TREASUREHUNT: Transients and Variability Discovered with HST in the JWST North Ecliptic Pole Time-domain Field

Abstract

The James Webb Space Telescope (JWST) North Ecliptic Pole (NEP) Time-domain Field (TDF) is a >14′ diameter field optimized for multiwavelength time-domain science with JWST. It has been observed across the electromagnetic spectrum both from the ground and from space, including with the Hubble Space Telescope (HST). As part of HST observations over three cycles (the “TREASUREHUNT” program), deep images were obtained with the Wide Field Camera on the Advanced Camera for Surveys in F435W and F606W that cover almost the entire JWST NEP TDF. Many of the individual pointings of these programs partially overlap, allowing an initial assessment of the potential of this field for time-domain science with HST and JWST. The cumulative area of overlapping pointings is ∼88 arcmin2, with time intervals between individual epochs that range between 1 day and 4+ yr. To a depth of m AB ≃ 29.5 mag (F606W), we present the discovery of 12 transients and 190 variable candidates. For the variable candidates, we demonstrate that Gaussian statistics are applicable and estimate that ∼80 are false positives. The majority of the transients will be supernovae, although at least two are likely quasars. Most variable candidates are active galactic nuclei (AGNs), where we find 0.42% of the general z ≲ 6 field galaxy population to vary at the ∼3σ level. Based on a 5 yr time frame, this translates into a random supernova areal density of up to ∼0.07 transients arcmin−2 (∼245 deg−2) per epoch and a variable AGN areal density of ∼1.25 variables arcmin−2 (∼4500 deg−2) to these depths.

Glimmers in the Cosmic Dawn: A Census of the Youngest Supermassive Black Holes by Photometric Variability*

“…We avoid regions toward the edge of the image where the dithering pattern has led to excess noise and consider only a central pointing of the HUDF covering 123″ × 139″. The photometric comparison follows a similar method to that of O'Brien et al (2024): for each of the six WFC3/IR images, we identify sources using Source Extractor (Bertin & Arnouts 1996). In contrast to performing "global" galaxy photometry (e.g., with Kron-like or moment-centered apertures as Source Extractor would do), we seek to obtain photometry centered upon the brightest unresolved sources within each galaxy.…”

“…This ensures that, for example, 31.7% of our sources have Δm/σ m 1, 4.5% of sources have Δm/σ m 2, etc. See O'Brien et al (2024) for a more detailed discussion.…”

“…Here we argue that the photometric variability that results from changes in the mass accretion rate of SMBHs can provide a completely independent and complementary probe of AGN. Monitoring for variability selects AGN from imaging data directly by phenomena related to the SMBH, without any biases of photometric preselection (color, luminosity, compactness, etc), and has been used successfully at intermediate redshifts in deep Hubble Space Telescope (HST) imaging (Cohen et al 2006;Pouliasis et al 2019;O'Brien et al 2024). However, the highest redshifts require monitoring at infrared wavelengths in order to even detect galaxies: JWST would obviously be the best tool, but as such a recent observatory, it will take years to build the requisite time baselines when accounting for cosmological time dilation.…”