Two stories from the life of binaries: getting bigger and making magnetars Sergei Popov, Mikhail Prokhorov (SAI MSU) This week SAI celebrates its 175 anniversary IASF, Milano. 14 December 2006

I. 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>2M sun 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 Progenitor mass vs. NS mass

Woosley et al Core mass vs. initial mass

NS+NS binaries Pulsar Pulsar mass Companion mass B B C B J J (PSR+companion)/2 J J J (David Nice 2005)

NS Masses We know several candidates to NS with high masses (M>1.8 M sun ): Vela X-1, M=1.88±0.13 or 2.27±0.17 M sun (Quaintrell et al., 2003) 4U , M=2.4±0.3 M sun (Clark et al., 2002) 2S /V395 Car, M= M sun [1  ] (Shahbaz et al., 2004) J , M= /-0.5M sun (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 M max =2.95M sun for a non-rotating configuration and M max =3.45M sun 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 M sun < Very Massive NS < 3.5 M sun 1.8M sun : Upper limit of Fe-core/young NS according to modeling of supernova explosions (Woosley et al. 2002). ~3.5M sun : Upper limit of rapidly rotating NS with Skyrme EOS (Ouyed 2004).

EvolutionEvolutionEvolutionEvolution 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 binaries was calculated in every Population Synthesis set ~10 4 very massive NS in the Galaxy (formation rate ~ /yr) in the model with kick [ stars and the corresponding formation rate ~ /yr for the zero kick]. State of NSwith kick zero kick Ejector32%39% Propeller+Georotator 2% 8% Accretor66%53%

Results 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. 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: 5-10 % of NSs are expected to be binary (for moderate and small kicks) 5-10 % of NSs are expected to be binary (for moderate and small kicks) All known magnetars (or candidates) are single objects. 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. At the moment from the statistical point of view it is not a miracle, however, it’s time to ask this question. Why do all magnetars are isolated? Two possible explanations Large kick velocities Particular evolutionary path

Theory of magnetars 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. ( ) 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. 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 α q 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 α q, q=M 2 /M 1 <1 We use α q =0 (flat distribution, i.e. all variants of mass ratio are equally probable) and α q =2 (close masses are more probable, so numbers of NS and BH progenitors are increased in comparison with α q =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