 1500 1750 thermonuclear bursts observed by NASA’s Rossi X-ray Timing Explorer during it’s 14-year mission (still going strong!) Galloway et al. 2008, ApJS, 179, 360

•  Analysis results consisting of time-resolved X-ray spectral analyses covering each burst, as well as high-time resolution lightcurves for pulsation searches

•  Continuing to extend the catalog with additional newly public RXTE bursts, BeppoSAX/WFC bursts (+2200) and also INTEGRAL/JEM-X (+1000?) bursts -> MINBAR


“Conventional” analysis•  For the vast majority of

bursts the X-ray spectra throughout are consistent with a Planck (blackbody) spectrum

•  Such spectra are characterised by two parameters: the temperature and the radius of the emitting object

•  We observe a flux at the earth which depends also upon the distance (assuming isotropy)

•  The spectrum also is distorted slightly so we must correct based on assumptions about the photosphere

•  Blackbody normalisation Rbb measured throughout tail… not always constant, or consistent…


Nuclear reactions and stability•  Interest has focussed on the CNO cycle “breakout” reaction 15O(α,γ)19Ne reaction

•  Has a significant impact on the stability of burning; (largely) sets the critical accretion rate above which bursts cease

•  Prior to measurements of the rate, limits were determined based on observations; existence of bursts at all provided a lower limit Fisker et al. 2006, ApJ 650, 332

•  Subsequently, measurements made to reduce uncertainties Tan et al. 2007, 2009, PRC 79, 055805

•  Confirmed long-standing identification of Eddington rate as the critical threshold, but more work to be done


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, “Thermonuclear X-ray bursts as probes of nuclear burning in neutron stars”

… 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 α 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)


An admission

•  Bursts from other sources are almost without exception, neither regular nor consistent

•  Above about 0.1 of the Eddington accretion rate, the burst rate is observed to decrease, contrary to predictions

•  At the same time, bursts become shorter, indicating less H, and we see mHz QPOs, suggesting quasi-stable burning

•  There are also short wait time bursts, occurring in groups of up to 4, separated by a few minutes e.g. Keek et al. 2010, ApJ 718, 292

-> there is a lot more going on than we understand


New regimes of burning

•  Intermediate duration bursts; arising from ignition of a deep He-layer (usually) at low accretion rates

•  Work has focussed on understanding the ignition conditions and influence of the crust Cumming et al. 2006 ApJ 646, 429

•  Also “superbursts”

48 M. Falanga et al.: Intermediate long bursts from SLX 1737-282

Table 2. Properties of the most powerful intermediate long bursts.

Source SLX1737-282 SLX 1735-269 2S 0918-549 IGR J17254-3257 GX 3+1 GX 17+2 4U 1708-23

instrument WFC JEM-X JEM-X WFC JEM-X JEM-X PCA SAS-3precursor burst no no yes no yes no no no no yesduration (min) 15 25 30 20 33 40 15 30 15–30 25!rise (s) 1 2 2 2 100 1 20 1.3 0.4–1.3 20!exp (min) 10 5.0 5.4 4.5 10 3.9 3.7 10.8 3.2–8.3 5.5kTmax (keV) 3.0 3.0 2.8 2.3 2.9 3.0 1.6 2.3 1.8–2.4 2.5La

peak (1038 erg s!1) 3.8 2.5 3.8 3.6 5.1 3.5 0.9 0.8 1.6–2.0 3.0Eb (1040 erg) 19 7 12 10 20 9 2.0 2.1 5.1–7.9 9.7! " Eb/Lpeak (min) 8.4 4.7 5.4 4.7 6.5 4.3 3.6 4.4 5.3–6.6 5.4Lb

pers (%LEdd) 0.4 0.5 0.5 0.5 1.0 0.6 0.2 6.0 75–80 0.3distance (kpc) 8 7.3 7.3 7.3 8.5 5.4 8 5 10 6references [1] [2] [2] [2] [3] [4] [5] [6] [7, 8, 9] [10]

a Unabsorbed bolometric peak (black-body) luminosity.b We used the bolometric unabsorbed flux from spectral fits; the observed maximum flux during radius-expansion bursts. 1. in ’t Zand et al. (2002);2. this work; 3. Molkov et al. (2005), (see also Suzuki & Kawai 2005; Sguera et al. 2007b); 4. in ’t Zand et al. (2005); 5. Chenevez et al. (2007);6. Chenevez et al. (2006); 7. Tawara et al (1984); 8. Kuulkers et al. (2002); 9. Galloway et al. (2006); 10. Ho!man et al. (1978).

100 1000Time (s)

1

10

Flux

(10-8

erg

cm

-2 s

-1)

Burst 2Burst 3

Fig. 6. Comparison of the observed decay of the bolometric black bodyflux (open circle burst 2 and circle burst 3) with a theoretical model(Cumming & Macbeth 2004) for the cooling rate of a column of depth7# 109 g cm!2 and a nuclear energy release of 1.6# 1018 erg g!1, whichis expected for helium burning to iron. The model is shown in dashedand dot-dashed curve for bursts 2 and 3, respectively.

report the properties of the most powerful and recently stud-ied intermediate long bursts. In Fig. 7, we also added interme-diate long bursts with ! $ 100 s from the following sources:GRS 1747-312, EXO 0748-676, and GX 17+2 (in ’t Zandet al. 2003; Galloway et al. 2006). The burst properties ofSLX 1737-282 are similar to the other intermediate long bursts.The intermediate long burst from SLX 1735-269 (Molkov et al.2005) is the only one showing a remarkably long rise time. Thiswas due to an extended photospheric radius expansion phasewith a well separated precursor. Molkov et al. (2005) inter-preted the long decay as most probably due to the mixed burningof H/He. However, in ’t Zand et al. (2007) suggested this sourceis an ultra-compact X-ray binary system and, therefore, a purehelium burst cannot be ruled out at this relatively low accretionrate. Note that only the two high accretion rate sources, GX 3+1and GX 17+2, show all three kinds of bursts, the latter being theonly source that shows intermediate long bursts at the Eddingtonmass accretion rate (most likely resulting from mixed burning

10!3 0.01 0.1 1

110

100

1000

104

105

! =

Eb/L

peak

(s)

Lpers/LEdd

Super BurstsLong BurstsNormal Bursts

Fig. 7. Bursts e!ective durations vs. persistent luminosity for normalbursts (points) observed with RXTE (see Galloway et al. 2006), inter-mediate long bursts (open circle, and triangle for this work; see Table 2and for the ! $ 100 s bursts see Galloway et al. 2006), and Superbursts(open square, see e.g., Kuulkers 2004; in ’t Zand et al. 2004, see alsoSect. 3.4).

of H/He, Chenevez et al. 2006; Galloway et al. 2006; Kuulkerset al. 2002; in ’t Zand et al. 2004; Kuulkers 2002).

The superbursts are observed between 0.1–0.3 MEdd and theintermediate long bursts are observed between 0.002–0.01 MEdd,except again for GX 17+2 at %1 MEdd and GX 3+1 for thepeculiar two-phase intermediate long burst at %0.06 MEdd (seeTable 2). For Fig. 7, we derived the persistent bolometric lu-minosity and burst duration for the superbursts 4U 0614+09(Kuulkers 2005) and 4U 1608-52 (Remillard & Morgan 2005)and found Lpers $ 0.013 and 0.14 LEdd and ! = 0.15#1042/0.2#1038 $ 2.01 hr and ! = 2 # 1042/0.6# 1038 < 9.2 h, respectively(see also for 4U 1608-52 Keek et al. 2007). For the first time asuperburst, from 4U 0614+09, has been observed at %0.01 MEddmass accretion rate, which diverges from the current predic-tion that superbursts with carbon ignition on the hot NS crustrequire an accretion rate >0.1 MEdd (see, e.g., Strohmayer &Bildsten 2006). A consideration of these observations will bepresented in Kuulkers (2008). However, one puzzling issue is to


Falanga et al. 2008, A&A 484, 43

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…

But models overpredict ignition column & can’t produce the required C fraction

4U 1636-536


Ignition is too easy

Whether you look at •  frequent bursts from He-rich donors; e.g.

Misanovic et al. 2010, ApJ 718, 947 & Cumming 2003, ApJ 595, 1077

•  infrequent (intermediate duration) bursts;•  superbursts;

Ignition appears to be taking place earlier than is predicted by theory

This perhaps says something fundamental about the physical conditions and processes in the neutron star crust e.g. Cumming et al. 2006 ApJ 646, 429


So where’s the nuclear physics?

•  At the moment we are effectively limited to using bursts from a single source to probe nuclear reactions in bursts

•  Several groups are working on sensitivity analyses of models for various reactions, with a view to–  identifying the most important reactions–  assessing the observational consequences of varying reaction rates

•  With so very many isotopes and reactions, this is a challenge! But progress is being made See e.g. Amthor et al. 2006, Nuclei in the Cosmos IX proceedings; Fisker et al. 2008, ApJS 174, 261; Parikh et al. 2008, ApJS 178, 110; José et al. 2010, ApJ S 189, 204; & also Cyburt et al. 2010, ApJS 189, 240


Breaking news: Terzan 5 X2X-ray bursts from IGR J17480–2446 5

Fig. 1.— Light curves of 1 s resolution of portions of six RXTE PCU-2 continuous data sets from IGR J17480–2446. The date of observationof each data set is mentioned. This figure shows that during the rise of the 2010 outburst, as the non-burst flux increases, the burst peak fluxand the burst interval gradually decrease. The initial burst properties return as the source intensity decays (§ 3).

•  A new transient outburst of a previously unknown globular cluster LMXB Atel #2919

•  11 Hz pulsations (<< typical freq); 21-hr orbit Atel #2919

•  Frequent bursts occurred closer and closer as the luminosity approached Eddington -> quasi-stable burning

•  First time this transition has been observed

e.g. Chakraborty et al. 2011, arXiv:1101.0181; Motta et al., arXiv:1102.1368; Linares et al., arXiv:1102.1455; Cavecchi et al., arXiv:1102.1548 etc. etc.


Summary and future work

•  There is much about thermonuclear burning on neutron stars that we don’t understand

•  Remarkably however, there is at least one source where this lack of understanding doesn’t seem to matter

•  Observationally, it is a high priority to –  understand better what is special about 1826-like bursts

–  gather and analyse additional examples•  This work has begun last year at Monash and hopefully will continue (via the Multi-INstrument Burst ARchive project – see http://users.monash.edu.au/~dgallow/minbar)



Time dependent burst models•  Ignition by unstable He burning in a mixed H/He environment

•  Includes a realistic adaptive reaction network with thousands of individual isotopes

(simulations from Woosley et al. ‘04)

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)


Summary and future work•  A few examples of burst behaviour which show “ideal” behaviour and have been successfully compared to ignition models of various sophistication

•  These bursts are particularly high priority for more stringent tests, for example to constrain poorly-known individual reaction rates

•  The broader sample of burst sources are more problematic, and there are poorly-understood additional factors which contribute significantly to the burst behaviour

•  Potentially a productive area, but little effort right now?


Documents

Thermonuclear X-ray bursts as probes of nuclear physics€¦ · time resolution lightcurves for pulsation searches • Continuing to extend the catalog with additional newly public