Searching for Short-timescale Variability in the Ultraviolet with the GALEX gPhoton Archive. I. Artifacts and Spurious Periodicities

Abstract

In order to develop and test a methodology to search for UV variability over the entire GALEX database down to the shortest time scales, we analyzed time-domain photometry of ∼ 5000 light curves of ∼ 300 bright (m FUV , m NUV ≤ 14) and blue (m FUV − m NUV < 0) GALEX sources. Using the gPhoton database tool, we discovered and characterized instrumentally-induced variabilities in time-resolved GALEX photometry, which may severely impact automated searches for short-period variations. The most notable artifact is a quasi-sinusoidal variation mimicking light curves typical of pulsators, seen occasionally in either one or both detectors, with amplitudes of up to 0.3 mag and periods corresponding to the periodicity of the spiral dithering pattern used during the observation (P∼120 sec). Therefore, the artifact may arise from small-scale response variations. Other artifacts include visit-long "sagging" or "hump" in flux, occurring when the dithering pattern is not a spiral, or a one-time change in flux level during the exposure. These instrumentally-caused variations were not reported before, and are not due to known (and flagged) artifacts such as hot spots, which can be easily eliminated. To characterize the frequency and causality of such artifacts, we apply Fourier transform analysis to both light curves and dithering patterns, and examine whether artificial brightness variations correlate with visit or instrumental parameters. Artifacts do not correlate with source position on the detector. We suggest methods to identify artifact variations and to correct them when possible.

The Near Ultraviolet Transient Surveyor (NUTS): An ultraviolet telescope to observe variable sources

“…We made use of the observations from the variability catalogs (Welsh et al, 2005;Wheatley et al, 2008) and time domain survey (Gezari et al, 2013), as well as additional catalogs of novae outbursts (Wils et al, 2010) and M dwarf flares (Welsh et al, 2007). In addition, there have been new archival studies of short timescale UV variability (de la Vega & Bianchi, 2018;Brasseur et al, 2019) using the gPhoton package (Million et al, 2016). These catalogs were used to determine the median NUV flux levels of these events, which were then converted to their equiv-alent counts as would be seen by NUTS.…”

New Time-Resolved, Multi-Band Flares In The GJ 65 System With gPhoton

“…This does not work for shorter integration times; if the light from a source intersects a masked detector region, its flux density will be incorrectly measured. In sub-visit light curves, the interference of a hotspot mask can produce significant non-astrophysical variability over the period of the GALEX ∼120 second dither pattern (Boudreaux et al 2017;de la Vega, & Bianchi 2018). The gPhoton software flags any light curve time bin where the photometric aperture contains any photon events falling within one pixel (∼6 ) of the flat field map of a masked region, and it is recommended that researchers exercise additional skepticism when using any flagged data.…”