Abstract Details
| Name: Aman Kaushik Affiliation: Tata Institute of Fundamental Research Conference ID: ASI2025_410 Title : The 2022 outburst of SAX J1808.4–3658. Authors and Co-Authors : Aman Kaushik 1, Yash Bhargava 1, Sudip Bhattacharyya 1 Abstract Type : Poster Abstract Category : High Energy Phenomena, Fundamental Physics and Astronomy Abstract : Accretion powered millisecond pulsars (AMXPs) are fast rotating neutron stars orbiting in binary systems and are believed to be the predecessors of radio millisecond pulsars. The intriguing observation of rapidly spinning radio pulsars with weak magnetic fields (∼ 10^8 G) in the 1980s prompted the hypothesis that these are “recycled” pulsars. This theory of “recycling scenario” was confirmed with the observation of an actively accreting millisecond pulsar in a binary system, namely SAX J1808.4–3658. We investigate the spectral and timing properties of SAX J1808.4–3658 during its 2022 outburst using the observations with NICER and AstroSat. Our spectral modelling suggests that the emission from the source can be well described by a combination of multicolour blackbody disk and thermal Comptonization by a cloud of hot electrons. Studying the evolution of the spectral parameters showed that covering fraction and photon index (Γ) were degenerate. Additionally, the evolution of disk and corona flux showed that majority of the estimated flux is provided by the disk but at higher energies corona flux dominates. The aperiodic timing analysis hinted at the presence of two broad components with characteristic frequencies varying within 0.63 ± 0.02 to 1.40 ± 0.08 Hz and from 9.22 ± 1.58 to 51.27 ± 22.98 Hz as the source evolves during the outburst. We assume that these high frequency fluctuations are caused by the corona. We also observe that 1.5–10.0 keV energy band lags behind the 0.5–1.5 keV band (hard lag) by a few milliseconds for 0–2 Hz frequency range and almost no lag for 10–100 Hz frequency range, indicating that the low energy and low frequency fluctuations from the disk are propagating into the corona and are getting up-scattered into higher energies. We will describe the evolution of the system connecting it with our spectral and timing results, (Kaushik et al. 2024 in prep). |