ISS/NICER Observations of Comet 88P/Howell in the Equatorial Solar Wind

We present the preliminary results of our observation of comet 88P/Howell during its 2020 apparition. We employed the X-ray timing instrument (XTI) of the Neutron star Interior Composition Explorer (NICER) on board the International Space Station for the observation of 88P and identified a reliable strategy for future comet observations using NICER’s XTI. The primary mechanism for X-ray emission in comets is solar wind charge exchange, which emits primarily between 0.2 and 2 keV. We chose to use NICER’s XTI to observe the comet because it collects photons over a large effective area in the soft X-ray regime, which includes photons with energies between 0.2 and 12 keV. We observed 88P between September 6, 2020 and November 1, 2020, a range which included the comet’s perihelion passage. During this time, 88P was at a heliocentric latitude that was a few degrees below the Sun’s equator and near to the coronal streamer belt; therefore, the source of its X-ray emission was its atmosphere’s interaction with the hot and highly variable slow solar wind. As comets and their atmospheres are extended sources (~100,000 km), subtracting the X-ray photon contribution from background sources is often complicated; however, since solar wind charge exchange is not expected to occur at energies above 2 keV and since the X-ray flux is generally stable within this energy range, we may take separate background measurements and compare the count rates of this spectral area over time to isolate the photon contribution from 88P. To further constrain the background contribution, we included two blank-sky exposures to accompany each observation of 88P, one before and one after the comet passed through the field of view. Our preliminary results suggest that using NICER’s XTI alongside a robust background-observation strategy to observe a comet is a viable method of collecting high-resolution data.