Last Updated: July 2025 Current Maintainer: Maxwell A. Fine email: maxwell.fine@mail.mcgill.ca

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Overview

Exposure refers to the observation duration of a given sky region above a defined system sensitivity threshold, and is essential for estimating the all-sky FRB rate, studying repeaters, and planning multi-wavelength follow-up. Accurate exposure estimates enable robust population modeling and help track variability across the electromagnetic spectrum.

To first order, CHIME/FRB operates 24/7. Assuming constant sensitivity, the exposure is calculated as the transit time of sky positions across the FWHM of the CHIME beam(s) at 600 MHz. Currently it is not possible to calculate exposure for a different reference frequency, 600 MHz is used as the beams do not overlap.

However, CHIME/FRB in reality does not operate truly 24/7 — there are sometimes issues with parts of the CHIME/FRB detection pipeline (L0–L4), which are monitored by Grafana metrics and the FRB-Tsars. There can also be maintenance at the site, power failures, node failures, thermal shutdowns, etc.

The sensitivity also changes due to outside temperature and other environmental effects, and this variability is tracked using pulsar metric files. The general idea is that for a population of pulsars, their average signal should remain roughly constant. CHIME/FRB can identify days of poor sensitivity by comparing the observed S/N values against a historical average derived from a reference catalog of pulsars. Days with 10% higher then the median relative RMS noise are excised. The reference time is -.