1Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015 Numerical simulations of...

1Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015

Numerical simulations of propeller accretion regime

and the variability ofIGR J18245-2452

Carlo FerrignoUniversity of GenevaMarina Romanova Cornell University

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General view

• Matter of the disk is stopped by the magnetic pressure• Accretion and outflows depend on amount of matter accretion

rate• Diffusivity at the disk-magnetosphere boundary

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Terminology

• Corotation radius: radius at which the magnetic field rotates at the local Keplerian speed

• Alfven radius: distance from a non- rotating star where the free-fall of a quasi-spherical accretion flow is stopped.

• Magnetospheric radius: radius at which magnetic pressure overcomes the ram pressure and flow is trapped by magnetic field in discs rm = f rco f~0.4

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Propeller accretion

• Inner disk rotates faster than magnetosphere and plasma is funnelled to the surface by B-field

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Accretion Propeller• Magnetos

phere rotates faster than accretion flow and matter can be centrifugally ejected.


Theory

• Study of the effective Alfven radius, which flickers in and out the corotation radius

• Matter and angular momentum flow due to coupling of magnetic field and plasma

• Transitions from propeller to accretion may be stochastic or chaotic in nature, with triggering due to small variations in the accretion flow or in the magnetic field configuration.

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Illarionov & Sunyaev (1975); Lovelace, Romanova and Bisnovatyi-Kogan (1999)

Illarionov & Sunyaev (1975); Lovelace, Romanova and Bisnovatyi-Kogan (1999)


The MHD problem

• Reference frame corotating with the star• Solving for 8 variables: B field (3), plasma speed (3), density,

energy density

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No shocks

Ideal

γ=5/3 Adiabatic

Stress tensor

e.g., Utsyugova (2006)e.g., Utsyugova (2006)

Viscosity

Diffusivity


Scalable simulations

• Adimensional variables. • Scalable to objects with small magnetosphere

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Set-up of propeller simulations

• Gudonov type MHD code: 2.5D simulations of propeller

• Laminar α- disks. Spherical coordinates, 2.5D – also top-bottom symmetry– αv=0.1-0.3, αd=0.1

• Turbulent MRI-driven disks. Cylindrical coordinates– Viscosity is determined by MRI. – Diffusivity is free parameter

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Utsyugova et al. (2006)

Utsyugova et al. (2006)

Lii et al. (2014)Lii et al. (2014)


Accretion excursus

• α-disk model. 3-D simulations to study the Reyleigh-Taylor instabilities.

• Heavy matter pervades the light medium across magnetic field line and produces accretion tongues (low viscosity 0.02)

• Reduction of significance of coherent pulsations9

Romanova et al. (2008) Kuulkarni & Romanova (2008) Spruit et al. (1995); Lubow & Spruit (1993); Kaisig, Tajima,

Lovelace (1992)

Romanova et al. (2008) Kuulkarni & Romanova (2008) Spruit et al. (1995); Lubow & Spruit (1993); Kaisig, Tajima,

Lovelace (1992)

geff = g – Ω2r


Stable/Unstable accretion

• High accretion rate, unstable accretion.

• Inclination stabilizes.

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Blinova et al. (2013)Blinova et al. (2013)

3D-slice-movie-short.mov


Dependencies

• Hollow column or crescent shape

• Dependency on accretion rate is less steep than standard theory (1/5 < 2/7)

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Kuulkarni & Romanova (2013)

Kuulkarni & Romanova (2013)


Propeller

• Conical winds dominate the outwards mass outflow• Strong magnetic tower produces a more collimated Pointing

dominated jet.• Accretion continues on episodic fashion

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rm > rco


Slow and fast rotators

• Outflows are present in both cases together with accretion in funnels.

• In propeller, there is a fast axial jet, which does not form in conical wind only regime

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Slow rotator Fast rotatorpropeller

Romanova et al. (2005)Romanova et al. (2005)


Cyclic accretion

• Matter accumulates and then is accreted in cyclic fashion• Contemporary ejections of material

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Episodic accretion cycle

• Observed sporadically in outburst of AMSP, but at different time scale and duty cycle

• Lower diffusivity may reconcile the observations.

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SAX J1808, Patruno et al. 2009;

NGC 6440 X-2 Patruno & D’Angelo 2013

D’Angelo & Spruit 2010;

SAX J1808, Patruno et al. 2009;

NGC 6440 X-2 Patruno & D’Angelo 2013

D’Angelo & Spruit 2010;


Strong propeller outflow

• one-sided outflow half-opening angle: ~20-40°, with continued collimation further out

• time averaged ejection efficiency: 50-90% depending on Ω*

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• Always a fraction is accreted


Dependencies

• Strong .......................... Weak

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rm/rco=2.5 rm/rco=1.5 rm/rco=1.1

• Diffusivity quenches bursts of accretion because of diffusive penetration through the boundary.



IGR J18245/M 28I

• In the globular cluster M 28: it is at 5.5 kpc.

• In 2013: one bright accretion driven outburst. Coherent pulsation. Strong spectral and timing variability.

• Intermediate accretion events: X-ray & optical brightening. Mode switch variability.

• Faint radio pulsar with irregularly eclipses due to outflows.

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Papitto+ (2013)Papitto+ (2013)


IGR J18245/M 28I in accretion phase

• Only a few days after the last X-ray detection !21

Papitto+ (2013)Papitto+ (2013)


XMM-Newton light curve

• Very interesting variability, unique among AMSP.

• Episodes of enhanced hardness at low flux

• No orbital dependency.

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Hard 3.5-10 keVSoft 0.5-3.5 keV

Ferrigno + (2014)Ferrigno + (2014)

time Bins >= 200 s


Two branches

• Blue: higher flux, limited Hardness variation• Magenta: lower flux, swings of hardness, what are they?

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time Bins >= 200 s


Always pulsed

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Light curve - Blue state

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• Strong second-scale variability.

• red points are bins of 200 s


Wavelets

• Try to understand if there are particular time scales in the variability of this source.

• Try to check when they appear, if they appear.• Use a wavelet power spectrum:

– continuous wavelet transform – Morlet wavelet with index 6 investigating time scales from 2dt for 8

octaves

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Wavelet investigation

• Stripe with period at ~20 s.

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20 s


Zoomed wavelet

• There seems to be a typical time scale around 20 s

• Short peaks are highlighted, but do not represent a true periodicity.

• Wavelet are sensitive to shot noise !

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A faster variability

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sec. scale~2 s scale


MHD modelling

• Semi-periodic Flaring αd=0.1

• Not what we observe in IGR J18245

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ms


Wavelet

• A clear periodicity is detected at ~60 s and peaks are highlighted by shorter term power.

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ms


Higher diffusivity

• αd=0.1• With

higher viscosity flaring is less regular

• It is what we observe in IGR J18245

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ms


Wavelet 2

• No clear periodicity.

• Short-time variability at strong peaks

• Longer term variability at ~250 s (windowing problems).

• Irregular.

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ms


Conclusions

• Numerical simulations are a unique tool to study accreting system behavior in terms of realistic physical conditions for various scales of objects

• MRI simulations evidence that in propeller regime both accretion and outflows are present both in weak and strong propeller. Strong propeller (rm/rco~2-3), Mwind>Mstar ; weak propeller (rm/rco~1), Mstar>Mwind

• High magnetic diffusivity plays an important role in smoothing spikes. Observed variability might provide a mean to narrow down the parameter space.

• Wavelet is a powerful tool to investigate the intermittent quasi-periodic signal in light curve. Work in progress to identify possible quasi periodic behavior.

• IGR J18245 has a pronounced variability never observed in aMSP: peculiar of transitional pulsars?

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1Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015 Numerical simulations of...

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