RXTE FITS Tables
The time stamps presented by RXTE FITS tables as generated by XFF are in TT.
It has been agreed that the Spacecraft Clock Seconds (SCCS), or Mission Elapsed Time (MET), will represent true elapsed seconds since January 1, 1994, at 0h0m0s UTC, which corresponds to MJD = 49353.0 (UTC).
Raw time stamps in the FITS tables created by XFF will be on this (SCCS or MET) time system. However, FITS times are to be taken as the sum of a Time value and the values of the MJDREF and TIMEZERO keywords. The MJDREF keyword will incorporate the transformation from UTC, via TAI, to TT (equaling an offset of 60.184 s). Thus, for January 1, 1994, at 0h UTC:
MJD (Jan 1, 1994, 0h UTC) = 49353.0 (UTC) = 49353.000696574074 (TT)
The TIMEZERO keyword will provide a nominal clock correction that ensures an absolute accuracy of 60 µs for the time stamps.
The goal is to provide reasonably accurate absolute time with a minimum effort. Generically, the value of TIMEZERO may be described as "the fractional part of UTCF".
[UTCF is an institutional dinosaur that can be found in the telemetry. Its purpose is to provide a conversion to UTC: when the sum of SCCS and UTCF is divided by 86400, the integer part of the result provides the number of UTC days since January 1, 1993, at 0h UTC, while the fractional part indicates the current UTC. Thus, UTCF is the sum of the number of seconds in 1993 (31,536,001) and the clock correction, minus the number of leap seconds since January 1, 1993.]
To summarize the times in the FITS tables:
- Raw time stamps are in SCCS = MET.
- Time stamps plus TIMEZERO provide time in TAI, as seconds since January 1, 1994, at 0h 0m 28s (TAI).
- Time stamps plus TIMEZERO plus MJDREF provides absolute time in TT.
- All RXTE science time stamps refer to the start of the bin, i.e. TIMEPIXR=0.0. In the case of event lists, the EDS rounds time values downward to the nearest time bin edge. In other words, the least significant time bits of event times are truncated by the EDS.
- All time-related quantities are in seconds, except for MJDREF which is in days.
Thus, to compute the modified Julian days (MJD) from raw XTE FITS
files, referred to the TT time system, one should use the
following formula,
MJD(TT) = (MJDREFI+MJDREFF)+(TIME+TIMEZERO+FINECLOCK)/86400
As noted above, this will compute the time at the start of
light curve bin or the truncated event time.
To compute a time which is properly centered on each time bin,
use this modified expression:
MJD(TT) = (MJDREFI+MJDREFF)+(TIME+TIMEZERO+FINECLOCK+(0.5-TIMEPIXR)*TIMEDEL)/86400
where TIMEPIXR is always 0 for RXTE data, and TIMEDEL is the time resolution of the event data, or bin size for light curve data.
In order to recover the most precise absolute times, the fine clock
corrections found in the tdc.dat file must be applied, which is the
FINECLOCK term listed in the formulae above. The fine clock
corrections are based on ground station estimates of the RXTE clock
offset and drift rate. These values are not a value which is
immediately available in the observation data, but rather are
estimated after the fact by the RXTE GOF and delivered approximately
monthly. See the RXTE Absolute
Timing page for more details about fine clock corrections, and
which tools incorporate these. See the Timing Digest for a figure of
fine clock corrections as a function of time. The maximum attainable
absolute time precision, using the tdc.dat file corrections, is about
2.5 microseconds for most of the mission (ref. Jahoda et al. 2006,
ApJS, 163, 401).
It may be possible to reach even higher accuracies, to the level of a few microseconds, but this will require a knowledgeable guest observer fine-tuning the clock corrections "by hand". No SOC support will be extended for this, beyond providing the guest observer with all data readily available to the SOC.
The relevant FITS table keywords are summarized below:
Keyword Value Unit Comment
TTYPE1 'TIME' s Column 1 contains raw time in SCCS
TIMESYS 'TT' Defines TT as fundamental time system
MJDREF 49353.000696574074 d 1994.0 (UTC) in TT
TIMEZERO frac(UTCF) s Clock correction
Various transformations between MET, TT, and UTC in a variety of formats
(seconds, calendar date/time, year-day-time, JD, MJD) can be performed
using the xTime web tool.
Barycentering
For high precision timing work, one often has to account for the fact
that the spacecraft and earth are moving through the solar system.
Since the speed of light is finite, the arrival time of a photon from
a target will be delayed or advanced, depending on the position of the
observatory within the solar system. The amount of delay or advance
varies appxroximately sinusoidally with time, with a period of one
year and an amplitude of about 8 minutes. Any astrophysical observed
variations from the target will be advanced or delayed by the same
amount.
Removing these observatory-related variations is known as
barycentering. Photon arrival times are adjusted so they represent
the effective arrival time to a ficticious observer, located at the
solar system barycenter. The solar system time scale is known as
Barycentric Time. The software tasks 'fxbary', 'faxbary' and
'barycorr' are capable of performing this correction on RXTE data.
The adjusted barycentered times will be written to either the 'TIME'
or 'BARYTIME' column, depending on the options used.
After barycentering, the 'TIME' or 'BARYTIME' column contains the
photon arrival time in TDB seconds, since the reference epoch, MJDREF,
now also expressed in the TDB system. Thus, to compute the MJD time
from barycentered files, referred to the TDB system, one should use
one of the following formulae,
MJD(TDB) = (MJDREFI+MJDREFF)+(BARYTIME+TIMEZERO)/86400 (or)
MJD(TDB) = (MJDREFI+MJDREFF)+(TIME+TIMEZERO)/86400
depending on task the settings used. If the TIMEZERO keyword is not
present, the default is zero.
After barycentering, the contents of the FITS table will be transformed,
as follows:
Keyword Value Unit Comment
TTYPE1 'TIME' or 'BARYTIME' s Column name depends on task settings
TIMESYS 'TDB' Time system now in Barycentric Time
MJDREF 49353.000696574074 d 1994.0 (UTC) in TDB
TIMEZERO 0 or undefined s Should have been absorbed into time column
