Abstract
We perform empirical fits to the Chandra and XMM-Newton spectra of three ultraluminous X-ray sources (ULXs) in the edge-on spiral galaxy NGC 891, monitoring the region over a 17-year time window. One of these sources was visible since the early 1990s with ROSAT and was observed multiple times with Chandra and XMM-Newton. Another was visible since 2011. We build upon prior analyses of these sources by analyzing all available data at all epochs. Where possible 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.
Highlights
Ultraluminous X-ray sources (ULXs) are point-like sources found far from the nucleus of external galaxies, with X-ray luminosities that exceed 1039 erg s−1 [1]
Given the stability of the spectra of ULX-1 and ULX-2 over time, the pn, MOS1, and MOS2 spectra taken over five observations in 2017 are merged for each camera, using the SAS task epicspeccombine
Three ULXs appeared over the course of nearly 30 years in the edge-on spiral galaxy
Summary
Ultraluminous X-ray sources (ULXs) are point-like sources found far from the nucleus of external galaxies, with X-ray luminosities that exceed 1039 erg s−1 [1]. ULXs were first discovered in the 1980s by the Einstein Observatory, the spectral resolution of ROSAT was needed to study their spectral properties [2,3]. If emission is instead arising from an accretion disk surrounding the compact object, which is presumably in a binary system, the luminosty can exceed the Eddington limit [8,9,10]. The latter model, where a supercritical accretion disk forms, and accretion occurs in super-Eddington mode, seems to be more favored nowadays [10]
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