Stellar X-ray sources in the eROSITA early data release

The incidence (or space-density) of "eROSITA-stars"

I took the occasion of the first eROSITA data release (in the 150sqdeg full-depth field) to make a few plots to wrap my mind around 'which stars emit X-ray emission'.

I started with the primary optical identification catalog, then took the objects that have Gaia counter-parts, then took those that had an expected plxSN>3 (by means of mag and plx cut, see Rix,Hogg+21) and looked at them: a CAMD with the points color-coded  by L_X/L_G

Note that the L_G is simply the Gaia G-band flux multiplied by a (bandpass width) of 4000A (rough).

Clear pattern! But to which extent does that reflect a "selection effect"?

If we just look at the fluxes, we get:

And if we color-code the CAMD by just X-ray luminosity, we get:

Is this interesting?

• what's the role of the vast majority of X-ray undetected sources?
• calculate space density = f(M,color | L_x) or f(M,color | L_x/L_G)
• look-up the X-ray spectral diagnostics (in the eROSITA papers): accretion disk, vs chromospheres, etc.., as different physical processes play a role here.