Transient location

[David Swan]

Hello,

I took some images of transient AT 2018fzl. It is real – there is nothing at all visible on the corresponding DSS image.

My plate solution gives a position of the transient that is a full 1 arcsec in Dec away from the position reported on the Transient Name Server. The plate solution is otherwise excellent in terms of star matching.

Any ideas?

David

The FITS:

https://1drv.ms/u/s!Agvxu8wNOxpAfRoq0IUC9GK4664

Attachments:

1. 

   AT2018fzl_20180909_201659_dswan.png
   

[Dr Paul Leyland]

Could it be a blend?

The residuals plot after the (presumably) Gaussian profile has been subtracted appears asymmetrical to me, with the left hump being somewhat larger than the right.

https://arxiv.org/abs/1809.00018 came to my attention a few days ago which is what made me think of this possible explanation.

Paul

[Robin Leadbeater]

A potential target for a spectrum. mag 15 is just about bright enough for a standard ALPY 600 and a modest (say> 10 inch) aperture.

Robin

[David Swan]

Hi Paul,

The abstract is most interesting – thanks for pointing this out. I’ll read the article proper this eve. I hope Robin or someone else takes up the spectro.

David

[Dr Paul Leyland]

Not entirely sure what to make of this …

I took your FITS image and fed it through IRAF’s DAOPHOT pipeline because I knew that it could compute PSFs and subtract them from images.  The image below shows an 8x enlarged view of the region of the transient after computed stars had been removed.  With the exception of saturated stars and a scattering of doubles, all the bright stars were invisible, meaning that they had been successfully modeled and removed from the image.  A good number of faint stars (one is very obvious in the snapshot)  had not been removed because they were too faint for accurate photometry with the parameters I chose.

The double next to the transient is very clearly marked as  such: the PSF of a single star has been removed from the mid-point.  The transient has been removed nicely but two wings remain.  I’d say that was fairly conclusive reason for your astrometric result but for the fact that the other two bright stars also show wings.

As I said, I’m not sure what to make of this but post it for your interpretation which may well be better than mine.

BTW, the image below is inverted N/S with respect to your because IRAF counts pixels from the bottom up.  Causes no end of confusion …

Paul

[Andy Wilson]

Hi Paul,

I was interested to see you post a link to this paper as I have spent the past couple of years working in the same office as Tom Wilson, with Tim supervising both of us.

I can’t claim to be an expert in his research but it is eye opening to discover how faint contaminant stars can affect astrometry.

Best wishes,
Andy

[David Swan]

Thanks for the image analysis. So with several ‘winged’ bright stars nearby, it looks like PSF contamination is an unlikely explanation. Maybe an artifact produced by the optics or by something weird associated with frame stacking? Hopefully someone else will image this transient at a good scale and post astrometry.

[Dr Paul Leyland]

“something weird associated with frame stacking”.  Ah, I wonder if perhaps some frames have become rotated with respect to the others?  If your stacking performs only rectilinear shifts then those near the center of rotation will be stacked perfectly but those further away will be smeared into short arcs.  Removing a circularly symmetric synthetic star from the center of an arc will result in wings.

Could you try stacking, say, half the images and see if the result changes?  Repeat with a differently chosen half, and again.

[David Swan]

Yes. Thanks Paul.

The raw FITS files are archived on my external drive, so I can go back and do this. It would be nice to identify the source of the problem, as I may need to modify my workflow.

D

[David Swan]

I have re-stacked with more stringent quality criteria (FWHM and roundness). The noise is higher, but the stars look more round. Problem sorted I think! I’ve looked back at my determinations of transient locations, and the difference from the survey telescope position in this case isn’t unusual. Experimenting with the fit order doesn’t really change the measured position either.

[Dr Paul Leyland]

“Problem sorted I think!”

Excellent, I’m happy to have been of help.

[David Swan]

Appreciated. David

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