Study of recently discovered Be/X-ray pulsar MAXI J0655-013 using NuSTAR

We study the recently discovered Be/X-ray pulsar MAXI J0655-013 using the 2022 NuSTAR observations. This paper is the first detailed study of the timing and spectral properties of the source. The pulse profiles of the pulsar vary with energy. The pulsed fraction is found to increase monotonically with energy. In between the two NuSTAR observations, a large spin-up rate of ~-1.23 s d^-1 is observed, which can be due to large spin-up torque acting on the pulsar during an outburst. Such a large spin-up rate is observed for the first time in an X-ray pulsar during an outburst. The variation of the spin period with time can be employed to obtain the orbital parameters of the binary system, and we found the orbital period to be ~27.9 d. The second NuSTAR observation is done in a low-luminosity state (L_X ~ 3.9 × 10³⁴ $\text{erg}\, \text{s}^{-1}\,$). We have detected the pulsation of the pulsar in such a low-luminosity state. In such a low-luminosity state, the pulsar MAXI J0655-013 might be accreting from the cold disc.