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 10 4 year).

At least about 10% of all NSs (or more, as transient magnetars can be numerous).

(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

   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) α 2 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

      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)

2003 Autumn 2003 Spring 2004 Autumn 2004 Mereghetti et al 2005

SGR 1806-20 - II

      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 value s/s, higher than the “historical” 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 (L X 10 36 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 (Thompson, Lyutikov & Kulkarni 2002) Thompson & Duncan 2001

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 Possible presence of proton cyclotron line All these features are consistent with an only during bursts

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   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 binary systems (astro-ph/0505406, 0905.3238).

AXP in Westerlund 1 most probably has a very massive progenitor >40 Msolar.

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

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.

We use

α q f(q)~q αq , q=M 2 /M

ratio are equally probable) and

α

are increased in comparison with

q 1 <1

=0 (flat distribution, i.e. all variants of mass =2 (close masses are more probable, so numbers of NS and BH progenitors

α q

=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 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 large.

Astronomy Reports vol. 53, p. 325 (2009)

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 (P orb <~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=V 0 (0.001/P NS ) 0.001

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