# AstroSat view of MAXI J1535-571: broadband spectro-temporal features

## TL;DR

This paper reports AstroSat observations of the black hole binary MAXI J1535-571, analyzing its broadband spectral and timing features, including QPOs and accretion rates, revealing high flux and near Eddington accretion.

## Contribution

First broadband spectral and timing analysis of MAXI J1535-571 with AstroSat, including QPO characterization and physical modeling of accretion processes.

## Key findings

- Detected QPOs with frequencies 1.85-2.88 Hz and harmonics.
- Estimated black hole mass between 5.14 and 7.83 solar masses.
- Observed high flux corresponding to near Eddington accretion rates.

## Abstract

We present the results of Target of Opportunity (ToO) observations made with AstroSat of the newly discovered black hole binary MAXI J1535-571. We detect prominent C-type Quasi-periodic Oscillations (QPOs) of frequencies varying from 1.85 Hz to 2.88 Hz, along with distinct harmonics in all the AstroSat observations. We note that while the fundamental QPO is seen in the 3 - 50 keV energy band, the harmonic is not significant above ~ 35 keV. The AstroSat observations were made in the hard intermediate state, as seen from state transitions observed by MAXI and Swift. We attempt spectral modelling of the broadband data (0.7-80 keV) provided by AstroSat using phenomenological and physical models. The spectral modelling using nthComp gives a photon index in the range between 2.18-2.37 and electron temperature ranging from 21 to 63 keV. The seed photon temperature is within 0.19 to 0.29 keV. The high flux in 0.3 - 80 keV band corresponds to a luminosity varying from 0.7 to 1.07 L_Edd assuming the source to be at a distance of 8 kpc and hosting a black hole with a mass of 6 M$_{\odot}$. The physical model based on the two-component accretion flow gives disc accretion rates as high as ~ 1 $\dot{m}_{Edd}$ and halo rate ~ 0.2 $\dot{m}_{Edd}$ respectively. The near Eddington accretion rate seems to be the main reason for the unprecedented high flux observed from this source. The two-component spectral fitting of AstroSat data also provides an estimate of a black hole mass between 5.14 to 7.83 M$_{\odot}$.