Flemming R. Ovesen / Torben Taustrup

Last update: January 2023

M1 Dim
ABSTRACT

Sudden dimming of a star in The Crab Nebula

TOC Observatory which is established in 1989 is a private observatory. The instrument, 12,5" - f:5 Newtonian is primarily used for astro photography.

For the past fifteen years we have been taking images of the Crab Nebula in order to document the expansion of the nebula. Almost every year these images has been put together in an animation, which shows the expansion quite clearly.

During the years we have caught some asteroids in the field. In the winter of 2019 an image from January 2 showed that something else had happened - a star in the nebula had become much fainter. The animation shows this - it comprises images from 2018 and 2019:

The event forced us to take up the task of measuring star magnitudes. By using the program Maxim DL we could measure, that the star, which we call Dim, (position: 5 34 34,38 +21 59 56,8) had become approximately 0,8 magnitude fainter than the year before. Dim is around one third further away than M1.

The main part of the exposures are made through a clear filter. After the discovery we have however made exposures both with red and blue filter (not Johnson filters), and we also have a limited number of exposures with these filters from before 2019.
I has been established, that the main part of the dimming is in blue light. For magnitude reference we have used four stars ( 3UC225-038420 - 3UC224-037485 - Ref and Sibling), two outside the nebula and two close to Dim. Reference to stars used. These have magnitudes close to the measured magnitude of Dim.

Measuring star magnitudes in a nebula is a challenge, as the background value in the nebula is far from the reference background level at the reference star outside the nebula. However, the magnitude of the star Ref. in the nebula seems to be quite stable and remains within +/- 0,1 magnitude.
Apparently Dim is situated at a position, where the dust is moving fast in the nebula. This is a very detailed animation made by Detlef Hartmann

The discovery has been reported both to AAVSO and IAU in 2019, but we have received no response so far.

The matter has been discussed in danish Astro-Forum , and a couple of theories has been put forward - and among these that the dimming was caused by a dusty globule passing in front of the star.
In another article it is stated, that much smaller globules might exist in the Crab Nebula and that these not are detectable with instruments that are available at the moment

The star is not registered as a variable, so another theory concerning this sudden dimming is needed.

December 2020
Flemming R. Ovesen and Torben Taustrup
TOC Observatory

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