X-ray Luminosity Lx Research Articles

X-ray study on the core collapse supernova 2008ax using XMM-Newton and Swift/XRT

We studied the x-ray evolution of the core collapse, Type IIb, supernova SN 2008ax. In addition to the four Swift/XRT observations previously published, we included, for the first time, two observations from XMM-NEWTON to our analysis that have not been published before. By focusing on the early observations of the SN 2008ax, we were able to explore the time evolution of both x-ray luminosity (Lx) and temperature (Tx) during the radiative phase. The analysis showed that the evolution of Lx and Tx can be expressed as a power law as Lx ∝ t −0.87 and Tx ∝ t −0.46, respectively. Based on the observed x-ray luminosity, we estimated the mass-loss rate ( Ṁ ) for the progenitor star to be approximately 0.64 ± 0.15 × 10−5 M⊙ yr −1, for a wind velocity of 10 km s−1. This finding is in good agreement with the previous studies.

Exploring the relationship between X-ray binaries and compact star clusters (I): NGC 4490 and NGC 4214

ABSTRACT We conducted a study to investigate the relationship between X-ray binaries (XRBs) and compact star clusters in the nearby star-forming galaxies NGC 4490 and NGC 4214. By analysing archival data from Chandra, we identified a total of 32 XRBs with X-ray luminosities Lx > 1036 erg s−1 in both galaxies. Utilizing data from HST/WFC3/UVIS, we presented a new catalogue of 715 compact star clusters in NGC 4490 and identified 312 such clusters in NGC 4214. In the case of NGC 4490, 14 of the 17 XRBs are associated with clusters, whereas in NGC 4214 all 15 XRBs are connected to clusters within a radius of <200 pc. Furthermore, among the XRBs in NGC 4490, 12 out of the 17 have optical counterparts. Similarly, in NGC4214, 11 out of the 15 XRBs exhibit optical counterparts. We have evidence for a substantial population of high-mass X-ray binaries (HMXBs) associated with clusters, accounting for ∼70 per cent and 40 per cent of the detected XRBs in NGC 4490 and NGC 4214, respectively. HMXBs are often found near young, less massive clusters, while low-mass X-ray binaries (LMXBs) are associated with older, more massive clusters in these galaxies. Intermediate-mass XRBs in NGC 4214, on the other hand, tend to be close to young clusters, suggesting that their formation mechanism is more similar to HMXBs than to LMXBs.

A multiband look at ultraluminous X-ray sources in NGC 7424

ABSTRACT We studied the multiband properties of two ultraluminous X-ray sources (2CXO J225728.9−410211 = X-1 and 2CXO J225724.7−410343 = X-2) and their surroundings, in the spiral galaxy NGC 7424. Both sources have approached X-ray luminosities LX ∼ 1040 erg s−1 at some epochs. Thanks to a more accurate astrometric solution (based on Australia Telescope Compact Array and Gaia data), we identified the point-like optical counterpart of X-1, which looks like an isolated B8 supergiant (M ≈ 9M⊙, age ≈30 Myr). Instead, X-2 is in a star-forming region (size of about 100 pc × 150 pc), near young clusters and ionized gas. Very large telescope long-slit spectra show a spatially extended region of He ii λ4686 emission around the X-ray position, displaced by about 50 pc from the brightest star cluster, which corresponds to the peak of lower ionization line emission. We interpret the He ii λ4686 emission as a signature of X-ray photoionization from the ULX, while the other optical lines are consistent with UV ionization in an ordinary He ii region. The luminosity of this He++ nebula puts it in the same class as other classical photoionized ULX nebulae such as those around Holmberg II X-1 and NGC 5408 X-1. We locate a strong (5.5 GHz luminosity $\nu \, L_{\nu } \approx 10^{35}$ erg s−1), steep-spectrum, unresolved radio source at the peak of the low-ionization lines, and discuss alternative physical scenarios for the radio emission. Finally, we use WISE data to obtain an independent estimate of the reddening of the star-forming clump around X-2.

Studying the variations of the cyclotron line in Cen X-3 using Insight-HXMT

We investigate the cyclotron resonant scattering features (CRSFs) of the accreting X-ray pulsar Cen X-3 and significantly detect the 29 keV cyclotron line features in the hard X-ray averaged spectroscopy studies based on the recent Insight-HXMT observations in 2022, when Cen X-3 has X-ray luminosity LX>∼5×1037 erg s−1 in the bands of 2 – 60 keV. We do not find a harmonic line in the average spectra based on different continuum models. We showed that the CRSF energies have no correlation with time or luminosity in the average spectra. In addition, by performing a pulse phase-dependent spectral analysis, we revealed the fundamental line with the centroid energy ranging from 25 to 29 keV with a high significance over the spin phases. The evolution of the cyclotron line centroid energies over pulse phase is similar to the shape of pulse profiles, illustrating a positive correlation between the energy of CRSFs and the pulse phase flux.

The nature of very-faint X-ray binaries: Near-infrared spectroscopy of 1RXH J173523.7–354013 reveals a giant companion

Abstract Very-faint X-ray binaries (VFXBs) are a sub-class of black holes and neutron stars in binaries that appear to be accreting at a very low rate. In addition to providing interesting constraints on poorly understood forms of accretion, elucidating the nature of VFXBs is particularly interesting for binary evolution and population modeling. Through near-infrared (nIR) spectroscopy, we here investigate the nature of the bursting neutron star and VFXB 1RXH J173523.7–354013 (J1735), which persistently accretes at an X-ray luminosity of LX ∼ 1034 − 1035 erg s−1. Our analysis shows that the nIR emission is dominated by that of the companion star, which we find to be a late G or early K-type giant, making this the second neutron star identified as a VFXB found to have a giant companion. We discuss how several of the system properties are difficult to reconcile with a wind-fed symbiotic X-ray binary. We therefore also propose an alternative scenario wherein J1735 is a wide binary system (supported by the discovery of a 7.5 d modulation in the nIR light curves) with a quiescent luminosity of LX ∼ 1034 − 1035 erg s−1, in which the donor star is overflowing its Roche lobe. This raises the possibility that J1735 may, every century or more, exhibit very long and very bright outbursts during which it reaches accretion rates around the Eddington limit like the neutron star Z sources.

Modelling the expected very high energy γ-ray emission from accreting neutron stars in X-ray binaries

ABSTRACT The detection of γ-ray emission from accreting pulsars in X-ray binaries (XRBs) has long been sought after. For some high-mass X-ray binaries (HMXBs), marginal detections have recently been reported. Regardless of whether these will be confirmed or not, future telescopes operating in the γ-ray band could offer the sensitivity needed to achieve solid detections and possibly spectra. In view of future observational advances, we explored the expected emission above 10 GeV from XRBs, based on the Cheng & Ruderman model, where γ-ray photons are produced by the decay of π0 originated by protons accelerated in the magnetosphere of an accreting pulsar fed by an accretion disc. We improved this model by considering, through Monte Carlo simulations, the development of cascades inside of and outside the accretion disc, taking into account pair and photon production processes that involve interaction with nuclei, X-ray photons from the accretion disc, and the magnetic field. We produced grids of solutions for different input parameter values of the X-ray luminosity (Lx), magnetic field strength (B), and for different properties of the region where acceleration occurs. We found that the γ-ray luminosity spans more than five orders of magnitude, with a maximum of ∼1035 erg s−1. The γ-ray spectra show a large variety of shapes: some have most of the emission below ∼100 GeV, others are harder (emission up to 10–100 TeV). We compared our results with Fermi/LAT and VERITAS detections and upper limits of two HMXBs: A0535+26 and GRO J1008−57. More consequential comparisons will be possible when more sensitive instruments will be operational in the coming years.

Benchmarks and explanations for deep learning estimates of X-ray galaxy cluster masses

ABSTRACTWe evaluate the effectiveness of deep learning (DL) models for reconstructing the masses of galaxy clusters using X-ray photometry data from next-generation surveys. We establish these constraints using a catalogue of realistic mock eROSITA X-ray observations which use hydrodynamical simulations to model realistic cluster morphology, background emission, telescope response, and active galactic nucleus (AGN) sources. Using bolometric X-ray photon maps as input, DL models achieve a predictive mass scatter of $\sigma _{\ln M_\mathrm{500c}} = 17.8~{{\ \rm per\ cent}}$, a factor of two improvements on scalar observables such as richness Ngal, 1D velocity dispersion σv,1D, and photon count Nphot as well as a 32 per cent improvement upon idealized, volume-integrated measurements of the bolometric X-ray luminosity LX. We then show that extending this model to handle multichannel X-ray photon maps, separated in low, medium, and high energy bands, further reduces the mass scatter to 16.2 per cent. We also tested a multimodal DL model incorporating both dynamical and X-ray cluster probes and achieved marginal gains at a mass scatter of 15.9 per cent. Finally, we conduct a quantitative interpretability study of our DL models and find that they greatly down-weight the importance of pixels in the centres of clusters and at the location of AGN sources, validating previous claims of DL modelling improvements and suggesting practical and theoretical benefits for using DL in X-ray mass inference.

Serendipitous discovery of the magnetic cataclysmic variable SRGE J075818−612027

We report the discovery of SRGE J075818−612027, a deep stream-eclipsing magnetic cataclysmic variable found serendipitously in SRG/eROSITA calibration and performance verification phase (CalPV) observations of the open cluster NGC 2516 as an unrelated X-ray source. An X-ray timing and spectral analysis of the eROSITA data is presented and supplemented by an analysis of TESS photometry and SALT spectroscopy. X-ray photometry reveals two pronounced dips repeating with a period of 106.144(1) min. The 14-month TESS data reveal the same unique period. A low-resolution identification spectrum obtained with SALT displays hydrogen Balmer emission lines on a fairly blue continuum. The spectrum and the stability of the photometric signal led to the classification of the new object as a polar-type cataclysmic variable. In this context, the dips in the X-ray light curve are explained by absorption in the intervening accretion stream and by a self-eclipse of the main accretion region. The object displays large magnitude differences on long timescales (months) both at optical and X-ray wavelengths, which are interpreted as high and low states and thus support its identification as a polar. The bright phase X-ray spectrum can be reflected with single temperature thermal emission with 9.7 keV and bolometric X-ray luminosity LX ≃ 8 × 1032 erg s−1 at a distance of about 2.7 kpc. The X-ray spectrum lacks the pronounced soft X-ray emission component prominently found in ROSAT-discovered polars.

Disc-halo gas outflows driven by stellar clusters as seen in multiwavelength tracers

ABSTRACT We consider the dynamics of and emission from growing superbubbles in a stratified interstellar gaseous disc driven by energy release from supernovae explosions in stellar clusters with masses Mcl = 105 − 1.6 × 106 M⊙. Supernovae are spread randomly within a sphere of rc = 60 pc, and inject energy episodically with a specific rate $1/130~\mathrm{M}_\odot ^{-1}$ proportional to the star formation rate (SFR) in the cluster. Models are run for several values of SFR in the range 0.01 to 0.1 M⊙ yr−1, with the corresponding average surface energy input rate ∼0.04–0.4 erg cm−2 s−1. We find that the discrete energy injection by isolated SNe are more efficient in blowing superbubbles: Asymptotically they reach heights of up to 3 to 16 kpc for Mcl = 105 − 1.6 × 105 M⊙, correspondingly, and stay filled with a hot and dilute plasma for at least 30 Myr. During this time, they emit X-ray, Hα and dust infrared emission. X-ray luminosities LX∝SFR3/5 that we derive here are consistent with observations in star-forming galaxies. Even though dust particles of small sizes a ≤ 0.03 μm are sputtered in the interior of bubbles, larger grains still contribute considerably ensuring the bubble luminosity $L_{\rm IR}/{\rm SFR}\sim 5\times 10^7 \, \mathrm{L}_\odot \, \mathrm{M}_\odot ^{-1} ~{\rm yr}$. It is shown that the origin of the North Polar Spur in the Milky Way can be connected with activity of a cluster with the stellar mass of ∼105 M⊙ and the SFR ∼ 0.1 M⊙ yr−1 some 25–30 Myr ago. Extended luminous haloes observed in edge-on galaxies (NGC 891 as an example) can be maintained by disc spread stellar clusters of smaller masses M* ≲ 105 M⊙.

Parallel tracks of kHz QPOs: implication of the bimodal luminosity components in neutron star low-mass X-ray binaries

ABSTRACT We employ the bimodal luminosity components, i.e. the disc luminosity Ld and non-disc luminosity Lr, to interpret the parallel track phenomena in the relation between the kilohertz quasi-periodic oscillation (kHz QPO) frequency ν and X-ray luminosity LX observed in the neutron star low-mass X-ray binaries: For an individual source with a constant non-disc luminosity Lr, the increase of its disc accretion rate $\dot{M}_{\rm d}$ can arise the shrink of neutron star magnetosphere-disc radius that results in the increase of the kHz QPO frequency. Then, with the enhancements of both the kHz QPO frequency and total X-ray luminosity, LX = Ld + Lr, the source will trace out an oblique track in ν–LX relation. Furthermore, for the different sources with various constant non-disc luminosities, they trace out series of parallel tracks in ν–LX relation. Our model presents an approximate power-law relation between the upper kHz QPO frequency ν2 and LX (i.e. $\nu _2\sim L_{\rm X}^k$), where the power-law index k decreases with the ratio parameter of $\mathcal {R}\equiv L_{\rm d}/L_{\rm r}$. Moreover, the fittings of $\nu _2{\!-\!}L_{\rm X}^k$ relation with the observation data indicate a range of ∼0.1–2.7 for the k values, which is basically compatible with the model prediction. In addition, we discussed the physical factors that affect the validity of the model.

Redshift evolution of galaxy group X-ray properties in the Simba simulations

ABSTRACT We examine the evolution of intragroup gas rest-frame X-ray scaling relations for group-sized haloes (M500 = 1012.3–1015 M⊙) in the Simba galaxy formation simulation. X-ray luminosity LX versus M500 shows increasing deviation from self-similarity from z = 3 → 0, with M500 < 1013.5 M⊙ haloes exhibiting a large reduction in LX and slight increase in X-ray luminosity-weighted temperature TX. These shifts are driven by a strong drop in fgas with time for these haloes, and coincides with the onset of Simba’s black hole (BH) jet feedback, occurring when MBH > 107.5 M⊙ and Eddington ratio <0.2, in group haloes at z ∼ 1.5. The connection with BH feedback is corroborated by fBH ≡ MBH/M500 in M500 < 1013.5 M⊙ haloes being strongly anticorrelated with LX and fgas at $z\lesssim 1.5$. This is further reflected in the scatter of LX − TX: haloes with small fBH lie near self-similarity, while those with the highest fBH lie furthest below. Turning off jet feedback results in mostly self-similar behaviour down to z = 0. For the X-ray weighted metallicity ZX, stellar feedback impacts the enrichment of halo gas. Finally, halo profiles show that jet feedback flattens the electron density and entropy profiles, and introduces a core in X-ray surface brightness, particularly at M500 < 1013.5 M⊙. This argues that in Simba, intragroup X-ray evolution is largely driven by jet feedback removing hot gas from the cores of massive groups, and expelling gas altogether in less massive groups.

The black hole X-ray binary MAXI J1348–630 in quiescence

ABSTRACT The properties of the disc/jet coupling in quiescent black hole low mass X-ray binaries (BH LMXBs) are still largely unknown. In this paper, we present the first quasi-simultaneous radio and X-ray detection in quiescence of the BH LMXB MAXI J1348–630, which is known to display a hybrid disc/jet connection that depends on the accretion rate. We performed deep X-ray and radio observations using the Chandra X-ray Observatory and the Australia Telescope Compact Array. MAXI J1348–630 is detected for the first time in quiescence at an X-ray luminosity LX = (7.5 ± 2.9) × 1030(D/2.2 kpc)2 erg s−1: one of the lowest X-ray luminosities observed for a quiescent BH LMXB, possibly implying a short orbital period for the system. MAXI J1348–630 is also detected in radio at LR = (4.3 ± 0.9) × 1026(D/2.2 kpc)2 erg s−1. These detections allow us to constrain the location of MAXI J1348–630 on the radio/X-ray diagram in quiescence, finding that the source belongs to the standard (radio-loud) track in this phase. This provides a strong confirmation that hybrid-correlation sources follow the standard track at low luminosities and down to quiescence, thus improving our knowledge of the disc/jet connection in BH LMXBs.

Explaining the ‘Outliers’ Track in Black Hole X-ray Binaries with a BZ-Jet and Inner-Disk Coupling

In this paper, we investigate the black hole (BH) spin contribution to jet power, especially for the magnetic arrested disk (MAD), where only inner accretion disk luminosity is closely coupled with the spin-jet power, and try to explain the ‘outliers’ track of the radio LR to X-ray luminosity LX in two black hole X-ray binaries (BHXBs). Our results suggest that the BZ-jet and the inner-disk coupling could account for the ‘outliers’ track of the radio/X-ray correlation in two BHXBs, H1743-322 and MAXI J1348-630. Although the accretion disk of H1743-322 in the outburst could be in the MAD state, there is a lower probability that MAXI J1348-630 is in the MAD state due to its low jet production efficiency. The difference in the inner-disk bolometric luminosity ratio of the two sources implies that these two BHXBs are in different inner-disk accretion states. We further investigate the phase-changing regime of MAXI J1348-630 and find that there is a phase transition around LX/LEdd∼10−3. The assumption of sub-MAD is discussed as well.

Radio and X-ray spectral properties of gamma-ray bursts and pulsar wind nebulae

ABSTRACT Both gamma-ray bursts (GRBs) and pulsar wind nebula (PWNe) are energetic outflows that are produced in powerful high-energy astrophysical environments. Based on the correlations between the radio luminosity LR and X-ray luminosity LX as well as between the multiband spectral slope αRX and radio flux FR for compiled GRB and PWN samples, the radio and X-ray spectral properties are statistically investigated. The results show that (1) the correlations between LR and LX are consistent for GRBs and PWNe, within the uncertainties; and (2) for the whole sample, FR is related to αRX, and GRBs have lower αRX than PWNe. It may be concluded that GRBs and PWNe have the same or similar particle acceleration mechanisms and radiation processes, i.e. synchrotron radiation, to produce the radio-to-X-ray band emission.

XMM-Newton X-Ray Observations of the Unusual Wolf–Rayet Star WR 66

Abstract We summarize results of an XMM-Newton X-ray observation of the WN8h-type Wolf–Rayet star WR 66 which shows unusual 3.5–4.1 hr optical variations. One possible explanation is a compact companion (cc) which, if present, could give rise to luminous X-ray emission. WR 66 was marginally detected and its inferred X-ray luminosity (L x ) is orders of magnitude below that expected from wind accretion onto a neutron star, but is comparable to that of the WN8h stars WR 16 and WR 124. If luminous X-ray emission from an accreting WR+cc system is present then it is absorbed and escapes detection. Other explanations of the faintly detected X-ray emission of WN8h stars are not ruled out.

Determining the range of validity of quasar X-ray and UV flux measurements for constraining cosmological model parameters

ABSTRACT We use six different cosmological models to study the recently released compilation of X-ray and UV flux measurements of 2038 quasars (QSOs) which span the redshift range 0.009 ≤ z ≤ 7.5413. We find, for the full QSO data set, that the parameters of the X-ray and UV luminosities LX−LUV relation used to standardize these QSOs depend on the cosmological model used to determine these parameters, i.e. it appears that the full QSO data set includes QSOs that are not standardized and so cannot be used for the purpose of constraining cosmological parameters. Subsets of the QSO data, restricted to redshifts z ≲ 1.5–1.7 obey the LX−LUV relation in a cosmological-model-independent manner, and so can be used to constrain cosmological parameters. The cosmological constraints from these lower z, smaller QSO data subsets are mostly consistent with, but significantly weaker than, those that follow from baryon acoustic oscillation and Hubble parameter measurements.

The radiative efficiency of neutron stars at low-level accretion

ABSTRACT When neutron star low-mass X-ray binaries (NS-LMXBs) are in the low-level accretion regime (i.e. $L_{\rm X}\lesssim 10^{36}\ \rm erg\ s^{-1}$), the accretion flow in the inner region around the NS is expected to exist in the form of the hot accretion flow, e.g. the advection-dominated accretion flow (ADAF) as that in black hole X-ray binaries. Following our previous studies in Qiao & Liu (2020a, b) on the ADAF accretion around NSs, in this paper, we investigate the radiative efficiency of NSs with an ADAF accretion in detail, showing that the radiative efficiency of NSs with an ADAF accretion is much lower than that of $\epsilon \sim {\dot{M} GM\over R_{*}}/{\dot{M} c^2}\sim 0.2$ despite the existence of the hard surface. As a result, given an X-ray luminosity LX (e.g. between 0.5 and 10 keV), $\dot{M}$ calculated by $\dot{M}=L_{\rm X}{R_{*}\over {GM}}$ is lower than the real $\dot{M}$ calculated within the framework of the ADAF accretion. The real $\dot{M}$ can be more than two orders of magnitude higher than that calculated by $\dot{M}=L_{\rm X}{R_{*}\over {GM}}$ with appropriate model parameters. Finally, we discuss that if applicable, the model of ADAF accretion around a NS can be applied to explain the observed millisecond X-ray pulsation in some NS-LMXBs (such as PSR J1023+0038, XSS J12270−4859, and IGR J17379−3747) at a lower X-ray luminosity of a few times of $10^{33}\ \rm erg\ s^{-1}$, since at this X-ray luminosity the calculated $\dot{M}$ with the model of ADAF accretion can be high enough to drive a fraction of the matter in the accretion flow to be channelled on to the surface of the NS forming the X-ray pulsation.

Chandra and HST studies of six millisecond pulsars in the globular cluster M13

ABSTRACT We analyse 55 ks of Chandra X-ray observations of the Galactic globular cluster (GC) M13. Using the latest radio timing positions of six known millisecond pulsars (MSPs) in M13 from Wang et al. (2020), we detect confident X-ray counterparts to five of the six MSPs at X-ray luminosities of LX(0.3–8 keV)∼3 × 1030–1031 erg s−1, including the newly discovered PSR J1641+3627F. There are limited X-ray counts at the position of PSR J1641+3627A, for which we obtain an upper limit LX < 1.3 × 1030 erg s−1. We analyse X-ray spectra of all six MSPs, which are well described by either a single blackbody (BB) or a single power-law model. We also incorporate optical/UV imaging observations from the Hubble Space Telescope (HST) and find optical counterparts to PSR J1641+3627D and J1641+3627F. Our colour–magnitude diagrams indicate the latter contains a white dwarf, consistent with the properties suggested by radio timing observations. The counterpart to J1641+3627D is only visible in the V band; however, we argue that the companion to J1641+3627D is also a white dwarf, since we see a BB-like X-ray spectrum, while MSPs with non-degenerate companions generally show non-thermal X-rays from shocks between the pulsar and companion winds. Our work increases the sample of known X-ray and optical counterparts of MSPs in GCs.

AstroSat observations of the first Galactic ULX pulsar Swift J0243.6+6124

ABSTRACT Swift J0243.6+6124, the first Galactic ultraluminous X-ray pulsar, was observed during its 2017–2018 outburst with AstroSat at both sub- and super-Eddington levels of accretion with X-ray luminosities of LX ∼ 7 × 1037 and 6 × 1038 erg s−1, respectively. Our broad-band timing and spectral observations show that X-ray pulsations at ${\sim}9.85~\rm {s}$ have been detected up to 150 keV when the source was accreting at the super-Eddington level. The pulse profiles are a strong function of both energy and source luminosity, showing a double-peaked profile with pulse fraction increasing from ∼$10{{{\ \rm per\ cent}}}$ at $1.65~\rm {keV}$ to 40–80 ${{\ \rm per\ cent}}$ at $70~\rm {keV}$. The continuum X-ray spectra are well modelled with a high-energy cut-off power law (Γ ∼ 0.6–0.7) and one or two blackbody components with temperatures of ∼0.35 and $1.2~\rm {keV}$, depending on the accretion level. No iron line emission is observed at sub-Eddington level, while a broad emission feature at around 6.9 keV is observed at the super-Eddington level, along with a blackbody radius ($121\!-\!142~\rm {km}$) that indicates the presence of optically thick outflows.

VLA Limits on Intermediate-mass Black Holes in 19 Massive Globular Clusters

Abstract The NSF’s Karl G. Jansky Very Large Array (VLA) was used at 3 cm to search for accretion signatures from intermediate-mass black holes (IMBHs) in 19 globular star clusters (GCs) in NGC 3115, an early-type galaxy at a distance of 9.4 Mpc. The 19 GCs have stellar masses M ⋆ ∼ (1.1–2.7) × 106 M ⊙, with a mean . None were detected. An IMBH accretion model was applied to the individual GCs and their radio stack. The radio-stacked GCs have an IMBH mass and mass fraction , with each limit being uncertain by a factor of about 2.5. The latter limit contrasts with the extremes of some stripped nuclei, suggesting that the set of stacked GCs in NGC 3115 is not a set of such nuclei. The radio luminosities of the individual GCs correspond to X-ray luminosities L X < (3.3–10) × 1038 erg s−1, with a factor of about 2.5 uncertainty. These limits predicted for putative IMBHs in the GCs are consistent with extant Chandra observations. Finally, a simulated observation with a next-generation VLA demonstrates that accretion signatures from IMBHs in GCs can be detected in a radio-only search, yet they elude detection in an X-ray-only search due to confusion from X-ray binaries in the GCs.