N. J. Cornish and E. P. S. Shellard- Formation of very strongly magnetized neutron stars: implications for gamma-ray bursts

8/3/2019 N. J. Cornish and E. P. S. Shellard- Formation of very strongly magnetized neutron stars: implications for gamma-ray bursts

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Magnetar

Recoil ofMagnetar

Soft Gamma Repeater Relation with AXPs

Galactic or Cosmological Burst?

Dynamo in a Naked Neutron Star:Implications for Cosmological GRBs


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Magnetar

A conceivable mechanism for magnetar formation is simple

magnetic flux conservation in the accretion-induced collapse of avery strongly magnetized white dwarf or in the collapse of a strongly

magnetized core of massive star.

Indeed, some white dwarfs with fields approaching ~ billion Gauss

are known, although these stars are isolated rather than accreting.

Magnetars are neutron stars with unusually strong magnetic dipolefields,

14 1510 10dipoleB G


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A neutron star undergose vigorous convection during the first ~ 30s

after its formation. When coupled with rapid rotations, this makesthe star a likely site for dynamo action.

Convective instability sets in when the quantity.

First, the outgoing shock weakens as it dissociated heavy nuclei,

creating a negative radial entropy gradient. Second, the outermost

layers of the star lose entropy and lepton number faster than the

interior, and negative gradients in these quantities are established.

,

l

t P

dYdS Sdr Y dr

V

x x

is negative.


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A key parameter for the success of large scale dynamos

is the Rossby number, defined as the ratio of the rotation

period P to the convective overturn time. In a turbulentfluid with Rossby number of order unity of less, an

efficient dynamo results.

Mixing-length theory implies that the overturn time of a

convective cell is only

The Rossby number is therefore

1 3

391con F msX

01 1R P ms

Near the base of the convection zone.


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During the vigorous, high Rossby number convective

episode which follows the formation of the neutron star.

In the progenitor stars, during low Rossby number, main-

sequence core convection.

The dipole field of an ordinary pulsar born with period much

longer than 1ms can be generated in two ways:


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The Recoil ofMagnetar

The star undergoes some form of anisotropic mass loss. For example,

young magnetar ejects a significant amount of material in an

anisotropic magnetized wind or jet. Even in the absence of mass loss,

off-center magnetic dipole radiation will generate a kick

A second class of recoil mechanism is anisotropic neutrino emission,

which can be induced in a number of ways by a strong magnetic field.

A fractional anisotropy in the radiated scalar momentum,

21400( / 0.16)( / 1 )rocket iV P ms kms

!

0.03p pH would produce a ~ 1000 km s^-1 recoil.


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A strong magnetic field locally suppresses convective energytransport and thus depresses the neutrino flux at the stellar surface,

creating the neutron star analog of sunspots. Dark spots induce arecoil larger than 1000km s^-1 if

A uniform magnetic field can also induce anisotropic neutrinoemission via weak interaction effects. This implies a momentum-loss

asymmetry parallel to the field of:

Most of the kick mechanisms considered above are totally ineffective

for pulsars, becoming important only in the magnetar regime.

2 2 310

coolR t f p pP X H

u

2

152~ 0.02

0.3 5 0.3

e

e

e

e

E p aeB aB

p E MeV

R R R

R

H X X X

X

!


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The Soft Gamma Repeater

On March 5, 1979, 10:51 A.M. EST, a pulse of gamma radiation hit

two Soviet spacecraft, Venera 11 and 12. Later it was detected by

many detectors

The pulse of gamma rays was 100 times as intense as any previous

burst of gamma rays. The hard pulse was followed by a fainter glow oflower-energy, which steadily faded over the subsequent three minutes.

Over the ensuing four years, 16 bursts coming from the same

direction were detected. They varied in intensity, but all were fainter

and shorter than the March 5 burst.

In mid-1980s it was realized that similar outbursts were coming fromtwo other areas of the sky.


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Each spacecraft had recorded the time of arrival of the hard initial

pulse. These data allow astronomers to triangulate the burst source.The position coincided with the Large Magellanic Cloud, 170,000light years away. Its associated with a young supernova remnantwith a age of 5000 years ago.

The source is a million times brighter than the Eddington limit. In 0.2

second the March 1979 event released as much energy as the sunradiates in roughly 10000 years.

The burster, with an rotation period of eight seconds, was spinningmuch more slowly than any radio pulsar that known

Even when not bursting, the object emitted a steady glow of x-rayswith more radiant power than could be supplied by the rotation of aneutron star.


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We can estimate the period derivative from the age of the SN

remnant. Approximating the spindown torque as being due tomagnetic dipole radiation implies, a surface dipole field

The estimated recoil velocity is:

14 1/ 2

46 10 B t Gv

1 2 1 1

4(3 2) (1100 650)transV V t kms ! ! s

Its doubtful that the star could have remained bound in a binarysystem after suffering a recoil this large.


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If the N49 neutron star does not have an unusually strong dipole

field, then it must have been born rotating several hundred timesmore slowly than a typical peculiar propensity to burst.

Note that magnetars receive kicks via a number of mechanismswhich are ineffective for stars with fields and rotation periods

characteristic of ordinary young pulsars, and possibly only a smallfraction receive kicks small enough to remain localized in the disk orin the near halo.

Evidence that the SGRs are in a transient phase of frequent burstactivity is provided by the sequence ofSGR bursts from the N49

source following the 1979 March 5 event, and its apparent cessationin 1983.


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Two otherSGRs, both within

ourMilky Way galaxy, went off

in 1979 and have remained

active, emitting hundreds of

bursts in the year since. Afourth SGR was located in

1998. Three of these four

objects have possible, but

unproved, associations with

young supernova remnants.


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X-ray emission and soft gamma burst

In the case a young magnetar (with age less than a few times 10000

years), its surface is so hot that is glows brightly in X-rays.

The shifting magnetic field outside the star must drive electrical

currents along arched magnetic field lines. The streaming chargedparticles inevitably impart energy to X-ray photons by scattering

against them.

Streaming charged particles also slam against the star when the

reach the footpoints of magnetic field lines, heating patches on thesurface, which glow brightly.


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As the tremendous magnetic field drifts through the solid

crust of the magnetar, it stresses the crust with magneticforces which get stronger than the solid can bear. This

causes shifts in the crust structure, leading to bright

outbursts.

Simultaneously, the magnetic field rearranges itself to astate of lower energy. In the process called magnetic

reconnection, magnetic energy is released.

The accompanying release of magnetic energy creates a

dense cloud of electrons and positrons, as well as a

sudden burst of soft gamma rays.


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It is suggested that the fireball was trapped by the

magnetic field lines and held close to the star. Thetrapped fireball gradually shrank and then evaporated,

emitting x-rays all the while. Greater than 100 trillion

gauss are needed to confine the enormous fireball

pressure.

In 1992, it is noted that x-rays can slip through a cloud of

electrons more easily if the charged particles are

immersed in a very intense magnetic. For the x-rays

during the burst to have been so bright, the magneticfield must have been stronger than 100 trillion Gauss.


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Relation with AXPs

Anomalous X-ray pulsars are spinning down pulsars, with a soft X-

ray spectrum, apparently not powered by accretion from a

companion star, with a luminosity larger than the available rotational

energy loss of a neutron star, and period between 6 and 12s.

The main different between SGRs and AXPs is that AXPs had not

been observed to burst.

Recently, however, bursts from two of the seven known AXPs are

detected.


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Evidence of Long Rotation Periods

X-ray tails of some GRBs exhibit a strong overall trend of

decreasing intensity and spectral hardness, but in many cases the

intensity rises to a second, spectrally soft maximum after ~ 50s.

Note the monotonic decrease in hardness of the three peaks, it is

suggested that the 33s periodicity is due to rotation. Without rotation, one must invoke discrete energy injection events of

progressively diminishing hardness, with the coincidence that the

successive peaks are evenly spaced in time with comparable widths.

It is possible that bursting stars with periods of about 30s are very

old, spun-down pulsars which have retained 3 trillion Gauss dipolefields for nearly a Hubble time; alternatively, they may be young

magnetars with quadrillion Gauss magnetic field with an age of

about 50000 yr.


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Galactic or Cosmological Burst?

Pair annihilation lines with neutron star-like redshifts

Cyclotron lines

Thermal X-ray tails with Galactic distance limits Bounds on recurrence times from archival plates

Complex variability of bursts on millisecond time scales.

Many observations indicate that perhaps some GRBsarise from neutron stars:

Based on the observed recoil and age of the soft

gamma repeater in N49, suggests that there exists a

population of >10000 ofSGRs throughout the

Galactic halo within a radius 100 kpc.


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Magnetars contain a tremendous reservoir of magneticenergy

Models of flarelike bursts involving pair cascades in

subcritical magnetic fields produce reasonable gamma-ray spectra.

Bounds on the anisotropy of GRB sources placesignificant, but perhaps not prohibitive, constraints on anyGalactic halo model for GRB sources

Whether magnetars are responsible for most GRBs ornot, they probably do exist, and they very plausiblyaccount for the SGRs. Thus magnetars could play asupporting role in cosmological GRB scenarios byexplaining the SGRs, which have positions clearlyindicating a Galactic origin.

47 2

153 10 B ergsv


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If all magnetar flares are as bright as the March 5th

event, there should be about a dozen such events listed

in the catalog of GRBs, with considerable uncertainty.

In order to determine how many short-duration GRBs are

magnetar flares, we need to obtain precise positions of

these events on the sky. NASAs Swift satellite,scheduled for launch in December 2003, is designed to

do just that.


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Dynamo in a Naked Neutron Star:

Implications for Cosmological GRBs

AIC provides a promising route to a very strongly manetized neutron

star. It is difficult to avoid a significant amount of baryonic pollution.

The star will lose energy to magnetic torques,

This energy is not quite sufficient to power a gamma-ray burst atcosmological distances, unless the magnetic wind is highly

collimated into a jet.

Shock waves in such a jet are capable of accelerating nonthermal

particles at large distances from the star, and thence generatingsome hard gamma rays via Compton scattering and pion decay.

50 2

1510 B ergs

in the first 10 s.


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Relation with UHECRs

UHECRs cannot travel vast distance because they tend to lose

energy buy scattering against the CMB. This means that very distant

quasars or AGNs cannot be the sources of UHECRs.

Jonathan Arons suggested that newborn magnetars may thesources. With their very strong electric fields that could accelerate

particles to ultra-high energies.

Otherwise the UHECRs would lose too much energy by interacting

with the hot gas of the exploding star. Arons argues that thepowerful wind of particles and radiation blown out from a young

magnetar at nearly the speed of light is fully capable of punching

out.


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Documents

N. J. Cornish and E. P. S. Shellard- Formation of very strongly magnetized neutron stars: implications for gamma-ray bursts