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
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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)
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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
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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
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7. Spectra  pure black body
Strohmayer & Bildsten 2006
Strohmayer & Brown 2002
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8. Nuclear reactions: CNO cycle and 3-alpha
Hot CNO cycle: for solar metal, takes 1 d to burn H
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9. Nuclear reactions: the rp-process
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10. Courtesy Andrew Cumming
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11. Burning regimes Fujimoto et al. 1981
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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
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13. Courtesy Andrew Cumming
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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
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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
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16. Burst peak fluxes & fluences
>pf/Ntot
< 0,2 Crab
10%
> 1 Crab
60%
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17.  intermediate duration bursts (UCXBs)  ‘superbursts’ (carbon)
Burst durations
 intermediate duration bursts (UCXBs)
 ‘superbursts’ (carbon)
He bursts 
mixed H/He bursts 
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18. Discoveries in
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19. Textbook burster (Kong et al. 2007)
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20. Textbook burster (Galloway et al. 2004)
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21. Textbook burster (Ubertini et al. 1999; Galloway et al. 2004)
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22. Why?  accretion histories of prolific bursters
RXTE-PCA Gal. Bulge scans, courtesy Craig Markwardt
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23. Texbook burster (in 't Zand et al. 2009)
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24. Burning regimes in transient KS 1731-260
(Cornelisse et al. 2003)
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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
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26. Burst oscillations
(Strohmayer et al. 1996/7, review by Watts 2012, cc Zhang et al. 2012)
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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
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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)
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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)
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33. Superburst problem (Altamirano et al. 2012)
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34. Also: normal burst problem
(in 't Zand et al. 2002, Wijnands et al. 2009)
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35. Intermediate duration bursts
(in 't Zand et al. 2005, 2007, Falanga et al. 2008)
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36. Intermediate duration bursts
Half look like
UCXBs
(in 't Zand et al. 2007)
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37. Superexpansion (Galloway et al. 2008)
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38. Superexpansion Hoffman et al. 1977
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39. Superexpansion Van Paradijs et al. 1990
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40. Superexpansion Van Paradijs et al. 1990
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41. Flux drops to below pre-burst levels
Superexpansion in 4U
Flux drops to below pre-burst levels
(Molkov et al. 2000)
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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)
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45. Superexpansion (in 't Zand et al. 2012)
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46. (Weinberg et al. 2008)
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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)
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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)
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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)
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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?
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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
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63. X-ray burst research is very much alive!
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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
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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)
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