Anonymous VOEvent client for receiving GCN/TAN notices in XML format
The Gamma-ray Coordinates Network/Transient Astronomy Network (GCN/TAN) is a system for distributing astronomical alerts, largely focused on operations of and detections from high-energy satellite missions.
GCN/TAN disseminates both Notices (prompt, machine-readable alerts) and Circulars (human-readable correspondence) through a handful of delivery methods and formats.
This package implements a simple client that listens for VOEvent XML format notices over the custom TCP/IP VOEvent Transport Protocol. By default, it connects to one of the anonymous GCN/TAN server, so no sign-up or configuration is necessary to begin receiving alerts.
To install PyGCN, simply run:
$ pip install --user pygcn
PyGCN provides an example script called pygcn-listen that will simply write
all VOEvents that it receives to files in the current directory. To try it out,
simply run:
$ pygcn-listen
and then type Control-C to quit.
You can also write your own handler that performs a custom action for every GCN
that is received. A handler function takes two arguments: payload, the raw
content of the GCN, and root, the root element of the XML document as parsed
by lxml.etree. Here is a basic example:
#!/usr/bin/env python
import gcn
# Define your custom handler here.
def handler(payload, root):
# Get the IVORN, or unique VOEvent ID, and print it.
print(root.attrib['ivorn'])
# Print all of the event attributes.
for param in root.findall('./What/Param'):
name = param.attrib['name']
value = param.attrib['value']
print('{} = {}'.format(name, value))
# Listen for VOEvents until killed with Control-C.
gcn.listen(handler=handler)You can also filter events by notice type using
gcn.include_notice_types or gcn.exclude_notice_types.
Here is an example:
#!/usr/bin/env python
import gcn
# Define your custom handler here.
@gcn.include_notice_types(
gcn.notice_types.FERMI_GBM_FLT_POS, # Fermi GBM localization (flight)
gcn.notice_types.FERMI_GBM_GND_POS, # Fermi GBM localization (ground)
gcn.notice_types.FERMI_GBM_FIN_POS) # Fermi GBM localization (final)
def handler(payload, root):
# Look up right ascension, declination, and error radius fields.
pos2d = root.find('.//{*}Position2D')
ra = float(pos2d.find('.//{*}C1').text)
dec = float(pos2d.find('.//{*}C2').text)
radius = float(pos2d.find('.//{*}Error2Radius').text)
# Print.
print('ra = {:g}, dec={:g}, radius={:g}'.format(ra, dec, radius))
# Listen for VOEvents until killed with Control-C.
gcn.listen(handler=handler)
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