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Swift J1858.6-0814 is a recently discovered X-ray binary notable forextremely strong variability (by factors $>100$ in soft X-rays) in itsdiscovery state. We present the detection of five thermonuclear (Type I) X-raybursts from Swift J1858.6-0814, implying that the compact object in the systemis a neutron star. Some of the bursts show photospheric radius expansion, sotheir peak flux can be used to estimate the distance to the system. The peakluminosity, and hence distance, can depend on several system parameters; forthe most likely values, a high inclination and a helium atmosphere,$D=12.8_{-0.6}^{+0.8}$ kpc, although systematic effects allow a conservativerange of $9-18$ kpc. Before one burst, we detect a QPO at $9.6\pm0.5$ mHz witha fractional rms amplitude of $2.2\pm0.2$% ($0.5-10$ keV), likely due tomarginally stable burning of helium; similar oscillations may be present beforethe other bursts but the light curves are not long enough to allow theirdetection. We also search for burst oscillations but do not detect any, with anupper limit in the best case of 15% fractional amplitude (over $1-8$ keV).Finally, we discuss the implications of the neutron star accretor and thisdistance on other inferences which have been made about the system. Inparticular, we find that Swift J1858.6-0814 was observed at super-Eddingtonluminosities at least during bright flares during the variable stage of itsoutburst.

Discovery of thermonuclear (Type I) X-ray bursts in the X-ray binary Swift J1858.6-0814 observed with NICER and NuSTAR