| Name: | Sandeep Rout |
| Affiliation: | New York University Abu Dhabi |
| Conference ID: | ASI2025_589 |
| Title: | Discovering hidden variability components in Cygnus X-1 using AstroSat/LAXPC |
| Authors: | Sandeep K. Rout, Federico Garcia, Mariano Mendez |
| Authors Affiliation: | 1 Sandeep K. Rout (New York University Abu Dhabi, UAE)
2 Federico Garcia (Institute of Radio Astronomy, UNLP, Buenos Aires, Argentina)
3 Mariano Mendez (Kapteyn Astronomical Institute, University of Groningen, the Netherlands) |
| Mode of Presentation: | Oral |
| Abstract Category: | High Energy Phenomena, Fundamental Physics and Astronomy |
| Abstract: | The power spectrum of an X-ray binary can be decomposed into a multi-Lorentzian model that are coherent in different energy bands but incoherent with each other. By simultaneously fitting the power spectra in two bands and the real and imaginary parts of the cross spectrum with such a model one can also predict the phase lag and coherence. By using this novel technique on NICER observations of black-hole binaries, recently it has been shown that there exists variability components that have a strong imaginary part but a weak real part, hence are not significant in the power spectrum. A characteristic feature of this “hidden” imaginary component is an abrupt drop in coherence and rise in the phase lag within a narrow frequency band. These imaginary components in the cross-spectrum were initially thought to arise only due to the soft X-ray coverage provided by NICER. In this work we carry out a systematic search of these components in Cygnus X-1 with data from AstroSat/LAXPC during the 2016-17 state transition. In order to reduce the dead-time effects at higher frequencies, we evaluated all the Fourier products from the cross-spectra between two LAXPC units (also called co-spectra). This resulted in a much improved correction of Poisson noise compared to the traditional methods. By analyzing ten observations during the transition from the hard to soft state we discovered, for the first time, the aforementioned narrow imaginary components in the cross-spectral products of higher energy bands, i.e. 3-5 and 6-20 keV. The frequency of these components spanned 0.01-0.1 Hz and increased with decrease in hardness. The presence of these components suggest that the power spectrum likely consists of several independent additive components originating from different regions, thus providing important implications for the various models of variability. |
