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Making magnetars:
isolated from binaries
Sergei Popov
Alexei Bogomazov, Mikhail Prokhorov
(astro-ph/0505406; arXiv:0905.3238)
Magnetars in the Galaxy
5 SGRs, >10 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).
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(see a recent review in arXiv:0804.0250 )
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.
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, it’s
time to ask this question.
Two possible explanations
• Large kick velocities
• Particular evolutionary path
Theory of magnetars
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Thompson, Duncan ApJ 408, 194 (1993)
Entropy-driven convection in young NSs
generate strong magnetic field
Twist of magnetic field lines
Generation of the magnetic
field
The mechanism of the magnetic
field generation is still unknown.
Turbulent dynamo
α-Ω dynamo (Duncan,Thompson)
α2 dynamo (Bonanno et al.)
or their combination
In any case, initial rotation of a
protoNS is the critical parameter.
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:
1. 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.
2. 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)
Magnetic field estimates
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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
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)
Bursts / day
(IPN)
 enhanced burst rate
 increased persistent luminosity
20-60 keV flux (INTEGRAL IBIS)
The 2004 December 27 Event
Spring
2003
Autumn
2003
Spring
2004
Autumn
2004
Mereghetti et al 2005
SGR 1806-20 - II
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Four XMM-Newton observations before the
burst (the last one on October 5 2004,
Mereghetti et al 2005)
Pulsations clearly detected in all observations
Ṗ ~ 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: Γ ~ 1.5 vs. Γ ~ 2
The 2-10 keV luminosity almost doubled (LX ~
1036 erg/s)
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
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Thompson & Duncan 2001
(Thompson, Lyutikov & Kulkarni 2002)
Growing twist
(images from Mereghetti arXiv: 0804.0250)
A Growing Twist in SGR 1806-20 ?
Evidence for spectral
hardening AND
enhanced spin-down
 Γ-Pdot and Γ-L
correlations
 Growth of bursting
activity
All these features are
 Possible presence of
consistent
with
an
proton cyclotron line
increasingly twisted
only during bursts
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
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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
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
AXP in Westerlund 1 most probably has
binary systems
a very massive progenitor >40 Msolar.
(astro-ph/0505406,
0905.3238).
The question is still on the list.
Progenitor mass for a SGR
0910.4859
Red supergiants in the cluster
Cluster are is between
13 and 15 Myr.
Studies of other stars
in the cluster confirm
this age estimate.
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.
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)
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.
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.
Parameters
We run the code for two values of the parameter
αq 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 αq , q=M2/M1<1
We use αq=0 (flat distribution, i.e. all variants of mass
ratio are equally probable) and αq=2 (close masses are
more probable, so numbers of NS and BH progenitors
are increased in comparison with αq=0).
Results of calculations
Results of calculations-details
Most of “magnetars” appear after coalescences or
from secondary companions after RLO by primaries.
They are mostly isolated.
Intermediate conclusions
• We made population synthesis of binary systems to derive
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the relative number of NSs originated from progenitors with
enhanced rotation -``magnetars''.
With an inclusion of single stars (with the total
number 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)
Problems and questions
In these calculations we assume that since a star obtained additional
angular momentum, then it is effectively transferred to the core,
and it doesn’t 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.
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 operating
in 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 similar
to the fraction of GRBs among BH-forming SNae.
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
Astronomy Reports vol. 53, p. 325 (2009)
large.
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 (Porb<~10 days).
We used two laws for stellar wind
A. Standard wind
C. Enhanced wind for massive stars
(classification following arXiv: 0704.1387)
Kicks
In this study we use two variants
of the velocity absolute value distribution:
• maxwellian
• delta-function
We used different options for direction:
• isotropic
• along the spin axis (see Kuranov et al. 2009 MNRAS 395, 2087)
Different kicks and mass loss
(1) isotropic kick,
type A wind scenario;
(2) 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 maxwellian
distribution
Delta-function kick
(1) isotropic kick,
type A wind scenario;
(2) 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
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
V=V0 (0.001/PNS)
0.001<PNS<0.005
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.
Companions
Most of companions are
-main-sequence stars (49%) and
-black holes (46%).
The remaining 5% are roughly
equally divided among:
-white dwarfs (2%),
-Wolf–Rayet 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%).
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 related
to their origin, but this is unclear.
If a magnetar appears in a
very close binary system, then
an analogue of a polar can be
formed.
The secondary star is inside
the huge magnetosphere of a
magnetar.
This can lead to interesting
observational manifestations.
Magnetor
Binaries with magnetars -magnetors
Can RCW 103 be a prototype?
6.7 hour period (de Luca et al. 2006)
Possible explanations:
1. Magnetar, spun-down by disc
2. Double NS system
3. Low-mass companion + magnetar=
magnetor
arXiv:0803.1373
(see also astro-ph/0610593)
RCW 103
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