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

C. Malacaria, S. N. Pike, A. D'Aì, G. L. Israel, L. Ducci, R. E. Rothschild, L. Stella, R. Amato, E. Ambrosi, J. B. Coley, F. Fürst, M. Imbrogno, P. Kretschmar, D. K. Maniadakis, A. Papitto, P. Pradhan, A. Rouco Escorial, A. Simongini, J. Stierhof, B. F. West, N. Zalot

TL;DR

MAXI J0655-013 is studied in its low-luminosity XRP state using XMM-Newton and NuSTAR, revealing a double-hump spectrum best modeled by two CompTT components, consistent with a two-region atmosphere scenario and different polarization modes. A phase-connected timing solution yields $P=1081.86\pm0.02$ s and supports a persistent Be/X-ray binary with an upper magnetic-field limit of $B\lesssim9\times10^{13}$ G, while the pulse profile exhibits a single broad peak with a dip likely caused by absorption and energy-dependent pulsed fractions that rise from $\sim$72% (3–30 keV) to $\sim$100% (10–30 keV). Pulsed fractions show a flat $\mathrm{PF_{\mathrm{rms}}}$ slope (~60%) across energy, and the pulsed-fraction spectrum reveals an iron-feature at high luminosity but no confirmed cyclotron line. These results constrain the emission geometry and accretion physics in the low-rate regime of XRPs and corrobor the double-hump interpretation of the spectrum with implications for magnetic-field–driven accretion processes in Be/X-ray binaries.

Abstract

Accreting X-ray pulsars (XRPs) undergo different physical regimes depending on the mass accretion rate. Recent observations have shown a dramatic change in the emission properties of this class of sources observed at low luminosity. We explore the timing and spectral properties of the XRP MAXI J0655-013 observed in the low-luminosity regime (about 5x$10^{33}$ erg/s) to witness the corresponding spectral shape and pulse profiles. We employ recent $XMM$ and $NuSTAR$ pointed observations of the MAXI J0655-013 X-ray activity during the low-luminosity stage. We explore several spectral models to fit the data and test theoretical expectations of the dramatic transition of the spectral shape. We study the pulsating nature of the source and find a phase-connected timing solution. We explore the energy-resolved pulse profiles and the derived energy-dependence of different pulsed fraction estimators ($PF_{minmax}$ and $PF_{rms}$). We also obtain $NuSTAR$ pulsed fraction spectra (PFS) at different luminosity regimes. MAXI J0655-013 spectrum is well fitted 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 measure a spin period of $1081.86\pm0.02$ s, consistent with the source spinning-up compared to previous observations, yielding an upper limit for the magnetic field strength of B<9x$10^{13}$ G. The pulse profiles show a single broad peak interrupted by a sharp dip that coincides with an increase in the hardness ratio. For the low-luminosity observation, the $PF_{minmax}$ increases with energy up to $\sim100\%$ in the 10-30 keV band, while the $PF_{rms}$ remains steady at $\sim60\%$. The PFS obtained at high luminosity shows evidence of an iron $Kα$ emission line but no indications of a cyclotron line.

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

TL;DR

MAXI J0655-013 is studied in its low-luminosity XRP state using XMM-Newton and NuSTAR, revealing a double-hump spectrum best modeled by two CompTT components, consistent with a two-region atmosphere scenario and different polarization modes. A phase-connected timing solution yields s and supports a persistent Be/X-ray binary with an upper magnetic-field limit of G, while the pulse profile exhibits a single broad peak with a dip likely caused by absorption and energy-dependent pulsed fractions that rise from 72% (3–30 keV) to 100% (10–30 keV). Pulsed fractions show a flat slope (~60%) across energy, and the pulsed-fraction spectrum reveals an iron-feature at high luminosity but no confirmed cyclotron line. These results constrain the emission geometry and accretion physics in the low-rate regime of XRPs and corrobor the double-hump interpretation of the spectrum with implications for magnetic-field–driven accretion processes in Be/X-ray binaries.

Abstract

Accreting X-ray pulsars (XRPs) undergo different physical regimes depending on the mass accretion rate. Recent observations have shown a dramatic change in the emission properties of this class of sources observed at low luminosity. We explore the timing and spectral properties of the XRP MAXI J0655-013 observed in the low-luminosity regime (about 5x erg/s) to witness the corresponding spectral shape and pulse profiles. We employ recent and pointed observations of the MAXI J0655-013 X-ray activity during the low-luminosity stage. We explore several spectral models to fit the data and test theoretical expectations of the dramatic transition of the spectral shape. We study the pulsating nature of the source and find a phase-connected timing solution. We explore the energy-resolved pulse profiles and the derived energy-dependence of different pulsed fraction estimators ( and ). We also obtain pulsed fraction spectra (PFS) at different luminosity regimes. MAXI J0655-013 spectrum is well fitted 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 measure a spin period of s, consistent with the source spinning-up compared to previous observations, yielding an upper limit for the magnetic field strength of B<9x G. The pulse profiles show a single broad peak interrupted by a sharp dip that coincides with an increase in the hardness ratio. For the low-luminosity observation, the increases with energy up to in the 10-30 keV band, while the remains steady at . The PFS obtained at high luminosity shows evidence of an iron emission line but no indications of a cyclotron line.
Paper Structure (14 sections, 2 equations, 6 figures, 2 tables)

This paper contains 14 sections, 2 equations, 6 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Observations and data reduction
  3. XMM-Newton observations
  4. NuSTAR
  5. Swift/XRT
  6. Spectral analysis
  7. Timing analysis
  8. Pulsed fraction spectra
  9. Discussion
  10. The double-hump spectrum
  11. Discrete spectral features
  12. Timing results
  13. Pulse profiles and pulsed fraction
  14. Conclusions

Figures (6)

  • Figure 1: XMM-Newton (0.5-10 keV) and NuSTAR (4-79 keV) both at 100 s binning light curves of MAXI J0655. Count rates were rescaled according to model predictions and effective area accounting for the observed spectral energy distribution.
  • Figure 2: Unfolded spectra from XMM-Newton pn, MOS 1 and 2 (black, red and green, respectively) and NuSTAR FPMA and FPMB (blues and cyan, respectively) from the MAXI J0655 observations in the low-luminosity regime analyzed in this work. Dashed lines represent the two Comptonization components for each instrument. Bottom panel: residuals to the best-fit double-CompTT model.
  • Figure 3: Top: 3-30 keV pulse profile measured by NuSTAR. Middle: Hard (10-30 keV, purple) and soft (3-10 keV, orange) pulse profiles, scaled and normalized for clarity. Bottom: phase-resolved hardness ratio between the hard and soft pulse profiles.
  • Figure 4: Energy-resolved pulse profiles measured with NuSTAR (red) and XMM-Newton-pn (green). The pulse profile shape varies with energy. The sharp dip around $\phi=0.85$ in particular shows a strong dependence on energy, being most prominent in the 3-6 keV range.
  • Figure 5: Energy-resolved pulsed fraction $\mathrm{PF_\mathrm{minmax}}$ as measured by XMM-Newton (green) and NuSTAR (red). The pulsed fraction estimator exhibits a high overall value which strongly correlates with photon energy.
  • ...and 1 more figures