Timing analysis of <i>Swift</i> J0243.6+6124 with <i>NICER</i> and <i>Fermi</i>/GBM during the decay phase of the 2017-2018 outburst

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Serim M. M., Dönmez Ç. K., Serim D., Ducci L., Baykal A., Santangelo A.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, vol.522, no.4, pp.6115-6122, 2023 (SCI-Expanded, Scopus)

  • Publication Type: Article / Article
  • Volume: 522 Issue: 4
  • Publication Date: 2023
  • Doi Number: 10.1093/mnras/stad1407
  • Journal Name: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus, Academic Search Premier, Aerospace Database, Applied Science & Technology Source, Communication Abstracts, Compendex, INSPEC, Metadex, zbMATH, DIALNET, Civil Engineering Abstracts
  • Page Numbers: pp.6115-6122
  • Keywords: accretion, accretion discs, methods: data analysis, pulsars: individual: Swift J0243.6+6124
  • Open Archive Collection: AVESIS Open Access Collection
  • Middle East Technical University Affiliated: Yes

Abstract

We present a timing and noise analysis of the Be/X-ray binary system Swift J0243.6+6124 during its 2017-2018 super-Eddington outburst using NICER/XTI observations. We apply a synthetic pulse timing analysis to enrich the Fermi/GBM spin frequency history of the source with the new measurements from NICER/XTI. We show that the pulse profiles switch from double-peaked to single-peaked when the X-ray luminosity drops below similar to 7 x10(36) erg s(-1). We suggest that this transitional luminosity is associated with the transition from a pencil beam pattern to a hybrid beam pattern when the Coulomb interactions become ineffective to decelerate the accretion flow, which implies a dipolar magnetic field strength of similar to 5 x10(12) G. We also obtained the power density spectra (PDS) of the spin frequency derivative fluctuations. The red noise component of the PDS is found to be steeper (omega(-3.36)) than the other transient accreting sources. We find significantly high noise strength estimates above the super-Eddington luminosity levels, which may arise from the torque fluctuations due to interactions with the quadrupole fields at such levels.