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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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Thanks!