Newbie Nova Question

https://www.legacysurvey.org//viewer/?ra=144.9170&dec=31.6703&layer=ls-dr9&zoom=15

So when I pull the individual exposures for this galaxy, I see that the red dot in the lower arm of the galaxy is not showing in the majority of the exposures. Does that give any indication that the red dot could be a nova?

A couple of things to point out:

  • check out the filter/band of the individual exposure. They are sorted in order g, r, z, where z is the reddest. You probably wouldn’t expect to see that red dot in the g filter.
  • the individual exposures are shown in whatever orientation the individual cameras produce their pixels, so in this case, it looks like the galaxy appears in different orientations (and at different scales, because the 90prime camera has larger pixels).
  • individual exposures have different noise levels and seeing, so sometimes it’s just a matter of a less sensitive image.

In this caes, if I look at the DECam z-band exposures (the bottom 5) I see it in all of them – barely in the first one (603125) because the seeing is blurry.

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Thanks for the information Dustin- very helpful!

No this gives not an indication. The red blob is a foreground star. You have to look at the date of the ‘single exposures’. If it is visible in exposures from more then one year then it can’t be a transient. That’s the case here. It is visible in ‘single image exposures’ from more then one year. It is visible in 2016 and 2017 as well in 2018. Some supernovae last longer then one year but they are very rare.

There is another important factor that comes into play as to whether an object can be a supernova candidate or not. Look at the different layers. If it is visible in DECaLS and SDSS then it cannot be a supernova anyway.

SDSS is a search from more than ten years ago. A supernova doesn’t last that long. You should also look at the color if it is green with blue or blue with red then it is probably an asteroid. Supernova candidate can appear in a wide range of colors. Deep blue aqua blue, mint green, magenta, yellow, white, deep red, orange etc.

First you always have to check check whether it is an asteroid or not. You can click on the link below the exposures. It is connected with the JPL small body database. If it doesn’t work you can do it manually via a tool named JPL small body identifier here
But to use this tool you first have to convert the coordinates (Ra, dec) with this tool here

You also need more than one exposure. With different dates but from the same year, preferably in different bands (g,r,iz) One exposure is not enough to determine if it could be a supernova candidate or not.

And always look if the object is visible in PanSTARRS or not regarding the first appearance of a transient. You need the first exposure image (fits file) to calculate an Apparent magnitude with APT and an equation. PanSTARRS images could have earlier images of the transient.

Also always check NED, Simbad Search, The Transient Name Server and the Rochester Bright Supernova pages whether a candidate has already been discovered or not.

Kind regards from Ine :stars::dizzy:

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Fantastic info Ine- thank you.

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