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Projects

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Blazars as astrophysical neutrino sources?

Blazars are candidate sources for astrophysical neutrinos and ultra-high energy cosmic rays. We recently published a paper where we found a 2σ association between ~1000 blazars observed by the Owens Valley Radio observatory and public neutrino events from IceCube (arXiv:2009.10523). Surprisingly, half the blazars possibly associated with neutrinos are γ-ray dark i.e., not detected by the Fermi gamma-ray space telescope. This motivated us to continue the monitoring as well as look at other wavelengths that will help us definitively answer the question: Do blazars emit high-energy astrophysical neutrinos?

X-ray polarization

The Imaging X-ray Polarimetry Explorer (IXPE) has recently opened a new window to the Universe by measuring X-ray polarization for many different sources, a lot of which are blazars! I led the detectability study (arXiv:1906.01647) that helped IXPE's Blazar and Radio galaxy group to select targets for the first two years of observations. I also led the first detection of X-ray polarization from a supermassive black hole (Mrk 501, arXiv:2209.06227).  I am currently the coordinator of the multiwavelength polarization follow up observations of IXPE-blazars.

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Tidal disruption events

TDEs offer a unique view on the early stages of  accretion disk and jet formation. I recently lead a team that produced the first optical polarization light curve of a TDE and the discovery of the most polarized TDE so far! The results (arXiv:2208.14465) can help us understand the origin of the emission in optical TDEs. There are still a lot of open questions, but there are also more polarization observations of TDEs underway!

Supermassive black hole binaries

SBHBs are the elusive link in hierarchical galaxy structure formation and prime targets for current pulsar timing arrays and future space-borne gravitational wave experiments. I am currently using light curves for ~2000 sources from the Owens Valley Radio Observatory blazar monitoring program looking for sources whose light curves show periodic behavior (see preliminary example to the right, Liodakis et al., 2023 in prep.). The new methodologies we are developing will be particularly useful for CMB-S4 and the Legacy of Space and Time of the Vera Rubin Observatory that will be monitoring  thousands of sources and will let us discover many more!

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Origin of the high-energy emission in blazars

The origin of the high-energy emission in blazars is still a mystery even after decades of systematic study. I led the analysis of the large sample to date looking at the cross-correlation between low and high-energies and the energetic requirements of different models in a series of papers (arXiv:1808.05625, 1905.11418, 2003.10460). The most likely scenario is that the γ-ray emission in blazars comes from leptonic processes (i.e., inverse-Compton scattering) with a possible, small, hadronic contribution which is at most 0.3/flares/yr/source.

Light curve modeling

Blazars are extremely variable across the electromagnetic spectrum. Their variability holds the key to understanding a lot of the jet physical processes, as well as the relativistic effects that dominate their emission and distort our view of their intrinsic properties. I have led several efforts (arXiv:1503.04780, 1701.01452, 1702.05493, 1809.08249) using a variety of different tools to quantify blazar variability and model flares. My work was crucial in understanding the plasma conditions during radio flares, showing that the maximum intrinsic brightness temperature is less than the often assumed inverse-Compton catastrophe limit. As a by product, I also constrained the relativistic effects for over 1000 blazars!

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Optical polarization

I am a member of RoboPol, an international collaboration dedicated to understand the optical polarization signatures of astrophysical objects. Through RoboPol I had the opportunity to study blazars, stars, X-ray binaries, gamma-ray bursts and more! I am currently leading the NOn-stop Polarization Experiment (NOPE), a novel experiment involving 15 telescopes around the world aiming to produce 24-hour-long uninterrupted polarization observations of blazars. These unique observations might finally give us evidence of the particle acceleration processes in the jets.

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