Speaker
Description
Fast X-ray transients (FXTs) are extragalactic flashes of X-rays, lasting from seconds to hours, whose nature remains largely unknown. Several progenitor mechanisms have been proposed to explain their origin, for instance, neutron star mergers, tidal disruption events involving a white dwarf and an intermediate-mass black hole, supernova shock break-out, and gamma-ray burst cocoon-like/jet breakout emission. Some FXTs have also been detected in wavelengths other than X-rays, from radio to gamma-rays, showing also optical and near-infrared (OIR) counterparts.
Until January 2024, 31 FXTs had been identified via data mining of the Chandra and XMM Newton archives, and only one of them had a known optical counterpart serendipitously observed. The launch of Einstein Probe (EP), in early 2024, has completely changed this scenario, unveiling the presence of more than a hundred FXTs during its first year of operation. The early transient alerts provided by EP, typically issued within a few minutes to some hours after the event, have enabled the discovery of the OIR counterparts for $\sim$55% of FXTs before the decay of the OIR flux to non-detectable levels. While this has led to the identification of progenitors for some FXTs and has constrained physical properties of others, a significant fraction of the FXTs discovered by EP remains without a known OIR counterpart. In some cases, the lack of a OIR counterpart might be because it has not been possible to carry out a proper follow-up of the event due to reasons beyond our control, such as late alerts or bad weather. However, in many others, the OIR follow-ups showed no counterparts associated to these events up to certain limiting magnitudes, which are very deep in some cases, within a few hours after the trigger.
These so-called "dark fast X-ray transients" are an important part of the intriguing FXTs population. Constraining their physical properties and their nature might be critical for disentangling the observational properties displayed by these events. In this talk, we will analise observations of a FXTs sample with no detected OIR counterpart. These observations have been carried out with some of the most powerful ground-based facilities available today, such as the 10.4m diameter Gran Telescopio Canarias. We will then discuss the implications of these non-detections within the upper limits set by the observations, how they constrain possible progenitor models, and ultimately, how they shed light on the nature of these elusive events.
| Talk category | NOVA Network 3 |
|---|---|
| Preference for a talk or poster | Talk |
