X-ray spectroscopy of newly identified ULXs associated with M87’s globular cluster population

Abstract

<jats:title>Abstract</jats:title> <jats:p>We have identified seven ultraluminous X-ray sources (ULXs) which are coincident with globular cluster candidates (GCs) associated with M87. ULXs in the old GC environment represent a new population of ULXs, and ones likely to be black holes. In this study we perform detailed X-ray spectroscopic followup to seven GC ULXs across a wealth of archival Chandra observations and long time baseline of 16 years. This study brings the total known sample of GC ULXs to 17. Two of these sources show variability in their X-ray luminosity of an order of magnitude over many years, and one of these sources shows intra-observational variability on the scale of hours. While the X-ray spectra of the majority of globular cluster ULXs are best fit by single component models, one of the sources studied in this paper is the second GC ULX to be best fit by a two component model. We compare this new sample of GC ULXs to the previously studied sample, and compare the X-ray and optical properties counterparts across the samples. We find that the clusters that host ULXs in M87 have metallicities from g − z=1.01 to g − z=1.70. The best-fit power-law indices of the X-ray spectra range from Γ=1.37-2.21, and the best fit inner black-body disk temperatures range from kT=0.56-1.90 keV.</jats:p>

A repeating fast radio burst source in a globular cluster

“…In some cases, ultraluminous X-ray sources (ULXs) 37 have been shown to be NSs accreting at hyper-Eddington rates 38 , though some may be systems with a more massive BH primary 39 . We note that ULXs have been associated with extragalactic globular clusters 40 but such systems are ruled out by our X-ray limit unless their luminosity varies in time by more than two orders of magnitude. Additionally, the association with a globular cluster rules out a high mass X-ray binary origin of FRB 20200120E and the projected offset of ≈ 2 pc from the centre of light of [PR95] 30244 excludes the association of FRB 20200120E with, e.g., a massive ( 10M ) stellar mass BH or a putative intermediate mass black hole at the core of [PR95] 30244.…”

Three Ultraluminous X-ray Sources Hosted by Globular Clusters in NGC 1316

“…Lastly, we extract background subtracted light-curves using and find no evidence of strong variability (see also analysis by Allak et al (2020) for lack of variability in GCULX3). This is typical for most GCULXs, currently only three of them show strong variability (see Maccarone et al (2007); Shih et al (2010); Dage et al (2020) for more details).…”

“…Of these, only GCULX2 was able to be fit by a two component model over a single component model 3 , but in either case the F-test probability was high, meaning that it is not statistically reasonable to add the extra component. Very few GCULXs can be well-fit by a two-component model, only two known sources (RZ2109 (Dage et al 2018), and M87-GCULX1 (Dage et al 2020) have any evidence for the extra component statistically being a better fit.…”

“…These sources were analysed in the same manner as Dage et al (2019aDage et al ( , 2020, in order to compare the X-ray properties of all of the 20 identified GC ULXs to each other. The sources were extracted in each observation using , and the background regions were created by placing regions of a similar size to the source region near the source, but avoiding any nearby point sources.…”

“…Seven more GC ULXs were identified by Dage et al (2020), associated with the large GC population of M87. The X-ray spectra of these sources behaved like the larger GC ULX sample (and unlike the optical emission line GC ULXs).…”