Astrophysics 2: Stellar and Circumstellar Physics

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Astrophysics 2Stellar and Circumstellar Physics
7. X-ray Binaries (3)
http//www.arc.hokkai-s-u.ac.jp/
okazaki/astrophys-2/
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7.5 X-ray outbursts in Be/X-ray binaries

• Most Be/X-ray binaries show only transient X-ray
  activity
• Periodical (or Type I) outbursts, separated by
  the orbital period.
• Giant (or Type II) outbursts, which last longer
  than Type I and show no orbital modulation.

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Type I/II X-ray outbursts(2S 1417-62)
(taken from Bildsten et al. 1997)
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7.6. Accretion disk simulations of Be/X-ray
binaries
7.6.1. Numerical model

• 3D SPH code (Bate et al. 1995)
• Be star and NS as sink particles
• Constant mass ejection from the the Be star
  (unless otherwise noted)
• Isothermal

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7.6.2 Accretion in low eccentricity systems
low e
strong truncation (i.e., large gap between Be
disk and compact object)
low mass-transfer rate
low accretion rate
No Type I X-ray outbursts
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Decretion disk simulation
(Okazaki et al. 2002)
Mass transfer rate
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Accretion disk simualtions
(Hayasaki Okazaki 2004, 2005, 2006)
Accretion disks evolve through three distinct
phases

• Developing phase Evolution towards a Kepelrian
  disk.
• Transition phase Accretion rate increases as
  disk grows.
• Quasi-equilibrium phase accretion rate balanced
  with mass-transfer rate.

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In general, viscous time-scale is much longer
than the orbital period
Persistent accretion disk with orbital modulation
e0.34, i0
(Hayasaki Okazaki 2006)
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Evolution of the accretion rate (e0.34)

• Accretion disk evolves towards a
  quasi-equilibrium state
• Accretion rate too low for Type I's

(Hayasaki Okazaki 2006)
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Evolution of the accretion rate (2)
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Enhanced accretion by an inwardly propagating,
one-armed density wave
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7.6.3 Accretion in high eccentricity systems
high e
weak truncation (i.e., small gap between Be disk
and compact object)
high mass-transfer rate
high accretion rate
Type I X-ray outbursts
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Decretion disk simulation
(Romero et al. 2007)
Mass transfer rate
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Accretion disk simulation
Accretion rate
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7.6.4 Summary on the origin of Type I outbursts

• Low-eccentricity systems are unlikely to show
  regular Type I outbursts. They can show transient
  Type I's when the Be disk is strongly disturbed.
• Highly eccentric systems (with short orbital
  periods) are capable of causing regular Type I
  X-ray outbursts.

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7.7 Origin of Type II X-ray outbursts
Observed characteristics

• Giant ( ) outbursts which
  last much longer than Type Is
• Strong disturbances in the Be disk, e.g.,
• Disk warping before Type II outbursts (4U011563)
• Reduction/growth of the Be disk size
  before/during Type II outbursts (A053526)
• Be disk is often lost after a Type II

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Profile variations in 4U011563
(Negueruela et al. 2001)
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Interpretation
Precessing warped disk
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A Scenario for Type II Outbursts
Formation and growth of the Be disk
Strong disturbances in Be disk

• Radiation driven warping?
• Strong disk elongation by global density waves?

A series of Type I and II outbursts
Loss of Be disk
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Summary on the origin of Type II outbursts

• Long-term, multi-wavelength observations of a few
  Be/X-ray binaries have revealed that strong
  disturbances in Be disks, e.g., disk warping,
  trigger Type II X-ray outbursts (and Type I's
  associated with them).
• The mechanism(s) that causes strong disturbances
  and subsequent large mass supply to NS is unknown.

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7.8 Gamma-ray Binary LS I 61 303

• One of three HMXBs emitting TeV gamma-rays
• A Be/X-ray binary (B0Ve NS or BH
• 26.5 d, e0.72)
• Weak X-rays ( )
• Does accretion occur?
• Cometary tail in VLBA maps
• Jet or colliding wind region?

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Detection of TeV gamma-rays with MAGIC
(Albert et al. 2006)
LS I 61 303
Phase 0.2-0.3
Phase 0.4-0.7
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VLBA observations of LS I 61 303
(Dhawan et al. 2006, astro-ph/0611628)
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3 HMXBs emitting TeV gamma-rays
LS 5039 O6.5V NS or BH, 3.9 d, e0.35. B
125963 B2Ve NS, 3.4 yr, e0.87. LS I 61
303 B0Ve NS or BH, 26.5 d, e0.72.
colliding winds
colliding winds
colliding winds or accretion/ejection?
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No way to explain the radio map with colliding
wind regions
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SPH simulation of colliding winds in LS I 61 303
x-y plane (orbital plane)
x-z plane (y0)
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SPH simulation of the interaction between the Be
disk and the companion
Periastron passage
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Accretion disk simulation
Regular orbital modulation
Shrinking at periastron ( )
Enhanced accretion by density wave
Gradual expansion until next periastron
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Mosaic of Be disk and accretion disk simulations
Periastron passage
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Accretion rate profile
Detection with MAGIC
Optically thick
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Summary of LS I 61 303 sims

• The shape of wind interaction surface doesnt
  agree with the VLBA maps.
• Accretion/ejection scenario is still viable as a
  model for LS I 61 303
• Models with a highly-inclined Be disk are worth
  studying, but they are likely to give much lower
  accretion rates than the coplanar model does.