Extended Data Fig. 6: X-ray and optical constraints on accretion in ZTFJ1539 + 5027.

These constraints on mass transfer result from the non-detection of any signatures of accretion in both the optical and X-ray bands. The upper limits are expressed in terms of the mass accretion rate contributing to the accretion luminosity of a hypothetical hotspot. The solid red curve illustrates the constraint imposed by the XMM EPIC-pn X-ray non-detection, which rules out statistically significant mass transfer contributing to a hotspot with temperatures greater than about 150,000 K, while the green dotted line illustrates a weaker upper limit imposed by the non-detection in a SWIFT XRT observation. We constructed the dashed blue curve, which represents the optical constraint, by requiring that any accretion luminosity originating from a hotspot should contribute <10% to the luminosity in the band ranging from 320 nm to 540 nm, as we know from the optical spectrum (Fig. 3) that this light is dominated by the approximately 50,000-K photosphere of the hot primary, and also we see no signature of a hotspot in the CHIMERA lightcurve (Fig. 1). We chose the threshold of <10% because, given the SNR of the spectra, we expect we should be able to detect optically thin emission with an amplitude at the 10% level. Other white dwarfs with such a hotspot (such as HM Cancri) exhibit such emission, particularly in lines associated with ionized helium.