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\IBVShead{5996}{17 August 2011}

\IBVStitletl{Reference frame and time standard}{ used in INTEGRAL/OMC datasets}

\IBVSauth{Zejda, Miloslav$^1$; Domingo, Albert$^2$}

\IBVSinst{Department of Theoretical Physics and Astrophysics, Masaryk University, Kotl\'a\v{r}sk\'a 2, CZ 611 37  Brno, Czech Republic. e-mail: zejda@physics.muni.cz}

\IBVSinst{Centro de Astrobiolog{\'{\i}}a (INTA-CSIC)
POB 78, 28691 Villanueva de la Ca\~{n}ada, Madrid, Spain. e-mail: albert@cab.inta-csic.es}


\IBVSkey{JD OMC INTEGRAL time scale}

\IBVSabs{Short description of time coordinate used in INTEGRAL/OMC data is given.}
\IBVSabs{The variable-star community uses observations from the INTEGRAL/OMC satellite,}
\IBVSabs{but it seems that with incorrect interpretation of time given there.}


\begintext

The INTEGRAL satellite (Winkler et al. 2003) started its operations in October 2002 and
is dedicated to search and study of all kinds of high-energy sources using gamma-ray and
X-ray instruments. In addition, there is an Optical Monitoring Camera (Mas-Hesse et al.
2003, hereafter OMC) consisting of a passively cooled CCD ($1056\times2061$ pixels,
imaging area: $1024\times1024$ pixels) working in frame transfer mode. The CCD is located
in the focal plane of a 50 mm (diameter) lens including a Johnson $V$-filter to cover the
500--600 nm wavelength range. The OMC obtains very good long-term series of photometric
observations. The public OMC Archive at \texttt{http://sdc.cab.inta-csic.es/omc/}
(Guti\'errez et al. 2004) contains not only optical measurements of the original
high-energy targets, but also many "common" variable stars observed with OMC. However,
some OMC data used in the past were unfortunately interpreted in the incorrect time
standard. For this reason we decided to write the following short description to help
people working in the variable star community.

The data in the public archive are given as binary tables in fits format. The
time information is given in two columns in terms of INTEGRAL Julian Date (IJD), which is
defined as JD-2\,451\,544.5 or MJD-51\,544.0. The time standard adopted by the INTEGRAL
Project, as recommended by IAU Division I, is Terrestrial Time (TT). So, IJD starts on 1
January 2000, but expressed in TT and not in UTC. Since TT differs from UTC by 32.184 sec
+ 32 leap seconds at the start of year 2000, the UTC origin of the IJD is actually
1999-12-31 T23:58:55.816 (=JD 2\,451\,544.49925713 UTC).

The first time column is "TFIRST", where no barycentric or heliocentric correction is
applied. So, "TFIRST" is the INTEGRAL Julian Date measured in the satellite reference
frame and expressed in TT. The second time column "BARYTIME" includes the time after
applying the barycentric correction to transform from the accelerated coordinate system
of the INTEGRAL satellite into the coordinate system of the solar system barycenter.
Following the IAU recommendations for barycentric ephemerides (IAU 2006 NFA glossary,
prepared by the IAU Division I Working Group), "BARYTIME" is the Barycentric INTEGRAL
Julian Date expressed in the Barycentric Dynamical Time standard (TDB). The difference
TDB--TT has a maximum amplitude of 3.4 ms (Eastman et al. 2010), which is orders of
magnitude lower than the OMC timing accuracy.


We notice that authors use heliocentric instead of barycentric correction in most papers
devoted to variable stars. Heliocentric correction is only accurate to 8 seconds (see
Eastman et al. 2010 for details). This difference is negligible for the majority of
periodic variable stars with periods of days and longer, but it brings redundant and
senseless noise to the data.

In addition to the above considerations, the OMC data user must take into account that
times in both columns "TFIRST" and "BARYTIME" are the "starting" time of the
integration. Thus, for the commonly used "middle" time of the integration,
one must add "TELAPSE"/2 and be aware that TELAPSE
is in seconds. An OMC integration consists of one or several individual exposures.
Information in the TELAPSE column gives the length of the entire integration in seconds
(which can include several exposures), while the EXPOSURE column gives the effective
exposure in seconds.
The number of exposures co-added to form a given OMC integration is controlled by the
"Sampling Time" query parameter shown on the OMC Archive Web page. This parameter must be
understood as a code to designate the type of light curve is obtained. There are three
different sampling times used for data in the OMC Archive: 1, 630 and 9000 seconds. With
1 second sampling we get a light curve with one photometric point per exposure. With 630
second sampling individual exposures are co-added to obtain roughly one photometric point
each 10 minutes, but images with exposures less than 20 seconds are rejected to increase
the signal-to-noise ratio. For the 9000 seconds sampling all exposures in a given
INTEGRAL pointing are co-added, and exposures shorter than 60 seconds are rejected to
increase the signal-to-noise ratio. It should be borne in mind that longer exposures lead
to saturation effects for bright sources. Consequently only the shortest possible
exposure times should be used for such bright sources.

The OMC Archive is a rich source of photometric information on many variable stars.
However, before using the information for a long term study of astronomical events,
substantial attention should be devoted to the time scales and reference frames of data
used in the analysis. As Bastian (2000), Eastman et al. (2010) and others showed,
researchers should take into account the difference between JD based on UTC and TT (or
TDB), which can introduce systematic errors of over 1 minute.

More information can be found at the ISDC\footnote{The INTEGRAL Science Data
Centre} FAQ list:  \\
\texttt{http://www.isdc.unige.ch/integral/support/faq}.


\bigskip
{\it Acknowledgements:} "Based on data and information from the OMC Archive at CAB-LAEFF,
pre-processed by ISDC".


\reference

Bastian, U., 2000, IBVS 4822

Eastman, J., Siverd, R., \& Gaudi, B.~S., 2010, PASP, 122, 935

Guti\'errez, R., Solano, E., Domingo, A., Garc\'ia, J., 2004, ASP Conf. Ser.,
314, 153 

Mas-Hesse, J.~M., Gim\'enez, A., Culhane, J.~L., et al., 2003, A\&A, 411, L261

Winkler, C., Courvoisier, T.~J.-L., Di Cocco, G., et al., 2003, A\&A, 411, L1


\endreferences

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