ROSAT All Sky Survey Which Stars are XRay Emitters

Within the framework of the ROSAT All-Sky Survey (RASS), it was possible to carry out a sensitive and unbiased survey of X-ray emission from all types of stars.

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Fig. 10.4 Simultaneous optical and ROSAT X-ray observation of the flare star EQ Peg during a flare

Quite a number of stellar surveys had previously been carried out with the Einstein Observatory [52], and many of the key results of stellar X-ray astronomy had already been obtained with such data. The big surprise of the first data obtained with the Einstein Observatory was the fact that X-ray emission was produced by many different types of stars at levels exceeding the X-ray output of the Sun by many orders of magnitude. If stars emitted X-rays at the level observed from the Sun, only stars in the immediate neighborhood of the Sun would be detectable. The characteristic value for the sensitivity limit of the RASS is a limiting X-ray flux of «2 x 10~13 erg cm~2 s_1, which implies that X-ray emission at solar-like levels of Lx«2 x 1027 erg s^1 can be detected only out to distances of 10pc in the RASS.

As a starting point of our investigations of the X-ray properties of the solarlike stars, let us consider the brightest stars contained in the Bright Star Catalog

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optical and ROSAT X-ray observation of the flare star

(BSC) and the nearest stars as contained in the Gliese catalog ([12,18]). The BSC comprises all stars down to a visual magnitude of 6.5 mag, and has around 10000 entries. Being a magnitude-limited catalog, its composition in terms of spectral type is biased toward intrinsically bright stars, and consequently contains many stars of spectral type A and F as well as giant stars, with a deficit of intrinsically faint but nearby G, K, and M dwarfs. The Gliese catalog on the other hand is a volume-limited catalog of all stars («3 200) known within a distance of 25 pc around the Sun. Note that the latter catalog is incomplete at the very faintest magnitudes and especially among ultracool dwarfs. By its very construction, the Gliese catalog is composed mostly of late-type dwarf stars of spectral type K and M, while its content of earlier type stars overlaps with the BSC.

In the left panel of Fig. 10.5, we plot a color-magnitude diagram of all RASS detected stars contained in the BSC and/or Gliese catalogs. As is apparent, all types of stars commonly placed in the color-magnitude diagram - with the exception of white dwarfs - are found to be X-ray emitters. For most stars shown in Fig. 10.5 (left panel) trigonometric distances are known and thus reliable X-ray luminosities can be computed. Figure 10.5 (right panel) shows X-ray luminosity vs. B — V color, which is used as an indicator of the stars' effective temperature, for the BSC stars (green) and Gliese stars (red). A huge spread of X-ray luminosities of up to four

BSC/Gliese detections -to

BSC/Gliese detections 33

BSC/Gliese detections -to

BSC/Gliese detections 33

-0.50.0 0.5 1.0 1.5 2.0 B-V Color

Fig. 10.5 Left: Color-magnitude diagram of RASS detected BSC and/or Gliese catalog stars from [14,19]. For main-sequence stars, the B-V color is a measure of spectral type. Color coding refers green Lx = 1028 — 1029 erg s—1, blue

Fig. 10.5 Left: Color-magnitude diagram of RASS detected BSC and/or Gliese catalog stars from [14,19]. For main-sequence stars, the B-V color is a measure of spectral type. Color coding refers green Lx = 1028 — 1029 erg s—1, blue and red L

to the X-ray luminosity: yellow denotes Lx < 1028 erg s 1

Lx = 1029 — 1030 erg s—1 BSC and Gliese stars vs. B V color

,x > 1030 erg s 1. Right: X-ray luminosity for RASS detected orders of magnitude from stars with given B -V color is apparent. Since most of the stars shown in Fig. 10.5 are main-sequence stars, it is also clear that the "fractional" X-ray luminosity, i.e., the ratio Lx/Lbol, also varies over many orders of magnitude from star to star. An understanding of the cause of these variations is one of the central themes of stellar X-ray astronomy.

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