Predicting electromagnetic counterparts using low-latency gravitational-wave data products

Searches for gravitational-wave counterparts have been going in earnest since GW170817 and the discovery of AT2017gfo. Since then, the lack of detection of other optical counterparts connected to binary neutron star or black hole–neutron star candidates has highlighted the need for a better discrimination criterion to support this effort. At the moment, low-latency gravitational-wave alerts contain preliminary information about binary properties and hence whether a detected binary might have an electromagnetic counterpart. The current alert method is a classifier that estimates the probability that there is a debris disc outside the black hole created during the merger as well as the probability of a signal being a binary neutron star, a black hole–neutron star, a binary black hole, or of terrestrial origin. In this work, we expand upon this approach to both predict the ejecta properties and provide contours of potential light curves for these events, in order to improve the follow-up observation strategy. The various sources of uncertainty are discussed, and we conclude that our ignorance about the ejecta composition and the insufficient constraint of the binary parameters by low-latency pipelines represent the main limitations. To validate the method, we test our approach on real events from the second and third Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO)–Virgo observing runs

Compact binary merger searches in the era of multimessenger high energy astrophysics

Les étoiles à neutrons et les trous noirs stellaires sont créés par l’effondrement gravitationnel qui survient à la fin des cycles de fusion nucléaire ayant lieu au sein des étoiles massives. Ils se trouvent soit isolés gravitationnellement soit en systèmes binaires, formant des binaires d’étoiles à neutrons (BNS), des systèmes trou noir - étoile à neutrons (NSBH) et des trous noirs binaires (BBH). Dans le dernier cas, lorsque les objets compacts sont en orbite l’un autour de l’autre, le système perd du moment cinétique par émission d’ondes gravitationnelles (GW). Ainsi la séparation diminue au cours du temps et, si la séparation initiale n’était pas trop grande, ceci pourrait conduire à une fusion durant l’âge de l’Univers. L’amplitude des GWs augmente avec l’approche du temps de la fusion, les rendant détectables par les interféromètres à GWs Advanced LIGO et Advanced Virgo. Si en plus, au moins un des membres de la binaires est une étoile à neutrons, une émission électromagnétique (EM) à haute énergie sous la forme d’un jet relativiste, appelé sursaut gamma court (GRB), activé par la chute de matière dans le disc d’accrétion entourant le trou noir nouvellement né, est attendue dans les secondes suivant ou précédant la fusion. Une radiation plus isotrope dans le spectre optique et infrarouge proche, appelée kilonova et alimentée par la désintégration radioactive des éléments lourds synthétisés lors du processus r, est émise à l’échelle des jours. Enfin, les contrecoups (afterglows) des GRBs courts, couvrant une large portion du spectre d’énergie allant des rayons X aux ondes radio, pourraient être détectés sur des échelles variant d’heures aux mois. GW170817 a été un événement astrophysique remarquable, montrant clairement l’existence de toutes ces signatures de GW/EM. En outre, des travaux théoriques prédisent également l’existence d’autres signaux lors de ces phénomènes violents, tels que l’émission des neutrinos et les précurseurs des GRBs. Dans cette thèse, plusieurs procédures sont présentées, dont le but est d’augmenter les chances de la détection de certains signaux de GW/EM, liés à la fusion d’une binaire compacte. On propose une méthode statistique pour la détection de l’association entre l’émission rapide du GRB, identifiable dans les données de Fermi-GBM, et le trigger de GW, rapporté par Advanced LIGO et/ou Advanced Virgo. L’idée derrière cela est de tirer profit de l’avantage du faible taux de coïncidence des bruits présents dans deux détecteurs distincts. On introduit aussi un outil destiné à la recherche d’un signal gamma modulé, détectable par les scintillateurs de Fermi-GBM, antérieurement à l’identification de l’événement de GW, par les interféromètres de seconde génération. Tandis que les émissions de rayonnement gamma et GWs devraient ếtre presque simultanées, la contrepartie kilonova est postérieure à la fusion, peut durer des jours et, la plupart du temps, est détectée par les télescopes terrestres. On propose ici deux méthodes visant à aider la communauté d’observateurs de EM, dans son but de détecter des kilonovas. D’abord on suggère l’utilisation d’un algorithme à base d’apprentissage automatique pour distinguer parmi des courbes de lumière photométriques attribuées aux kilonovas ciblées, aux supernovas dominant la liste de candidats et aux autres transients optiques non désirés. En second lieu, des courbes de lumière photométriques, pour des kilonovas, sont proposées, avec des délais de l’ordre des minutes, à partir des données de GWs, de faible latence. Le futur proche sera caractérisé par une multitude de signaux de natures différentes, rendant l’interprétation des données très difficile. Dans ce monde passionnant, il n’y aura de place que pour les méthodes d’analyse de données les plus efficaces.Neutron stars and stellar mass black holes are the remnants of massive stars, caused by the gravitational collapse taking place at the end of their nuclear fusion based life cycles. They can be found gravitationally isolated or in binary systems, forming binary neutron stars (BNS), neutron star-black holes (NSBH) and binary black holes (BBH). In the latter case, when the compact objects orbit each other, the system is loosing angular momentum by emission of gravitational waves (GW). Thus the orbital separation shrinks over time and, if the initial separation is not too high, it might lead to a merger during the Universe lifetime. The amplitude of the GWs increases with the approach of the coalescence time, making them detectable by the GW interferometers Advanced LIGO and Advanced Virgo. If in addition, at least one of the binary components is a neutron star, a relativistic beamed high energy electromagnetic (EM) emission, called short gamma ray burst (GRB), activated by the infalling matter into the accretion disc surrounding the newly born black hole, is expected in the seconds following or preceding the merger. A more isotropic radiation in both the optical and near infrared spectrum, called kilonova and powered by the radioactive decay of the synthesized r-process heavy elements, is emitted on days timescale. Finally the short GRB afterglows spanning a wide energy spectrum going from X-rays to radio wavelengths might be detectable on timescales varying from hours to months. GW170817 was a remarkable astrophysical event, showing clear evidence for all these GW/EM signatures. Moreover, theoretical work predict the existence of some other signals emitted by these violent phenomena such as the emission of neutrinos or gamma-ray burst precursors. In this thesis, several procedures whose aim is to increase the chances of detecting some GW/EM signals linked to a compact binary merger are presented. We propose a statistical method for the detection of the association between the prompt GRB emission, identifiable in the Fermi-GBM data, and the GW trigger, reported by Advanced LIGO and/or Advanced Virgo. The idea behind it is to take advantage of the low coincident rate of backgrounds in two distinct detectors. We also introduce a tool aimed to search for a modulated gamma-ray signal detectable by the Fermi-GBM scintillators, prior to a GW event identified by the Advanced interferometers. Whereas the emissions of gamma-ray radiation and GWs are expected to be almost simultaneous, the kilonova counterpart follows the merger, can last for days and is tracked most of the time by the terrestrial telescopes. We propose here two methods aiming to help the EM observers community in trying to detect kilonovae. Firstly, we suggest the use of a machine learning based algorithm in order to distinguish between photometric lightcurves attributed to the desired kilonovae, the dominating background supernovae and other background optical transients. Secondly, GW low-latency based kilonovae photometric lightcurves are proposed with delays of the order of minutes. The near future will be characterized by a plethora of signals of different natures, making the data interpretation very challenging. In this exciting world, there will be place only for the most efficient data analysis. methods

Recherches de fusions d’objets compacts à l’ère de l’astrophysique multi-messagers des hautes énergies

Neutron stars and stellar mass black holes are the remnants of massive stars, caused by the gravitational collapse taking place at the end of their nuclear fusion based life cycles. They can be found gravitationally isolated or in binary systems, forming binary neutron stars (BNS), neutron star-black holes (NSBH) and binary black holes (BBH). In the latter case, when the compact objects orbit each other, the system is loosing angular momentum by emission of gravitational waves (GW). Thus the orbital separation shrinks over time and, if the initial separation is not too high, it might lead to a merger during the Universe lifetime. The amplitude of the GWs increases with the approach of the coalescence time, making them detectable by the GW interferometers Advanced LIGO and Advanced Virgo. If in addition, at least one of the binary components is a neutron star, a relativistic beamed high energy electromagnetic (EM) emission, called short gamma ray burst (GRB), activated by the infalling matter into the accretion disc surrounding the newly born black hole, is expected in the seconds following or preceding the merger. A more isotropic radiation in both the optical and near infrared spectrum, called kilonova and powered by the radioactive decay of the synthesized r-process heavy elements, is emitted on days timescale. Finally the short GRB afterglows spanning a wide energy spectrum going from X-rays to radio wavelengths might be detectable on timescales varying from hours to months. GW170817 was a remarkable astrophysical event, showing clear evidence for all these GW/EM signatures. Moreover, theoretical work predict the existence of some other signals emitted by these violent phenomena such as the emission of neutrinos or gamma-ray burst precursors. In this thesis, several procedures whose aim is to increase the chances of detecting some GW/EM signals linked to a compact binary merger are presented. We propose a statistical method for the detection of the association between the prompt GRB emission, identifiable in the Fermi-GBM data, and the GW trigger, reported by Advanced LIGO and/or Advanced Virgo. The idea behind it is to take advantage of the low coincident rate of backgrounds in two distinct detectors. We also introduce a tool aimed to search for a modulated gamma-ray signal detectable by the Fermi-GBM scintillators, prior to a GW event identified by the Advanced interferometers. Whereas the emissions of gamma-ray radiation and GWs are expected to be almost simultaneous, the kilonova counterpart follows the merger, can last for days and is tracked most of the time by the terrestrial telescopes. We propose here two methods aiming to help the EM observers community in trying to detect kilonovae. Firstly, we suggest the use of a machine learning based algorithm in order to distinguish between photometric lightcurves attributed to the desired kilonovae, the dominating background supernovae and other background optical transients. Secondly, GW low-latency based kilonovae photometric lightcurves are proposed with delays of the order of minutes. The near future will be characterized by a plethora of signals of different natures, making the data interpretation very challenging. In this exciting world, there will be place only for the most efficient data analysis. methods.Les étoiles à neutrons et les trous noirs stellaires sont créés par l’effondrement gravitationnel qui survient à la fin des cycles de fusion nucléaire ayant lieu au sein des étoiles massives. Ils se trouvent soit isolés gravitationnellement soit en systèmes binaires, formant des binaires d’étoiles à neutrons (BNS), des systèmes trou noir - étoile à neutrons (NSBH) et des trous noirs binaires (BBH). Dans le dernier cas, lorsque les objets compacts sont en orbite l’un autour de l’autre, le système perd du moment cinétique par émission d’ondes gravitationnelles (GW). Ainsi la séparation diminue au cours du temps et, si la séparation initiale n’était pas trop grande, ceci pourrait conduire à une fusion durant l’âge de l’Univers. L’amplitude des GWs augmente avec l’approche du temps de la fusion, les rendant détectables par les interféromètres à GWs Advanced LIGO et Advanced Virgo. Si en plus, au moins un des membres de la binaires est une étoile à neutrons, une émission électromagnétique (EM) à haute énergie sous la forme d’un jet relativiste, appelé sursaut gamma court (GRB), activé par la chute de matière dans le disc d’accrétion entourant le trou noir nouvellement né, est attendue dans les secondes suivant ou précédant la fusion. Une radiation plus isotrope dans le spectre optique et infrarouge proche, appelée kilonova et alimentée par la désintégration radioactive des éléments lourds synthétisés lors du processus r, est émise à l’échelle des jours. Enfin, les contrecoups (afterglows) des GRBs courts, couvrant une large portion du spectre d’énergie allant des rayons X aux ondes radio, pourraient être détectés sur des échelles variant d’heures aux mois. GW170817 a été un événement astrophysique remarquable, montrant clairement l’existence de toutes ces signatures de GW/EM. En outre, des travaux théoriques prédisent également l’existence d’autres signaux lors de ces phénomènes violents, tels que l’émission des neutrinos et les précurseurs des GRBs. Dans cette thèse, plusieurs procédures sont présentées, dont le but est d’augmenter les chances de la détection de certains signaux de GW/EM, liés à la fusion d’une binaire compacte. On propose une méthode statistique pour la détection de l’association entre l’émission rapide du GRB, identifiable dans les données de Fermi-GBM, et le trigger de GW, rapporté par Advanced LIGO et/ou Advanced Virgo. L’idée derrière cela est de tirer profit de l’avantage du faible taux de coïncidence des bruits présents dans deux détecteurs distincts. On introduit aussi un outil destiné à la recherche d’un signal gamma modulé, détectable par les scintillateurs de Fermi-GBM, antérieurement à l’identification de l’événement de GW, par les interféromètres de seconde génération. Tandis que les émissions de rayonnement gamma et GWs devraient ếtre presque simultanées, la contrepartie kilonova est postérieure à la fusion, peut durer des jours et, la plupart du temps, est détectée par les télescopes terrestres. On propose ici deux méthodes visant à aider la communauté d’observateurs de EM, dans son but de détecter des kilonovas. D’abord on suggère l’utilisation d’un algorithme à base d’apprentissage automatique pour distinguer parmi des courbes de lumière photométriques attribuées aux kilonovas ciblées, aux supernovas dominant la liste de candidats et aux autres transients optiques non désirés. En second lieu, des courbes de lumière photométriques, pour des kilonovas, sont proposées, avec des délais de l’ordre des minutes, à partir des données de GWs, de faible latence. Le futur proche sera caractérisé par une multitude de signaux de natures différentes, rendant l’interprétation des données très difficile. Dans ce monde passionnant, il n’y aura de place que pour les méthodes d’analyse de données les plus efficaces

Differentiating the signal from the noise: towards optimal choices of wide field-of-view telescope transient follow-up

With the advent of the follow-up of large sky localization regions from gravitational-wave detectors and gamma-ray burst telescopes with wide field-of-view telescopes, the need for efficient follow-up of the many identified candidates is required. Due to limited telescope time, it is important to create prioritized lists of the many candidates identified. Towards this end, we use \text{\astrorapid}, a multi-band photometric lightcurve classifier, to differentiate between kilonovae, supernovae and other possible transients. We demonstrate our method on both ideally sampled, simulated lightcurves, as well as the photometric observations of real events. We show that after only a few days of observations of an astronomical object, it is possible to rule out candidates as supernovae and other known transient

Using machine learning for transient classification in searches for gravitational-wave counterparts

International audienceThe large sky localization regions offered by the gravitational-wave interferometers require efficient follow-up of the many counterpart candidates identified by the wide field-of-view telescopes. Given the restricted telescope time, the creation of prioritized lists of the many identified candidates becomes mandatory. Towards this end, we use astrorapid, a multiband photometric light-curve classifier, to differentiate between kilonovae, supernovae, and other possible transients. We demonstrate our method on the photometric observations of real events. In addition, the classification performance is tested on simulated light curves, both ideally and realistically sampled. We show that after only a few days of observations of an astronomical object, it is possible to rule out candidates as supernovae and other known transients

Searches for Modulated γ-Ray Precursors to Compact Binary Mergers in Fermi-GBM Data

International audienceGW170817 is the only gravitational-wave event for which a confirmed γ-ray counterpart, GRB 170817A, has been detected. Here, we present a method to search for another type of γ-ray signal, a γ-ray burst precursor, associated with a compact binary merger. If emitted shortly before the coalescence, a high-energy electromagnetic (EM) flash travels through a highly dynamical and relativistic environment, created by the two compact objects orbiting each other. Thus, the EM signal arriving at an Earth observer could present a somewhat predictable time-dependent modulation. We describe a targeted search method for light curves exhibiting such a modulation, parameterized by the observer-frame component masses and binary merger time, using Fermi-GBM data. The sensitivity of the method is assessed based on simulated signals added to GBM data. The method is then applied to a selection of potentially interesting compact binary mergers detected during the second (O2) and third (O3) observing runs of Advanced LIGO and Advanced Virgo. We find no significant modulated γ-ray precursor signal associated with any of the considered events

Deep Multimessenger Search for Compact Binary Mergers in LIGO, Virgo, and Fermi/GBM Data from 2016–2017

GW170817–GRB 170817A provided the first observation of gravitational waves from a neutron star merger with associated transient counterparts across the entire electromagnetic spectrum. This discovery demonstrated the long-hypothesized association between short gamma-ray bursts and neutron star mergers. More joint detections are needed to explore the relation between the parameters inferred from the gravitational wave and the properties of the gamma-ray burst signal. We developed a joint multimessenger analysis of LIGO, Virgo, and Fermi/GBM data designed for detecting weak gravitational-wave transients associated with weak gamma-ray bursts. As such, it does not start from confident (GWTC-1) events only. Instead, we take the full list of existing compact binary coalescence triggers generated with the PyCBC pipeline from the second Gravitational-Wave Observing Run (O2), and reanalyze the entire set of public Fermi/GBM data covering this observing run to generate a corresponding set of gamma-ray burst candidate triggers. We then search for coincidences between the gravitational-wave and gamma-ray burst triggers without requiring a confident detection in any channel. The candidate coincidences are ranked according to a statistic combining each candidate’s strength in gravitational-wave and gamma-ray data, their time proximity, and the overlap of their sky localization. The ranking is then converted to a false alarm rate using time shifts between the gravitational-wave and gamma-ray burst triggers. We present the results using O2 triggers, which allowed us to check the validity of our method against GW170817–GRB 170817A. We also discuss the different configurations tested to maximize the significance of the joint detection

Predicting electromagnetic counterparts using low-latency gravitational-wave data products

International audienceSearches for gravitational-wave counterparts have been going in earnest since GW170817 and the discovery of AT2017gfo. Since then, the lack of detection of other optical counterparts connected to binary neutron star or black hole–neutron star candidates has highlighted the need for a better discrimination criterion to support this effort. At the moment, low-latency gravitational-wave alerts contain preliminary information about binary properties and hence whether a detected binary might have an electromagnetic counterpart. The current alert method is a classifier that estimates the probability that there is a debris disc outside the black hole created during the merger as well as the probability of a signal being a binary neutron star, a black hole–neutron star, a binary black hole, or of terrestrial origin. In this work, we expand upon this approach to both predict the ejecta properties and provide contours of potential light curves for these events, in order to improve the follow-up observation strategy. The various sources of uncertainty are discussed, and we conclude that our ignorance about the ejecta composition and the insufficient constraint of the binary parameters by low-latency pipelines represent the main limitations. To validate the method, we test our approach on real events from the second and third Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO)–Virgo observing runs

Search for Lensing Signatures in the Gravitational-Wave Observations from the First Half of LIGO–Virgo’s Third Observing Run

International audienceWe search for signatures of gravitational lensing in the gravitational-wave signals from compact binary coalescences detected by Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) and Advanced Virgo during O3a, the first half of their third observing run. We study: (1) the expected rate of lensing at current detector sensitivity and the implications of a non-observation of strong lensing or a stochastic gravitational-wave background on the merger-rate density at high redshift; (2) how the interpretation of individual high-mass events would change if they were found to be lensed; (3) the possibility of multiple images due to strong lensing by galaxies or galaxy clusters; and (4) possible wave-optics effects due to point-mass microlenses. Several pairs of signals in the multiple-image analysis show similar parameters and, in this sense, are nominally consistent with the strong lensing hypothesis. However, taking into account population priors, selection effects, and the prior odds against lensing, these events do not provide sufficient evidence for lensing. Overall, we find no compelling evidence for lensing in the observed gravitational-wave signals from any of these analyses

Search for intermediate-mass black hole binaries in the third observing run of Advanced LIGO and Advanced Virgo

International audienceIntermediate-mass black holes (IMBHs) span the approximate mass range 100−105 M⊙, between black holes (BHs) that formed by stellar collapse and the supermassive BHs at the centers of galaxies. Mergers of IMBH binaries are the most energetic gravitational-wave sources accessible by the terrestrial detector network. Searches of the first two observing runs of Advanced LIGO and Advanced Virgo did not yield any significant IMBH binary signals. In the third observing run (O3), the increased network sensitivity enabled the detection of GW190521, a signal consistent with a binary merger of mass ∼150 M⊙ providing direct evidence of IMBH formation. Here, we report on a dedicated search of O3 data for further IMBH binary mergers, combining both modeled (matched filter) and model-independent search methods. We find some marginal candidates, but none are sufficiently significant to indicate detection of further IMBH mergers. We quantify the sensitivity of the individual search methods and of the combined search using a suite of IMBH binary signals obtained via numerical relativity, including the effects of spins misaligned with the binary orbital axis, and present the resulting upper limits on astrophysical merger rates. Our most stringent limit is for equal mass and aligned spin BH binary of total mass 200 M⊙ and effective aligned spin 0.8 at 0.056 Gpc−3 yr−1 (90% confidence), a factor of 3.5 more constraining than previous LIGO-Virgo limits. We also update the estimated rate of mergers similar to GW190521 to 0.08 Gpc−3 yr−1.Key words: gravitational waves / stars: black holes / black hole physicsCorresponding author: W. Del Pozzo, e-mail: [email protected]† Deceased, August 2020