The Virgo detector: status and first experimental results

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
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

• 50s-60s 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
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 Range GW 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 Webers 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
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
Virgo foreseen sensitivity
12
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 lt 5 ppm, absorption lt 1 ppm
• Reflectivity gt 99.995
• Radius of curvature 3450 m (4.5 mm sagitta)
• Laser power 20 W
• Seismic noise attenuation gt 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
Virgo central interferometer (CITF)
16
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 (l1l2)/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
Cavity Control
Fabry Perot cavity
Characteristic quantity the finesse F

• Linear around resonance
• Linear region width ? 1 / F
• Slope increasing with F

Pound-Drever error signal
The higher F, the more difficult the cavity
control
A finesse of 400 (aligned CITF) is high for a
suspended cavity
19
First control of the Michelson
20
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)

Channel type Physics Control Monitoring
Data fraction 2 61 37
Engineering Run ER0 ER1 ER2 ER3 ER4
Duty Cycle 98 85 98 96 77
All sources of control losses understood ?
Improvements in progress
22
CITF sensitivity improvements
June 2001 ? July 2002
ER Best Sensitivity m/?Hz
E0 8 10-12 (_at_ 500 Hz)
E1 5 10-12 (_at_ 500 Hz)
E2 10-14 (_at_ 1 kHz)
E3 5 10-15 (_at_ 1 kHz)
E4 10-16 (_at_ 1 kHz)
Factor 103 improvement _at_ 10 Hz
Factor 105 improvement _at_ 1 kHz
Room for many more Improvements
Virgo foreseen sensitivity
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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
October-November 2002
10-7
June-August 2002
10-12
LIGO
TAMA
10-20
10-20
10 Hz
1 Hz
10 kHz
10 kHz

• All sensitivities in m/?Hz
• ? Comparable plots!
• Improvements still needed!
• Record sensitivity Tama
• 10-18 m/?Hz _at_ 1 kHz
• _at_ 10 Hz, the CITF has the
• best sensitivity 10-13 m/?Hz

10-7
Virgo CITF
July 2002
10-20
5 kHz
1 Hz
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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 191316)
• black holes
• supernovae
• pulsars
• stochastic backgrounds
• The corresponding signals have very different
  features
• ? various data analysis
  techniques

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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

now ACIGA
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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
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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
Interferometers 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
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

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 f1f2
• Amplitude Spectrum
  Density (unit )

Log-log scales graph
Detector Sensitivity
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Compact binary coalescences
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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,
  T1019 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 _at_ 907 Hz
  Explorer/Nautilus

with