arXiv subjects

David M. Russell

Publications and source records attributed to David M. Russell.

At least 19 recordsLinked to original sources

arXiv2026

Scaling Up: A Global Optical/X-ray Correlation For Reprocessing Across Black Hole Masses

This study extends the optical/X-ray correlation from black hole low-mass X-ray binaries (XRBs) to active galactic nuclei (AGN), bridging scales from stellar-mass to supermassive black holes (SMBHs). In XRBs, optical emission predominantly originates from the thermal reprocessing of high-energy photons on the surface of the geometrically thin, optically thick accretion disk. The reprocessed optical luminosity follows a power-law relation of the form $L_{V} \propto L_{X}^β a$, with $β\sim 0.5$, where $L_X$ is the X-ray luminosity and the orbital separation $a$ serves as a proxy for the disk size. To test the universality of this relationship across all mass scales, we analyzed quasi-simultaneous X-ray and optical flux data from a sample of XRBs and AGN that have estimates of the radius of the optically emitting disk. To bridge the structural differences between these populations, we utilized the characteristic optical/X-ray time lag derived from reverberation mapping as a physical proxy for the reprocessing radius. Our analysis reveals a tight, global correlation across all black hole masses, described by $\log L_V = 0.48\log L_X + 0.96\log a_R + 6.5$ with a scatter of $\sim 0.20$ dex, where $a_R$ is the reprocessing radius derived from the lag. Incorporating a geometric correction for the disk inclination angle ($\cos i$) slightly reduces this scatter to $0.18$ dex. These results suggest that X-ray reprocessing is scale-invariant across accretion disks, from stellar-mass black holes to SMBHs. This scaling relation provides new constraints for models of X-ray emission regions in accreting black holes.

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arXiv2026

The compact jets of the black hole X-ray binary Swift J1753.5-0127 during its 2023-2024 outburst

X-ray binaries are excellent laboratories to study the coupling between accretion and ejection phenomena around compact objects through multiwavelength observations. Swift J1753.5-0127 is a black hole low-mass X-ray binary discovered in outburst in 2005 that remained active for 11 years, followed by two mini-outbursts. We investigate the behavior of the compact jets of Swift J1753.5-0127 during its 2023-2024 outburst. We report a dense multiwavelength monitoring in the radio, NIR/optical/UV and X-ray bands. Using new and archival luminosity measurements, we construct a refined, smoother radio/X-ray plane that covers the steep/outlier and shallow/transition branches of the radio/X-ray correlation. We also present spectral energy distributions (SEDs) constructed using quasi-simultaneous observations from four epochs. Swift J1753.5-0127 underwent a faint, hard-only, double-peaked outburst, followed by a re-brightening in the optical. The updated radio/X-ray plane offers a clear view on the region occupied by this source, similar to MAXI J1348-630, but at lower radio luminosities than H 1743-322, and we confirm its hybrid behavior. The broad-band SED fitting reveals that a cool, truncated accretion disk dominates the NIR/optical/UV band. We find that the compact jets have a variable but weak contribution in the NIR, with a break frequency $ν_\mathrm{break} \lesssim 5 \times 10^{12}~\mathrm{Hz}$, providing additional evidence that radio-quiet sources are also infrared-quiet. We also find evidence that the radio spectral index evolves with X-ray luminosity, transitioning from inverted values near quiescence to flatter or negative values at higher luminosities. In the context of adiabatic jet models, this trend may indicate that the jet geometry becomes increasingly confined as the accretion rate rises.

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arXiv2026

A Multi-Wavelength Picture of the Surprisingly Short 2024/25 Outburst from EXO 0748-676

After sixteen years in quiescence, EXO 0748-676 unexpectedly returned to outburst in March 2024. However, in stark contrast to its previous outburst (1985 - 2008), EXO 0748-676 returned to quiescence in January 2025, after ~ 10 months in outburst. Here, we collate data from nine observing facilities, covering wavelengths from radio to X-ray, to provide the first multi-wavelength study of the 2024/25 outburst. Unlike the 1985 outburst, both the rise and decay are fully captured at X-ray and optical wavelengths. The light curves do not exhibit the hallmarks of a canonical transient low-mass X-ray binary outburst. Instead, the X-ray and optical fluxes remain reasonably stable during the 2024/25 outburst. The wealth of optical data of EXO 0748-676 reveals a skewed optical phase curve and an irradiation-dominated optical light curve which lasts until the onset of hydrogen recombination in the outer disc at ~11000K, triggering a rapid fade at the end of the outburst. X-ray observations by XMM-Newton and NICER revealed that EXO 0748-676 was heavily obscured early on, with spectra exhibiting a dampened soft X-ray contribution. We find that a spectral model including a clumpy, ionised absorber describes the obscuration, although the exact cause of the early-time obscuration remains uncertain. X-ray colour analysis indicates that EXO 0748-676 spent most of the ~10 month outburst in a faint, hard state. EXO 0748-676 increased marginally in brightness towards the end of the outburst, but never entered a soft spectral state. This behaviour is characteristic of a failed transition outburst.

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arXiv2026

JWST/MIRI Spectroscopy of the Quiescent Black Hole X-ray Binary XTE J1118+480: Constraints on Jet and Circumbinary Dust Contributions to the Mid-infrared Emission

We report on JWST/MIRI low-resolution spectroscopy ($5$-$12 μ$m) and imaging ($15$-$25 μ$m) of the black hole X-ray binary XTE~J1118+480 in quiescence, at an X-ray luminosity nine orders of magnitude sub-Eddington, together with contemporaneous optical monitoring and a radio upper limit of $4.5~μ$Jy at 6 GHz. With the donor subtracted, the $5$-$12 μ$m continuum is curved, turning over near $6~μ$m; a single power law is rejected in favor of a $764\pm 9$ K blackbody. Longward of the turnover the spectral index is $α= +1.21\pm0.36$, steeper at the $2σ$ level than the flat to slightly inverted spectrum of a compact jet. Interpreted as synchrotron, the turnover implies equipartition fields of $\sim$$10^{4}$ G on scales of tens of gravitational radii, but the MIRI photometry at $15$-$21~μ$m-taken $2.6$ hr before the spectrum-lies $30$-$90$ per cent above its extrapolation, suggesting a second self-absorbed zone. A single partially self-absorbed jet spectrum can reach the photometry only at the expense of the spectral curvature and the $25.5~μ$m upper limit, so a steady compact jet is disfavored, although not excluded. Representative self-consistent irradiated-disk models instead reproduce the principal features of the $5$-$25~μ$m spectral energy distribution, with a directly illuminated dust wall at $\simeq 5$ binary separations, outside the tidal-truncation radius, and a minimum dust mass of order $10^{22}$ g. This is the first spectroscopic evidence, though not yet conclusive, favoring circumbinary dust over a jet in a quiescent black hole X-ray binary.

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arXiv2026

Radio flares and X-ray hardening embedded in the long soft state of 4U 1543-475

We present a comprehensive multi-wavelength study of the black hole X-ray binary 4U 1543-475 during its 2021 outburst, focusing on radio flaring episodes that are commonly interpreted as signatures of episodic jet production and are embedded within states when the X-ray emission was dominated by an accretion disk component. The radio monitoring reveals at least two discrete flares that coincide with periods of enhanced Comptonized X-ray emission. Broadband spectral modelling shows a significant decrease in the reflection-to-disk flux ratio (by a factor of ~3-4) during these episodes, consistent with a temporary change in the geometry of the inner accretion flow, although the data do not allow the causal sequence to be firmly established. Optical photometry exhibits variability that broadly tracks the reflection fraction, consistent with changes in the illuminating component. The accompanying spectral hardening indicates that the radio flares were associated with short-lived excursions toward a "harder" state, departing from the soft state. X-ray timing analysis suggests that the radio flares may be associated with changes in the fractional rms variability; however, no consistent or unified pattern can be firmly established across different events. These results provide a multi-wavelength observational example of radio flaring activity in a black hole binary and highlight the complex interplay between accretion flow geometry, coronal emission, and jet-related phenomena.

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arXiv2026

A Real-Time Jet Laboratory in Swift J1727.8-1613

We present the results of our intensive VLBI campaign on the black-hole low-mass X-ray binary (LMXB) Swift J1727.8-1613 during its 2023-2024 outburst. We observed the repeated quenching and re establishment of the highly-extended continuous jet during several transitions between hard-intermediate and soft-intermediate states, and the repeated ejection of transient jets. Using time-dependent visibility model fitting, we tracked the motion of nine discrete jet knots, obtaining some of the most precise measurements of transient jet proper motions and ejection dates in an LMXB. These ejecta were only detectable for a short time with VLBI, and some showed rapid intra-observation flux density variability that was not captured in image reconstructions. For the first time, we use time-dependent visibility modelling to fit a piecewise model for the jet knot flux densities, allowing us to create complex, non-parametric light curves of their intra-observation variability. We observed the launching of multiple ejecta across several state transitions, however, we could not identify a consistent signature of jet ejection in the available X-ray intensity or hardness data. We constrained the intrinsic speeds and bulk Lorentz factors of the jet knots, finding that Swift J1727.8-1613 launched both mildly relativistic and highly relativistic ejecta throughout its outburst. We used their proper motions to constrain a posterior distribution for the maximum inclination angle of the jet axis, which had 50th, 84th, and 99th percentiles of 40°, 50°, and 66°, respectively. These unique observations of the repeated ejection of transient jets by a single LMXB reveal that fixed parameters such as black-hole mass, black-hole spin, and spin-orbit misalignment do not uniquely determine the varying properties of transient jets, particularly their speeds and Lorentz factors.

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arXiv2026

Superorbital variability in the quiescent black hole X-ray transient A0620-00

Quiescent black hole low-mass X-ray binaries provide a key setting for probing accretion physics at low luminosities. A0620-00, the archetypal system in this class, has remained in X-ray quiescence for decades and exhibits complex optical variability, yet the long-term behaviour of its accretion flow remains poorly understood. Here, we report an analysis of long-term optical monitoring of A0620-00 from ZTF, LCO, and ATLAS. The full dataset spans nearly two decades, with the ZTF light curve providing the primary $\sim 2760$-day baseline for the period analysis. We identify a superorbital cycle with a period of $P = 261.9 \pm 9.4$ d and a peak-to-peak amplitude of $\sim 0.2$ mag. The signal is recovered independently across all three surveys, and red-noise simulations indicate that it is unlikely to arise from stochastic variability alone. Furthermore, the relative occurrence of the \textit{passive} and \textit{active} quiescent states displayed by the system seems to depend on the superorbital phase, with passive states concentrated near the cycle minimum and active states more common near maximum. We find that, among the possible interpretations, retrograde nodal precession of a hot inner accretion flow might be able to explain the observed long-term modulation. In this interpretation, the periodic signal may arise from cyclic reorientation of the inner flow, which modulates the photometric contribution from the innermost regions. The inferred modulation period would correspond to a characteristic dynamical radius of $\sim0.13a$ ($\sim10^4~R_{\rm g}$), where $a$ is the binary semi-major axis, broadly consistent with the expected transition between the outer thin disc and the inner hot accretion flow.

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arXiv2026

A cooler look at the environment of Cygnus X-1: Searching for dynamical interactions within cold molecular gas

We present IRAM--30m observations aimed at identifying potential outflow-interstellar medium interaction sites in the vicinity of the black hole X-ray binary Cygnus X--1, which displays persistent relativistic jets and a prominent stellar wind. Using this dataset, we construct molecular line emission maps, identifying a never before seen molecular structure potentially linked to X-ray binary-driven feedback. This structure, surrounding the source, exhibits properties consistent with an interaction powered primarily by the stellar wind of the massive stellar companion and further sculpted by the relativistic jets. Our results indicate that multiple outflow mechanisms (stellar winds and relativistic jets) may simultaneously be shaping the interstellar medium around Cygnus X--1, and that molecular line imaging can help to disentangle complex feedback processes in environments where multiple outflows take place.

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arXiv2026

On The Nature of Einstein Probe Transient EP250916a: Insights from X-ray, Optical, and Radio Observations

We report multi-wavelength studies of the transient EP250916a, detected by the Einstein Probe on 2025 September 16. Located at low Galactic latitude, the source exhibited a rapid X-ray brightening, reaching an unabsorbed 0.5--10 keV flux of $(6.4 \pm 0.1) \times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$, followed by a plateau and a two-stage decay lasting over 40 days. Swift/XRT monitoring shows a persistently hard spectrum ($Γ\approx 1.6$--2.2) with only modest softening during decay, while a NuSTAR observation confirms a hard-state continuum extending up to 70 keV. Timing analysis of XMM-Newton data reveals a weak quasi-periodic oscillation (QPO) at $\sim$13 Hz. No other coherent pulsations or thermonuclear bursts are detected. Broadband spectral modeling favors a nonthermal power-law continuum with partial-covering absorption, and shows no significant thermal disk component. Optical imaging obtained with NOT/ALFOSC, LCO, and GaiaDR3 identifies two faint sources within the 2 arcsec Swift/XRT positional uncertainty. A MeerKAT observation at 1.28 GHz yielded no radio counterpart, with a 3$σ$ upper limit of 60 $μ$Jy beam$^{-1}$. The combination of a long-lasting outburst, a hard nonthermal X-ray spectrum, a weak QPO detection, the absence of coherent timing features, and faint potential optical counterparts disfavors a stellar-flare or extragalactic origin and supports an accreting compact-object scenario. Comparisons with similar faint, hard-state transients place EP250916a within a growing population of low-luminosity, hard-state black hole X-ray binary candidates.

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arXiv2026

Exotic optical variability in the black hole X-ray binary IGR J17091-3624

IGR J17091-3624 is a distinctive black hole X-ray binary exhibiting exotic variability, including complex "heartbeat" oscillations in its X-ray light curves, similar to those observed in GRS 1915+105, a system renowned for its structured, rapid X-ray variability but heavily obscured at optical wavelengths. In contrast, IGR J17091-3624 is less obscured, making it a more accessible target for optical investigations. Due to its weak radio emission, optical and infrared data are essential to probe the jet and outer disc behavior of IGR J17091-3624. This study presents the first long-term optical monitoring of IGR J17091-3624, using data from the Las Cumbres Observatory (LCO) over its 2011, 2016, and 2022 outbursts. We combine these observations with quasi-simultaneous X-ray data from Swift/XRT, RXTE, and NICER, employing light curve and variability analysis, spectral energy distributions, color-magnitude diagrams, and optical/X-ray correlations to investigate optical emission mechanisms. We find that the optical and X-ray fluxes are significantly correlated, following a power-law relation with the index 0.40\pm0.04, suggesting that the optical emission in IGR J17091-3624 is dominated by an X-ray-irradiated accretion disk. Based on optical spectral slope constraints, we estimate the extinction toward IGR J17091-3624 A_V = 4.3 to 6.6 mag. The global optical/X-ray correlation suggests a distance estimate of 8-17 kpc, in line with previous findings. High-cadence optical observations show tentative evidence of optical oscillations that may arise from reprocessed X-ray modulations, although confirming this will require higher time-resolution optical data.

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arXiv2025

Constraining the Jet Energetics of the Transient X-ray Binaries MAXI J1348-630 and MAXI J1820+070 through Calorimetry

We present Atacama Large Millimeter/Submillimeter Array (ALMA) observations aimed at identifying potential jet-ISM interaction sites in the vicinity of the transient black hole X-ray binaries MAXI J1348-630 and MAXI J1820+070, both of which have recently undergone an outburst, and displayed powerful large scale jets. Using this dataset, we construct molecular line emission maps. By analyzing the morphological, spectral, and kinematic properties of the detected emission, we identify a molecular structure that provides compelling evidence for a jet-driven cavity in the local environment of MAXI J1348-630 but find no significant emission in the local environment of MAXI J1820+070. We use the properties of the detected molecular emission surrounding MAXI J1348-630 to constrain the jet power, finding our results to be consistent with other independent studies of this source, and further validating the utility of astrochemistry for constraining jet energetics. Additionally, our findings provide the first assessment on the formation timescales for jet-ISM interaction regions in the transient black hole X-ray binary population.

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arXiv2025

A Hidden Pulse: Uncovering a New Timing Signal in Cygnus X-1 with AstroSat

The study of fast variability properties in X-ray binaries advances our understanding of the physical processes and geometric properties of the accretion flow around the compact object. In this work, we study the evolution of the timing properties of Cygnus X-1 with AstroSat/LAXPC, during the transition of the source from the hard to soft state in 2017. We use a novel frequency-segmented technique to fit simultaneously the cross spectra and parts of the power spectra and coherence function with a multi-Lorentzian model and predict the phase-lags and the complementary parts of the power spectra and coherence function. We study the evolution of the frequency and power of the main variability components that are present throughout all the states. In particular, we identify previously undetected variability components, one of which manifests as a narrow dip in the coherence function and a broad drop in the phase-lag spectrum at the same frequency. This dip in coherence, which we detected for the first time in Cygnus X-1 at energies above 3 keV, appears in a state in which the source shows high-amplitude radio variability and significant hard X-ray polarization. While the contribution of the compact jet in X-rays is debated in the literature, this study provides a new avenue for investigating jet properties as well as the geometry of the Comptonizing medium.

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arXiv2025

The Accretion-Ejection Connection in the Black Hole X-ray Binary MAXI J1820$+$070

The black hole X-ray binary MAXI J1820$+$070 began its first recorded outburst in March 2018, and remained an active radio, X-ray, and optical source for over four years. Due to the low distance to the source and its intrinsically high luminosity MAXI J1820$+$070 was observed extensively over this time period, resulting in high-cadence and quasi-simultaneous observations across the electromagnetic spectrum. These data sets provide the opportunity to probe the connection between accretion and the launch of jets in greater detail than for the majority of black hole X-ray binaries. In this work we present radio (Arcminute Microkelvin Imager Large Array, MeerKAT), X-ray (Swift), and optical (Las Cumbres Observatory) observations of MAXI J1820$+$070 throughout its entire outburst, including its initial hard state, subsequent soft state, and further hard-state-only re-brightenings (covering March 2018 to August 2022). Due to the regularity and temporal density of our observational data we are able to create a Radio - X-ray - Optical activity plane where we find a high degree of correlation between the three wave bands during the hard states, and observe hysteresis as MAXI J1820$+$070 enters and exits the soft state. Based on the morphology of the optical light curves we see evidence for optical jet contributions during the soft-to-hard state transition, as well as fading optical emission well before the hard to soft transition. We establish that the remarkably similar profiles of the re-brightening events are broadly consistent with modified disk instability models where irradiation from the inner accretion disk is included.

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arXiv2025

Evidence for enhanced mass transfer in the disc preceding the transition to the soft state in MAXI J1820+070

We investigate the 2018-2019 main outburst and the subsequent mini-outbursts of the black hole low-mass X-ray binary MAXI J1820+070 using optical/ultraviolet data from the Las Cumbres Observatory (LCO), the American Association of Variable Star Observers (AAVSO), and $\textit{Swift}$/UVOT, as well as X-ray data from $\textit{Insight}$-HXMT and $\textit{Swift}$/XRT. Given the high-cadence observations, we identify a broad dip-like feature in both the optical and X-ray light curves preceding the transition to the soft state, with the X-ray dip lagging the optical dip by approximately 10 days. We propose that the dip is caused by a brief decrease followed by an increase in the mass accretion rate as it propagates through the disc, ultimately triggering the transition to the soft state. This might be a potential tool to predict impending hard-to-soft state transitions, although such a dip has not yet been observed in many sources. Additionally, we find that optical colour ($g^{\prime}-i^{\prime}$) becomes bluer and less variable before the transition to the intermediate state, preceding a dramatic change in the hardness ratio. This appears to be an unusual case, differing from the typical scenario where the optical colour changes usually along with the transition to the soft state. Finally, we explore the implications of the complex evolution of optical/X-ray correlation during both main outbursts and mini-outbursts. In particular, we find a loop-like evolutionary track before the transition to the soft state, which is linked to the optical/X-ray dips in the light curves.

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arXiv2025

Comprehensive Radio Monitoring of the Black Hole X-ray Binary Swift J1727.8$-$1613 during its 2023$-$2024 Outburst

This work presents comprehensive multi-frequency radio monitoring of the black hole low-mass X-ray binary Swift J1727.8$-$1613, which underwent its first recorded outburst after its discovery in August 2023. Through a considerable community effort, we have coalesced the data from multiple, distinct observing programs; the light curves include ${\sim} 10$ months and 197 epochs of monitoring from 7 radio facilities with observing frequencies ranging from (approximately) 0.3$-$230GHz. The primary purpose of this work is to provide the broader astronomical community with these light curves to assist with the interpretation of other observing campaigns, particularly non-radio observing frequencies. We discuss the phenomenological evolution of the source, which included: (i) multiple radio flares consistent with the launching of discrete jet ejections, the brightest of which reached $\sim$ 1 Jy; (ii) temporally evolving radio spectral indices ($α$), reaching values steeper than expected for optically-thin synchrotron emission ($α{<} -1$) and emission with significant radiative cooling ($α< -1.5$). We have published a digital copy of the data and intend for this work to set a precedent for the community to continue releasing comprehensive radio light curves of future low-mass X-ray binary outbursts.

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arXiv2025

A Multi-wavelength Characterization of the 2023 Outburst of MAXI J1807+132: Manifestations of Disk Instability and Jet Emission

Several phenomenological aspects of low-luminosity neutron star transients, such as atolls, remain poorly understood. One such source, MAXI J1807+132, entered its latest outburst in July 2023. To thoroughly characterize this outburst, we conducted an extensive observational campaign spanning radio to X-ray wavelengths. Here, we present the results of this campaign, which covered the period from before the outburst to the return to quiescence. We detected a delay between the X-ray and optical rise times, which is consistent with the predictions of the disk instability model with a truncated disk. The color evolution and optical/X-ray correlations, along with infrared and radio detections, support the presence of jet synchrotron emission during the gradual decay phase following the peak. We also report for the first time in an X-ray binary a near-orthogonal rotation of the optical polarization just before a small flare, after which the jet is thought to be quenched. The main outburst is followed by several high-amplitude, rapid reflares in the optical, ultraviolet, and X-ray bands, the origin of which remains difficult to constrain.

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arXiv2025

Gone with the Wind: JWST-MIRI Unveils a Strong Outflow from the Quiescent Stellar-Mass Black Hole A0620-00

We present new observations of the black hole X-ray binary A0620-00 using the Mid-Infrared Instrument on the James Webb Space Telescope, during a state where the X-ray luminosity is 9 orders of magnitude below Eddington, and coordinated with radio, near-infrared and optical observations. The goal is to understand the nature of the excess mid-infrared (MIR) emission originally detected by Spitzer red-ward of 8 $μ$m. The stellar-subtracted MIR spectrum is well-modeled by a power law with a spectral index of $α=0.72\pm0.01$, where the flux density scales with frequency as $F_ν\propto ν^α$. The spectral characteristics, along with rapid variability--a 40% flux flare at 15$μ$m and 25% achromatic variability in the 5-12 $μ$m range--rule out a circumbinary disk as the source of the MIR excess. The Low Resolution Spectrometer reveals a prominent emission feature at 7.5 $μ$m, resulting from the blend of three hydrogen recombination lines. While the contribution from partially self-absorbed synchrotron radiation cannot be ruled out, we argue that thermal bremsstrahlung from a warm (a few $10^4$ K) wind accounts for the MIR excess; the same outflow is responsible for the emission lines. The inferred mass outflow rate indicates that the system's low luminosity is due to a substantial fraction of the mass supplied by the donor star being expelled through a wind rather than accreted onto the black hole.

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arXiv2025

The Ejection of Transient Jets in Swift J1727.8-1613 Revealed by Time-Dependent Visibility Modelling

High angular resolution radio observations of relativistic jets are necessary to understand the causal connection between accretion and jet ejection in low mass X-ray binaries. Images from these observations can be difficult to reconstruct due to the rapid intra-observational motion and variability of transient jets. We have developed a time-dependent visibility model fitting and self-calibration procedure and applied it to a single four-hour VLBA observation of the low-mass X-ray binary Swift J1727.8-1613 during the bright flaring period of its 2023 outburst. This allowed us to detect and model a slightly resolved self-absorbed compact core, as well as three downstream transient jet knots. We were able to precisely measure the proper motion and flux density variability of these three jet knots, as well as (for the first time) their intra-observational expansion. Using simultaneous multi-frequency data, we were also able to measure the spectral index of the furthest downstream jet knot, and the core, as well as the frequency-dependent core shift between 2.3 and 8.3 GHz. Using these measurements, we inferred the ejection dates of the three jet knots, including one to within $\pm40$ minutes, which is one of the most precise ever measured. The ejection of the transient jet knots coincided with a bright X-ray flare and a drastic change in the X-ray spectral and timing properties as seen by HXMT, which is the clearest association ever seen between the launching of transient relativistic jets in an X-ray binary and a sudden change in the X-ray properties of the accretion inflow.

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