Photometry on Supernovae with bright host galaxies?

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    Graeme Coates

    This might be an involved question…I have some calibrated V filtered images from last night of SNs 2020jfo and hfv – a question on reduction of this data… (put this in variable stars as seemed most sensible place…)

    I’m used to running multi-aperture photometry on variable stars using AstroImageJ. However, if you run this in this situation (esp on 2020jfo in M61!) there’s significant contribution from the host galaxy in the background annulus which gets subtracted away as “sky background”. If I run photometry with the normal aperture settings, and compare to the output when setting the size of the background annulus to zero, I get answers that are 0.4mag apart.

    What’s the best way of running photometry here to get a realistic answer, and how to best deal with errors…? 


    Robin Leadbeater

    Yes the sky background has to be subtracted somehow. The same problem crops up in spectroscopy of supernovae and some programs have various ways of fitting a model to the sky background in the surrounding region. In my limited experience of photometry I think programs which model the star point spread function against the background work better in this situation, though there  could still be a bright/or dark patch directly under the target of course.  The transient survey instruments get round this problem by subtracting an image taken when the transient was not there. 


    Graeme Coates

    Thanks Robin – yep – certainly don’t have the luxury of subtracting either image pre-SN like the surveys 😉 

    I wonder if anyone has a suggestion on software to allow background modelling to allow me to reduce the data I have? AIJ doesn’t look to have this feature (it allows aperture photometry with background subtraction via an annulus, and (I suppose by setting the inner and outer background radii to the same value) PSF photometry without any background correction – possibly more accurate than the former, but then still prone to inaccuracies… 


    Andy Wilson

    I can’t offer a perfect way to do this, but you may be helped by looking at this another way.

    The photometry aperture containing the light of the supernova will itself contain background light from the host galaxy. That disc containing the light of the supernova will be huge in terms of the galaxy, covering many millions if not billions of stars. Hence the trick is not to subtract a sky background without the galaxy, but to have an annulus that is representative of the background light you are collecting with the supernova photons.


    Robin Leadbeater

    Hi Graeme,

    You definitely need to set up an annulus of some width to measure and subtract the background. If not, as  Andy says you include light from the galaxy (and light pollution etc) with the supernova in the brightness measurement. Too narrow though and the background level measurement becomes noisy. To wide (or too far out) and it becomes unrepresentative of the background where the target is.  Perhaps you could try  annuluses (annuli?) of different non zero widths and see how much the result varies ?

    Paul Leyland

    APT has a variety of sky background models. You might find one that works well in this situation.

    APT is the workhorse of my photometry though I’ve never used it where the “sky” varies that much on such a small scale. It appears to work fine where the background gradient is relatively small, such as variables in M31 and M33.

    Otherwise I would suggest PSF-fitting photometry such as IRAF/DAOPHOT. Again I’ve never used it in such extreme circumstances but the documentation has reassuring words about what to do where a star is positioned within nebulosity. Be warned: this package has a steep learning curve and is generally labour intensive.

    Philip Masding

    Hi Graeme, 

    Have you tried Astrometrica? It’s free for 100 days then 25 euros. It models the star’s PSF I think. I have used it for the SN in M61 and NGC 4568. The latter is very much within the galaxy so background subtraction is important . Attached is a screen shot of the NGC4568 results. 


    David Boyd

    I use AIP4WIN for aperture photometry. My pragmatic approach in trying to measuring SNe is to choose an aperture for the SN itself which extends out to where the SN light distribution blends into the background but no further. Then a small gap of a couple of pixels followed by an annulus which includes at least 10 times as many pixels as contained in the central aperture. This will inevitably include light from the surrounding galaxy but the distribution of this light is likely to be a gradual slope as you move away from the galaxy core. The annulus will record an average of this background intensity which should be reasonably representative of the galaxy background under the SN itself. You can try varying the various radii involved to see how stable the result is. As I say this is a pragmatic solution and not perfect but in my experience it seems to give a result consistent with other reports.


    Graeme Coates

    Great – thanks all for your thoughts. I played around a little with the varying radii of the annulus and it actually seemed to be reasonably stable in terms of calculated magnitudes. I’ll tabulate the results and pop them here in when I get chance to sit down and do it.

    Interestingly, the figures I see in running this initially are a good half magnitude lower than other estimates I’ve seen – clearly this will be due to subtraction of flux from M61 – and it will disagree with visual estimates that have no way of doing that! 

    Bart Staels

    SNOoPY might help when the code gets publicly available: . The tutarial mentioned is interesting.


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