# The dependence of the estimated luminosities of ULX on spectral models

## TL;DR

This study analyzes how the choice of spectral models affects the estimated luminosities of ULX sources in nearby galaxies, revealing that model selection significantly influences luminosity functions and identifying two distinct classes of extremely luminous X-ray sources.

## Contribution

It demonstrates the dependence of luminosity estimates on spectral models and classifies extremely luminous X-ray sources into two spectral types with different physical interpretations.

## Key findings

- Luminosity functions depend on spectral model choice, especially above 10^{40} ergs/s.
- Identified four sources exceeding 10^{40} ergs/s, called ELX, with two spectral classes.
- Supersoft ELX may harbor intermediate-mass black holes, while hard ELX likely involve saturated inverse Comptonization.

## Abstract

Data from {\it Chandra} observations of thirty nearby galaxies were analyzed and 365 X-ray point sources were chosen whose spectra were not contaminated by excessive diffuse emission and not affected by photon pile up. The spectra of these sources were fitted using two spectral models (an absorbed power-law and a disk blackbody) to ascertain the dependence of estimated parameters on the spectral model used. It was found that the cumulative luminosity function depends on the choice of the spectral model, especially for luminosities $> 10^{40}$ ergs/s. In accordance with previous results, a large number ($\sim 80$) of the sources have luminosities $> 10^{39}$ ergs/s (Ultra-Luminous X-ray sources) with indistinguishable average spectral parameters (inner disk temperature $\sim 1$ keV and/or photon index $\Gamma \sim 2$) with those of the lower luminosities ones. After considering foreground stars and known background AGN,we identify four sources whose minimum luminosity exceed $10^{40}$ ergs/s, and call them Extremely Luminous X-ray sources (ELX). The spectra of these sources are in general better represented by the disk black body model than the power-law one. These ELX can be grouped into two distinct spectral classes. Two of them have an inner disk temperature of $< 0.5$ keV and hence are called ``supersoft'' ELX, while the other two have temperatures $\gtrsim 1.3$ keV and are called ``hard'' ELX. The estimated inner disk temperatures of the supersoft ELX are compatible with the hypothesis that they harbor intermediate size black holes, which are accreting at $\sim 0.5$ times their Eddington Luminosity. The radiative mechanism for hard ELX, seems to be Inverse Comptonization, which in contrast to standard black holes systems, is probably saturated.