1. The Virgo detector: status and first experimental results Nicolas Arnaud NIKHEF June 20 th, 2003

2. Outline The quest for gravitational waves (GW): a long history Detection principle  Interferometric detectors Description of the Virgo interferometer Optical scheme Main features of the instrument Foreseen sensitivity Experimental control of the Central Interferometer (CITF) CITF description and CITF commissioning goals Experimental results (spring 2001  summer 2002) Virgo versus the other GW interferometric detectors  The LIGO interferometers (USA) + TAMA (Japan) Main GW sources and filtering techniques

3. «J'ai été d'abord conduit à supposer que la propagation de la gravitation n'est pas instantanée mais se fait à la vitesse de la lumière (…) Quand nous parlerons donc de la position ou de la vitesse du corps attirant, il s'agira de cette position ou de cette vitesse à l'instant où l'onde gravifique est partie de ce corps (…)» [Italics of the author] 50’s-60’s: back in the footlights GW theoretical framework developped (Pirani & Isaacson) Do gravitational waves exist? GW existence predicted by Einstein in 1918 A difficult first appearance  Validity of the General Relativity linearization ?! «GW travel at the speed of mind » Sir A.S. Eddington First «imagined» by Poincaré in 1905 The breakthrough: the binary pulsar PSR 1913+16 (1974) Indirect evidence that GW exist Hulse & Taylor (Nobel 1993) [& Damour] 20 years of measurement Yes they do!

4. GW main characteristics Perturbations of the Minkowski metric Quadrupolar emission Extremely weak!!! Luminosity  G/c 5  10 -53 W -1 Ex: Jupiter radiates 5.3 kW as GW during its orbital motion  over 10 10 years: E GW = 2  10 21 J  E kinetic  2  10 35 J A good source of GW must be: asymetric compact (R ~ R Schwartzchild = 2GM/c 2 ) relativistic No Hertz experiment possible! Astrophysical sources required

5. GW detectable effect GW effect : differential modification of lengths L L +  L The detector sensitivity volume should ultimately extend beyond the Virgo cluster (~ 20 Mpc  65  10 6 light years) h: dimensionless amplitude h  1 / distance Two main categories of detectors: resonant bars giant interferometers, Earth-based or space-based Virgo LISA

6. A very large GW frequency domain Extremely Low Frequencies 10 -18  10 -15 Hz Very Low Frequencies 10 -9  10 -7 Hz Low Frequencies 10 -4  10 -1 Hz High Frequencies 1  10 4 Hz LISA Earth-based detectors Resonant bars or IFOs CMB polarization Pulsar timing Frequency Range GW ‘Probe’

7. First GW detectors: Joe Weber’s pioneering work – see Phys. Rev. 117 360 (1960) Resonator: supraconducting coupled with cylindrical bar a transducer Network of bars working for years with high duty cycles Narrow-band sensitivities limited by noises difficult to beat Resonant bars GW deposit energy inside the bar Vibrations modulate DC voltage

8. Interferometric detection Incident GW Optical path modification Variation of the power P det at the IFO output port Sensitivity : Suspended Michelson Interferometer Mirrors used as test masses

9. The Virgo optical scheme  To increase the arm length : 1 m  3 km  To add Fabry-Perot cavities (Finesse = 50  Gain = 30)   To add a recycling mirror (P = 1 kW on the Beam Splitter) Sensitivity : Sensitivity : h sens ~ Detection Photodiode Laser Gain : 3000  30~ 10 6 10 -17 3 10 -21 10 -23 10 -22 White fringe Laser power: P in = 20 W Sensitivity

10. Dual role: Passive seismic isolation Mirror active control only 0.4 N needed for a 1 cm motion The Virgo SuperAttenuator Length ~ 7 m; Mass ~ 1 ton Structure in inverted pendulum Seismic Attenuation: ~ 10 14 at 10 Hz  f res ~ 30 mHz - INFN Pisa

11. Virgo foreseen sensitivity Minimum ~ 3 10 -23 between ~ 500 Hz et 1 kHz «Seismic Wall» Thermal noise Tail of the 0.6 Hz marionetta/ mirror resonance Shot noise Thermal noise mirrors Violin modes

12. Full Virgo configuration The Virgo detector Half-Arm Buildings 1.5 km North Arm West Arm 3 km Mode-Cleaner 144 m Central Building Control Building

13. Virgo in numbers Arm length: 3 km  6800 m 3 in ultra-high vacuum (10 -10 mbar) Very high quality mirrors: Diffusion < 5 ppm, absorption < 1 ppm Reflectivity > 99.995% Radius of curvature 3450 m (4.5  m sagitta) Laser power: 20 W Seismic noise attenuation: > 10 14 above 10 Hz Foreseen sensitivity range: 4 Hz  10 kHz Best sensitivity ~ 3  10 -23 /  Hz around 1 kHz Control accuracy Length: down to 10 -12 m Angular: from 10 -6 to 10 -9 radians Fabry-Perot end mirrors

14. Status of Virgo Spring 2001-Summer 2002: Successful commissioning of the central interferometer (CITF) CITF: Virgo without the 3-km Fabry-Perot arms But : Same suspensions Same control chain  Ideal benchmark for the complete Virgo interferometer From autumn 2002: upgrade to Virgo March 2003: first beam in the 3-km arm The Full Virgo commissioning will start after summer First Physical Data: 2004 or a bit later…

15. Virgo central interferometer (CITF) CITF commissioning = 1 rst step of Virgo commissioning Recycled and suspended Michelson Interferometer Uses the technology developped for the Virgo control system CITF commissioning goals: check the different component performances validate control algorithms test data management (acquisition, storage…) Arm lengths ~ 6 m The CITF is not sensitive enough: no hope to collect data with GW signal!!! «North» Mirror «West» Mirror Recycling Mirror

16. CITF and working point Best sensitivity : Michelson on dark fringe  control arm asymmetry: l 2 -l 1 Recycling cavity resonant (maximize the stored power)  control IFO mean length: l 0 + (l 1 +l 2 )/2 Very narrow Working Point In addition: residual low frequency motion of mirrors (0.6 Hz)  CITF active controls needed (local and global) Goal : Longitudinal control «Locking »  Resonant cavities  l ~ 10 -10 – 10 -12 m Angular control «Alignment »  Aligned mirrors  ~10 -9 – 10 -7 rad

17. The steps of the Virgo control Decreasing the residual motion separately for each mirror  Local controls + First alignment of mirrors Lock acquisition of the cavities Check working point control stability Switch on the angular control  Automatic Alignment Switching from local controls to global controls Control aim: to go from an initial situation with random mirror motions to the Virgo working point

18. Cavity Control L M 1 (r 1, t 1 ) M 2 (r 2, t 2 ) Characteristic quantity: the finesse F Linear around resonance Linear region width  1 / F Slope increasing with F A finesse of 400 (aligned CITF) is high for a suspended cavity Pound-Drever error signal The higher F, the more difficult the cavity control Fabry Perot cavity

19. Global Control First control of the Michelson Fringe interval ~ 0.5  m Error signal Interferometer power output Fringe Counting Time (s) AC Power DC Power Dark fringe June 13 th 2001

20. P max ~ 5.8 W  Gain ~ 70 (P laser ~ 80 mW) Dark fringe less «dark»  unperfect contrast Large fluctuations of the stored power: low feedback gain misalignments December 16 th 2001 IFO output power Stored Power West correction Recycling correction A complex problem: Two lengths to be controlled instead of one  coupled error signals Narrow resonance of the recycling cavity (high finesse) Limited force available to act on mirrors Error signal ~ to the electronic noise outside resonance [weak laser power + Recycling mirror reflectivity = 98.5%] Main issues: To select the right resonance [trigger on the stored power] Simultaneous acquisition of the 2 cavity controls Fast damping of the 0.6 Hz pendulum resonance excited each time the locking attempt fails First control of the recycled CITF

21. CITF main steps 5 Engineering Runs 3 days duration (24h/24h) ~ 1 TB data collected / Engineering Run ~ 5 MBytes/s ~ 160 TB/an The 2 first in Michelson configuration (9/01 and 12/01) The 3 others Recycled configuration (4/02, 5/02 and 7/02) Channel type «Physics»ControlMonitoring Data fraction 2 %61 %37 % Engineering RunER0ER1ER2ER3ER4 Duty Cycle98%85%98%96%77% All sources of control losses understood  Improvements in progress

22. CITF sensitivity improvements ER Best Sensitivity m/  Hz E0 8 10 -12 (@ 500 Hz) E1 5 10 -12 (@ 500 Hz) E2 10 -14 (@ 1 kHz) E3 5 10 -15 (@ 1 kHz) E4 10 -16 (@ 1 kHz) Factor 10 3 improvement @ 10 Hz Factor 10 5 improvement @ 1 kHz June 2001  July 2002 Room for many more Improvements Virgo foreseen sensitivity

23. From the CITF to the full Virgo CITF commissioning completed Large improvements in sensitivity in only one year Gain in ‘experimental experience’  many upgrades for Virgo CITF  Virgo will provide ‘free’ sensitivity improvements: Arm length: 6 m  3 km  gain of a factor 500 in h Fabry-Perot cavities: factor 30 in addition Reduction of laser frequency noise  In reality, such gains are unfortunately not automatic: Some noises do not depend on the laser optical path Noise hunting is a very long work  Virgo scheme more complicated (4 lengths instead of 2)  Control acquisition procedures  from CITF (under study)  Virgo can benefit from the other detector experiences

24. Virgo versus other interferometers LIGO TAMA June-August 2002 Virgo CITF July 2002 All sensitivities in m/  Hz  Comparable plots! Improvements still needed! Record sensitivity: Tama 10 -18 m/  Hz @ 1 kHz @ 10 Hz, the CITF has the best sensitivity: 10 -13 m/  Hz 10 Hz 10 kHz 5 kHz 1 Hz 10 kHz October-November 2002 10 -20 10 -12 10 -7 1 Hz 10 -20 10 -7

25. One word about LISA Earth-based detectors limited by seismic noise below few Hz Strong sources certainly exist in the mHz range Constellation of 3 satellites 3 semi-independent IFOs Optimal combinations to maximize SNR or study noise Search periodical sources Expected lifetime: 5 years Approved by NASA/ESA To be launched in 2011 Seismic wall

26. Preparing the GW Data Analysis Activity parallel to the experimental work on detectors  1 international conference / year (GWDAW) Large number of potential GW sources: compact binary coalescences (PSR 1913+16) black holes supernovae pulsars stochastic backgrounds … The corresponding signals have very different features  various data analysis techniques

27. Coincidence detections Why ? Some detectors will be working in the future LIGO : 4 km VIRGO : 3 km GEO : 600 m TAMA : 300 m ACIGA : 500 m Coincidence = only way to separate a GW (‘global’ in the network) from transient noises in IFOs Coincidences may allow to locate the source position in sky Coïncidences with other emissions: , now ACIGA

28. Interferometer angular response Reduction of a factor ~ 2 in average of the amplitude 2 maxima GW perpendicular to detector plane 4 minima blind detector! e.g. when the GW comes along the arm bissector Right ascension  Declination 

29. Example of the Virgo-LIGO network Spatial responses  in a given direction Similarities between the maps of the two LIGO interferometers Complementarity Virgo / LIGO  Good coverage of the whole sky  Double or triple coincidences unlikely

30. Summary Many interferometers are currently under developpement  Worldwide network in the future All instruments work already although they did not prove yet there can fulfill their requirements  Control of complex optical schemes with suspended mirrors All sensitivities need to be significally improved to reach the amplitude of GW theoretical predictions Many different GW sources  various data analysis methods in preparation In the two last years, the Virgo experiment became real The different parts of the experiment work well together Successful commissioning of the CITF 2003: CITF  Full Virgo First ‘physically interesting’ data expected for 2004 !?!?!

31. GW: a never ending story The future of gravitational astronomy looks bright. 1972 That the quest ultimately will succeed seems almost assured. The only question is when, and with how much further effort. 1983 [I]nterferometers should detect the first waves in 2001 or several years thereafter (…) 1995 Kip S. Thorne Km-scale laser interferometers are now coming on-line, and it seems very likely that they will detect mergers of compact binaries within the next 7 years, and possibly much sooner. 2002

32. References about Virgo and GW Virgo web site: www.virgo.infn.it Virgo-LAL web site (burst sources): www.lal.in2p3.fr/recherche/virgo Source review: C. Cutler - K.S. Thorne, gr-qc/0204090 Some other GW experiment websites: LIGO: www.ligo.caltech.edu GEO: www.geo600.uni-hannover.de TAMA: www.tamago.mtk.nao.ac.jp/tama.html IGEC (bar network): igec.lnl.infn.it LISA: sci.esa.int/home/lisa Moriond 2003: moriond.in2p3.fr/J03 «Gravitational Waves and Experimental Gravity» Recent status of all detectors: bars, IFOs and LISA

33. Detector noise characterization Gaussian noise characterization: Power Spectrum Density (PSD) If the noise is dimensionless, the PSD unit is Hz -1 RMS in the bandwidth [f 1 ;f 2 ]: Amplitude Spectrum Density (unit ) FT: Fourier Transform one-sided PSD (only positive frequencies) with Autocorrelation function Detector Sensitivity: Frequency (Hz) S n or s n ~ Log-log scales graph

34. Compact binary coalescences Example: PSR 1913+16 Coalescence expected in a few hundred million years Virgo will (?!?) be sensitive to the last minutes… Waveform analytically estimated by developments in v/c  Wiener filtering used for data analysis Optimal but computationally expensive Chirp signal: amplitude and frequency increase with time until the final coalescence The signal knowledge ends before the coalescence when approximations used for the computation are no more valid.  large theoretical work to go beyond this limit!

35. Impulsive sources (‘bursts’) Examples: Merging phase of binaries Supernovae Black hole ringdowns GW main characteristics: Poorly predicted waveforms  model dependent Short duration (~ ms) Weak amplitudes  Need to develop  filters : robust (efficient for a large class of signals) sub-optimal (/ Wiener filtering) online (first level of event selection) Zwerger / Müller examples of simulated supernova GW signals

36. Pulsars GW signal: permanent, sinusoidal, possibly 2 harmonics Weak amplitude  detection limited to the galaxy Matched filtering-like algorithms using FFT periodograms Idea: follow the pulsar freq. on large timescales (~ months)  compensation of frequency shifts: Doppler effect due to Earth motion, spindown… Very large computing power needed (~ 10 12 Tflops or more)  Hierarchical methods are being developped  1 TFlop  Need to define the better strategy: search only in the Galactic plane, area rich of pulsars uniform search in the sky not to miss close sources focus on known pulsars Permanent signal  coincident search in a single detector: compare candidates selected in 2 different time periods

37. Described by an energy density per unit logarithmic frequency normalized to the critical density of the universe: Two main origins: Cosmological Emission just after the Big Bang: ~10 -44 s, T~10 19 GeV Detection  informations on the early universe Astrophysical Incoherent superposition of GW of a given type emitted by sources too weak to be detected separately. Detection requires correlations between 2 detectors After 1 year integration: h 0 2  stoch  10 -7 (1 rst generation) 10 -11 (2 nd generation) Theoretical predictions: ~ 10 -13  10 -6 Current best limit:  stoch  60 @ 907 Hz [Explorer/Nautilus] Stochastic backgrounds with