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NICER / ISS Science Nugget
for December 16, 2021



NICER tracks the evolution of a rare X-ray-bright Dwarf Nova

On November 29, 2021, NICER responded to a Target of Opportunity (TOO) request to observe the new optical transient AT2021afpi that was found by several ground-based optical telescopes. Initially, the optical discovery seemed coincident with a neutrino detection by the IceCube Neutrino Observatory. However, AT2021afpi was found to be bright before the neutrino detection. Optical spectra have determined that AT2021afpi is a WZ Sagittae (WZ Sge)-type dwarf nova, a type which has not been commonly tracked in X-rays.

A WZ Sge dwarf nova is a binary star system consisting of a white dwarf (an a bit less than the mass of the Sun and about the size of the eearth) and a lower-mass companion, somewhat less than 1/10th that of the Sun. Outbursts occur when the low-mass star and an accretion disk around the white dwarf interact, which suddenly dumps a large amount of matter onto the white dwarf's surface.

NICER observations continue to show spectral evolution in AT2021afpi, from being a very soft X-ray source at the outset, then hardening, and now slowly softening again (see below). NICER observations combined with extensive ground-based optical measurements should yield new insight in these systems.

Top: The measured X-ray intensity as a function of time of AT2021afpi shows a gradual decline over the past two weeks. Bottom: Evolution of the spectral hardness, which relates the total number of counts detected in the 350-3500 eV range to the counts in the 200-350 eV range. This shows that the source started very soft, then hardened, and now has just begun to soften again.



Figure: Top: The measured X-ray intensity as a function of time of AT2021afpi shows a gradual decline over the past two weeks. Bottom: Evolution of the spectral hardness, which relates the total number of counts detected in the 350-3500 eV range to the counts in the 200-350 eV range. This shows that the source was very soft at the start, then hardened, and now has just begun to soften again.



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