Making magnetars: Making magnetars:

isolated from binariesisolated from binaries

Sergei PopovSergei Popov Alexei Bogomazov, Mikhail Alexei Bogomazov, Mikhail

ProkhorovProkhorov(astro-ph/0505406; arXiv:0905.3238)

Magnetars in the GalaxyMagnetars in the Galaxy 5 SGRs, >10 AXPs, plus candidates, plus 5 SGRs, >10 AXPs, plus candidates, plus

radio pulsars with high magnetic fields…radio pulsars with high magnetic fields… Young objects (about 10Young objects (about 1044 year). year). At least about 10% of all NSs (or more, as At least about 10% of all NSs (or more, as

transient magnetars can be numerous).transient magnetars can be numerous).

(see a recent review in arXiv:0804.0250 )

Origin of magnetars: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: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

Generation of the Generation of the magnetic fieldmagnetic fieldThe mechanism of the magnetic field generation is still unknown.

Turbulent dynamo

α-Ω dynamo (Duncan,Thompson) α2 dynamo (Bonanno et al.) or their combination

In any case, initial rotation of aprotoNS is the critical parameter.

Strong field via flux Strong field via flux conservationconservation

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 estimatesMagnetic field estimates

Spin downSpin down Long spin periods Long spin periods Energy to support Energy to support

burstsbursts Field to confine a Field to confine a

fireball (tails)fireball (tails) Duration of spikes Duration of spikes

(alfven waves)(alfven waves) Direct measurements Direct measurements

of magnetic field of magnetic field (cyclotron lines)(cyclotron lines)

Ibrahim et al. 2002

SGR 1806-20 - I 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)

enhanced burst rate increased persistent luminosity

The 2004 December 27 EventSpring2003

Spring2004

Autumn2003 Autumn

2004

Bursts / day (IPN)

20-60 keV flux (INTEGRAL IBIS)

Mereghetti et al 2005

SGR 1806-20 - IISGR 1806-20 - II

Four XMM-Newton observations before the Four XMM-Newton observations before the burst (the last one on October 5 2004, burst (the last one on October 5 2004, Mereghetti et al 2005Mereghetti et al 2005))

Pulsations clearly detected in all observationsPulsations clearly detected in all observations Ṗ ~ 5.5x10Ṗ ~ 5.5x10-10-10 s/s, higher than the “historical” s/s, higher than the “historical”

valuevalue Blackbody component in addition to an Blackbody component in addition to an

absorbed power law (kT ~ 0.79 keV)absorbed power law (kT ~ 0.79 keV) Harder spectra: Harder spectra: ΓΓ ~ 1.5 vs. ~ 1.5 vs. ΓΓ ~ 2~ 2 The 2-10 keV luminosity almost doubled (LThe 2-10 keV luminosity almost doubled (LXX

~ 10~ 103636 erg/s) erg/s)

Twisted Magnetospheres – Twisted Magnetospheres – II

The magnetic field inside a magnetar The magnetic field inside a magnetar is “wound up”is “wound up”

The presence of a toroidal component The presence of a toroidal component induces a rotation of the surface layersinduces a rotation of the surface layers

The crust tensile strength resists The crust tensile strength resists A gradual (quasi-plastic ?) deformation A gradual (quasi-plastic ?) deformation

of the crustof the crust The external field twists up The external field twists up (Thompson, Lyutikov & Kulkarni 2002)(Thompson, Lyutikov & Kulkarni 2002)

Thompson & Duncan 2001

Growing twistGrowing twist

(images from Mereghetti arXiv: 0804.0250)

A Growing Twist in SGR 1806-A Growing Twist in SGR 1806-20 ?20 ?

Evidence for spectral Evidence for spectral hardening AND hardening AND enhanced spin-downenhanced spin-down

ΓΓ-Pdot -Pdot and and ΓΓ-L -L correlationscorrelations

Growth of bursting Growth of bursting activityactivity

Possible presence of Possible presence of proton cyclotron line proton cyclotron line only during bursts only during bursts

All these features are consistent with an increasingly twisted magnetosphere

Twisted magnetospheresTwisted magnetospheres Twisted magnetosphere model, within Twisted magnetosphere model, within

magnetar scenario, in general agreement with magnetar scenario, in general agreement with observationsobservations

Resonant scattering of thermal, surface Resonant scattering of thermal, surface photons produces spectra with right propertiesphotons produces spectra with right properties

Many issues need to be investigated furtherMany issues need to be investigated further Twist of more general external fieldsTwist of more general external fields Detailed models for magnetospheric currentsDetailed models for magnetospheric currents More accurate treatment of cross section including More accurate treatment of cross section including

QED effects and electron recoil (in progress)QED effects and electron recoil (in progress) 10-100 keV tails: up-scattering by (ultra)relativistic 10-100 keV tails: up-scattering by (ultra)relativistic

(e(e±±) particles ?) particles ? Create an archive to fit model spectra to Create an archive to fit model spectra to

observationsobservations

What is special about What is special about magnetarsmagnetars??

Link withLink with massive starsmassive starsThere are reasons to suspect that magnetars are connected to massive stars (astro-ph/0611589).

Link to binary starsThere is a hypothesis that magnetars are formed in close binary systems (astro-ph/0505406, 0905.3238).

The question is still on the list.

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

Progenitor mass for a SGRProgenitor mass for a SGR

0910.4859

Red supergiants in the Red supergiants in the clustercluster

Cluster are is between13 and 15 Myr.

Studies of other stars in the cluster confirmthis age estimate.

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 first calculationsThe first calculations

• 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)

An optimistic scenario

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/ +arXiv:

0704/1387)

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 calculationsResults of calculations

Results of calculations-Results of calculations-detailsdetails

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

They are mostly isolated.

Intermediate conclusionsIntermediate 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 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)

Problems and questionsProblems and questionsIn these calculations we assume that since a star obtained additionalangular 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 circulationwhich result in slowing down the core rotation.

Let us consider more conservative scenarios.

GRBs and magnetarsGRBs 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 operatingin 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 similarto the fraction of GRBs among BH-forming SNae.

Magnetars, Gamma-ray Bursts, and Very Close Binaries

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

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.

Model assumptionsModel 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 (Porb<~10 days).

We used two laws for stellar windA. Standard windC. Enhanced wind for massive stars (classification following arXiv: 0704.1387)

KicksKicksIn 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 Different kicks and mass lossloss

(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 maxwelliandistribution

Delta-function kickDelta-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 Delta-function + spin influenceinfluence

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 wind2- type C wind

V=V0 (0.001/PNS)0.001<PNS<0.005

Orbital periodsOrbital 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.

CompanionsCompanions

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 Are there magnetars in binaries?binaries?

Magnetor

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 relatedto their origin, but this is unclear.

If a magnetar appears in a very close binary system, thenan analogue of a polar can be formed.The secondary star is insidethe huge magnetosphere of a magnetar.This can lead to interestingobservational manifestations.

Binaries with magnetars -Binaries with magnetars -magnetorsmagnetorsCan RCW 103 be a prototype?

6.7 hour period (de Luca et al. 2006)

RCW 103

Possible explanations:1. Magnetar, spun-down by disc2. Double NS system3. Low-mass companion + magnetar= magnetor

arXiv:0803.1373(see also astro-ph/0610593)

ConclusionsConclusions• 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