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

Abstract

Quasi-periodic oscillations (QPOs) are often present in the X-ray flux from accreting stellarmass black holes (BHs). If they are due to relativistic (Lense-Thirring) precession of an inner accretion flow which is misaligned with the disc, the iron emission line caused by irradiation of the disc by the inner flow will rock systematically between red and blue shifted during each QPO cycle. Here we conduct phase-resolved spectroscopy of a ∼ 2.2 Hz type-C QPO from the BH X-ray binary GRS 1915+105, observed simultaneously with NICER and NuSTAR. We apply a tomographic model in order to constrain the QPO phase-dependent illumination profile of the disc. We detect the predicted QPO phase-dependent shifts of the iron line centroid energy, with our best fit featuring an asymmetric illumination profile (> 2𝜎 confidence). The observed line energy shifts can alternatively be explained by the spiral density waves of the accretion-ejection instability model. However we additionally measure a significant (> 3𝜎) modulation in reflection fraction, strongly favouring a geometric QPO origin. We infer that the disc is misaligned with previously observed jet ejections, which is consistent with the model of a truncated disc with an inner precessing hot flow. However our inferred disc inner radius is small (𝑟 in ∼ 1.4 𝐺 𝑀/𝑐 2 ). For this disc inner radius, Lense-Thirring precession cannot reproduce the observed QPO frequency. In fact, this disc inner radius is incompatible with the predictions of all well-studied QPO models in the literature.

Disk Tearing Leads to Low and High Frequency Quasi Periodic Oscillations in a GRMHD Simulation of a Thin Accretion Disk

“…There is increasing evidence for the geometric origin of some QPOs, such as the dependence of LFQPO amplitude on the inclination of the binary orbit (Heil et al 2015;Motta et al 2015), phase lag dependence on inclination for type-C LFQPOs (van den Eijnden et al 2017) and the modulation of the equivalent width of the iron line with the phase of a type-C QPO detected in H1743-322 (Ingram et al 2016) (also GRS 1915+105, see Ingram & van der Klis 2015;Ingram et al 2016;Nathan et al 2022). Models based on geometric effects invoke a wide range of mechanisms for QPO production often resulting from the misalignment between the BH spin axis and the binary orbit.…”

Are Low-Frequency Quasi-Periodic Oscillations seen in accretion flows the disk response to a jet instability?

“…Motta et al 2015;Heil et al 2015;van den Eijnden et al 2017). But according to the above discussion, we doubt that solid body LT-precession could be the generic physical mechanism responsible for Type-C QPOs in all XrBs and believe that another mechanism needs therefore to be found (see also Nathan et al 2022).…”