Super-massive black holes in active galaxies can accelerate particles to relativistic energies, producing jets with associated gamma-ray emission. Galactic 'microquasars', which are binary systems consisting of a neutron star or stellar-mass black hole accreting gas from a companion star, also produce relativistic jets, generally together with radio flares. Apart from an isolated event detected in Cygnus X-1, there has hitherto been no systematic evidence for the acceleration of particles to gigaelectronvolt or higher energies in a microquasar, with the consequence that we are as yet unsure about the mechanism of jet energization. Here we report four gamma-ray flares with energies above 100 MeV from the microquasar Cygnus X-3 (an exceptional X-ray binary that sporadically produces radio jets). There is a clear pattern of temporal correlations between the gamma-ray flares and transitional spectral states of the radio-frequency and X-ray emission. Particle acceleration occurred a few days before radio-jet ejections for two of the four flares, meaning that the process of jet formation implies the production of very energetic particles. In Cygnus X-3, particle energies during the flares can be thousands of times higher than during quiescent states.

Cygnus X‐3 is one of the brightest X‐ray and radio sources in the Galaxy and is well known for its erratic behaviour in X‐rays as well as in the radio, occasionally producing major radio flares associated with relativistic ejections. However, even after many years of observations in various wavelength bands Cyg X‐3 still eludes clear physical understanding. Studying different emission bands simultaneously in microquasars has proved to be a fruitful approach towards understanding these systems, especially by shedding light on the accretion disc/jet connection. We continue this legacy by constructing a hardness–intensity diagram (HID) from archival Rossi X‐ray Timing Explorer data and linking simultaneous radio observations to it. We find that surprisingly Cyg X‐3 sketches a similar shape in the HID to that seen in other transient black hole X‐ray binaries during outburst but with distinct differences. Together with the results of this analysis and previous studies of Cyg X‐3, we conclude that the X‐ray states can be assigned to six distinct states. This categorization relies heavily on the simultaneous radio observations and we identify one new X‐ray state, the hypersoft state, similar to the ultrasoft state, which is associated with the quenched radio state during which there is no or very faint radio emission. Recent observations of GeV flux observed from Cyg X‐3 during a soft X‐ray and/or radio quenched state at the onset of a major radio flare hint that a very energetic process is at work during this time, which is also when the hypersoft X‐ray state is observed. In addition, Cyg X‐3 shows flaring with a wide range of hardness.

Aims. We address the problem where the X-ray emission lines are formed and investigate orbital dynamics using Chandra HETG observations, photoionizing calculations and numerical wind-particle simulations. The aims were to set constraints on the masses of the components of this close binary system consisting of a Wolf-Rayet (WR) star and a compact component and to investigate the nature of the latter (neutron star or black hole). The goal was also to investigate P Cygni signatures in line profiles.Methods. The observed Si xiv (6.185 Å) and S xvi (4.733 Å) line profiles at four orbital phases were fitted with P Cygni-type profiles consisting of an emission and a blue-shifted absorption component. Numerical models were constructed using photoionizing calculations and particle simulations. In the models, the emission originates in the photoionized wind of the WR companion illuminated by a hybrid source: the X-ray radiation of the compact star and the photospheric EUV-radiation from the WR star.Results. Spectral lines with moderate excitation (such as Si xiv and S xvi) arise in the photoionized wind. The emission component exhibits maximum blue-shift at phase 0.5 (when the compact star is in front), while the velocity of the absorption component is constant (around −900 km s −1 ). Both components, like the continuum flux, have intensity maxima around phase 0.5. The simulated Fe xxvi Lyα line (1.78 Å, H-like) from the wind is weak compared to the observed one. We suggest that it originates in the vicinity of the compact star, with a maximum blue shift at phase 0.25 (compact star approaching). By combining the mass function derived with that from the infrared He i absorption (arising from the WR companion), we constrain the masses and the inclination of the system. Conclusions. The Si xiv and S xvi lines and their radial velocity curves can be understood in the framework of a photoionized wind involving a hybrid ionizer. Constraints on the compact star mass and orbital inclination (i) are given using the mass functions derived from the Fe xxvi line and He i 2.06 μm absorption. Both a neutron star at large inclination (i ≥ 60 degrees) and a black hole at small inclination are possible solutions. The radial velocity amplitude of the He ii 2.09 μm emission (formed in the X-ray shadow behind the WR star) suggests i = 30 degrees, implying a possible compact star mass between 2.8-8.0 M . For i = 60 degrees the same range is 1.0-3.2 M .

Accreting black holes are responsible for producing the fastest, most powerful outflows of matter in the universe. The formation process of powerful jets close to black holes is poorly understood, and the conditions leading to jet formation are currently hotly debated. In this paper, we report an unambiguous empirical correlation between the properties of the plasma close to the black hole and the particle acceleration properties within jets launched from the central regions of accreting stellar-mass and supermassive black holes. In these sources the emission of the plasma near the black hole is characterized by a power law at X-ray energies during times when the jets are produced. We find that the photon index of this power law, which gives information on the underlying particle distribution, correlates with the characteristic break frequency in the jet spectrum, which is dependent on magnetohydrodynamical processes in the outflow. The observed range in break frequencies varies by five orders of magnitude in sources that span nine orders of magnitude in black hole mass, revealing a similarity of jet properties over a large range of black hole masses powering these jets. This correlation demonstrates that the internal properties of the jet rely most critically on the conditions of the plasma close to the black hole, rather than other parameters such as the black hole mass or spin, and will provide a benchmark that should be reproduced by the jet formation models.