Evidence for Pulsar-like Emission Components in the Broadband ULX Sample

TL;DR

This study analyzes broadband X-ray data of ultraluminous X-ray sources (ULXs) and finds spectral evidence suggesting many may host neutron star accretors with pulsar-like emission components, expanding understanding of ULX nature.

Contribution

It provides the first broadband spectral analysis linking ULX emission features to pulsar-like accretion columns, suggesting many ULXs could be neutron star systems.

Findings

Pulsed emission in NGC5907 ULX resembles known ULX pulsars.

Hard excesses are common across the ULX sample.

Accretion column models can explain the observed spectra.

Abstract

We present broadband X-ray analyses of a sample of bright ultraluminous X-ray sources with the goal of investigating the spectral similarity of this population to the known ULX pulsars, M82 X-2, NGC7793 P13 and NGC5907 ULX. We perform a phase-resolved analysis of the broadband XMM-Newton+NuSTAR dataset of NGC5907 ULX, finding that the pulsed emission from the accretion column in this source exhibits a similar spectral shape to that seen in both M82 X-2 and NGC7793 P13, and that this is responsible for the excess emission observed at the highest energies when the spectra are fit with accretion disk models. We then demonstrate that similar 'hard' excesses are seen in all the ULXs in the broadband sample. Finally, for the ULXs where the nature of the accretor is currently unknown, we test whether the hard excesses are all consistent with being produced by an accretion column similar to those present in M82 X-2, NGC7793 P13 and NGC5907 ULX. Based on the average shape of the pulsed emission, we find that in all cases a similar accretion column can successfully reproduce the observed data, consistent with the hypothesis that this ULX sample may be dominated by neutron star accretors. Compared to the known pulsar ULXs, our spectral fits for the remaining ULXs suggest that the non-pulsed emission from the accretion flow beyond the magnetosphere makes a stronger relative contribution than the component associated with the accretion column. If these sources do also contain neutron star accretors, this may help to explain the lack of detected pulsations.