The extremely sensitive Chandra Deep Field-North ( CDF-N ) pencil-beam X-ray survey is used to identify and characterize the X-ray emission from old high-latitude main sequence Galactic stars .
Our principal goal is to investigate the expected long-term decay of magnetic activity of late-type stars due to the gradual spindown of stellar rotation from a magnetized stellar wind .

Thirteen X-ray sources are associated with late-type stars ; 11 of these constitute a well-defined sample for statistical analysis .
This sample consists of 2 G , 2 K0 - K4 , and 7 M2 - M5 stars with median V -band magnitude around 19 and median distance around 300 pc .
X-ray luminosities are typically \log L _ { x } \simeq 27 
erg s ^ { -1 } but are substantially higher in two cases .
The combination of large-amplitude variations on timescales of hours and plasma temperatures around 5 - 30 MK indicates that the observed X-ray emission is dominated by magnetic reconnection flares rather than quiescent coronal emission .
These X-ray properties are quantitatively similar to those seen in the active contemporary Sun .

The CDF-N stellar sample is compared to simulations based on convolution of X-ray luminosity functions ( XLFs ) with the known spatial distribution of old disk stars .
The model indicates that the CDF-N stars are the most magnetically active old disk stars .
A substantial decline in X-ray luminosities over the 1 < t < 11 Gyr age interval is required : 39 rather than 11 stars should have been detected if the XLF does not evolve over this time interval .
This is the first demonstration that the coronal and flaring components of stellar magnetic activity – and presumably the interior magnetic dynamos responsible for the reconnecting fields at the stellar surface – exhibit long-term decay over the age of the Galactic disk .
The model that best fits the magnitudes , spectral types and X-ray luminosities of the sample has L _ { x } \propto t ^ { -2 } erg s ^ { -1 } which is faster than the t ^ { -1 } decay rate predicted from widely accepted rotational spindown rates and X-ray-activity relations .