The obscured X-ray binaries V404 Cyg, Cyg X-3, V4641 Sgr, and GRS 1915+105

Aims. V404 Cyg, Cyg X-3, V4641 Sgr, and GRS 1915+105 are among the brightest X-ray binaries and display complex behavior in their multiwavelength emission. Except for Cyg X-3, the other three sources have large accretion disks, and there is evidence of a high orbital inclination. Therefore, any large-scale geometrical change in the accretion disk can cause local obscuration events. On the other hand, Cyg X-3 orbits its Wolf-Rayet companion star inside the heavy stellar wind obscuring the X-ray source. We study here whether the peculiar X-ray spectra observed from all four sources can be explained by local obscuration events.Methods. We used spectra obtained with the Nuclear Spectroscopic Telescope Array and Rossi X-ray Timing Explorer to study the spectral evolution of the four luminous hard X-ray sources. We fit the time-averaged spectra, and also time-resolved spectra in case of V404 Cyg, with two physically motivated models describing either a scenario where all the intrinsic emission is reprocessed in the surrounding matter or where the emitter is surrounded by a thick torus with variable opening angle.Results. We show that the X-ray spectra during specific times are very similar in all four sources, likely arising from the high-density environments where they are embedded. The fitted models suggest that a low-luminosity phase preceding an intense flaring episode in the 2015 outburst of V404 Cyg is heavily obscured, but intrinsically very bright (super-Eddington) accretion state. Similar spectral evolution to that of V404 Cyg is observed from the recent X-ray state of GRS 1915+105 that presented unusually low luminosity. The modeling results point to a geometry change in the (outflowing) obscuring matter in V404 Cyg and GRS 1915+105, which is also linked to the radio (jet) evolution. Within the framework of the models, all sources display obscured X-ray emission, but with different intrinsic luminosities ranging from lower than 1% of the Eddington luminosity up to the Eddington limit. This indicates that different factors cause the obscuration. This work highlights the importance of taking the reprocessing of the X-ray emission in the surrounding medium into account in modeling the X-ray spectra. This may well take place in other sources as well

Simultaneous NICER and NuSTAR Observations of the Ultracompact X-Ray Binary 4U 1543-624

We present the first joint NuSTAR and NICER observations of the ultracompact X-ray binary (UCXB) 4U 1543−624 obtained in 2020 April. The source was at a luminosity of L0.5−50 keV = 4.9(D/7 kpc)2 × 1036 erg s−1 and showed evidence of reflected emission in the form of an O viii line, Fe K line, and Compton hump within the spectrum. We used a full reflection model, known as xillverCO, that is tailored for the atypical abundances found in UCXBs, to account for the reflected emission. We tested the emission radii of the O and Fe line components and conclude that they originate from a common disk radius in the innermost region of the accretion disk (Rin ≤ 1.07 RISCO). Assuming that the compact accretor is a neutron star (NS) and the position of the inner disk is the Alfvén radius, we placed an upper limit on the magnetic field strength to be B ≤ 0.7(D/7 kpc) × 108 G at the poles. Given the lack of pulsations detected and position of Rin, it was likely that a boundary layer region had formed between the NS surface and inner edge of the accretion disk with an extent of 1.2 km. This implies a maximum radius of the neutron star accretor of RNS ≤ 12.1 km when assuming a canonical NS mass of 1.4 M⊙

The nova-like nebular optical spectrum of V404 Cygni at the beginning of the 2015 outburst decay

We report on FORS2 optical spectroscopy of the black hole X-ray binary V404 Cygni, performed at the very beginning of its 2015 outburst decay, complemented by quasi-simultaneous Swift X-ray and ultraviolet as well as Rapid Eye Mountain near-infrared observations. Its peculiar spectrum is dominated by a wealth of emission signatures of H i, He i, and higher ionization species, in particular Fe ii. The spectral features are divided between broad redshifted and narrow stationary varieties, the latter being emitted in the outer regions. Continuum and line variability at short time-scale is high, and we find Baldwin effect-like anticorrelations between the full widths at half-maximum and equivalent widths of the broad lines with their local continua. The Balmer decrement H ?/H ? is also abnormally large at 4.61 ± 0.62. We argue that these properties hint at the broad lines being optically thick and arising within a circumbinary component in which shocks between faster optically thick and slower optically thin regions may occur. We associate it to a nova-like nebula formed by the cooling remnant of strong accretion disc winds that turned off when the mass-accretion rate dropped following the last major flare. The Fe ii lines likely arise from the overlap region between this nebula and the companion star winds, whereas we favour the shocks within the nebula as responsible for the optical continuum via self-absorbed optically thin bremsstrahlung. The presence of a near-infrared excess also points towards the contribution of a strongly variable compact jet or a dusty component

Physical Constraints from Near-infrared Fast Photometry of the Black Hole Transient GX 339–4

We present results from the first multi-epoch X-ray/IR fast-photometry campaign on the black hole transient GX 339–4, during its 2015 outburst decay. We studied the evolution of the power spectral densities finding strong differences between the two bands. The X-ray power spectral density follows standard patterns of evolution, plausibly reflecting changes in the accretion flow. The IR power spectral density instead evolves very slowly, with a high-frequency break consistent with remaining constant at 0.63 ± 0.03 Hz throughout the campaign. We discuss this result in the context of the currently available models for the IR emission in black hole transients. While all models will need to be tested quantitatively against this unexpected constraint, we show that an IR-emitting relativistic jet that filters out the short-timescale fluctuations injected from the accretion inflow appears as the most plausible scenario