In Lyman-alpha images taken by the SWAN experiment aboard the SOHO spacecraft several people noticed a moving object during Mar. 29 – Apr. 1, 2015, which was situated near the circlet of Pisces, moving northward. Before a public posting was done, Xing Gao reported the independent discovery of the comet on images, taken by Guoyou Sun with a 11cm-refractor in the course of the XINGMING sky survey during twilight on Apr. 4. The Chinese obervers estimated comet C/2015 F5 (SWAN-XINGMING) to be of magnitude 11.5, showing a 1' coma and a 4.5' tail in p.a. 300°. Additional CCD observers reported a rather diffuse, 3' coma and a 6' tail. Visual observations on Apr. 8 and 10 reported a total magnitude of 8.5 mag and a coma up to 5'. The comet orbits the Sun with a period of 60.5 years and passed perihelion on Mar. 28. Despite a solar distance of only 0.35 AU it has to be assumed that the comet was not more than half a magnitude brighter in those days, due to its very faint absolute magnitude (CBET 4091 / MPEC 2015-H37). Until the opening of May its altitude above the northwestern evening will increase (then reaching 40°), with the comet fading rapidly, being of expected magnitude 12 at that time. Thereafter it sinks down to the horizon in a similar rapid way, expected to reach 16 mag before it disappears above the northwestern horizon.
The comet was only visible for a short period of time, which is the reason why only 6 observations by 3 members of the German Comet Section and 40 international observations could be taken into account. The short apparition was primarily caused by the proximity of the comet to Earth and the unfavorable orientation of the comet's orbital plane relative to Earth. It was not the result of the activity of the comet, because the derived parameters m0=11.9 mag / n=2 rather indicate a slow brightness evolution. The maximum brightness of 9.4 mag should have been reached in mid-April (around the time of discovery). By mid-May the comet had already faded to 13.0 mag.
Total Brightness and Coma Diameter
The coma diameter decreased during the apparition from 3.5' (100.000 km) to less than 1' (45.000 km). In parallel the coma became ever more diffuse; the degree of condensation decreased from DC 4-5 at to DC 1. No visual tail observations had been recorded.
Andreas Kammerer
