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ACCRETING X-RAY MILLISECOND PULSARS

M A U R I Z I O F A L A N G A

&

E R I N W. B O N N I N G

NS day, ParisJune 27, 2007

Service d‘Astrophysique, CEA –SaclayLUTH

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MSPs hosted in LMXBs

SXT:L ~ 1031-1032 erg/s in quiescentL ~ 1036-1038 erg/s in outburst, recurence time 2-5 yr.

Close X-ray binaries: Companion: M << Msun, Accretion disk, Compact object NS: B~108G

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.)

7 SXT which show X-ray millisecond coherent modulation.Spin frequencies lye between 180 and 600 Hz. (see review by Wijnands 2004, astro-ph/0403409)

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.)

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…now we know 7 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)

The growing family of the X-ray millisecond pulsars

NS day, ParisJune 27, 2007

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Companion mass Mc/Msun

Com

pani

on r

adiu

s R

c/R

sun

Brown dwarfs0.1 Gyr

5 Gyr

1 Gyr

White dwarfs

XTE J0929-314XTE J1751-305XTE J1807-294

IGR J00291+5934SAX J1808..4-3658XTE J1814-338

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 J00291+5934 SAX J1808.4-3658 H-rich donor, brown dwarfXTE J1814-338XTE J0292-314XTE J1751-305 H-poor, highly evolved dwarf XTE J1807-294

NS day, ParisJune 27, 2007

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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 ac

cretio

n

Accreting NS in LMXBs are conventionally thought to be the progenitors of millisecond or „recycled“ radio pulsars (Alpar et al. 1982)

NS day, ParisJune 27, 2007

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INTEGRAL IBIS/ISGRI Observation

IGR J00291+5934

(20 - 40 keV)V709 Cas

Cas Gamma

2S0114+650

Cas A

IGR J00291+5934

(80 - 200 keV)

(40-80 keV) significance level ~51σ

(80-200 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´

NS day, ParisJune 27, 2007

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OUTBURST PROFILE

Discovery (Eckert et al. 2004)

From RXTE (Galloway et al. 2005)

(Falanga, Kuiper, Poutanen et al. 2005)

ISGRI 20-100 keV

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

NS day, ParisJune 27, 2007

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Geometry of the emission region

XTE J1807-294

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 ~ 108 G

Rm

(Falanga, Bonnet-Bidaud, poutanen et al. 2005)

θ

NS day, ParisJune 27, 2007

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Pulsed fraction and Time lag : IGR J00291+5934

(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 index Γ(E) = Γ0 + β(E/Ec)β

(Falanga, Kuiper, Poutanen et al. 2005)

NS day, ParisJune 27, 2007

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Time/Phase Lag Model Accretioncolumn

Disk

Disk soft photons

Soft photonsNeutron Star

Hard photons1-Cill

Hard photonsCill

θhot

θref

Compton cloud

(Falanga & Titarchuk 2007)

∆t(Cill,ref,hot,neref,ne

hot) =

upscattering lag + downscattering lag

NS day, ParisJune 27, 2007

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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

υ = + 3.7 × 10-13 (L37/η-1I45) (Rm/Rco)1/2 (M/1.4Msun) (υspin/600)-1/3 Hz s-1

NS day, ParisJune 27, 2007

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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 YearOur observation

υ = + 5.8 × 10-14 Hz s-1

0.2 arcsec0.092 arcsec 0.2 arcsec

0.092 arcsec

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

YES

NS day, ParisJune 27, 2007

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Pulsar spin-up Animation

NASA, D. Barry

NS day, ParisJune 27, 2007

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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

NS day, ParisJune 27, 2007

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Thank You…

NS day, ParisJune 27, 2007

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Spectral analysis IGR J00291+5934

JEM-X/ISGRI Model ComppsNH (cm-2) 0.28 x 1022 (f) kTe (keV) 49 ± 4kTseed (keV) 1.49 ± 0.24Optical depth 1.12 ± 0.05Aseed (km2) 20.7 ± 8.5Cos θ 0.6 ± 0.07L(0.1-300keV) (10

36 erg s

-1) 3.7 NS day, ParisJune 27, 2007

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HETE J1900.1-2455

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SAX J1808.4-3658

First millisecond accretion powered X-ray pulsar

Rotation period: 2.5 ms Orbital period: 2 hrs

Low mass companionWijnands & van del Klis 1998

Chakrabarty & Morgan 1998 Light curve folded at the spin period

For a millisecond pulsar, e.g. SAX-J1808.4-3658, can take ~ 20% of 2.5 ms pulse period the time of flight delay cannot be neglected

Geometry illustration

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Constraints on the neutron star mass-radius relation obtained by fitting the pulse profile of SAX J1808.4-3658 (filled circles with error bars) toghether with a set of equations of state ans strange star

(Poutantn & Gierlinski 2003)

NS day, ParisJune 27, 2007

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OUTBURST PROFILE

XTE J1807-294

Discovery (Eckert et al. 2004)

From RXTE (Galloway et al. 2005)

(Falanga et al. 2005)(Falanga et al. 2005)

(Wijnands 2005, astro-ph/0403409)

XTE J1807-294

ISGRI 20-100 keV

Outburst are extended as a consequence of

X-ray irradiation of the disk? (King & Ritter 1998)

Distinct knee

Pulsar Workshop, IAP, ParisMay 3-4, 2007

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Pulse profile IGR J00291+5934

ISGRI 9.1σ 20-35 keV

ISGRI 7.3 σ 35-60 keV

ISGRI 5.0 σ 60-100 keV

Rev 261/262/263, ~205

Porbit = 2.457 hr

Ps = 1.67 ms

Pdot = +8.4 x 10-13 Hz/s

ISGRI 2.0 σ 100-150 keV

HEXTE

1.1 σ 100.9-151.1 keV

HEXTE

3.3 σ 60.1-100.9 keV

HEXTE

8.3 σ 35.2-60.1 keV

HEXTE

11.4 σ 20.3-35.2 keV

JEM-X 4.0 σ 5-10 keV

(Falanga et al. 2005)

Pulsar Workshop, IAP, ParisMay 3-4, 2007

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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=(GMRm)1/2

M

The process goes on until the pulsar reaches the keplerian velocity at Rm (equilibrium period); Pmin when Rm = Rns

The conservation of AM tells us how much mass is necesssary to reach Pmin starting from a non-rotating NS

Accretion regime

Rm < Rcor

(Illarionov & Sunyaev 1975)

R(corotation)

Propeller regime

Rm > Rcor

Pulsar Workshop, IAP, ParisMay 3-4, 2007

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(Falanga et al. 2005)

Hard X-ray

Soft X-ray

Hot corona Accretion disk

Time lag IGR J00291+5934

Compton scattering model

Time lag are normally hard

The energy spectra often observed in LMXRBs 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)

Pulsar Workshop, IAP, ParisMay 3-4, 2007

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INTEGRAL CODED MASK (IBIS, SPI & JEM -X)

Observation

Coded Mask Shadow

Deconvolved Image Corrected Image

End Image

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« Une étoile cannibale »

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