Grupo de Astrofísica de Rayos X

NICER observations of Cygnus X-1 reveal a previously undetected type-C QPO, mainly observable in the imaginary part of the cross-spectrum. This "imaginary QPO" appears exclusively in intermediate states, coinciding with the soft-to-hard decay transition seen in LMXB outbursts. It manifests as a coherence drop and a steep phase-lag increase at ~1–6 Hz, requiring a narrow Lorentzian with a large phase lag to explain. Its properties closely match confirmed type-C QPOs in the LMXBs MAXI J1348-630 and MAXI J1820+070, suggesting that such QPOs may be more frequent across accreting black holes than previously recognized. These results highlight the power of Fourier-domain techniques in uncovering hidden variability components and suggest a deeper connection between black holes in HMXBs and LMXBs.

During the soft-to-hard transition of MAXI J1820+070, we observe an abrupt change in the lags and a narrow drop in the coherence. At the same frequency, we detect an “imaginary” QPO that explains the observed features and whose properties resemble those of type-C QPOs. Hidden in the power spectra, this QPO is unveiled when analysing the cross-spectrum. The study of the evolution of the imaginary QPO provides new insights into this rarely observed BH state transition.

We investigated NGC 4190 ULX-1, an ultraluminous X-ray source, using XMM-Newton, NICER, and NuSTAR. Through temporal and spectral analyses, we identified no pulsations. The ULX spectrum, resembling typical ULX emission, suggests a super-Eddington black hole, possibly obscured by a dense wind, with emission mechanisms explained by geometric beaming and Comptonization.

We studied the binary 4FGL J1405.1-6119, a high-mass gamma-ray emitter, with NuSTAR and XMM-Newton. We use a parabolic and slightly relativistic lepto-hadronic jet scenario to explain the spectral energy distribution, suggesting that 4FGL J1405.1-6119 could be a supercritical microquasar similar to SS433.

We studied the neutron-star binary GX 13+1 with NuSTAR, finding its transition from normal to flaring states. Spectral analysis revealed evidence of relativistic reflection, constraining inner disc radius and magnetic field strength. A hot wind with photo-ionised absorption of Fe and Ni was detected. The study suggests high electron densities in the accretion disk, challenging previous models and impacting our understanding of GX 13+1's physical conditions.

We studied the famous microquasar SS433 using 10 NuSTAR observations that span almost 2 precessional cycles of the system. Using a combination of a thermal leptonic jet and relativistic reflection from an accretion disk, we found that the precessional motion of the system successfully explains the modulation seen in several spectral parameters.

We introduce a dynamic Comptonization model, explaining low-frequency quasi-periodic oscillations in black hole LMXBs. Modeling the accretion disc as a multi-temperature blackbody, we compare it with a spherical blackbody model. Applying our model to BH LMXBs (e.g., MAXI J1438-630, GRS 1915 + 105) shows better fits and compatibility, affirming its ability to reproduce QPO characteristics.

The model is openly available on GitHub.

We explore the role of inverse Compton processes in shaping X-ray spectra in black hole low-mass X-ray binaries, focusing on GRS 1915+105. Our spectral-timing Comptonization model, applied to 398 observations, reveals consistent trends in corona properties over 15 years. Connecting with 15 GHz radio monitoring, we propose disc-corona interactions influencing the radio jet launch.