Abstract

Dr. Aaron Tohuvavohu is a physicist, astronomer, and explorer designing the next generation of space telescopes. He has designed missions and experiments across the electromagnetic and multi-messenger spectrum, with expertise spanning black holes, relativistic explosions, UV and X-ray instrumentation, and space systems engineering. Most recently, he led an 11-month sprint from clean sheet to launch of the highest-performance UV detector in orbit, and drove major upgrades to NASA’s Swift Observatory, significantly expanding its scientific reach, impact, and efficiency.

Abstract

Solar-stellar flares release a portion of the stored magnetic free energy in their outer atmosphere. Strong flares can accelerate particles to very high energies and produce large-scale coronal mass ejections (CMEs), both of which can destabilize planetary atmospheres and impact habitability. Unlike the Sun, stellar observations lack both spatial resolution to probe the physical scales of the active region and a means to directly probe the space-weather impacts of major flares, namely energetic particle events (EPEs) and CMEs. Decades of solar observations offer a means to explore multi-waveband sun-as-a-star parameters with high space weather diagnostic potential. Of the various multi-waveband sun-as-a-star observables, radio burst types (type-II, III, and IV) form the direct diagnostics of flare-accelerated electron beams, fundamental to the standard flare model. The occurrence of different solar burst types is very closely linked to that of EPEs and CMEs propagating into the interplanetary space. However, the detection of these bursts has been quite rare in other stars, despite several hours to days of monitoring in multiple active stars. In my talk, I will present the comprehensive sun-as-a-star catalogs of the three radio burst types using simultaneous data from multiple spacecraft in the decameter-hectometric (DH) band sensitive to EPEs and CMEs in interplanetary space heights. I will present the results on the impact of the line-of-sight effects in burst detection and the resultant impact on space weather diagnostics. I will discuss how insights from the Sun help choose the targets for stellar radio burst monitoring. I will also present our discovery of the first type-III and IV bursts in a young active M-dwarf, AD Leo.

Abstract

Recent work on long-term X-ray variability in the nearby spiral galaxy M83 has revealed an unexpected result: a significant fraction of sources classified as supernova remnants (SNRs) exhibit substantial variability on decade timescales. This is surprising because soft X-ray emission from remnants older than ~100 years is expected to evolve smoothly, changing little over such periods. The observed light curves and spectra are difficult to reconcile with standard SNR evolution models, and may point to the presence of accretion-powered sources physically associated with the remnants. Possible explanations include fallback accretion onto the compact object formed in the explosion, or an X-ray binary associated with the remnant — though neither interpretation is without difficulty. The variable sources are statistically correlated with high-mass stars, and their luminosities and spectral properties are broadly consistent with cool accretion disks around stellar-mass black holes, but the current data do not uniquely determine the nature of these systems. The origin of the variability remains an open question.

I will close with a brief look at preliminary efforts to find similar systems in other galaxies. Sources with broadly similar properties have been identified in M51, and we are using eROSITA to search for soft, variable sources near known SNRs in the Large Magellanic Cloud. Several candidates have emerged for further study, though none yet constitute a convincing match to the class of sources seen in M83. The frequency of such systems in the LMC appears to be substantially lower than in M83.

Abstract

Since the discovery of cosmic rays over a century ago, their origin has remained an open question. While the IceCube neutrino observatory has revealed that the most extreme cosmic hadron accelerators are likely associated with faraway AGN, cosmic rays with energies up to the "knee" at a few PeV are widely believed to originate from Galactic particle accelerators, often referred to as Galactic PeVatrons. Rapid discoveries and breakthroughs in ultra-high-energy gamma-ray astronomy and neutrino astronomy since 2023 have led us into a new era where pursuing individual Galactic PeVatrons becomes possible. A series of on-going projects aims to answer the following big questions: What kind of Galactic sources are able to accelerate charged particles into PeV energies? Which types of particles can they accelerate, leptons or hadrons? Where, within or around a Galactic source, are the particle accelerated? In this talk, I will present our on-going multi-messenger effort to identify Galactic PeVatron candidates, by coming multi-wavelength data with neutrino observations from IceCube. Especially, I will highlight a few recent successful source identifications of PeVatron candidates discovered by the LHAASO observatory.