Monday, October 21, 2013

Outburst of comet C/2012 X1 (LINEAR)

Cbet No. 3674, issued on 2013 October 21, reports an outburst in brightness of comet C/2012 X1 (LINEAR). The magnitude of the comet was measured by  H. Sato on on Oct. 20.5 to be total mag 8.5 (as measured within a circular aperture of diameter 85".2) with a brighter center about 10" across.  The predicted H_10 magnitude for C/2012 X1 (LINEAR) would be around 14 now.

We performed follow-up measurements of this object on 2013 October 21.51.  Below you can see our image of this comet, stacking of 3x20-seconds unfiltered exposures, obtained remotely from MPC code H06 (iTelescope Observatory, New Mexico) through a 0.50-m f/6.8 astrograph + CCD + f/4.5 focal reducer. At the moment of the imaging session, the comet was just +16 degree above the horizon and the Sun was -11 degree. Click on it for a bigger version.

The comet shows a morphology similar to the outburst of comet 17P/Holmes in 2007: the C/2012 X1 disk-like coma diameter is about 105" and the sharp central condensation is ~15" across with magnitude ~11.

Below you can see an elaboration of the original image with the MCM filter. This filter creates an artificial coma, based on the photometry of the original image, and subtract the original image itself in order to highlight the internal zones of different brightness that are very close to the inner core and that would normally be hidden from the diffuse glow of the comet.

Below you can see the magnitude graph of C/2012 X1 (click on the image for a bigger version).

Credit: S. Yoshida

UPDATE - October 28, 2013

Using our C/2012 X1 images of October 21 + the C/2012 X1 images of October 25 (kindly provided by Gianluca Masi) we calculated the dust shell expansion rate and compared it to that of 17P/Holmes during the 2007 outburst.

The coma diameter increases from 113 arcsec to 202 arsec in about 88 hours (3.66 days). This corresponds to a projected radial speed of 1.01 arcsec/hour ( or 24.3 arsec/day) and, at the distance of 2.95 AU, to a speed of about 0.6 km/s. We can compare this speed with that of 17P/Holmes during its 2007 outburst where the expansion rate of the dust shell, also projected on the plane of the sky, has been found to be constant at a rate of approximately 0.554 +/- 0.005 km/s from October 25.8 to November 1.6 (Yi Lin et al., The Astronomical Journal 138 (2009) 625).

by Ernesto Guido, Nick Howes and Martino Nicolini

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