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We present MAXI and NuSTAR observations of the Be X-ray binary, MAXIJ0655-013, in outburst. NuSTAR observed the source once early in the outburst,when spectral analysis yields a bolometric (0.1--100 keV), unabsorbed sourceluminosity of $L_{\mathrm{bol}}=5.6\times10^{36}\mathrm{erg\,s^{-1}}$, and asecond time 54 days later, by which time the luminosity dropped to$L_{\mathrm{bol}}=4\times10^{34}\,\mathrm{erg\,s^{-1}}$ after first undergoinga dramatic increase. Timing analysis of the NuSTAR data reveals a neutron starspin period of $1129.09\pm0.04$ s during the first observation, which decreasedto $1085\pm1$ s by the time of the second observation, indicating spin-up dueto accretion throughout the outburst. Furthermore, during the first NuSTARobservation, we observed quasiperiodic oscillations with centroid frequency$\nu_0=89\pm1$ mHz, which exhibited a second harmonic feature. By combining theMAXI and NuSTAR data with pulse period measurements reported by Fermi/GBM, weare able to show that apparent flaring behavior in the MAXI light-curve is anartifact introduced by uneven sampling of the pulse profile, which has a largepulsed fraction. Finally, we estimate the magnetic field strength at theneutron star surface via three independent methods, invoking a tentativecyclotron resonance scattering feature at $44$ keV, QPO production at the inneredge of the accretion disk, and spin-up via interaction of the neutron starmagnetic field with accreting material. Each of these result in a significantlydifferent value. We discuss the strengths and weaknesses of each method andinfer that MAXI J0655-013 is likely to have a high surface magnetic fieldstrength, $B_{s}>10^{13}$ G.

Accretion spin-up and a strong magnetic field in the slow-spinning Be X-ray binary MAXI J0655-013