X-ray Spectral Signatures of the Photon Bubble Model for Ultraluminous X-ray Sources

TL;DR

This paper analyzes the X-ray spectral signatures of the photon bubble model for ultraluminous X-ray sources, predicting spectral features and reflection characteristics to distinguish it from other models.

Contribution

It provides the first detailed spectral analysis of the photon bubble model for ULXs, including predictions of observable features and reflection signatures.

Findings

Reflection components are generally weak and often unobservable.

Spectra are dominated by Comptonization down to a few keV.

A potential 9 keV spectral feature may indicate photon bubbles.

Abstract

The nature of ultraluminous X-ray sources in nearby galaxies is one of the major open questions in modern X-ray astrophysics. One possible explanation for these objects is an inhomogeneous, radiation dominated accretion disk around a $\sim 10 M_{\odot}$ black hole -- the so-called ``photon bubble'' model. While previous studies of this model have focused primarily on its radiation-hydrodynamics aspects, in this paper, we provide an analysis of its X-ray spectral (continuum and possible edge and line) characteristics. Compton reflection between high and low density regions in the disk may provide the key to distinguishing this model from others, such as accretion onto an intermediate mass black hole. We couple a Monte Carlo/Fokker-Planck radiation transport code with the XSTAR code for reflection to simulate the photon spectra produced in a photon bubble model for ULXs. We find that reflection components tend to be very weak and in most cases not observable, and make predictions for the shape of the high-energy Comptonizing spectra. In many cases the Comptonization dominates the spectra even down to $\sim$ a few keV. In one simulation, a $\sim 9 \kev$ feature was found, which may be considered a signature of photon bubbles in ULXs; furthermore, we make predictions of high energy power-laws which may be observed by future instruments.