A Method to Measure Photometries of Moderately Saturated UVOT Sources

Abstract

For bright transients such as gamma-ray bursts (GRBs), the Ultra-Violet/Optical Telescope (UVOT) operates under event mode at early phases, which records incident positions and the arrival time for each photon. The event file is able to be screened into many exposures to study the early light curve of GRBs with a high time resolution, including the rapid brightening of the UV/optical emission in particular. Such a goal, however, is hampered for some extremely bright GRBs by the saturation in UVOT event images. For moderately saturated UVOT sources, in this work we further develop the method proposed in Jin et al. to recover their photometries. The basic idea is to assume a stable point-spread function of UVOT images, for which the counts in the core region (i.e., an aperture of a radius of 5″) and the wing region (i.e., an annulus ranging from 15″–25″) should be constant, and the intrinsic flux can be reliably inferred with data in the ring. We demonstrate that in a given band, a tight correlation does hold among the background-removed count rates in the core and the wing. With the new method, the bright limit of measuring range for UVOT V and B bands increases by ∼1.7 mag, while only by ∼0.7 mag for the U band due to the lack of bright calibration sources. Systematic uncertainties are ∼0.2 mag for the V, B, and U bands.