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\nprecision with which certain astrophysical events can be timed becomes limited\nnot by the data themselves, but by the manner, standard, and uniformity with\nwhich time itself is referenced. While some areas of astronomy (most notably\npulsar studies) have required absolute time stamps with precisions of\nconsiderably better than 1 minute for many decades, recently new areas have\ncrossed into this regime. In particular, in the exoplanet community, we have\nfound that the (typically unspecified) time standards adopted by various groups\ncan differ by as much as a minute. Left uncorrected, this ambiguity may be\nmistaken for transit timing variations and bias eccentricity measurements. We\nargue that, since the commonly-used Julian Date, as well as its heliocentric\nand barycentric counterparts, can be specified in several time standards, it is\nimperative that their time standards always be reported when accuracies of 1\nminute are required. We summarize the rationale behind our recommendation to\nquote the site arrival time, in addition to using BJD_TDB, the Barycentric\nJulian Date in the Barycentric Dynamical Time standard for any astrophysical\nevent. The BJD_TDB is the most practical absolute time stamp for\nextra-terrestrial phenomena, and is ultimately limited by the properties of the\ntarget system. We compile a general summary of factors that must be considered\nin order to achieve timing precisions ranging from 15 minutes to 1 microsecond.\nFinally, we provide software tools that, in principal, allow one to calculate\nBJD_TDB to a precision of 1 microsecond for any target from anywhere on Earth\nor from any spacecraft.\n