Grupo de Astrofísica de Rayos X

Ultraluminous X-ray sources (ULXs) continually push the boundaries of our understanding of extreme astrophysics. Located on the outskirts of local galaxies, these sources emit radiation at levels far exceeding the Eddington limit for typical stellar-mass black holes. Initially classified as elusive intermediate-mass black holes due to their unusual nature, the groundbreaking discovery of X-ray pulsations in several ULXs revealed that many of these cosmic engines are actually neutron stars. In order to explain how they achieve this extreme super-Eddington accretion, models often invoke either intense geometric beaming or colossal magnetic fields, or both. However, finding direct observational evidence of these magnetic fields on the surface of ULXs has proven remarkably difficult. In this work, we present an essential observational study of these extreme environments. By analysing the X-ray spectrum of the NGC 4656 ULX-1 source, we discovered a distinct, narrow absorption feature around 3.29 keV, alongside a strong pulsation candidate at 0.97 Hz. After rigorously testing this feature against different continuum models and confirming its statistical significance, we interpret this absorption as a proton cyclotron resonant scattering feature. This discovery is significant: it suggests the existence of an extremely strong local magnetic field near the surface of the neutron star. As this is only the second ULX neutron star in which such a feature has been found, our findings provide compelling, direct evidence of a link between magnetar-strength fields and the extreme super-Eddington regimes observed in ULXs.

In X-ray binary systems, accretion onto compact objects generates significant variability across a broad frequency spectrum. This work examines the phenomenology of such accretion, where thermal disk emission is processed via Comptonization within a hot electron corona.

By employing Fourier analysis techniques, spectral-timing studies allow for the measurement of lags and signal coherence, revealing critical information regarding the flow geometry and radiation propagation in strong gravitational fields. In this Invited Article of the Bulletin of the Argentine Astronomical Society, we present recent results derived from the development of a time-dependent Comptonization model, which is essential for inferring the physical properties of the accreted material.

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 present a spectral and timing analysis of two ULXs in ESO 501-023 and IC 5052 using XMM-Newton and NuSTAR data. No pulsations have been detected (PF < 11%). Both sources exhibit typical ULX spectra, well described by two thermal components plus a high-energy comptonization tail. Based on comparison with a sample of known ULXs, we classify ESO 501-023 as a broadened disk and IC 5052 as a hard ULX. Our results suggest that both systems are likely to be nonmagnetic objects undergoing supercritical accretion driven by companion stars.

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.