observed ingress/egress BJD times don't match predicted

[james.lowenthal]

At Smith College Observatory we got a nice light curve for the Hot Jupiter K2-34b on the night of 3 Mar 2022 (UT 4 mar 2022).  We took the UT start/mid/end times from the ETD, and converted them to BJD using the AIJ Coordinate Converter.  In AIJ, we reduced the data and plotted the curve in BJD_TDB, and plotted the predicted ingress/egress BJD times.  The transit is very clearly visible.  But the timing is off: the transit is about 30 min late compared to the predicted start/mid/end times.

We also observed a similar offset in our light curve of XO-2b on the night of 4 Mar 2022 (UT 5 Mar 2022): there, the transit came early compared to the prediction.

Any suggestions?

[karenacollins]

Hi James,

It looks like the most recent observation posted to the ETD for K2-34 b is ~20 minutes late. I'm not familiar enough with ETD to know if the ephemeris they provide considers the BJD_TDB offsets. Maybe your measurement is good as long as your observatory computer clock was accurate when the observations were taken? In any event, it appears the ephemeris for K2-34 b on ETD is a bit stale relative to the last three measurements that were uploaded.

Karen