X-Ray Monitoring of Ultraluminous X-Ray Sources

TL;DR

This study presents X-ray monitoring of ultraluminous X-ray sources revealing spectral states, potential periodicities, and a significant nuclear outburst, contributing to understanding their variability and nature.

Contribution

It provides new observational data on ULX spectral states, potential periodic signals, and a rare nuclear outburst, enhancing knowledge of their behavior and properties.

Findings

Holmberg IX X-1 remains in the hard spectral state with flux variations up to a factor of 7.

Detected potential periodicities near 60 and 28 days in Holmberg IX X-1.

Observed a nuclear outburst in NGC 4395 with luminosity exceeding previous records.

Abstract

X-ray monitoring observations were performed with the Swift observatory of the ultraluminous X-ray sources Holmberg IX X-1, NGC 5408 X-1, and NGC 4395 X-2 and also of the nuclear X-ray source in NGC 4395. Holmberg IX X-1 remains in the hard X-ray spectral state as its flux varies by a factor of 7 up to an (isotropic) luminosity of 2.8E40 erg/s. This behavior may suggest an unusually massive compact object. We find excess power at periods near 60 days and 28 days in the X-ray emission from Holmberg IX X-1. Additional monitoring is required to test the significance of these signals. NGC 5408 X-1 and NGC 4395 X-2 appear to remain in the soft spectral state found by Chandra and XMM with little variation in spectral hardness even as the luminosity changes by a factor of 9. We found an outburst from the nuclear source in NGC 4395 reaching an X-ray luminosity of 9E40 erg/s, several times higher than any previously reported.