Research

X-ray binaries

X-ray binaries are binary systems composed of a compact object (white dwarf, neutron star, or black hole) “stealing” material from a companion star in the main sequence. Since this material carries angular momentum, it does not fall directly onto the surface of the compact object but instead forms an accretion disk around it. The disk is one of the prominent structures we observe in these binaries (see Figure 1).

These systems also display an interesting transient behavior. In quiescence, the X-ray luminosity is relatively faint due to the low level of accretion. On the contrary, the outburst phase is characterized by a rapid increase in the X-ray luminosity and accretion levels that vary over the course of this phase (giving rise to different spectral states). I have studied neutron stars and stellar-mass black holes in the context of X-ray binaries. Both compact objects are the remnants of massive stars that end their lives in violent explosions known as supernovae. That said, here is a summary of my work.

Stellar mass black holes (BHs)

Figure 1 - The components of a Black Hole X-ray Binary system, indicating the frequency band in which each element emits radiation. Background image credit: Dana Berry/NASA Goddard Space Flight Center.

Perhaps one of the most puzzling events happening during the outburst phase is the detection of a relativistic jet, an energetic beam of material that is expelled away from the BH. Jets are present in diverse astrophysical scenarios: formation of young stars, accreting compact objects, actively accreting supermassive BHs in the centers of galaxies, and more recently, during the collision of two neutron stars. However, despite the wide range of scenarios in which jets are observed, very little is known about the physical processes involved in their launching, the composition of the jet material, and the amount of energy carried away from the system.

What can we learn from them?

BH X-ray binaries in our Galaxy are ideal targets to study jets. Their proximity and rapid variability (hours to a few days) allow us to observe how the system evolves in real time. Notably, we can study the connection between accretion inflow and jet outflow. Because these binaries emit across different bands of the electromagnetic spectrum (see Figure 1), observing and studying the system involves collecting data from various telescopes!

My work focuses on one BH X-ray binary known as MAXI J1820+070, which was first detected in outburst in March 2018. This Galactic BH was intensively monitored by several telescopes throughout its full outburst, making it one of the most well-sampled sources to date! I use these complete data sets to characterize the broad-band spectrum of the system. Tracking the broad-band evolution provides insight into how the structure of the jet and the accretion flow is changing during the outburst, which could indicate a connection between the two. For more details of this work see Echiburú-Trujillo et al. 2024.

Neutron stars (NSs)

Figure 2 - Scheme of neutron star interior. Image credit: NASA’s Goddard Space Flight Center.

The fundamental difference between BHs and NSs is that we can observe neutron stars directly. NSs are pretty small objects: spheres of ~10 km radius enclosing approximately the mass of the Sun. If you were to take one tablespoon of NS material, it would weigh as much as Mount Everest!

What can we learn from them?

NSs are one of the most extreme objects in the Universe due to their high density. Particularly in their cores (see Figure 2), densities overcome that of the nucleus of an atom. Since those conditions are difficult to reproduce in Earth laboratories, we do not understand how matter behaves at such extreme densities. That behavior is described with an equation of state. Measuring the macroscopic properties of NSs (e.g., mass and radius) can constrain these equations, and that is why NSs provide a unique place to study the equation of state of ultra-dense matter. One promising method to measure neutron star radii is the study of low-mass X-ray binaries in quiescence (qLMXB). X-ray emission comes primarily from the star in this phase, allowing us to characterize its surface emission!

My work used Chandra X-ray observations of the globular cluster M30, which hosts a qLMXB known as A1. With this data, we were able to characterize the thermal emission of the NS with light-element composition atmosphere models (hydrogen or helium), and from there, constrain the mass-radius relation for both models. Comparing this relation to that obtained from different equations of state allows us to favor (or not) theoretical models of nuclear physics. For more details of this work see Echiburú et al. 2020.