Advanced Virgo results and the dawn of gravitational multimessenger astronomy

F. Garufia,b,1,∗

aUniversita degli Studi di Napoli Federico II - Dipartimento di Fisica ’Ettore Pancini’, Complesso Universitario di Monte S. Angelo - via Cintia,80126 Napoli, Italy

bINFN Sezione di Napoli, Complesso Universitario di Monte S. Angelo - via Cintia, 80126 Napoli, Italy

Abstract

We report on the results obtained by Advanced Virgo in the last joint run with LIGO, the observation of firstGW source together with gamma, X satellites and astronomical observatories and the scientific outcomes of thisrevolutionary observation. Finally we report on the status and perspectives of the upgrades of Advanced Virgo.

Keywords: VIRGO, Gravitational Waves

1. Introduction

Gravitational Waves (GW) are ripples in the space-time fabric propagating at the speed of light producedwhen a distrubution of mass is accelerated with a nonvanishing quadrupole moment. Given the weakness ofgravitational coupling constant, to produce GWs de-tectable by present day detectors extreme stellar eventsare needed, such as black hole or neutron star clashes.

The effect of a GW coming orthogonally to a planeof a ring of free-falling masses is to stretch the ring in adirection and squeeze it in the orthogonal direction. Thechange of the relative distance of two masses in free fall,caused by a typical gravitational wave is of the orderof h = ΔL/L = 10−22, which corresponds to a changeof the distance Earth-Sun by less than the diameter ofan atom. A natural detector for this type of displace-ments is a Michelson interferometer: the light of a laseris split in two orthogonal beams, which, after traveling akilometric distance, are reflected and recombined at thebeam splitter; the gravitational wave lengthens one armand shortens the other, and thus creates a path lengthdifference, which can be detected in the output interfer-ence fringe. Three of such large interferometers are, todate, operating in the World: the two LIGO [1] in the

∗Corresponding author1On Behalf of the Virgo Collaboration.

USA, at Hanford (WA) and Livingston (LA) and Virgo[2] in Italy at Cascina (PI), and performed a joint sci-entific run (O2) in the summer of 2017 as they were asingle observatory with tree telescopes.

Due to the weakness of gravitational interaction withrespect to all other forces in nature, to prduce GW de-tectable by instruments on Earth, extreme events areneeded, such as dense stellar (supernovae, neutron stars–NS –, black holes – BH –) collapses or coalescencesare needed. In fact, the first events detected by the ob-servatories are BH-BH and NS-NS coalescences, andgave us the chance to shed light on the physics of suchdense objects.

2. The interferometers

As outlined in section 1, Virgo is a gravitational wavedetector with 3 km long arms, located near Pisa in Italy.It was built by the initially French-Italian Virgo col-laboration, which was then joined by The Netherlands,Poland, Hungary and Spain. The European Gravita-tional Observatory (EGO), a French-Italian consortium,hosts the infrastructure and fosters European collabora-tion.

To increase the detector sensitivity, the effective armlength is increased by means of Fabry-Perot cavities in-stalled into the arms, so that light escapes the cavity

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in resonance after having bounced back and forth sev-eral times (order 100). The interferometer is tuned todark fringe in the output port, so that light is mostly re-flected towards the laser input, thus a power recyclingmirror is placed between the laser and the beam splitterin order to increase the power entering the interferom-eter so to decrease the shot noise. A schematic of theVirgo interferometer is shown in Fig.2. Apart from thedescribed features, one recognizes Input Mode Cleanerfor improving the quality of the beam entering/leavingthe interferometer and eliminating higher-order modecontaminations. Mirrors motion must be taken under

Figure 1: Schematic layout of initial Virgo in its design configuration.The main optical elements are the Power Recycling Mirror (PRM),the Beam Splitter (BS), the North and West arm Input and End mirrors(NI, NE, WI, WE), and Input Mode Cleaner (IMC).

control in order to minimise any possible contaminationthat can mimic the effects of a GW, thus, all mirrorsare suspended to 10 m high multi-pendular suspensionscalled superattenuators that filter out the sesmic motion.Superattenuators (see Fig. 2) consist of an inverted pen-dulum and five intermediate filter stages; the last stageis the payload, including an intermediate body calledmarionette, allowing to steer the mirror suspended fromit. The motion of superattenuators is also electronically’hierarchically’ controlled at three different stages tofurther reduce the residual motion at different frequen-cies. The whole optical system is under vacuum, includ-ing the 3 km long arm tubes and the towers hosting themirrors with their suspension chains.

3. Virgo evolution toward Advanced Virgo

Initial interferometers (2001-2007) sensitivity, asshown in Fig. 3 has evolved during the years, due to acontinue work on the interferometer and controls. Afterthe initial period, Virgo underwent a substantial upgrade

Figure 2: The superattenuator mirror suspension system of the Virgointerferometer

to Virgo+ [3] and its sensitivity improved by roughly afactor 1.5. To give a number that represents sensitiv-ity, it is customary in GW field to quote the distance(horizon) a coalescence of two 1.4M� neutron stars canbe detected with a signal-to-noise ratio (SNR) of 8, al-though this unit accounts only for the part of sensitivitycurve around its minimum and it is almost insensitive tolow and high frequency part. In this unit, from Virgo toVirgo+ the horizon passed from typically 8 to 12 Mpc,although Virgo best achieved sensitivity is around 10MPc.

After 2011, with Virgo Scientific Run 4 (VSR4)Virgo+ was partially dismantled to be upgraded toAdvanced Virgo (AdV) [4], implementing several im-provements and new features. The works lasted up to2016 when AdV started its commissioning that was de-signed to led to an improvement in sensitivity of a factor10. The principal differences between Virgo+ and AdVare:

• Larger beam: 2.5 times larger;

• Heavier mirrors: 2 times heavier

• Higher quality optics: residual roughness <0.5 nm;

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Figure 3: The evolution of Virgo sensitivity curves

• Improved mirror coatings for lower losses: absorp-tion < 0.5 ppm, scattering < 10 ppm reducing shotnoise: arm finesse of cavities are 3 times largerthan in Virgo+;

• Thermal control of aberrations: compensate forcold and hot defects on the core optics actuatingon the mirrors with CO2 lasers and ring heaters;

• Stray light control: suspended optical benches invacuum, and new set of baffles and diaphragms tocatch diffuse light;

• Improved vacuum: 10−9 mbar instead of10−7 mbar.

A schematic sketch of AdV layout is shown in Fig. 4,where it can be noticed the insertion of new optical el-ements to increase the power circulating in the cavities,clean the signal and compensate thermal effects and thesuspended benches (SNEB, SWEB, SIBx, SDBx) to de-crease the influence of ground motion in the injectionand detection.

Unfortunately, the AdV interferometer suffered someaccidents that prevented it to reach its design sensitiv-ity, although it improved Virgo+ sensitivity by a factor2 to 3, reaching 28 MPc at best and thus allowing theparticipation to the joint LIGO-Virgo observation runO2. In particular one major problem was encountered:detector integration was troubled by repeated breakageof fused silica fibers when test masses were suspendedin vacuum. This lead to decision of suspending themwith steel wires in order not to stop commissioningprogress until the cause and a solution is found. Even-tually the cause was found: dust particle generated byscroll pumps and blown at high speed towards the fibersduring a venting of the vacuum chamber. The solution,

Figure 4: Schematic layout of Advanced Virgo in its design config-uration. The main differences with the initial detector design are thesignal recycling mirror (SRM), the output mode cleaner (OMC), thepick-off plate (POP) and the compensation plates (CP). The SRM in-stallation is foreseen in 2020

to be implemented after the O2 run, was to upgradethe vacuum system by scroll pumps replacement, mod-ifications of the venting pipes and installation of fiberguards.

4. The scientific run O2

After the first observing run O1,the Advanced LIGOinterferometers in Handford (H1) and Livingston (L1)started a second observing run – O2 – on November 302016. AdV interferometer joined the run on August 1and the run O2 ended on August 25. The sensitivity ofthe detectors participating to the run is shown in Fig.5. The two LIGO detectors, having started operationearlier than Virgo, have horizons of 60 100 MPc; inthe figure it is also shown the GEO600 detector [5] lo-cated in Hannover, Germany, which, due to a differentconfiguration, is mostly sensitive in the high-frequencyregion.

The insertion of AdV in the network allows a moreprecise localization of a GW source in the sky. Dur-ing the observational run O2, the first GW source de-tected by the three interferometers has been observed:GW170814 [6], a BH-BH coalescence. The third de-tector in play, allowed to restrict the sky region forGW170814 to a size of only 60 square degrees, morethan 10 times better than what could be gone with thetwo LIGO interferometers alone. Also, since Virgo doesnot respond in the exactly same way to passing gravita-

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Figure 5: Sensitivity curves of Virgo, the two LIGO interferometersand the GEO600 detector in Hannover, Germany, at the beginning ofthe O2 observational run

tional waves as the LIGO detectors because of its orien-tation on Earth, it was possible to test GW polarizationsto check whether there were deviations from what Gen-eral Relativity predicts. The data analysis shows thatEinstein's prediction is strongly favoured over all con-sidered alternative theories.

Three days after the first “network” observation, thefirst NS-NS coalescence ever – GW170817 – was ob-served by the three interferometers [7], marking a his-torical benchmark in GW astronomy. The signal, Fig.6,is visible by the three detectors with different SNR:Hanford: 18.8; Livingston: 26.4; Virgo: 2.0. In AdVthe signal is very low but not exactly null, although thesource is estimated to be widely in the detector Horizon.This is due to the detector orientation and its geometri-cal response (antenna pattern) that make the source fallnear a detector’s blind zone. This circumstance allowsto restrict the sky region to the intersection of the LIGOestimated zone with the “blind spot” of AdV sensitivity,so to obtain a reduction from 190 to 28 square degreesas shown in Fig.7.

The precision in pointing and the simultaneous ob-servation of a Gamma Ray Burst by the Fermi/GBMsatellite in the same sky zone [8], triggered gamma. op-tical, X and radio telescopes to watch the interested skyregion. Pointing optical telescopes to the zone indicatedby GW and GRB triggers, resulted in the identificationby the SWOPE telescope of an optical transient in theNGC4993 galaxy , that was compatible with the dis-tance as computed from GW signal [9]. in Fig. 7, theimages of the host galaxy by SWOPE is shown togetherwith the same galaxy by DLT taken some days before,showing the appearance of a bright dot and its position-ing on the LIGO/Virgo/Fermi sky map. Subsequent ob-servations in different EM radiation frequency bands –X, optical, IR, radio – all showed evidence of emissionfrom the identified source and, from time evolution andenergy spectra, led to the identification of this event as

Figure 6: Representation in the time-frequency plane of theGW170917 NS-NS coalescence signal as observed by the three in-terferometers.

a (so called) kilonova, compatible with the NS-NS coa-lescence as seen by GW observatories. This marks thisobservation as the birth of GW multimessenger astron-omy. The observation of light spectra and their temporalevolution showed that in this merging event, elementsheavier than Iron are produced by the so called R pro-cesses. No high energy neutrino was detected by anyobservatory on Earth in coincidence with this event, butthis is compatible with a jet emission off-axis with Earthdirection by the merging object, as estimated by otherobservations [10].

Since the GW amplitude at the production point isknown from the signal time evolution, the coincidentelectromagnetic signal observation allowed to constrainthe graviton speed to the speed of light within some partin 10−17 and the Hubble constant [11] to a value com-patible within the errors with what found by Plank [12].

5. Post O2 improvements

After the end of the O2 run in August 2017, the nextcommon observation run O3 is foreseen to start early

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Figure 7: The participation of a third detector to the GW interferome-ters network allowed the prompt pinpointing of the direction of arrivalof the signal coded GW170817. This served as a trigger for electro-magnetic telescopes which were able to detect the optical counterpartof the GW source in the Galaxy NGC4993. In the pictures, NGC4993before (bottom) and after (top) the detection of GW170817 by LIGOand AdV

2019. The period between the runs is used for upgradingthe detectors and improving the sensitivities.

First of all the ”monolitic” silica fiber suspensionhave been installed to replace the steel wires, togetherwith a newly designed vacuum system and fiber wards.Multi-root pumps were installed instead of the scrollpumps, the venting system was changed, and the in-side of the vacuum system was carefully cleaned andis monitored with improved dust detection systems ateach intervention.

A new 100W laser amplifier have been installed inplace of the previous 70W, with a new pre-mode cleanerand a suspended injection bench, to further decrease thenoise due to vibrations. Considering the losses in theinjection system, the maximum power that can be deliv-ered to the interferometer input is now about 60 W.

Increasing the laser power for further reducing shotnoise is difficult while maintaining the required highbeam quality and low noise. Besides the thermal ef-fects on the mirrors, that can be compensated by anappropriate system, another potential serious problemare parametric instabilities, excitations of internal vi-brations of the mirrors interacting with high-order res-onance modes of the arm cavities, which can emerge athigh-power operation. Reduced shot noise without in-creased laser power can be achieved by using squeezedlight, a technology that uses quantum effects for con-trolling light fluctuations by using nonlinear optical pro-cesses that reduce phase noise at the expenses of ampli-tude noise (i.e. squeeze the ellipse that represents thedetected beam with its noise in a phase-amplitude planealong the phase direction and stretch it along the ampli-

tude direction). A squeezed light source developed atthe Albert Einstein Institute in Hannover [13] has beeninstalled and is now being commissioned.

The interferometer restarted its operations in April2018 and in May performed a first Engineering run dur-ing a weekend. The commissioning will continue to theend of 2018 in order to reduce noise and glitches and toincrease the interferometer lock robustness before thethird joint observation run O3, now scheduled for mid2019.

6. Conclusions

The Virgo detector was upgraded to the second gen-eration Advanced Virgo configuration. In August 2017took part to the O2 joint observative run with the LIGOobservatories and could detect gravitational waves fromBH-BH and the first NS-NS coalescence ever observed.Advanced Virgo is now in a upgrading and commission-ing phase that will lead to the next observative run O3during 2019.

References

[1] B. Abbott, et al., Ligo: the laser interferometer gravitational-wave observatory, Rep. Prog. Phys. 72 (2009) 076901.doi:10.1088/0034-4885/72/7/076901.URL http://stacks.iop.org/0034-4885/72/i=7/a=076901

[2] T. Accadia, et al., Virgo: a laser interferometer to detectgravitational waves, JOURNAL OF INSTRUMENTATION 7(2012) i–124. doi:10.1088/1748-0221/7/03/P03012.URL http://iopscience.iop.org/1748-0221/7/03/P03012

[3] T. Accadia, et al., Status and perspectives of the virgo gravita-tional wave detector, Journal of Physics: Conference Series 203(2010) 012074. doi:10.1088/1742-6596/203/1/012074.

[4] F. Acernese, et al., Advanced virgo: a second-generation inter-ferometric gravitational wave detector, Class. Quantum Grav. 32(2015) 024001. doi:10.1088/0264-9381/32/2/024001.

[5] B. Willke, et al., The geo 600 gravitational wave detector,Class. Quantum Grav. 19 (2002) 1377–1387. doi:10.1088/0264-9381/19/7/321.

[6] B. Abbott, et al., GW170814: A three-detector ob-servation of gravitational waves from a binary blackhole coalescence, Physical Review Letters 119 (14).doi:10.1103/PhysRevLett.119.141101.

[7] B. P. Abbott, et al., GW170817: Observation of gravitationalwaves from a binary neutron star inspiral, Physical Review Let-ters 119. doi:10.1103/PhysRevLett.119.161101.

[8] V. Connaughton, GBM-LIGO Group, et al., Fermi GBM trigger524666471/170817529: LIGO/Virgo identification of a possiblegravitational-wave counterpart (2017).

[9] B. P. Abbott, et al., Multi-messenger Observations of a BinaryNeutron Star Merger, ASTROPHYSICAL JOURNAL LET-TERS 848 (2). doi:10.3847/2041-8213/aa91c9.

[10] A. Albert, et al., Search for high-energy neutrinos from bi-nary neutron star merger gw170817 with antares, icecube, andthe pierre auger observatory, The Astrophysical Journal Letters850 (2) (2017) L35.

F. Garufi / Nuclear and Particle Physics Proceedings 303–305 (2018) 86–9190

[11] B. P. Abbott, et al., A gravitational-wave standard siren mea-surement of the Hubble constant, Nature Letter 551 (2017) 85–87. doi:10.1038/nature24471.

[12] P. A. R. Ade, et al., Planck 2015 results XIII. Cosmo-logical parameters, Astronomy & Astrophysics 594 (A13).doi:10.1051/0004-6361/201525830.

[13] H. Vahlbruch, M. Mehmet, K. Danzmann, R. Schnabel, De-tection of 15 dB Squeezed States of Light and their Appli-cation for the Absolute Calibration of Photoelectric Quan-tum Efficiency, PHYSICAL REVIEW LETTERS 117 (11).doi:10.1103/PhysRevLett.117.110801.

F. Garufi / Nuclear and Particle Physics Proceedings 303–305 (2018) 86–91 91