Last week, NuSTAR performed a two-day long observation of the symbiotic binary star system, RT Cru. Previous observations of the X-ray emission from the accreting white dwarf star in this system indicate that is likely to be very massive, close to the Chandrasekhar gravitational collapse limit, and so is a candidate progenitor for a Type Ia supernova. The NuSTAR observation is scheduled to be simultaneous with an observation by the NASA/JAXA X-ray observatory XRISM which, with its high spectral sensitivity microcalorimeter instrument Resolve, has the potential to allow a direct measurement of the gravitational redshift on the white dwarf's surface and hence an accurate measurement of the mass of the star. To achieve this, the wide energy sensitivity of NuSTAR will be used to support this measurement by providing an observation that accurately characterizes the broad-band X-ray spectrum of RT Cru. The NuSTAR observation will also supply an independent mass estimate of the white dwarf through modeling of the X-ray continuum. This summer NuSTAR will be performing a series of observations coordinated with the XRISM observing schedule, and more observations are planned over the next year that combine the unique capabilities of NuSTAR and XRISM.

Also over the course of last week, NuSTAR performed a series of observations of the supernova SN2025mvn in coordination with NASA's Neil Gehrels Swift Observatory. SN2025mvn is a recent stellar explosion in the nearby galaxy NGC 5033, at a distance of 40 million light years, which was discovered at a very early stage of evolution of the supernova. SN2025mvn showed spectroscopic signatures of strong shock interaction in its optical spectra and so researchers triggered Target-of-Opportunity observations with NuSTAR and Swift. These spectroscopic features are caused by the interaction of the explosion's shock with a dense medium created by the mass lost through winds from the star in the decades before its explosive demise; a phase of stellar evolution that is poorly explored. NuSTAR observations of SN2025mvn, acquired within just a few days after the explosion, allow an exploration of this pristine part of the parameter space of supernova events. Specifically, NuSTAR observations of SN2025mvn will allow constraints to be placed on the physical parameters of the radiating electrons, which directly depend on a combination of the stellar explosion's parameters and its environment, as well as the fundamental physics of strong shocks. This is a unique set of observations, enabled by the recently-developed fast repointing capabilities of NuSTAR, the unique hard X-ray frequency coverage of NuSTAR, and the coordination with the Swift observatory.

Authors: Karl Forster (NuSTAR Science Operations Manager), Raffaella Margutti (Associate Professor, UC Berkeley)