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Observations of thermonuclear X-ray bursts - an overview

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1 Observations of thermonuclear X-ray bursts - an overview
Jean in ‘t Zand

2 The X-ray burst phenomenon is omnipresent
1 day looking at 40x40 degrees around the Galactic center (BeppoSAX WFC) The X-ray burst is the brightest phenomenom from the NS surface Observations of thermonuclear X-ray bursts / EWASS 2013

3 Talk outline Introduction 'Recent' discoveries Summary & Outlook
History Principles 'Recent' discoveries Textbook burster Burst oscillations Superbursts Intermediate duration bursts Superexpansion/absorption edges Etc. Summary & Outlook Observations of thermonuclear X-ray bursts / EWASS 2013

4 History: observations
44 years First one detected in 1969 with Vela 5b Still the brightest X-ray burst ever: 1.4 x 10-6 erg s-1 cm-2 (50 x Crab !; bright enough to disturb earth’s ionosphere) Published in 1972 (Belian et al.), explained as an accretion event First cited in 1976 From Cen X-4, the nearest LMXB 1975: first promptly followed-on burst detection with Astronomical Netherlands Satellite (Grindlay & Heise 1975) Observations of thermonuclear X-ray bursts / EWASS 2013

5 History: theory 40 years ago
Rosenbluth, Ruderman, Dyson, Bahcall, Shaham, Ostriker (1973) predict for first time nuclear fusion on accreting NSs Van Horn and Hansen (1974) predict unstable fusion to explain what we now know are BH transients Maraschi & Cavaliere (1976), after first acknowledged burst detection, connect burst phenomenon to theoretical prediction by Van Horn & Hansen (1974) Woosley & Taam (1976) first model for X-ray burst, although proposed as model for Gamma-Ray Bursts, using carbon as fuel Joss (1977) and Lamb & Lamb (1978) first to propose helium as dominant fuel Wallace & Woosley (1981) explained hydrogen/helium flashes 40 years ago Observations of thermonuclear X-ray bursts / EWASS 2013

6 Local accretion rate in low-B NSs 10 to 105 gr s-1 cm-2
After hours to days, accumulate columns of y=108 gr cm-2 (cf, 103 for earth atmosphere) Pressure (y*g) builds up to ignition condition for runaway triple-alpha and CNO processes Can result in thermonuclear shell flash if Layer heats up to 109 K and then cools radiatively through 107 K photosphere  X-ray burst Based on slide from Andrew Cumming Observations of thermonuclear X-ray bursts / EWASS 2013

7 Spectra  pure black body
Strohmayer & Bildsten 2006 Strohmayer & Brown 2002 Observations of thermonuclear X-ray bursts / EWASS 2013

8 Nuclear reactions: CNO cycle and 3-alpha
Hot CNO cycle: for solar metal, takes 1 d to burn H Observations of thermonuclear X-ray bursts / EWASS 2013

9 Nuclear reactions: the rp-process
Observations of thermonuclear X-ray bursts / EWASS 2013

10 Courtesy Andrew Cumming
Observations of thermonuclear X-ray bursts / EWASS 2013

11 Burning regimes Fujimoto et al. 1981
Observations of thermonuclear X-ray bursts / EWASS 2013

12 Short H flash + long He flash Mixed H/He flash Short pure He flash
H-poor donor? (UCXB?) n y M-dot>3% Edd? M-dot>10% Edd? y n n M-dot<1% Edd? M-dot>10%? M-dot>1% Edd? y y n y M-dot > 100%? n n y y n Short H flash + long He flash Mixed H/He flash Short pure He flash Mixed H/He flash Short pure He flash No flash Long He flash No flash Observations of thermonuclear X-ray bursts / EWASS 2013

13 Courtesy Andrew Cumming
Observations of thermonuclear X-ray bursts / EWASS 2013

14 Basic inferences Fluence  amount of energy  amount of fuel burnt
Decay time  thickness of fuel layer If at Eddington limit: peak flux  distance (d=√Ledd/4πFpeak) Peak luminosity  amount of fuel X production rate of nuclear energy Flux + distance  radius (r=d √F/σT4 = Stefan Boltzmann) Alpha  fuel composition Observations of thermonuclear X-ray bursts / EWASS 2013

15 History: observations of X-ray bursts
Instrument Operational time Type of observations Number of bursts Burster discoveries Vela 5b Scan tens? 2 ANS Pointed few 1 OSO-8 Scan+pointed ? 4 Ariel V SAS-3 100s 12 Hackucho ~100? EXOSAT ~150 5 Mir/COMIS-TTM Wide FOV ~50 Granat ART-P ~30 ASCA ~15 3 BeppoSAX WFC 2213 23 HETE II ~1200 RXTE PCA ~2100 8 RXTE ASM ~2000 INTEGRAL JEMX 2002- INTEGRAL IBIS Wide FOV (>10 keV) ~200 Swift XRT/BAT 2004- 100? 11 SuperAgile 2007- Wide FOV(>10 keV) Total ~11,000 102* White: past missions; red: current missions *not all 102 discoveries in this table Observations of thermonuclear X-ray bursts / EWASS 2013

16 Burst peak fluxes & fluences
>pf/Ntot < 0,2 Crab 10% > 1 Crab 60% Observations of thermonuclear X-ray bursts / EWASS 2013

17  intermediate duration bursts (UCXBs)  ‘superbursts’ (carbon)
Burst durations  intermediate duration bursts (UCXBs)  ‘superbursts’ (carbon) He bursts  mixed H/He bursts  Observations of thermonuclear X-ray bursts / EWASS 2013

18 Discoveries in Observations of thermonuclear X-ray bursts / EWASS 2013

19 Textbook burster (Kong et al. 2007)
Observations of thermonuclear X-ray bursts / EWASS 2013

20 Textbook burster (Galloway et al. 2004)
Observations of thermonuclear X-ray bursts / EWASS 2013

21 Textbook burster (Ubertini et al. 1999; Galloway et al. 2004)
Observations of thermonuclear X-ray bursts / EWASS 2013

22 Why?  accretion histories of prolific bursters
RXTE-PCA Gal. Bulge scans, courtesy Craig Markwardt Observations of thermonuclear X-ray bursts / EWASS 2013

23 Texbook burster (in 't Zand et al. 2009)
Observations of thermonuclear X-ray bursts / EWASS 2013

24 Burning regimes in transient KS 1731-260
(Cornelisse et al. 2003) Observations of thermonuclear X-ray bursts / EWASS 2013

25 Bursts with too short recurrence times
150 hours over 7 observations 76 bursts, 15 in 5 triples, 28 in doubles Boirin et al. 2007 Keek et al. 2010 Observations of thermonuclear X-ray bursts / EWASS 2013

26 Burst oscillations (Strohmayer et al. 1996/7, review by Watts 2012, cc Zhang et al. 2012) Observations of thermonuclear X-ray bursts / EWASS 2013

27 Superbursts (Cornelisse et al. 2000, Cumming & Bildsten 2001, Strohmayer & Brown 2002, Strohmayer & Markwardt 2002, in 't Zand et al. 2003) Observations of thermonuclear X-ray bursts / EWASS 2013

28 Superburst population
21 superbursts from 14 superbursters (3/2 questionable) All superbursters are normal bursters as well 4 recurrent superbursters (few months to 10 years recurrence time) Object Instr. Porb (min) # SB Accretion (fraction of Eddington) Dur. (hr) PkLum (1038 erg/s) Reference SB discovery 4U ASM 05 50? 1 0.01 >1.5 >0.1 Kuulkers05 4U WFC 99 236 0.13 14 0.4 Zand03 4U ASM+HETE 05 773? 0.03 (tr.) ~15 0.5 Keek08 4U ASM 96/97/98/01 228 3 0.1 6 1.3 Stroh02, Wij03, Kuu09 KS WFC 97 0.1 (tr.) 12 1.4 Kuulkers02 SLX MAXI 2 >2 Negoro12 4U WFC 96 279 0.25 7 1.5 Cor00 GX 3+1 ASM 99 0.2 >3.3 0.8 GX 17+2 WFC 96-01 10d? 4 1.8 Zand04 EXO MAXI/BAT <0.01 (tr.) 20 0.7 Altamirano12 SAX J JEM-X+MAXI 0.15 (tr.) Chenevez11 4U PCA99,MAXI+ASM10 11 >2.5 3.4 Stroh02,Zand11 Ser X-1 WFC 97/ASM 99/08 1.6 Cor02, Kuu09 SAX J <0.01 Asada11 Observations of thermonuclear X-ray bursts / EWASS 2013

29 Observations of thermonuclear X-ray bursts / EWASS 2013

30 Superburst – normal burst quenching
(Kuulkers et al. 2004, Cumming & Macbeth 2004, Keek et al. 2011) Observations of thermonuclear X-ray bursts / EWASS 2013

31 Observations of thermonuclear X-ray bursts / EWASS 2013

32 Superburst - problem 1.8 MeV/nucl
4/12 superbursters accrete at low average values (<1% Eddington). Implied recurrence time is ~20 yrs, inconsistent with number detected Implied crustal heating much higher Chemical freeze out at bottom ocean? 1.8 MeV/nucl (Keek et al. 2008, Medin & Cumming 2010) Observations of thermonuclear X-ray bursts / EWASS 2013

33 Superburst problem (Altamirano et al. 2012)
Observations of thermonuclear X-ray bursts / EWASS 2013

34 Also: normal burst problem
(in 't Zand et al. 2002, Wijnands et al. 2009) Observations of thermonuclear X-ray bursts / EWASS 2013

35 Intermediate duration bursts
(in 't Zand et al. 2005, 2007, Falanga et al. 2008) Observations of thermonuclear X-ray bursts / EWASS 2013

36 Intermediate duration bursts
Half look like UCXBs (in 't Zand et al. 2007) Observations of thermonuclear X-ray bursts / EWASS 2013

37 Superexpansion (Galloway et al. 2008)
Observations of thermonuclear X-ray bursts / EWASS 2013

38 Superexpansion Hoffman et al. 1977
Observations of thermonuclear X-ray bursts / EWASS 2013

39 Superexpansion Van Paradijs et al. 1990
Observations of thermonuclear X-ray bursts / EWASS 2013

40 Superexpansion Van Paradijs et al. 1990
Observations of thermonuclear X-ray bursts / EWASS 2013

41 Flux drops to below pre-burst levels
Superexpansion in 4U Flux drops to below pre-burst levels (Molkov et al. 2000) Observations of thermonuclear X-ray bursts / EWASS 2013

42 Compilation of 37 cases from 10 sources
(24 WFC, 8 PCA, 1 INTEGRAL, 4 literature) All continue to show small expansion factors after the superexpansion ALL from hydrogen-deficient UCXBs (in 't Zand et al. 2010, 2011, 2012) Observations of thermonuclear X-ray bursts / EWASS 2013

43 SE and ME durations versus burst duration
Moderate expansion duration tme Superexpansion duration tse Why flat?? Burst ‘duration’ τ (in 't Zand et al. 2010) Observations of thermonuclear X-ray bursts / EWASS 2013

44 Explanation: expulsion of shell
κ = κ0 / (1+(T/4.5x108K)0.86)  Ledd increases ~5 times from photosphere to ignition depth Transition initially blows away column/shell where L>Ledd On energetic arguments shell thickness can be up to 1% of yb , or g cm-2 Shell will dilute as (r/10 km)2 and become optically thin after a time (vt/10)2=106 or t=104/v km s-1 t < 10 s  v>103 km s-1 Consistent with preliminary measurements This needs to be confirmed by time-dependent model calculations Like a classical nova (RS Oph simulation by A. Beardmore) (in 't Zand et al. 2010) Observations of thermonuclear X-ray bursts / EWASS 2013

45 Superexpansion (in 't Zand et al. 2012)
Observations of thermonuclear X-ray bursts / EWASS 2013

46 (Weinberg et al. 2008) Observations of thermonuclear X-ray bursts / EWASS 2013

47 Most beautiful data set (bright 7 Crab burst, 5 PCUs on): 4U 0614+091 – a very short 50 ms precursor
(Kuulkers et al. 2010) Observations of thermonuclear X-ray bursts / EWASS 2013

48 Time-resolved spectroscopy of 4U 0614+09
(in 't Zand et al. 2010) Observations of thermonuclear X-ray bursts / EWASS 2013

49 Improvement by including reflection against disk (end of burst)
(in 't Zand et al. 2010) Observations of thermonuclear X-ray bursts / EWASS 2013

50 Improvement by including edges (end of superexpansion)
(in 't Zand et al. 2010) Observations of thermonuclear X-ray bursts / EWASS 2013

51 Time-resolved spectroscopy of 4U 0614+09
8.5 keV when R is close to NS  redshifted from 11 keV  near to H-like (10.8) and He-like (10.3 keV) Ni K-edges. Would be consistent with expectation for ashes early energies not explained. Possibly other elements and/or ionization states (in 't Zand et al. 2010) Observations of thermonuclear X-ray bursts / EWASS 2013

52 SAX J1808.4-3658 2011 November outburst
Brightest burst detected with Chandra and one-but brightest with RXTE (peak flux erg/s/cm2) moderate photospheric expansion NO absorption edges detected But soft excess Turns out to be prototypical behavior of many PRE bursts (Worpel et al. 2013) (in 't Zand et al. 2013) Observations of thermonuclear X-ray bursts / EWASS 2013

53 Burst / accretion disk interaction
(in 't Zand et al. 2011, Bagnoli et al., in prep., Ji et al. 2013, in 't Zand et al. 2013, Worpel et al. 2013) Observations of thermonuclear X-ray bursts / EWASS 2013

54 Burst/disk interaction
(Zhang et al. 2010, 2012) Observations of thermonuclear X-ray bursts / EWASS 2013

55 Marginally stable burning
(Revnivtsev et al. 2004, Heger et al. 2007, Altamirano et al. 2009) Observations of thermonuclear X-ray bursts / EWASS 2013

56 Sudden stop bursts (Chenevez et al. 2010, in 't Zand et al. 2012)
Observations of thermonuclear X-ray bursts / EWASS 2013

57 Summary Observations of thermonuclear X-ray bursts / EWASS 2013

58 Burster statistics Brightest burst: 50 Crabs (Cen X-4, 1969)
11,000 bursts detected thus far 20% of all bursts are Eddington limited 102 Galactic X-ray bursters (plus 2 in M31) 60% are transients 40% have orbital periods (11 min – 25 days) 20% are burst oscillators ( Hz) 10% are superbursters as well (4 multiple) 5% are millisecond pulsars 5% are mHz oscillators Reviews: Lewin et al. (1993), Bildsten (1998), Strohmayer & Bildsten (2006), Galloway et al. (2008) (see also Observations of thermonuclear X-ray bursts / EWASS 2013

59 Classification of X-ray bursts (based on M-dot and composition accreted material)
Type Fuel Duration Recurrence time Number per yr in Galaxy Short & frequent Helium 10 s Hours 4000 Longer & frequent Hydrogen + helium 102 s 8000 Intermediately long 103 s Days 100 Superbursts with H-rich donors Thinned Carbon mixture 104 s Years 20 Superbursts with hydrogen-poor donors Pure Carbon layer Decades 0.1 (red: Eddington limited) Observations of thermonuclear X-ray bursts / EWASS 2013

60 New phenomena New physics Burst oscillations
Superbursts, intermediate duration bursts Superexpansion Incomplete bursts Premature bursts Burst/accretion disk interaction Marginally stable burning Non-Planckian spectra Absorption edges New physics Nuclear processes, particularly carbon burning. Dependig on M-dot, spin Heating processes in ocean and crust 3D details (along surface, with depth): flames, mixing, dynamic magnetic fields.. Observations of thermonuclear X-ray bursts / EWASS 2013

61 Questions on physics (thanks Kuulkers, Brown, Heger, Keek, Falanga, Thielemann, Chenevez, Poutanen, Galloway, Linares) Why do superbursts ignite so easy / what heating processes are we missing? Why is the threshold between stable and unstable burning where it is? How do short waiting times come about / how is burning cut off prematurely? How do flashes start off and develop (flame spreading) as a function of spin and surface conditions? What is the cause of burst oscillations in burst tails? What is the importance of gating in the accretion flow and fuel confinement; how important are quadrupolar components of the magnetic field; how does the accretion flow affect the location of ignition? What are the particularities of X-ray bursts at low M-dot (<1% Eddington). Is sedimentation important? Are there pure hydrogen bursts? What can we learn about classical novae from X-ray bursts? Observations of thermonuclear X-ray bursts / EWASS 2013

62 Wish list for future observations (thanks Kuulkers, Brown, Heger, Keek, Falanga, Thielemann, Chenevez, Poutanen, Galloway, Linares) Measure donor compositions  optical/nir follow up with ELT? Detect absorption edges in superexpansion bursts  detect one with Chandra grating spectrometer (if edge is deep) or wait for bigger instruments such as LOFT (if edge is shallow) Search for narrow features with better than 10% spectral resolution  Chandra and Swift-XRT Measure dynamic response of accretion flow to X-ray bursts by measuring >30 keV emission accurately  Nustar (perhaps) and LOFT Probe burst oscillations in finer detail and lower amplitudes, study dependence on accretion state better  bigger instrument such as LOFT, and much exposure time Measure much more superbursts, to at least constrain recurrence time  wide field / large duty cycle instrument such as on LOFT Detect pure hydrogen flashes at low M-dot  big instrument such as LOFT Measure M/R! Either through non-Planckian spectra, burst oscillations or narrow spectral features from identified isotopes/ionization states Observations of thermonuclear X-ray bursts / EWASS 2013

63 X-ray burst research is very much alive!
Observations of thermonuclear X-ray bursts / EWASS 2013

64 Thank you Observations of thermonuclear X-ray bursts / EWASS 2013

65 What do we want to measure?
Burst oscillations in greater detail: changes during burst onset: study flame spreading as a function of spin (see presentations by Strohmayer, Johnson, Mirach) during burst tails: study as a function of B (or, pulsar presence) Bursts at small persistent M-dot detect more bursts: search for trends; confirm that most are from UCXBs, search for pure H-bursts investigate differences with high M-dot Superbursts recurrence times: probe ignition conditions and crust heating quenching times: study cooling precursors: verify Weinberg et al. model more accurate fluences: better burst parameters Spectroscopy find constraints on ashes and nuclear processes through absorption edges Investigate imprints of accretion disk Find another slow burster, find out how unique Terzan 5 11 Hz system is Determine more accurately M-dot changes during burst, by measuring >30 keV flux Observations of thermonuclear X-ray bursts / EWASS 2013

66 What can LOFT do? LAD Find more (fainter) burst oscillations (in both rises and tails) Follow from oscillation to oscillation, see flame spreading Measure absorption edges down to <1s resolution (see Galloway presentation) Search precursors shorter than 10 ms, indicative of weak relativistic shell expansion Trigger on superbursts within ¼ day on occasion (if we get substantial amounts in WFM) WFM Measure better recurrence times of rare burst types (superbursts, burst-only sources, intermediate duration bursts) Measure quench times after superburst Detect 10 to 20,000 bursts in a 4-year mission, more than doubling the number. Make new discoveries of rare phenomena Great advantage in more WFM units (up to 2x more bursts) Observations of thermonuclear X-ray bursts / EWASS 2013


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