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Spectroscopic Alerts for the Time-Domain Era

Alejandra Melo, Paula Sanchez-Saez, Valentin D. Ivanov, Richard I. Anderson, Amelia Bayo, Avraham Binnenfeld, Sofia Bisero, Dragana Ilić, Andjelka B. Kovačević, Fatemeh Zahra Majidi, Jaroslav Merc, Anna Pala, Swayamtrupta Panda, Sarath Satheesh-Sheeba, Fabian Schüssler, Susanna D. Vergani

Abstract

Time-domain astronomy is entering an era of unprecedented discovery driven by wide-field, high-cadence surveys such as LSST, Roman, Euclid, SKA, and PLATO. While some of these facilities will generate enormous photometric alert streams, the physical interpretation of variability and transients often requires spectroscopy, which encodes changes in ionisation state, kinematics, and accretion that are inaccessible to photometry alone. A critical gap is therefore emerging: next-generation surveys may produce up to $\sim10^9$ alerts per year, whereas global spectroscopic follow-up is limited to only $\sim10^4$--$10^5$ transient spectra annually. We present the concept of spectroscopic alerts: real-time notifications triggered by significant spectral evolution, enabling spectroscopy to act as a discovery channel rather than solely as follow-up. We outline the key science cases enabled by this capability and describe the instrumental and operational requirements of a wide-field, highly multiplexed spectroscopic facility capable of delivering real-time spectral discovery for 2040s time-domain and multi-messenger astronomy.

Spectroscopic Alerts for the Time-Domain Era

Abstract

Time-domain astronomy is entering an era of unprecedented discovery driven by wide-field, high-cadence surveys such as LSST, Roman, Euclid, SKA, and PLATO. While some of these facilities will generate enormous photometric alert streams, the physical interpretation of variability and transients often requires spectroscopy, which encodes changes in ionisation state, kinematics, and accretion that are inaccessible to photometry alone. A critical gap is therefore emerging: next-generation surveys may produce up to alerts per year, whereas global spectroscopic follow-up is limited to only -- transient spectra annually. We present the concept of spectroscopic alerts: real-time notifications triggered by significant spectral evolution, enabling spectroscopy to act as a discovery channel rather than solely as follow-up. We outline the key science cases enabled by this capability and describe the instrumental and operational requirements of a wide-field, highly multiplexed spectroscopic facility capable of delivering real-time spectral discovery for 2040s time-domain and multi-messenger astronomy.

Paper Structure

This paper contains 2 figures.

Figures (2)

  • Figure 1: Comparison of all Transient Name Server (TNS) alerts and the subset with spectra (2017–2025). The large gap between alerts and spectroscopic classifications highlights the significant global shortfall in follow-up capacity.
  • Figure 2: Examples of pronounced and evolving spectral variability. Left: Simulation showing the detectability of the Beta Pictoris' exocomets with the expected WST high resolution mode. Individual epochs are shown as gray lines; additional redshifted and some blue shifted components appear and disappear on time scales of hours to days (WST collaboration, private communication). Right: Changing-look AGN candidates exhibiting changes in Balmer and [O iii] emission-line strengths between SDSS and follow-up spectra (Adapted from 2022Lopez-Navas).