November 18
The Landscape of Relativistic Stellar Explosions in the Einstein Probe Era
Speaker: Dr. Brendan O'Connor, UMD Astronomy
Abstract: Fast X-ray transients (FXTs) are elusive high-energy events lasting minutes to hours. Many FXTs were identified in archival Chandra and XMM–Newton observations, but this delay prevented rapid multi-wavelength follow-up and left their origins ambiguous, with a wide range of proposed progenitors. The Einstein Probe (EP) has changed this landscape. As a wide-field soft X-ray monitor, EP is revealing a dynamic soft X-ray sky that was largely missed by traditional high-energy monitors, detecting FXTs at an unprecedented rate of 100 events per year over the past three years and enabling follow-up within hours across the electromagnetic spectrum.
A key open question is how EP FXTs relate to the broader population of more common cosmological gamma-ray bursts (GRBs), particularly because many EP events show no detected gamma-ray emission. To investigate this connection, we compare the redshift distribution of extragalactic EP FXTs with that of long-duration GRBs and find them consistent with being drawn from the same parent population. This link is further supported by EP FXTs consistency with the Amati relation between spectral peak energy and isotropic-equivalent energy.
At the same time, EP is also identifying a subset of peculiar transients (e.g., EP240408a, EP240414a, EP241021a, EP250108a, EP250304a, EP250827b, EP260321a, etc.) with no gamma-ray trigger and stringent limits on prompt gamma-ray emission. These events exhibit multi-wavelength behavior distinct from typical GRBs, yet overlap in key respects, including their association with a Type Ic-BL supernova. In particular, their early blue optical emission is inconsistent with a standard afterglow and instead points to emission from a mildly relativistic cocoon formed as a jet interacts with an extended stellar envelope. In these systems the jet likely fails to break out, potentially due to enhanced circumstellar material, producing soft X-ray transients that occupy a parameter space that is inaccessible to gamma-ray triggered samples. Put together, these results strongly support that the majority of extragalactic EP FXTs are closely related to long GRBs and arise from the massive star (collapsar) channel, with EP uncovering a previously poorly sampled population of lower Lorentz factor explosions and failed or weak jets that radiate primarily in soft X-rays.
Resolving the Rise and Fall of Star Formation across Nearby Galaxies
Speaker: Dr. Yu-Hsuan Teng, UMD Astronomy
Abstract: Star formation drives the evolution of galaxies. As stars are born from the cold and dense molecular clouds, star formation is governed by the amount of molecular gas and how efficiently molecular gas is converted into stars (i.e., the star formation efficiency, SFE). However, the physical mechanisms that control the rise and fall of star formation in galaxies are not well understood, partially due to a lack of precise knowledge in molecular gas mass and SFE variations across galaxies at different evolutionary stages. In this talk, I will present a series of my work that tackle these challenges, using state-of-the-art telescopes including ALMA, JWST, and GBT. I will also talk about ongoing ALMA and JWST programs which aim to uncover the exact molecular gas conditions and feedback processes that drive the star formation quenching of galaxies.
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