VOEvent - Pasadena1 Space-Time Coordinate Metadata Arnold Rots Harvard-Smithsonian CfA / CXC T HE US N ATIONAL V IRTUAL O BSERVATORY

VOEvent - Pasadena2 Overview Justification: what is driving this? The simple requirements Design: how do we solve this? Implementation

VOEvent - Pasadena3 Scope The following coordinate axes are closely intertwined: –Space –Time –Spectral (frequency, wavelength, energy) –Redshift (Doppler velocity) Time is bound to a position and positions are time-variable Spectral and redshift data are tied to reference frames that may or may not be time-variable

VOEvent - Pasadena4 Scope (2) Context-dependent defaults are fine –Issues are well-defined and clear for single- observatory observations – even when not all is explicitly specified But there are no global defaults –In the VO all implicit assumptions need to be made explicit since they will not be obvious anymore One must be able to transform the coordinates of two observations to a common coordinate system –Including far-field/near-field transformations

VOEvent - Pasadena5 Requirement The requirement for Space-Time Coordinate metadata is that they: –provide sufficient and necessary information –are self-consistent For an observation two sets of metadata are required: –Location of the observatory –Location that the observation pertains to

VOEvent - Pasadena6 The Metadata Components Coordinate system –Consists of one or more frames Frames typically consist of a reference position (origin) and a reference frame Coordinate values –Refers to a coordinate system Coordinate areas or ranges –To define a volume in coordinate space –Special case: Regions Specifically for spatial coordinates

VOEvent - Pasadena7 Coordinate System Time frame –Reference position & time scale Spatial frame –Reference position and coordinate frame Spectral frame –Reference position in phase space Redshift frame –Definition and reference position

VOEvent - Pasadena8 Reference Frames and Positions Examples of spatial Reference Frames: –FK4, FK5, ICRS, Ecliptic, Galactic, Geocentric, Geodetic, various solar and planetary frames, unknown, custom, … Time scales: –TT, TAI, UTC, TDB, TEB, TCG, TCB, … Reference Positions: –Topocenter, Geocenter, Heliocenter, Barycenter, Galactic center, LSR-K, LSR-D, planetary centers, unknown, custom, …

VOEvent - Pasadena9 Coordinates A coordinate object contains a reference to a coordinate system It is a composite object that may hold –Name- Resolution –Value- Size –Error- Pixel size All quantities are scalar except for spatial (>1D) –In that case errors and resolutions get more complicated All coordinates include their units Spatial may include position and velocity (PM) Time has options (absolute or elapsed)

VOEvent - Pasadena10 Coordinate Area Defines the volume in coordinate space that is occupied by the Observation –Not necessarily relevant for VOEvent Consists of one or more ranges in individual coordinates Spatial position has more options –Sphere –2-D Regions Shapes: polygon, sector, ellipse, convex, convex hull Operations: intersection, union, negation

VOEvent - Pasadena11 Implementations STC-X: XML schemata, current version: 1.20 STC-S: String version ObsDataLocation ObservatoryLocation Position GEO_D TOPOCENTER SPHER ObservationLocation Time UTC TOPOCENTER T23:58:55 Error 5 Position ICRS TOPOCENTER Error Spectral TOPOCENTER 460 unit nm Resolution 40

VOEvent - Pasadena12 STC-X XML Example The physical XML documents can be simplified through use of XInclude files that allow inclusion of frequently used elements, referenced through standardized IDREFs –Constructors can just insert the single lines –Parsers that do not care about coordinate systems or observatory positions can just ignore them and optionally rely on the IDREFs, while the document remains rigorously correct The event described was observed at KPNO in blue light, just before midnight tomorrow, and is known to be located within 1 degree of the nucleus of M81 file:///EventKPNOinc.xml