Quiet, but not silent: uncovering quiescent state properties of two transient high-mass X-ray binaries

We present the first set of broad-band spectral and timing studies of two transient X-ray pulsars, MXB 0656-072 and MAXI J1409-619 using NuSTAR observations conducted during quiescence. Despite being captured at one of their lowest luminosity states, both these targets show signs of ongoing low-level accretion. Results from the time-averaged spectral analysis indicate for the first time, the presence of a strong soft power-law component along with thermal emission from the neutron star hotspots. For both targets, the quiescent thermal X-ray emission is consistent with the deep crustal heating model. In MXB 0656-072, we do not detect any pulsation or indication of a cyclotron line during quiescence. However, in MAXI J1409-619 we detect strong pulsations at 502 s with a pulsed fraction of ∼66 per cent, which adds this pulsar to the list of a handful of quiescent-state pulsating systems.