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Constraints on cyclotron features and accretion regime in the high-mass X-ray binary 4U 1700--37 from NuSTAR

Lautaro West-Ocampo, Federico A. Fogantini, Enzo A. Saavedra, Jorge A. Combi, Federico García, Pedro L. Luque-Escamilla, Josep Martí, Sylvain Chaty, Juan F. Albacete-Colombo

TL;DR

The paper analyzes two NuSTAR epochs of 4U 1700-37 to test the reality of candidate CRSFs and to constrain the neutron-star magnetic field and the wind-fed accretion regime. No coherent pulsations are detected, and the time-averaged spectrum is best described by a two-component continuum with an Fe K$\\alpha$ line; residuals near $\sim$19.5 and $\sim$52 keV suggest CRSFs but remain below the $3\sigma$ threshold under the preferred continuum, though more significant under some alternate continua. Interpreting these features within quasi-spherical subsonic accretion yields $B\sim(1.7-4.4)\times10^{12}$ G and an equilibrium spin period $P_{eq}\approx 1.9$ ks, placing 4U 1700-37 in the sgHMXB domain of the Corbet diagram. The work highlights the strong continuum dependence of marginal CRSFs in wind-fed systems and demonstrates the need for multi-epoch, simulation-based significance assessments to robustly constrain magnetic fields and accretion physics.

Abstract

4U 1700-37 is a wind-fed high-mass X-ray binary hosting a compact object, likely a neutron star, accreting from O6.5 Iaf+ supergiant HD 153919. Coherent pulsations not firmly detected; magnetic field strength remains uncertain. We analyze NuSTAR observations to characterize hard X-ray timing and spectral properties, test robustness of candidate cyclotron features, and constrain magnetic field and accretion regime. We perform timing and spectral analysis of two observations, modeling spectra with continua used for accreting pulsars, and use simulations to assess significance of features. No coherent pulsations detected; pulsed fraction constrained below 1.5\%. Spectra are well described by absorbed blackbody plus cutoff power-law continuum, showing residuals around 20 keV and 40--50 keV. Features improve fits but do not constitute firm cyclotron detections. Intensity-resolved spectroscopy suggests possible shifts of apparent line centroid. Results favor neutron-star magnetic field of 1.7--4.4 $\times 10^{12}$ G and quasi-spherical subsonic accretion regime with equilibrium spin period $\sim 1.9$ ks. Analysis provides quantitative constraints on magnetic field and accretion physics, helping reconcile discrepant line-energy measurements.

Constraints on cyclotron features and accretion regime in the high-mass X-ray binary 4U 1700--37 from NuSTAR

TL;DR

The paper analyzes two NuSTAR epochs of 4U 1700-37 to test the reality of candidate CRSFs and to constrain the neutron-star magnetic field and the wind-fed accretion regime. No coherent pulsations are detected, and the time-averaged spectrum is best described by a two-component continuum with an Fe K line; residuals near 19.5 and 52 keV suggest CRSFs but remain below the threshold under the preferred continuum, though more significant under some alternate continua. Interpreting these features within quasi-spherical subsonic accretion yields G and an equilibrium spin period ks, placing 4U 1700-37 in the sgHMXB domain of the Corbet diagram. The work highlights the strong continuum dependence of marginal CRSFs in wind-fed systems and demonstrates the need for multi-epoch, simulation-based significance assessments to robustly constrain magnetic fields and accretion physics.

Abstract

4U 1700-37 is a wind-fed high-mass X-ray binary hosting a compact object, likely a neutron star, accreting from O6.5 Iaf+ supergiant HD 153919. Coherent pulsations not firmly detected; magnetic field strength remains uncertain. We analyze NuSTAR observations to characterize hard X-ray timing and spectral properties, test robustness of candidate cyclotron features, and constrain magnetic field and accretion regime. We perform timing and spectral analysis of two observations, modeling spectra with continua used for accreting pulsars, and use simulations to assess significance of features. No coherent pulsations detected; pulsed fraction constrained below 1.5\%. Spectra are well described by absorbed blackbody plus cutoff power-law continuum, showing residuals around 20 keV and 40--50 keV. Features improve fits but do not constitute firm cyclotron detections. Intensity-resolved spectroscopy suggests possible shifts of apparent line centroid. Results favor neutron-star magnetic field of 1.7--4.4 G and quasi-spherical subsonic accretion regime with equilibrium spin period ks. Analysis provides quantitative constraints on magnetic field and accretion physics, helping reconcile discrepant line-energy measurements.
Paper Structure (13 sections, 2 equations, 4 figures, 2 tables)

This paper contains 13 sections, 2 equations, 4 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Observation and Data Analysis
  3. Swift/BAT data
  4. NuSTAR data
  5. Results
  6. Temporal analysis
  7. Spectral analysis
  8. Testing for CRSFs
  9. Intensity-resolved spectroscopy
  10. Discussion
  11. Comparison with previous X-ray campaigns
  12. Accretion regime in 4U1700
  13. Conclusions

Figures (4)

  • Figure 1: Swift/BAT folded light curve of 4U1700 using 64 orbital phase bins. The data were folded using a orbital period of $3.411581$ d Falanga2015AA...577A.130F and a reference epoch $T_0 [\rm MJD]= 57446.55$Bala2020MNRAS.493.3045B. The NuSTAR observations of 4U1700 used in this work are represented by the purple and blue colored stripes, spanning $\sim$39% and $\sim$25% of the orbital period, respectively.
  • Figure 2: NuSTAR background-subtracted light curves of 4U1700 extracted with a bin time of $50~\mathrm{s}$. Top panel: 3--8 $\mathrm{keV}$ light curve. Middle panel: 8--79 $\mathrm{keV}$ light curve. Bottom panel: hardness ratio.
  • Figure 3: Top panel: FPMA/B background-subtracted spectra of 4U1700. Background spectra are also shown in dashed lines. Second-to-last panels: residuals obtained from fitting different continuum models. The corresponding model label is indicated in each panel. All models contain a Gaussian line component at ${\sim}$6.3 keV. The total $\chi^{2}$ and degrees of freedom are shown on the bottom left part of each panel. A modulation in the residuals above 30 keV can be seen for all the tested models.
  • Figure 4: NuSTAR residuals of 4U1700's using the cutoffpl model as baseline. The top panel shows only the residuals of the continuum and Fe line contributions ($\mathcal{H}_0$). The middle panel shows the residuals of the continuum affected by a single gabs component ($\mathcal{H}_1$) at ${\sim}50$ keV. The bottom panel also includes a second gabs component ($\mathcal{H}_2$) at ${\sim}20$ keV. Total $\chi^{2}$ and dof are indicated on the bottom left part of each panel. The vertical gray stripes indicate the CSRFs candidates with their corresponding centroids and widths. The data were re-binned for better visualization.