Winds of Change: XRISM Resolve X-Ray Spectroscopy of NGC 4051

NGC 4051 is a nearby (16.7 Mpc), Narrow Line Seyfert 1 galaxy (NLS1), which has a low black hole mass of 106 M⊙. It is also known for its rapid X-ray variability, on timescales of kiloseconds, and has a complex, multicomponent wind in both the soft X-ray and Fe K bands. Here we present the first high-resolution XRISM Resolve spectrum of NGC 4051, which was captured in a historically bright state for a 150 ks exposure. XRISM resolves two blueshifted Fe K shell absorption troughs in the mean spectrum, which can be ascribed to H-like iron and arises from two outflow components with outflow velocities of 0.025c and 0.04c. A time-dependent spectral analysis shows that the iron K absorption is variable on timescales of less than a day, increasing in velocity over the duration of the observation. The velocity changes may be explained either by the passage of two separate transiting absorbers, of different velocities, or by a single accelerating outflow of approximately constant column density. In the latter case, the wind acceleration is likely to be too large to be caused by radiation pressure, and instead, magnetic driving is favored to accelerate the wind up to 0.04c. The outflow can originate from an accretion disk wind, whose kinetic power is sub-Eddington, in contrast to recent examples of winds from powerful, luminous quasars observed by XRISM.