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Modelling and Detecting Kilonovae in the Rubin Observatory Era
Stockholm University, Faculty of Science, Department of Physics.ORCID iD: 0000-0002-7439-2735
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Survey astronomy is a powerful tool for discoveries in astrophysics and cosmology. In the coming years, this field will be revolutionised with the start of the ten-year Legacy Survey of Space and Time (LSST), to be conducted at the Vera C. Rubin Observatory. This survey, with its unique capabilities in temporal sampling, single-image depth and covered sky-area, will explore a new discovery space for astrophysical transients in the Universe. The 2017 discovery of an electromagnetic and gravitational-wave transient presents a unique opportunity to influence the design of the LSST observing strategy for the detection of binary neutron star (BNS) mergers. This will be scientifically beneficial, not only for studies of the astrophysics of these sources, but also for developing new cosmological probes. Given the sensitivity of the Rubin Observatory, it is expected that this instrument will detect these binary neutron star mergers to greater distances than detectable by current and near-term gravitational wave detectors. This presents further opportunities to study the characteristics of the BNS population that will be selected into these surveys. If we understand the underlying BNS merger population and associated electromagnetic emission, it may also be possible to recover the previously undetected counterpart gravitational wave signals.

In this thesis I discuss kilonovae (kNe) from BNS mergers with a focus on detection of kNe in the LSST survey. I will discuss the physics and modelling of kNe, including my work incorporating a viewing-angle dependence in the optical light curve modelling of BNS kNe. After setting the context for the Rubin Observatory and the LSST, I will describe work on optimising the observing strategy of the LSST to detect kNe from BNS mergers and the observing strategy features that impact detection. This work also indicates that a portion of the BNS mergers associated with kN detections in the LSST will be below the threshold for detection of their gravitational wave emission. Furthermore, I will discuss modelling a population of kNe from BNS mergers that is consistent with each merger’s associated gravitational-wave signal. This modelling includes a dependence of the kN on nuclear physics calibrated with detailed emulation of radiation-transport simulations. I conclude by summarising the scientific impact of this research and discussing future directions, such as: studying the BNS multi-messenger observational selection function for the LSST and concurrent gravitational wave detectors, detection of subthreshold signals, and the problem of classifying kN light curves.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2024. , p. 114
Keywords [en]
cosmology, transient surveys, kilonovae, neutron stars, population modelling
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-227381ISBN: 978-91-8014-711-8 (print)ISBN: 978-91-8014-712-5 (electronic)OAI: oai:DiVA.org:su-227381DiVA, id: diva2:1844032
Public defence
2024-04-30, sal FB54, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2024-04-05 Created: 2024-03-12 Last updated: 2024-03-22Bibliographically approved
List of papers
1. Optimizing the LSST Observing Strategy for Dark Energy Science: DESC Recommendations for the Wide-Fast-Deep Survey
Open this publication in new window or tab >>Optimizing the LSST Observing Strategy for Dark Energy Science: DESC Recommendations for the Wide-Fast-Deep Survey
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2018 (English)Manuscript (preprint) (Other academic)
Abstract [en]

Cosmology is one of the four science pillars of LSST, which promises to be transformative for our understanding of dark energy and dark matter. The LSST Dark Energy Science Collaboration (DESC) has been tasked with deriving constraints on cosmological parameters from LSST data. Each of the cosmological probes for LSST is heavily impacted by the choice of observing strategy. This white paper is written by the LSST DESC Observing Strategy Task Force (OSTF), which represents the entire collaboration, and aims to make recommendations on observing strategy that will benefit all cosmological analyses with LSST. It is accompanied by the DESC DDF (Deep Drilling Fields) white paper (Scolnic et al.). We use a variety of metrics to understand the effects of the observing strategy on measurements of weak lensing, large-scale structure, clusters, photometric redshifts, supernovae, strong lensing and kilonovae. In order to reduce systematic uncertainties, we conclude that the current baseline observing strategy needs to be significantly modified to result in the best possible cosmological constraints. We provide some key recommendations: moving the WFD (Wide-Fast-Deep) footprint to avoid regions of high extinction, taking visit pairs in different filters, changing the 2x15s snaps to a single exposure to improve efficiency, focusing on strategies that reduce long gaps (>15 days) between observations, and prioritizing spatial uniformity at several intervals during the 10-year survey.

Publisher
p. 20
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-177889 (URN)
Available from: 2020-01-09 Created: 2020-01-09 Last updated: 2024-03-12
2. Serendipitous discoveries of kilonovae in the LSST main survey: maximizing detections of sub-threshold gravitational wave events
Open this publication in new window or tab >>Serendipitous discoveries of kilonovae in the LSST main survey: maximizing detections of sub-threshold gravitational wave events
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2019 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 485, no 3, p. 4260-4273Article in journal (Refereed) Published
Abstract [en]

We investigate the ability of the Large Synoptic Survey Telescope (LSST) to discover kilonovae (kNe) from binary neutron star (BNS) and neutron star-black hole (NSBH) mergers, focusing on serendipitous detections in the Wide-Fast-Deep (WFD) survey. We simulate observations of kNe with proposed LSST survey strategies, focusing on cadence choices that are compatible with the broader LSST cosmology programme. If all kNe are identical to GW170817, we find the baseline survey strategy will yield 58 kNe over the survey lifetime. If we instead assume a representative population model of BNS kNe, we expect to detect only 27 kNe. However, we find the choice of survey strategy significantly impacts these numbers and can increase them to 254 and 82 kNe over the survey lifetime, respectively. This improvement arises from an increased cadence of observations between different filters with respect to the baseline. We then consider the detectability of these BNS mergers by the Advanced LIGO/Virgo (ALV) detector network. If the optimal survey strategy is adopted, 202 of the GW170817-like kNe and 56 of the BNS population model kNe are detected with LSST but are below the threshold for detection by the ALV network. This represents, for both models, an increase by a factor greater than 4.5 in the number of detected sub-threshold events over the baseline strategy. These subthreshold events would provide an opportunity to conduct electromagnetic-triggered searches for signals in gravitational-wave data and assess selection effects in measurements of the Hubble constant from standard sirens, e.g. viewing angle effects.

Keywords
gravitational waves, surveys, binaries: general, stars: neutron, stars: black holes, cosmology: observations
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-172069 (URN)10.1093/mnras/stz506 (DOI)000474902000096 ()2-s2.0-85066984323 (Scopus ID)
Available from: 2019-08-22 Created: 2019-08-22 Last updated: 2024-03-12Bibliographically approved
3. The Impact of Observing Strategy on Cosmological Constraints with LSST
Open this publication in new window or tab >>The Impact of Observing Strategy on Cosmological Constraints with LSST
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2022 (English)In: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 259, no 2, article id 58Article in journal (Refereed) Published
Abstract [en]

The generation-defining Vera C. Rubin Observatory will make state-of-the-art measurements of both the static and transient universe through its Legacy Survey for Space and Time (LSST). With such capabilities, it is immensely challenging to optimize the LSST observing strategy across the survey's wide range of science drivers. Many aspects of the LSST observing strategy relevant to the LSST Dark Energy Science Collaboration, such as survey footprint definition, single-visit exposure time, and the cadence of repeat visits in different filters, are yet to be finalized. Here, we present metrics used to assess the impact of observing strategy on the cosmological probes considered most sensitive to survey design; these are large-scale structure, weak lensing, type Ia supernovae, kilonovae, and strong lens systems (as well as photometric redshifts, which enable many of these probes). We evaluate these metrics for over 100 different simulated potential survey designs. Our results show that multiple observing strategy decisions can profoundly impact cosmological constraints with LSST; these include adjusting the survey footprint, ensuring repeat nightly visits are taken in different filters, and enforcing regular cadence. We provide public code for our metrics, which makes them readily available for evaluating further modifications to the survey design. We conclude with a set of recommendations and highlight observing strategy factors that require further research.

Keywords
Cosmology, Observational cosmology, Optical telescopes, Sky surveys
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-204482 (URN)10.3847/1538-4365/ac5033 (DOI)000778730700001 ()
Available from: 2022-05-09 Created: 2022-05-09 Last updated: 2024-03-12Bibliographically approved
4. Modelling populations of kilonovae
Open this publication in new window or tab >>Modelling populations of kilonovae
2023 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 520, no 2, p. 2829-2842Article in journal (Refereed) Published
Abstract [en]

The 2017 detection of a kilonova coincident with gravitational-wave emission has identified neutron star mergers as the major source of the heaviest elements and dramatically constrained alternative theories of gravity. Observing a population of such sources has the potential to transform cosmology, nuclear physics, and astrophysics. However, with only one confident multi-messenger detection currently available, modelling the diversity of signals expected from such a population requires improved theoretical understanding. In particular, models that are quick to evaluate and are calibrated with more detailed multi-physics simulations are needed to design observational strategies for kilonovae detection and to obtain rapid-response interpretations of new observations. We use grey-opacity models to construct populations of kilonovae, spanning ejecta parameters predicted by numerical simulations. Our modelling focuses on wavelengths relevant for upcoming optical surveys, such as the Rubin Observatory Legacy Survey of Space and Time (LSST). In these simulations, we implement heating rates that are based on nuclear reaction network calculations. We create a Gaussian-process emulator for kilonova grey opacities, calibrated with detailed radiative transfer simulations. Using recent fits to numerical relativity simulations, we predict how the ejecta parameters from binary neutron star (BNS) mergers shape the population of kilonovae, accounting for the viewing-angle dependence. Our simulated population of BNS mergers produce peak i-band absolute magnitudes of −20 ≤ Mi ≤ −11. A comparison with detailed radiative transfer calculations indicates that further improvements are needed to accurately reproduce spectral shapes over the full light curve evolution. 

Keywords
transients: neutron star mergers, stars: neutron, opacity, radiative transfer, methods: numerical
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-216283 (URN)10.1093/mnras/stad257 (DOI)000943150500010 ()
Available from: 2023-04-13 Created: 2023-04-13 Last updated: 2024-03-12Bibliographically approved

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