Gravitational Wave Astrophysics, Compact Binaries, and Numerical Relativity

• Joan Centrella

• Gravitational Astrophysics Laboratory

• NASA/GSFC

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…

A Different Type of Astronomical Messenger Gravitational Waves . . .

• 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

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

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) = ΔL/L

• Source waveforms scale as h(t) ~ 1/r

Estimating Gravitational Wave frequencies . . .

• 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

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...

Significant progress in ground-based GW detectors....

• 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!

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

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 Pathfinder’s major milestone, the Preliminary Design Review, is nearly complete. ESA’s LISA Test Package has built and tested engineering models. NASA’s 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.

Gravitational Reference Sensor

Interferometry

LISA / LIGO Relationship

• Complementary observations, different frequency bands

• Different astrophysical sources

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....

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

• M87: giant elliptical galaxy with jet

• Cyg-A: radio source: jet extends ~ 7 x 105 ly

Massive Black Holes....

• Good evidence for “massive dark objects” with masses 106 Msun < M < 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 ~ σ4 – 5

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 > 5 or more

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

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)

IMBH/IMBH binaries….

• IMBHs can form in dense stellar clusters (Miller, Freitag,...)

• stellar dynamics
• collisions
• core collapse of cluster
• runaway  form IMBH

• Can > 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 < 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

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 > 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 > 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....

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….

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 ringdown…and 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

What powers short Gamma-ray bursts?

• Gamma-Ray Bursts (GRBs) come in 2 types: long (> 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 Swift’s γ-ray (blue) & X-ray (red) instruments

• Right: GRB 050724 observed by Swift’s X-ray telescope (red) and the small circles and crosses are from optical, X-ray (Chandra) and radio observations

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

“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.”

• “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.”