Magnetars origin and progenitors with enhanced rotation

S.B. Popov, M.E. Prokhorov

(Sternberg Astronomical Institute)

(astro-ph/0505406)

Abstract

• • 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 13-16 %.

• 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 .

Magnetars in the Galaxy

• 4 SGRs, 8 AXPs, plus candidates, plus radio pulsars with high magnetic fields … • Young objects (about 10 4 yrs).

• Probably about 10% of all NSs.

A question:

Why do all magnetars are isolated?

• 10 % of NSs are expected to be binary. • 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

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

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. • 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 αq , q=M 1 /M 2 <1

We use with

α q α q

=0 (flat distribution, i.e. all variants of mass ratio are equally probable) and =0).

α q

=2 (close masses are more probable, so numbers of NS and BH progenitors are increased in comparison

Results of calculations

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 total number equal to the total number of binaries) the fraction of ``magnetars'‘ is ~13-16%.

• 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.