How does one convert the variation in “normalized flux” to variation in magnitudes? Attached light curve shows about +/- 0.006 in normalized flux for HD 21551.
To convert the difference between the normalized flux (which has mean value 0, as shown in the figure you have attached) and, say, the peak of the flux values in the left-most section (which have a value about 0.004 units), use the following:
mag(top flux) - mag(mean flux) = -2.5 * log10( 1.0 + peak_flux )
= -2.5 * log10( 1.0 + 0.004 )
= -2.5 * log10( 1.004 )
= -2.5 * 0.0017
= -0.0043
The magnitude of the object at peak flux is the magnitude of the object at mean flux, minus 0.0043. In other words, if the object has a mean mag of 7.600, then the object at peak flux has a mag of 7.5957.
That’s basically what I was trying. I found the result didn’t align well with V band variations in VSX so I was puzzled.
Hi Tom,
HD 21551 is not in VSX, so I am confused by your comments.
I see it is an SPB variable discovered in 2020 so I will add it to VSX.
But are you sure you are referring to this object? It is a 5.83 Vmag. star with a mean TESS magnitude of 5.876 in the TESS Input Catalog.
If you are checking some star in VSX, then you are talking about a different object. Can you check?
PS: it is easy to get magnitudes using the TESS VStar plugins. VStar will do it for you.
Cheers,
Sebastian
The star in question was actually delta Scuti (HD 21551 just happened to be a TESS light curve I had handy for the above screenshot).
Here is part of the del Sct TESS curve…
Taking the “swing” from zero to be 0.05, I get an amplitude range of about 0.11 magnitudes. VSX lists the variation range as nearly 0.2 magnitudes. That’s why I was puzzled.
Tom
The amplitude of delta Scuti varies quite a bit over time, and depends on the passband in which one observes. Measurements by Templeton et al. (https://articles.adsabs.harvard.edu/pdf/1997AJ....114.1592T) in y-band in 1997 show an amplitude of +/- 0.06 mag, while those of Cugier and Monier (https://ui.adsabs.harvard.edu/abs/1993AcA....43...39C/abstract) have amplitude of +/- 0.07 mag in 1989.
I’d say that there’s nothing wrong with your conclusion that the TESS data shows an amplitude of +/- 0.06 mag.
MWR
The challenge here is that TESS was designed with a broad filter biased to the red since much of the planet hunting was on redder and fainter stars. They wanted to get as much light as possible. The filter doesn’t match any of the standard filters (Johnsons, Cousins, Sloan etc.). The filter goes from roughly 600 nm to 1000 nm. The effective wavelength of the filter is similar to a Cousins I filter but is much wider. The TESS T magnitude doesn’t match any specific traditional filter. Some have said that it tracks with GAIA magnitudes.
Otherwise, the equations given by other on this feed give the transformation to a relative magnitude from the relative flux.