XRISM Time-resolved Fe K$α$ Spectroscopy of NGC 4395: Time-variable Inner-disk Emission

Abstract

We report the first XRISM observation of the low-mass AGN in the nearby dwarf galaxy NGC 4395 ($M_{\rm BH}\sim10^{4-5}\,M_\odot$), complemented by a simultaneous NuSTAR observation. We constrained the continuum by jointly fitting the XRISM/Resolve (2-12 keV) and NuSTAR (3-30 keV) spectra while excluding the Fe K band (5.5-7.5 keV). Relative to this baseline continuum, the time-averaged Resolve spectrum revealed an unresolved neutral Fe K$α$ core with a velocity width of $\lesssim$110 km s$^{-1}$ and an adjacent redward wing. The red wing was well reproduced by an additional relativistically broadened Fe K component. Furthermore, time-resolved spectroscopy with $\approx$87 ks bins showed that the diskline profile varied significantly over the $\sim$400 ks observation. This evolution can be interpreted in terms of changes in the inner radius of the line-emitting region, together with a possible inclination modulation with a period of $\approx$210 ks. If interpreted as Lense-Thirring precession of a tilted inner flow, the observed period would favor the low end of the black hole mass estimates ($M_{\rm BH}\approx9\times10^3\,M_\odot$) and imply a moderate spin ($a\gtrsim0.6$). These results highlight the capability of XRISM to track relativistic disk dynamics in AGNs.