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On Dec. 4, 2013 the PanSTARRS project discovered a comet positioned in the northern part of Gemini. Comet C/2013 X1 (PanSTARRS) showed a miniscule coma of magnitude 20.0 and a 2" tail towards p.a. 30° on the discovery image. Follow-up observations showed a medium-condensed 6" coma. The comet will reach perihelion in mid-April 2016, when it could reach magnitude 7. Mid-European observers can follow the comet approaching the Sun between mid-July 2015 (14-15 mag) and mid-February 2016 (9-10 mag). During this period it moves through the constellations Auriga, Perseus, Andromeda, Pegasus into Pisces, where it will disappear above the western evening horizon. It will then plunge into the Southern Sky (CBET 3736 / MPEC 2014-C43).
The comet showed an activity clearly above average, as is demonstrated by 50 observations from 10 members of the German Comet Section and 385 international observations. Between Jan. 4 and 25 the comet experienced a short-lived outburst. In the course of this outburst the brightness reached 7.7 mag on Jan. 7, yielding an amplitude of 1.5 mag. According to the derived pre-perihelion brightness parameters the comet should have peaked at about 5.5 mag during the days of perigee (mid-June at 0.64 AU). However, post-perihelion the comet showed an absolute magnitude nearly one magnitude fainter, resulting in a peak brightness of only 6.4 mag. Neglecting the short-lived outburst the brightness evolution can be rather well described by the following formulae:
pre-perihelion: m = 3.8 mag + 5×log D + 14.0×log r post-perihelion: m = 4.4 mag + 5×log D + 15.1×log rUntil mid-October 2015 the coma measured less than 0.7' (125.000 km), but increased thereafter, reaching 2.5' (200.000 km) in early December and short of 5' (400.000 km) in early January. During the outburst the coma reached a diameter of almost 12' (1 mio. km) temporarily. In February it was reported at 6' (600.000 km). Post-perihelion the apparent coma diameter increased due to the approach of the comet to Earth from 4' at the end of April to 17' in mid-June. Thereafter it shrunk in an equal fast manner. At the start of June it measured 12', at the start of August 5'. In absolute dimensions the coma increased from 300.000 km in mid-April to 580.000 km at the end of May. It shrunk to 400.000 km until the start of August.
Total Brightness and Coma Diameter
Pre-perihelion the comet was well-condensed (DC 4-5). Surprisingly the coma did not condense during the outburst. Between mid-April und mid-May the degree of condensation was in the order of DC 5, but decreased to DC 3-4 until the start of August. Visually a faint tail could be detected, measuring 0.25° (3.5 Mio. km) in Dec./Jan. and mostly 0.3° post-perihelion. However at the end of July the observers measured a length of 0.7° (5 Mio. km). It pointed towards E to ENE.
Andreas Kammerer
