Compact Object Binaries in the Multiwavelength and Time Domain Sky

Author: Rodriguez, Antonio Cuevas

Year: 2025

Degree: Dissertation (Ph.D.)

Advisor: Kulkarni, Shrinivas R.

Committee Members: Fuller, James; El-Badry, Kareem J.; Prince, Thomas A.; Harrison, Fiona A.; Kulkarni, Shrinivas R.

Option: Astrophysics

DOI: 10.7907/8qf6-q175

Abstract

Compact objects are natural laboratories to study the most extreme physics under conditions unable to be replicated anywhere on Earth. White dwarfs (WDs) are the most abundant compact objects, and when located in binaries, it becomes possible to measure physical properties such as mass and density. Remarkable phenomena result from interacting WD binaries; for instance, Type Ia supernovae, which established the existence of dark energy, likely result from the coalescence of two WDs or the accretion of matter by a WD in a binary.

This thesis focuses primarily on interacting WD binaries, in the form of cataclysmic variables (CVs) and their ultracompact cousins, AM CVns. The main product of this thesis is the deepest X-ray survey of CVs and AM CVns to date, assembled using a multiwavelength crossmatch of the SRG/eROSITA all-sky X-ray catalog, \textit{Gaia}, and time-domain photometry from the Zwicky Transient Facility (ZTF). I present a rejuvenated version of a tool used for the discovery of such systems in the X-ray + optical sky that would be missed in purely optical surveys. I calculated CV and AM CVn space densities and luminosity functions, and showed that 1) observations indeed reveal a dearth of accreting WDs compared to population synthesis predictions, and 2) the mean X-ray luminosity of CVs was overestimated by a factor of 10--100 in the past.

Along the way, several single-object papers shine a new light on the diverse physics of binary star evolution. I report the discovery of the second-nearest eclipsing AM CVn and, by constraining binary parameters, show that the "evolved CV" formation channel, which only involves one episode of common envelope evolution, is most likely. I include work on multiwavelength follow-up of the nearest black holes to Earth, Gaia BH1 and BH2, and show that the the lack of a detection confirms predictions of hot accretion flows in the extreme sub-Eddington regime. Later, I present a multi-year, multiwavelength campaign of an enigmatic accreting WD, and argue that it is a missing link between rapidly spinning WD pulsars and slowly rotating polars. This object serves as strong evidence for the dynamo theory of WD magnetism in CVs, where a WD must be spun up by accretion to generate a strong magnetic field. Finally, I report optical spectroscopy of a new radio source pulsing on a 2.9-hr timescale, the slowest at the time of publication. I show that this is a WD + M dwarf binary with a particularly massive (~ M☉) WD, likely representing a new subclass of long period radio transients.

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