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

¹ INAF-Osservatorio Astronomico di Roma Via Frascati 33 I-00078 Monte Porzio Catone (RM), Italy
² Department of Astronomy and Astrophysics, University of California San Diego CA 92093, USA
³ INAF – IASF Palermo Via Ugo La Malfa 153 90146 Palermo, Italy
⁴ Department of Astronomy and Astrophysics, University of California San Diego La Jolla CA 92075, USA
⁵ Institut für Astronomie und Astrophysik, Universität Tübingen Sand 1 D-72076 Tübingen, Germany
⁶ Institute of Space Sciences (ICE, CSIC), Campus UAB Carrer de Can Magrans s/n E-08193 Barcelona, Spain
⁷ Institut d’Estudis Espacials de Catalunya (IEEC) 08860 Castelldefels Barcelona, Spain
⁸ European Space Agency (ESA), European Space Astronomy Centre (ESAC) Camino Bajo del Castillo s/n 28692 Villanueva de la Cañada Madrid, Spain
⁹ Università Tor Vergata, Dipartimento di Fisica Via della Ricerca Scientifica 1 I-00133 Rome, Italy
¹⁰ Department of Physics and Astronomy, Howard University Washington DC 20059, USA
¹¹ CRESST and NASA Goddard Space Flight Center, Astrophysics Science Division 8800 Greenbelt Road Greenbelt MD, USA
¹² Dipartimento di Fisica e Chimica E. Segrè, Università degli Studi di Palermo Via Archirafi 36 90123 Palermo, Italy
¹³ Physics Department, United States Naval Academy Annapolis MD 21402, USA
¹⁴ Department of Physics and Astronomy, Embry-Riddle Aeronautical University 3700 Willow Creek Road Prescott AZ 86301, USA
¹⁵ Dr. Karl-Remeis-Observatory and Erlangen Centre for Astroparticle Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg Sternwartstr. 7 96049 Bamberg, Germany

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Received: 27 October 2025
Accepted: 22 December 2025

Abstract

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 × 10³³ 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 × 10¹³ 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 Kα emission line but no indications of a cyclotron line.