The Unusual Broadband X-ray Spectral Variability of NGC 1313 X-1 seen with XMM$-$Newton, Chandra and NuSTAR

TL;DR

This study analyzes the complex broadband X-ray spectral variability of ULX NGC 1313 X-1 using multi-epoch data from XMM-Newton, Chandra, and NuSTAR, revealing unusual patterns and potential accretion flow stratification.

Contribution

It introduces two-component accretion disc models to interpret the spectral variability of ULX NGC 1313 X-1, considering both black hole and neutron star scenarios, and explores the implications of radial stratification in accretion flows.

Findings

Spectral variability is suppressed above 10-15 keV.

Two distinct luminosity-temperature tracks follow L∝T^4 and L∝T^2 relations.

Possible evidence for radial stratification and wind effects in accretion flows.

Abstract

We present results from the major coordinated X-ray observing program on the ULX NGC 1313 X-1 performed in 2017, combining $XMM$-$Newton$, $Chandra$ and $NuSTAR$, focusing on the evolution of the broadband ($\sim$0.3-30.0 keV) continuum emission. Clear and unusual spectral variability is observed, but this is markedly suppressed above $\sim$10-15 keV, qualitatively similar to the ULX Holmberg IX X-1. We model the multi-epoch data with two-component accretion disc models designed to approximate super-Eddington accretion, allowing for both a black hole and a neutron star accretor. With regards to the hotter disc component, the data trace out two distinct tracks in the luminosity-temperature plane, with larger emitting radii and lower temperatures seen at higher observed fluxes. Despite this apparent anti-correlation, each of these tracks individually shows a positive luminosity-temperature relation. Both are broadly consistent with $L\propto{T}^{4}$, as expected for blackbody emission with a constant area, and also with $L\propto{T}^{2}$, as may be expected for an advection-dominated disc around a black hole. We consider a variety of possibilities for this unusual behaviour. Scenarios in which the innermost flow is suddenly blocked from view by outer regions of the super-Eddington disc/wind can explain the luminosity-temperature behaviour, but are difficult to reconcile with the lack of strong variability at higher energies, assuming this emission arises from the most compact regions. Instead, we may be seeing evidence for further radial stratification of the accretion flow than is included in the simple models considered, with a combination of winds and advection resulting in the suppressed high-energy variability.