A. Hempelmann 1 - J. H. M. M. Schmitt 1 - S. L. Baliunas 2 - R. A. Donahue 2
1 - Universität Hamburg, Hamburger Sternwarte,
Gojenbergsweg 112, 21029 Hamburg, Germany
2 -
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 15, Cambridge, MA 02138, USA
Received 2 May 2003 / Accepted 11 June 2003
Abstract
We investigate a four-and-one-half year time-series of ROSAT HRI pointed observations
of 61 Cyg A and B and compare the X-ray light curves with the chromospheric Ca HK variability.
The ROSAT sampling rate was two pointings per year and typical errors lie in the range
of 5-10%.
The chromospheric cycles are well-known for both stars from the Mt. Wilson Ca HK survey. Although
the time basis of our ROSAT observations is shorter than the 7-and 12-year cycles of components A and B,
respectively,
we find the long-term trend of coronal activity in close correlation with the chromospheric activity
during the observation period, between 1993 and 1998.
The chromospheric activity increased through maximum activity down to a minimum for component A,
and from maximum to minimum activity for component B. The same behaviour is observed for the X-ray
light curves but with much higher amplitudes by factors 2.5-3. The remaining scatter observed around low-order
regression curves of coronal activity is small. We conclude that both stars do
show coronal cycles and that coronal cycles are the dominant source of variability for 61 Cygni.
Key words: stars: late-type - stars: activity - stars: coronae - stars: chromospheres
For stars rotating below the mass-dependent saturation limit,
the level of coronal activity of cool stars is determined by stellar rotation (Pizzolato et al. 2003).
Hempelmann et al. (1996) investigated the scatter around a relation
between X-ray stellar surface flux ()
and Rossby number (the Rossby number is the rotation
scaled by the convective turnover time) for a sample of stars with
clear chromospheric cycles as determined from the Mt. Wilson Ca HK survey (Baliunas et al. 1995).
They correlated the residuals to that relation with the known phases of
chromospheric cycles, and found (with a confidence of 2
)
that the X-ray residuals depend on the cycle phase
of chromospheric activity and claimed this as a hint of coronal cycles inherent the data.
One of those stars investigated was the cool star binary 61 Cygni.
61 Cygni is a well-studied nearby binary consisting of two late-type stars of spectral types K5V and K7V with an angular separation of only 26 arcsec.
Consequently it was observed with the ROSAT High Resolution Imager (HRI).
The result of these observations is
subject of this paper.
The ROSAT HRI observations of 61 Cygni were started in the middle of 1993 and stopped at the end
of the ROSAT operations in 1997. We obtained a total number of 9 pointings with
typical exposure times of a few kiloseconds giving typical Poisson
errors of 5-10% for each
flux measurement.
Concerning variations on time-scales of years, the typical sampling
of half a year should be sufficient.
The two stellar components are clearly resolved by the HRI field of view, making a standard reduction process possible: corrections for dead time, vignetting and
background subtraction. A count-rate to flux conversion factor of
ergs/cm2/counts
and a distance of 3.48 pc (Perryman et al. 1997) are
used to calculate X-ray luminosities. Their uncertainties will not affect our results because
we do not investigate absolute brightness but its variability.
The data are given in Table 1; they are also available from the ROSAT data
archive and from the ROSAT Database for Nearby X-ray and extreme UV emitting Stars, NEXXUS
(Schmitt & Liefke 2003).
Time | HRI count-rate | Errror | ![]() |
year | cts/s | cts/s | erg/s |
Component A | |||
1993.44 | 0.061 | 0.005 |
![]() |
1994.86 | 0.079 | 0.008 | 2.795 |
1994.86 | 0.071 | 0.003 | 2.506 |
1995.44 | 0.087 | 0.004 | 3.075 |
1995.95 | 0.063 | 0.003 | 2.224 |
1996.39 | 0.063 | 0.003 | 2.210 |
1996.87 | 0.048 | 0.003 | 1.690 |
1997.34 | 0.031 | 0.003 | 1.096 |
1997.84 | 0.046 | 0.005 | 1.602 |
Component B | |||
1993.44 | 0.028 | 0.003 |
![]() |
1994.86 | 0.026 | 0.005 | 9.042 |
1994.86 | 0.024 | 0.002 | 8.581 |
1995.44 | 0.026 | 0.002 | 9.239 |
1995.95 | 0.016 | 0.002 | 5.587 |
1996.39 | 0.013 | 0.002 | 4.563 |
1996.87 | 0.014 | 0.002 | 4.870 |
1997.34 | 0.014 | 0.002 | 5.017 |
1997.84 | 0.011 | 0.002 | 3.789 |
![]() |
Figure 2: As Fig. 1 but for the stellar component B. |
![]() |
Figure 3: Detail of Fig. 1 concerning the period of ROSAT HRI observations. The curve is a fourth order polynomial fit. |
![]() |
Figure 4: Time-series of the ROSAT HRI pointed observations of stellar component A. The curve is a second order polyxnomial fit. |
![]() |
Figure 5:
Linear correlation between ![]() |
Unfortunately, more than half of the ROSAT data points lie in the seasonal gaps of the Ca HK observations which hampers
a direct correlation analysis. In order to find a suitable Ca H+K S value for a comparison with
a ROSAT observation, we therefore attempted three methods.
First we took the Ca HK observation that is closest to a ROSAT data point.
Second, we made a linear interpolation between the two neighboured points and, third, we fit the whole
curve by a polynomial (regression curve in Fig. 3) and compared the value of the regression curve
at the time of ROSAT observation with the corresponding X-ray measurement. The results are given in Table 2.
The correlation between
and the interpolated S value (derived by help of the third method) is
demonstrated in Fig. 5.
The amplitude of the long-term X-ray variability is factors 2.5-3 larger than the factors 1.1-1.4 which were observed for the Ca HK cycles between 1993 and 1998. (However, the measure S of Ca HK activity is contaminated by constant sources like photospheric contributions, which will damp the observed amplitudes.)
Schmitt & Liefke (2003) compared the ROSAT PSPC RASS count-rates with count-rates of ROSAT PSPC
pointed observations of a sample of Gliese catalogue stars. There the scatter around unity is interpreted
as caused by stellar variability. They find a factor of four between maximium and minimum deviations.
Thus the cycle amplitudes of 61 Cygni A and B lie well inside this scatter band.
Method of comparison with Ca H+K S | A | B |
Closest S data point | 0.58 | 0.73 |
Interpolation between S neighbours | 0.68 | 0.74 |
S-curve fit value | 0.93 | 0.88 |
![]() |
Figure 6: Detail of Fig. 2 concerning the period of ROSAT HRI observations. The curve is a second order polyxnomial fit. |
There are a few stars of the Schmitt & Liefke (2003) sample which lie outside the scatter band mentioned above.
All these stars are known flare stars.
![]() |
Figure 7: Time-series of the ROSAT HRI pointed observations of stellar component B. The curve is a second order polyxnomial fit. |
![]() |
Figure 8:
Linear correlation between ![]() |
Acknowledgements
This work was supported by funds from the National Aeronautics and Space Administration (NAG5-7635) and the Air Force Office of Scientific Research (AF 49620-02-1-0194).