Nine years of UVIT: assessing sensitivity variation

TL;DR

This study evaluates whether the FUV channel of UVIT has experienced sensitivity degradation over nine years in orbit, using frequent NGC 188 observations and the calibration star HZ 4 as references after the NUV channel failed in 2018. The analysis employs aperture photometry with carefully reversed flat-fielding, followed by aperture and saturation corrections, to produce corrected flux measurements across epochs. Results show no significant FUV sensitivity decline at the field center, with a 95% confidence upper limit of about 4% over nine years, and short-term episodic variations in NGC 188 constrained to ≲5%. Together with the implied long-term stability of the NUV channel, the findings support the continued reliability of UVIT for long-term UV photometry and calibration for AstroSat.

Abstract

The Ultra-Violet Imaging Telescope (UVIT) is one of the five payloads onboard the first Indian multiwavelength astronomical observatory, AstroSat, launched by the Indian Space Research Organisation on 28 September 2015. UVIT, designed for simultaneous imaging in the far-ultraviolet (FUV; 1300-1800 Å) and near-ultraviolet (NUV; 2000-3000 Å) channels, has completed nine years in orbit in 2024 despite the failure of the NUV channel in 2018. As the FUV optics is subject to possible reduction in sensitivity due to microscopic amounts of contaminants, we used the FUV data acquired by UVIT over the past nine years on the open cluster NGC 188 and the white dwarf HZ 4 to study sensitivity variations in the UVIT FUV channel. Our findings indicate no significant reduction in the sensitivity of the FUV channel over the last nine years, with no significant episodic variations due to unknown causes.

Figures (9)

• Figure 1: A pseudo-colour image of the NGC 188 field. The blue and yellow colours represent the observations conducted in the UVIT's FUV F148W (exposure time = 36484 seconds) and NUV N279N (exposure time = 14954 seconds) filters, respectively. The image has been processed to bring out the faint features.
• Figure 2: The three sources (green circles) selected in the NGC 188 field to carry out UVIT sensitivity checks are marked in the UVIT F148W image. The image has been processed to bring out the faint features.
• Figure 3: All steps followed to get the corrected flux for a point source.
• Figure 4: The FUV light curve for HZ4. The solid and dashed lines mark the best fit line and the global average of the data, respectively. Green minor ticks and labels mark the times of the first and last observations.
• Figure 5: The FUV light curves for the selected three sources in the field of NGC 188. The dashed line marks the line of constant fit. Green minor ticks and labels mark the times of the first and last observations.