On Dec. 8, 2012 the LINEAR project discovered an asteroidal object of magnitude 19.5 in the northwestern corner of Leo, which showed its cometary nature in the course of follow-up observations. Comet C/2012 X1 (LINEAR) displayed a 13" coma of total magnitude 18.5 and a broad, 11" tail in p.a. 310°. The comet will pass perihelion of its 2.000 year orbit in February 2014, expected to reach magnitude 11-12 (CBET 3340 / MPEC 2013-C52). It should be brighter than magnitude 16 between summer 2013 and fall 2014. Mid-European observers can follow the comet in the morning sky between October 2013 and April 2014 (at altitudes of less than 35°). It will move from the constellation Coma Berenices into Aquarius.

Before its conjunction with the Sun the comet evolved as predicted. However, when it reappeared in the morning sky in mid-October 2013 it surprised the observers. Instead of being a 14 mag object on Oct. 20 it showed a disk-like coma of diameter 1.6' and magnitude 8.2 with a 10" central condensation, surrounded by a fainter, 5' outer coma. Thus its appearance was very similar to comet 17P/Holmes shortly after its cataclysmic dust release in fall 2007 (CBET 3675). This comparison was supported by the rapidly increasing coma, which grew ever more diffuse.

After the outburst it was expected that the comet would return to the state prior to the outburst, which means that it would have been of magnitude 11-12 around perihelion. However, it evolved much differently, implying that the outburst started a continuing activity on the nucleus. Based on 22 observations by 8 members of the German Comet Section and 400 international observations the comet peaked at 7.8 mag on Nov. 6, 2013. Thereafter the comet faded, as expected. However, the fading stopped on Nov. 28, when the comet was magnitude 9.6. Thereafter the comet showed a very uniform brightness evolution, reaching a secondary maximum of magnitude 8.2 at the start of March 2014, slowly fading to 9.0 mag in mid-June. The slow fading was due to the fact, that the comet recedes from the Sun since Feb. 22, but approached Earth until the end of June with the distance to Earth decreasing from 2.0 AU to 1.6 AU during this period. According to the observers in the Southern hemisphere the comet reached 12.5 mag at the end of October. After Nov. 28, 2013 the brightness evolution can be well described by the following standard formula

The apparent coma diameter showed a linear increase to 13.5' (1.6 Mill. km) until Nov. 8, implying Oct. 18 as the date of the start of the outburst. In absolute terms the debris cloud increased 75.000 km per day or 0.85 km/s. After Nov. 8 it began to shrink rapidly, reaching 4.5' (500.000 km).

Total Brightness and Coma Diameter

The decrease was due to the fact that ever larger parts of the diluting debris cloud could no longer be discerned from the sky background (which was brightened due to the low altitudes at this time), with the proper coma ever more dominating the scene. The following weeks showed a constant apparent coma diameter of 4.5', which eventually increased to 7' between mid-March and the end of April 2014. It then decreased, measuring 5' at the beginning of June and short of 2' in mid-October. This implies a slow shrinkage of the coma from 500.000 km to 450.000 km between the end of November 2013 and mid-March 2014. Thereafter it increased, reaching 550.000 km at the end of April, then eventually decreased, reaching 375.000 km at the start of June and 225.000 km in mid-October. The debris cloud diluted rapidly, documented by the degree of condensation which decreased from DC 4 to DC 1-2 during the outburst. The proper coma was medium-condensed, showing a constant degree of condensation of DC 4-5 for a very long time. In mid-October it had decreased to DC 2-3. Visually a tail was reported between mid-December 2013 and the end of May 2014. Throughout this period the observers reported a length of about 9' (implying a shrinkage from 2.5 Mio. km to 1.5 Mio km). Its orientation changed from NW to WSW.

Andreas Kammerer

FGK observations