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Two stories from the life of binaries:
getting bigger and making magnetars
Sergei Popov, Mikhail Prokhorov
(SAI MSU)
IASF, Milano. 14 December 2006
This week SAI
celebrates its
175 anniversary
I. Getting bigger
We use a population synthesis code to estimate
numbers of very massive neutron stars on different
evolutionary stages.
A neutron star increases its mass by accretion from
a secondary companion. Significant growth of a
neutron star mass due to accretion is possible only for
certain values of initial parameters of the binary.
Here we show that significant part of massive
neutron stars with M>2Msun can be observed as
millisecond radio pulsars, as X-ray sources in pair with
white dwarfs, and as accreting neutron stars with very
low magnetic fields.
A&A vol. 434, p. 649 (2005)
Progenitor mass vs. NS mass
Woosley et al. 2002
Core mass vs. initial mass
Woosley et al. 2002
NS+NS binaries
Pulsar
Pulsar mass
B1913+16
B2127+11C
B1534+12
J0737-3039
J1756-2251
Companion mass
1.44
1.35
1.33
1.34
1.40
1.39
1.36
1.35
1.25
1.18
(PSR+companion)/2
J1518+4904
J1811-1736
J1829+2456
(David Nice 2005)
1.35
1.30
1.25
NS Masses
We know several candidates to NS with high masses (M>1.8 Msun):
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Vela X-1, M=1.88±0.13 or 2.27±0.17 Msun (Quaintrell et al., 2003)
4U 1700-37, M=2.4±0.3 Msun (Clark et al., 2002)
2S 0921-630/V395 Car, M=2.0-4.3 Msun [1] (Shahbaz et al., 2004)
J0751+1807, M=2.1+0.4/-0.5Msun(Nice,Splaver,2004) binary radiopulsar!
In 1999 Ouyed and Butler discussed an EOS based on the model by
(Skyrme 1962). A NS with such EOS has Mmax=2.95Msun for a non-rotating
configuration and Mmax=3.45Msun for extreme rotation. This model defines the
upper mass limit for our study.
We will discuss formation of very massive NS due to accretion processes in
binary systems.
What is «Very Massive NS» ?
1.8 Msun < Very Massive NS < 3.5 Msun
• 1.8Msun:
Upper limit of Fe-core/young NS according to modeling of
supernova explosions (Woosley et al. 2002).
• ~3.5Msun: Upper limit of rapidly rotating NS with Skyrme EOS (Ouyed 2004).
E
v
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l
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t
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For our calculations we use
the “Scenario Machine’’ code
developed at the SAI.
Description of most of parameters
of the code can be found in
(Lipunov,Postnov,Prokhorov 1996)
Results
1 000 000 binaries was calculated in every
Population Synthesis set
~104 very massive NS in the Galaxy
(formation rate ~6.7 10-7 1/yr)
in the model with kick
[6 104 stars and the corresponding formation
rate ~4 10-6 1/yr for the zero kick].
State of NS
Ejector
Propeller+Georotator
Accretor
with
kick
zero
kick
32%
39%
2%
8%
66%
53%
Results II
Mass distribution of very massive NS
Luminosity distribution of
accreting very massive NS
Dashed line: Zero natal kick of NS ( just for illustration).
Solid line:
Bimodal kick similar to (Arzoumanian et al. 2002).
Conclusions-I.
• Masses of compact objects can be increased up to >3 solar masses
due to accretion in a binary system
• Most massive NS are expected in systems with WD donors
• It can be difficult to find such NSs as radio pulsars if
the magnetic field significantly decays
A&A vol. 434, p. 649 (2005)
II. 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.
• The fraction of such neutron stars in our calculations
is 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)
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
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 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.
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-1
Results of calculations-2
Most of “magnetars” appear after coalescences or
from secondary companions after RLO by primaries.
They are mostly isolated.
Conclusions.II.
• 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 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)
Conclusions
It is possible to make very massive NS
Most massive NSs are expected to be
accreting from WDs
 If rapid rotation is necessary for magnetar
formation, then they can be products of
binary evolution
 Discovery of magnetars in binary
systems can have strong impact on
models of their formation