MAGNETARS

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MAGNETARS

• Ultra-Strong Magnetic Fields producing Gamma Ray
  flashes and Persistent X-rays.

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Scheme of the Presentation

• Soft Gamma Repeaters (SGR) Bursts Events.
• Phenomenological Burst Theory.
• Magnetic Field in the Magnetar Model.
• Requiring and Making the Magnetic Field Work.
• Year of Magnetar breakthrough 1998.
• Modeling the X-Ray emission.
• Population Information.

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A Fantastic Start

• SGR 180620 (January 7, 1979) 0.25 sec. pulse of
  soft gamma rays measured by Venera spacecraft.
• SGR 052666 (March 5, 1979) Extremely bright
  spike peaking at 1045 ergs-s-1 followed by
  3min. train of coherent 8 sec. pulsation,
  quasi-exponential flux decay.
• SGR 190014 (March, 1979) 3 bursts in two days
  and quiet for nearly two decades.
• These events were considered variants of the
  usual GRBs until intense reactivation of SGR
  1806-20 in 1983.

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Characteristics of SGR

• The emission of Soft Gamma rays (Hard X-rays)
  the repetition of Bursts distinguishes them from
  GRB.
• Energy radiated 107 1012 Lsun though in a
  different band. 103 104 more energetic than
  other galactic burst sources (BH-XRT, XRB, Novae
  etc.).
• Burst activity lasts 1000 yrs. Youngest few
  detected, probably millions undetected in our
  galaxy.
• Supernovae/GRB are one shot events more powerful
  than SGR, mostly extra-galactic.
• March 5, SGR event briefly brighter than a
  Supernova.

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Brightest SGR and its phenomenological
implications.

• The March 5, SGR 052666 burst sent the detectors
  aboard some 5 space-crafts and ISEE (pointed
  away) off-scale. Einstein XRT registered a
  strong signal.
• Hard pulse of Gamma Rays lasting 0.2s, followed
  by 3mins. fainter soft-tail of gamma rays with
  a cycle period of 8.0s observed in many
  detectors.
• Light curve of the March 5th event, as recorded
  by gamma-ray detectors aboard the Venera 12 space
  probe. (From E.P. Mazets et al., 1979, Nature
  282, p. 587.)

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• Burst repeated 18 times with more time between
  consecutive re-activation and fainter than the
  first giant-flare.
• Timing Location information of burst detection
  from various satellites/spacecrafts pointed to a
  source covered by a SNR N49 in the LMC.
• X-ray map of N49 in LMC from ROSAT. White box
  calculated location of the march 5, 1979. The
  point source near the centre of the white box is
  the SGR. ?
• Intrinsic Brightness recalculated to 1012
  Lsun.

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MAGNETAR MAGNETIC FIELD

• Neutron Star magnetic fields.
• From Pulsar observations, the pulsation periods
  show a slow increase over time suggesting
  spin-down.
• Reason for spin down The rotating dipole
  magnetic field emits EM waves as do the charged
  particles following the field lines sending out
  energy and causing spin-down.

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The Creation of Magnetic Field.

• Starting with a very hot neutron star, the dense
  fluid of neutrons roils and churns to help
  carry out heat. These convection currents depend
  on the electron density in this nuclear fluid
  too.
• Ultra dense neutron star fluid - good conductor
  -gt Charged electrons carry electric current,
  trapped magnetic field lines follow convection
  current.
• Dynamo Action Requires a fast rotating star
  with the embedded electric current and dragged
  magnetic fields. Similar to earth and sun. Ideal
  efficiency can generate 1016G.
• On cooling, convection -gt Dynamo stops (t 10-20
  s)

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• Built-up magnetic field can remain trapped in the
  heavy, stratified liquid of neutrons and protons.
• Conclusion Pulsars were neutron stars with
  failed large-scale dynamos. The initial spins
  were not fast enough for magnetic field build-up.
• (Crab Pinitial 10ms not enough for LS dynamo)
• Succeeding Dynamo.
• Neutron stars with gt 200 rps., dynamo action
  buids up magnetic field fast and strong.
• Efficient removal of rotational energy for high
  magnetic fields. Magnetars quickly spin-down to
  low speeds but trap the high B field.

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Magnetar model basics

• Radio pulsars slowly loses its rotational energy
  passively facilitated by magnetic fields.
• Rotational Energy being lesser in a Magnetar,
  Magnetic field can directly cause energy output.
  Stellar material (Crust/interior) pushed around
  by magnetic field releasing energy ( 10000
  yrs.)

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• Ref. Duncan and Thompson, Ap.J. 392, L(9),
  M.N.R.A.S. 275, 255
• A strong magnetic field allows quick spin
  dissipation by magnetic waves, magnetic
  vibrations and exterior field twists.
• Neutrino Rocket-effect causes high proper and
  spin velocities.

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Trapped Fireballs

• Brightness/Hardness of SGR activity Pure
  explosion of energy fireball powered by a
  magnetic flare.
• Negligible baryons but ubiquitous e- e pairs.
  Baryons steal energy for motion.
• Dispensed fireball Residual hot gas of charged
  leptons trapped to strong magnetic field.
• Inside fireball Particles gyrate along B lines,
  gamma/X rays passing across field lines are
  scattered, more pair creation. Photon dissipation
  from fireball surface, photon from annihilation
  and particle bombardment on star surface.

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Fireball Evolution Verification

• Fireball loses energy from outer layer photon
  escape, more annihilation favored causing photon
  emission -gt Shrinking fireball as layers empty
  out.
• Explains the diminishing flux of March5
  light-curve. Fireball rotating with star every
  8sec., Brightness vary according to orientation
  of fireball, steady shrinking lead to dimming.
  Complete evaporation of fireball in about 3 mins.
• Magnetic field needs to be strong enough for
  charge confinement. X-ray measurement of soft
  tail summed over the 3min. duration gives total
  energy in trapped gas. Energy of particles
  indicate required field for confinement.

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Subsequent Progress

• FFWD 1998 Observation of 7.5s pulsation in X-ray
  emission for SGR 1806-20 interpreted as rotation
  of hotspots on the magnetar Calculation of
  field strength from braking 8 x 1014 Gauss.
• Detection of 5.16s pulsation in SGR 190014, B
  5 x 1014G.
• Aug.1998 Giant flaring on SGR 190014 exceeding
  flaring of 1979 in apparent brightness. Saturated
  7 different detectors. Recorded well by KONUS,
  Beppo-SAX, GR detector aboard Ulysses. RXTE
  caught strong signal pointing the other direction.

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August 27, Giant Flare

• Very similar to March 5 giant flaring Initial
  Hard spike followed by soft oscillating tail.
• Regular 5.16 second oscillation.
• A closer source hence bright. Complete
  termination of the flare recorded at 380 seconds
  past its activation -gt Complete Fireball
  evaporation.
• Cooling Hot-Spots on surface to fade away
  shifting to lower energies.

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• Details in the light-curve
• - Emergence of strong four-peaked pattern after
  about 40 seconds.
• Persistence of the four-peak pattern for rest of
  the flare with 5.16 second period.
• Inference Complicated geometry of the magnetic
  field regions near the star surface, exposed
  after the outer, smoother zones have cleared.

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Magnetic Twist induced X-Ray emission

• Compression of crust material impossible by
  Magnetic forces which are feeble compared to
  gravity or pressure. Shearing horizontal
  movements more favorable. Incompressibility from
  degeneracy pressure, shear-ability from weaker
  electrostatic restoring forces of lattice.
• Tendency of Magnetic field towards low energy
  configuration causes strong magnetic forces on
  crust. Drifting magnetic field lines stress
  from interior.
• Result Twisting of Magnetic field lines anchored
  to the crust. Amperes law-current induction,
  electron-positrons along field lines, current
  loops and impacting the photon or star emit
  X-rays as observed in SGR and AXP.

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Population information

• Predictions about Magnetars
• A dozen known SGR and AXP in Milky way in LMC
  with periods 5-12sec. High spin down implies
  younger population lt 10000 yrs old.
• 10 Magnetars younger than 10,000yrs
• gt Formation rate of 1 per 1000 yrs. (probably
  higher)
• From SNR associations, Birthrate 1 per 100
  yrs.
• Even a 1 per 300 yrs brithrate implies a
  population of 30 million magnetars for the 10b
  year old galaxy.

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• Dead Magnetars X-ray dark and burst/flare quiet.
  Not powered by magnetic dissipation.
• Location depends on initial recoil velocities
  drifted to halo for large kicks as in present
  SGRs.
• Theory suggests strong residual magnetism
  helpful for detection of it sweeps up ISM
  material, though requires high velocities. Not
  too hopeful.
• Rare episode of reactivation possible due to
  gradually gathered magnetic instabilities but
  rarity hinders chance of detection.