Science with a large field-of-view polarization survey: The Large Array Survey Telescope Polarization Node (LAST-P)

TL;DR

LAST-P introduces a modular, wide-field optical polarimetry node designed to surmount the small-field limitation of current polarimeters. By deploying 48 telescopes with four fixed polarization filters to achieve $88.8\,\mathrm{deg}^2$ instantaneous coverage, LAST-P enables high-cadence polarization monitoring of thousands of sources, including AGN, GRBs, SNe, TDEs, FBOTs, and Galactic transients, as well as large-scale ISM polarization mapping. The paper presents the instrument design, throughputs, and SNR-based polarimetric sensitivity, and outlines three survey strategies alongside detailed science cases. The anticipated capabilities include sub-percent PD precision for bright sources and percent-level MD P for a broad magnitude range, enabling time-resolved polarization studies that can distinguish emission mechanisms, jet geometries, and dust properties. If realized, LAST-P will significantly enhance multi-messenger and multi-wavelength campaigns by providing rapid, wide-field polarization data and expanding the catalog of polarized sources across the sky.

Abstract

Optical polarimetry provides information on the geometry of the emitting region, the magnetic field configuration and the properties of dust in astrophysical sources. Current state-of-the-art instruments typically have a small field of view (FoV), which poses a challenge for conducting wide surveys. We propose the construction of the Large Array Survey Telescope Polarization Node (LAST-P), a wide-field array of optical polarimeters. LAST-P is designed for high-cadence ($\lesssim 1$ day) polarization monitoring of numerous astrophysical transients, such as the early phases of gamma-ray bursts, supernovae, and novae. Furthermore, LAST-P will facilitate the creation of extensive polarization catalogs for X-ray binaries and white dwarfs, alongside a large FoV study of the interstellar medium. In survey mode, LAST-P will cover a FoV of 88.8 deg$^2$. With a 15 x 1-minute exposure, the instrument will be capable of measuring polarization of sources as faint as Gaia Bp-magnitude $\sim$20.9. The precision on the linear polarization degree (PD) will reach 0.7\%, 1.5\%, and 3.5\% for sources with magnitudes 17, 18, and 19, respectively, for a seeing of 2.7 arcsec, air mass of about 1 for observations in dark locations. We propose three distinct non-simultaneous survey strategies, among them an active galactic nuclei (AGN) strategy for long-term monitoring of $\sim$200 AGN with $<$1-day cadence. In this paper, we present the predicted sensitivity of the instrument and outline the various science cases it is designed to explore.

Figures (9)

• Figure 1: System throughput as function of the wavelength for various instrumental configurations. Configurations plotted include: 'Camera only' (bare sensor QE), 'R Filter + Polarizer', 'R Filter Only', and 'Polarizer Only'. The legend for filter configurations displays their transmittance, representing the system's lambda-weighted throughput relative to the 'Camera only' baseline. The 'Gaia $Bp$-band' curve shows its "transmission" normalized to the peak of the 'R filter + Polarizer Only' system. On the right axis we show the polarizer extinction ratio (purple curve), obtained from thorlabs_lpvisc100, showing an excellent performance in the region of interest.
• Figure 2: Polarization degree of AGN from the RoboPol AGN catalog robol_cat. Gray points indicate individual observations, while colored points show the median polarization degree for each source. The color scale represents the minimum exposure time required for our instrument (assuming four telescopes, i.e., one unit) to achieve the sensitivity needed to detect that polarization, while black points represent AGN that fall outside the 15 x 1 min curve for one unit. Colored dashed and solid lines indicate the MDP curves of LAST-P for different exposure times using survey mode (one unit) and deep mode (four units), respectively. Typical seeing of 2.7$\arcsec$ and air mass of approximately 1 are assumed.
• Figure 3: Spatial distribution of AGNs observed by RoboPol robol_cat. Colored dots represent AGNs, where the color encodes the minimum exposure time needed to measure their PD. Dashed gray circles show the FoV of our instrument ($88.8\,\text{deg}^2$) centered on each AGN. The gray region below declination $-$30$^\circ$ is hardly accessible to LAST-P due to its assumed future geographic location.
• Figure 4: Top panel: Evolution of optical afterglow PD for a GRB sample. Data points are sized according to the apparent R-magnitude measurement closest in time, with brighter magnitudes corresponding to larger points. Points falling below the relevant sensitivity threshold for their associated magnitude are shown unfilled. On the left side we consider the sensitivity (MDP) for an exposure of 60 seconds and on the right for an exposure of 15 x 1 minute in the deep mode of observation. The dashed lines represent the MDP for sources with magnitudes 18, 17, 16, 15 (in the $<$10 minutes region) and with magnitudes 20 and 19 (in the $>$10 minutes region) from top to bottom. Bottom panel: The corresponding apparent magnitude (R or r according to available data) . Data compiled from: GRB 090102 2009Natur.462..767S10.1111/j.1365-2966.2010.16601.x, GRB 091018 2012MNRAS.426....2W, GRB 091208B Cano2009GCN102622012ApJ...752L...6U, GRB 101112A 2017ApJ...843..143S, GRB 120308A 2013Natur.504..119M2017ApJ...843..143S, GRB 131030A 2014MNRAS.445L.114K, GRB 141220A 2021MNRAS.505.2662J, GRB 180618A Jordana-Mitjans_2022, GRB 190114C 2020ApJ...892...97J, GRB 191221B 2021MNRAS.506.4621BChen_2024.
• Figure 5: Top panel: PD as function of time for the supernova sample Nagao2024. Data points are sized according to the apparent magnitude measurement closest in time extracted from AstroCats_Software, with brighter magnitudes corresponding to larger points. Points falling below the relevant sensitivity threshold for their associated magnitude are shown partially transparent. Dashed lines represent the MDP for the a 15 x 1 minute exposure in deep mode of observation for sources with magnitudes 16, 15, 14, 13, 12 from top to bottom. Bottom panel: Apparent $R$-band magnitude evolution is shown for all objects, except for SN2008bk, for which the $V$-band magnitude is plotted.