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Utilizing archival Chandra X-ray Observatory data and Hubble Space Telescope globular cluster catalogues, we probe the time-domain properties of the low mass X-ray binary population in the elliptical galaxy NGC 4261. Of the 98 unique X-ray sources identified in this study, 62 sources are within the optical field of view and, of those, 33 per cent are aligned with an optical cluster counterpart. We find twenty X-ray sources coincident with globular clusters; two are previously discovered ultra-luminous X-ray sources (ULXs) and eighteen are low mass X-ray binaries (GCLMXBs) with LX < 1039 erg s−1. ULXs are a heterogeneous class of extremely bright X-ray binaries (LX > 1039 erg s−1) and ULXs located in globular clusters (GCULXs) and may be indicators of black holes. Identifying these unusually X-ray bright sources and measuring their optical properties can provide valuable constraints on the progenitors of gravitational wave sources. We compare observations of these sources to the twenty previously studied GCULXs from five other early-type galaxies, and find that GCULXs in NGC 4261 are of similar colour and luminosity and do not significantly deviate from the rest of the sample in terms of distance from the galaxy centre or X-ray luminosity. Both the GCULX and GCLMXB populations of NGC 4261 show long-term variability; the former may have implications for fast radio bursts originating in globular clusters and the latter will likely introduce additional scatter into the low mass end of GCLMXB X-ray luminosity functions. [ABSTRACT FROM AUTHOR]

Owing to their quiet evolutionary histories, nearby dwarf galaxies (stellar masses |$M_\star \lesssim 3 \times 10^9 \, \mathrm{M}_\odot$|⁠) have the potential to teach us about the mechanism(s) that 'seeded' the growth of supermassive black holes, and also how the first stellar mass black holes formed and interacted with their environments. Here, we present high spatial resolution observations of three dwarf galaxies in the X-ray (Chandra), the optical/near-infrared (Hubble Space Telescope), and the radio (Karl G. Jansky Very Large Array). These three galaxies were previously identified as hosting candidate active galactic nuclei on the basis of lower resolution X-ray imaging. With our new observations, we find that X-ray sources in two galaxies (SDSS J121326.01+543631.6 and SDSS J122111.29+173819.1) are off-nuclear and lack corresponding radio emission, implying they are likely luminous X-ray binaries. The third galaxy (Mrk 1434) contains two X-ray sources (each with L X ≈ 1040 erg s−1) separated by 2.8 arcsec, has a low metallicity [12 + log(O/H)  = 7.8], and emits nebular He  ii λ4686 line emission. The northern source has spatially coincident point-like radio emission at 9.0 GHz and extended radio emission at 5.5 GHz. We discuss X-ray binary interpretations (where an ultraluminous X-ray source blows a 'radio bubble') and active galactic nucleus interpretations (where an |$\approx 4\times 10^5 \, \mathrm{M}_\odot$| black hole launches a jet). In either case, we find that the He  ii emission cannot be photoionized by the X-ray source, unless the source was ≈30–90 times more luminous several hundred years ago. [ABSTRACT FROM AUTHOR]

Weakly accreting black hole X-ray binaries launch compact radio jets that persist even in the quiescent spectral state, at X-ray luminosities ≲ 10−5 of the Eddington luminosity. However, radio continuum emission has been detected from only a few of these quiescent systems, and little is known about their radio variability. Jet variability can lead to misclassification of accreting compact objects in quiescence, and affects the detectability of black hole X-ray binaries in next-generation radio surveys. Here we present the results of a radio monitoring campaign of A0620 − 00, one of the best-studied and least-luminous known quiescent black hole X-ray binaries. We observed A0620 − 00 at 9.8 GHz using the Karl G Jansky Very Large Array on 31 epochs from 2017 to 2020, detecting the source |$\sim 75{{\ \rm per\, cent}}$| of the time. We see significant variability over all time-scales sampled, and the observed flux densities follow a lognormal distribution with μ = 12.5 μJy and σ = 0.22 dex. In no epoch was A0620 − 00 as bright as in 2005 (51 ± 7 μJy), implying either that this original detection was obtained during an unusually bright flare, or that the system is fading in the radio over time. We present tentative evidence that the quiescent radio emission from A0620 − 00 is less variable than that of V404 Cyg, the only other black hole binary with comparable data. Given that V404 Cyg has a jet radio luminosity ∼20 times higher than A0620 − 00, this comparison could suggest that less luminous jets are less variable in quiescence. [ABSTRACT FROM AUTHOR]

Radio continuum observations offer a new window on compact objects in globular clusters compared to typical X-ray or optical studies. As part of the MAVERIC survey, we have used the Australia Telescope Compact Array to carry out a deep (median central noise level ≈4  |$\mu$| Jy beam-1) radio continuum survey of 26 southern globular clusters at central frequencies of 5.5 and 9.0 GHz. This paper presents a catalogue of 1285 radio continuum sources in the fields of these 26 clusters. Considering the surface density of background sources, we find significant evidence for a population of radio sources in seven of the 26 clusters, and also identify at least 11 previously known compact objects (six pulsars and five X-ray binaries). While the overall density of radio continuum sources with 7.25-GHz flux densities ≳ 20  |$\mu$| Jy in typical globular clusters is relatively low, the survey has already led to the discovery of several exciting compact binaries, including a candidate ultracompact black hole X-ray binary in 47 Tuc. Many of the unclassified radio sources near the centres of the clusters are likely to be true cluster sources, and multiwavelength follow-up will be necessary to classify these objects and better understand the demographics of accreting compact binaries in globular clusters. [ABSTRACT FROM AUTHOR]

We present an analysis of the ultraluminous X-ray source (ULX) population in 75 Virgo cluster late-type galaxies, including all those with a star formation rate ≳1 M⊙ yr−1 and a representative sample of the less star forming ones. This study is based on 110 observations obtained over 20 yr with the Chandra X-ray Observatory Advanced Camera for Imaging Spectroscopy. As part of a Large Chandra Program, new observations were obtained for 52 of these 75 galaxies. The data are complete to a sensitivity of ≈1039 erg s−1, with a typical detection limit of ≈3 × 1038 erg s−1 for the majority of the sources. The catalogue contains about 80 ULXs (0.3–10 keV luminosity >1039 erg s−1), and provides their location, observed flux, de-absorbed luminosity, and (for the 25 most luminous ones) simple X-ray spectral properties. We discuss the ULX luminosity function in relation to the mass and star formation rate of the sample galaxies. We show that recent models of low-mass plus high-mass X-ray binary populations (scaling with stellar mass and star formation rate, respectively) are mostly consistent with our observational results. We tentatively identify the most luminous X-ray source in the sample (a source in IC 3322A with L X ≈ 6 × 1040 erg s−1) as a recent supernova or its young remnant. The properties of the sample galaxies (morphologies, stellar masses, star formation rates, total X-ray luminosities from their point-source population) are also summarized. [ABSTRACT FROM AUTHOR]

Transitional millisecond pulsars are millisecond pulsars that switch between a rotation-powered millisecond pulsar state and an accretion-powered X-ray binary state, and are thought to be an evolutionary stage between neutron star low-mass X-ray binaries and millisecond pulsars. So far, only three confirmed systems have been identified in addition to a handful of candidates. We present the results of a multiwavelength study of the low-mass X-ray binary NGC 6652B in the globular cluster NGC 6652, including simultaneous radio and X-ray observations taken by the Karl G. Jansky Very Large Array and the Chandra X-ray Observatory, and optical spectroscopy and photometry. This source is the second brightest X-ray source in NGC 6652 (⁠|$L_{\textrm {X}}\sim 1.8 \times 10^{34}{\, \mathrm{erg\, s}^{-1}}$|⁠) and is known to be variable. We observe several X-ray flares over the duration of our X-ray observations, in addition to persistent radio emission and occasional radio flares. Simultaneous radio and X-ray data show no clear evidence of anticorrelated variability. Optical spectra of NGC 6652B indicate variable, broad H α emission that transitions from double-peaked emission to absorption over a time-scale of hours. We consider a variety of possible explanations for the source behaviour, and conclude that based on the radio and X-ray luminosities, short time-scale variability and X-ray flaring, and optical spectra, NGC 6652B is best explained as a transitional millisecond pulsar candidate that displays prolonged X-ray flaring behaviour. However, this could only be confirmed with observations of a change to the rotation-powered millisecond pulsar state. [ABSTRACT FROM AUTHOR]

We report on Atacama Large Millimeter Array (ALMA) continuum observations of the black hole X-ray binary A0620–00 at an X-ray luminosity nine orders of magnitude sub-Eddington. The system was significantly detected at 98 GHz (at 44 ± 7  |$\mu$| Jy) and only marginally at 233 GHz (20 ± 8  |$\mu$| Jy), about 40 d later. These results suggest either an optically thin sub-mm synchrotron spectrum, or highly variable sub-mm jet emission on month time-scales. Although the latter appears more likely, we note that, at the time of the ALMA observations, A0620–00 was in a somewhat less active optical-IR state than during all published multiwavelength campaigns when a flat-spectrum, partially self-absorbed jet has been suggested to extend from the radio to the mid-IR regime. Either interpretation is viable in the context of an internal shock model, where the jet's spectral shape and variability are set by the power density spectrum of the shells' Lorentz factor fluctuations. While strictly simultaneous radio–mm-IR observations are necessary to draw definitive conclusions for A0620–00, the data presented here, in combination with recent radio and sub-mm results from higher luminosity systems, demonstrate that jets from black hole X-ray binaries exhibit a high level of variability – either in flux density or intrinsic spectral shape, or both – across a wide spectrum of Eddington ratios. This is not in contrast with expectations from an internal shock model, where lower jet power systems can be expected to exhibit larger fractional variability owing to an overall decrease in synchrotron absorption. [ABSTRACT FROM AUTHOR]

We present coordinated multiwavelength observations of the high Galactic latitude (b = +50°) black hole X-ray binary (BHXB) Swift J1357.2−0933 in quiescence. Our broad-band spectrum includes strictly simultaneous radio and X-ray observations, and near-infrared, optical, and ultraviolet data taken 1-2 d later. We detect Swift J1357.2-0933 at all wavebands except for the radio (f5GHz < 3.9 μJy beam-1; 3σrms). Given current constraints on the distance (2.3-6.3 kpc), its 0.5-10 keV X-ray flux corresponds to an Eddington ratio LX/LEdd = 4 × 10-9-3 × 10-8 (assuming a black hole mass of 10 M⊙). The broad-band spectrum is dominated by synchrotron radiation from a relativistic population of outflowing thermal electrons, which we argue to be a common signature of short-period quiescent BHXBs. Furthermore, we identify the frequency where the synchrotron radiation transitions from optically thick-to-thin (νb ≈ 2-5 × 1014 Hz), which is the most robust determination of a 'jet break' for a quiescent BHXB to date. Our interpretation relies on the presence of steep curvature in the ultraviolet spectrum, a frequency window made observable by the low amount of interstellar absorption along the line of sight. High Galactic latitude systems like Swift J1357.2-0933 with clean ultraviolet sightlines are crucial for understanding black hole accretion at low luminosities. [ABSTRACT FROM AUTHOR]

The nature of black hole jets at the lowest detectable luminosities remains an open question, largely due to a dearth of observational constraints. Here, we present a new, nearly simultaneous broad-band spectrum of the black hole X-ray binary (BHXB) XTE J1118+480 at an extremely low Eddington ratio (LX ~ 10-8.5LEdd). Our new spectral energy distribution (SED) includes the radio, near-infrared, optical, ultraviolet, and X-ray wavebands. XTE J1118+480 is now the second BHXB at such a low Eddington ratio with a well-sampled SED, thereby providing new constraints on highly sub-Eddington accretion flows and jets, and opening the door to begin comparison studies between systems. We apply a multizone jet model to the new broad-band SED, and we compare our results to previous fits to the same source using the same model at 4-5 decades higher luminosity. We find that after a BHXB transitions to the so-called quiescent spectral state, the jet base becomes more compact (by up to an order of magnitude) and slightly cooler (by at least a factor of 2). Our preferred model fit indicates that jet particle acceleration is much weaker after the transition into quiescence. That is, accelerated non-thermal particles no longer reach high enough Lorentz factors to contribute significant amounts of synchrotron X-ray emission. Instead, the X-ray waveband is dominated by synchrotron self-Compton emission from a population of mildly relativistic electrons with a quasi-thermal velocity distribution that are associated with the jet base. The corresponding (thermal) synchrotron component from the jet base emits primarily in the infrared through ultraviolet wavebands. Our results on XTE J1118+480 are consistent with broad-band modelling for A0620-00 (the only other comparably low Eddington ratio BHXB with a well-sampled SED) and for Sgr A* (the quiescent supermassive black hole at the Galactic centre). The above could therefore represent a canonical baseline geometry for accreting black holes in quiescence. We conclude with suggestions for future studies to further investigate the above scenario. [ABSTRACT FROM AUTHOR]

ABSTRACT The Fundamental Plane of black hole activity is a relation between X-ray luminosity, radio luminosity and black hole mass for hard-state Galactic black holes and their supermassive analogues. The Fundamental Plane suggests that, at low-accretion rates, the physical processes regulating the conversion of an accretion flow into radiative energy could be universal across the entire black hole mass scale. However, there is still a need to further refine the Fundamental Plane in order to better discern the radiative processes and their geometry very close to the black hole, in particular the source of hard X-rays. Further refinement is necessary because error bars on the best-fitting slopes of the Fundamental Plane are generally large, and also the inferred coefficients can be sensitive to the adopted sample of black holes. In this work, we regress the Fundamental Plane with a Bayesian technique. Our approach shows that sub-Eddington black holes emit X-ray emission that is predominantly optically thin synchrotron radiation from the jet, provided that their radio spectra are flat or inverted. X-ray emission dominated by very radiatively inefficient accretion flows is excluded at the >3σ level. We also show that it is difficult to place Fanaroff-Riley type I (FR I) galaxies on to the Fundamental Plane because their X-ray jet emission is highly affected by synchrotron cooling. On the other hand, BL Lac objects (i.e. relativistically beamed sub-Eddington AGN) fit on to the Fundamental Plane. Including a uniform subset of high-energy peaked BL Lac objects from the Sloan Digital Sky Survey, we find sub-Eddington black holes with flat/inverted radio spectra follow log LX= (1.45 ± 0.04)log LR− (0.88 ± 0.06)log MBH− 6.07 ± 1.10, with σint= 0.07 ± 0.05 dex. Finally, we discuss how the effects of synchrotron cooling of jet emission from the highest black hole masses can bias Fundamental Plane regressions, perhaps leading to incorrect inferences on X-ray radiation mechanisms. [ABSTRACT FROM AUTHOR]