O3 is running!

The first month has already rewarded scientists with a rich pool of observations

May 2, 2019

Simulation of a binary neutron star merger. From top left, clockwise: the two neutron stars (drawn in white) spiral around each other, come into contact, and merge into one heavy neutron star. Some of the material (with color-coded density) pollutes the surrounding environment and forms a thick accretion disk around the remnant object. The whole sequence covers about 0.03 s of evolution. In most cases, the heavy neutron star cannot survive its own gravity for much longer, eventually collapsing into a black hole.
Simulation & visualisation credits: Ciolfi, Giacomazzo (Virgo Collaboration), Kastaun (LIGO Scientific Collaboration).
The first month of the new LIGO-Virgo survey for gravitational waves (GWs) has already rewarded scientists with a rich pool of observations; as well as plenty of work in terms of their interpretation. O3, as the observation run is called, started on the 1st of April and is planned to last for 12 months. Improvements achieved in the sensitivity of the detectors and the fact that the three LIGO-Virgo instruments have been operating simultaneously since day one, are now enabling unprecedented opportunities. In addition, for the first time, LIGO and Virgo are providing public alerts. These are delivered shortly after the detection of credible transient gravitational waves candidates. This strategy aims to facilitate follow-up observations by other telescopes and enhance the extraordinary potential of multi-messenger observations.
“I couldn’t have dreamed of a better moment to be on shift!”, says Olivier Minazzoli, researcher at Centre Scientifique de Monaco, currently collaborating with the ARTEMIS laboratory in Nice, France, and who has been on shift for the characterisation of the Virgo detector during the past week. “I was expecting to see one binary black hole candidate, at best, certainly not two binary neutron star candidates, and even less the potential first neutron star-black hole candidate ever!”.
Five LIGO-Virgo public alerts have been issued since the 1st of April, and they can be freely accessed by visiting the Gravitational Wave Candidate Event Database. Three are classified as candidate mergers of Binary Black Hole (BBH) systems. Full assessment requires more analysis, which is already ongoing. If confirmed, these candidates would add to the catalogue of 10 BBH mergers, detected by LIGO-Virgo in previous runs, and would help to improve our understanding of the formation processes of these extreme compact objects and on the nature of gravity, space and time.
Two more events require deeper investigations by the LIGO-Virgo teams, as well by a larger scientific community. They point to coalescences of binary systems involving at least one Neutron Star (NS), the densest form of matter of which we have proof.
On the 25th of April, at about 08:18 (UTC) one likely merger of a Binary Neutron Star (BNS) system was observed (called S190425z – more information is available at this link). This follows the famous GW170817, the first BNS, which was detected two years ago and which gave birth to multi-messenger astronomy using GWs. The quest for possible associated sources in the electromagnetic spectrum, counterparts of S190425z, is still ongoing. This challenge is much harder than it was for GW170817, as this time the source is four times more distant and the sky localisation provided by LIGO-Virgo is much more uncertain. In fact, S190425z occurred while only two detectors were operating: LIGO-Livingston and Virgo.
The other candidate signal involving a NS (named S190426c – more information is available at this link) occurred on the 26th of April at about 15:22 (UTC). All three of the LIGO-Virgo detectors were observing, but, due to the signal being fainter, there is still some remaining probability that it may not be a genuine astrophysical signal. S190426c is an extremely interesting signal, because its morphology hints that it may have been emitted by a mixed system of a NS merging into a more massive BH. If this is confirmed, it will be another unprecedented discovery. The investigation to understand S190426c will require more time and involve exciting work by the LIGO-Virgo teams.
“I am especially excited”, says Tanja Hinderer, postdoc at the University of Amsterdam (NL), “to use GWs and multi-messenger observations to learn about the nature of matter in Neutron Stars at the highest densities, which would cause a teaspoon of NS material to weigh as much as a billion tonnes. The GWs encode the properties of the merging objects, while the eventual electromagnetic/neutrino counterparts probe the merger remnant. Having information from multiple messengers is key to understanding these extreme phenomena. The public alerts are making the hunt for multi-messenger counterparts highly exciting, and I am very curious about the diverse signatures associated with different BNS and NS-BH events in O3 and what insights we can gain from them.”
“The latest LIGO-Virgo observing run is proving to be the most exciting one so far,” says David Reitze, executive director of LIGO at Caltech. “We’re already seeing hints of the first observation of a black hole swallowing a neutron star. If it holds up, this would be a trifecta for LIGO and Virgo. But we’ve learned that claims of detections require a tremendous amount of painstaking work—checking and rechecking — so we’ll have to see where the data takes us.”
“Dealing with three heterogeneous detectors is challenging work”, says Florian Aubin, PhD student at the University Savoie Mont Blanc, France. “But it is also a great opportunity to identify the sky position of the source and search for electromagnetic / neutrino counterparts. I am very excited for the coming run. These two NS merger candidates and the other three BBH merger candidates, in less than a month, promise a full year of interesting discoveries. It is really rewarding for me to be here, after two years of tough work.”
Before starting O3, all detectors in the network, namely the two Advanced LIGO interferometers at Livingston and Hanford in the USA, and the Advanced Virgo interferometer at Cascina (Pisa, Italy), undertook an intense period of improvements. Advanced Virgo almost doubled its sensitivity with respect to the second observing run, O2, which ended in 2017.
“It has been 18 months of hard work since September 2017 to improve the Advanced Virgo sensitivity and its robustness against external disturbances”, says Irene Fiori, physicist at the European Gravitational Observatory (EGO, Italy) and in charge of Virgo Environmental Noise Studies. “It has been a great collaborative work with experts in many fields: high quality laser beams, super fine alignments of optics, seismic isolation, compensation of thermal defects, and more. Even the production of squeezed light!”
On average, Advanced Virgo can now reliably observe the merger of a BNS at a distance from the Earth of approximately 160 million light years, and the merger of a BBH system at a distance of about 2 billion light years (for BH masses equal to 30 solar masses). Currently, Advanced Virgo is the detector in the network with the highest percentage of time spent observing the Universe. 90% of its operational time has been spent in observation mode; a remarkable success, which has mainly been limited only by maintenance work, which is planned in coordination with LIGO in order to optimise the network performance, and by occasional strong environmental disturbances. This very high duty-cycle reflects the accuracy with which Advanced Virgo is controlled and the stability of the instrumental noise. It also makes Virgo more likely to contribute to GW detections that are still to come during O3.
“In the Virgo Control room”, Fiori adds, “we shared hard work, long scientific discussions and strong feelings. On a daily basis, you could tell from people’s moods whether Virgo was progressing or stuck. There was great joy when, on the 1st of April, we inaugurated, along with LIGO, the start of the O3 run, with double the sensitivity with respect to 2017. ‘We did it!’, was a common cry. Now, each time we harvest a new GW signal there is renewed satisfaction.”
The very high duty cycle of Advanced Virgo, combined with its distance from and different orientation with respect to, the two Advanced LIGO interferometers, increases the capabilities of the detector network to localise GW sources in the sky and to fully understand the characteristics of the GW signals. Indeed a good sky localisation is the key factor that can lead to a successful campaign of electromagnetic observations and the wealth of scientific results that can follow.
Many detections are expected as Advanced LIGO and Advanced Virgo continue observations during O3, which is scheduled to last one year: more mergers of binaries of Black Holes and Neutron Stars, but hopefully, also new GW signals emitted by other types of astrophysical systems. Both the live data analysis methods that scan the data as soon as they are acquired, and the offline analyses that process big chunks of data using all the information available at the time, will continue the search for GW signals, expected or not.

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