# Spectro-timing analysis of MAXI J1535-571 using AstroSat

## TL;DR

This study analyzes AstroSat observations of the black hole candidate MAXI J1535-571, focusing on spectral and timing evolution during its Hard Intermediate state, revealing correlations between QPO frequencies and spectral properties.

## Contribution

It provides a detailed spectro-timing analysis of MAXI J1535-571 during its outburst, highlighting the relationship between QPOs and spectral index in a hard intermediate state.

## Key findings

- QPO centroid frequency fluctuated between 1.7-3.0 Hz.
- Strong correlation between QPO frequency and spectral index.
- No correlation between QPO frequency and source flux.

## Abstract

We report the results of the analysis of an AstroSat observation of the Black Hole candidate MAXI J1535-571 during its Hard Intermediate state. We studied the evolution of the spectral and timing parameters of the source during the observation. The observation covered a period of $\sim$5 days and consisted of 66 continuous segments, corresponding to individual spacecraft orbits. Each segment was analysed independently. The source count rate increased roughly linearly by $\sim$30 %. We modelled the spectra as a combination of radiation from a thermal disk component and a power-law. The timing analysis revealed the presence of strong Quasi Periodic Oscillations with centroid frequency $\nu_{\rm{QPO}}$ fluctuating in the range 1.7-3.0 Hz. We found a tight correlation between the QPO centroid frequency $\nu_{\rm{QPO}}$ and the power-law spectral index $\Gamma$, while $\nu_{\rm{QPO}}$ appeared not to be correlated with the linearly-increasing flux itself. We discuss the implications of these results on physical models of accretion.