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The Virgo detector: status and first experimental results

The Virgo detector: status and first experimental results. Nicolas Arnaud NIKHEF June 20 th , 2003. Outline. The quest for gravitational waves ( GW ): a long history Detection principle Interferometric detectors Description of the Virgo interferometer Optical scheme

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The Virgo detector: status and first experimental results

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  1. The Virgo detector: status and first experimental results Nicolas Arnaud NIKHEF June 20th, 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. The breakthrough: • the binary pulsar PSR 1913+16 (1974) • Indirect evidence that GW exist • Hulse & Taylor(Nobel 1993) • [& Damour] 20 years of measurement Do gravitational waves exist? • First «imagined» by Poincaré in 1905 «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] • 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 • 50’s-60’s: back in the footlights • GW theoretical framework developped(Pirani & Isaacson) Yes they do!

  4. GW main characteristics • Perturbations of the Minkowski metric • Quadrupolar emission • Extremely weak!!! Luminosity  G/c5  10-53 W-1 Ex:Jupiter radiates 5.3 kW as GW during its orbital motion •  over 1010 years:EGW = 2  1021 J  Ekinetic  2  1035 J • A good source of GW must be: • asymetric • compact (R ~ RSchwartzchild = 2GM/c2) • relativistic No Hertz experiment possible! Astrophysical sources required

  5. L L + DL GW detectable effect GW effect : differential modification of lengths h: dimensionless amplitude h  1 / distance The detector sensitivity volume should ultimately extend beyond the Virgo cluster (~ 20 Mpc  65106 light years) • Two main categories of detectors: • resonant bars • giant interferometers, Earth-based or space-based • Virgo LISA

  6. A very large GW frequency domain Frequency RangeGW ‘Probe’ • 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  104 Hz CMB polarization Pulsar timing LISA Earth-based detectors Resonant bars or IFOs

  7. Resonant bars • 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 GW deposit energy inside the bar Vibrations modulate DC voltage

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

  9. White fringe 10-17 The Virgo optical scheme Laser power: Pin = 20 W Sensitivity Gain : 3000  30 ~ 106 Laser Sensitivity : hsens ~ 3 10-21 10-23 10-22 Detection Photodiode  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)

  10. The Virgo SuperAttenuator INFN Pisa Length ~ 7 m; Mass ~ 1 ton Structure in inverted pendulum -  fres ~ 30 mHz • Dual role: • Passive seismic isolation • Mirror active control • only 0.4 N needed • for a 1 cm motion Seismic Attenuation: ~ 1014 at 10 Hz

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

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

  13. Virgo in numbers • Arm length:3 km •  6800 m3 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 mm sagitta) • Laser power:20 W • Seismic noise attenuation: > 1014 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. CITF commissioning = 1rst step of Virgo commissioning • Recycled and suspendedMichelson 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…) «West» Mirror Arm lengths ~ 6 m Recycling Mirror «North» Mirror The CITF is not sensitive enough: no hope to collect data with GW signal!!! Virgo central interferometer (CITF)

  16. Very narrow Working Point In addition: residual low frequency motion of mirrors (0.6 Hz)  CITF active controls needed (local and global) Longitudinal control «Locking »  Resonant cavities dl ~ 10-10 – 10-12 m Angular control «Alignment »  Aligned mirrors dq ~10-9 – 10-7 rad Goal : CITF and working point • Best sensitivity : • Michelson on dark fringe control arm asymmetry:l2-l1 • Recycling cavity resonant (maximize the stored power) •  control IFO mean length:l0 + (l1+l2)/2

  17. The steps of the Virgo control Control aim: to go from an initial situation with random mirror motions to the Virgo working point • 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

  18. M2 (r2, t2) M1 (r1, t1) L Fabry Perot cavity Cavity Control Characteristic quantity: the finesseF Pound-Drever error signal • Linear around resonance • Linear region width  1 / F • Slope increasing with F The higher F, the more difficult the cavity control A finesse of 400 (aligned CITF) is high for a suspended cavity

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

  20. Stored Power • Pmax ~ 5.8 W •  Gain ~ 70 • (Plaser ~ 80 mW) • Dark fringe • less «dark» •  unperfect contrast • Large fluctuations of • the stored power: • low feedback gain • misalignments December 16th 2001 IFO output power Recycling correction West correction First control of the recycled CITF • 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

  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) All sources of control losses understood  Improvements in progress

  22. CITF sensitivity improvements June 2001  July 2002 Factor 103 improvement @ 10 Hz Factor 105 improvement @ 1 kHz 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 10-7 October-November 2002 June-August 2002 10-12 LIGO TAMA 10-20 10-20 1 Hz 10 Hz 10 kHz 10 kHz 10-7 • 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 Virgo CITF July 2002 10-20 5 kHz 1 Hz

  25. One word about LISA • 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 • Earth-based detectors • limited by seismic noise • below few Hz • Strong sources certainly • exist in the mHz range

  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: g, n now ACIGA

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

  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 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 Kip S. Thorne

  32. 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 References about Virgo and GW

  33. FT: Fourier Transform one-sided PSD (only positive frequencies) Autocorrelation function with Sn or sn ~ Frequency (Hz) 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 [f1;f2]: • Amplitude Spectrum Density (unit ) Log-log scales graph Detector Sensitivity:

  34. 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! 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 Compact binary coalescences

  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 Zwerger / Müller examples of simulated supernova GW signals •  Need to develop  filters : • robust (efficient for a large class of signals) • sub-optimal (/ Wiener filtering) • online (first level of event selection)

  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 (~ 1012 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. Stochastic backgrounds • 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~1019 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: h02stoch  10-7 (1rst generation) • 10-11 (2nd generation) • Theoretical predictions: ~ 10-13 10-6 • Current best limit:stoch  60 @ 907 Hz [Explorer/Nautilus] with

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