The geometry and physical conditions of black hole accretion discs are expected to change dramatically as a function of mass accretion rate and the resulting X-ray luminosity. Theoretical models predict that these changes become particularly relevant as the luminosity approaches the Eddington limit (LEdd), a regime where the standard thin-disc solution is thought to break down in...
Outbursts from transient Low-Mass X-ray Binaries (LMXBs) have been historically caught by All-Sky X-ray monitors at relatively high X-ray luminosities, i.e. above ~1e36 erg/s; observations of these sources during the early rise of the outbursts, the stage where the X-ray luminosity increases by three or more orders of magnitude with respect to the quiescence regime, have been so far very...
MAXI J1348-630 is a black hole X-ray transient discovered in January 2019 that underwent a reflare and several mini-outbursts at the end of its main outburst. In this talk, we present the first multiwavelength study of the system during outburst, reflare and mini-outbursts based on the long-term optical monitoring of the system with the Las Cumbres Observatory (LCO) network. The full optical...
The irradiation of the innermost accretion disk by X-rays emitted by the corona produces characteristic reflection features. Besides depending on the density and ionization of the disk itself, also the spin of the black hole and the corona itself is imprinted on the reflection spectrum due to strong relativistic effects. When fitting observational data of black hole X-ray binaries and AGN,...
Superluminal ejections were first reported from microquasars almost thirty years ago. Their apparent superluminal motion is due to discrete, relativistic, collimated ejections of plasma launched from the vicinity of the black hole, during hard-to-soft state transitions. It is suspected, but not definitively proven, that these jets are fed from particles in the highly energetic corona, rather...
The 2023/24 NICER monitoring campaign of the 7 Crab bright black hole X-ray binary Swift J1727.8-1613 covered the outburst in almost all states. High-quality data are available in the high-Eddington-fraction bright hard state, the soft intermediate state, the soft state decay, and the poorly understood back-transition into the dim hard state, making it an ideal dataset to compare the accretion...
Swift J1727.8-1613 is a recently discovered black hole low-mass X-ray binary (BH LMXB) that began its first recorded outburst in the autumn of 2023. The source rapidly became one of the brightest X-ray sources in the sky — reaching an X-ray flux nearly an order of magnitude greater than that of the Crab Nebula — and prompted a comprehensive multi-wavelength observing campaign. The radio...
Jet emitting disks (JEDs) provide the theoretical framework of mathematically exact steady-state accretion and ejection solutions. A large-scale vertical magnetic field threads a turbulent, strongly magnetized, accretion disk driving laminar, bipolar, super-Alfvénic jets. In previous treatments of JEDs, turbulence has been considered to provide only anomalous transport coefficients, namely...
Mini-HAWKs is a 50 sqr deg pathfinder of the Galactic Plane down to r≃21 that employs 3 custom Hα filters, optimized to select targets with very broad Hα emission lines. These are the hallmark of strong gravitational fields as they are typically formed in accretion discs around black holes (BHs). Mini-HAWKs will prove a novel photometric strategy that, when extended to the entire northern...
I will present the results of a unique multi-wavelength campaign focused on the recently discovered black-hole transient Swift J1727. This system exhibited the brightest X-ray source in the sky ever observed, and it was the first to be observed with the X-ray polarimeter (IXPE) throughout an entire outburst. Our observations covered simultaneously the full frequency range from X-ray to radio...
Measuring the Lorentz factors of jets from black holes is essential for understanding the jet launching mechanisms and overall energy budget of the black holes themselves. However, this is a notoriously difficult task due to special relativistic effects which are exacerbated for off-axis sources, namely X-ray binary (XRB) jets. In fact, from kinematics alone we find that, remarkably, we can...
Matter accreting onto the surface of a neutron star can ignite in a thermonuclear explosion visible as a ~1-min long bright flash of X-ray emission. Such thermonuclear X-ray bursts radiate a total energy of ~E39 erg and can recur on a timescale of hours. Every time an explosion goes off, a strong radiation field suddenly floods the direct environment of the neutron star and observations have...
Currently, the only accretion flows than can be imaged directly -- supermassive black holes (BHs) at the centres of galaxies via EHT -- evolve on timescales much greater than a human lifetime. In contrast, X-ray binaries (XRBs) in our Galaxy provide ideal observational laboratories. Their frequently recurring bright outbursts present a unique opportunity to study an evolving astrophysical...
Astrophysical systems housing compact objects produce high-energy transient events that can be leveraged to study the processes of accretion and jet ejection. In particular, thermonuclear fusion on the surface of accreting neutron stars produces bursts of X-ray radiation that not only strongly impact the accretion structures in these systems, but as a recent novel experiment has shown, can...
The geometry of the accretion flow around accreting black holes is highly variable and remains highly unconstrained. In their hard states, the X-ray spectrum is dominated by a hot inflow located above or within the accretion disk. Conversely, in their soft states, the spectrum is dominated by soft emission from the geometrically thin accretion disk. The geometry of the hot inflow and its...
Accreting low-mass X-ray binaries (LMXBs) provide a unique testbed to investigate the complex interplay between accretion flows and relativistic jets. In both neutron star and black hole systems LMXBs, X-ray and radio emissions are observed to be correlated, though neutron star systems exhibit significantly lower radio fluxes at comparable X-ray luminosities. Among neutron star LMXBs,...
4U 1630-47 is a peculiar transient X-ray binary believed to host a black hole as its compact object. Its behavior differs from that of other BH-XRBs, as it shows recurrent outbursts with a fairly regular recurrence period and lacks bright hard states. In contrast, its soft-state spectra above 10 keV are characterized by a strong power-law component extending up to 200 keV without any...
We report new constraints on the distance to 4U 1630−47 by analyzing its dust scattering halo (DSH) using high-resolution X-ray (Chandra) and millimeter (APEX) observations. DSHs, produced when X-rays scatter off interstellar dust, encode information about both the source distance and the distribution of intervening material. Previous distance estimates for 4U 1630−47 (4.9 kpc and 11.5 kpc)...
In recent years, spins of merging black holes have been relatively accurately measured based on their gravitational-wave signals. Their are generally low, with the estimated average effective spin parameter as low as 0.06. On the other hand, spins of many accreting black-hole binaries have been measured to be high, some close to the maximum spin parameter of 1, e.g. greater than 0.9985 at 3...
