Double-hump spectrum, pulse profile dip, and pulsed fraction spectra from the low-accretion regime in the X-ray pulsar MAXI J0655-013

Context. Accreting X-ray pulsars (XRPs) undergo different physical regimes depending on different mass accretion rates, with related changes in the emitted X-ray spectral properties. Recent observations have shown a dramatic change in the emission properties of this class of sources observed at low luminosity. Aims. We explore the timing and spectral properties of the XRP MAXI J0655-013 observed in the low-luminosity regime (about 5 × 10 33 erg s −1 ) to witness the corresponding spectral shape and pulse profiles. Methods. We employed recent XMM-Newton and NuSTAR pointed observations of the MAXI J0655-013 X-ray activity during the low-luminosity stage. We explored several spectral models to fit the data and test theoretical expectations of the dramatic transition of the spectral shape compared to the higher luminosity regime. We studied the pulsating nature of the source and found a precise timing solution. We explored the energy-resolved pulse profiles and the derived energy dependence of different pulsed fraction estimators (PF minmax and PF rms ). We also obtained NuSTAR pulsed fraction spectra (PFS) at different luminosity regimes. Results. The MAXI J0655-013 spectrum is well fit by a double Comptonization model, in agreement with recent observational results and theoretical expectations that explain the observed spectrum as being composed of two distinct bumps, each dominated by different polarization modes. We measured a spin period of 1081.86 ± 0.02 s, consistent with the source spinning up compared to previous observations, yielding an upper limit for the magnetic field strength of B ≲ 9 × 10 13 G. The pulse profiles show a single broad peak interrupted by a sharp dip that coincides with an increase in the hardness ratio, and thus likely due to absorption. For the low-luminosity observation, the PF minmax increases with energy up to ∼100% in the 10–30 keV band, while the PF rms remains steady at ∼60%. The PFS obtained at high luminosity shows evidence of an iron K α emission line but no indications of a cyclotron line.