Identification and photometric classification of extragalactic transients in the Vera C. Rubin Observatory's Data Preview 1

TL;DR

Rubin DP1 provides an early demonstration of detecting extragalactic transients with LSST-like data and highlights the challenge of photometric classification without spectroscopy. The authors process DP1 DIA objects, apply multi-stage quality cuts, correct flux biases with archival templates, and perform photometric classification with the Superphot+ framework using redshift-informed filter mapping. They identify 11 extragalactic transients (8 new, 3 previously reported) and obtain tentative sub-classifications, with only two objects exceeding $95\%$ confidence. Their rate predictions using skysurvey yield a total of $15\pm4$ all-classes transients for the DP1 conditions, consistent with the observed sample. The work validates DP1 data quality and suggests LSST LSSTCam-based main survey will fulfil its time-domain discovery potential, albeit with ongoing challenges in classification and template fidelity.

Abstract

The Vera C. Rubin Observatory will soon survey the southern sky, delivering a depth and sky coverage that is unprecedented in time domain astronomy. As part of commissioning, Data Preview 1 (DP1) has been released. It comprises a LSSTComCam observing campaign between November and December 2024 with multi-band imaging of seven fields, covering roughly 0.4 square degrees each, providing a first glimpse into the data products that will become available once the Legacy Survey of Space and Time begins. In this work, we search three fields for extragalactic transients. We identify eight new likely supernovae, and three known ones from a sample of 369,644 difference image analysis objects. Photometric classification using Superphot+ assigns sub-classes with >95% confidence to only one SN Ia and one SN II in this sample. Our findings are in agreement with supernova detection rate predictions of $15\pm4$ supernovae from simulations using simsurvey. The supernova detection rate in the data is possibly affected by the lack of suitable templates. Nevertheless, this work demonstrates the quality of the data products delivered in DP1 and indicates that the Rubin Observatory's Legacy Survey of Space and Time (LSST) is well placed to fulfill its discovery potential in time domain astronomy.

Figures (6)

• Figure 1: Distributions of the single-visit $5\sigma$ limiting magnitudes in the Rubin DP1 fields ECDFS, EDFS and LELF. The dotted lines show the median values for each filter: $u=23.4$, $g=24.6$, $r=24.34$, $i=23.9$, $z=23.1$ and $y=21.94$.
• Figure 2: Coadded images of the location of 2024aigj from DECaLS (bottom left) and the Rubin DP1 templates (bottom right). We show these images in color using $gri$ filters and indicate the location of the transient with yellow cross-hairs. The transient is present in the Rubin DP1 templates but not in the DECaLS images. Top: Difference images of the boxed region, using the DECaLs template, isolate the transient to correct DIA object fluxes. In these images, a dark source corresponds to a positive flux detection.
• Figure 3: Identified extragalactic transients in Rubin DP1. Photometric redshift measurements are obtained from the Legacy Survey DR10 photometric redshift catalog 2022MNRAS.512.3662D.
• Figure 4: Multi-band light curves of extragalactic transients identified in this search. Each data point is the result of binning all DIA forced PSF photometry from the ForcedSourceOnDiaObject catalog 10.71929/rubin/2570321 from all the visits over a single epoch. A flux offset, obtained using the method described in Section \ref{['sec:flux_offsets']}, has been applied to each light curve.
• Figure 5: (continued)