__Examining the Time Dependence of the C-K edge__

The fits to many observations of several sources allowed
N_{H} to be free, as well as the C-K, N-K,
O-K and F-K edges to be unconstrained and, perhaps, negative. As in my
previous analysis, I scaled the uncertainties by
sqrt(chi^{2}) to account for unknown
systematic errors in a dubious statistical manner. The __new model__ of the C-K edge was used for
consistency. Fig. 1 shows the results from the fits and the linear
trend law that the optical depths follow.

Fig. 1. The C-K edge optical depth as a function of time for 11
observations of essentially featureless sources observed with the LETG
and ACIS. The solid line is a linear regression that is not forced to
go through zero, while the dashed line is a model with similar
asymptotic behavior that is forced to zero at ACIS door opening.

The data are well fit by a linear model but a perhaps more physical
model was adopted to allow the optical depth to be zero at the time of
the opening of the ACIS door. The equation for the time dependence
is

tau = a(t-y0) - a(t0-y0)exp( [t-t0]/q ),

where tau is the C-K optical depth, a is the asymptotic rate of increase
(0.455 opt. depths per year), t0 is the date (1999.70) at which tau is
forced to be zero, y0 is the year (1998.32) at which the linear
regression gives tau=0, and q is the estimated timescale on which the
optical depth approaches the asymptotic form: 0.15 yr.

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Herman Marshall

hermanm@space.mit.edu

Last updated July 7, 2003