1 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 Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008

2 MSPs hosted in LMXBs  SXT: L ~ erg/s in quiescent L ~ erg/s in outburst, recurence time 2-5 yr. Close X-ray binaries: Companion: M << M sun, Accretion disk, Compact object NS: B~10 8 G  Rich time variability, such as twin QPOs at kHz frequencies ( Hz, increasing with Mdot); kHz QPOs are thought to reflect Kepler at the inner accretion disk. (Van der Klis, 2000, astro-ph/ ) (The Power spectra obtained for SAX J during 2002 outburst.)  8 SXT which show X-ray millisecond coherent modulation.  Spin frequencies lie between 180 and 600 Hz. (see review by Wijnands 2004, astro-ph/ )  Type-I X-ray bursts, with nearly coherent oscillations in the range Hz.  Burst oscillations reflect the NS spin frequency (D. Chakrabarty, Nature, 2003) (Burst oscilation from SAX J during 2002 outburst.) Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008

3 …now we know 8 LMXBs (transients) which show X- ray millisecond coherent modulation: SAX J : P s = 2.5ms, P orb = 2hr (Wijnands & van der Klis 1998) XTE J : P s = 2.3ms, P orb = 42min (Markwardt et al. 2002) XTE J : P s = 5.4ms, P orb = 43.6min (Galloway et al. 2002) XTE J : P s = 5.3ms, P orb = 40min (Markwardt et al. 2003) XTE J : P s = 3.2ms, P orb = 4.3hr (Markwardt et al. 2003) IGR J : Ps = 1.67ms, Porb = 2.46hr (Eckert et al. 2004) HETE J : Ps = 2.65ms, Porb = 1.4hr (Markwardt et al. 2005) Swift J : Ps = 5.49ms, Porb = 54.7min (Krimm et al. 2007) The growing family of the X-ray millisecond pulsars Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008

4 Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008 A Decade of Accreting millisecond X-ray Pulsars Amsterdam, April 2008 SAX J

5 Companion mass Mc/Msun Companion radius Rc/Rsun Brown dwarfs 0.1 Gyr 5 Gyr 1 Gyr White dwarfs XTE J XTE J XTE J IGR J SAX J XTE J Assuming that the companion star should fill its Roche lobe to allow sufficient accretion on the compact star (Bildsten & Chakrabarty, 2001) Companion Star  Brown dwarf models at different ages (Chabrier et al. 2000, Deloye&Bildsten, 2003)  Cold low-mass white dwarfs with pure-helium composition  IGR J  SAX J H-rich donor, brown dwarf  XTE J  HETE J  XTE J  XTE J H-poor, highly evolved dwarf  XTE J  Swift J Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008

6 L  R 2 T 4 α = (F pers /f b )∆t Helium-rich Thermonuclear Bursts and the Distance to the Accretion-powered MSP L edd ~ 3.8 x erg s -1 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 Strohmayer et al (1996); Strohmayer, Markwardt (1999) Cool discs, hot flows, Funäsdalen, SwedenMarch 28, U (D. Chakrabarty, Nature, 2003) Milliseconds Bursts oscillations SAX J XTE J (Kaaret et al. 2007)

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

9 Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008 Outburst are extended as a consequence of X-ray irradiation of the disk (King & Ritter 1998) (Powell, Haswell & Falanga, 2007) SAX J XTE J U 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. Theory: dwarf novae, SXT Hot viscose state R h < R disc

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

11 Intermittent MSP (Altamirano et al. 2007) ISSI BernDecembre 3, 2007 (Casella et al. 2007) SAX J (Globular Cluster NGC 6440), P spin = 2.26 ms, P orb = 8.3 hr AQL X-1, P spin = 1.8 ms, P orb = 19 hr

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+τ c ) or (τ c > 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 J IGR J ( keV) V709 Cas Cas Gamma 2S Cas A IGR J ( keV) (40-80 keV) significance level ~51σ ( keV) significance level ~17σ December 2004 Outburst Exposure 343 ks (20-40 keV) significance level ~88σ derived angular distance: 18´ Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008

14 Geometry of the emission region XTE J Thermal disk emission 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. Seed photons from the hotspot Thermal Comptonization in plasma of Temperature ~ 40 keV B ~ 10 8 G RmRm (Falanga, Bonnet-Bidaud, Poutanen et al. 2005) θ Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008

15 XTE J IGR J disc spot kT bb =0.66keV kT e = 60 keV t T = 0.9 Spectral energy distribution HETE J Gierlinski & Poutanen 2003 Poutanen & Gierlinski 2001 Falanga et al 2005 Falanga et al 2007

16 Spectral evolution Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008 IGR J XTE J Gierlinski & Poutanen 2003Falanga et al. 2005

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

18 Pulsed fraction and Time lag : IGR J (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 -(Γ 0-1 ) exp(-[E/E c ] β ), Componization photon index Γ(E) = Γ 0 + β(E/E c ) β (Falanga, Kuiper, Poutanen et al. 2005) Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008

19 (Falanga et al. 2005) Hard X-ray Soft X-ray Hot coronaAccretion disk Time lag IGR J Compton scattering model Time lag are normally hard 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 see Poutanen 2001) Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008

20 Time/Phase Lag Model Accretion column Disk Disk soft photons Soft photons Neutron Star Hard photons 1-C ill Hard photons C ill θ hot θ ref Compton cloud (Falanga & Titarchuk 2007) ∆t(C ill,  ref,  hot,n e ref,n e hot ) = upscattering lag + downscattering lag Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008

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

22 Spin-up IGR J υ = × Hz s -1 υ = × (L 37 /η -1 I 45 ) (R m /R co ) 1/2 (M/1.4M sun ) (υ spin /600) -1/3 Hz s -1 Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008 (Burderi et al. 2007) (Falanga et al. 2005) We measured for the first time a spin-up for an accreting X-ray millisecond Pulsar

23 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 1 Year Our observation υ = × Hz s arcsec arcsec 0.2 arcsec arcsec 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 YES Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008

24 « Une étoile cannibale » « Star eats companion »

25 Pulsar spin-up Animation NASA, D. Barry

26 kT disc =0.43 keV kT seed = 0.75 keV A seed = 26 km 2 kT e = 37 keV t T = 1.7 disc Strohmayer et al. (2003) Kirsch et al. (2003) XTE J XTE J XTE J Gierlinski & Poutanen 2003 Modeling the Pulse Profile

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 (Schwarzschild metric) 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 

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

30 slow pulsar (dashes) fast pulsar, =401 Hz F sc,E (fast) ~ d 3+ G F sc,E (slow) F bb (fast) = F bb (slow) x d 5 Poutanen & Gierliński (2003) Doppler boosting I obs =d 4 I em Aberration cos a obs =  d cos a e Light curves of MSP d =1/ g (1 - b cos q ) – Doppler factor g =1/  1-b 2 – Lorentz factor

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

32 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 )  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 Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008

33 Thank You… Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008

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

35 Pulse profile IGR J ISGRI 9.1σ keV ISGRI 7.3 σ keV ISGRI 5.0 σ keV Rev 261/262/263, ~205 Porbit = hr Ps = 1.67 ms Pdot = +8.4 x Hz/s ISGRI 2.0 σ keV HEXTE 1.1 σ keV HEXTE 3.3 σ keV HEXTE 8.3 σ keV HEXTE 11.4 σ keV JEM-X 4.0 σ 5-10 keV (Falanga et al. 2005) Cool discs, hot flows, Funäsdalen, SwedenMarch 28, 2008

36 INTEGRAL CODED MASK (IBIS, SPI & JEM -X) Observation Coded MaskShadow Deconvolved Image Corrected Image End Image