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Swift J1858.6-0814 was discovered by Swift-BAT on October 25, 2018. Here wereport on the first follow-up NuSTAR observation of the source, which showsvariability spanning two orders of magnitude in count rate on timescales of~10-100 s. The power-spectrum of the source does not show any quasi-periodicoscillations or periodicity, but has a large fractional rms amplitude of147%$\pm3$%, exhibiting a number of large flares throughout the observation.The hardness ratio (defined as $R_{10-79 \rm keV}/R_{3-10 \rm keV}$) of theflares tends to be soft, while the source spans a range of hardness ratiosduring non-flaring periods. The X-ray spectrum of the source shows strongreflection features, which become more narrow and peaked during the non-flaringintervals. We fit an absorbed relativistic reflection model to the sourcespectra to place physical constraints on the system. Most notably, we find thatthe source exhibits a large and varying intrinsic absorbing column density($N_{\rm H}=1.4-4.2\times10^{23}$ cm$^{-2}$). This large intrinsic absorptionis further supported by the energy spectra extracted from two flares observedsimultaneously by NuSTAR and NICER. We find that the inner accretion disk ofthe source has a low inclination, $i<29^{\circ}$ ( 3$\sigma$ upper-limit),while the iron abundance in the disk is close to solar, $A_{\rm Fe}=1.0\pm0.3$.We set a 90% confidence upper limit on the inner radius of the accretion diskof $r_{\rm in}<8 \ r_{\rm ISCO}$, and, by fixing $r_{\rm in}$ to be at $r_{\rmISCO}$, a 90% confidence lower-limit on the spin of the black hole of$a^{*}>0.0$. Lastly, we compare the properties of Swift J1858.6$-$0814 to thoseof V404 Cygni and V4641 Sgr, which both show rapid flaring and a strong andvariable absorption.

NuSTAR observations of the Transient Galactic Black Hole Binary Candidate Swift J1858.6-0814: A New Sibling of V404 Cyg and V4641 Sgr?