2020MNRAS.496.1768D

We probe the properties of the transient X-ray pulsar MAXI J1409-619 through RXTE and Swift follow-up observations of the outburst in 2010. We are able to phase-connect the pulse arrival times for the 25 d episode during the outburst. We suggest that either an orbital model (with P_orb ≃ 14.7(4) d) or a noise process due to random torque fluctuations (with S_r ≃ 1.3 x 10^–18 Hz² s^–2 Hz^–1) is plausible to describe the residuals of the timing solution. The frequency derivatives indicate a positive torque-luminosity correlation, which implies temporary accretion disc formation during the outburst. We also discover several quasi-periodic oscillations in company with their harmonics whose centroid frequencies decrease as the source flux decays. The variation of the pulsed fraction and spectral power-law index of the source with X-ray flux is interpreted as the sign of transition from a critical to a sub-critical accretion regime at the critical luminosity within the range of 6 x 10³⁷-1.2 x 10³⁸ erg s^–1. Using pulse-phase-resolved spectroscopy, we show that the phases with higher flux tend to have lower photon indices, indicating that the polar regions produce spectrally harder emission.