Last week, NuSTAR supported a multi-mission monitoring of the interstellar comet 3I/ATLAS. This observation was performed under Director’s Discretionary Time (DDT), with the NuSTAR observations providing the strongest upper limits on any X-ray emission at energies above a few keV. Comets primarily emit soft X-rays (<2 keV) through charge exchange between the gasses in the cometary atmosphere and the solar wind. This mechanism is not expected to generate hard X-rays observable by NuSTAR; however, alternative emission mechanisms have been considered, including the reflection of solar X-rays by dust grains in the coma of the comet. NuSTAR tracked this comet (only the fourth solar system object observed by NuSTAR after the Sun, the Moon, and Jupiter) for 18 NuSTAR orbits as the comet emerged from the XMM-Newton solar exclusion zone only a few weeks after its periapsis. The NuSTAR Science Operations team worked with the Principal Investigator team to compute the apparent position of the comet every orbit to determine the optimal pointing of the satellite. This allowed NuSTAR to capture the entire coma of the comet in its field of view as it moved across the sky. Fortunately, the Sun did emit an M-class flare during this observation that will be used to search for reflected solar X-rays from the comet.
Author: Brian Grefenstette (NuSTAR Instrument Scientist, Caltech)
