Accretion Spin-up and a Strong Magnetic Field in the Slow-spinning Be X-Ray Binary MAXI J0655-013

<jats:title>Abstract</jats:title> <jats:p>We present Monitor of All-sky X-ray Image (MAXI) and Nuclear Spectroscopic Telescope Array (NuSTAR) observations of the Be X-ray binary, MAXI J0655−013, in outburst. NuSTAR observed the source once early in the outburst, when spectral analysis yields a bolometric (0.1–100 keV), unabsorbed source luminosity of <jats:italic>L</jats:italic> <jats:sub>bol</jats:sub> = 5.6 × 10<jats:sup>36</jats:sup> erg s<jats:sup>−1</jats:sup>, and a second time 54 days later, by which time the luminosity had dropped to <jats:italic>L</jats:italic> <jats:sub>bol</jats:sub> = 6.5 × 10<jats:sup>34</jats:sup> erg s<jats:sup>−1</jats:sup> after first undergoing a dramatic increase. Timing analysis of the NuSTAR data reveals a neutron star spin period of 1129.09 ± 0.04 s during the first observation, which decreased to 1085 ± 1 s by the time of the second observation, indicating spin-up due to accretion throughout the outburst. Furthermore, during the first NuSTAR observation, we observed quasi-periodic oscillations (QPOs) with centroid frequency <jats:italic>ν</jats:italic> <jats:sub>0</jats:sub> = 89 ± 1 mHz, which exhibited a second harmonic feature. By combining the MAXI and NuSTAR data with pulse period measurements reported by Fermi/GBM, we are able to show that apparent flaring behavior in the MAXI light curve is an artifact introduced by uneven sampling of the pulse profile, which has a large pulsed fraction. Finally, we estimate the magnetic field strength at the neutron star surface via three independent methods, invoking a tentative cyclotron resonance scattering feature at 44 keV, QPO production at the inner edge of the accretion disk, and spin-up via interaction of the neutron star magnetic field with accreting material. Each of these result in a significantly different value. We discuss the strengths and weaknesses of each method and infer that MAXI J0655−013 is likely to have a high surface magnetic field strength, <jats:italic>B</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub> > 10<jats:sup>13</jats:sup> G.</jats:p>