Making magnetars: isolated from binaries

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Making magnetars isolated from binaries

• Sergei Popov Alexei Bogomazov, Mikhail Prokhorov

(astro-ph/0505406 arXiv0905.3238)
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Magnetars in the Galaxy

• 5 SGRs, gt10 AXPs, plus candidates, plus radio
  pulsars with high magnetic fields
• Young objects (about 104 year).
• At least about 10 of all NSs (or more, as
  transient magnetars can be numerous).

(see a recent review in arXiv0804.0250 )
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Origin of magnetars
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.
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A question

• Why do all magnetars are isolated?

• 5-10 of NSs are expected to be binary (for
  moderate and small kicks)
• 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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Theory of magnetars

• Thompson, Duncan ApJ 408, 194 (1993)
• Entropy-driven convection in young NSs generate
  strong magnetic field
• Twist of magnetic field lines

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Generation of the magnetic field
The mechanism of the magnetic field generation
is still unknown. Turbulent dynamo
a-O dynamo (Duncan,Thompson) a2 dynamo (Bonanno
et al.) or their combination
In any case, initial rotation of a protoNS is the
critical parameter.
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Strong field via flux conservation
There are reasons to suspect that the magnetic
fields of magnetars are not due to any kind of
dynamo mechanism, but just due to
flux conservation

• Study of SNRs with magnetars (Vink and Kuiper
  2006).
• If there was a rapidly rotating magnetar
  then a huge
• energy release is inevitable. No traces of
  such energy
• injections are found.
• There are few examples of massive stars with
  field
• strong enough to produce a magnetars due to
  flux
• conservation (Ferrario and Wickramasinghe
  2006)

Still, these suggestions can be criticized
(Spruit arXiv 0711.3650)
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Magnetic field estimates

• Spin down
• Long spin periods
• Energy to support bursts
• Field to confine a fireball (tails)
• Duration of spikes (alfven waves)
• Direct measurements of magnetic field (cyclotron
  lines)

Ibrahim et al. 2002
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SGR 1806-20 - I

• SGR 1806-20 displayed a gradual increase in the
  level of activity during 2003-2004 (Woods et al
  2004 Mereghetti et al 2005)
• enhanced burst rate
• increased persistent luminosity

Bursts / day (IPN)
20-60 keV flux (INTEGRAL IBIS)
The 2004 December 27 Event
Mereghetti et al 2005
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SGR 1806-20 - II

• Four XMM-Newton observations before the burst
  (the last one on October 5 2004, Mereghetti et al
  2005)
• Pulsations clearly detected in all observations
• ? 5.5x10-10 s/s, higher than the historical
  value
• Blackbody component in addition to an absorbed
  power law (kT 0.79 keV)
• Harder spectra G 1.5 vs. G 2
• The 2-10 keV luminosity almost doubled (LX 1036
  erg/s)

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Twisted Magnetospheres I

• The magnetic field inside a magnetar is wound
  up
• The presence of a toroidal component induces a
  rotation of the surface layers
• The crust tensile strength resists
• A gradual (quasi-plastic ?) deformation of the
  crust
• The external field twists up
• (Thompson, Lyutikov Kulkarni 2002)

Thompson Duncan 2001
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Growing twist
(images from Mereghetti arXiv 0804.0250)
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A Growing Twist in SGR 1806-20 ?

• Evidence for spectral hardening AND enhanced
  spin-down
• G-Pdot and G-L correlations
• Growth of bursting activity
• Possible presence of proton cyclotron line only
  during bursts
  

All these features are consistent with an
increasingly twisted magnetosphere
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Twisted magnetospheres

• Twisted magnetosphere model, within magnetar
  scenario, in general agreement with observations
• Resonant scattering of thermal, surface photons
  produces spectra with right properties
• Many issues need to be investigated further
• Twist of more general external fields
• Detailed models for magnetospheric currents
• More accurate treatment of cross section
  including QED effects and electron recoil (in
  progress)
• 10-100 keV tails up-scattering by
  (ultra)relativistic (e) particles ?
• Create an archive to fit model spectra to
  observations

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What is special about magnetars?
Link with massive stars There are reasons to
suspect that magnetars are connected to massive
stars (astro-ph/0611589). Link to binary
stars There is a hypothesis that magnetars are
formed in close binary systems (astro-ph/0505406,
0905.3238).
AXP in Westerlund 1 most probably hasa very
massive progenitor gt40 Msolar.
The question is still on the list.
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Progenitor mass for a SGR
0910.4859
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Red supergiants in the cluster
Cluster are is between13 and 15 Myr. Studies of
other stars in the cluster confirmthis age
estimate.
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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

Rem Now there are claims (Vink et al.,
Ferrario et al.) that magnetars can be
born slowly rotating, so the field is fossil.
We do not discuss this ideas here.
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The first calculations
An optimistic scenario
We present population synthesis
calculations of binary systems using
optimistic assumptions about spinning up of
stellar cores and further evolution of
their rotation rate. The fraction of neutron
stars born from stellar cores with enhanced
rotation is estimated to be about 8-14 .
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.
MNRAS vol. 367, p. 732 (2006)
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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/
  arXiv 0704/1387)

• 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 without a
  common envelope.
• 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 , qM2/M1lt1
• 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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Results of calculations-details
Most of magnetars appear after coalescences or
from secondary companions after RLO by
primaries. They are mostly isolated.
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Intermediate 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
  8-14.
• 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.

MNRAS vol. 367, p. 732 (2006)
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Problems and questions

• In these calculations we assume that since a star
  obtained additionalangular momentum, then it is
  effectively transferred to the core,and it
  doesnt loose in afterwards.This is too
  optimistic.
• There are three processes (Hirschi et al. 2004,
  2005)
• convection,
• shear diffusion,
• meridional circulation
• which result in slowing down the core rotation.
• Let us consider more conservative scenarios.

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GRBs and magnetars
It is important to remember that a similar
problem necessity of rapid core rotation is
in explanation of GRB progenitors. We
hypothesize that a similar channel is
operatingin binary systems to produce rapidly
rotating pre-SN. If then a BH is born we have a
GRB.If a NS we have a magnetars. The fraction
of magnetars among NSs is similarto the fraction
of GRBs among BH-forming SNae.
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Magnetars, Gamma-ray Bursts, and Very Close
Binaries
We consider the possible existence of a common
channel of evolution of binary systems, which
results in a GRB during the formation of a BH or
the birth of a magnetar during the formation of a
NS. We assume that the rapid rotation of the
core of a collapsing star can be explained by
tidal synchronization in a very close binary. The
calculated rate of formation of rapidly rotating
neutron stars is qualitatively consistent with
estimates of the formation rate of magnetars.
However, our analysis of the binarity of
newly-born compact objects with short rotational
periods indicates that the fraction of binaries
among them substantially exceeds the
observational estimates. To bring this fraction
into agreement with the statistics for magnetars,
the additional velocity acquired by a magnetar
during its formation must be primarily
perpendicular to the orbital plane before the
supernova explosion, and be large.
Astronomy Reports vol. 53, p. 325 (2009)
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Model assumptions
Here we consider only tidal synchronization on
late stages(end of helium burning, or carbon
burning).I.e. a core gets additional momentum
not long before the collapse.This is possible
only in very narrow systems (Porblt10 days).
We used two laws for stellar windA. Standard
windC. Enhanced wind for massive stars
(classification following arXiv 0704.1387)
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Kicks

• In this study we use two variants
• of the velocity absolute value distribution
• maxwellian
• delta-functionWe used different options for
  direction
• isotropic
• along the spin axis (see Kuranov et al. 2009
  MNRAS 395, 2087)

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Different kicks and mass loss

• isotropic kick, type A wind scenario
• isotropic kick, type C wind scenario
• (3) Kick along the spin axis, type A wind
  scenario
• (4) Kick along the spin axis, type C wind scenario

Single maxwelliandistribution
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Delta-function kick

• isotropic kick, type A wind scenario
• isotropic kick, type C wind scenario
• (3) kick along the spin axis, type A wind
  scenario
• (4) Kick along the spin axis, type C wind scenario

Delta-function kick
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Delta-function spin influence
The absolute value of the kick depends on the
initial rotational period of the young neutron
star.Kick always along the spin. 1- type A
wind 2- type C wind
VV0 (0.001/PNS) 0.001ltPNSlt0.005
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Orbital periods
Distribution of orbital periods just before the
collapse in the systems in which neutron stars
originate. If a neutron star originates in a
disrupted system, the orbital period at the time
of disruption is taken into account. The type A
evolutionary scenario is adopted.
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Companions
Most of companions are -main-sequence stars
(49) and -black holes (46). The remaining 5
are roughly equally divided among -white
dwarfs (2), -WolfRayet stars (1), -stars
filling their Roche lobes (0.7), -helium stars
filling their Roche lobes (the BB stage),
-hot white dwarfs (0.7), -neutron stars (0.6).
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Are there magnetars in binaries?
At the moment all known SGRs and AXPs are
isolated objects. About 10 of NSs are expected
to be in binaries. The fact that all known
magnetars are isolated can be relatedto their
origin, but this is unclear.
If a magnetar appears in a very close binary
system, thenan analogue of a polar can be
formed. The secondary star is insidethe huge
magnetosphere of a magnetar. This can lead to
interestingobservational manifestations.
Magnetor
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Binaries with magnetars -magnetors
Can RCW 103 be a prototype? 6.7 hour period (de
Luca et al. 2006)

• Possible explanations
• Magnetar, spun-down by disc
• Double NS system
• Low-mass companion magnetar
• magnetor

arXiv0803.1373(see also astro-ph/0610593)
RCW 103
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Conclusions

• We made population synthesis of binary stars to
  explore the evolution and products of stars
  with enhanced rotation
• In the optimistic scenario we easily explain the
  fraction of magnetars an the fact that they
  are isolated
• In a more conservative scenario we need large
  kicks to explain the fact that all known
  magnetars are isolated
• Without detailed data about spatial velocities
  of magnetars it is difficult to make
  conclusions
• Still, it is possible that the channel for
  magnetar formation is the same as for
  GRB-progenitors formation, most probably in
  close binary systems