1. 1 Gravitational Wave Astrophysics, Compact Binaries, and Numerical Relativity Joan Centrella Gravitational Astrophysics Laboratory NASA/GSFC Numerical Relativity 2005: Compact Binaries November 2 – 4, 2005

2. 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 20 th century science.

3. 3 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 h x 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. 4 Characteristic amplitude –r = distance to source –R Sch = 2GM/c 2 –Q = (trace-free) quadrupole moment of source –v = characteristic nonspherical velocity in source Amplitudes of Gravitational Wave Sources... Estimate upper limits: 1.4 M Sun 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 10 6 M sun MBH at r = 3000 Mpc, h ~ 10 -16  Strongest sources have large masses moving with velocities v ~ c

5. 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. 6 Estimating Gravitational Wave frequencies... Natural frequency 1.4 M Sun NS, R = 10 km f o ~ 2 kHz 10 M Sun BH f o ~ 1 kHz 4 x 10 6 M Sun MBH f o ~ 3 mHz Binary orbital frequency –M = M 1 + M 2, M 1 = M 2 –a = separation NS/NS, a = 10 R f GW ~ 200 Hz BH/BH, a = 10 M f GW ~ 100 Hz MBH/MBH, a = 10 M f GW ~ 3 x 10 -4 Hz

7. 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...

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10. 10 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!

11. 11 LISA: Laser Interferometric Space Antenna NASA/ESA collaboration detect low frequency GW 3 spacecraft –equilateral triangle –orbits Sun at 1 AU –20 o behind Earth in its orbit arm length L = 5 x 10 6 km optical transponders receive and re-transmit phase locked light launch ~ 2015 Typical sources: MBH/MBH, Galactic compact binaries, NS/MBH, BH/MBH

12. 12 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.

13. 13 Gravitational Reference Sensor Engineering model of the gravitational reference sensor for LISA Pathfinder

14. 14 Interferometry Engineering model of the interferometer for LISA Pathfinder

15. 15 LISA / LIGO Relationship Complementary observations, different frequency bands Different astrophysical sources

16. 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 ≥ 10 5 M sun –intermediate mass black holes (IMBHs): 10 2 M sun ≤ M ≤ 10 4 M sun –stellar black holes: M ≤ 10 2 M sun 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....

17. 17 Massive Black Holes...first found in active galaxies.. M87: giant elliptical galaxy with jet Cyg-A: radio source: jet extends ~ 7 x 10 5 ly optical (AURA/NRAO/NSF) VLA(top left), HST (top right), VLBI (bottom) (NASA,NRAO/NSF,STScI/JHU, AUI) (NRAO/AUI)

18. 18 Massive Black Holes.... Good evidence for “massive dark objects” with masses 10 6 M sun < M < 10 10 M sun 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) M BH ~ σ 4 – 5 LISA observations of GW from compact objects inspiralling into these objects can falsify the hypothesis that they are actually Kerr BHs

19. 19 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 10 8 yrs  LISA could observe ~ several tens per year, out to redshifts z > 5 or more

20. 20 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 (Merritt & Ekers, Science, 2002) (Image courtesy of NRAO/AUI & Inset: STScI)

21. 21 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 M sun (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.)

22. 22 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 m 1 /m 2 < 10 LISA could see as many as several inspirals per year, for masses in the range M ~ few x 100 M sun – 10 3 M sun Advanced LIGO could see binaries with masses in the range M ~ (10s – 100s)M sun

23. 23 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 M sun (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 M sun  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)

24. 24 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…. (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…

25. 25 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 M 1 ≠ M 2  astrophysics

26. 26 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

27. 27 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)

28. 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

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