Real Time Transient Notification Projects

As mentioned earlier, information on new transient events needs to be notified to all interested recipients as soon as it is detected. Therefore, some of the projects are focused on implementing the computational architecture in order to providing real rime transients’ notification to all interested recipients.

The GCN (Gamma-ray Coordinates Network) [107] [108] is a first system that utilizes the real time notification concept for transient data delivered, it is designed by the NASA scientific group, and consists of three components; Notices, GCN Circulars, and Reports. The main objective of the GCN system is to receive and distribute information about the GRBs and other transients to interested recipients (robotic telescopes, human operated telescopes, amateurs and professional astronomers) in an automatic manner.

The Notices [109] component is typically designed for the needs of real time observations. It collects the location information of GRBs that are detected by various

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spacecraft or instruments, and then automatically distributes this information to all interested parties via either socket connections or email notification. The data flow of the Notices feature can be divided into the following steps:

1. The users register a New Site on the GCN website, providing all of the necessary information to receive the GRB location information from the Notices; such as an email address, socket IP address, filter condition, and what type of Notice the users wish to receive. The Site information will be stored in the GCN system once a New Site is successfully built;

2. When the GRB triggers the detector instrument on the spacecraft, the location information for the GRB will be sent down to the ground station. The ground station will then transfer the GRB location to the GCN at GFSC (NASA Goddard Space Flight Center).

3. The GCN will process the GRB location into a standard format using custom hardware and software. Then the Notices component uses either sockets or emails to automatically send a notification about the post process of the GRB location to specific Sites or emails based on a variety of pre-defined filtering conditions.

The GCN Circulars [115] collect timely information about optical, radio, x-ray and gamma-ray from the follow up community and sends this information to all interested recipients. The process flow of Circulars is much simpler than that of Notices. The follow up observers send their Circulars to a central location via email; the Circulars are then automatically forwarded to all recipients on the email distribution list in the form of a prose-style message. In addition, the email distribution list in the Circulars component is completely separate from the list in the Notices component, because some of the recipients (users and systems) might only be interested in the follow up reports, rather than the location of GRBs.

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The Reports [116] portion distributes the final reports of each burst to the entire community via email. The final report is a mini paper describing the full details of the observation of each burst; this report is distributed later than the Circulars portion, giving the observers enough time to do a full analysis and make corrections to the measurements on each burst. Furthermore, the Reports portion uses the same email message style (prose-style) and distribution list as the Circulars portion.

Spacecrafts Instruments

High Energy Transient Explorer (HETE) Wide-Field X-ray Monitor (WXM) Soft X-ray Camera (SXC)

International Gamma-Ray Astrophysics Laboratory (INTEGRAL)

IBIS (Imager on-Board the INTEGRAL Satellite)

ISGRI (Integral Soft Gamma-Ray Imager) PICsIT (Pixellated Caesium-lodide

Telescope)

SPI (Spectrometer for INTEGRAL)

Swift Gamma-Ray Burst Mission (Swift)

Burst Alert Telescope (BAT) X-ray Telescope (XRT)

Ultraviolet/Optical Telescope (UVOT)

Astro-Rivelatore Gamma a Immagini Leggero (AGILE)

Gamma Ray Imaging Detector (GRID) SuperAGILE (SA) hard X-ray monitor Mini-Calorimeter (MCAL)

Anti-Coincidence System (AC) Fermi Gamma-ray Space Telescope

(FGST)

Large Area Telescope (LAT) Gamma-ray Burst Monitor (GBM)

Table 2: Spacecrafts and onboard instrumentation that detects and provides location information of GRBs to the GCN system [108] [110] [111] [112] [113] [114].

Gamma ray bursts are a very short duration transient phenomenon, and the duration of each gamma ray burst ranges from a few seconds to a few minutes [218][219][220][221], which is much shorter than a gravitational microlensing event. Thus, the GCN system reduces the time delay for delivery of the location information about GRBs from the

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spacecraft instruments to the follow up observation telescope. When a new GRB event has been detected, the GCN system can provide a notification regarding the coordination of this new GRB event to follow up telescopes and observers within a few seconds [222]; this means that follow up telescopes and observers are able to makes follow up observations while the GRB is still bursting. In addition, the GCN system also allows the GRB follow up parties to make maximum use of limited resources (labour and telescope time) by communicating what has already been done, or what is going to be done soon.

Figure 11: The overview and processing flows of the GCN architecture.

The GCN system also cooperates with VO projects (such as SkyAlert, NOAO (National Optical Astronomy Observatory [117]), and eStar) to form a system called the VO-GCN VOEvent Network. The VO-GCN system uses information on GRBs, or any other transients that are collected by the GCN, and processes this information into VOEvent packets. Once the VOEvent packet has been created, the VO-GCN system will

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distribute the new VOEvent packet to two different types of backbone servers (Jabber and JBOSS); each cooperating VO project has a software agent connected to one of these two backbone servers. When the new VOEvent packet is available in the backbone server, the backbone server will forward this new VOEvent packet to all online software agents via the Pub/Sub service [117] [223].

The GCN system reduces the time delay for delivery of the location information about GRBs from the spacecraft instruments to the follow up observation telescope. It also allows the GRB follow up parties to make maximum use of limited resources (labour and telescope time), by communicating what has already been done or is going to be done soon.

The GCN system now has more than 600 sites, and has successfully made more than 7,500 observations on around 1,300 GRBs, with a system up time of more than 98% over its 19 years of operation [118].

The VOEventNet [100][101] project is another typical example of a real time transient notification architecture project that aims to transport VOEvent packets from surveys to all interested astronomers, robotic telescopes and other astronomical computer systems in near real time. The VOEventNet project has three major concepts; broker, publisher, and subscriber. The broker is a server that maintains a list of event streams (each event stream represents a single survey), receives VOEvent packets from publishers and broadcasts to all subscribers. The broker also has the ability to know what type of VOEvent packet each subscriber wants to receive.

The surveys that fill the role of publisher are responsible for generating and publishing the VOEvent packet to the broker; each survey will publish their own VOEvent packets to the corresponding event stream on the broker.

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The astronomers, robotic telescopes and astronomical computer systems fill the role of subscribers. They establish a long term connection with the broker, subscribing to the event streams which interest them and waiting for new incoming VOEvent packets.

The VOEventNet architecture is built upon these three concepts and all transient event streams on the broker are publicly available. Moreover, the VOEventNet also implements a number of open source software clients for publication, subscription and reception of VOEvent packets to and from the broker; these clients are implemented based on mainstream program languages and can be download from the VOEventNet website.

After operating for a few years, the VOEventNet project has been replaced by another project called SkyAlert [104]. SkyAlert is a clearing house and repository of information about astronomical transients; each transient is described by a collection of VOEvent packets; people and machines can access information on recent and past transients through using either a Web page interface or a Web service interface. In fact, the SkyAlert project not only aims to provide a way to browse the historical information on transients, but it also aims to provide a system that can work with all transients in general. The advantages of a general transient system are increased interoperability and the avoidance of duplicated efforts in implementing similar systems. The core features of the SkyAlert system [105] [106] are:

x It acts as a web author publishing system. The survey’s role as author is that it is responsible for publishing information on newly discovered transients. However, SkyAlert requires that each author (survey) creates a sample event at the initial stage; the sample event must contain all of the necessary parameters and full metadata that might be present in the real event. After the sample event has been created, SkyAlert will create a new event stream on its server for the author. The author can use either the web form or the web service interface to publish information on newly discovered transients once the new event stream is online;

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x A web subscription and broadcast system. This is a list of event streams that are available for public subscription. Subscribers can obtain information on all new transients through the event stream they choose. Moreover, the system also allows subscribers to create custom feeds providing a Python expression on the values of parameters, so that only those transients that satisfy the expressions will be delivered to the subscribers, rather than delivering every new transient to the subscribers;

x It is a repository that stores all published transients, and it also allows users to query particular events on the web form;

Stream Name Belong to Project

AAVSO AAVSI Alerts and Special Notices

CRTS, CRTS2, CRTS3 Catalina Real time Transient Survey

CSS_NEO Catalina Sky Survey (Moving objects)

MOA MOA Microlensing Survey

OGLE OGLE Microlensing Survey

SWIFT SWIFT GRB alerts

HST_MCT CANDELS or CLASH (Supernova)

Table 3: Some available event streams on SkyAlert, the full list of event streams can found at the SkyAlert website [104].

x A multi-layer web page for browsing recent and past information of transients. Each transient has its own web page called a portfolio. The portfolio is a new concept invented by SkyAlert, and it is a collection of information about the transient, including the finder chart, event parameters and the VOEvent packet. The event portfolio also inherits the citation mechanism from the VOEvent packet. If an event cites another event, it will join the portfolio to which the other transient belongs. Otherwise, an event will have its own portfolio page. Moreover, the event portfolio page provides three different options to users browsing information about the transient; an overview, the parameters, and XML. The overview shows the finder chart, the light curve of the selected transient and

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photometry data of the transient, which is also provided through a clickable link. The params presents all parameters of the transients in a tabular format, while the XML option shows the actual VOEvent packet that was received by SkyAlert; and

x An open development platform allows for the implementation of a local client application, as well as the web based application.

The SkyAlert aggregates different sources of transient events into a single platform, but it is not only a repository system; it is also a transient notification system that allows surveys to share new information of transients with worldwide astronomers, amateur astronomers and telescopes in almost real time. This real time sharing feature is very important in the context of time critical astronomical research.

The concepts and protocols that SkyAlert, VOEventNet and GCN use are not the only way to providing real time transient notification. In the remaining sections of this chapter, we will discuss some concepts and protocols that are relevant in implementing a real time transient notification architecture.