Source ranking

In some XRT observations of unknown transients, we may rank the sources according to how likely they are to be the counterpart to the original trigger. The ranks run from 1—4, with 1 indicating that the source is very likely to be the counterpart, and 4 indicating that the source is very unlikely to be the counterpart. The details of how these ranks are defined are below.

Conversions from counts to flux or between different X-ray instruments are calculated assuming a typical AGN spectrum: an absorbed power-law with N_H=3×10²⁰ cm^-2, Γ=1.7. Where we define a source as ‘(un)catalogued in X-rays’, this is compared to the Master X-ray catalogue at HEASARC and LEDAS; and the LSXPS catalogue. Other X-ray catalogues not currently in HEASARC are not searched at the present time.

Rank 1

Lies within 200 kpc of a known galaxy from the GWGC catalogue (distance in projection, at the galaxy's distance from Earth).
AND is either:
- Uncatalogued in X-rays, and at least 5-σ above the 3-σ upper limit from RASS, XMM or LSXPS (whichever is deepest).
- OR Catalogued in X-rays, but with a flux at least 5-σ above the catalogued flux.

Rank 2

The source is either:

Uncatalogued in X-rays, and at least 3-σ above the 3-σ upper limit from RASS, XMM or LSXPS (whichever is deepest).
OR Catalogued in X-rays, but with a flux at least 3-σ above the catalogued flux.

Rank 3

The source is uncatalogued in X-rays, but meets none of the above criteria, i.e. it cannot be distinguished from an unrelated background object from the available data.

Rank 4

The source is a catalogued X-ray source, and the measured XRT flux is consistent with or below the catalogued flux.