My current research mainly focuses on binary stars composed of two white dwarf stars. We know from theoretical studies that these double white dwarf binaries should be very numerous and widespread in the Milky Way, yet few have been found. However, when two white dwarfs form a very tight binary (e.g. with orbital period of less than a couple of hours), this binary will emit strong gravitational wave radiation. In the future, double white dwarfs will be detected in large numbers through gravitational wave emission by the Laser Interferometer Space Antenna (LISA), a European Space Agency-led mission planned for launch in mid 2030s. Composed of three spacecrafts in an equilateral configuration  separated by 2.5 million kilometers and connected by lasers along the sizes of the triangle, LISA is going to be one of the most exciting experiments of the XXI century!

The goal of my research is to explore science of the Milky Way with LISA. In particular, I would like to establish a tight link between observation of binaries composed of white dwarfs - but also of neutron stars and black holes -  using electromagnetic telescopes and the future observations with LISA. I want to show that, when we combine information extracted from optical and GW observations, these binaries will have the potential to address a wealth of outstanding questions such as: What is the final fate of double white dwarfs; and what physical process determines that fate? What would the Milky Way look like in gravitational waves? Can we use gravitational waves to study the structure and the past history of our Galaxy?

Selected publications

A gap in the double white dwarf (DWD) separation distribution: astrometric evidence from Gaia

Characterising the Galactic DWD population is technically challenging, even though the sample now amounts to about 150 binaries. The Gaia mission offers an opportunity to identify unresolved DWD systems in bulk, significantly boosting our statistics. This is possible because the trajectory of the centre of light of an unresolved binary is different from that of its centre of mass. Binary-induced stellar centroid wobbling can therefore be detected as an excess in the goodness-of-fit of the single-star astrometric model, and the wobble amplitude can be related to the separation. In this way we can access orbital separations between approximately 0.01 and 2 au, where theoretical models predict a gap in the DWD separation distribution caused by the common envelope phase(s) prior to DWD formation. In this paper we show that unresolved in the Gaia data reveal the side of a deep gap in the DWD separation distribution. This feature can be now used for constraining DWD evolution models, and, more specifically, the physical processes responsible for carving the gap. 

This work is published in MNRAS, arXiv:2203.03659 

How can LISA probe a population of GW190425-like binary neutron stars in the Milky Way?

The nature of GW190425, a merger detected by the LIGO/Virgo Scientific Collaboration through gravitational wave radiation, is a mystery.  If it is a binary neutron star merger, then its estimated total mass of 3.4 Solar masses is significantly larger than that of the Milky Way's binary neutron star population, which we observe at radio wavelengths.  Luckily, binary neutron stars with orbital periods less than a few hours – i.e. progenitors of LIGO/Virgo mergers and descendants of radio detections at the same time – are prime targets for the LISA gravitational wave experiment. In this work we discuss whether LISA's observations can probe a population of GW190425-like binary neutron stars in our Milky Way and help us to understand their origin. Specifically, we show that LISA's measurement errors will allow us to identify GW190425-like binaries as a sub-population of a broader more radio-like looking population.

This work is published in MNRAS, arXiv:2012.03070 

Weighing Milky Way Satellites with LISA

In this paper we propose to use gravitational wave signals from double white dwarf binaries detectable by LISA for measuring the total stellar mass of the Milky Way satellite galaxies. Building upon the analogy with simple stellar population models, we demonstrated how the total stellar mass of the Milky Way satellites can be estimated from the number of associated LISA events. Using a fiducial double white dwarf evolution model we showed that satellite masses inside the Milky Way virial radius can be recovered within 1) a factor two if the star formation history is known and 2) within an order of magnitude even when marginalising over alternative star formation history models.  Importantly, we found that the accuracy of our method in some cases can match that of the mass estimated based on electromagnetic observations. Thus, with the addition of independent measurements based on gravitational wave observations, we can place stronger constraints on satellite masses, and we can gain a deeper understanding of their properties and co-evolution with the Milky Way.

This work is published in MNRAS Letters, arxiv:2010.05918

The data underlying this article and our code will be publicly available here.

Milky Way Satellites Shining Bright in Gravitational Waves

In two papers Populations of double white dwarfs in Milky Way satellites and Milky Way Satellites Shining Bright in Gravitational Waves  our LISA group at the University of Birmingham explores the size and the properties of the population of gravitational wave sources in the Milky Way satellite galaxies and their detectability with LISA. We find that double white dwarfs - the most numerous among LISA sources - emitting at frequencies higher than 3 mHz can be detected in satellites at large distances. The number of these high frequency binaries per satellite primarily depends on its mass, distance, age, and star formation history, and only mildly depends on the other assumptions regarding their evolution such as metallicity. We find that satellite galaxies with a total stellar mass higher than one million Solar masses can host detectable double white dwarfs, and that the number of detections scales linearly with the satellite’s mass. We forecast that out of the known satellites in the Local Group, Sagittarius, Fornax, Sculptor, and the Magellanic Clouds can be detected with LISA.

These two papers has now been published in ApJL 894, L15, (2020), arXiv:2002.10465 and A&A 638, A153 (2020), arXiv:2002.10462 

The Milky Way's bar structural properties from gravitational waves

In this paper, led by my former master student Martijn Wilhelm, we combine a realistic population of double white dwarfs with a high-resolution Galactic simulation in good agreement with current observations of the Milky Way. We then model gravitational wave signals from our synthetic double white dwarf population and reconstruct the structure of the Galaxy from mock LISA observations. Our results show that the stellar bar will clearly appear in the LISA map, however because of the low density contrast between the disc and the spiral arms the spiral structure of the Milky Way will be less clear. We find that gravitational wave observation will allow us to accurately define the bar's geometry by measuring its physical length, axis ratio and orientation angle with respect to us.

This work has been published in MNRAS, arXiv:2003.11074 

The data underlying this article is publicly available here

Prospects for detection of detached double white dwarf binaries with Gaia, LSST and LISA

Binary systems made of two white dwarf stars in orbits of less than a couple of hours represent a unique opportunity for multi-messenger (electromagnetic + gravitational wave) studies. On one hand, we can detect their electromagnetic radiation with optical telescopes if they are eclipsing. On the other hand,  they can also be detected through gravitational wave radiation with LISA. In this paper, we compute the size of the sample of Galactic multi-messenger detections that could be observed with current and future optical facilities such as the Gaia mission and the Vera Rubin Observatory, and LISA in the next two decades. We show that Gaia, LSST and LISA have the potential to detect, respectively, around a few hundred, a thousand and tens of thousands double white dwarf binaries, out of which about 100 will be multi-messenger sources.

This work has been published in MNRAS, arXiv:1703.02555 

Selected talks

Prospects for multi-messenger studies of the Milky Way with LISA

09.12.2021 | Synergies@Prague 2021 workshop, invited talk

White dwarf stars are a well-established tool for studying Galactic stellar populations. Two white dwarfs in a tight orbit forming a double white dwarf (DWD) binary offer us an additional messenger - gravitational waves - for exploring the Milky Way and its immediate surroundings. Gravitational waves emitted by DWDs can be detected by the future Laser Interferometer Space Antenna (LISA). While awaiting the LISA’s launch, DWDs can be discovered as faint electromagnetic transients in wide-field sky optical surveys.

I will discuss what we will learn about our Galaxy from the multi-messenger sample of DWDs. In particular, I will demonstrate how well the density distribution of DWDs constrains scale parameters of the Milky Way's bulge, disc and central bar. I will highlight how the power to constrain the overall properties of the Galactic baryonic potential will be significantly enhanced by using gravitational waves in combination with electromagnetic observations. Finally, I will show that massive Galactic satellites can be seen in the gravitational wave sky and I will present which of their properties we will be able to investigate with LISA.

Listening to the Universe with LISA

23.08.2021 | 20th Lomonosov Conference, invited plenary talk

Gravitational wave observations have opened a whole new way of studying compact objects throughout the Universe. The future LISA mission, sensitive to low frequency gravitational waves between 0.1 - 100 mHz, will enable the detection of a strikingly large variety of gravitational wave sources ranging from stellar binaries in our own Galaxy to mergers between nascent massive black holes at the cosmic dawn. Thus, it is expected to revolutionize our understanding of these astrophysical sources by allowing the reconstruction of their demographics and dynamical evolution, as well as the discovery of new types of sources, including some that have already been hypothesized by theorists but not yet detected by conventional means.

Characterizing the Galactic DWD population 

joint talk with Na'ama Hallakoun

30.05.2021 | KITP Online Conference: White Dwarfs from Physics to Astrophysics  

The characterization of the double white dwarf (DWD) population is crucial to our understanding of multiple questions, from stellar evolution, through the progenitors of Type-Ia supernovae (SNe Ia), to gravitational-wave sources. In this talk we will discuss the current status of the observed DWD population and the future prospects for the upcoming gravitational-wave observations with the Laser Interferometer Space Antenna (LISA).

First, we will present a statistical analysis of the local DWD population using two large, multi-epoch, spectroscopic samples: SDSS (Badenes & Maoz 2012), and SPY (Maoz & Hallakoun 2017). By combining the results from these complementary samples, more precise information on the DWD population and on its (gravitational-wave-driven) merger rate can be obtained (Maoz, Hallakoun, & Badenes 2018), indicating that about ~10% of the WDs are in DWD systems with separations <4 au. The implied Galactic WD merger rate is ~1e-11 per year per WD, which is 4.5-7 times higher than the Milky Way's specific SN Ia rate. We will present these implications in detail.

Next, we will use these results to forecast the number and the properties of DWDs detectable by LISA. We will show that through gravitational-waves emitted by DWDs LISA will access the entire volume of the Milky Way and its massive satellite galaxies. A large number of detections (~50e3) will then make it possible to use DWDs as Galactic tracers. In conclusion, we will present what we can learn about our Galaxy and the local neighbourhood by combining gravitational wave and optical observations of DWDs.

Galactic Astronomy with LISA

26.02.2020 | High Energy Division Seminar, Center for Astrophysics, Harvard & Smithsonian (CfA), Cambridge (MA), USA 

Gravitational wave signals from ultra-short period double white dwarf binaries are unique Galactic stellar tracers. They can be detected by the upcoming Laser Interferometer Space Antenna (LISA). 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, I demonstrate that 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 Milky Way's classical dwarf satellites and potentially discover new ones.

Slides of the presentation

Constraining the Milky Way's potential with double white dwarfs

27.07 2018 | 21st European Workshop on White Dwarfs, Austin, Texas, USA

The upcoming LISA mission is the only experiment that will allow us to study the Milky Way’s structure using gravitational wave signals from Galactic double white dwarfs (DWDs). The total number of expected detections exceeds ten of thousand. Furthermore, up to a hundred DWDs can be simultaneously detected in both gravitational and optical radiation (e.g. with Gaia and LSST as eclipsing), making DWDs ideal sources for performing a multi-messenger tomography of the Galaxy. We show that LISA will detect DWDs everywhere, mapping also the opposite side of the Galaxy. This complete coverage will: 1) provide precise and unbiased constraints on the scale radii of the Milky Way’s bulge and disc, and 2) allow us to compute the rotation curve and derive competitive estimates for the bulge and disc masses, when combining gravitational wave and optical observations.

Slides of the presentation