Accreting neutron star spins and the equation of state

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Accreting neutron star spins and the equation of
state

• Duncan Galloway
• Monash University
• Deepto Chakrabarty
• Center for Space Research, MIT
• Andrew Cumming
• McGill University, Canada

MAD 07, November 2007
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What is a neutron star made of?

• Theory predicts in detail the likely structure in
  outer layers
• Crust (1-2km) nuclei (mainly 56Fe) plus neutrons
  (1S0 superfluid?)
• Composition of inner outer cores (up to 99 of
  mass) however EXTREMELY uncertain Hyperons?
  Pions? Kaons? Quark matter?
• How can we find out?
• (Figure courtesy D. Page)

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Constraining the equation of state

• Basically we want a simultaneous mass and radius
  measurement from a neutron star
• This can be achieved by
• Detection of rapidly-rotating neutron stars (in
  LMXBs or elsewhere)
• Measurements of the neutron-star radius from the
  spectrum of thermonuclear (type-I) burst tails
• Measurement of the Eddington limit from
  radius-expansion bursts
• Detecting discrete spectral features from the
  neutron star surface
• Measurement of the heat flux from the crust via
  thermonuclear burst recurrence times energetics

?
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Low-mass X-ray binaries
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(Some) present-day instruments
RXTE, launched 1995 (NASA) large effective area
and very high timing resolution but no imaging
capability 2-200 keV Chandra, launched 1999
(NASA), small effective area but very high
spatial and spectral resolution (courtesy
transmission gratings) 0.5-10 keV XMM-Newton,
launched 1999 (ESA), moderate effective area,
spatial and spectral resolution (reflection
gratings) optical monitor 0.5-10
keV INTEGRAL, launched 2002 (ESA), primarily
gamma-ray instrument but also wide-field X-ray
and optical capability 4 keV - 10 MeV
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Constraints via rapid spin

• Allowed region is to the left of the dashed
  curves
• While the robust maximum spin rate is still 716
  Hz
• a neutron star spinning at 1122 Hz or faster
  would rule out many EOSs

Lattimer Prakash 2007

• So its not enough to find neutron stars spinning
  above the present maximum we really want to be
  well above 1000 Hz
• Given the present limit of 716 Hz, this seems
  unlikely

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Gravitational wave observatories

• There are currently 4 interferometric
  gravitational wave observatories operational
  worldwide
• Candidate sources include binary inspirals,
  stochastic background and rapidly rotating
  neutron stars
• Hotly anticipated first detection of
  gravitational waves, perhaps within the next
  decade!
• http//www.ligo.caltech.edu

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Examples of X-ray bursts from RXTE
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Constraints from thermonuclear bursts

• We can measure the radius of the neutron star
  from the (blackbody) emission in the burst tail
• Burst recurrence time depends on the heat flux
  from the interior -gt composition
• We have lots of good quality (archival) data!
  (RXTE catalog, MINBAR project)

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For more high-energy fun

• Monthly High-Energy Astrophysics Teleconference
  (HEAT) http//users.monash.edu.au/dgallow/heat
  email Stephen Ng (ncy_at_physics.usyd.edu.au) to
  join the mailing list
• High-energy Astrophysics Workshop, Kialoa, NSW,
  April 20-22 details TBA
• http//www.physics.monash.edu.au/research/astronom
  y research areas honours/PhD projects
• http//users.monash.edu.au/dgallow