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Postdoctoral Researcher FNRS.

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Research Associate FNRS.

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Research Director FNRS.

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... MSP18

The MSP numbers come from the numbering scheme devised by Moffat et al. (1991).

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... website

http://cxc.harvard.edu/

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... catalog

http://cdsarc.u-strasbg.fr/viz-bin/Cat?I/297

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... fluxes

For a typical source with absorbed thermal emission ( $N^{\rm H}_{\rm avg~West2}=10^{22}$ cm^-2 and kT = 0.3 keV), a photon flux of 1 photon cm^-2 s^-1 corresponds at the time of our observations to a count rate of 350 cts s^-1, an absorbed flux of $1.7\times10^{-9}$ erg cm^-2 s^-1 and an unabsorbed flux of $5.7\times10^{-8}$ erg cm^-2 s^-1.

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...)

Note that the values of the photon rates given in that table are raw, i.e. uncorrected for the encircled energy fraction, whereas the X-ray luminosity, derived from spectral fits, include that correction.

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... branch

Note that we need to be rather cautious about the results from the first observation, since during this exposure WR20a moved in and out of CCD2 due to the spacecraft's dithering.

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... bands

Since WR20a is heavily absorbed, there are insufficient counts to produce a lightcurve in the soft band.

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... zone

The $\chi$ cooling parameter of Stevens et al. (1992) amounts to about 0.02 for WR20a.

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... value

Note that due to the steep dependence on the pre-shock velocity, adopting v = 500 km s^-1 would imply a column density about 60 times larger, and a pre-shock velocity of 1500 km s^-1 a value ten times lower.

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... sources

Note that source A falls outside the field-of-view covered by our photometric data.

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