I'm Magdalena Siwek

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Final Year PhD Candidate at Center for Astrophysics | Harvard & Smithsonian

About Me

About Me

I'm Magdalena Siwek, a PhD Candidate at Center for Astrophysics | Harvard & Smithsonian

I am a 5th year PhD candidate at the Center for Astrophysics | Harvard & Smithsonian (CfA). I develop hydrodynamic simulations of accretion disks around binary systems using the moving mesh code Arepo. My simulations shed light on the fate of the most massive black hole binaries (MBHBs): Do they merge, or do they stall? What is the effect of gas on their orbital properties, and on the gravitational waves they emit? As a full member of the NANOGrav collaboration, I utilize results from my binary simulations to inform the evolution of nano-Hertz gravitational wave sources by co-developing the MBHB population synthesis code holodeck.

I am originally from Germany, but completed my undergraduate (BSc) and graduate (MSci) studies in physics and astronomy at the University of Glasgow, before starting my PhD at Harvard University in 2018. When I am not busy with my thesis work, I run the CfA's very own community garden. I am also an avid mushroom hunter.

  • Name:Magdalena Siwek
  • Email:magdalena.siwek@
  • Research Interests:Multi-Messenger Astrophysics, Massive Black Hole Binaries, Hydrodynamics
  • Current Position:PhD Candidate
  • PhD Advisor:Lars Hernquist
  • Education:
    2024 - PhD in Astrophysics, Harvard University
    2018 - MSci, University of Glasgow
    2017 - BSc, University of Glasgow
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Oct 2023 (in prep)

Circumbinary disk-driven eccentricity in multi-messenger studies of Massive Black Hole Binaries

Magdalena Siwek, Luke Zoltan Kelley

In this work, we combine the CBD models from my suite of hydrodynamic simulations and apply it to a MBHB population synthesis model based on galaxy merger rates from the cosmological simulation Illustris. We find that MBHB populations detected in PTAs, LSST and even LISA show a significant (1-2 orders of magnitude) increase in their eccentricity distributions when CBD models are applied. Our results suggest that detections of eccentric MBHBs are the rule rather than the exception in upcoming transient surveys, provided that CBDs regularly form in MBHB systems. This is due to the eccentricity pumping effects found in my simulations. We also find that eccentric harmonics are much more likely to be detected in LISA with the influence of CBD accretion.

June 2023

Orbital Evolution of Binaries in Circumbinary Disks

Magdalena Siwek, Rainer Weinberger, Lars Hernquist

Binaries on many scales encounter circumbinary disk (CBD) driven evolution at some point in their lives. But how does the presence of a CBD affect the orbital elements of the binary? In this work, we ran the largest to-date parameter study over binary mass ratio (q) and eccentricity (e), evaluating how the interaction with the CBD is affected by varying the parameters q and e. We discovered that mass ratio and eccentricity become correlated very quickly in the presence of a CBD, that is, the eccentricity evolves to an equilibrium value that is determined by the mass ratio of the system. I am excited to see whether we can find evidence of this dynamical effect in massive black hole or stellar binary populations.

Feb 2023

Preferential accretion and circumbinary disk precession in eccentric binary systems

Magdalena Siwek, Rainer Weinberger, Diego Muñoz, Lars Hernquist

Accretion from circumbinary disks (CBDs) has been known to increase the mass ratio of the binary system, by preferentially accreting onto the secondary (lower mass) component. But this statement has only been proven in special cases: binaries on circular orbits. In this work, we scanned the parameter space of eccentric, unequal mass ratio binary systems and investigated their accretion behaviour. We find that when compared with circular binaries, mass ratios grow more efficiently in binaries on moderately eccentric orbits while high eccentricities suppress mass ratio growth. We further identify three regimes of CBD evolution: free and forced precession, and disk 'locking'.

Oct 2020

The effect of differential accretion on the gravitational wave background and the present-day MBH binary populations

Magdalena Siwek, Luke Zoltan Kelley, Lars Hernquist

Pulsar Timing Arrays (PTAs) have now found compelling evidence for the existence of a Gravitational Wave Background (GWB). However, their evolution histories, including accretion phases, are still uncertain. Here, we investigate the influence of circumbinary disk accretion on MBHB merger rates, chirp masses, and the resulting GWB spectrum. We find that CBD accretion can increase the median mass ratio of coalescing MBHBs by up to a factor 3.6, the coalescence rate by 52.3 per cent, and the GWB amplitude by a factor 4.0. We further predict binary separation and mass ratio distributions of stalled MBHBs in the low-redshift Universe, and find that these depend sensitively on binary accretion models. This presents the potential for combined electromagnetic and GW observational constraints on merger rates and accretion models of MBHB populations.