LISA Astrophysics Working Group Meeting

9 Mar 2020, 12:00 → 11 Mar 2020, 13:45 Europe/Amsterdam

Chapel (Berchmanianum, Nijmegen)

Gijs Nelemans (Radboud University), Lucio Mayer (University of Zurich), Marta Volonteri (IAP), Shane Larson (Northwestern University)

This is the second meeting of the LISA Astrophysics Working Group. The aims of this meeting are

• discuss LISA science
• to provide up to date information about the status of ESA's LISA mission and the LISA consortium
• make plans for the WG activities
• provide a forum for exchange of ideas and building of research collaborations

The meeting will take place at the campus of Radboud University, Nijmegen and can be reached by train or by plane via Amsterdam Shiphol, Düsseldorf or Eindhoven airports.

You can submit an abstract if you would like to present something, but we will need a lot of time for information and discussions, so the space for contributions is limied!

Monday, 9 March

12:00 → 13:00 lunch

13:00 → 13:40 LISA and the consortium (zoom)

Speaker: Martin Hewitson

13:40 → 14:00 The LISA Early Career Scientists

Speakers: Dr Thomas Kupfer (The Kavli Institute for Theoretical Physics), Dr Valeriya Korol (University of Birmingham)

Slides LECS_LISA_Early_Career_Scientist_Group_(1).pdf

14:00 → 14:30 The AstroWG and LISA work packages

Speaker: Elena Rossi

14:30 → 15:00 The Astrophysics WG

Speaker: Chairs

15:00 → 15:20 coffee/tea

15:20 → 17:30 First discussion white paper: assign section captains

Tuesday, 10 March

09:00 → 09:15 Investigating Coalescing Neutron-Star-White-Dwarf Binaries for LISA (zoom)

Prime candidate sources for LISA are the numerous Galactic tight binaries of white dwarfs (WDs) and neutron stars (NSs), many of which will coalesce and undergo mass transfer, leading to simultaneous emission of X-rays and gravitational waves (GWs). I present ongoing work on modelling these sources and demonstrate evolutionary tracks of their characteristic strain amplitude and dynamical chirp masses. I show how a precise detection of the chirp masses in detached systems allows determination of the NS mass to an accuracy of a few percent. Finally, I discuss dual-line GW sources.

Speaker: Prof. Thomas Tauris (Aarhus University)

Paper Tauris_2018_PRL.pdf

Slides Nijmegen_LISA-2020_Tauris.pdf

09:15 → 09:30 Galactic astronomy with LISA

Gravitational wave signals from ultra-short period double white dwarf binaries detectable by LISA are unique Galactic stellar tracers. I will show that as an all-sky survey that does not suffer from contamination and dust extinction LISA will map the Milky Way and environs. In particular, the density distribution of DWDs detected by LISA can be used to constrain scale parameters of the Milky Way's bulge, disc and central bar. Finally, I will show that LISA can detect known classical dwarf satellites and potentially discover new ones.

Speaker: Dr Valeriya Korol (University of Birmingham)

Slides Korol_Galactic_Astronomy_with_LISA.pdf

09:30 → 09:45 IMBH Binary Evolution in Nucleated Dwarf Galaxies (zoom)

Speaker: Dr Fazeel Mahmood Khan (Institute of Space Technology Islamabad)

09:45 → 10:00 The Zwicky Transient Facility high-cadence Galactic Plane survey

The Zwicky Transient Facility (ZTF) started science operation in March 2018. 40% of ZTF observing time is dedicated to two public surveys: one covering the entire Northern sky every three nights in g and r passbands and one visiting the Galactic Plane every night in g and r. In addition to the public Galactic Plane survey, ZTF has conducted a dedicated high-cadence survey of selected Galactic Plane fields to find and study ultracompact Galactic binaries. So far we have covered about 4900 sqd. In this talk, I will present an overview of the ZTF Galactic Plane observations and present some science highlights from the first two years, including a new class of ultracompact He-star binaries which are detectable for LISA.

Speaker: Dr Thomas Kupfer (The Kavli Institute for Theoretical Physics)

Slides lisa_wg_nijmegen.pdf

10:00 → 10:15 An Optical Survey for LISA Detectable Binaries

We give an overview of a survey we are conducting using the Zwicky Transient Facility to discover new LISA detectable binary systems. This survey makes use of large scale periodicity searches, with follow up to confirm promising candidates using high-speed photometry and phase-resolved spectroscopy, when feasible. Thus far, we have discovered and confirmed 14 new LISA detectable binary systems. The discovery engine we have developed will be directly applicable to LSST, and we discuss simulations we have performed both estimating the completeness of our search in ZTF data and how complete we anticipate such searches being in LSST, given its significantly lower cadence.

Speaker: Jan van Roestel

10:15 → 10:45 coffee/tea

10:45 → 11:00 Sparsity Based Recovery of Galactic Binaries Gravitational Waves

The detection of galactic binaries as sources of gravitational waves promises an unprecedented wealth of information about these systems, but also raises several challenges in signal processing. In particular the large number of expected sources and the risk of misidentification call for the development of robust methods. We describe here an original non-parametric reconstruction of the imprint of galactic binaries in measurements affected by instrumental noise typical of the space-based gravitational wave observatory LISA. We assess the impact of various approaches to sparsity and focus on adaptive structured block sparsity. We carefully show that a sparse representation gives a reliable access to the physical content of the interferometric measurement. In particular we check the successful fast extraction of the gravitational wave signal on a simple yet realistic example involving verification galactic binaries recently proposed in LISA data challenges.

Speaker: Ms Aurore Blelly (CEA/IRFU)

11:00 → 11:15 Improved gravitational radiation time-scales: significance for LISA and LIGO-Virgo sources

We compute a revised version of Peters' (1964) time-scale for the gravitational-wave (GW) induced decay of two point masses, by taking into account post-Newtonian (PN) perturbations of the orbital motion. In the standard approach of neglecting the time evolution of the eccentricity, the corrected time-scale can be approximated by multiplying Peters' estimate by the simple factor $Q = 1 + 5 (r_{\rm S}/p)$, where $p$ is the periapsis and $r_{\rm S}$ the Schwarzschild radius of the system. The difference between the Newtonian and the corrected result increases if large eccentricities (-0.5) are evolved self-consistently, and we provide both a fit and a simple recipe to compute the correct time-scale. We apply the revised time-scalereds to a set of typical sources for existing ground-based laser interferometers and for the future Laser Interferometer Space Antenna (LISA), at the onset of their GW driven decay. We argue that our more accurate model for the orbital evolution will affect current event- and detection-rate estimates for mergers of compact object binaries, with stronger deviations for LISA sources, such as extreme and intermediate mass-ratio inspirals and supermassive black hole binaries. We propose the correction factor $Q$ as a simple analytical prescription to quantify decay time-scales more accurately in future population synthesis models.

Speaker: Lorenz Zwick (ICS UZH)

11:15 → 11:30 A phase shift of gravitational waves induced by aberration (zoom)

The velocity of a gravitational wave (GW) source provides crucial information about its formation and evolution processes. Previous studies considered the Doppler effect on the phase of GWs as a potential means to detect a time-dependent velocity of the source. However, the Doppler shift only accounts for the time component of the wave vector, and in principle motion also affects the spatial components. The latter effect, known as "aberration'' for light, is analyzed in this Letter for GWs and applied to the waveform modeling of an accelerating source. We show that the additional aberrational phase shift could be detectable in two astrophysical scenarios, namely, a recoiling binary black hole (BBH) due to GW radiation and a BBH in a triple system. Our results suggest that adding the aberrational phase shift in the waveform templates could significantly enhance the detectability of moving sources.

Speaker: Alejandro Torres-Orjuela (Peking University)

11:30 → 11:45 LISA Waveform development and you

Waveform templates are necessary prerequisite for detecting gravitational waves and extracting information from them. The sources that LISA will observe set various new challenges for waveform development related to mass-ratio, eccentricity, and required waveform accuracy. I will review the waveform development activities in the LISA consortium, including both LISA science group work packages, and the activities of the LISA Waveform Working group. In particular, I will focus on aspects where input of the Astrophysics Working Group is needed.

Speaker: Maarten van de Meent (AEI Potsdam)

Slides mvdm_AstroWG2020.pdf

12:00 → 13:30 lunch

13:30 → 13:45 The stochastic pairing of massive black hole binaries in the young Universe

Massive black hole (MBH) binaries will be among the brightest sources of gravitational waves in the LISA band. LISA will capture signals from coalescing binaries up and above z~20, probing the clustering of MBHs at the onset of galaxy formation. In this talk, I will present the results from a zoom-in cosmological simulation exploring the large-scale pairing of MBHs in a main-sequence, turbulent galaxy at z>6. I will show that the dynamical-friction induced pairing has to compete with stochastic gravitational torques arising from the non-symmetric and warped host system; in particular, my simulation shows that the development of a galactic bar either accelerates or ultimately hinders the MBH inspiral. I will highlight the importance of accounting for such stochasticity when inferring the rates of MBH mergers, especially those occurring in high-redshift galaxies embedded in a realistic cosmological environment.

Speaker: Dr Elisa Bortolas (University of Zurich)

13:45 → 14:00 Analytical fits for parameter estimation of inspiralling MBH binaries in LISA (zoom)

The gravitational wave signal of coalescing massive binary black holes carries exquisite information on the physical parameters of the source. These long-living signals enable the estimate of the source parameters on flight, i.e. when the signal-to-noise-ratio is still rising and the uncertainties are narrowing down. For a set of binaries, we provide analytical fit to the running uncertainties of the sky localization, luminosity distance, chirp mass, and mass ratio as a function of their total mass, redshift and time to coalescence, one month to one hour prior to merging. Our aim is at guiding observers to program in advance LISA protected observing periods, to alert astronomers for a potential electromagnetic counterpart detectable during the inspiral phase and in proximity of the merger, taking advantage of the knowledge of the intrinsic parameters at the source and its coalescence time.

Speaker: Mr Alberto Mangiagli (University of Milan - Bicocca)

Slides mangiagli_astroWG.pdf

14:00 → 14:15 Black hole binary catalogs from the LGalaxies SAM model

I will present updates on the L-Galaxies semi-analytical model focusing on the new prescriptions for BH seeding and the tracking of spin evolution. The model is currently being used to generate catalogs of BH binaries and to study their environment.

Speaker: Dr Silvia Bonoli (DIPC)

14:15 → 14:30 Supermassive black hole merger rates, masses, and the mophological evolution of their host galaxies

With detections of supermassive black hole mergers expected in the near future from LISA and PTAs, now is the ideal time to make predictions on what we can expect from them. Using high-resolution cosmological simulations, I will discuss the expected merger and detection rates for such black hole mergers, including the need to incorporate inspiral/hardening times, with particular emphasis on the impact the merger timescale has on the merging masses. Furthermore, I will discuss what we can expect for the morphologies of the galaxies which host SMBH mergers (important for multimessenger followup observations to gravitational wave detections), and show that we can expect the host morphologies to exhibit evidence of recent galaxy mergers, with the strongest morphological signals at z~0.5 and typically surviving on the order of ~500 Myr.

Speaker: Dr Colin DeGraf (University of Cambridge)

14:30 → 17:30 session 3: White paper and projects discussion

18:00 → 21:00 conference dinner

Wednesday, 11 March

09:00 → 09:15 Stochastic background of extreme mass ratio inspirals (zoom)

Extreme mass ratio inspirals (EMRIs), i.e. binary systems comprised by a compact stellar-mass object orbiting a massive black hole, represent primary gravitational wave (GW) targets for the forthcoming LISA mission. Unfortunately, the astrophysical processes leading to the formation of such systems still remain poorly understood and, consequently, this translates into very large uncertainties for the detection rate of these GW sources, spanning at least three orders of magnitude. In this talk, by considering different astrophysical formation scenarios available in the literature, we present estimates of the stochastic GW background produced by EMRIs. In several formation scenarios, resulting in hundreds of detections during the timespan of the LISA mission, a large unresolved population of EMRIs may generate a significant stochastic noise in the LISA frequency band, that in the most extreme cases, can substantially limit the performance of the detector in the bucket of the sensitivity curve.

Speaker: Dr Matteo Bonetti (University of Milano-Bicocca)

09:15 → 09:30 Nuclear Stellar Clusters, Supermassive Black Holes, and Massive Star Production in Galactic Nuclei (zoom)

We review the ecology of galactic nuclei considering how supermassive black holes may form and grow in galactic nuclei and also how massive stars may be produced from low-mass stars via Bondi-Hoyle accretion within gaseous accretion discs. We suggest this is how the massive stars seen in the centre of the Milky Way were produced. Supermassive black holes are found in most galactic nuclei. A large fraction of these nuclei also contain a nuclear stellar cluster surrounding the black hole. Here we consider the idea that the nuclear stellar cluster formed first and that the supermassive black hole grew later. In particular we consider the merger of stellar clusters to form a nuclear stellar cluster, where some of these clusters contain a single intermediate-mass black hole (IMBH). In the cases where multiple clusters contain IMBHs, we discuss whether the black holes are likely to merge and whether such mergers are likely to result in the ejection of the merged black hole from the nuclear stellar cluster. In some cases, no supermassive black hole will form as any merger product is not retained. This is a natural pathway to explain those galactic nuclei that contain a nuclear stellar cluster but apparently lack a supermassive black hole; M33 being a nearby example. Alternatively, if an IMBH merger product is retained within the nuclear stellar cluster, it may subsequently grow, e.g. via the tidal disruption of stars, to form a supermassive black hole. Gaseous accretion discs may be produced in galactic nuclei for example by the tidal shredding of giant molecular clouds by supermassive black holes. We discuss how low-mass stars which find themselves inside such an accretion disc will grow by Bondi-Hoyle accretion producing massive stars, many of which will ultimately produce stellar-mass black holes. This process may explain the observed massive stars in the centre of the Milky Way. In addition this channel enhances enormously the number of stellar-mass black holes found in the very centres of galactic nuclei and thus will affect the EMRI rate.

Speaker: Prof. Melvyn Davies (Lund University)

09:30 → 09:45 Torques on gas-embedded intermediate mass ratio inspirals

Black hole coalescences may occur within the dense accretion disks of active galactic nuclei. For extreme/intermediate mass ratio inspirals that last for years in the LISA frequency band, interaction with the gas imparts a phase drift (a `gas imprint’) in the GW waveform that may be detectable and carry information on disk properties. I will present a suite of simulations with the hydrodynamical moving-mesh code DISCO of a two-dimensional, viscous disk with an embedded inspiraling BH, and show how the torques exerted by the gas are different from previous analytical estimates in their strength, direction, and temporal evolution, all of which are sensitive to the BH mass and disk parameters. I will discuss how torques can evolve throughout an inspiral depending on the system properties, under what conditions the gas imprint is detectable, and some critical next steps.

10:00 → 12:00 session 4: White paper discussion/writing assignment

12:00 → 13:00 lunch