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

Understand

Swift J1858.6-0814 was discovered by Swift-BAT on October 25, 2018.

• Here we report on the first follow-up NuSTAR observation of the source, which shows variability 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-periodic oscillations or periodicity, but has a large fractional rms amplitude of 147%$\pm3$%, 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 the flares tends to be soft, while the source spans a range of hardness ratios during non-flaring periods.