On images taken with the 28cm-astrograph of the SONEAR observatory (Brasil) on Mar. 27, 2015 C. Jacques discovered a comet on the border of the constellations Telescopium/Sagittarius. Comet C/2015 F4 (Jaqcues) showed an 8" coma of total magnitude 16.5 and a 17" tail in p.a. 238°. Observations on Mar. 28 showed a strongly condensed coma of diameter 12x18" and of total magnitude 15.5, which displayed a sharp central condensation, and an 80" tail in p.a. 250°. The comet will pass perihelion (at solar distance: 1.64 AU) at the opening of August, but will pass Earth at a distance of 0.82 AU three weeks in advance. In consequence it will display a maximum brightness of 12 mag between end of June and end of August (CBET 4085 / MPEC 2015-H37), fading below 16.0 mag not until the end of the year. For mid-European observers it will appear above the southeastern morning horizon at the opening of June (constellation Capricornus), then passing the constellations Aquarius, Aquila, Sagitta, Vulpecula, Lyra and Hercules until the end of the year.
The comet showed a very rapid brightness increase as well as decrease, as can be derived by 34 observations of 6 members of the German Comet Section and 146 international ones. They indicate an evolution according to the formula
m = 6.6 mag + 5×log D + 21.5×log rindicating a maximum brightness of 10.8 mag around July 25, 2015. In mid-May the comet was only of magnitude 14.0 and at the start of October had already faded to 13.0 mag.
Total Brightness and Coma Diameter
At the beginning of the apparition the visual coma diameter was reported as measuring only 0.6' (35.000 km), but peaked at 2.5' (95.000 km) around July 25, decreasing thereafter, measuring 1.5' (70.000 km) at the opening of October. On images the coma diameter could be traced out to a diameter of 8' (300.000 km). The degree of condensation increased from DC 3 to DC 5 at the final days of June. Thereafter it decreased steadily, reaching DC 3 again in mid-October. A stubby tail with an estimated length of 2' was recognizable visually between July and mid-September.
Andreas Kammerer
