Nicole M. Lloyd-Ronning

The most luminous objects in our universe, gamma-ray bursts, can spectacularly outshine millions of galaxies while they are active. The emission from these objects - brightest in gamma-rays but covering the entire electromagnetic spectrum - is believed to come from dissipation processes in a relativistic jet, launched by a highly spinning black hole-accretion disk system left over after a cataclysmic event associated with stellar collapse or the merger of two compact objects. There are a number of fascinating and surprising mysteries that remain in understanding the progenitors of (and physics behind) these brilliant events. In this talk, we presented a multi-code effort to model gamma-ray bursts across many scales in length and time. We described recent progress in unveiling the physical processes that power these extreme objects, from the launch of the relativistic jet, to the microphysics of the dissipation processes, to new exciting results on their circumstellar environments and progenitor systems. Finally, we showed how different intrinsic properties of these systems evolve as a function of redshift, and how we can use them to learn about massive stellar evolution over cosmic time.

It has recently been suggested that a variety of data on active galactic nuclei (AGN) can be explained if AGN disks are supported against gravitational fragmentation by magnetic fields that are advected into the disk from the surrounding galaxy. Here we derive the maximum timescales over which accretion onto a black hole (BH) powering an AGN can be maintained at a given rate, both with and without magnetic disk support. We then compare these timescales to the lifetimes of episodes of sustained luminous accretion that are inferred from measurements of the photoionized proximity zones around high-redshift quasars. While some of the shortest inferred quasar lifetimes are consistent with pure gas pressure support, we find that some additional magnetic support is likely required to explain the longest inferred quasar lifetimes of >10⁴ yr. For these longest-lived AGN, we find that magnetic pressure in their disks can be up to a 100 times higher than the gas pressure. In addition, the lack of inferred quasar lifetimes that are definitively >10⁶ yr is consistent with gas pressure and advected magnetic fields being the principal sources of disk support. This adds to the body of evidence that magnetic fields play an important role in sustaining the rapid growth of supermassive BHs in the early Universe.

We present the distribution of the intrinsic duration of gamma-ray burst (GRB) emission. This expands upon the analysis of O. Bromberg et al. (2012, 2013), who showed evidence for collapsar progenitors based on the presence of a plateau in the distribution of T ₉₀ , the duration over which 90% of the prompt emission is observed. We confirm the presence of this plateau in the distribution of duration corrected for cosmological time dilation but shifted to smaller timescales by a factor of 1/(1 + z _av ) ∼ 1/3, where z _av is the average GRB redshift. More significantly, we show that this plateau is only present in the sample of GRBs with redshifts greater than (1 + z ) ∼ 2. This result aligns with suggestions that the low-redshift population of GRBs has a significant contribution from noncollapsar progenitors (while the high-redshift sample is dominated by collapsars). We also show the difference in this distribution between spectrally hard and soft GRBs, confirming that a plateau is only present for the soft subset of GRBs. However, when we separate the soft GRBs into low- and high-redshift subsets, we find that only the high-redshift soft GRBs show evidence of a plateau, while the low-redshift soft GRBs do not. This suggests that there exists a significant subset of spectrally soft noncollapsar progenitors at low redshift . Finally, we use the end time of the plateau to constrain the GRB progenitor density profile and radius and show that the maximum size of a collapsar is a few tenths of a solar radius.

Although the association of gamma-ray bursts (GRBs) with massive stellar death is on firm footing, the nature of the progenitor system and the key ingredients required for a massive star to produce a GRB remain open questions. Here, we investigate the evolution of a 15–25 M _⊙ massive star with a 10–15 M _⊙ black hole (BH) using Modules for Experiments in Stellar Astrophysics. We quantify companion-influenced angular-momentum evolution over a stellar lifetime for orbital periods where tides are significant, varying stellar and BH masses, initial stellar spin, and accretion and dynamo prescriptions while tracking mass loss and angular momentum. The final spin is set by tidal torques versus stellar winds. For binaries that initially avoid Roche-lobe overflow, tides can spin up the star, but a late-stage expansion can drive tidal stripping; associated mass and angular-momentum loss can suppress spin-up. We find that massive-star BH binaries at comparable mass ratios may be potential GRB progenitors for short orbital periods (∼20–5 × 10 ² days) and long orbital periods (∼2 × 10 ³ –4 × 10 ³ days), where our suite of lifetime simulations reveals a favored parameter space with negligible mass loss and enough spin angular momentum to power a GRB jet. For initially nonrotating stars, this provides a lower limit on the final spin above a threshold estimate consistent with forming a postcollapse BH mass of 5–10 M _⊙ with a spin parameter ≥ 0.5. For initially rapidly rotating stars, tidal interactions may sustain high spin when mass loss is negligible because the binary is not tidally synchronized.

Recent detections of kilonova-like emission following long-duration gamma-ray bursts GRB 211211A and GRB 230307A have been interpreted as originating from the merger of two neutron stars. In this work, we demonstrate that these observations are also consistent with nucleosynthesis originating from a collapsar scenario. Our model is capable of reproducing the observed optical and infrared light curves using a single, weak r -process component. The absence of lanthanide-rich material in our model, consistent with the data, challenges the prevailing interpretation that a red evolution in such transients necessarily indicates the presence of heavy r -process elements.

We present an analysis of gamma-ray burst (GRB) progenitor classification through their positions on a Uniform Manifold Approximation and Projection plot, constructed by M. Negro et al., from Fermi-GBM waterfall plots. The embedding plot has a head–tail morphology, in which GRBs with confirmed progenitors (e.g., collapsars versus binary neutron star mergers) fall in distinct regions. We investigate the positions of various proposed sub-populations of GRBs, including those with and without radio afterglow emission, those with the lowest intrinsic luminosity, and those with the longest lasting prompt gamma-ray duration. The radio-bright and radio-dark GRBs fall in the head region of the embedding plot with no distinctive clustering, although the sample size is small. Our low luminosity GRBs fall in the head/collapsar region. A continuous duration gradient reveals an interesting cluster of the longest GRBs (T ₉₀ > 100 s) in a distinct region of the plot, possibly warranting further investigation.

We propose that neutrons may be generated in high-energy, high-flux photon environments via photo-induced reactions on pre-existing baryons. These photohadronic interactions are expected to occur in astrophysical jets and surrounding material. Historically, these reactions have been attributed to the production of high-energy cosmic rays and neutrinos. We estimate the photoproduction off of protons in the context of gamma-ray bursts, where it is expected there will be sufficient baryonic material that may be encompassing or entrained in the jet. We show that typical stellar baryonic material, even material completely devoid of neutrons, can become inundated with neutrons in situ via hadronic photoproduction. Consequently, this mechanism provides a means for collapsars and other astrophysical sites containing substantial flux of high-energy photons to be favorable for neutron-capture nucleosynthesis.

We present particle-in-cell simulations of one dimensional relativistic electromagnetic shocks in a uniform magnetic field, for a range of magnetic field strengths, plasma temperatures and numerical initial conditions. We show that the particle energy distributions of these shocks can develop a state of population inversion in the precursor and shock regions, which may allow for synchrotron maser (or maser-like, coherent) emission. Our set-up is applicable to conditions expected in models of fast radio bursts and therefore lends credence to the synchrotron maser model for these transients. We also show, for the first time, how a newly developed ``analytic particle pusher'' for kinetic simulations gives similar results to the commonly-used Boris pusher, but for larger timesteps and without the need to resolve the gyro-radius and gyro-period of the system. This has important implications for modeling astrophysical plasmas in extreme magnetic fields as well as for bridging scales between kinetic and fluid regimes.

Although there is strong evidence that many long GRBs are associated with the collapse of a massive star, tantalizing results in recent years have upended the direct association of all long GRBs with massive stars. In particular, kilonova signals in some long GRB light curves as well as a suggested uptick in the rate density of long GRBs at low redshifts (deviating significantly from the star formation rate) suggest that compact object mergers may be a non-negligible fraction of the long GRB population. Here we investigate the contribution of white dwarf-black hole mergers to the long GRB population. We present evidence for the deviation of the long GRB rate density from the star formation rate at low redshifts, and provide analytic and numerical arguments for why a white dwarf-black hole merger system may be a viable progenitor to explain this deviation. We show the range of parameter space in which the durations, energetics, and rates of these systems can account for a significant sub-population of low-redshift long GRBs.