Ultraluminous X-ray sources (ULXs) represent the most extreme accreting X-ray binaries in the local Universe, with luminosities up to $ \sim10^{41}\ \rm erg/s$. They are now widely interpreted as stellar-mass compact objects accreting at super-Eddington rates in majority, as indicated by their soft X-ray spectral components, coherent pulsations in multiple systems, and the presence of...
NGC 5907 ULX-1, the most luminous ULX pulsar (peak luminosity 10⁴¹ erg/s), shows super-orbital modulation whose physical origin has implications for the magnetic field strength required to sustain super-Eddington accretion. From >10 years of Swift XRT monitoring (2014–2025), we characterise this modulation using Bayesian mixture models that properly marginalise over intermediate-flux states...
Quasi-periodic oscillations in the mHz range (mHz QPOs) have been detected in different X-ray binaries, hosting either a neutron star or a black hole. Nonetheless, the presence of mHz QPOs in a few ULXs was initially interpreted as the footprint of accreting intermediate-mass black holes. The detection of mHz QPOs first in M82 X-2, and then in M51 ULX-7 (two known pulsating ULXs), however, has...
Blazars exhibit strong variability across multiwavelength observations and multiple timescales. We present a systematic study of the X-ray spectral and timing variability of the TeV blazar PKS 2155-304 using Chandra and NuSTAR observations spanning 25 years (1999-2024). Timing analysis and flux distribution studies reveal moderate variability in most light curves and variable power spectral...
Ultraluminous X-ray sources (ULXs) are among the most luminous non-nuclear X-ray emitters in nearby galaxies, with luminosities exceeding the Eddington limit expected for stellar-mass black holes. They were historically considered promising candidates for hosting intermediate-mass black holes (IMBHs). However, the discovery of coherent pulsations in several ULXs, together with detailed...
X-ray pulsars (XRPs) provide a unique laboratory for studying accretion under extreme magnetic fields and strong-gravity conditions. Most are found in high-mass X-ray binaries, particularly systems with Be-star companions (BeXRBs), where episodic mass transfer can trigger bright X-ray outbursts reaching luminosities of ≳1E+38 erg/s. These systems probe the physics of magnetically channeled...
Nebular He II λ4686 in low-metallicity star-forming galaxies requires a hard ionizing spectrum beyond that produced by stellar populations. I present new deep Chandra and XMM-Newton observations that quantify the relative contributions of ULXs and hot diffuse plasma to this ionizing field in nearby metal-poor starbursts. Chandra’s sub-arcsecond imaging resolves individual ULXs, deconfuses them...
We present spatially resolved JWST NIRSpec and MIRI integral-field spectroscopy of the nebula surrounding the ultraluminous X-ray source Holmberg II X-1, providing the first direct view of high-ionization infrared emission on scales of only tens of parsecs in this system. By combining emission-line surface-brightness maps, diagnostic line ratios, and kinematic profiles, we detect intense [Ne...
