The American amateur R.D. Matson and the Australian comet observer M. Mattiazzo independently discovered a moving object in the constellation Hydra in the SWAN data, beginning on June 20, 2006. Based on this information the Australian comet observer T. Lovejoy found comet C/2006 M4 (SWAN) on an image taken with a telephoto lens on June 30, displaying a 0.5' coma of magnitude 12. An image taken by R.H. McNaught with a 0.5m telescope on July 12 showed a strongly condensed coma of magnitude 12 and an 80" tail in p.a. 155°. According to the first orbital calculation the comet would have passed the Sun extremely close at the end of August, thereby remaining unobservable for mid-European observers (IAUC 8729/31). Actually it will pass perihelion at the end of September and will become visible in the morning sky for mid-European observers as a 8-9 mag object around that date (MPEC 2006-N38). From Oct. 10 onwards it will be better positioned in the evening sky, peaking at 8.0 mag. During the most interesting weeks it will move from Ursa Major through Canes Venatici, Bootes and Corona Borealis into Hercules. CCD images taken by N. Teamo and Sebastian Hönig with a 35 cm reflector on Tahiti on July 17 showed a 45" coma of magnitude 12.7 with a central condensation of 8" diameter, as well as a 2.1' tail in p.a. 160° (IAUC 8733).
After its conjunction with the Sun the surveillance by international observers began around Sep. 20 and by members of the German Comet Section on Sep. 30. Altogether 100 observations by 11 members of the German Comet Section were received. For the analysis 350 international observations could be added. Increasing still a bit in brightness during the first two weeks the brightness remained almost constant at 6.0 mag for the next two weeks. Then on Oct. 24 an outburst occurred, which increased the brightness from 6.0 mag to 4.2 mag on Oct. 25/26. The outburst phase lasted one week. Before and after that week the brightness evolution can be very well described by the following formulae:
pre-outburst: m = 6.5m + 5×log D + 10×log r
post-outburst: m = 5.5m + 5×log D + 14×log r
If the outburst had not occured the maximum brightness would have been only 5.8 mag in mid-October. Observations in the micro-wave region on Nov. 7 showed a secondary component in a distance of 3.4" (p.a. 431°), which was elongated (3" x 1.5") along the same position angle. The primary itself could not be resolved, because it was embedded within a diffuse central condensation (IAUC 8772). Calculations by Z. Sekanina indicate that the secondary actually was a short-lived cloud of debris. The position relative to the primary on Nov. 7 is compatible with the assumption that it was released by the primary on Oct. 23.9 UT, causing the short-lived outburst (CBET 738).
Total Brightness and Coma diameter
The coma diameter was in the order of 3-4' (300.000 km) between the end of July and end of September. During the following weeks it steadily increased to a maximum of 14' (600.000 km) during the last days of September. Thereafter it shrinked in a similar way, reaching 4' (300.000 km) in mid-December. A week-long period of slightly increased coma diameters, due to the outburst, can only be discerned via floating 3-days means. After the outburst the absolute coma diameter tended to be significantly larger than before.
The degree of condensation measured DC 7-8 between mid-September and the beginning of October, maybe influenced by the mediocre observation conditions during these weeks (which probably influenced the coma diameter estimates too). Thereafter it decreased to DC 6 shortly before the outburst. In the course of the outburst it increased to DC 7 and continued to show significantly larger values for one week. At the beginning of November the degree of condensation was 5, decreasing to DC 2-3 until mid-December.
Visually a tail was observed shortly after the start of the autumnal apparition. Until the outburst it was about 1° (4-5 mio. km) long, then increasing for a few days to 3-4° (10 mio. km). Around Nov. 10 the length was estimated as 0.8° (3 mio. km), decreasing to 0.1-0.2° (1.5 mio. km) in mid-November. A tail of a few arcminutes length could be still glimpsed in mid-December.
Andreas Kammerer
FGK observations