VOEvent workshop report

[Roy Williams]

The IVOA VOEvent workshop [1] has concluded with an agreement by an 
international team on an information infrastructure to support the 
burgeoning field of event-based astronomy.

Preamble: As we measure the sky with ever-increasing depth and breadth, 
the Universe seems filled with violent, distant explosions, 
trans-Neptunian objects, and asteroids that come all too close to the 
Earth. The detection and understanding of these transient and immediate 
phenomena requires new kinds of surveys, fast data reduction, and and 
fast response by the astronomical community. Providers of the event 
stream include the Swift and Integral satellites for gamma-ray bursts, 
supernova "factories", and synoptic surveys coming online now and 
planned for the future. There are robotic telescope projects that will 
respond in seconds to these discovery events, giving a comprehensive, 
panchromatic view. Until now, events have been distributed in various 
formats and protocols, so that aggregation and federation have been 
difficult. Users of such events have been forced to create specialized 
software to receive such streams. The objective of the VOEvent working 
group is to build an open standard for exchanging messages about these 
immediate astronomical events, including publication, archiving, query, 
subscription, and aggregation.

The VOEvent standard has been agreed in rough form at the workshop, and 
includes  the following features:

-- Buy-in for the new standard from projects that include: GCN, LSST, 
Pan-STARRS, Palomar-Quest, LIGO, eStars, Raptor, Pairitel, ATEL, and 
Hands-On Universe.

-- The possibility of simplicity (the mantra "Publisher, RA, Dec, 
Magnitude"), and also the possibility of positional and semantic accuracy,

-- Division into sections: Who, Wherewhen, What, How, Hypothesis, and 
Citations.

-- Contact and publisher information so that consumers of events can 
restrict event subscription to trusted sources,

-- Rich semantic content through the IVOA UCD vocabulary [2] to express 
the meaning of project-specific data in an interoperable way,

-- Integration with the IVOA Space-Time Coordinate schema[3], so that 
locations and timing can be defined either very simply or with 
sophistication and precision,

-- Integration with the Remote Telescope Markup Language (RTML) so that 
VOEvents can be rapidly converted to scheduling instructions for a 
telescope,

-- A vocabulary of event types that can be used to express a hypothesis 
about the astronomical meaning of the event, a way to express 
association with, and positional offset from a given object,

-- A global identifier structure for events so they can be cited into 
the future,

-- Message types such as Discovery, Followup, Retraction, and Supercede 
to provide a coherent picture of distributed knowledge about a discovery,

-- Modular XML syntax, allowing the use of pre-built tools for parsing, 
storage, and filtering, as well as a clear path to extensibility.

The meeting concluded with a number of action items, including initial 
implementations and documentation, and produced two "sample" event 
documents [5].

[1] http://www.ivoa.net/twiki/bin/view/IVOA/IvoaVOEvent
 Presentations are available at
http://www.ivoa.net/twiki/bin/view/IVOA/VOEventSchedule

[2] Unified Content Descriptors
http://www.ivoa.net/Documents/latest/UCD.html
http://cdsweb.u-strasbg.fr/UCD/ucd1p-words.txt

[3] Space-Time Coordinate Metadata for the Virtual Observatory
http://www.ivoa.net/Documents/latest/STC.html

[4] Remote Telescope Markup Language
http://www.uni-sw.gwdg.de/~hessman/RTML/

[5] Two example VOEvents are here:
http://www.ivoa.net/forum/voevent/att-0089/voevent_example.xml
http://www.ivoa.net/forum/voevent/att-0089/voevent_example_smallest.xml
(if this XML renders badly in your browser, choose "View/Source")