A Long-term Study of Ultraluminous X-ray Sources in NGC 891

TL;DR

This study analyzes 17 years of X-ray data from multiple telescopes to monitor and model ultraluminous X-ray sources in NGC 891, revealing long-term stability and a new transient source.

Contribution

It provides a comprehensive long-term spectral analysis of ULXs in NGC 891, including the discovery of a new transient ULX and comparison of multiple spectral models.

Findings

Known ULXs show consistent luminosities over years

A new transient ULX was identified and monitored

Multiple spectral models fit the data similarly well

Abstract

We perform empirical fits to the \emph{Chandra} and \emph{XMM-Newton} spectra of three ultraluminous X-ray sources (ULXs) in the edge-on spiral galaxy NGC 891, monitoring the region over a seventeen year time window. One of these sources has been visible since the early 1990s with \emph{ROSAT} and has been observed multiple times with \emph{Chandra} and \emph{XMM-Newton}. Another has been visible since 2011. We build upon prior analyses of these sources by analyzing all available data at all epochs. Where possible \emph{Chandra} data is used, since its superior spatial resolution allows for more effective isolation of the emission from each individual source, thus providing a better determination of their spectral properties. We also identify a new transient ULX, CXOU J022230.1+421937, which faded from view over the course of a two month period from Nov 2016 to Jan 2017. Modeling of each source at every epoch was conducted using six different models ranging from thermal bremsstrahlung to accretion disk models. Unfortunately, but as is common with many ULXs, no single model yielded a much better fit than the others. The two known sources had unabsorbed luminosities that remained fairly consistent over five or more years. Various possibilities for the new transient ULX are explored.