Phase-resolved spectroscopy of a quasi-periodic oscillation in the black hole X-ray binary GRS 1915+105 with <i>NICER</i> and <i>NuSTAR</i>

Quasi-periodic oscillations (QPOs) are often present in the X-ray flux from accreting stellar-mass black holes (BHs). If they due to relativistic (Lense-Thirring) precession of an inner accretion flow which is misaligned with disc, iron emission line caused by irradiation disc will rock systematically between red and blue shifted during each QPO cycle. Here we conduct phase-resolved spectroscopy a $\sim2.2$ Hz type-C BH binary GRS 1915+105, observed simultaneously NICER NuSTAR. We apply tomographic model order constrain phase-dependent illumination profile disc. detect predicted shifts centroid energy, our best fit featuring asymmetric ($>2{\sigma}$ confidence). The energy can alternatively be explained spiral density waves accretion-ejection instability model. However additionally measure significant ($>3{\sigma}$) modulation reflection fraction, strongly favouring geometric origin. infer that previously jet ejections, consistent truncated precessing hot flow. inferred radius small ($r_\text{in}{\sim} 1.4 GM/c^2$). For this radius, Lense-Thirring cannot reproduce frequency. In fact, incompatible predictions all well-studied models literature.