Ultraluminous X-ray Sources in Extragalactic Globular Clusters

Ultraluminous X-ray sources (ULXs) are X-ray binaries that produce luminosities that exceed the Eddington Limit of the accretor (where the external radiation pressure is balanced by the internal gravitational forces). Sources that exceed their Eddington Limit are thought to be in an unusual accretion state, and likely undergoing more extreme physics than their less luminous counterparts. ULXs have long been associated with star-forming regions of galaxies, but recent studies have shown that there may be a comparable number of ULXs in older globular cluster (GC) populations. The nature of ULXs is likely heterogeneous: due to either neutron star or black hole accretors. Some may even be ultra-compact X-ray binaries, with a degenerate white dwarf donor star and orbital periods on the order of five minutes. Thus, the study of the accretion physics and host environment provides crucial clues to the binary nature, and can provide far reaching impacts into different sub-fields of astronomy, such as Gravitational Wave science. My research involves both detailed studies of the accretion physics of black hole candidate ULXs (like RZ2109) and neutron star ULX analogues (like SMC X-1), as well as searching galaxies for more globular clusters and their strange multiwavelength counterparts.

RZ2109

RZ2109 is a ULX and black hole candidate located in a globular cluster near NGC 4472. We have analyzed new and archival data in both optical and x-ray, going back to the year 2000. Previous work indicates it is likely to be an ultra compact x-ray binary; the presence of the completely forbidden [OIII] transition line in the optical points towards the binary companion being a white dwarf. Long-term optical and X-ray monitoring of this unique object will help shed light on the nature of the source. The combination of the strong [OIII] line and the strongly variable, as well as the X-ray brightness make RZ2109 very different from other known globular cluster X-ray sources. See these papers for X-ray studies and optical studies.

Globular Cluster Ultra-Luminous X-ray Sources

Prior to my thesis work, only a handful of globular cluster ULXs had been studied in any detail, but this changed with my 2019 study which compared the X-ray properties of the nine known sources. My subsequent studies of M87 and NGC 1316 have already doubled the known number of globular cluster ULXs. By increasing these numbers, I am now able to use properties of the sources to carry out pilot population studies of globular clusters that host black holes. See these papers on X-ray studies of globular cluster ULXs in NGC 4472, NGC 4649 and NGC 1399, M87 and NGC 1316. We have now discovered 20 GC ULXs, and their X-ray and optical properties can be viewed at this catalogue .

SMC X-1

SMC X-1 is a high mass X-ray binary situated in the Small Magellenic Cloud. It can reach high X-ray luminosities close to the Eddington Limit, and due to its proximity to us, it can serve as a proxy towards understanding the accretion powering the more distant neutron star ULXs. In the past, I've looked for a link between the change in the super-orbital period (thought to be due to a warped, precessing accretion disk) and the change in the pulse period of the high mass X-ray binary SMC X-1. While the data showed a tentative link between these two observational phenomena, the data at the time was not sufficient to prove a correlation in a statistically compelling manner. However, in 2021, I led a proposal to monitor the source with NICER, and our team will use this data to thoroughly probe the physics of the accretion disk as the super-orbital period changes (see the MOOSE homepage for updates on the monitoring program).

Contact

Email: kcdage$@$msu.edu

Wayne State University

666 W. Hancock St.

Detroit, MI, USA

NHFP