Two stories from the life of Two stories from the life of binaries: getting bigger and binaries: getting bigger and

making magnetarsmaking magnetars

Sergei Popov, Mikhail Sergei Popov, Mikhail ProkhorovProkhorov

(SAI MSU)(SAI MSU)

This week SAIcelebrates its 175 anniversary

IASF, Milano. 14 December 2006

I. Getting biggerI. Getting bigger

A&A vol. 434, p. 649 (2005)

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.

Woosley et al. 2002

Progenitor mass vs. NS Progenitor mass vs. NS massmass

Woosley et al. 2002

Core mass vs. initial massCore mass vs. initial mass

NS+NS binariesNS+NS binariesPulsar Pulsar mass Companion mass

B1913+16 1.44 1.39B2127+11C 1.35 1.36B1534+12 1.33 1.35J0737-3039 1.34 1.25J1756-2251 1.40 1.18

(PSR+companion)/2

J1518+4904 1.35J1811-1736 1.30J1829+2456 1.25

(David Nice 2005)

NS MassesNS Masses

We know several candidates to NS with high masses (M>1.8 Msun):

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 What is «Very Massive «Very Massive NS» 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).

EEvvoolluuttiioonn

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)

ResultsResults

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 with kick

zero kick

Ejector 32% 39%

Propeller+Georotator

2% 8%

Accretor 66% 53%

Results IIResults II

Mass distribution of very massive NS

Dashed line: Zero natal kick of NS ( just for illustration).Solid line: Bimodal kick similar to (Arzoumanian et al. 2002).

Luminosity distribution of accreting very massive

NS

Conclusions-I.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: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:A question:

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

• • All known magnetars (or All known magnetars (or candidates) are single candidates) are single objects.objects.

• • At the moment from the At the moment from the statistical point of view it statistical point of view it is not a miracle, however, is not a miracle, however, it’s time to ask this it’s time to ask this question.question.

Why do all magnetars are isolated?

Two possible explanations

• Large kick velocities

• Particular evolutionary path

Theory of magnetarsTheory of magnetars Thompson, Duncan ApJ 408, 194

(1993) Entropy-driven convection in young

NSs generate strong magnetic field Twist of magnetic field lines

Magnetars originMagnetars 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 codeThe code

We use the “Scenario Machine” code.Developed in SAI (Moscow) since 1983by Lipunov, Postnov, Prokhorov et al.(http://xray.sai.msu.ru/~mystery/articles/review/(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 modelThe model Among all possible evolutionary paths that result in Among all possible evolutionary paths that result in

formation of NSs we select those that lead to angular formation of NSs we select those that lead to angular momentum increase of progenitors.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.

ParametersParameters 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-1Results of calculations-1

Results of calculations-2Results of calculations-2

Most of “magnetars” appear after coalescences or from secondary companions after RLO by primaries.

They are mostly isolated.

Conclusions.II.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 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)

ConclusionsConclusions

It is possible to make very massive NSMost 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