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X-ray bursters

This course explores X-ray bursters, their observations, and the explanations behind their behavior. Topics include compact objects, classification, observational methods, and spectral analysis. Presented by Jérôme Chenevez.

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X-ray bursters

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  1. X-ray bursters Course on Compact Objects Niels Bohr Institute (KU) 29 May 2007 Jérôme Chenevez

  2. OVERVIEW INTRODUCTION OBSERVATIONS EXPLANATIONS X-ray bursters | Jérôme Chenevez | Compact Objects | page 2

  3. X-ray binaries X-ray bursters | Jérôme Chenevez | Compact Objects | page 3

  4. 2˚ N GP X-ray binaries in the GC region 84 type I X-ray bursters known to date; ~2/3 located in the Galactic Bulge X-ray bursters | Jérôme Chenevez | Compact Objects | page 4

  5. Characteristics Classification after the mass of the companion MComp > MCO MComp < MCO IMXB: Intermediate-Mass X-ray Binaries (Mcomp 1-10 M) X-ray bursters | Jérôme Chenevez | Compact Objects | page 5

  6. Black Holes have no surface  no X-ray bursts! X-ray bursters | Jérôme Chenevez | Compact Objects | page 6

  7. X-ray bursters Type I X-ray bursts are thermonuclear explosions in the surface layers of a neutron star accreting H and/or He from the envelope of a companion star. Their emission is described by blackbody radiation with peak temperature ~2 keV and X-ray softening during the decay. X-ray bursters | Jérôme Chenevez | Compact Objects | page 7

  8. TYPE II TYPE I X-ray bursters | Jérôme Chenevez | Compact Objects | page 8

  9. Only 2 Type II X-ray bursters known so far: MXB 1730-335 (Rapid Burster) and the Bursting Pulsar GRO J1744-28 (no type I). X-ray bursters | Jérôme Chenevez | Compact Objects | page 9

  10. OBSERVATIONS X-ray bursters | Jérôme Chenevez | Compact Objects | page 10

  11. Example 1: X-ray burst detection in JEM-X images X-ray bursters | Jérôme Chenevez | Compact Objects | page 11

  12. Example 2: JEM-X detector light curve (30s bins) X-ray bursters | Jérôme Chenevez | Compact Objects | page 12

  13. (Burst 2) ← slew! X-ray bursters | Jérôme Chenevez | Compact Objects | page 13

  14. IGR J17254-3257 X-ray burst light curves Short burst Long burst /5s bins /20s bins 17 February 2004 1 October 2006 X-ray bursters | Jérôme Chenevez | Compact Objects | page 14

  15. Light curves and hardness profiles GX 17+2 (Kuulkers et al., 2002) Hardness = hard flux / soft flux X-ray bursters | Jérôme Chenevez | Compact Objects | page 15

  16. Fluence: (bolometric) Shorter tails at harder energies: Softening Exponential decay (e-folding time: t) due to thermal conduction (Newton’s law)  Cooling X-ray bursters | Jérôme Chenevez | Compact Objects | page 16

  17. (2005) Long rise Soft precursor Very soft emission (UV) due to cooling caused by large radius expansion followed by contraction. Progressive hardening JEM-X X-ray bursters | Jérôme Chenevez | Compact Objects | page 17

  18. Investigation method T i m e r e s o l v e d s p e c t r a l a n a l y s i s • Standard method: modelling of the net burst emission by blackbody (BB) • 2-component method: modelling of the total burst emission by BB+PL • (PL is fixed by pre-burst persistent emission) X-ray bursters | Jérôme Chenevez | Compact Objects | page 18

  19. Blackbody radiation Type I X-ray bursts are characterized by a 2 keV (T ≈ 25∙106 K) blackbody emission and exponential decay with cooling. Spectral intensity ≡ Planck Function: Wien displacement law: hnMax = 2.82 kT  Maximum burst emission (5-6 keV) in JEM-X X-ray bursters | Jérôme Chenevez | Compact Objects | page 19

  20. ), to obtain: X-ray bursters | Jérôme Chenevez | Compact Objects | page 20

  21. Blackbody emission from a neutron star Flux conservation: L = F (Stefan’s law) Caveats: Burst emission is assumed isotropic (x=1) Gravitational redshift effects What is actually observed is a “colour temperature”… X-ray bursters | Jérôme Chenevez | Compact Objects | page 21

  22. Deviations from blackbody emission of hot neutron stars (kT > kTEdd ≈ 2.4 keV) • Nakamura et al., 1989: • High energy tail due to comptonization of photons in a hot plasma around NS • Lewin et al., Space Sci. Rev. 62 (1993): • Modification of BB emission by electron scattering in the atmosphere of NS • ► Tcol≈ 1.5 Teff R understimated by factor ≈ 2 • Strohmayer & Brown, ApJ 566 (2002): • Reflection from accretion disk of 4U 1820-30 • [suggested by Day & Dove, MNRAS 253 (1991)] X-ray bursters | Jérôme Chenevez | Compact Objects | page 22

  23. kTBB RBB FBB Model Fit X-ray bursters | Jérôme Chenevez | Compact Objects | page 23

  24. ≠ time intervals ≠ kTBB… X-ray bursters | Jérôme Chenevez | Compact Objects | page 24

  25. Results (Example 1) Bolometric luminosity (assuming d = 5 kpc) The time resolved spectral analysis of GX 3+1 long X-ray burst reveals variations in the temperature and inferred blackbody radius which indicate expansion and contraction of the emission region. Blackbody temperature Inferred blackbody radius X-ray bursters | Jérôme Chenevez | Compact Objects | page 25

  26. Example 2:IGR J17254-3257 X-ray burst spectral analysis X-ray bursters | Jérôme Chenevez | Compact Objects | page 26

  27. Photospheric Radius Expansion X-ray bursters | Jérôme Chenevez | Compact Objects | page 27

  28. Radius expansion burst from GX 354-0 observed by INTEGRAL (Falanga et al., 2006) X-ray bursters | Jérôme Chenevez | Compact Objects | page 28

  29. Radius expansion burst from GX 3+1 (Kuulkers & van der Klis, 2000) X-ray bursters | Jérôme Chenevez | Compact Objects | page 29

  30. Blackbody cooling track and Stefan’s law: F~ST4 Slope = 4 X-ray bursters | Jérôme Chenevez | Compact Objects | page 30

  31. Long radius expansion burst from GX 17+2 (Kuulkers et al., 2002) Touchdown 3 2 2 3 1 4 1 Expansion/contraction phase Cooling track 4 X-ray bursters | Jérôme Chenevez | Compact Objects | page 31

  32. Eddington Limit For any luminous object, there is a maximum luminosity beyond which radiation pressure will overcome gravity, and material outside the object will be forced away from it rather than falling inwards. For canonical NS parameters (RNS= 10 km, MNS= 1.4 M ) • Eddington luminosity • Eddington temperature • Eddington accretion rate X-ray bursters | Jérôme Chenevez | Compact Objects | page 32

  33. X-ray bursters | Jérôme Chenevez | Compact Objects | page 33

  34. X-ray bursters | Jérôme Chenevez | Compact Objects | page 34

  35. For pure He: LEdd =2.9x1038 erg s-1 X-ray bursters | Jérôme Chenevez | Compact Objects | page 35

  36. 2 = 105 g cm-2 s-1 Per unit area: Peak Temperature (at “touchdown”): Note: The persistent luminosity L is a direct measure of the accretion rate. X-ray bursters | Jérôme Chenevez | Compact Objects | page 36

  37. Applications Relativistic formula for LEdd : X : H fraction, 2.2x10-9 K-1 : e- scattering opacity coefficient of the atmosphere Observationally (globular clusters ): LEdd 3.8x1038 erg/s (Kuulkers et al., 2003) • Determination of the redshift z  MNS • X-ray bursts as standard candles: if L= LEdd  d : upper limit to distance X-ray bursters | Jérôme Chenevez | Compact Objects | page 37

  38. Burst intervals and burst energy The released energy is limited by PRE and indicates limited nuclear fuel due to steady nuclear burning between bursts. X-ray bursters | Jérôme Chenevez | Compact Objects | page 38

  39. Recurrence time • Bursts are fuelled by accreted material at rate • Mass burned during a burst is given by: • Recurrence time between bursts is then: Energy Dt (Lpers)-1 should reflect the time needed to accumulate the nuclear burning fuel But things are not that simple… X-ray bursters | Jérôme Chenevez | Compact Objects | page 39

  40. 10<a<103 : a measure of burst energetics : burst strength relative to persistent emission Effective burst duration: ~ e-folding decay time Note: Burst parameters Fluence X-ray bursters | Jérôme Chenevez | Compact Objects | page 40

  41. Burst parameter relationships: t vs. g The decrease of burst duration with persistent luminosity indicates that hydrogen becomes less important in the energetics of the burst as the mass accretion rate increases (Van Paradijs et al, 1988). X-ray bursters | Jérôme Chenevez | Compact Objects | page 41

  42. Burst parameter relationships: a vs. g The correlation of a with g is consistent with previous conclusion and seems to indicate that steady nuclear burning limiting the burst energy release does increase with accretion rate (Van Paradijs et al, 1988). X-ray bursters | Jérôme Chenevez | Compact Objects | page 42

  43. Preliminary interpretation • The previous records seem to indicate a transition between two nuclear burning regimes. • Evidences of increasing time between bursts as persistent emission increases may indicate an increase of the accretion area implying that the local accretion rate per unit area, , actually decreases with the accretion rate . • The influence of the accretion rate per unit area is an indication that only a fraction of the NS is covered by freshly accreted fuel. X-ray bursters | Jérôme Chenevez | Compact Objects | page 43

  44. X-ray burst oscillations Power spectrum showing millisecond variability during X-ray bursts Recently, Kaaret et al. (2007) has observed a record breaking oscillation at 1122 Hz in the tail of an X-ray burst from XTE J1739-285, previously identified as a burster by the JEM-X team. X-ray bursters | Jérôme Chenevez | Compact Objects | page 44

  45. Spin modulation Oscillations are associated with a hot spot expanding on the NS surface like a deflagration flame and modulated by the NS rotation. Coriolis forces X-ray bursters | Jérôme Chenevez | Compact Objects | page 45

  46. THEORY X-ray bursters | Jérôme Chenevez | Compact Objects | page 46

  47. More or less long bursts Ordinary H bursts Unusual long bursts Ordinary He bursts Superbursts X-ray bursters | Jérôme Chenevez | Compact Objects | page 47

  48. Burst Energetics 1.6 Mev/nucleon X-ray bursters | Jérôme Chenevez | Compact Objects | page 48

  49. Power of accretion: Nuclear vs. gravitation Energy release of nuclear burning to heavy elements is 1.6 Mev/nucleon for pure He and 5 Mev/nucleon for solar composition material  Ineffective process compensated by accumulation X-ray bursters | Jérôme Chenevez | Compact Objects | page 49

  50. Nuclear burning regimes H unstability • : Mixed H/He burning triggered by thermally unstable H ignition. Long burst duration (> 100s - 1000s) due to rp- process. a150. • : H stable burning (hot CNO cycle) to He  Pure He flash (3-a). Frequent PRE. a200. • : Mixed H/He burning triggered by thermally unstable He ignition. Burst duration > 10s due to rp- process. a~20-100. • : No bursts (e.g. pulsars). • Pure He accretion (e.g. from white dwarf)  powerful pure He bursts. • Deep Carbon burning in superbursts. He unstability X-ray bursters | Jérôme Chenevez | Compact Objects | page 50

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