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 Collaborators: J. Poutanen, L. Kuipers, E. Bonning W. Hermsen, J. M. Bonnet-Bidaud, L. Stella ISSI BernDecembre 3, 2007

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 lye 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.) ISSI BernDecembre 3, 2007

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) The growing family of the X-ray millisecond pulsars ISSI BernDecembre 3, 2007

4 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 Companion Star  Brown dwarf models at different ages (Chabrier et al. 2000)  Cold low-mass white dwarfs with pure-helium composition  IGR J  SAX J H-rich donor, brown dwarf  XTE J  XTE J  XTE J H-poor, highly evolved dwarf  XTE J ISSI BernDecembre 3, 2007

5 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) ISSI BernDecembre 3, 2007

6 INTEGRAL IBIS/ISGRI Observation IGR J ( keV) V709 Cas Cas Gamma 2S Cas A IGR J ( keV) (40-80 keV) significance level ~51σ ( keV) significance level ~17σ Rev 261/262/263/264 ; Exposure 343 ks December 2004 Outburst (20-40 keV) significance level ~88σ derived angular distance: 18´ ISSI BernDecembre 3, 2007

7 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 ISSI BernDecembre 3, 2007

8 HETE J Type I X-ray burst ISSI BernDecembre 3, 2007

9 OUTBURST PROFILE Discovery (Eckert et al. 2004) From RXTE (Galloway et al. 2005) (Falanga, Kuiper, Poutanen et al. 2005) ISGRI keV PULSE PROFILE IGR J Rev 261/262/263, ~205 Porbit = hr Ps = 1.67 ms Pdot = +8.4 x Hz/s ISSI BernDecembre 3, 2007

10 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 (Powell, Haswell & Falanga, 2007) ISSI BernDecembre 3, 2007

11 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) ISSI BernDecembre 3, 2007

12 (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) ISSI BernDecembre 3, 2007

13 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 ISSI BernDecembre 3, 2007

14 Spin-up IGR J We measured for the first time a spin-up for an accreting X-ray millisecond Pulsar 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 ISSI BernDecembre 3, 2007

15 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 ISSI BernDecembre 3, 2007

16 « Une étoile cannibale » « Star eats companion »

17 Pulsar spin-up Animation NASA, D. Barry

18 SAX J Rotation period: 2.5 ms Orbital period: 2 hrs Low mass companion Wijnands & van del Klis 1998 Chakrabarty & Morgan 1998 Light curve folded at the spin period For a millisecond pulsar, e.g. SAX-J ,  can take ~ 20% of 2.5 ms pulse period the time of flight delay cannot be neglected Geometry illustration

19 Constraints on the neutron star mass-radius relation obtained by fitting the pulse profile of SAX J (filled circles with error bars) toghether with a set of equations of state ans strange star (Poutantn & Gierlinski 2003) ISSI BernDecembre 3, 2007

20 Intermittent MSP: 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 = ? (Altamirano et al. 2007) ISSI BernDecembre 3, 2007 (Casella et al. 2007)

21 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 ISSI BernDecembre 3, 2007

22 Thank You… ISSI BernDecembre 3, 2007