README.md

xmatch

Implementation of cross-matching algorithms for astronomical catalogs.

Cross-match is a daily procedure in astrophysics, and although it's been around virtually since ever it is a non-trivial hot topic when multiwavelength catalogs are in hands.

Cross-matching has on its basis the relative position of objects in the projected sky: the same object seeing by two different instruments should have the same (or very close) registered positions in the sky. This is an absolute true when if/when the object emits light similarly in the frequencies both instruments observe, but may get a bit non-linear when the object differently in different wavebands or the instruments observe in quite different resolutions.

The xmatch tool is python interface to the cross-matching procedure, implmenting the most commong algorithms in use:

• bidirectional nearest-neighbour (nn)
• great-circle (gc): nn with maximum distance threshold
• maximum likelihood estimator (mle): a feature column is used to evaluate multiple counterpart candidates

Install

# python setup.py install

or

# pip install .

Using it

Consider we have two catalogs "A" and "B" with at least the columns ra, dec, id (for right ascension, declination, identifier, resp.) that we want to cross-match.

If all we want is to "find the nearest object in catalog 'B' (aka, counterpart) for each object from 'A' (target), if it is within a limiting 'radius'; if it is not, say none was found", then we run xmatch on its default:

>>> radius = 5 # arcsec
>>> from xmatch import xmatch
>>> out_x = xmatch(catalog_A[['ra','dec','id']],
                   catalog_B[['ra','dec','id']],
                   radius=radius)

out_x provides a table containing all entries from catalog_A and the corresponding counterpart -- if any -- from catalog_B.

Examples

Check out xmatch/docs/notebooks for some examples.

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