1

Gravitational Wave Astrophysics, Compact Binaries,

and Numerical Relativity

Joan Centrella

Gravitational Astrophysics LaboratoryNASA/GSFC

Numerical Relativity 2005: Compact Binaries November 2 – 4 , 2005

2

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.

3A 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

• Hulse-Taylor binary pulsar PSR 1913+16– Orbital period decay agrees

with GR to within the obs errors of < 1%

– Nobel Prize 1993

4

• Characteristic amplitude

– r = distance to source– RSch = 2GM/c2

– Q = (trace-free) quadrupole moment of source

– v = characteristic nonspherical velocity in source

Amplitudes of Gravitational Wave Sources . . .

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

2

2

4 ~~cv

rR

rQ

cGh Sch

Strongest sources have large masses moving with velocities v ~ c

5

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

(graphic courtesy of B. Barish, LIGO-Caltech)

6

Estimating Gravitational Wave frequencies . . .

• Natural frequency

• 1.4 MSun NS, R = 10 kmfo ~ 2 kHz

• 10 MSun BHfo ~ 1 kHz

• 4 x 106 MSun MBHfo ~ 3 mHz

• 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

2/1

3

2/1

43~

4~

RGMGfo

2/1

3orbGW12

aGMff

7

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...

Hz10 Hz 10 4GW f

8

9

10Significant 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!

11LISA: 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

Hz1 Hz 10 GW4 f

12Recent 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.

13Gravitational Reference Sensor

Engineering model of the gravitational reference sensor for LISA Pathfinder

14

Interferometry

Engineering model of the interferometer for LISA Pathfinder

15

LISA / LIGO Relationship

• Complementary observations, different frequency bands• Different astrophysical sources

16

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....

17Massive Black Holes...first found in active galaxies..

• M87: giant elliptical galaxy with jet

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

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

(NRAO/AUI)

18Massive 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

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

19MBH/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

20Evidence 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

(Merritt & Ekers, Science, 2002)(Image courtesy of NRAO/AUI & Inset: STScI)

21IMBHs....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.)

22IMBH/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

23Stellar 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....(Ihle 2004)

24Final 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….

(graphic courtesy of Kip Thorne)

measure masses

and spins of binary

BHs

detect normal modes of

ringdown to identify final

Kerr BH

strong-field spacetime dynamics, spin flips

and couplings…

25Focus 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

26What 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

27Short 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)

28

“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)

Gravitational Waves . . .a new kind of cosmic messenger

29