Seminario DAS: The “soft”, the “faint” and the “early”: a brand-new look at Galactic X-ray transients by Einstein Probe

Speaker: Dr. Alessio Marino
Affiliation:  Institute of Space Sciences (ICE-CSIC), Barcelona, Spain

Abstract: The entire population of X-ray sources in our Galaxy — including X-ray binaries, cataclysmic variables, magnetars, and others — is characterized by rapid and extreme variability spanning several orders of magnitude in X-ray luminosity. All-sky X-ray monitors — instruments capable of observing a large fraction of the X-ray sky with a single “glance” — are therefore essential for catching new events or outbursts from these sources and for enabling follow-up observational campaigns to study them in detail. Launched in 2024 by the Chinese Academy of Sciences in partnership with the European Space Agency (ESA) and the Max Planck Institute, Einstein Probe (EP) — the most sensitive all-sky X-ray monitor ever flown — has significantly enhanced our ability to study Galactic X-ray transients, allowing us to discover entirely new classes of X-ray transients or observing the already known ones during poorly explored phases of their accretion history. In this talk, I will present an overview of the first 1.5 years of EP operations, highlighting the transformational discoveries made for three specific classes of X-ray transients: 1) The “soft” — an exotic and elusive class of X-ray binaries in which white dwarfs accrete matter from high-mass Be-type stars (the Be–WD binaries). These systems undergo nova-like explosions whose emission peaks at very soft X-ray energies, below the sensitivity of past all-sky monitors. 2) The “faint” — a population of accreting black holes and neutron stars, known as Very Faint X-ray Transients (VFXTs), whose outbursts are significantly fainter than those of standard X-ray binaries, allowing them to linger for weeks to years at remarkably steady but low accretion rates. 3) The “early” — well-known, frequently observed X-ray binaries whose outbursts have been detected many times in the past but usually only once they reach high luminosities, leaving the earliest stages of the outburst and the physical mechanisms that trigger it largely unexplored. Finally, I will discuss how EP is expected to deliver unprecedented insights into low-level accretion, the physical processes responsible for outburst triggering, and the broader evolution of binary stars and compact object formation.