PyGCN

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.

Installation

To install PyGCN, simply run:

$ pip install --user pygcn

Usage

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.

Writing a custom GCN handler

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)

Filtering

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)

Threading

You can run the listener in a separate thread and communicate with it through Event and Queue instances. Here is an example:

#!/usr/bin/env python
import gcn
import threading
import queue # For python 2.7, import Queue

# Set up communications:
stopevent = threading.Event()
messagequeue = queue.Queue()
# Create a listen handler that enqueue the (payload, root) tuple
handler = gcn.handlers.queuehandlerfor(messagequeue)

# Create and start the thread.
thread = threading.Thread(target=gcn.listen,
            kwargs=dict(handler=handler, stopevent=stopevent))
thread.start()

# Wait for messages to come in, but do other things if they don't.
nothingcount=0
while thread.is_alive():
    try:
        # Use block=False if you want to timeout immediately 
        payload,root = messagequeue.get(timeout=10)
        print(root.attrib['ivorn'])
        nothingcount = 0
    except queue.Empty:
        # Do idle stuff here.
        print("Nothing...")
        nothingcount += 1
        if nothingcount > 10:
            print("Quitting due to inactivity")
            break

# Send a stop event to the listen thread, then wait for it to quit.
stopevent.set()
thread.join()
print("Done")