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Integrating VOEvents with AstroGrid through STAP Services

by Elizabeth Auden, Alasdair Allan, Kevin Benson, Silvia Dalla, Noel Winstanley

I. Introduction

The International Virtual Observatory Alliance has recommended the VOEvent specification to describe transient space events in an XML format that can be transmitted between data archives, astronomical applications, and robotic telescopes [1]. A VOEvent packet may describe, for example, a gamma ray burst, a coronal mass ejection, or a microlensing event. One of the primary characteristics of each such event is its finite duration; therefore, one useful query that can be applied to an archive of VOEvent packets is a query on start and stop times.

The AstroGrid Simple Time Access Protocol (STAP) specification has been developed by Silvia Dalla and Kevin Benson to describe web services that access data archives primarily through time queries [2]. STAP queries may specify right ascension (RA) and declination (dec) parameters in addition to mandatory start and stop time parameters. Astronomical VOEvent packets usually specify RA and dec as spatial coordinates, while solar VOEvent packets may specify a NOAA active region number or no spatial information. Because the RA and dec parameters are optional, STAP queries can be applied to both astronomical and solar VOEvent packets.

The AstroGrid AstroScope tool includes support for STAP queries against solar image and time series archives, and this support has been extended to STAP queries of VOEvent archives [3]. When an AstroScope user locates a VOEvent packet through AstroScope, the user can load VOEvent reference files directly into PLASTIC-enabled virtual observatory applications. In addition, metadata from the VOEvent packet, such as spatial and temporal coordinates, instrument information, or object names, can be used to create further AstroScope data searches of image, spectra, and time series archives.

II. Parsing VOEvents

In order to integrate VOEvent packet searches with AstroGrid search applications, VOEvent packets need to be archived in a format accessible by STAP web services. The AstroGrid STAP client can access data stored in a MySQL database, so metadata extracted from a single VOEvent packet can be used to populate one row of a MySQL table.

VOEvent packets are generated in several ways. The eSTAR manual injection system allows registered users to submit an HTML form that will generate an event packet [4]. The SolarSoft vobs:ontology IDL package uses the export_event.pro and struct4events.pro modules to generate event packets for use with the Heliophysics Knowledge Base; although these packets use VOEvent elements, at the time of writing the packets do not conform to the VOEvent schema [5]. VOEvent packet templates can be created manually from the schema using an XML editor, and such templates were used in conjunction with shell scripts to convert HTML-formatted solar flare catalogues into archives of VOEvent packets. VOEvent packet archives for the BATSE and GOES solar x-ray flares catalogues, along with the LASCO coronal mass ejection catalogue, have been generated using this method, and they are hosted at MSSL[6,7,8].

Once a VOEvent packet has been generated, the metadata can be extracted using element accessor methods from the VOEvent sourceforge project’s VOEvent.pm module. First, a perl script receives a VOEvent packet in one of two ways: either it listens for published packets transmitted by a VOEvent hub (such as the eSTAR hub at the University of Exeter), or it is invoked by a local shell script that generates packets from a legacy catalogue. The perl script then extracts the following metadata parameters using the relevant accessor methods from VOEvent.pm:

  • id – the “ivorn” attribute from the root VOEvent element
  • ra – right ascenscion
  • dec – declination
  • timeinstant – TimeInstant ISOTime value
  • starttime – StartTime ISOTime value
  • stoptime – StopTime ISOTime value
  • concept – concept value that describes type of event
  • name – event name according to the publisher or original catalogue’s nomenclature
  • contactname – contact name for the event publisher
  • contactemail – contact email for the event publisher
  • params – comma-separated list of parameters in the format “name=value units”
  • references – comma-separated list of reference files in the format “name=value”

Only the “id” value extracted from the VOEvent’s ivorn is mandatory; not all VOEvent packets include “Param” or “Reference” elements, and solar VOEvent packets will not include RA and dec values. Most packets will either specify TimeInstant or StartTime / StopTime, but not both. An additional value of “url” may be generated to point to a web-accessible copy of the original VOEvent packet.

III. Accessing VOEvents with STAP

The next stap is to create the MySQL tables that will be accessed by VOEvent STAP web services:

mysql> create table myTable (ivorn varchar(200), url varchar(200), ra float, dc float, start datetime, stop datetime, instant datetime, concept varchar(50), name varchar(50), contactname varchar(50), contactemail varchar(50), params varchar(500), refs varchar(500));

The twelve parameters listed above (plus the “url” value if applicable) are inserted into the table. A sensible rule of thumb is to create one MySQL table for each STAP web service that will be deployed. STAP services and their accompanying tables may be structured by VOEvent publisher, event type, or the instruments used to observe the events. Tables for all STAP services deployed within a single application server instance can be created within a single database.

The AstroGrid STAP web service listens for time queries in the format

http://my.server.com/my-stap/StapSearch?start=yyyy-mm-ddThh:mm:ss&end= yyyy-mm-ddThh:mm:ss&service=astrogrid_stap

Queries may include additional parameters such as INSTRUMENT-ID, DATA_ID, TIME_RESOLUTION, POS, SIZE, or BAND; the STAP specification contains further information on these parameters. A properties file distributed with the STAP web application contains the relevant MySQL table name, user access information, parameters to return, and instructions to convert the query URL’s parameters into an appropriate SQL query[10]. The results of the SQL query are formatted as a VOTable and returned to the user; the VOTable may be displayed in a browser or interpreted by a data searching tool such as AstroScope. The STAP web service should be registered with an AstroGrid registry so that it may be queried by other virtual observatory resources. A tutorial on the deployment of VOEvent STAP services with AstroGrid is available at http://wiki.astrogrid.org/bin/view/Astrogrid/VoEventSTAPtutorial.

At the time of writing, eight VOEvent STAP web services are hosted at MSSL and registered with AstroGrid:

  • stap-batse, generated from the BATSE solar x-ray flare catalogue
  • stap-essence, harvested from the CalTech ESSENCE supernovae search VOEvent feed
  • stap-gcn, harvested from the CalTech GCN gamma ray burst VOEvent feed
  • stap-lasco, generated from the LASCO coronal mass ejection catalogue
  • stap-noaa, generated from the NOAA GOES solar x-ray flare catalogue
  • stap-ogle, harvested from the eSTAR OGLE gravitational microlensing VOEvent feed
  • stap-robonet, harvested from the eSTAR RoboNet VOEvent feed
  • stap-sdss, harvested from the CalTech SDSS supernovae search VOEvent feed

IV. Displaying VOEvent STAP Results with AstroScope

The AstroScope tool, distributed with the AstroGrid VODesktop application, displays the federated results of position, object name, or time queries to data archives hosted in IVOA-compliant virtual observatories. Time queries are sent to STAP services registered with AstroGrid. VOEvent queries are isolated to STAP services listing “voevent” in their registry entries.

VOEvents are presented in the AstroScope window as bubbles labelled with the event’s start time (or time instant if start time is not available). The user can hover a mouse over the bubble to view all event metadata returned by the STAP service, or the user can click on the bubble to access any reference files such as images, time series, or spectra.

voevent_astroscope.jpg Figure 1: Screenshot of AstroScope tool listing results of VOEvent query

V. Future Work and Conclusions

VOEvent packets can now be integrated with AstroGrid search mechanisms by deploying a STAP web service configured to access a MySQL table containing VOEvent metadata. A Simple Event Access Protocol (SEAP) specification is under development that will describe more sophisticated queries to a VOEvent archive using parameters other than time[12]. As STAP or SEAP services are deployed with VOEvent archives at other institutions, fewer astronomical VOEvent STAP services will be hosted at MSSL. The focus of VOEvent STAP services hosted at MSSL will shift to solar events and the inclusion of SEAP search terms such as heliocentric coordinates and active region numbers.

VI. References

  1. VOEvent Specification http://www.ivoa.net/Documents/REC/VOE/VOEvent-20061101.html
  2. STAP specification http://wiki.astrogrid.org/bin/view/Astrogrid/SimpleTimeAccessProtocol
  3. Astroscope http://www2.astrogrid.org/science/documentation/data-explore/astroscope
  4. eSTAR manual injection http://www.estar.org.uk/wiki/index.php/VOEvent
  5. HKB http://www.lmsal.com/helio-informatics/hpkb/
  6. BATSE solar flare http://umbra.nascom.nasa.gov/batse/batse_flares_1991.html
  7. GOES solar flare ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SOLAR_FLARES/XRAY_FLARES
  8. LASCO cme http://cdaw.gsfc.nasa.gov/CME_list/UNIVERSAL/text_ver/
  9. VOEvent sourceforge http://voevent.sourceforge.net/
  10. Astrogrid staphttp://msslxx.mssl.ucl.ac.uk:8080/astrogrid-stap/war/astrogrid-stap.war
  11. VOTable http://www.ivoa.net/Documents/latest/VOT.html
  12. SEAP - http://wiki.astrogrid.org/bin/view/Astrogrid/SimpleEventAccessProtocol

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Topic revision: r1 - 2007-08-24 - ElizabethAuden
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