Excitation of the Neutron Star fmode in LMXBs

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Excitation of the Neutron Star f-mode in
LMXBs

• J. C. N. de Araujo, O. D. Miranda O. D. Aguiar
• INPE/Brazil

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Outline

• f-mode
• excitation of the f-mode
• LMXBs
• detectability of the Neutron Star f-mode in LMXBs
• conclusions

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f-mode

• Neutron Stars (NSs) present a host of pulsation
  modes. Only a few of them, however, is of
  relevance for GW detection.
• From the GW point of view the most important
  modes are the fundamental (f ) mode of fluid
  oscillation, the first few pressure (p) modes and
  the first GW (w) mode (Kokkotas and Schutz 1992).
• Among these three modes the pulsation energy is
  mostly stored in the f-mode in which the fluid
  parameters undergo the largest changes.
• It is worth mentioning that the r-mode can also
  be, under certain circumstances, a very important
  source of GWs (Andersson et al 2001).

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Kokkotas Andersson gr-qc/0109054
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Kokkotas Andersson gr-qc/0109054
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Excitation of the f-mode

• Through quakes that may be associated with the
  glitch phenomenon (see de Araujo, Miranda
  Aguiar 2005)
• Supernova explosion, which is expected to form a
  wildly pulsating neutron star .
• The neutron star may undergo a phase transition
  leading to a mini-collapse, which could lead to a
  sudden contraction during which part of the
  gravitational binding.
• During the coalescence of two neutron stars
  several oscillation modes could in principle be
  generated. Stellar oscillations being excited by
  the tidal fields of the two stars, for example.
• Fluid accretion onto neutron stars (see Nagar
  Diaz 2004).

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• In particular, for this last mechanism, Nagar
  Diaz performed numerical simulations of the GW
  emitted from neutron stars as a result of
  axisymmetric accretion of layers of perfect fluid
  matter.
• We argue that this result indicates that neutron
  stars in binary systems, in particular those
  which are accreting mass through disks, can also
  have the f-mode excited.
• Although, it is necessary to model such binaries
  taking into account their specific
  characteristics to realistically evaluate the GW
  production efficiency.
• Also, an interesting issue is that accreting
  neutron stars could well be a recurrent source of
  GWs.

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Nagar Diaz (gr-qc/0408041)
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LMXBs

• Among the binary systems containing a neutron
  star are the Low Mass X-ray Binaries (LMXBs).
• These systems are composed by a neutron star (or
  a black hole) and a low mass companion (lt MNS).
• The compact star accretes mass from the companion
  through a disk.
• The LMXBs are are found mostly towards the
  galactic center and in globular clusters.
• There are known 80 LMXBs in the Milk Way.

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? LMXBs ? HMXBs
Grimm, Gilfanov Sunyaev 2001
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• Several authors have shown that gravitational
  radiation can carry away substantial angular
  momentum from accreting neutron stars, driven by
  excitation of the r-mode instability in the
  neutron star core (Wagoner 1984).
• These losses can balance accretion torques for
  the relevant ranges of spin and mass accretion
  rate.
• We argue here that, besides being r-mode
  unstable, the neutron star in LMXBs can have its
  f-mode excited by the matter accreted from its
  companion.
• To evaluate the GW amplitude related to the
  f-mode, however, one needs the distance from the
  LMXBs to the earth, but only a few of them are
  known (see the next transparency).

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Grimm, Gilfanov Sunyaev 2001
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Detectability of the Neutron Star f-mode in LMXBs
The characteristic amplitude for the f-mode is
given by (Andersson Kokkotas 1998)
Considering sources within 10 kpc and a SNR of at
least 5, we calculate the minimum value of ??GW
for a given sensitivity.
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Conclusions

• We argue here that accreting neutron stars in
  LMXBs can become f-mode unstable.
• However, the efficiency of generation of GWs
  needs to be properly evaluated.
• The LMXBs may be a recurrent source of GWs
  through the f-mode instability. An issue to
  consider in this case is the recurrence time.