# Pulsator-like spectra from Ultraluminous X-ray sources and the search for more ultraluminous pulsars

## TL;DR

This study analyzes the spectral properties of ultraluminous X-ray sources (ULXs), finding that many can be modeled similarly to accreting neutron stars, suggesting some non-pulsating ULXs may also host neutron stars and encouraging pulsation searches.

## Contribution

The paper demonstrates that a simple continuum model used for accreting neutron stars fits most ULX spectra, implying many ULXs could host neutron stars rather than black holes.

## Key findings

- Most ULX spectra fit a neutron star-like model.
- Known pulsating ULXs have harder spectra above 2 keV.
- Two ULXs may host neutron stars based on spectral hardness.

## Abstract

Ultraluminous X-ray sources (ULXs) are a population of extragalactic objects whose luminosity exceeds the Eddington limit for a 10 Msun black hole (BH). Their properties have been widely interpreted in terms of accreting stellar-mass or intermediate-mass BHs. However at least three neutron stars (NSs) have been recently identified in ULXs through the discovery of periodic pulsations. Motivated by these findings we studied the spectral properties of a sample of bright ULXs using a simple continuum model which was extensively used to fit the X-ray spectra of accreting magnetic NSs in the Galaxy. We found that such a model, consisting of a power-law with a high-energy exponential cut-off, fits very well most of the ULX spectra analyzed here, at a level comparable to that of models involving an accreting BH. On these grounds alone we suggest that other non-pulsating ULXs may host NSs. We found also that above 2 keV the spectrum of known pulsating ULXs is harder than that of the majority of the other ULXs of the sample, with only IC 342 X-1 and Ho IX X-1 displaying spectra of comparable hardness. We thus suggest that these two ULXs may host an accreting NS and encourage searches for periodic pulsations in the flux.