| Name: | Birendra Chhotaray |
| Affiliation: | Physical Research Laboratory, Ahmedabad, Gujarat, India |
| Conference ID : | ASI2024_727 |
| Title : | Long-term Study of the First Galactic Ultraluminous X-ray Source Swift J0243.6+6124 Using NICER |
| Authors : | Birendra Chhotaray, Gaurava K. Jaisawal, Prantik Nandi, Sachindra Naik, Neeraj kumari, Mason Ng |
| Authors Affiliation: | Birendra Chhotaray, Prantik Nandi, Sachindra Naik, Neeraj kumari Affiliation (Physical Research Laboratory, Navrangpura, Ahmedabad - 380009, Gujarat, India)
Gaurava K. Jaisawal Affiliation (DTU Space, Technical University of Denmark, Elektrovej 327-328, DK-2800 Lyngby, Denmark)
Mason Ng Affiliation (MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA) |
| Mode of Presentation: | Poster |
| Abstract Category : | High Energy Phenomena, Fundamental Physics and Astronomy |
| Abstract : | We present the results obtained from detailed X-ray timing and spectral studies of X-ray pulsar Swift J0243.6+6124 during its giant and other normal X-ray outbursts between 2017 and 2023 observed by the Neutron star Interior Composition Explorer (NICER). We focused on the timing analysis of the post-giant normal outbursts including the recent one in 2023. A distinct break is found in the power density spectra of the source. The corresponding break frequency and slope of power laws around it vary with luminosity, indicating the change in accretion dynamics with mass accretion rate. Interestingly, we detected quasi-periodic oscillations within a specific luminosity range, providing further insights into the underlying physical processes. We also studied the neutron star spin period evolution and a luminosity variation in pulse profile during the recent 2023 outburst. The spectral analysis was conducted comprehensively for the giant and all other normal outbursts. We identified a double transition at L1 ∼ 8.6×10^37 and L2 ∼ 2.4×10^38 erg s−1 in the evolution of continuum parameters like photon index and cutoff energy with luminosity. This indicates three distinct accretion modes experienced by the source during the giant X-ray outburst. A soft blackbody component with a temperature of 0.1-0.7 keV is also detected in spectra. The observed temperature undergoes a discontinuous transition when the pulsar evolves from a sub-critical to a super-critical state. Notably, in addition to an evolving 6-7 keV iron line complex, a 1 keV emission line was observed during the super-critical state of the source, implying the X-ray reflection from the accretion disc or outflow material. |
