accreting x-ray millisecond pulsars in outburst

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 ~ 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.)

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)

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.)

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

3

…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)

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, 14 - 18 April 2008

SAX J1808.4-3658

5

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 (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 J00291+5934 SAX J1808.4-3658 H-rich donor, brown dwarfXTE J1814-338HETE J1900.1-2455XTE J0292-314XTE J1751-305 H-poor, highly evolved dwarf XTE J1807-294Swift J1756.9-2508

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

6

L R2T4

α = (Fpers/fb)∆t

Helium-rich Thermonuclear Bursts and the Distance to the Accretion-powered MSP

Ledd ~ 3.8 x 1038 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, 2008

4U 1728-34

(D. Chakrabarty, Nature, 2003)

Milliseconds Bursts oscillations

SAX J1808.4-3658 XTE J1739-285

(Kaaret et al. 2007)

8

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

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 J1808.4-3658

XTE J1751-3054U 1705-44

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, SXTHot viscose state

Rh < Rdisc

10

OUTBURST PROFILE

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

(Galloway et al. 2007

(Piro & Bildsten, 2005)

NEW: Intermittent Pulsation

HETE J1900.1-2455

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 J1748.9-2021 (Globular Cluster NGC 6440) , Pspin = 2.26 ms, Porb = 8.3 hr

AQL X-1, Pspin = 1.8 ms, Porb = 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+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

(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

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 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

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

15

XTE J1751-305

IGR J00291+5934

disc

spot

kTbb =0.66keVkTe = 60 keVtT = 0.9

Spectral energy distributionSpectral energy distribution

HETE J11900.1-2455

Gierlinski & Poutanen 2003Poutanen & Gierlinski 2001

Falanga et al 2005Falanga et al 2007

16

Spectral evolution

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

IGR J00291+5934

XTE J1751-305

Gierlinski & Poutanen 2003 Falanga et al. 2005

17

(Falanga, Kuiper, Poutanen et al. 2005)

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

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

18

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)

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

19

(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 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 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

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 ac

cretio

n

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 J00291+5934

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

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 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

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

24

« Une étoile cannibale »

« Star eats companion »

25

Pulsar spin-up Animation

NASA, D. Barry

26

kTdisc=0.43 keVkTseed = 0.75 keVAseed = 26 km2

kTe = 37 keVT = 1.7

disc

Strohmayer et al. (2003) Kirsch et al. (2003)

XTE J1751-305

XTE J1814-338XTE J1807-294

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 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 = Idd

29

Modeling the Pulse Profile: oblateness of rapidly rotating NS

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

Morsink et al. 2007

i = 20°, = 41°

i = 70°, = 49°

30

slow pulsar (dashes)

fast pulsar, =401 Hz

Fsc,E(fast) ~ 3+ Fsc,E(slow)

Fbb(fast) = Fbb(slow) x 5

Poutanen & Gierliński (2003)

Doppler boosting Iobs4 Iem

Aberration cos obs cos e

Light curves of MSPLight curves of MSP

=1/(1 - cos ) – Doppler factor

=1/

– Lorentz factor

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

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

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=(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

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

35

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)

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

36

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

Observation

Coded Mask Shadow

Deconvolved Image Corrected Image

End Image