Short-term variability of comet C/2012 S1 (ISON) at 4.8 AU from the Sun

TL;DR

This study monitored comet C/2012 S1 (ISON) at 4.8 AU from the Sun, detecting very shallow brightness variations likely due to nucleus rotation, and tentatively estimating a rotational period of about 14.4 hours.

Contribution

First photometric analysis of comet ISON at 4.8 AU, providing a tentative rotational period estimate based on differential photometry and periodogram analysis.

Findings

Detected shallow brightness variations with 0.03 mag amplitude.

Tentative rotational period of approximately 14.4 hours.

Seeing variations did not affect the periodicity analysis.

Abstract

We observed comet C/2012 S1 (ISON) during six nights in February 2013 when it was at 4.8 AU from the sun. At this distance and time the comet was not very active and it was theoretically possible to detect photometric variations likely due to the rotation of the cometary nucleus. The goal of this work is to obtain differential photometry of the comet inner coma using different aperture radii in order to derive a possible rotational period. Large field of view images were obtained with a 4k x 4k CCD at the f/3 0.77m telescope of La Hita Observatory in Spain. Aperture photometry was performed in order to get relative magnitude variation versus time. Using calibrated star fields we also obtained ISON's R-magnitudes versus time. We applied a Lomb-Scargle periodogram analysis to get possible periodicities for the observed brightness variations, directly related with the rotation of the cometary nucleus. The comet light curve obtained is very shallow, with a peak-to-peak amplitude of 0.03 $\pm$ 0.02 mag. A tentative synodic rotational period (single-peaked) of 14.4 $\pm$ 1.2 hours for ISON's nucleus is obtained from our analysis, but there are other possibilities. We studied the possible effect of the seeing variations in the obtained periodicities during the same night, and from night to night. These seeing variations had no effect on the derived periodicity. We discuss and interpret all possible solutions for the rotational period of ISON's nucleus.