Hello everyone,
I recently encountered an issue. For variable star entries like BD+60 2611, which only have TESS data, the “Revision History” section usually includes a note like “mean magnitude derived from XXX.”
I’m curious about how this average magnitude is calculated. I’ve noticed that many such variable stars in VSX have photometric data from Gaia or APASS, but instead, data from Tycho or GCPD is used as the so-called average magnitude.
Additionally, it seems that these values need to be corrected before they can be used for calibrating TESS data. How is this achieved?
Cheers, Lucas.
Hi Lucas,
We recommend adding a mean standard magnitude from a photometric catalogue for TESS submissions because TESS magnitudes are not standard (are similar to Ic) and since all stars observed by TESS are bright, they tend to have well-measured magnitudes in catalogs like the GCPD, Hipparcos, etc. This applies when you can’t combine TESS data with observations from other survey like ASAS-SN, ZTF, ASAS-3, etc. A large number of objects can only be detected as variable with TESS (or Kepler) because their variations are of very small amplitude.
So the mean magnitudes adopted are taken from the above sources (Gaia DR3 is a very good choice now if the star is not too bright and is in the GCPD or Hipparcos), and since the amplitudes are tiny (usually <0.01 mag.), then the mean values are basically okay.
We do not calibrate TESS data, for VSX submissions just a magnitude offset is applied if we adopt a zero point from another survey. If only TESS data are used, you can plot TESS magnitudes as they are or even normalize them to 0 and show amplitudes.
About “correcting” the magnitudes, the only correction we apply is usually related to light contamination, since blending may affect some of the survey data. Each survey has its own resolution (you can see a list of surveys and their resolution in the VSX guidelines page), so we correct the magnitudes for light contamination, using Gaia DR3 magnitudes, which are free from contamination to <1".
Cheers,
Sebastian
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One follow up, if you want to use Gaia DR3 magnitudes as the mean magnitude, it’s best you convert it to V. Usually you can do the conversion with Gaia DR3 G and BR-RP .
See table 5.9 in Gaia DR3 Documentation for the conversion:
https://gea.esac.esa.int/archive/documentation/GDR3/Data_processing/chap_cu5pho/cu5pho_sec_photSystem/cu5pho_ssec_photRelations.html
Also ensure the BP-RP is within the acceptable range for the conversion. See Table 5.10 for the specifics.
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Hi,
I attempted to download the data from TESS and opened it using the plugin of Vstar. I noticed that the time presented in Vstar is in BJD.
When reporting to VSX, do I need to convert the BJD or directly fill in the JD column with the BJD values?
Cheers, Lucas.
Hi Lucas,
What do you mean with “the JD column”?
Epochs in VSX are usually given in HJD, but the truth is that for every star with results from TESS, BJD is being added. The difference is small.
To be honest, stars imported from old catalogs that have not been revised, have JD dates, not even HJD. For the shortest period stars the difference between JD and HJD is large enough to cause problems.
We might discuss if we need to have multiple date type fields in the future VSX.
Cheers,
Sebastian
Hi, Sebastian.
I’m referring to the content at the “Epoch” when filling in the information for VSX.
I think that if I haven’t misunderstood the meaning, I can directly use the BJD from the TESS data to fill in the “Epoch” ?
Cheers, Lucas.
I actually do TESS BJD to HJD_UTC conversion for my submissions based on TESS data. The difference is small though.
There is a web site for the conversion:
https://astroutils.astronomy.osu.edu/time/bjd2utc.html
(strictly speaking it converts to JD_UTC of a given location. So it’s not quite HJD_UTC.)
I do the conversion in Python (with astropy ). The specifics can be found in this notebook, of which one can use in a web browser without Python installation.