Last week, NuSTAR performed an observation of a newly-appearing bright optical source ATO 2026ofr (also known as ATLAS26gre) in the nearby galaxy NGC 5102. NGC 5102 is a lenticular (that is, lens-shaped) galaxy, also known as Iota's Ghost, and is relatively nearby, at a distance of 3.4 Mpc. The optical transient was discovered by the ATLAS collaboration and could indicate the beginning of a Type Ia supernova in the host galaxy. If confirmed, this would be only the second nearby Type Ia (after SN 2024j in M82) close enough for follow-up by NuSTAR in the last 14 years. Type Ia supernovae are powered by the radioactive decay of nickel, which releases gamma-rays that heat the envelope of the star and power the optical emission from the explosion. As the stellar atmosphere expands, it starts to become transparent to high-energy X-rays, letting them leak out of the explosion to be observed by NuSTAR. Notification of this optical transient was circulated via the Transient Name Server (TNS) and additional discussions on the Time Domain Astronomy Slack channels. A preliminary NuSTAR DDT observation was triggered on late Friday (the same day as the transient's discovery), with the telescope arriving on target on Saturday afternoon. These observations provide a baseline measurement of the X-ray emission from the host galaxy (which has not yet been observed by NuSTAR) to compare with later observations if the source is identified as a Type Ia through optical spectroscopy. These early X-ray measurements of this transient will also be useful if it turns out to be either a foreground Galactic transient source or a nova in the host galaxy.

Author: Brian Grefenstette (NuSTAR Instrument Scientist, Caltech)