Discovery of spin-phase dependent QPOs in the super-critical accretion regime from the X-ray pulsar RX J0440.9+4431

TL;DR

This paper reports the discovery of transient, spin-phase dependent QPOs in the X-ray pulsar RX J0440.9+4431, observed during super-critical accretion, revealing new insights into accretion disk instabilities and emission geometry.

Contribution

It introduces the phenomenon of thrice transient QPOs (ATTO) in RX J0440.9+4431 and links their occurrence to super-critical accretion regime changes and emission region configuration.

Findings

QPOs appear only above certain luminosity levels.

QPOs are phase-dependent, occurring at pulse peaks.

QPOs are transient, lasting only a few oscillations.

Abstract

RX J0440.9+4431 is an accreting X-ray pulsar (XRP) that remained relatively unexplored until recently, when major X-ray outburst activity enabled more in-depth studies. Here, we report on the discovery of ${\sim}0.2$ Hz quasi-periodic oscillations (QPOs) from this source observed with $Fermi$-GBM. The appearance of QPOs in RX J0440.9+4431 is thricely transient, that is, QPOs appear only above a certain luminosity, only at certain pulse phases (namely corresponding to the peak of its sine-like pulse profile), and only for a few oscillations at time. We argue that this newly discovered phenomenon (appearance of thricely transient QPOs -- or ATTO) occurs if QPOs are fed through an accretion disk whose inner region viscosity is unstable to mass accretion rate and temperature variations. Such variations are triggered when the source switches to the super-critical accretion regime and the emission pattern changes. We also argue that the emission region configuration is likely responsible for the observed QPOs spin-phase dependence.