Super-massive black holes inhabit the cores of every galaxy in our Universe. In many cases, material can reach very close to them forming a flat, thin disc of hot material producing one of the most energetic and bright sources in the sky --- quasar --- observable all the way back to the formation of the first galaxies. As material spirals into the black hole it causes changes in the brightness that propagates outwards to the disc. As light travels further away the black hole, it echos at larger distances in the surrounding disc. From the earth, we observe this “echoes” of the inner parts of the disc at different colours of the electromagnetic spectrum shifted in time, as seen in the figure below.

This “echo mapping” of black holes is a fantastic technique that allows us to probe an otherwise inaccesible environment enabling us to measure the size of the disc and the mass of the black hole.

Echo mapping of the accretion disc in the nearby AGN --- Fairall 9, using the Neils Gehrel Swift Telescope and Las Cumbres Observatory. Quasar image by ESO/M. Kornmesser - http://www.eso.org/public/images/eso1122a/, CC BY 4.0, Link

Accreting White Dwarfs

My main field of study during my PhD was cataclysmic variables, a class of accreting white dwarfs (AWDs). These systems are semi-detached binaries in which one of them is a white dwarf (WD) and the other is a low-mass star. The donor star is losing mass via Roche-lobe transfer onto the surface of the WD, often via an accretion disc. These systems are fantastic laboratories to study the physics of accretion discs, nova explosions, sub-stellar atmospheres, stellar winds, among other...

Diagram of a cataclysmic variable. Credit: J. Hernández Santisteban CC BY 4.0, Link

In particular, I have focused on the late evolution of these AWDs, trying to directly identify and characterise a donor that has succesfully made the transition from stellar to a sub-stellar regime (after been stripped of most of it's mass by accretion). On top, the donor is subject to high irradiation flux from the compact object and changes the observed properties in the atmosphere, seen as an increased temperature. The possibility to measure the albedo and reprocessing efficiency in the atmosphere of the sub-stellar donors, allows us to provide a new observational benchmark for brown dwarf atmospheres and exo-planets. Check the dedicated website from my recent Nature paper on J1433.

Missing Link Pulsars

Millisecond pulsars (MSPs) are a sub–class of very old, rapidly rotating neutron stars with spin periods less than 10 ms. This apparent contradiction has been reconciled if the neutron star could be recycled or spun-up by accretion, the so-called "recycling scenario". As the companion star evolves and fill its Roche lobe, accretion onto the neutron star occurs. These systems, also known as low–mass X–ray binaries (LMXBs), can spin-up the neutron star by transferring angular momentum from the accreted material of the donor star.

We find evidence of this scenario in systems that switch between LMXB and MSP states. PSR J1023+0038 is one of only three confirmed systems where this transition has been observed. Only last year, this system turned on again as a LMXB, and the radio pulsations were no longer detected. I am part of a multiwavelength campaign to observe this system, in particular the ultraviolet spectra with the Hubble Space Telescope, where the accretion disc light dominates.