Speaker
Description
NewAthena will advance our understanding of accretion onto stellar‑mass compact objects by enabling high‑throughput pointed X‑ray observations of low‑mass X‑ray binaries (LMXBs), including the elusive early stages of outbursts, through the combination of WFI wide‑field imaging/fast timing and X‑IFU high‑resolution spectroscopy. However, it is very difficult to predict when an LMXB will undergo a new outburst, and they are usually only detected with X-ray all-sky monitors once they brighten above their sensitivity limits (a few mCrab on day timescales), when the outburst is already well underway. This causes a gap in the coverage of the rise of the outbursts, limiting our understanding of their initial evolution. We address this gap with the X‑ray Binary New Early Warning System (XB‑NEWS), an automated pipeline that processes Las Cumbres Observatory (LCO) optical observations in real time to detect the onset of LMXB outbursts. From our long‑term monitoring of ~50 LMXBs (2005-2023) with the Faulkes Telescopes and LCO, we are detecting the optical brightening of XRBs typically ~12 days before the outbursts are detected in X-rays with all-sky monitors like MAXI. In addition, we show that outbursts rise at shorter optical wavelengths before rising at longer wavelengths, consistent with an ionising heating wave propagating through the disk at the onset of the outburst. XB‑NEWS therefore provides days‑to‑weeks advance warning that can be used to prioritise time‑critical NewAthena follow‑up and organise simultaneous multi-wavelength campaigns, enabling WFI/X‑IFU observations at the very onset of LMXB outbursts, revealing the initial physical processes and conditions that trigger the subsequent accretion state transitions, and outflows such as winds and jets.
