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Abstract Using optical and near-infrared images of the Cassiopeia A (Cas A) supernova remnant covering the time period 1951–2022, together with optical spectra of selected filaments, we present an investigation of Cas A’s reverse shock velocity and the effects it has on the remnant’s metal-rich ejecta. We find the sequence of optical ejecta brightening and the appearance of new optical ejecta indicating the advancement of the remnant’s reverse shock in the remnant’s main shell has velocities typically between 1000 and 2000 km s⁻¹, which is ∼1000 km s⁻¹less than recent measurements made in X-rays. We further find that the reverse shock appears to move much more slowly and is nearly even stationary in the sky frame along the remnant’s western limb. However, we do not find the reverse shock to move inward at velocities as large as ∼2000 km s⁻¹as has been reported. Optical ejecta in Cas A’s main emission shell have proper motions indicating outward tangential motions ≃3500–6000 km s⁻¹, with the smaller values preferentially along the remnant’s southern regions, which we speculate may be partially the cause of the remnant’s faint and more slowly evolving southern sections. Following interaction with the reverse shock, ejecta knots exhibit extended mass ablated trails $0\stackrel{″}{.}2$– $0\stackrel{″}{.}5$in length, leading to extended emission indicating reverse shock induced decelerated velocities as large as ≃1000 km s⁻¹. Such ablated material is most prominently seen in higher ionization line emissions, whereas denser parts of ejecta knots show surprisingly little deceleration. 

Ultraluminous X-ray sources (ULXs) were once largely believed to be powered by super-Eddington accretion onto stellar-mass black holes, although in some rare cases, ULXs also serve as potential candidates for (sub-Eddington) intermediate-mass black holes. However, a total of eight ULXs have now been confirmed to be powered by neutron stars, thanks to observed pulsations, and may act as contaminants for the radio/X-ray selection of intermediate-mass black holes. Here, we present the first comprehensive radio study of seven known neutron star ULXs using new and archival data from the Karl G. Jansky Very Large Array and the Australia Telescope Compact Array, combined with the literature. Across this sample, there is only one confident radio detection, from the Galactic neutron star ULX Swift J0243.6+6124. The other six objects in our sample are extragalactic, and only one has coincident radio emission, which we conclude is most likely contamination from a background HII region. We conclude that with current facilities, neutron star ULXs do not produce significant enough radio emission to cause them to be misidentified as radio-/X-ray-selected intermediate-mass black hole candidates. Thus, if background star formation has been properly considered, the current study indicates that a ULX with a compact radio counterpart is not likely to be a neutron star. 

ABSTRACT The globular cluster ultraluminous X-ray source, RZ 2109, is a complex and unique system that has been detected at X-ray, ultraviolet, and optical wavelengths. Based on almost 20 yr of Chandra and XMM–Newton observations, the X-ray luminosity exhibits order of magnitude variability, with the peak flux lasting on the order of a few hours. We perform robust time series analysis on the archival X-ray observations and find that this variability is periodic on a time-scale of 1.3 ± 0.04 d. The source also demonstrates broad [O iii] λ5007 emission, which has been observed since 2004, suggesting a white dwarf donor and therefore an ultra-compact X-ray binary. We present new spectra from 2020 and 2022, marking 18 yr of observed [O iii] emission from this source. Meanwhile, we find that the globular cluster counterpart is unusually bright in the NUV/UVW2 band. Finally, we discuss RZ 2109 in the context of the eccentric Kozai–Lidov mechanism and show that the observed 1.3 d periodicity can be used to place constraints on the tertiary configuration, ranging from 20 min (for a 0.1 M⊙ companion) to approximately 95 min (for a 1 M⊙ companion), if the eccentric Kozai–Lidov mechanism is at the origin of the periodic variability.