X-Ray Spectro-Temporal Analysis of the LMXB 4U 1608-52: Insights from AstroSat and NICER Observations

We present a comprehensive analysis of the X-ray spectral and temporal properties of the neutron star low-mass X-ray binary (LMXB) 4U 1608-52, utilizing observations from AstroSat (LAXPC/SXT) and NICER during its 2016 and 2020 outbursts. The 0.7-20.0 keV spectra are well-described by a disk blackbody and thermal Comptonization model, yielding a best-fitting inner disk temperature of ∼1 keV and radius of ∼22.17-27.19 km. Notably, no significant evolution in the disk radius is observed through flux and time-resolved spectroscopy. Employing a multi-Lorentzian approach, we model the power density spectra and detect broad-band noise variability. We quantify the energy-dependent fractional root mean square and time-lag of the broad-band noise, and interpret these variations as a consequence of coherent fluctuations in disk emission and coronal heating rate. Our findings demonstrate consistency between rapid temporal modeling and longer-term spectral evolution, where the inner disk radius remains invariant. Instead, variations are attributed to accretion rate changes, which modulate the inner disk temperature and coronal heating rate. Reference: Bhattacherjee S., Nath A., Sarkar B., Beri A., Chattopadhyay S., Bhulla Y., Misra R., 2024, ApJ, 971, 154. doi:10.3847/1538-4357/ad583d