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+# Copyright (c) 2017 Weitian LI <liweitianux@live.com>
+# MIT license
+
+"""
+Chandra ACIS spectrum.
+"""
+
+
+from astropy.io import fits
+
+from acis import ACIS
+
+
+class Spectrum:
+ """
+ Chandra ACIS spectrum
+ """
+ def __init__(self, filepath):
+ self.filepath = filepath
+ self.fitsobj = fits.open(filepath)
+ ext_spec = self.fitsobj["SPECTRUM"]
+ self.header = ext_spec.header
+ # spectral data
+ self.channel = ext_spec.data.columns["CHANNEL"].array
+ self.counts = ext_spec.data.columns["COUNTS"].array
+ # spectral keywords
+ self.EXPOSURE = self.header.get("EXPOSURE")
+ self.BACKSCAL = self.header.get("BACKSCAL")
+
+ def calc_pb_flux(self, elow=9500, ehigh=12000):
+ """
+ Calculate the particle background flux:
+ flux = counts / exposure / area
+
+ Parameters
+ ----------
+ elow, ehigh : float, optional
+ Lower and upper energy limit for the particle background.
+ """
+ chlow = ACIS.energy2channel(elow)
+ chhigh = ACIS.energy2channel(ehigh)
+ counts = self.counts[(chlow-1):chhigh].sum()
+ flux = counts / self.EXPOSURE / self.BACKSCAL
+ return flux