We assess the contribution to the X-ray ( above 2 keV ) luminosity of the Milky Way from different classes of low-mass binary systems and single stars .
We begin by using the RXTE Slew Survey of the sky at |b| > 10 ^ { \circ } to construct an X-ray luminosity function ( XLF ) of nearby X-ray sources in the range 10 ^ { 30 } ~ { } { erg } ~ { } { s } ^ { -1 } < L _ { \mathrm { x } } < 10 ^ { 34 } ~ { } { erg } ~ { } { s% } ^ { -1 } ( where L _ { \mathrm { x } } is the luminosity over 2–10 keV ) , occupied by coronally active binaries ( ABs ) and cataclysmic variables ( CVs ) .
We then extend this XLF down to L _ { \mathrm { x } } \sim 10 ^ { 27.5 } ~ { } { erg } ~ { } { s } ^ { -1 } using the Rosat All-Sky Survey in soft X-rays and available information on the 0.1–10 keV spectra of typical sources .
We find that the local cumulative X-ray ( 2–10 keV ) emissivities ( per unit stellar mass ) of ABs and CVs are ( 2.0 \pm 0.8 ) \times 10 ^ { 27 } and ( 1.1 \pm 0.3 ) \times 10 ^ { 27 } erg s ^ { -1 } M _ { \odot } ^ { -1 } , respectively .
In addition to ABs and CVs , representing old stellar populations , young stars emit locally ( 1.5 \pm 0.4 ) \times 10 ^ { 27 } erg s ^ { -1 } M _ { \odot } ^ { -1 } .
We finally attach to the XLF of ABs and CVs a high luminosity branch ( up to \sim 10 ^ { 39 } erg s ^ { -1 } ) composed of neutron-star and black-hole low-mass X-ray binaries ( LMXBs ) , derived in previous work .
The combined XLF covers \sim 12 orders of magnitude in luminosity .
The estimated combined contribution of ABs and CVs to the 2–10 keV luminosity of the Milky Way is \sim 2 \times 10 ^ { 38 } erg s ^ { -1 } , or \sim 3 % of the integral luminosity of LMXBs ( averaged over nearby galaxies ) .
The XLF obtained in this work is used elsewhere ( Revnivtsev et al . )
to assess the contribution of point sources to the Galactic ridge X-ray emission .