Astromon Components

Cross-Matching Algorithms

The process of finding counterparts for detected x-ray sources proceeds in two major steps:

  • Rough match: Find all counterparts within a radius around the x-ray sources. These rough matches constitute a superset of all reasonable cross-matches. This is done by rough_match, which encapsulates all queries to the Tycho2, ICRS, USNO-B1.0, 2MASS, SDSS catalogs and selects matches within 3 arcsec.

  • Cross-match. Pair each x-ray source with at most one counterpart from the rough matches. This is done by the compute_cross_matches function, which in turn delegates to specific implementation functions.

The default cross-match set is identified as "astromon_21" (see Pre-computed queries). This set is produced using the “simple” matching algorithm implemented in simple_cross_match.

Available catalogs

These are the catalogs available for rough matches:

  • Tycho2. Reference catalog of 2.5 million stars observed by the Tycho instrument abord the ESA Hipparcos satellite. Astrometric accuracy ~25 mas with stars down to ~11.5 mag. More information available in the Guide to the Tycho2 catalog (PDF).

  • ICRF2. Sources from The Second Realization of the International Celestial Reference Frame by Very Long Baseline Interferometry Ma et al. 2009, IERS Technical Note No. 35 (pdf)

  • USNO-B1.0. Monet, D.G. et al. (2003), “The USNO-B Catalog”, The Astronomical Journal, vol. 125, no. 2, pp. 984-993.

  • 2MASS. Two micron all-sky survey: 162,213,354 million point sources from 19,600 square degrees of sky.

  • SDSS. The SDSS Photometric Catalog, Release 7

Database

The main data product of Astromon is a database that contains the following tables:

  • Observations (astromon_obs). Observation info from OBSPARS.

  • X-ray sources (astromon_xray_src). Point-sources detected in Chandra observations.

  • Catalog sources (astromon_cat_src). Optical/Radio catalog entries in the vicinity of the X-ray sources

  • Cross-matches (astromon_xcorr). Pairs of X-ray and catalog sources, where there is at most one catalog source associated with a given detected X-ray source.

  • Excluded regions (astromon_regions). Circular regions within which all x-ray sources are excluded from analysis.

Astromon data can be accessed through the astromon.db module using convenience functions that return the tables as an Astropy Table

from astromon import db
matches = db.get_cross_matches()
observations = db.get_table('astromon_obs')
xray_src = db.get_table('astromon_xray_src')
cat_src = db.get_table('astromon_cat_src')
xcorr = db.get_table('astromon_xcorr')
regions = db.get_table('astromon_regions')

Note

One can modify the behavior of astromon.db and specify a different database file by defining the ASTROMON_FILE environmental variable.

Observations

The access to proprietary information related to observations is encapsulated in the astromon.observation module. The main component is the Observation class, which encapsulates all calls to arc5gl, CIAO scripts, etc

The following statement instanciates an observation:

from astromon.observation import Observation
obs = Observation(8008)

Upon creation, if the files corresponding to the observation are not available locally, it will use arc5gl to download them. The files are stored locally in a temporary directory unless a working directory is given to the constructor:

from astromon.observation import Observation
obs = Observation(8008, workdir='./astromon/work')

in which case the files will persist after the observation instance is deleted.

The Observation class encapsulates several common operations with CIAO. The following runs a prescribed sequence of commands that should result in the creation of files with detected sources:

obs.process()