Long-term spectral variability and searching the pulsating nature of the ultra-luminous x-ray source M81 X-6

摘要

We studied the X-ray spectral properties of the ultra-luminous X-ray source (ULX) M81 X-6 using Suzaku, XMM-Newton and NuSTAR observations. M81 X-6 showed long-term X-ray spectral variability in the Suzaku and XMM-Newton observations performed during 2001-2015. The spectral variability is mainly characterized by the variable optical depth of the Comptonizing corona when X-ray spectra fitted with a standard multi-temperature disk and a thermal Comptonization component. In addition, we investigated the broadband X-ray properties of M81 X-6 using the simultaneous Suzaku and NuSTAR observations conducted in 2015. We searched for the pulsation from the source using standard tools. Unfortunately, we could not detect any strong pulsation from the source. To further investigate the pulsating nature of the source, we modeled the observed 0.6-20 keV broadband spectrum using the accreting magnetic neutron star (NS) continuum model and compared the broadband X-ray spectral properties with other known pulsating ULXs. These results suggest that M81 X-6 may be a candidate NS ULX.