Data analysis challenges in gravitational-wave astronomy Éric Chassande-Mottin* for the LIGO Scientific Collaboration and the Virgo Collaboration * CNRS, AstroParticule et Cosmologie, Paris France
Outline Gravitational waves Direct detection of GW with large-scale interferometers Searches for GW transients and related data analysis challenges Multimessenger astronomy with GW Future detectors Conclusions
Gravitational waves GW ● Propagating distorsions of space-time metric ● Predicted by General Relativity ● Propagate at the speed of light ● Transverse and quadrupolar (in far field) ● Two polarizations (+ and x) ● Dimensionless strain amplitude h ● Sources of GW ● Produced by accelerated mass ● Rapid changes in shape and orientation of massive objects ● Large mass and density, relativistic plus cross motion → astrophysical sources
Indirect evidence ● Binary pulsar PSR B1913+16 Orbital decay → energy loss due to GW ● In agreement with GR to ~0.2 % ● Hulse & Taylor's Nobel prize ● Binary orbit will continue to decay over 300 millions years until coalescence ● GWs from binary systems Estimate with quadrupole formula ● For a binary close to coalescence ● R=20 km, M=1.4 M sun , f=400 Hz, d=15 Mpc
Direct detection of GW ● Michelson interferometer ● test mass displacement due to GW→ phase shift measurement ● Sees mixture of both polarizations ● Large aperture: not directional ● more like a ear than an eye! ● 1D time series (not a 2D image)
Sensitivity of interferometric GW detectors ● High-precision metrology ● Measurement limitations Fundamental sensing and ● displacement noises “Technical” noises (controls, ● electronics, acoustic, etc.) ● Observable freq. band From few 10 Hz to few kHz ●
Detector highlights Long folded arms using Fabry- Long folded arms using Fabry- Long folded arms using Fabry- Long folded arms using Fabry- Perot cavities Perot cavities Perot cavities Perot cavities Suspended instrument Suspended instrument Suspended instrument Suspended instrument High-Q material High-Q material High-Q material High-Q material Ultra-high vaccum Ultra-high vaccum Ultra-high vaccum Ultra-high vaccum High-power stabilized laser High-power stabilized laser High-power stabilized laser High-power stabilized laser 3 km
Worldwide network of GW detectors GEO 600 Germany LIGO US Virgo Italy Since 2007, partnership and data exchange agreement
Sources of gravitational waves We will be interested in transient sources in this presentation Data analysis is challenging for the other GW sources too!
Sources of gravitational wave transients ● Catastrophic astrophysical events the “violent Universe” ● Efficient production of GWs compact objects: neutron stars (NS) ● or black holes (BH) bulk motion at relativistic velocities ● Some degree of asymmetry ● ● Binary mergers (BBH, BNS) post-Newtonian chirps + numerical relativity ● ● Supernova core collapses numerical simulations. no comprehensive ● view of the collapse. few predictions, robustness? ● … and others (e.g. star quakes, cosmic strings, etc)
Achieved sensitivity and data takings strain sensitivity (Hz -1/2 ) 10 -21 10 -23 10 10 2 10 3 frequency (Hz) 3 joint LIGO – Virgo science runs ● S5 S6 ~2 yrs total NS-NS = 1% total mass emitted in GW ● horizon is ~ 20 – 40 Mpc VSR 1 2 3 4 Core Collapse SN = 10 -8 M c2 ● galactic SN are observable
Searches for GW transients: basic ideas Time series analysis Time series analysis rare transients with low signal to noise ratio rare transients with low signal to noise ratio Expected signal is known known Expected signal is Expected signal is unknown unknown Expected signal is (inspiralling binaries) Excess in time-frequency maps Excess in time-frequency maps Matched filtering Matched filtering (wavelets) (wavelets)
Challenges with real-world data (1) ● Non-stationary and non- Gaussian loud glitches zoo of instrumental glitches → ● bulk of the glitch background has heavy tails population ● Data quality is a key issue Veto known artifacts ● Cross-correlation with >100 auxiliary ● channels Trade-off: maximize “efficiency” ● (fraction of glitches that get vetoed) and minimize “dead time” (volume of vetoed data) p o w e r Safety checks with “hardware” ● l a w injection of fake GW signals 70 DQ flags, efficiency 90% for loud ● glitches
Challenges with real-world data (2) ● Due to instrument complexity, comprehensive noise modelling is out of reach ● Background estimation is also a key issue: “time-slide” analysis Exploit availability of multiple detectors ● Apply non-physical (> 1 s) time-shifts to data stream and repeat analysis ● → Reference background distribution of noise-only events Compare distribution of non time-shifted (“zero-lag”) events to reference ● to get confidence (probability of occurrence) Limitation of the number of time-slides (1 s – 1 day) ●
Worldwide network of GW detectors all detectors receive the same polarizations but GW couples differently according to the antenna patterns orientation phase shift – scaling (antenna patterns) position time delay (propagation) time delay scale this property is specific to GW and can be factor phase shift used to eliminate noise events
Challenges with real-world data (3) Background rejection using multiple detectors require time coincidence and ● phase consistency “coherent veto”: signal vs null ● P is the projector onto noise or null space linear combining of data from each detector so that GW signatures cancelled in the sum coherent and incoherent projected energies Glitch: off-diag. GW: on and off-diag. terms much smaller terms cancels (are of than on-diag. same order) signal space is a 2D plane!
Selection of results ● Latest “all-sky” burst search S5-VSR1 & S6-VSR 2/3: 2 yrs ● observation total Transients (< 1s) in 64 Hz– 5 kHz ● Search with coherent WaveBurst ● standard candle No GW candidate event E GW =1 M sun c 2 ● Upper-limits on the rate of bursts ● estimated using generic waveforms detectable GW energy at a given distance 10 kpc: E GW = 3 x 10 -8 M sun c 2 (comparable to CC SN) 15 Mpc: E GW = 10 -1 M sun c 2 (comparable to black-hole binary merger)
Stress test with “blind injections” ● Blind injection challenge ● fake signals secretly added to the data to test the detector and analysis Envelope opening ● Nick name “Big Dog” Envelope opening ● Event (inspiral) successfully recovered as a detection candidate with a FAR < 1/7000 y. ● The process was a valuable end- to-end test of our analyses Analysis → Paper draft on event candidate → Internal review → Detection committee → Envelope opening arXiv:1111.7314 http://www.ligo.org/science/GW100916
Connection to high-energy astrophysics low medium high energy range electromagnetic electromagnetic Gamma-ray burst and their afterglow Soft-gamma repeaters Anomalous X-ray pulsars Pulsar glitches low high energy range neutrino neutrino
Prompt search for EM counterparts ● Low-latency analysis (30 mins) Transfer, qualify and search the data ● Select candidates, reconstruct source direction (3 ● GW GW detectors) Error region made of disconnected islands ● 10 to 100 sq degrees ● Wide-field robotic telescopes (but not only) alert message alert message Observe asap + subsequent nights ● Galaxy mass targetting, FOV ~ few sq deg ● EM EM Example of GW error region
2 nd generation of “advanced” detectors ● x 10 sensitivity improvement x 10 3 observable volume ● Neutron star binaries ~ 140 Mpc ● Typ. few tenths of detectable binaries ● per year Black binaries ~ 1 Gpc ● ● First science data ~ 2015 x 10 Installation is under way ● Kagra (Japan), LIGO India ● x 10 improvement in angular resolution ●
Summary ● A new window on the Universe opens with GW observations ● Our searches cover a wide range of expected sources ● We are prepared to detect a signal ● Developing synergy with high-energy astrophysics ● With the 2 nd generation of instruments, the next decade will probably see the 1 st direct detection of GW Stay tuned! Stay tuned!
Recommend
More recommend
