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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
1
2Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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
2
3Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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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4Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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.
5Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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.
5
Illarionov & Sunyaev (1975); Lovelace, Romanova and Bisnovatyi-Kogan (1999)
Illarionov & Sunyaev (1975); Lovelace, Romanova and Bisnovatyi-Kogan (1999)
6Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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
7Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
Scalable simulations
• Adimensional variables. • Scalable to objects with small magnetosphere
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8Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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
8
Utsyugova et al. (2006)
Utsyugova et al. (2006)
Lii et al. (2014)Lii et al. (2014)
9Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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
10Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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
11Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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)
12Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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
13Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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)
15Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
Cyclic accretion
• Matter accumulates and then is accreted in cyclic fashion• Contemporary ejections of material
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Lii et al. (2014)Lii et al. (2014)
16Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
Episodic accretion cycle
• Observed sporadically in outburst of AMSP, but at different time scale and duty cycle
• Lower diffusivity may reconcile the observations.
16
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;
17Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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
18Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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.
Lii et al. (2014)Lii et al. (2014)
20Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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.
20
Papitto+ (2013)Papitto+ (2013)
21Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
IGR J18245/M 28I in accretion phase
• Only a few days after the last X-ray detection !21
Papitto+ (2013)Papitto+ (2013)
22Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
XMM-Newton light curve
• Very interesting variability, unique among AMSP.
• Episodes of enhanced hardness at low flux
• No orbital dependency.
22
Hard 3.5-10 keVSoft 0.5-3.5 keV
Ferrigno + (2014)Ferrigno + (2014)
time Bins >= 200 s
23Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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
24Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
Always pulsed
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26Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
Light curve - Blue state
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• Strong second-scale variability.
• red points are bins of 200 s
27Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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
27
28Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
Wavelet investigation
• Stripe with period at ~20 s.
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20 s
29Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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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30Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
A faster variability
30
sec. scale~2 s scale
31Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
MHD modelling
• Semi-periodic Flaring αd=0.1
• Not what we observe in IGR J18245
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ms
32Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
Wavelet
• A clear periodicity is detected at ~60 s and peaks are highlighted by shorter term power.
32
ms
33Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
Higher diffusivity
• αd=0.1• With
higher viscosity flaring is less regular
• It is what we observe in IGR J18245
33
ms
34Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
Wavelet 2
• No clear periodicity.
• Short-time variability at strong peaks
• Longer term variability at ~250 s (windowing problems).
• Irregular.
34
ms
35Propeller and Variability IGR J18245-2452 - C. FerrignoEWASS 26.06.2015
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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