Searching for binary black holes in the Milky Way and its neighborhood with LISA

Rafia Sarwar 1,2,3 , Simone S. Bavera 1 , Anastasios Fragkos 1

  • 1 Observatory of Geneva, University of Geneva, Chemin Pegasi 51, 1290, Versoix, Geneva
  • 2 Department of Space Science, Institute of Space Technology(IST), 1, Islamabad Highway, Islamabad, 44000
  • 3 Space and Astrophysics Research Lab (SARL), National Center for GIS and Space Applications (NCGSA), Institute of SpaceTechnology (IST), 1, Islamabad Highway, Islamabad, 44000

Abstract

In 2034, within the rapidly changing landscape of gravitational-wave astronomy, the Laser interferometer Space Antenna will be the first space-based detector that will observe the gravitational spectra in the millihertz frequency band. It has recently been proposed that numerous LIGO/VIRGO sources will also be detectable by LISA. LISA will be able to detect binary black holes from our Milky Way galaxy and its neighborhood, evolving from their early inspiral stages. Interestingly, the sources that appear to be circular in the LIGO band may be eccentric in the LISA band, depending on the earlier stages of their evolution. We aim to explore the gravitational-waves emitted from black hole binaries in our Milky Way galaxy and its neighborhood, as they are expected to be observable with LISA. Here, I will present models that combine simulation of Milky Way-like galaxy formation, and specifically, the Latte simulation from the Feedback in relativistic environments (FIRE-2) project, with the new binary population synthesis code POSYDON to investigate the detectability of inspiraling binary black hole populations in both the LISA and the LIGO frequency bands, as a function of eccentricity and their horizon distances, using a Monte-Carlo approach. Furthermore, I will discuss how one can disentangle different formation channels of these binaries using LISA, and estimate the rate and observable properties with which these binaries form in the Milky Way galaxy and other nearby galaxies.

Introduction

Understanding the population of binary stars and their remnants is fundamental to unveil the formation of galaxies. Properties of binary systems like black holes, neutron stars, white dwarfs, and pulsars and X-ray binaries inform us about their progenitor stars, formation channels, environmental conditions, metallicities, and earlier histories of their mergers [1].

     We use the next-generation population synthesis code POSYDON [2] combine with the publicly available data from one of the most advanced galaxy formation simulations, the Latte simulation [3, 4] from the Feedback in relativistic environments (FIRE-2 [5]) project, which provides spatially resolved information about the star-formation history and metallicity evolution of Milky Way-type galaxies and their environments. These synthetic catalogues of the black hole binaries are then convolved with metallicity dependent star-formation history appropriate for the Milky Way neighbourhood and Laser Interferometer Space Antenna (LISA) selections effects [6] to finally produce predictions for observable properties and merger rates [7].

Formation channels of isolated BBHs

Results

Figure 1: The plot shows the signal to noise ratio (ρ) for black holes binaries of M1 = M2 = 10M☉ (lines) and M1 = M2 = 30M☉ (dotted) as a function of their merger times Tmerge(yrs) with LISA for different distances from 1 kpc to as far as 100 kpcs. The horizontal (dashed) red line represents here the detection threshold of SNR ρthr = 8. The vertically shaded black and grey regions represent LISA observing times Tobs = 4 years and Tobs = 10 years, respectively [9].

Would it be possible to detect merging Galactic black hole binaries with LISA?

By Figure 1, a black hole binary placed at 1 kpc can have a merger timescale up to 10 million years and still be detectable with LISA. This would correspond to a present-day orbital period of few hours. Even if we will not be able to have any merging binary black hole from the Milky Way galaxy during the LISA mission, we might be able to detect galactic binaries at different stages of their inspiral that might enable us to distinguish between their formation channels [10].

Article 4

Figure 2: The distribution of observable properties of black hole binaries chirp mass, effective spin parameter χeff and mass ratio q, that would merge within the next 20 years.

Would we be able to distinguish between the formation channels of Galactic black hole binaries using the LISA detector?

The observable properties of black hole binaries for the common envelope and stable mass transfer produces very similar distributions, except for the chirp mass where the stable mass transfer gives support for slightly higher masses.

     The most striking difference in Figure 2 is the lack of fast-spinning black holes in the common envelope channel in the Milky way populations. This is because the Milky Way has formed relatively fewer metal-poor stars, compared to the average Universe, and it is those very metal-poor stars that are forming the fast-spinning binary black holes.

 

Take away messages:

  • Merging binary black holes can form in the Milky Way Galaxy via isolated binary evolution
  • Their property distributions are not the same as the binary black holes of the whole Universe 
  • Preliminary results show that disentangling formation channels with LISA can be challenging. Observed eccentricity might be the key! 

Creating the catalog of Galactic BBHs

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References:

  1. A Lamberts et al., 2018, MNRAS, 480(2):2704–2718.
  2. Fragos et al., 2021, in prep.
  3. Wetzel A. R et al., 2016, ApJ, 827, L23.
  4. Hopkins P. F., 2015, MNRAS, 450, 53.
  5. Sanderson et al., 2020, ApJ, 246.
  6. Robson et al., 2019, Class.Quant.Grav., 36, 105011.
  7. Alberto Sesana, 2017, J. Phys.: Conference Series, 840:012018.
  8. Bavera et al., 2021, A&A 647, A153.
  9. Davide Gerosa et al., 2019, Phys. Rev. D, 99(10).
  10. K. Breivik., et al., 2016, ApJ , 830, L18.