Achieving Better Than 1 Minute Accuracy in the Heliocentric and Barycentric Julian Dates

Abstract

As the quality and quantity of astrophysical data continue to improve, the precision with which certain astrophysical events can be timed becomes limited not by the data themselves, but by the manner, standard, and uniformity with which time itself is referenced. While some areas of astronomy (most notably pulsar studies) have required absolute time stamps with precisions of considerably better than 1 minute for many decades, recently new areas have crossed into this regime. In particular, in the exoplanet community, we have found that the (typically unspecified) time standards adopted by various groups can differ by as much as a minute. Left uncorrected, this ambiguity may be mistaken for transit timing variations and bias eccentricity measurements. We argue that, since the commonly-used Julian Date, as well as its heliocentric and barycentric counterparts, can be specified in several time standards, it is imperative that their time standards always be reported when accuracies of 1 minute are required. We summarize the rationale behind our recommendation to quote the site arrival time, in addition to using BJD TDB , the Barycentric Julian Date in the Barycentric Dynamical Time standard for any astrophysical event. The BJD TDB is the most practical absolute time stamp for extra-terrestrial phenomena, and is ultimately limited by the properties of the target system. We compile a general summary of factors that must be considered in order to achieve timing precisions ranging from 15 minutes to 1 µs. Finally, we provide software tools that, in principal, allow one to calculate BJD TDB to a precision of 1 µs for any target from anywhere on Earth or from any spacecraft.

THEK2-ESPRINT PROJECT III: A CLOSE-IN SUPER-EARTH AROUND A METAL-RICH MID-M DWARF

“…Aperture sizes of 6.0, 8.0, and 6.0 pixels were selected for the J-, H-, and K s -band data to minimize the dispersion of the light curves with respect to the best-fit transit models. The time system of the light curves was converted from Julian Day (JD) to Barycentric JD (BJD) using the code from Eastman et al (2010). The resulting light curves are plotted in the top panel of Figure 3.…”

K s-band secondary eclipses of WASP-19b and WASP-43b with the Anglo-Australian Telescope★

“…We incorporated the uncertainties associated with each of these system parameters by drawing their values from Gaussian distributions with standard deviations as the literature uncertainty values. We convert the exposure time stamps to BJD-TDB using the UTC2BJD tool (Eastman et al 2010), such that they match with the literature T0 values. The best fit values and uncertainties are explored via a Markov chain Monte Carlo (MCMC) analysis, using the emcee ensemble sampler (Foreman-Mackey et al 2013).…”

The first eclipsing binary catalogue from the MOA-II data base

“…BJD − JD = ∆ R + ∆ C , where ∆ R is the Rømer delay (i.e. the time correction when the barycentre of the Solar system is taken as the reference point) and ∆ C is the clock correction from Coordinated Universal Time (UTC) to Barycentric Dynamical Time (TDB 1 ), varies over time and can be as much as about 9 minutes to date (Eastman et al 2010). Nevertheless, the systematic time errors that are induced when using JD instead of BJD are negligible comparing to a typical period of contact binaries, let alone detached binaries, even though such errors will induce spurious ETVs corresponding to the Earth's heliocentric motion which would smear out any signal associated with the LTTE in an EB.…”