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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]

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 near-infrared polarimetric observations of the black hole X-ray binaries Swift J1357.2-0933 and A0620-00. In both sources, recent studies have demonstrated the presence of variable infrared synchrotron emission in quiescence, most likely from weak compact jets. For Swift J1357.2-0933, we find that the synchrotron emission is polarized at a level of 8.0 ± 2.5 per cent (a 3.2σ detection of intrinsic polarization). The mean magnitude and rms variability of the flux (fractional rms of 19-24 per cent in KS band) agree with previous observations. These properties imply a continuously launched (stable on long time-scales), highly variable (on short time-scales) jet in the Swift J1357.2-0933 system in quiescence, which has a moderately tangled magnetic field close to the base of the jet. We find that for A0620-00, there are likely to be three components to the optical-infrared polarization; interstellar dust along the line of sight, scattering within the system, and an additional source that changes the polarization position angle in the reddest (H and KS) wavebands. We interpret this as a stronger contribution of synchrotron emission, and by subtracting the line-of-sight polarization, we measure an excess of ~1.25 ± 0.28 per cent polarization and a position angle of the magnetic field vector that is consistent with being parallel with the axis of the resolved radio jet. These results imply that weak jets in low-luminosity accreting systems have magnetic fields which possess similarly tangled fields compared to the more luminous, hard state jets in X-ray binaries. [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]

We present new Chandra х-ray observations of the transient black hole х-ray binary MAхI J1659-152 in quiescence. These observations were made more than one year after the end of the source's 2010-2011 outburst. We detect the source at a 0.5-10 keV flux of 2.8(8) × 10-15 erg s-1 cm-2, which corresponds to a luminosity of ∼1.2 × 1031 (d/6 kpc)2 erg s-1. This level, while being the lowest at which the source has been detected, is within factors of ∼2 of the levels seen at the end of the initial decay of the outburst and soon after a major reflare of the source. The quiescent luminosity of MAхI J1659-152, which is the shortest-orbital-period black hole х-ray binary (∼2.4 hr), is lower than that of neutron-star х-ray binaries with similar periods. However, it is higher than the quiescent luminosities found for black hole х-ray binaries with orbital periods ∼2-4 times longer. This could imply that a minimum quiescent luminosity may exist for black hole х-ray binaries, around orbital periods of ∼5-10 hr, as predicted by binary-evolution models for the mass transfer rate. Compared to the hard state, we see a clear softening of the power-law spectrum in quiescence, from an index of 1.55(4) to an index of 2.5(4). We constrain the luminosity range in which this softening starts to (0.18-6.2)×10-5 (d/6 kpc)2 (M/8M_) LEdd, which is consistent with the ranges inferred for other sources. [ABSTRACT FROM AUTHOR]

Most transient black hole X-ray binaries (BHXBs) spend the bulk of their time in a quiescent state, where they accrete matter from their companion star at highly sub-Eddington luminosities (we define quiescence here as a normalized Eddington ratio lx = L0.5-10 keV/LEdd < 10-5). Here, we present Chandra X-ray imaging spectroscopy for three BHXB systems (H 1743-322, MAXI J1659-152, and XTE J1752-223) as they fade into quiescence following an outburst. Multiple X-ray observations were taken within one month of each other, allowing us to track each individual system's X-ray spectral evolution during its decay. We compare these three systems to other BHXB systems. We confirm that quiescent BHXBs have softer X-ray spectra than low-hard-state BHXBs, and that quiescent BHXB spectral properties show no dependence on the binary system's orbital parameters. However, the observed anti-correlation between X-ray photon index (Г) and lx in the low-hard state does not continue once a BHXB enters quiescence. Instead, Γ plateaus to an average {Γ} = 2.08 ± 0.07 by the time lx reaches ~10-5. lx ~ 10-5 is thus an observationally motivated upper limit for the beginning of the quiescent spectral state. Our results are discussed in the context of different accretion flow models and across the black hole mass scale. [ABSTRACT FROM AUTHOR]

ABSTRACT The radio/X-ray correlation for hard and quiescent state black hole X-ray binaries is critically investigated in this paper. New observations of known sources, along with newly discovered ones (since 2003), have resulted in an increasingly large number of outliers lying well outside the scatter about the quoted best-fitting relation. Most of these outliers tend to cluster below the best-fitting line, possibly indicative of two distinct tracks which might reflect different accretion regimes within the hard state. Here, we employ and compare state of the art data clustering techniques in order to identify and characterize different data groupings within the radio/X-ray luminosity plane for 18 hard and quiescent state black hole X-ray binaries with nearly simultaneous multiwavelength coverage. Linear regression is then carried out on the clustered data to infer the parameters of a relationship of the form ℓr=α+βℓx through a Bayesian approach (where ℓ denotes logarithmic luminosities). We conclude that the two-cluster model, with independent linear fits, is a significant improvement over fitting all points as a single cluster. While the upper track slope (0.63 ± 0.03) is consistent, within the errors, with the fitted slope for the 2003 relation (0.7 ± 0.1), the lower track slope (0.98 ± 0.08) is not consistent with the upper track or with the widely adopted value of ≃1.4 for the neutron stars. The two luminosity tracks do not reflect systematic differences in black hole spins as estimated either from reflection- or continuum-fitting method. Additionally, there is evidence for at least two sources (H1743−322 and GRO J1655−500) jumping from the lower to the upper track as they fade towards quiescence, further indicating that black hole spin does not play any major role in defining the radio loudness of compact jets from hard black hole X-ray binaries. The results of the clustering and regression analysis are fairly insensitive to the selection of subsamples, accuracy in the distances and the treatment of upper limits. Besides introducing a further level of complexity in understanding the interplay between synchrotron and Comptonized emission from black hole X-ray binaries, the existence of two tracks in the radio/X-ray domain underscores that a high level of caution must be exercised when employing black hole luminosity-luminosity relations for the purpose of estimating a third parameter, such as distance or mass. [ABSTRACT FROM AUTHOR]

Correlations between the radio and X-ray bands in the hard state of black hole X-ray binaries (BHBs) have led to the discovery of the Fundamental Plane of black hole accretion, linking accretion-driven radiative attributes to black hole mass. Although this discovery has led to new constraints on radiative efficiencies, there is still significant degeneracy in terms of understanding the governing physics. I present several new results exploring the processes driving the Fundamental Plane over the black hole mass range. These include the first ever homogeneous fits of sources at approximately the same Eddington luminosity but millions of times different in mass, which I focus on for this proceeding article. [ABSTRACT FROM PUBLISHER]