Transcript GW signal - Virgo

The Virgo detector: status
and first experimental results
Nicolas Arnaud
CERN EP Seminar
17/03/2003
Outline
• The quest for gravitational waves (GW): a long history
• Detection principles
 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)
• Main GW sources and filtering techniques
• Virgo versus the other GW interferometric detectors
 The LIGO interferometers (USA) + TAMA (Japan)
Gravitational waves: a brief history
• First «imagined» by Poincaré in 1905
été d'abord
conduitby
à supposer
que
la propagation de
• «J'ai
GW existence
predicted
Einstein in
1918
la 
gravitation
n'est pas instantanée
mais Relativity
se fait à la(1915)
vitesse
Direct consequence
of the General
de la lumière (…) Quand nous parlerons donc de la position ou
•deBut
not immediately
la vitesse
du corps accepted:
attirant, il s'agira de cette position ou
 GW
appear
after
a linearization
ofgravifique
the Einstein
de
cette
vitesse
à l'instant
où l'onde
estequations
partie de
while non-linearity
the[Italics
key of of
their
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(…)»
thephysical
author] content
 Non linear correction terms cannot be calculated this way
«GW travelframework
at the speed
of mind »inSir
• Theoretical
developped
theA.S.
50’sEddington
and 60’s
(Pirani & Isaacson)
• Indirect evidence of the GW existence: PSR 1913+16
Hulse & Taylor (Nobel Prize 1993) [& Damour]
 GW must exist !
Gravitational waves main characteristics
• Perturbations of the Minkowski metric propagating at the speed of light
• 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
No Hertz experiment possible!
• A good source of GW must be: asymetric, compact, relativistic
GW effect : differential modification of lengths
h(t)  2 DL(t)
L
L
L + DL
GW amplitude:
h
1
source distance
 Detectors: IFO, resonant bars, LISA…
Detections expected up to the Virgo cluster (~ 20 Mpc)
Interferometric detection
Suspended
Michelson
Interferometer
Mirrors used as
test masses
Incident GW
Sensitivity :
hsens 
Optical path
modification
Variation of the
power Pdet at the
IFO output port
1
Arm length  Power incident on BS
The Virgo Interferometer
• French-Italian collaboration
(CNRS/INFN)
~ 50 physicists
~ 50 engineers
• Budget: ~ 75 M€
55% Italy, 45% France
Site: Cascina, near Pisa
Planning:
• Spring 2001 - Summer 2002:
successfully
Central Interferometer Commissioning  completed
•  2003: Shutdown and transition to the full detector
• From summer 2003: Full scale Virgo commissioning
• First Physical Data foreseen for 2004 … or later
The Virgo Detector
Laser power: Pin = 20 W
Sensitivity h  1 / Pin
Gain : 3000  30  50 ~ 106
White
fringe
Laser
Detection
Photodiode
-17 / Hz
-23
-22
Sensitivity : hsens ~ 3 10-21
 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)
The Virgo SuperAttenuator
Length ~ 7 m; Mass ~ 1 ton
Structure in inverted pendulum
INFN
Pisa
-
g
k
1


fres 2π m
l
 fres ~ 30 mHz
Dual role:
• Passive seismic isolation
Seismic Attenuation:
~ 1014 à 10 Hz
• Mirror active control
only 0.4 N needed
for a 1 cm motion
Virgo configuration
Full Virgo
configuration
Virgo foreseen sensitivity
Thermal
noise
Tail of the
0.6 Hz marionetta/
mirror resonance
Thermal
noise
Violin
mirrors
modes
Shot noise
«Seismic Wall»
Minimum ~ 3 10-23/
Hz between ~ 500 Hz et 1 kHz
Virgo central interferometer (CITF)
• CITF commissioning = 1rst step of Virgo commissioning
• Recycled and suspended Michelson Interferometer
• Uses the main technology developped for Virgo
• CITF commissioning goals:
 check the different component performances
 validate control algorithms
 test data management (acquisition, storage…)
«West» Mirror
Arm
lengths
~6m
Recycling
Mirror
«North»
Mirror
The CITF is not sensitive enough:
no hope to collect data with GW signal!!!
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
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
dl ~ 10-10 – 10-11 m
Angular control
«Alignment »
 Aligned mirrors
dq ~10-9 – 10-7 rad
The steps of the Virgo control
Control aim: to go from an initial
situation with random mirror
motions to the Virgo working point
Virgo
frame
• 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
First control of the Michelson
Fringe Counting
Fringe interval
Global~ 0.5 mm
Control
Time (s)
AC Power
Error signal
Time (s)
DC Power
Dark fringe
Time (s)
Interferometer
power output
June 13th 2001
First control of the recycled CITF
A complex problem:
Stored
• Two lengths to be controlled
instead
of~one
• Pmax
5.8 W
Power
 coupled error signals
 Gain ~ 70
• Narrow resonance of the recycling(Pcavity
(high finesse)
laser ~ 80 mW)
IFO
• Limited force available to act on mirrors
output
• Error signal ~ to the electronic noise
outside
• Dark
fringeresonance
power
[weak laser power + Recycling mirrorless
reflectivity
«dark» = 98.5%]
D5
 unperfect
contrast
Recycling
Main issues:
correction • Large fluctuations of
• To select the right resonance
[trigger on the stored power]
the stored power:
West
•
Simultaneous
acquisition of the 2 cavity
controls
 low
feedback gain
• Recycling
:
D5
photodiode
correction
•
Fast
dampingbyofthe
theBeam
0.6 Hz
pendulum
resonance
excited
misalignments
Signal reflected
Splitter
2ndface
(AR coating)
each
time the applied
locking on
attempt
fails
 Correction
the Recycling
Mirror
December 16th 2001
CITF Main steps
• 5 ER for the CITF commissioning Channel
«Physics» Control Monitoring
• 3 days duration (24h/24h)
type
• ~ 1 TB data collected / ER
Data
2%
61 %
37 %
~ 5 MBytes/s  ~ 160 TB/an
fraction
• The 2 first in Michelson configuration (9/01 and 12/01)
• The 3 others Recycled configuration (4/02, 5/02 and 7/02)
Engineering Run
Duty Cycle
ER0
ER1
ER2
ER3
ER4
98% 85% 98% 96% 77%
All sources of control losses understood
 Improvements already done or in progress
Main CITF improvements:
• Suspension hierarchical control (feedback splitting)
• Output Mode-Cleaner locking
• Laser frequency stabilization above few Hz
• West mirror linear alignment
CITF sensitivity improvements
ER
Best
Sensitivity
m/Hz
8 10-12
E0
(@ 500 Hz)
5 10-12
E1
(@ 500 Hz)
E2
10-14
(@ 1 kHz)
E3
5 10-15
(@ 1 kHz)
E4
10-16
(@ 1 kHz)
June 2001  July 2002
Factor 103
improvement
@ 10 Hz
Factor 105
improvement
Readout electronic
@ 1 kHznoise
Back scattered light
on laser bench
Alignment
Auxiliary laser
Noise:
frequency
noise
peaks
= qx
Room for
resonances
many Laser
more
frequency
noise
Laser
frequency
Improvements
due to vibrations
noise + some
of the input
test masses
Mode-Cleaner
resonantlength
modes
From the CITF to the full Virgo (1)
Question: what does the CITF teach us on the full Virgo?
Answer: many (encouraging) things but the commissioning
of the complete Virgo remains a major challenge
Main results of the CITF commissioning:
• Scientific program completed
• The different parts of the control chain work well
 seismic noise no more dominant above 1 Hz
 methods for resonance acquisition developped
 linear feedbacks (z and q) stable on large timescales
• Dark fringe control @ 10-12 m as requested for Virgo
• Large improvements in sensitivity in only one year
 Gain in ‘experimental experience’ allows one to prepare
several upgrades for the full Virgo in various systems
 Main concerns: input laser system
 CITF studies are direct benefits for Virgo
From the CITF to the full Virgo (2)
 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
 In reality, such gains are unfortunately not automatic:
 some noises do not depend on the laser optical path
 as soon as the main noise in a given bandwidth is
lowered, other sources previously hidden appear!
• Virgo optical scheme more complicated (4 lengths)
 Dark Fringe (difference in the arm lengths)
 Recycling Length (mean length of the interferometer)
 2 Fabry-Perot cavities
 Lock acquisition procedure  from CITF methods
more complex
 currently under study with simulations
 Virgo can benefit from the other detector experiences
First beam in the 3 km North arm
Thursday March 13th 2003:
First beam travelling in the 3km North Arm!!!
« As a first conclusion, the tube is straight! »
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
 Brief review of the main GW signals and methods
Compact binary coalescences
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!
Waveform analytically estimated by developments in v/c
 Wiener (optimal) filtering used for data analysis
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
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
Stochastic backgrounds
• Described by an energy density per unit logarithmic
frequency normalized to the critical density of the universe:
2
3
π f 3S stoch (f)
H
0
with  c  8G
• Two main origins: Ω stoch (f) 
2G ρc
 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]
Coincidence detections
Why ?
• Some detectors
will be working
in the future
now ACIGA
 LIGO : 4 km
 VIRGO : 3 km
 GEO : 600 m
 TAMA : 300 m
 ACIGA : 500 m
• Coincidence = only way to separate a real GW from
transient noises in a particular interferometer
• Coincidences may allow to locate the source position in sky
• Coïncidences with other emissions: g, n
Interferometer angular response
Declination d
The detectable GW amplitude is a linear combination of the
two GW polarizations h+ et h h(t) = F+ h+(t) + F h(t)
Beam Pattern
statistical
distribution
Angular
response
RMS ~ 0.45
Reduction of a factor ~ 2
in average of the amplitude
Right ascension a
• 2 maxima ( detector)
• 4 minima (blind detector)
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
Virgo versus other interferometers
10-7
June-August 2002
October-November 2002
10-12
LIGO
TAMA
10-20
1 Hz
10 kHz
10-7
Virgo CITF
10-20
10 Hz
10 kHz
• All sensitivities in m/Hz
 Comparable plots!
• Improvements still needed!
• Record sensitivity: Tama
10-18 m/Hz @ 1 kHz
10-20 July
1 Hz
2002
5 kHz
• @ 10 Hz, the CITF has the
best sensitivity: 10-13 m/Hz
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 !?!?!
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
References about Virgo and GW
• Virgo web site: www.virgo.infn.it
• Virgo-LAL web site (burst sources): www.lal.in2p3.fr
• Source review: (recent)
C. Cutler - K.S. Thorne, gr-qc/0204090
• Supernova signal simulation:
H. Dimmelmeier et al. astro-ph/0204288 and 89 (2002)
• 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