# First results from the use of the relativistic and slim disc model SLIMULX in XSPEC

## TL;DR

This paper introduces the SLIMULX model for XSPEC, which accurately fits thermal spectra of slim discs around stellar-mass black holes in super-Eddington accretion, incorporating relativistic effects, and applies it to NGC 5408 X-1 data.

## Contribution

The paper presents a new relativistic slim disc model, SLIMULX, for XSPEC, enabling more accurate spectral fitting of super-Eddington accreting stellar-mass black holes.

## Key findings

- SLIMULX successfully fits the X-ray spectrum of NGC 5408 X-1.
- The model accounts for relativistic effects in super-Eddington accretion.
- Results support stellar-mass black holes as ULX sources rather than IMBHs.

## Abstract

Ultra-Luminous X-ray sources (ULXs) are accreting black holes for which their X-ray properties have been seen to be different to the case of stellar-mass black hole binaries. For most of the cases their intrinsic energy spectra are well described by a cold accretion disc (thermal) plus a curved high-energy emission components. The mass of the black hole (BH) derived from the thermal disc component is usually in the range of 100-1000 solar masses, which have led to the idea that this might represent strong evidence of the Intermediate Mass Black Holes (IMBH), proposed to exist by theoretical studies but with no firm detection (as a class) so far. Recent theoretical and observational developments are leading towards the idea that these sources are instead stellar-mass BHs accreting at an unusual super-Eddington regime. In this paper we briefly describe the model SLIMULX that can be used in XSPEC for the fit of thermal spectra of slim discs around stellar mass black holes in the super-Eddington regime. This model consistently takes all relativistic effects into account. We present the obtained results from the fit of the X-ray spectra from NGC 5408 X-1.