Gravitational Wave Astrophysics, Compact Binaries, and Numerical Relativity

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Gravitational Wave Astrophysics, Compact
Binaries, and Numerical
Relativity

• Joan Centrella
• Gravitational Astrophysics Laboratory
• NASA/GSFC

Numerical Relativity 2005 Compact Binaries
November 2 4 , 2005
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Most of the information we have about the
Universe so far has come to us in the form of . .
.

• Electromagnetic radiation
• Visible light naked eye observations,optical
  telescopes
• Full electromagnetic spectrum radio, IR, UV,
  visible, X-rays, Gamma-rays
• Particle nuclear astrophysics, neutrinos,
  cosmic rays

These cosmic messengers provide a wealth of
information, making astronomy one of the crowning
glories of 20th century science.
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A Different Type of Astronomical
MessengerGravitational Waves . . .

• Hulse-Taylor binary pulsar PSR 191316
• Orbital period decay agrees with GR to within the
  obs errors of lt 1
• Nobel Prize 1993

• ripples in spacetime curvature
• travel at velocity v c
• generated by matter distributions w/
  time-changing quadrupole moments
• ? carry info about bulk motion
  
  
  
  
  
  of sources
• transverse ? act normal to propagation direction
• 2 polarization states, h and hx
• interact weakly with matter
• ? carry info about deep, hidden regions in the
  universe

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Amplitudes of Gravitational Wave Sources . . .

• Characteristic amplitude
• r distance to source
• RSch 2GM/c2
• Q (trace-free) quadrupole moment of source
• v characteristic nonspherical velocity
  in source

• Estimate upper limits
• 1.4 MSun NS at
• r 15 kpc, h 10-17
• r 15 Mpc, h 10-20
• r 200 Mpc, h 10-21
• r 3000 Mpc, h 10-22
• 4 x 106 Msun MBH at
• r 3000 Mpc, h 10-16

• Strongest sources have
• large masses moving
• with velocities v c

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Detecting gravitational waves. . .

• Resonant mass detectors, laser interferometers
• Detector of length scale L
• A passing gravitational wave causes distortion of
  detector that produces a strain amplitude h(t)
  ?L/L
• Source waveforms scale as h(t) 1/r

(graphic courtesy of B. Barish, LIGO-Caltech)
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Estimating Gravitational Wave frequencies . . .

• Binary orbital frequency
• M M1 M2, M1 M2
• a separation
• NS/NS, a 10 R
• fGW 200 Hz
• BH/BH, a 10 M
• fGW 100 Hz
• MBH/MBH, a 10 M
• fGW 3 x 10-4 Hz

• Natural frequency
• 1.4 MSun NS, R 10 km
• fo 2 kHz
• 10 MSun BH
• fo 1 kHz
• 4 x 106 MSun MBH
• fo 3 mHz

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Ground-based interferometers . . .

• detect high frequency GW
• broad band
• kilometer-scale arms
• Current projects
• LIGO Hanford, WA, and Livingston, LA L 4
  km
• VIRGO France/Italy, near
  Pisa L 3 km
• GEO600 Germany/Britain, Hanover L 600 m
• Typical sources NS/NS, NS/BH, BH/BH, stellar
  collapse, LMXBs...

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• Significant progress in ground-based GW
  detectors....
• LIGO
• has a set of running detectors
• data analysis process has matured
• the main initial LIGO science run S5
• to take 1 full year of integrated data
• set to begin later this year
• reorganization of LIGO lab and LSC into a single
  LIGO
• Advanced LIGO upgrade
• showing good technical progress
• optimistic about starting funding from NSF in
  2008
• VIRGO, GEO600
• also progressing
• ? the age of GW observations is beginning in
  earnest!

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LISA Laser Interferometric Space Antenna

• NASA/ESA collaboration
• detect low frequency GW
• 3 spacecraft
• equilateral triangle
• orbits Sun at 1 AU
• 20o behind Earth in its orbit
• arm length L 5 x 106 km
• optical transponders receive and re-transmit
  phase locked light
• launch 2015
• Typical sources MBH/MBH, Galactic compact
  binaries, NS/MBH, BH/MBH

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Recent LISA Accomplishments

• The LISA Project has been in the Formulation
  Phase one year.
• ESA has engaged a contractor for formulation
  studies. The Architecture Definition Phase of
  that contract is complete.
• The LISA Project team has mapped out 35 design
  studies, 13 are done, 5 are ongoing, and the rest
  to be finished by Apr. 06.
• LISA Pathfinders major milestone, the
  Preliminary Design Review, is nearly complete.
  ESAs LISA Test Package has built and tested
  engineering models. NASAs ST-7 has built and
  tested engineering models.
• Ground-based technology development is
  progressing on microthrusters, phasemeter,
  lasers, etc.
• LISA data analysis planning has started both in
  the U.S. and Europe.

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Gravitational Reference Sensor
Engineering model of the gravitational reference
sensor for LISA Pathfinder
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Interferometry
Engineering model of the interferometer for LISA
Pathfinder
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LISA / LIGO Relationship

• Complementary observations, different frequency
  bands
• Different astrophysical sources

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Astrophysical black holes....

• Black holes are formed throughout the universe as
  the extreme end states of collapse, accretion,
  mergers....
• There is good evidence for BHs in 3 mass ranges
• massive black holes (MBHs) M 105 Msun
• intermediate mass black holes (IMBHs) 102 Msun
  M 104 Msun
• stellar black holes M 102 Msun
• BHs are powerful cosmic engines, heating and
  accelerating gas and particles to produce
  impressive displays of electromagnetic energy...
• When occuring in a binary, BHs are also
  prodigious sources of gravitational waves....

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Massive Black Holes...first found in active
galaxies..

• M87 giant elliptical galaxy with jet
• Cyg-A radio source jet extends
  7 x 105 ly

VLA(top left), HST (top right), VLBI (bottom)
(NASA,NRAO/NSF,STScI/JHU, AUI)
optical (AURA/NRAO/NSF)
(NRAO/AUI)
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Massive Black Holes....

• Good evidence for massive dark objects with
  masses 106 Msun lt M lt
  1010 Msun at centers of few dozen galaxies
• Based on dynamical models, the case for these
  massive dark objects being MBHs is tight for 3
  galaxies...
• MBH masses correlate with bulge luminosity (left)
  and velocity dispersion (right)
  
  (Ferrarase Ford 2005)
• MBH s4 5

• LISA observations of GW from compact objects
  inspiralling into these objects can falsify the
  hypothesis that they are actually Kerr BHs

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MBH/MBH binaries.

• MBHs at the centers of most, if not all, galaxies
• Most galaxies undergo at least one merger
• ? MBH binaries
• Coalescence of MBH binary depends on stellar
  effects, gas, feedback....
• Chandra X-ray observatory found the first known
  system of 2 MBH starting to merge in the galaxy
  NGC 6240
• distance 120 Mpc ? close!
• BHs will merge in few x 108 yrs
• LISA could observe several tens per year, out
  to redshifts z gt 5 or more

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Evidence for MBH mergers....

• Jets emanating from centers of active galaxies
• believed to result from accretion onto central
  MBH
• jet directed along spin axis
• Mergers of spinning BHs can change orientation of
  BH spin axis
• ? sudden flip in jet direction
• X-type radio sources may be
  signature
  of MBH merger

(Image courtesy of NRAO/AUI Inset STScI)
(Merritt Ekers, Science, 2002)
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IMBHs....X-ray sources in dense stellar clusters

• M82 active star-forming galaxy ?
• many young, dense stellar clusters luminous
  X-ray sources (ULXs)
• associate cluster MGG 1 w/ ULX M82 X-1 (near
  center of image)
• Identify this w/ IMBH of mass M 350 Msun
  (Portegies Zwart, et al)

• M74 Optical image w/ Chandra X-ray image
  overlaid
• Sc spiral galaxy with ULX
• ULX is IMBH candidate

(Optical NOAO/AURA/NSF/T.Boroson X-ray
NASA/CXC/U. of Michigan/J.Liu et al.)
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IMBH/IMBH binaries.

• IMBHs can form in dense stellar clusters (Miller,
  Freitag,...)
• stellar dynamics
• collisions
• core collapse of cluster
• runaway ? form IMBH
• Can gt 1 IMBH form in a stellar cluster?
• recent simulations by John Fregeau and
  collaborators find multiple sites for runaway to
  occur in clusters
• ? multiple IMBHs form, with comparable masses
  m1/m2 lt 10
• LISA could see as many as several inspirals per
  year, for masses in the range M few x 100 Msun
  103 Msun
• Advanced LIGO could see binaries with masses in
  the range M (10s 100s)Msun

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Stellar Black Holes.

• Form as the end result of massive star evolution
• Type II supernova
• collapse of iron core in highly evolved massive
  star
• outer regions blasted away in supernova explosion
• core collapses to BH if mass of remnant core M gt
  3 Msun (maximum mass of NS)
• Evidence for BH strongest in low mass X-ray
  binaries (LMXBs)
• interacting binary systems with compact object
  and companion star
• accretion of material from companion onto compact
  object ? X-rays
• in 17 cases, compact object has
  mass M
  gt 3 Msun ? BH (Orosz)
• BH/BH binary
• forms if companion evolves to BH w/out
  disrupting binary
• no gas ? no EM emission
• but...detectable by GWs
• Source for ground based detectors....

(Ihle 2004)
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Final coalescence of BH binary proceeds in 3
stages . . .

• GW produced in all three phases of this evolution
  . . .
• Waveforms and dynamics scale with BH masses and
  spins
• ? source modeling applicable to
• stellar BHs, IMBHs MBHs.

strong-field spacetime dynamics, spin flips and
couplings
measure masses and spins of binary BHs
detect normal modes of ringdown to identify final
Kerr BH
(graphic courtesy of Kip Thorne)
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Focus on the merger stage

• Inspiral lasts until last stable orbit (LSO)
  ...then BHs leave quasi-static orbits
  and plunge together
• Need to evolve BH binary for few orbits near
  the LSO at the end of the inspiral,
  through merger and ringdownand extract the GW
  signature
• Expect several cycles of gravitational
  radiation from merger
• burst waveform, observable by LISA for
  minutes hours
• Strong, highly nonlinear, dynamical gravitational
  fields
• Importance of astrophysical initial data...
• Requires numerical solution of full Einstein eqs
  in 3-D time
• Merger can be phenomenologically rich
• effects of spin spin-spin and spin-orbit
  couplings, spin flips
• test of GR in the dynamical, nonlinear regime
• possible ejection of final BH for M1 ? M2 ?
  astrophysics

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What powers short Gamma-ray bursts?

• Gamma-Ray Bursts (GRBs) come in 2 types long (gt
  2 sec) and short
• The burst is followed by a fainter, longer lived
  afterglow
• By observing their afterglows, long GRBs are
  associated with the collapse of young, massive
  stellar cores
• Recent observations by HETE Swift allowed fast
  and precise localization of X-ray afterglows of
  some short GRBs
• Left GRB 050509b observed by Swifts ?-ray
  (blue) X-ray (red) instruments
• Right GRB 050724 observed by Swifts X-ray
  telescope (red) and the small circles and crosses
  are from optical, X-ray (Chandra) and radio
  observations

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Short GRBS....NS/NS and NS/BH mergers?

• These observations of short GRBs are consistent
  with models of NS/NS or NS/BH mergers
• Such events would also produce GWs that could be
  detectable by ground-based detectors such as LIGO
• Can tell us about the populations of such
  compact binaries, and the GRB mechanisms
• Will look for coincidences between Swift and
  HETE events and possible GW signals during the
  upcoming S5 science run

(Nature)
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Gravitational Waves . . . a new kind of cosmic
messenger

• Every time you build new tools to see the
  universe, new universes are discovered. Through
  the ages, we see the power of penetrating into
  space.

-- David H. DeVorkin (paraphrasing Sir William
Herschel)
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