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Introducing Einstein Probe, Astro Flavored Markdown, and Notices Schema v4.0.0. See news and announcements

GCN Notices

Notices are real-time, machine-readable alerts that are submitted by participating facilities and redistributed publicly.

GCN Classic and the new GCN offer multiple file formats and protocols for receiving notices. We strongly encourage use of the new GCN, as we gradually migrate all services, and new notice types will only be available via the new GCN.

ProtocolFile FormatSystemSubscriptionStatus
Kafka*160-byte binary, VOEvent XML, Text, JSONNew GCNSelf ServicePreferred
Email* ( binary, VOEvent XML, TextNew GCNSelf ServicePreferred
Socket160-byte binaryGCN ClassicManualActive but recommend Kafka instead
VOEventVOEvent XMLGCN ClassicManual or AnonymousActive but recommend Kafka instead
Email (capella2)TextGCN ClassicManualActive but recommend email via new GCN
SMSShort TextGCN ClassicManualActive (future new GCN enhancement)

*Note that GCN Classic attachments are not available via the New GCN.

  • For legacy applications

    GCN Classic

    Data flow diagram for GCN Classic

    Three formats, three protocols.

  • Recommended

    GCN Classic over Kafka

    Data flow diagram for GCN Classic over Kafka

    Three formats, one protocol.

  • Coming soon

    GCN Kafka

    Data flow diagram for GCN Kafka

    One format, one protocol.

Distribution Protocols

The new GCN is built upon the Kafka data streaming protocol. We encourage the use of Kafka for automated pipelines, robotic telescopes, and users who wish to stream the notices in real time. GCN also provides self-managed subscriptions to any of the legacy notice formats (binary, VOEvent XML, text) via email. All new notice types are available only via the new GCN in JSON formats over Kafka. Human-readable text versions of the JSON formats are in development.

GCN Classic will continue to maintain distribution of all legacy notice types via manual subscriptions to socket connections, VOEvent, and email, until at least the end of the O4 LVK observing run. However, we strongly encourage new sites to utilize the subscription methods of the new GCN, and for users to transition to the new system.

File Formats

JSON (JavaScript Object Notation) is a file that stores simple data structures and objects. They are text based, human readable, and easily parseable.

VOEvent is an XML-based text format generalized for astronomical transients. It was developed by the International Virtual Observatory Alliance (IVOA). VOEvent format Notices are available via the GCN VOEvent brokers or preferably via Kafka.

Binary is a custom 160-byte packet protocol developed for GCN Classic socket connections. The socket service remains active, but we encourage users to receive these messages via Kafka.

Text format is a human-readable message developed for GCN Classic and distributed via email.


Notices have fixed pre-defined schema.

GCN Classic Schema

GCN Classic binary and text schema are bespoke for each notice type with some commonality across missions. Detailed descriptions for each notice type are available in the mission descriptions. VOEvent schema are defined from the IVOA standard.

Unified Schema

The GCN team has created new JSON format custom schema designed specifically for the types of transients reported by current and upcoming GCN producer missions. The core schema are designed to be building blocks containing most of the common fields used by the majority of instruments. Additional parameters can be added, but we encourage only including fields useful for informing consumers about the properties of the event useful for alerting and follow-up observations. They should not include fields that can easily be calculated using common software packages (e.g. coordinate transformations, distance from Sun or Moon).

More details on schema are available on the individual Mission pages and in the Schema Browser.

For more details on how to use GCN Notices, see:

  • Consuming to receive GCN Notices by Kafka or Email
  • Producers for setting up new notice types
  • Archive for how to search for and browse GCN Notices
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