arXiv subjects

F. M. Vincentelli

Publications and source records attributed to F. M. Vincentelli.

At least 19 recordsLinked to original sources

arXiv2026

The strength of Type-C quasi-periodic oscillations in black hole X-ray binaries correlates with the jet inclination

X-ray quasi-periodic oscillations (QPOs) are a characteristic feature of low-mass X-ray binaries (LMXBs). These oscillations have been studied for decades and revealed a rich and complex phenomenology that is still not fully understood. RXTE archival studies have shown that the amplitude of these oscillations differs significantly between black holes (BH) with high or low inclination. Yet, the actual dependence on inclination has never been adequately estimated. Thanks to the improvement of inclination measurements through radio observations and the recent observations by the HXMT satellite, we quantified for the first time the dependence of Type-C QPO amplitudes on the jet inclination of individual BH LMXBs. Our analysis reveals the presence of a significant linear correlation up to 8 Hz, strengthening the case for a ''geometrical'' origin of the QPOs. In addition, for a given QPO frequency, we observe systematically lower amplitudes during the decay of outbursts compared to the rise. This data collection represents a key benchmark for any QPO model. Our comparison with the predictions from a precessing hot flow shows that the amplitude of the QPOs can be reproduced by this scenario if the spin-orbit misalignment is at least $\approx$10-15$^\circ$.

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arXiv2026

Even a precessing clock is right twice per orbit -- The super-periods of eRO-QPE2 and challenges for quasi-periodic eruption orbital models

We present O$-$C (``observed minus calculated'') timing analysis of the quasi-periodic eruption (QPE) source eRO-QPE2 with a multi-mission X-ray campaign, which includes 32 observed eruptions spanning a month (i.e. 325 QPE cycles). In relation to accretion (e.g. disk instability) models, the O-C is consistent with a damped random walk of the QPE recurrence, albeit with highly uncertain parameters. If instead an underlying orbital clock is present, eRO-QPE2 is consistent with a period of $P \sim 2.24$\,h and two hierarchical super-periodic modulations, with periods of $\sim 4.4$\,d ($\sim47$\,P) and $\approx 95$\,d ($\approx 1000$\,P). We found no negative period derivative, with $|\dot{P}| \lesssim 2 \times 10^{-6}$\,s/s at $3σ$. This disfavors high-eccentricity WDs and high-mass/eccentricity IMBHs via GW decay. For disk-collision models, where the $\dot{P}$ from gas drag and the QPE integrated energy provide bounds on the local disk density, a main-sequence star is disfavored as EMRI secondary unless stellar debris streams are present, while stripped stars remain allowed. The correlated odd/even O-C disfavors both disk crossings per orbit being observed. Interpreting the data with one \emph{observed} event per orbit, the short modulation is consistent with apsidal precession for $a \sim 140\,R_g$, $e \approx 0.1$, and $M_{\rm BH} \approx 1.5 \times 10^{5}\,M_\odot$. The longer modulation (much less constrained) is inconsistent with EMRI nodal precession and disk precession is allowed for a limited parameter volume, while there is a solution with a stable hierarchical triple system with an outer massive black hole at $\sim 0.4\,\mathrm{mpc}$ and mass $\sim(0.1-1) \times M_{\rm BH}$. However, no reliable solution can be found with more robust EMRI trajectory models, possibly due to narrow likelihood peaks in a multi-dimensional parameter space with sparse data.

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arXiv2026

Optical outburst evolution of the transient black hole X-ray binary Swift J1727.8-1613: Disc response to jet ejections and late-outburst emergence of powerful disc winds

Swift J1727.8$-$1613 is a newly discovered transient low-mass X-ray binary harbouring a stellar-mass ($\sim 10M_\odot$) black hole. We present state-resolved VLT/X-Shooter optical spectroscopy of its 2023 outburst, sampling the luminous hard-to-soft and late soft-to-hard transitions. During the onset of the brightest radio flare, He\,\textsc{ii} flux rises relative to adjacent epochs, with reduced peak-to-peak separation and full-width-half-maximum, consistent with enhanced irradiation shifting line emissivity to larger radii. We detect no contemporaneous change in the line base tracing the inner disc. The most dramatic change occurs at the onset of the dim-hard state, when strong, broad (higher-order) Balmer lines appear in absorption, and He\,\textsc{ii} remains in emission, but becomes highly asymmetric. While the hardening of the X-ray spectrum likely promotes the reappearance of an underlying disc photosphere, the kinematic alignment between the Balmer absorption ($v_w\sim-750\,\mathrm{km\,s^{-1}}$) and the suppressed blue peak of He\,\textsc{ii} suggests a unified origin in a massive, cool ($T\lesssim10^{4}\,\mathrm{K}$) accretion disc wind. Radiative transfer simulations demonstrate that such asymmetric He\,\textsc{ii} profiles are naturally produced in a rotating and accelerating outflow. Using the Sobolev approximation, we estimate the wind mass-loss rate to be $\dot{M}_w\gtrsim10^{-9}\,M_\odot\,\mathrm{yr^{-1}}$, comparable to the instantaneous accretion rate and a significant fraction of the secular mass-transfer rate from the donor. If persistent at quiescent-level X-ray luminosities, this outflow could strongly impact the system's secular evolution.

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arXiv2026

A solution to the tension of burning on neutron stars and nuclear physics

When neutron stars accrete matter from a companion star, this matter forms a disc around them and eventually falls on their surface. Here, the fuel can ignite into bright flashes called Type I bursts. Theoretical calculations based on state-of-the-art nuclear reactions are able to explain many features of the bursts. However, models predict that the bursts should cease at high accretion rates, whereas in many sources they disappear at much lower rates. Moreover, their recurrence times also show strong discrepancies with predictions. Although various solutions have been proposed, none can account for all the observational constraints. Here we describe a new model that explains all the contradictory behaviours within a single picture. We are able to reconstruct the conditions on the star surface that determine the burst properties by comparing data to new simulations. We find strong evidence that the physical mechanism driving the burst behaviour is the structure of the accretion disc in the regions closest to the star. This connection reconciles the puzzling burst phenomenology with nuclear physics and also opens a new window on the study of accretion processes around compact objects.

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arXiv2025

Accretion and Ejection Physics at High Time Resolution

Accretion onto compact objects is one of the most fundamental phenomena in the astrophysics, powering some of the most luminous objects in the sky. Along with this, accretion has also a key impact on the evolution of the Universe, through the launch of powerful outflows that affect the surrounding medium. In the last years sub-second optical-infrared observations of accreting X-ray binaries have opened a new window in the study of inflow-outflow connection, discovering a wide range of previously unkown phenomena. Here we review the key open questions in accretion and ejection physics and discuss how a dedicated facility, equipped with photon-counting detectors and high spectral resolution from the UV to the mid-infrared, can enable transformative advances in our understanding of accretion processes.

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arXiv2025

Intensive X-ray/UVOIR continuum reverberation mapping of the Seyfert AGN MCG+08-11-11

We present results from intensive (x3 daily), three-month-long X-ray, UV and optical monitoring of the bright Seyfert active galactic nucleus (AGN) MCG+08-11-11 with Swift, supported by optical-infrared ground-based monitoring. The 12 resultant, well-sampled, lightcurves are highly correlated; in particular, the X-ray to UV correlation r_max = 0.85 is, as far as we know, the highest yet recorded in a Seyfert galaxy. The lags increase with wavelength, as expected from reprocessing of central high-energy emission by surrounding material. Our lag spectrum is much shallower than that obtained from an optical monitoring campaign conducted a year earlier when MCG+08-11-11 was approximately 4 times brighter. After filtering out long-term trends in the earlier optical lightcurves we recover shorter lags consistent with our own - demonstrating concurrent reverberation signals from different spatial scales and the luminosity dependence of the measured lags. We use our lag spectrum to test several physical models, finding that disc reprocessing models cannot account for the observed 'excess' lags in the u and r-i-bands that are highly indicative of the Balmer and Paschen continua produced by reprocessing in the broad line region (BLR) gas. The structure seen in both the variable (rms) and lag spectra, and the large time delay between X-ray and UV variations (approximately 2 days) all suggest that the BLR is the dominant reprocessor. The hard X-ray spectrum (Gamma approximately 1.7) and faint, red, UV-optical spectrum both indicate that the Eddington accretion ratio is low: approximately 0.03. The bolometric luminosity then requires that the black hole mass is substantially greater than current reverberation mapping derived estimates.

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arXiv2025

Tracking optical variability and outflows across the accretion states of the black hole transient MAXI J1820+070

We present a study of the minute-timescale optical variability and spectroscopic outflow signatures in the black hole X-ray binary MAXI J1820+070 during its 2018 outburst and re-brightenings. Minute-cadence, multi-filter optical light curves were obtained with the Las Cumbres Observatory network and the Al Sadeem Observatory (UAE) over 2018-2020, complemented by X-ray data from Swift/BAT, XRT, and MAXI. We also acquired contemporaneous low-resolution optical spectra with the 2.1 m OAN San Pedro Martir and OAGH Cananea telescopes (Mexico) and the 1.5 m G.D. Cassini telescope at Loiano (Italy). The optical fractional rms peaks in the hard state and is dominated by short-timescale flickering that strengthens toward longer wavelengths, suggesting that the variability is jet-driven. In this scenario, inflow fluctuations inject velocity perturbations at the jet base (internal shock model). The variability is quenched in the soft state, with any residual signal likely linked to accretion-flow fluctuations. This behaviour supports the accretion-ejection coupling in black hole binaries and shows that the jet's variability signature extends to optical wavelengths in all hard states. In the faintest hard states, residual optical variability may instead trace cyclo-synchrotron emission from the hot flow. The spectra show double-peaked emission lines and tentative cold-wind signatures during the hard state. Such winds were reported during the main 2018 outburst; here we find evidence of their presence also in later re-brightenings. Their absence in the soft state likely reflects stronger disc ionisation from the higher X-ray flux, suppressing low-ionisation optical features.

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arXiv2025

Infrared Synchrotron Emission in the Soft State of GX 339-4 and the Mid-Infrared/X-ray Luminosity Plane of Black Hole X-ray Binaries

Progress in understanding the growth of accreting black holes remains hampered by a lack of sensitive coordinated multiwavelength observations. In particular, the mid-infrared (MIR) regime remains ill-explored except for jet-dominant states. Here, we present comprehensive follow-up of the black hole X-ray binary GX 339-4 during a disc-dominated state in its 2023/24 outburst as part of a multi-wavelength campaign coordinated around JWST/MIRI. The X-ray properties are fairly typical of soft accretion states, with a high-energy Comptonised tail. The source is significantly detected between 5-10$μ$m, albeit at a faint flux level requiring MIR compact jet emission to be quenched by a factor of $\sim$300 or more relative to previous hard-state detections. The MIRI spectrum can be described as a simple power-law with slope $α$ = +0.39$\pm$0.07 ($F_ν$ $\propto$ $ν^α$), but surprisingly matches neither the radio/sub-mm nor the optical broadband slopes. Significant MIR stochastic variability is detected. Synchrotron radiation from the same medium responsible for high-energy Comptonisation can self-consistently account for the observed MIRI spectral-timing behaviour, offering new constraints on the physical conditions in the soft-state accretion disc atmosphere/corona. Alternative explanations, including a circumbinary disc or emission from a warm wind, fail to cleanly explain either the spectral properties or the variability. Multiwavelength timing cross-correlations show a puzzlingly long MIR lag relative to the optical, though at limited significance. We compile archival MIR and X-ray luminosities of transient black hole systems, including previously unreported detections of GX 339-4. These trace the evolution of the MIR-to-X-ray flux ratio with accretion state, and also reveal high MIR luminosities for GX 339-4 across all states. (abridged)

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arXiv2025

Bridging the Gap: OPTICAM Reveals the Hidden Spin of the WZ Sge Star GOTO 065054.49+593624.51

WZ Sge stars are highly evolved accreting white dwarf systems (AWDs) exhibiting remarkably large amplitude outbursts (a.k.a. super-outbursts), typically followed by short rebrightenings/echo outbursts. These systems have some of the lowest mass transfer rates among AWDs, making even low magnetic fields dynamically important. Such magnetic fields are often invoked to explain the phenomenology observed in these systems, such as their X-ray luminosity and long periods of quiescence (30+ years). However, the detection of these is very elusive given the quenching of the accretion columns during outburst and the low luminosity of these systems during quiescence. Here we present high-cadence multi-band observations with {\it OPTICAM} of the recent outburst of the recently discovered WZ Sge star GOTO065054.49+593624.51, during the end of the main outburst and the dip in-between rebrightenings, covering 2 orders of magnitude in brightness. Our observations reveal the presence of a statistically significant signal with $P_ω\simeq148$ seconds in the bluer ($g$) band which is detected only during the dip between the main outburst and the rebrigthenings. We interpret this signal as the spin period of the AWD. If confirmed, GOTO 0650 would bridge the gap between intermediate- and fast-rotating intermediate polars (IPs) below the period gap.

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arXiv2025

Sub-second optical/near-infrared quasi-periodic oscillations from the black hole X-ray transient Swift J1727.8-1613

We report on the detection of optical/near-infrared (O-IR) quasi-periodic oscillations (QPOs) from the black hole X-ray transient Swift J1727.8-1613. We obtained three X-ray and O-IR high-time-resolution observations of the source during its intermediate state (2023 September 9, 15 and 17) using NICER, HAWK-I@VLT, HIPERCAM@GTC and ULTRACAM@NTT. We clearly detected a QPO in the X-ray and O-IR bands during all three epochs. The QPO evolved, drifting from 1.4 Hz in the first epoch, up to 2.2 Hz in the second and finally reaching 4.2 Hz at the third epoch. These are among the highest O-IR QPO frequencies detected for a black hole X-ray transient. During the first two epochs, the X-ray and O-IR emission are correlated, with an optical lag (compared to the X-rays) varying from +70 ms to 0 ms. Finally, during the third epoch, we measured for the first time, a lag of the $z_s$-band respect to the $g_s$-band at the QPO frequency ($\approx$+10 ms). By estimating the variable O-IR SED we find that the emission is most likely non-thermal. Current state-of-the-art models can explain some of these properties, but neither the jet nor the hot flow model can easily explain the observed evolution of the QPOs. While this allowed us to put tight constraints on these components, more frequent coverage of the state transition with fast multi-wavelength observations is still needed to fully understand the evolution of the disc/jet properties in BH LMXBs.

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arXiv2025

Flip-flop QPO changes during state transitions: a case study of GX339-4 and theoretical discussion

We analyse the 2021 outburst from the black hole X-ray binary GX339-4 observed by NICER around the hard to soft transition, when the system exhibits flip-flops between two distinct luminosity states: a bright state with a 5-6 Hz quasi-periodic oscillation (QPO) and a dim state showing only strong broadband noise. Despite the marked differences in variability patterns between these states, the spectral energy distributions remain strikingly similar, with only minor changes in the black body component in the soft X-ray range. We find that the QPO frequency correlates with the X-ray count rates and hardness, suggesting a tight coupling between the QPO mechanism and the accretion disc's spectral properties. Additionally, we demonstrate that flip-flops can occur on very short timescales, with almost 50 state changes within ~1200 s, while both states can also remain stable over longer periods (at least 1000 s). We explore various QPO models to explain these observations, including the possibility that the corona's accretion speed is near the sound speed, affecting the presence of QPOs. However, the exact mechanism driving the flip-flops and the QPOs remains unclear. Our findings emphasize the complexity of these phenomena and the necessity for further theoretical and observational studies to unravel the intricacies of QPO and flip-flop behaviours in X-ray binaries.

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arXiv2025

Accretion disc winds imprint distinct signatures in the optical variability spectrum of black hole transients

Quantifying the variability, measured as the root mean square (rms), of accreting systems as a function of energy is a powerful tool for constraining the physical properties of these objects. Here, we present the first application of this method to optical spectra of low-mass X-ray binaries. We use high-time-resolution data of the black hole transient V404 Cygni, obtained with the \textit{Gran Telescopio Canarias} during its 2015 outburst. During this event, conspicuous wind-related features, such as P-Cygni profiles, were detected in the flux spectra. We find that rms spectra are rich in spectral features, although they are typically morphologically different from their flux counterparts. Specifically, we typically observe absorption components in correspondence to the presence of emission lines in the flux spectra. Similarly, when analysing segments with significant variability in the optical flux, P-Cygni line profiles appear inverted in the rms spectra (i.e., enhanced variability in the blue-shifted region, accompanied by a decrease in that associated with the red component). We discuss the possible origin of these features, which resemble those found in other objects, such as active galactic nuclei. Finally, we highlight the potential of this technique for future searches for wind-type outflows in accreting compact objects.

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arXiv2024

X-ray and optical polarization aligned with the radio jet ejecta in GX 339-4

We present the first X-ray polarization measurements of GX 339-4. IXPE observed this source twice during its 2023-2024 outburst, once in the soft-intermediate state and again during a soft state. The observation taken during the intermediate state shows significant ($4σ$) polarization degree P = $1.3\% \pm 0.3\%$ and polarization angle $θ$ = -74\degree $\pm$ 7\degree only in the 3 - 8 keV band. FORS2 at VLT observed the source simultaneously detecting optical polarization in the B, V, R, I bands (between $0.1%$ and $0.7\%$), all roughly aligned with the X-ray polarization. We also detect a discrete jet knot from radio observations taken later in time; this knot would have been ejected from the system around the same time as the hard-to-soft X-ray state transition and a bright radio flare occurred $\sim$3 months earlier. The proper motion of the jet knot provides a direct measurement of the jet orientation angle on the plane of the sky at the time of the ejection. We find that both the X-ray and optical polarization angles are aligned with the direction of the ballistic jet.

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arXiv2024

Rapid Mid-Infrared Spectral-Timing with JWST. I. The prototypical black hole X-ray Binary GRS 1915+105 during a MIR-bright and X-ray-obscured state

We present mid-infrared (MIR) spectral-timing measurements of the prototypical Galactic microquasar GRS 1915+105. The source was observed with the Mid-Infrared Instrument (MIRI) onboard JWST in June 2023 at a MIR luminosity L(MIR)~10^{36} erg/s exceeding past IR levels by about a factor of 10. By contrast, the X-ray flux is much fainter than the historical average, in the source's now-persistent 'obscured' state. The MIRI low-resolution spectrum shows a plethora of emission lines, the strongest of which are consistent with recombination in the hydrogen Pfund (Pf) series and higher. Low amplitude (~1%) but highly significant peak-to-peak photometric variability is found on timescales of ~1,000 s. The brightest Pf(6-5) emission line lags the continuum. Though difficult to constrain accurately, this lag is commensurate with light-travel timescales across the outer accretion disc or with expected recombination timescales inferred from emission line diagnostics. Using the emission line as a bolometric indicator suggests a moderate (~5-30% Eddington) intrinsic accretion rate. Multiwavelength monitoring shows that JWST caught the source close in-time to unprecedentedly bright MIR and radio long-term flaring. Assuming a thermal bremsstrahlung origin for the MIRI continuum suggests an unsustainably high mass-loss rate during this time unless the wind remains bound, though other possible origins cannot be ruled out. PAH features previously detected with Spitzer are now less clear in the MIRI data, arguing for possible destruction of dust in the interim. These results provide a preview of new parameter space for exploring MIR spectral-timing in XRBs and other variable cosmic sources on rapid timescales.

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arXiv2023

Shedding far-ultraviolet light on the donor star and evolutionary state of the neutron-star LMXB Swift J1858.6-0814

The evolution of accreting X-ray binary systems is closely coupled to the properties of their donor stars. As a result, we can constrain the evolutionary track a system is by establishing the nature of its donor. Here, we present far-UV spectroscopy of the transient neutron-star low-mass X-ray binary Swift J1858 in three different accretion states (low-hard, high-hard and soft). All of these spectra exhibit anomalous N\,{\sc v}, C\,{\sc iv}, Si\,{\sc iv} and He\,{\sc ii} lines, suggesting that its donor star has undergone CNO processing. We also determine the donor's effective temperature, $T_{d} \simeq 5700$~K, and radius, $R_d \simeq 1.7~R_{\odot}$, based on photometric observations obtained during quiescence. Lastly, we leverage the transient nature of the system to set an upper limit of $\dot{M}_{\rm acc} \lesssim 10^{-8.5}~M_{\odot}~yr^{-1}$ on the present-day mass-transfer rate. Combining all these with the orbital period of the system, $P_{\rm orb} = 21.3$~hrs, we search for viable evolution paths. The initial donor masses in the allowed solutions span the range $1~M_{\odot} \lesssim M_{d,i} \lesssim 3.5~M_{\odot}$. All but the lowest masses in this range are consistent with the strong CNO-processing signature in the UV line ratios. The present-day donor mass in the permitted tracks are $0.5~M_{\odot}\lesssim M_{d,obs} \lesssim 1.3~M_{\odot}$, higher than suggested by recent eclipse modelling. Since $P_{\rm orb}$ is close to the so-called bifurcation period, both converging and diverging binary tracks are permitted. If Swift J1858 is on a converging track, it will end its life as an ultra-compact system with a sub-stellar donor star.

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arXiv2023

Sub-second infrared variability from the archetypal accreting neutron star 4U~1728-34

We report on the first simultaneous high-time resolution X-ray and infrared (IR) observations of a neutron star low mass X-ray binary in its hard state. We performed $\approx 2\,$h of simultaneous observations of 4U 1728-34 using HAWK-I@VLT, XMM-Newton and NuSTAR. The source displayed significant X-ray and IR variability down to sub-second timescales. By measuring the cross-correlation function between the infrared and X-ray lightcurves, we discovered a significant correlation with an infrared lead of $\approx 30-40\,$ms with respect to the X-rays. We analysed the X-ray energy dependence of the lag, finding a marginal increase towards higher energies. Given the sign of the lag, we interpret this as possible evidence of Comptonization from external seed photons. We discuss the origin of the IR seed photons in terms of cyclo-synchrotron radiation from an extended hot flow. Finally, we also observed the IR counterpart of a type-I X-ray burst, with a delay of $\approx7.2\,$s. Although some additional effects may be at play, by assuming that this lag is due to light travel time between the central object and the companion star, we find that 4U 1728-34 must have an orbital period longer than $3\,$h and an inclination higher than 8$^\circ$.

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arXiv2023

A shared accretion instability for black holes and neutron stars

Accretion disks around compact objects are expected to enter an unstable phase at high luminosity. One instability may occur when the radiation pressure generated by accretion modifies the disk viscosity, resulting in the cyclic depletion and refilling of the inner disk on short timescales. Such a scenario, however, has only been quantitatively verified for a single stellar-mass black hole. Although there are hints of these cycles in a few isolated cases, their apparent absence in the variable emission of most bright accreting neutron stars and black holes has been a lingering puzzle. Here we report the presence of the same multiwavelength instability around an accreting neutron star. Moreover, we show that the variability across the electromagnetic spectrum-from radio to X-ray-of both black holes and neutron stars at high accretion rates can be explained consistently if the accretion disks are unstable, producing relativistic ejections during transitions that deplete or refill the inner disk. Such new association allows us to identify the main physical components responsible for the fast multiwavelength variability of highly accreting compact objects.

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arXiv2023

The evolution of the UV/optical lag spectrum of NGC 7469 seen by the Liverpool Telescope

We present the results regarding the analysis of an intensive monitoring of the Active Galactic Nucleus (AGN) NGC 7469. We observed the source for 4 months with almost daily cadence in the ugriz bands, using the IO:O on the Liverpool Telescope. We measured the lags with respect to the u band and found a clear change of the lag spectrum between the first and the second half of the campaign. Given that the source varies on different timescales during these two segments, it is likely that different components are dominating the variability at different times. This result further confirms that reverberation models require a more complex geometry than a static illuminating point source and that particular attention has to be given in the interpretation of these delays.

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