Magnetars origin and progenitors with enhanced rotation

1
Magnetars origin and progenitors with enhanced
rotation

• S.B. Popov, M.E. Prokhorov
• (Sternberg Astronomical Institute)

(astro-ph/0505406)
2
Abstract

• We present population synthesis
  calculations of binary systems.
• Our goal is to estimate the number of
  neutron stars originated from progenitors with
  enhanced rotation, as such compact objects can be
  expected to have large magnetic fields, i.e. they
  can be magnetars.
• The fraction of such neutron stars in our
  calculations is about 13-16 .
• Most of these objects are isolated due to
  coalescences of components prior to a neutron
  star formation, or due to a system disruption
  after a supernova explosion.
• The fraction of such neutron stars in
  survived binaries is about 1 or lower, i.e.
  magnetars are expected to be isolated objects.
  Their most numerous companions
  are black holes.

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Magnetars in the Galaxy

• 4 SGRs, 8 AXPs, plus candidates, plus radio
  pulsars with high magnetic fields
• Young objects (about 104 yrs).
• Probably about 10 of all NSs.

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A question

• Why do all magnetars are isolated?

• 10 of NSs are expected to be binary.
• All known magnetars (or candidates) are
  single objects.
• At the moment from the statistical point of
  view it is not a miracle, however, its time to
  ask this question.

Two possible explanations Large kick
velocities Particular evolutionary path
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Magnetars origin

• Probably, magnetars are isolated due to
  their origin
• Fast rotation is necessary (Thompson,
  Duncan)
• Two possibilities to spin-up during evolution
  in a binary
• 1) Spin-up of a progenitor star in a binary
  via accretion or synchronization
• 2) Coalescence

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The code

• We use the Scenario Machine code.
• Developed in SAI (Moscow) since 1983
• by Lipunov, Postnov, Prokhorov et al.
• (http//xray.sai.msu.ru/mystery/articles/review/
  )

• We run the population synthesis of binaries to
  estimate the fraction of NS progenitors with
  enhanced rotation.

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The model

• Among all possible evolutionary paths that
  result in formation of NSs we select those that
  lead to angular momentum increase of progenitors.
• Coalescence prior to a NS formation.
• Roche lobe overflow by a primary.
• Roche lobe overflow by a primary with a common
  envelope.
• Roche lobe overflow by a secondary without a
  common envelope.
• Roche lobe overflow by a secondary with a
  common envelope.

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Parameters

• We run the code for two values of the
  parameter aq which characterizes the mass ratio
  distribution of components, f(q), where q is the
  mass ratio.
• At first, the mass of a primary is taken from
  the Salpeter distribution, and then the q
  distribution is applied. f(q)q
  aq , qM1/M2lt1
• We use aq0 (flat distribution, i.e. all
  variants of mass ratio are equally probable) and
  aq2 (close masses are more probable, so numbers
  of NS and BH progenitors are increased in
  comparison with aq0).

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Results of calculations
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Coalescence of helium stars
Fryer and Heger (2005) suggested a scenario in
which a GRB progenitor is formed after a
coalescence of two helium stars. We estimate the
rate of BH formation after a coalescence of
two helium stars as
10-6 yr-1 for aq 0
5 10-6 yr-1 for aq 2 It is too low to
explain the rate of GRB
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Conclusions

• We made population synthesis of binary
  systems to derive the relative number of NSs
  originated from progenitors with enhanced
  rotation -magnetars''.
• With an inclusion of single stars (with the
  totalnumber equal to the total number of
  binaries) the fraction of magnetars' is
  13-16.
• Most of these NSs are isolated due to
  coalescences of components prior to NS
  formation, or due to a system disruption after a
  SN explosion.
• The fraction of magnetars'' in survived
  binaries is about 1 or lower.
• The most numerous companions of magnetars''
  are BHs.