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We present a 5 month NICER X-ray monitoring campaign for two low luminosity active galactic nuclei (LLAGNs) -- NGC 4594 and IC 1459 -- with complementary Swift and NuSTAR observations. Utilizing an absorbed power law and thermal source model combined with NICER's SCORPEON background model, we demonstrate the effectiveness of joint source/background modeling for constraining emission from faint, background-dominated targets. Both sources are dominated by nuclear power law emission with photon indices $\Gamma \sim 1.5 - 2$, with NGC 4594 being slightly harder than IC 1459. The thermal contribution in both sources is fainter, but constant, with $kT \sim 0.5$ keV ($\sim 5 \times 10^6$ K). The power law flux and $\Gamma$ are strongly anti-correlated in both sources, as has been seen for other LLAGNs with radiatively inefficient accretion flows. NGC 4594 is the brighter source and exhibits significant aperiodic variability. Its variability timescale with an upper limit of $5 - 7$ days indicates emission originating from $< 100 R_{g}$, at the scale of the inner accretion flow. A spectral break found at $\sim 6$ keV, while tentative, could arise from synchrotron/inverse compton emission. This high-cadence LLAGN X-ray monitoring campaign underlines the importance of multi-wavelength variability studies for a sample of LLAGNs to truly understand their accretion and outflow physics., Comment: 18 pages, 5 figures, 5 tables. Accepted to ApJ

The time-variable emission from the accretion flow of Sgr A*, the supermassive black hole at the Galactic Center, has long been examined in the radio-to-mm, near-infrared (NIR), and X-ray regimes of the electromagnetic spectrum. However, until now, sensitivity and angular resolution have been insufficient in the crucial mid-infrared (MIR) regime. The MIRI instrument on JWST has changed that, and we report the first MIR detection of Sgr A*. The detection was during a flare that lasted about 40 minutes, a duration similar to NIR and X-ray flares, and the source's spectral index steepened as the flare ended. The steepening suggests synchrotron cooling is an important process for Sgr A*'s variability and implies magnetic field strengths $\sim$40--70 Gauss in the emission zone. Observations at $1.3~\mathrm{mm}$ with the Submillimeter Array revealed a counterpart flare lagging the MIR flare by $\approx$10 minutes. The observations can be self-consistently explained as synchrotron radiation from a single population of gradually cooling high-energy electrons accelerated through (a combination of) magnetic reconnection and/or magnetized turbulence., Comment: Accepted for publication ApJL

Using the Event Horizon Telescope (EHT), the gravitationally lensed rings around the supermassive black holes (SMBHs) in Messier 87 (M87) and Sagittarius A* (Sgr A*) have now been successfully imaged at a resolution under 10 gravitational radii (R$_{\rm g}$ $ = \rm{GM/c^2}$). To expand studies beyond M87 and Sgr A*, we have constructed the Event Horizon and Environs (ETHER) sample, a comprehensive database encompassing approximately 3.15 million SMBH mass estimates, $\sim$ 20,000 Very-Long Baseline Interferometry (VLBI) radio flux densities, and $\sim$ 36,000 hard X-ray flux densities. This database is designed to identify and optimize target selection for the EHT and its upgrades on the ground and in space. We have identified a Gold Sample (GS) of nearby low-luminosity Active Galactic Nuclei (AGNs) within it that are ideal for studying jet bases and potentially imaging black hole shadows. We observed 27 of these AGNs using the EHT from 2022 to 2024, providing an opportunity to resolve and image accretion flows and jets at resolutions of $\leq$ 100 R$_{\rm g}$. Only a few SMBHs have sufficiently high enough flux density to be imaged at scales of $\leq$ 50 R$_{\rm g}$ with the present EHT. Among these are M87, Sgr A*, NGC4594 (Sombrero/M104), NGC4261, and NGC4374 (Messier 84/M84). Of these, NGC4261, Sombrero, and M84 have been observed and/or are scheduled for deep imaging with EHT+ALMA from 2023 to 2025. Sombrero, NGC4261, M84, NGC4278, and NGC5232 are clearly detected in our EHT+ALMA observations in 2022, indicating that the 230 GHz flux density from the accretion flows is significantly high. Ongoing imaging of the ETHER GS will enable measurements of black hole mass and spin, help constrain General Relativity, and enrich our understanding of jet launching and accretion inflows across a broad multi-parameter space, including black hole mass, spin, accretion rate, and orientation., Comment: 9 pages, 6 figures, 1 table, published in Proceedings of the 16th EVN Symposium, Ed. E. Ros, P. Benke, S.A. Dzib, I. Rottmann, & J.A. Zensus, Bonn: Max-Planck-Institut f\"ur Radioastronomie, 2024, pages 75-84, https://cloud.mpifr-bonn.mpg.de/index.php/s/BkX2CC2Xjn2aKR4

In kilonovae, freshly-synthesized $r$-process elements imprint features on optical spectra, as observed in AT2017gfo, the counterpart to the GW170817 binary neutron star merger. However, measuring the $r$-process compositions of the merger ejecta is computationally challenging. Vieira et al. (2023) introduced Spectroscopic $r$-Process Abundance Retrieval for Kilonovae (SPARK), a software tool to infer elemental abundance patterns of the ejecta, and associate spectral features with particular species. Previously, we applied SPARK to the 1.4 day spectrum of AT2017gfo and inferred its abundance pattern for the first time, characterized by electron fraction $Y_e=0.31$, a substantial abundance of strontium, and a dearth of lanthanides and heavier elements. This ejecta is consistent with wind from a remnant hypermassive neutron star and/or accretion disk. We now extend our inference to spectra at 2.4 and 3.4 days, and test the need for multicomponent ejecta, where we stratify the ejecta in composition. The ejecta at 1.4 and 2.4 days is described by the same single blue component. At 3.4 days, a new redder component with lower $Y_e=0.16$ and a significant abundance of lanthanides emerges. This new redder component is consistent with dynamical ejecta and/or neutron-rich ejecta from a magnetized accretion disk. As expected from photometric modelling, this component emerges as the ejecta expands, the photosphere recedes, and the earlier bluer component dims. At 3.4 days, we find an ensemble of lanthanides, with the presence of cerium most concrete. This presence of lanthanides has important implications for the contribution of kilonovae to the $r$-process abundances observed in the Universe., Comment: Accepted to ApJ; 19 pages, 8 figures in body; 10 pages, 7 figures in appendix

Neutrinos drive core-collapse supernovae, launch outflows from neutron star merger accretion disks, and set the ratio of protons to neutrons in ejecta from both systems that generate heavy elements in the universe. Neutrinos of different flavors interact with matter differently, and much recent work has suggested that fast flavor instabilities are likely ubiquitous in both systems, but the final flavor content after the instability saturates has not been well understood. In this work we present particle-in-cell calculations which follow the evolution of all flavors of neutrinos and antineutrinos through saturation and kinematic decoherence. We conduct one-dimensional three-flavor simulations of neutrino quantum kinetics to demonstrate the outcome of this instability in a few example cases. We demonstrate the growth of both axially symmetric and asymmetric modes whose wavelength and growth rate match predictions from linear stability analysis. Finally, we vary the number density, flux magnitude, and flux direction of the neutrinos and antineutrinos and demonstrate that these factors modify both the growth rate and post-saturation neutrino flavor abundances. Weak electron lepton number (ELN) crossings in these simulations produce both slow growth of the instability and little difference between the flavor abundances in the initial and final states. In all of these calculations the same number of neutrinos and antineutrinos change flavor, making the least abundant between them the limiting factor for post-saturation flavor change. Many more simulations and multi-dimensional simulations are needed to fully probe the parameter space of the initial conditions., Comment: 15 pages, 10 figures, plus references and appendices. Accepted to PRD