Fang-Ming Gu (Graduate Institute of Astronomy, NCU); Yi Chou (Graduate Institute of Astronomy, NCU); Hung-En Hsieh (Graduate Institute of Astronomy, NCU); Yi-Hao Su (Graduate Institute of Astronomy, NCU)
MAXI J0911-655 (Swift J0911.9-6452), the 18th known accreting millisecond X-ray pulsar (AMXP) located in globular cluster NGC 2808, was discovered in 2016 with a pulsation period of 2.94 ms. The follow-up observations were made by XMM-Newton and NuSTAR. In this study, we attempted to detect the energy dependent pulse arrival time lags, which have been seen in other AMXPs. To obtain the pulse profile, precise orbital and spin parameters are essential. We first applied the orbital and spin parameters that yielded by previous study and then refined them using pulse arrival time delay technique for the 0.3 to 10 keV events collected by XMM-Newton. These photons were further divided into 10 energy bands and then folded with the best spin and orbital parameters to make the pulse profiles for these bands. The pulse arrival time lags respected to the softest energy band were evaluated through cross correlation of the best fitted pulse profiles. We found the lags can be up to 0.13 cycle (~378 μs) in the energy range of 0.3 to 10 keV. The analyzing of NuSTAR data, whose photon energy may reach to harder energy band (up to ~79 keV), is still processing.