Outburst of comet 46P/Wirtanen reported: inner coma brightens 1.5 mag in 10 days

Mark Kidger in today's THE ASTRONOMER Electronic Circular No 2446 reports on behalf of the "Observadores_cometas" group that 46P/Wirtanen has experienced a significant outburst which is evident in these light curves as well as direct images. Between May 6.9 and 16.9 UTC the magnitude in a 10" aperture rose from 16.8 to 15.1 mag, in 20" from 15.6 to 14.4 and in 30" from 15.2 ro 14.0 mag. in the R band. "Images and the multi-aperture photometry show that the inner coma has become significantly more condensed," the report says, "with the coma slope changing from considerably more extended than 1/r pre-outburst, to being 1/r or slightly steeper at the peak of the outburst." And Kidger advises that observers should "follow the evolution of this apparently unique light curve event in this object, which has shown highly consistent light curve behaviour in its previous apparitions" over the last three apparitions. Its dust production had been practically identical to the 2002 and 1996 apparitions - until the current outburst began.

In other news Kidger also published a detailled analysis of the (if you trust the noisy light curves) repeating outbursts of 17P/Holmes in The Astronomer (magazine) 44 #528 [2008 April] 320-1: He sees four outbursts, in Oct. (the BIG one) and Dec. 2007 and Jan. and March 2008, each with a inner coma amplitude of about 0.4 mag. And with a period of 45±1 days, "although the interval between mini-outbursts is slightly longer each time". If 45 days is the rotation period of the nucleus the increasing intervals "could be consistent with the decreased insolation taking longer to trigger activity." Here, by the way, is an independent near-nucleus light curve with ½ mag. scatter, though: this observer saw "no larger outbursts". Meanwhile Andreas Kammerer has extended his analysis of Holmes' visual behavior in Schweifstern 24 #126 [April 2008] 8-10, based on over 1000 reports. Peak brightness was 2.4 mag. on Oct. 30; in late March it had dropped to roughly 5.5 mag. And while the bright dust coma expanded 100,000 km/day til early January (when it appeared to stall and shrink, due to its low surface brightness), the faint gas coma expanded at 230,000 km/day until it was lost visually around Nov. 5.

• There is also a new comet C/2008 J6 (Hill) and another observation of Beshore which now has an asteroid-like orbit. • An optical transient in NGC 300 at around 14th mag. could be a peculiar star like V838 Mon. • The central star of the LMC-N66 nebula has undergone a new outburst; the nature of this star is uncertain, too. • As SOHO sees it today: the Sun close to the Plejades and Venus; there are three new old-cycle sunpots on the disk. • There have been rare halos galore in Finland after some hiatus - while Earth's magnetic field reversals could be more frequent (no connection implied). • The ISS transiting the Moon; good viewing opportunities will come soon to N America as well as Europe. • And here are some expectations for the LSST, the Large Synoptic Survey Telescope. Its smaller predecessor PanSTARRS 1 will start its 3½-year science run at the end of this calendar year, this blog was told recently - also promising a flood of discoveries near and far.