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ACCRETING X-RAY MILLISECOND PULSARS IN OUTBURST

March 28, 2008. Cool discs, hot flows, Funäsdalen, Sweden. ACCRETING X-RAY MILLISECOND PULSARS IN OUTBURST. M A U R I Z I O F A L A N G A. Service d‘Astrophysique, CEA –Saclay, France. Collaborators: J. Poutanen, L. Kuipers, J. M. Bonnet-Bidaud. March 28, 2008.

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ACCRETING X-RAY MILLISECOND PULSARS IN OUTBURST

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  1. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden ACCRETING X-RAY MILLISECOND PULSARS IN OUTBURST M A U R I Z I O F A L A N G A Service d‘Astrophysique, CEA –Saclay, France Collaborators: J. Poutanen, L. Kuipers, J. M. Bonnet-Bidaud

  2. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden MSPs hosted in LMXBs Close X-ray binaries: Companion: M << Msun, Accretion disk, Compact object NS: B~108G • 8 SXT which show X-ray millisecond coherent modulation. • Spin frequencies lie between 180 and 600 Hz.(see review by Wijnands 2004, astro-ph/0403409) • Rich time variability, such as twin QPOs at kHz frequencies (400 - 1300 Hz, increasing with Mdot); kHz QPOs are thought to reflect Kepler at the inner accretion disk.(Van der Klis, 2000, astro-ph/00001167) (The Power spectra obtained for SAX J1808.4-3658 during 2002 outburst.) • Type-I X-ray bursts, with nearly coherent oscillations in the range 300-600 Hz . • Burst oscillations reflect the NS spin frequency (D. Chakrabarty, Nature, 2003) (Burst oscilation from SAX J1808.4-3658 during 2002 outburst.) • SXT: L ~ 1031-1032 erg/s in quiescent • L ~ 1036-1038 erg/s in outburst, recurence time 2-5 yr.

  3. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden The growing family of the X-ray millisecond pulsars …now we know 8 LMXBs (transients) which show X-ray millisecond coherent modulation: SAX J1808.4-3658: Ps = 2.5ms, Porb = 2hr (Wijnands & van der Klis 1998) XTE J1751-306: Ps = 2.3ms, Porb = 42min (Markwardt et al. 2002) XTE J0929-314: Ps = 5.4ms, Porb = 43.6min (Galloway et al. 2002) XTE J1807-294: Ps = 5.3ms, Porb = 40min (Markwardt et al. 2003) XTE J1814-388: Ps = 3.2ms, Porb = 4.3hr (Markwardt et al. 2003) IGR J00291+5934: Ps = 1.67ms, Porb = 2.46hr (Eckert et al. 2004) HETE J1900.1-2455: Ps = 2.65ms, Porb = 1.4hr(Markwardt et al. 2005) Swift J1756.9-2508: Ps = 5.49ms, Porb = 54.7min(Krimm et al. 2007)

  4. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden A Decade of Accreting millisecond X-ray Pulsars SAX J1808.4-3658 Amsterdam, 14 - 18 April 2008

  5. IGR J00291+5934 SAX J1808..4-3658 XTE J1814-338 Brown dwarfs Companion radius Rc/Rsun 0.1 Gyr 1 Gyr XTE J0929-314 XTE J1751-305 XTE J1807-294 5 Gyr White dwarfs Companion mass Mc/Msun March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden Companion Star Assuming that the companion star should fill its Roche lobe to allow sufficient accretion on the compact star (Bildsten & Chakrabarty, 2001) • Brown dwarf models at different ages (Chabrier et al. 2000, Deloye&Bildsten, 2003) • Cold low-mass white dwarfs with pure-helium composition • IGR J00291+5934 • SAX J1808.4-3658 H-rich donor, brown dwarf • XTE J1814-338 • HETE J1900.1-2455 • XTE J0292-314 • XTE J1751-305 H-poor, highly evolved dwarf • XTE J1807-294 • Swift J1756.9-2508

  6. Helium-rich Thermonuclear Bursts and the Distance to the Accretion-powered MSP Ledd ~ 3.8 x 1038 erg s-1 L R2T4 α= (Fpers/fb)∆t The time between bursts was long enough for hot CNO burning to significantly deplete the accreted hydrogen, so that ignition occurred in a pure helium layer underlying a stable hydrogen burning shell.

  7. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden Milliseconds Bursts oscillations XTE J1739-285 SAX J1808.4-3658 4U 1728-34 (D. Chakrabarty, Nature, 2003) Strohmayer et al (1996); Strohmayer, Markwardt (1999) (Kaaret et al. 2007)

  8. Distinct knee Outburst are extended as a consequence of X-ray irradiation of the disk?(King & Ritter 1998) March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden OUTBURST PROFILE Discovery(Eckert et al. 2004) XTE J1807-294 XTE J1807-294 From RXTE (Galloway et al. 2005) ISGRI 20-100 keV (Falanga et al. 2005) (Falanga et al. 2005) (Wijnands 2005, astro-ph/0403409)

  9. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden Outburst are extended as a consequence of X-ray irradiation of the disk(King & Ritter 1998) Theory:dwarf novae, SXT XTE J1751-305 4U 1705-44 Hot viscose state Rh < Rdisc SAX J1808.4-3658 Central object prevents the disk to cool down due to Irradiation, on a viscous time-scale, accounting for the exponential decay of the outburst on a timescale τ~20–40 d. (Powell, Haswell & Falanga, 2007)

  10. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden OUTBURST PROFILE HETE J1900.1-2455 (Falanga et al. 2007) After 60 days: Period disappeared (Kaaret, et al. 2007) NEW: Intermittent Pulsation (Galloway et al. 2007 (Piro & Bildsten, 2005)

  11. Decembre 3, 2007 ISSI Bern Intermittent MSP SAX J1748.9-2021 (Globular Cluster NGC 6440), Pspin = 2.26 ms, Porb = 8.3 hr (Altamirano et al. 2007) AQL X-1, Pspin = 1.8 ms, Porb = 19 hr (Casella et al. 2007)

  12. The reason for the lack of coherent pulsations in the persistent emission from LMXBs • Different explanations: • Gravitational deflection (lensing effect)(Wood et al.1988) • Electron scattering (Brainerd & Lamb, 1987; Titarchuk et al. 2002) ~1/(1+τc) or (τc > 4) • Weak surface magnetic fields due to magnetic screening (e.g., Cumming et al. 2001) • Rayleigh-Taylor instability:Depending on the accretion rate, a star may be in the stable or unstable regime of accretion.(Kulkarni & Romanova 2008)

  13. INTEGRAL Observation IGR J00291+5934 IGR J00291+5934 March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden (80 - 200 keV) December 2004 Outburst Exposure 343 ks IGR J00291+5934 2S0114+650 Cas Gamma Cas A V709 Cas (20 - 40 keV) (20-40 keV) significance level ~88σ derived angular distance: 18´ (40-80 keV) significance level ~51σ (80-200 keV) significance level ~17σ

  14. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden Geometry of the emission region The plasma is heated by the accretion shock as the material collimated by the hotspot on to the surface. The seed photons for Comptonization are provided by the hotspot. θ B ~ 108 G Seed photons from the hotspot Thermal Comptonization in plasma of Temperature ~ 40 keV XTE J1807-294 Rm Thermal disk emission (Falanga, Bonnet-Bidaud, Poutanen et al. 2005)

  15. Spectral energy distribution Gierlinski & Poutanen 2003 Poutanen & Gierlinski 2001 spot kTbb =0.66keV kTe = 60 keV tT = 0.9 XTE J1751-305 disc Falanga et al 2007 Falanga et al 2005 HETE J11900.1-2455 IGR J00291+5934

  16. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden Spectral evolution IGR J00291+5934 XTE J1751-305 Gierlinski & Poutanen 2003 Falanga et al. 2005

  17. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden PULSE PROFILE IGR J00291+5934 Rev 261/262/263, ~205 Porbit = 2.457 hr Ps = 1.67 ms Pdot = +8.4 x 10-13 Hz/s (Falanga, Kuiper, Poutanen et al. 2005)

  18. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden Pulsed fraction and Time lag : IGR J00291+5934 (Falanga, Kuiper, Poutanen et al. 2005) (Falanga et al. 2005) If the spectrum has a sharp cutoff, the rms amplitude of the pulse at energies above the cutoff increases dramatically. F(E) ≈E-(Γ0-1) exp(-[E/Ec]β),Componization photon indexΓ(E) = Γ0 + β(E/Ec)β

  19. Hard X-ray Soft X-ray Hot corona Accretion disk March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden (Falanga et al. 2005) Time lag IGR J00291+5934 Time lag are normally hard Compton scattering model The energy spectra often observed in LMXBs suggests that the dominant radiative mechanism in the system is Compton scattering of soft photons in a hot plasma. (For a review of models for spectral variability and time lags seePoutanen 2001)

  20. Accretion column θhot Hard photons 1-Cill Hard photons θref Disk soft photons Cill Neutron Star Soft photons Disk Compton cloud March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden (Falanga & Titarchuk 2007) Time/Phase Lag Model ∆t(Cill,ref,hot,neref,nehot) = upscattering lag + downscattering lag

  21. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden Recycling model for MSPs • LMXB phase preceding the MSP stage; • mass transfer stops; • the radio MSP switches on Old Neutron stars spin up by accretion from a companion Spin up by mass accretion • Most binary MSPs have short orbital • periods and mass function identifying the • companions as low mass evolved dwarfs Radio Pulsar Millisecond Radio Pulsar Accreting NS in LMXBs are conventionally thought to be the progenitors of millisecond or „recycled“ radio pulsars(Alpar et al. 1982) X-ray transients can be the missing link between LMXBs and MSPs!

  22. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden Spin-up IGR J00291+5934 We measured for the first time a spin-up for an accreting X-ray millisecond Pulsar υ= + 8.4 ×10-13 Hz s-1 (Falanga et al. 2005) (Burderi et al. 2007) υ= + 3.7 ×10-13 (L37/η-1I45) (Rm/Rco)1/2 (M/1.4Msun) (υspin/600)-1/3 Hz s-1

  23. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden YES Is the Spin-up real? An error in the source coordinates can give rise to timing error which may introduce a spurious spin-up or spin-down 0.2 arcsec 0.092 arcsec 0.2 arcsec 0.092 arcsec 1 Year Our observation 0.092 arcsec source position error would introduce a non-existant spin-up rate of Such an apparent spin-up would require a fairly large ~ 0.7 arcsec source position error during our observation υ= + 5.8 ×10-14 Hz s-1

  24. « Star eats companion » «Une étoile cannibale »

  25. Pulsar spin-up Animation NASA, D. Barry

  26. Modeling the Pulse Profile XTE J1751-305 disc Gierlinski & Poutanen 2003 XTE J1807-294 XTE J1814-338 kTdisc=0.43 keV kTseed = 0.75 keV Aseed = 26 km2 kTe = 37 keV tT = 1.7 Kirsch et al. (2003) Strohmayer et al. (2003)

  27. Strong-field General Relativity is required to describe the lightcurve observed at infinit. (Gierlinski & Poutanen, 2005) (Morsink et al. 2007)

  28. The geometry of the model • Motion of Matter • (Time-like geodesics) • Curved photon trajectories • (Null-like geodesics) • Doppler shift : (1 + z) • The solid angle : d(R,d,i,db) • (Gravitational lensing effect) • Travel time delay • The observed flux : F =  Idd (Schwarzschild metric)

  29. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden Modeling the Pulse Profile: oblateness of rapidly rotating NS i = 70°,  = 49° i = 20°,  = 41° Morsink et al. 2007

  30. Light curves of MSP Fbb(fast) = Fbb(slow) xd5 Doppler boostingIobs=d4Iem Aberration cos aobs=dcos ae slow pulsar (dashes) fast pulsar, =401 Hz Fsc,E(fast) ~d3+GFsc,E(slow) d=1/g(1 - b cos q) – Doppler factor g=1/1-b2 – Lorentz factor Poutanen & Gierliński (2003)

  31. Constraints on the neutron star mass-radius relation obtained by fitting the pulse profile of SAX J1808.4-3658 • Complications: • Shape of the star • Shape of the spot • Influence of the accretion disk (Poutantn & Gierlinski 2003)

  32. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden Important Questions • Missing link between LXMB and ms radio pulsar ? • Analysis suggests that the spin frequency is limited to 760 Hz (95% confidence; Chakrabarty et al. 2003) • Several have suggested that gravitational radiation from a non-spherical neutron star might limit the maximum fraquency (Bildsten et al. 1998) • Detection by LISA? • Detecting more of these source with more instrument than before

  33. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden Thank You…

  34. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden Pulsar spin-up M R(corotation) Accretion regime Rm< Rcor The accreting matter transfers its specific angular momentum (the Keplerian AM at the magnetospheric radius) to the neutron star: L=(GMRm)1/2 R(magnetosphere) Propeller regime Rm> Rcor The process goes on until the pulsar reaches the keplerian velocity at Rm (equilibrium period); Pmin when Rm = Rns (Illarionov & Sunyaev 1975) The conservation of AM tells us how much mass is necesssary to reach Pmin starting from a non-rotating NS

  35. March 28, 2008 Cool discs, hot flows, Funäsdalen, Sweden Pulse profile IGR J00291+5934 HEXTE 11.4 σ 20.3-35.2 keV ISGRI 9.1σ 20-35 keV HEXTE 8.3 σ 35.2-60.1 keV ISGRI 7.3 σ 35-60 keV HEXTE 3.3 σ 60.1-100.9 keV ISGRI 5.0 σ 60-100 keV JEM-X 4.0 σ 5-10 keV Rev 261/262/263, ~205 Porbit = 2.457 hr Ps = 1.67 ms Pdot = +8.4 x 10-13 Hz/s HEXTE 1.1 σ 100.9-151.1 keV ISGRI 2.0 σ 100-150 keV (Falanga et al. 2005)

  36. INTEGRAL CODED MASK (IBIS, SPI & JEM -X) Observation CorrectedImage DeconvolvedImage End Image Coded Mask Shadow

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