An Estimate of the Sun’sROSAT‐PSPC X‐Ray Luminosities UsingSNOE‐SXP Measurements

Abstract

Using solar soft X-ray irradiance measurements from the SXP instrument on the SNOE satellite, we relate the solar surface flux densities and their variability to those of stars as measured with the PSPC instrument on ROSAT. We translate SNOE-SXP measurements into equivalent ROSAT-PSPC counts using model spectra calculated from the CHIANTI package. Using the SNOE-SXP measurements has significant advantages over earlier studies: the absolute calibration is known to AE25%, SNOE measures the Sun as if it were an unresolved star, it has operated over a significant fraction of the solar cycle, and its three wavelength channels overlap substantially with that of the ROSAT-PSPC instrument. The predicted solar X-ray luminosities and surface flux densities are compared with measurements from the ROSAT database. We find that we can estimate the luminosity of the Sun as seen in the 0.1-2.4 keV ('' RASS '') passband of ROSAT-PSPC to within AE50%, not counting sources of systematic uncertainty mentioned in an appendix: the result lies between 10 27:1 and 10 27:75 ergs s À1 (measured in the existing data set, only partially covering a full solar cycle) and between 10 26:8 and 10 27:9 ergs s À1 (extrapolated to the full activity range of a typical solar cycle). The solar luminosities lie close to the median behavior found for a volume-limited (d < 13 pc) sample of G stars studied in 1997 by Schmitt, revealing the Sun to be a normal or slightly inactive G dwarf. A factor of 1.5 peak-to-peak variation in the RASS passband is predicted due simply to rotational modulations (i.e., those filtered to include periods P < 81 days). The ratio of maximum/minimum RASS luminosities from the magnetic activity cycle (filtered to include periods P > 81 days) are estimated to be 0.7-0.8 in log 10 L RASS , a ratio of 5 or 6. These variations are much smaller than both recent estimates of solar X-ray variability and the range of X-ray luminosities seen within Schmitt's sample. It is suggested that the reported absence of '' solar-like '' cyclic emission in stellar X-rays might partly arise because the Sun is less variable than assumed in some earlier work. Repeated ROSAT observations of Cen A during 1995-1998 show X-ray behavior reminiscent of the Sun during activity minimum conditions.

Coronal activity cycles in nearby G and K stars

“…The full Sun has at activity minimum, adapting log L X = 26.8 erg s −1 (Judge et al 2003), an average surface flux of log F X = 4.0, i.e. slightly above the minimum value of log F X = 3.9.…”

“…The Sun exhibits an average cycle period of 11 yr and has an X-ray luminosity range of L X ≈ 0.5−6 × 10 27 erg s −1 , corresponding to an average log L X /L bol = −6.2 (see e.g. Judge et al 2003). Sorting the three G2 stars by increasing activity, i.e.…”

“…27.5 erg s −1 in the ROSAT 0.1−2.4 keV energy band (Peres et al 2000). However, for the Sun the calculation of a disk-integrated X-ray flux as it is observed for stars is far from trivial, as can be concluded from the divergent X-ray brightness estimations presented by Judge et al (2003). While at harder X-ray energies the amplitude of the variations can easily be of the order of hundred as observed by the Yohkoh satellite (Orlando et al 2001), in the chromospheric Ca ii H+K lines or in transition region lines observed at UV wavelengths these variations are much more scarce, ranging from several tens of percent up to a factor of a few (e.g.…”

X-ray emission regimes and rotation sequences in M 35

“…with log(L X, /L ) = −6.24 (Judge et al 2003), P = 26.09 days (Donahue et al 1996), and τ c, = 14.45 days (Wright et al 2011). I considered two models of X-ray luminosity distributions in M 35 using two different hypotheses.…”

The Cycles of Alpha Centauri

“…Consequently, the solar HK index varies only about 25% over the solar cycle, while coronal X-rays can change by factors of several to a hundred, depending on the energy band, although the details remain in some dispute. For example, Judge, Solomon, & Ayres (2003) deduced a modest factor of ∼6 change in the solar 0.1-2.4 keV X-ray luminosity over the rise of current Cycle 23, based on Cycles of α Cen 147 ultra-soft X-ray photometry from the SNOE satellite; compared with the more extreme ∼ 30× variations inferred from higher energy experiments like Yohkoh SXT (Acton 1996).…”