An Australian-led 8m-class optical transient discovery imager with minutes later fast spectroscopic follow up

[jeffcooke]

This issue is to gauge interest in an Australian-led 8m-class discovery optical imager with fast spectroscopic follow up for the transient community to be operational near the start of this Decadal Plan timeframe.

The Australian-led Wide-Field Imager (WFI) - WFI is a 1 degree field of view deep, wide-field 8m-class optical imager, with high blue sensitivity (and narrowband filters) that fills the large gap left by Rubin and existing instruments. No other 8m-class wide-field imager is planned for 2+ decades. WFI can be built by 2028-2029. WFI probes ~200x more volume than 1m-class telescopes and ~15x more volume than 4m-class telescopes.

• WFI has full, year-round target-of-opportunity capability for transients.
• WFI can tile each square degree down to mag ~ 26 in 60 seconds (it can tile shallower depths faster, e.g., mag ~ 25 in 10s), even in the u-band (for kilonova blue rise, early transient detections, etc.), and is UV-sensitive down to 3000A for key transient UV science.
• WFI has fast ~10s readout and ~10s filter change.
• WFI also offers a multiplexing mode that enables deep wide-field imaging, real-time data analysis and transient identification and, with a flip of a mirror, minutes-later spectroscopic follow up using optical and IR instruments on the same telescope.
• WFI probes GW170817-like kilonovae to ~1200 Mpc and is sensitive to fainter events at closer distances. WFI will capture ~90% of the kilonovae missed with current and all future optical facilities. It can also search for kilonovae well beyond the GW detector horizons to better understand the full population and evolution with redshift.
• WFI reaches depths of m ~ 28, including u-band, in ~2 hrs to ~2 nights to get elusive faint events and distant events (e.g., NSBH kilonovae and BBH merger flares, radio, high-energy and particle sources, their host galaxies, high redshift transients such as the deaths of the first generation Pop II stars, etc.).
• WFI will be the only optical imager capable of detecting sources for the next generation radio, high energy, particle, and GW facilities.

No other existing or future telescope can do what WFI can, not even the ELTs or any space-based telescope - enabling Australia to lead globally in many areas of transient science.

Why Keck: The Keck Observatory is the only observatory that can host such an instrument as a result of its modular design for a prime focus instrument, its high elevation (4100 m) and superior atmospheric UV transmission, and its large aperture for the necessary extreme depths. The ESO VLT and other 8m-class telescopes require significant telescope and observatory restructuring that incur high costs, especially for the installation of a deployable secondary mirror, and are at lower elevations and have poorer atmospheric UV-transmission.

Other options? The only deep wide-field optical imagers available for the next 2+ decades (ground- or space-based) are DECam and the upcoming Rubin (see note about Subaru HSC). DECam is a good wide-field imager on a 4m telescope, but probes about 15x less volume than WFI and is largely insensitive to NSBH kilonovae and other faint or high redshift transients. DECam doesn't have crucial minutes-later deep spectroscopic follow up. Importantly, DECam is 12 years old now and may not be operational throughout this Decadal Plan time window. Rubin will only follow up ~50 hrs of transient science per year (e.g., 3-4 kilonovae/yr) and will not follow up any of the other 99% of transient classes. Rubin has very poor u-band throughput and cannot provide crucial minutes-later deep spectroscopic follow up. Transient astronomy is an all-sky science and DECam and Rubin are co-located in Chile. Keck's latitude covers all of the Northern sky and ~2/3rds of the Southern sky.

Note: Subaru HSC does not perform target of opportunity observations. HSC has no u-band, requires 30 minutes for filter changes (a showstopper for fast transient follow up), is 2-3x less sensitive than WFI, and its optics are used for the Prime Focus Spectrograph, resulting in little availability in the future. Subaru HSC is 12 years old now and may likely be retired within the next decade.

Status: WFI is currently under development and a partnership between Swinburne, AAO-Macquarie, ANU-AITC, Caltech and the University of California to be on sky by ~2028-2029. WFI will be accessible to Australian researchers via a significant amount of guaranteed time as part of the instrument and via Swinburne partnership time, NASA and NOIRLab access time, Australian purchased time, and collaboration with Keck community and international researchers.

[ashruiter]

Thanks Jeff for the nice summary and comparison with other instruments. WFI sounds amazing. Addressing a major gap in observational capability and also leveraging national and international partnerships!

[tzafarastro]

Thanks Jeff. A note is that Subaru/ULTIMATE will be a near-infrared WFI with 14x14 arcmin, doesn't go to shorter wavelengths of course but also leverage international partnerships and synergies in between.

[jeffcooke]

Thanks, Eric! Here, the topic is an instrument that can survey the sky quickly at needed depths (and, here, optical) and/or go very deep for high redshift and faint events. While the near IR is also good for transients (however, most are blue at outburst, and IR imaging is inherently shallower due to the bright night sky), with a 14' x 14' field of view, it would take around 19 pointings for each square degree. A wide-field imager would need about 5x that field of view (ideally more) to be practical, especially, given limited source visibilities on the sky. For GW counterparts as an example, trying to obtain a minimum two filters per pointing with ULTIMATE, and imaging the full area twice to detect source change, would take too long for anything with more than, say, ~2 deg^2 localisation. But I agree that ULTIMATE can be good for some specific cases and synergy is good!

I note that WFI leverages synergy with Subaru (and other international partners). Subaru are highly interested in WFI and they're involved in the project, including sharing their narrowband filters (which have > $2 million value).