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diff --git a/python/randomize_events.py b/python/randomize_events.py
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+#!/usr/bin/env python3
+#
+# Randomize the (X,Y) position of each X-ray photon events according
+# to a Gaussian distribution of given sigma.
+#
+# References:
+# [1] G. Scheellenberger, T.H. Reiprich, L. Lovisari, J. Nevalainen & L. David
+#     2015, A&A, 575, A30
+#
+#
+# Aaron LI
+# Created: 2016-03-24
+# Updated: 2016-03-24
+#
+
+from astropy.io import fits
+import numpy as np
+
+import os
+import sys
+import datetime
+import argparse
+
+
+CHANDRA_ARCSEC_PER_PIXEL = 0.492
+
+def randomize_events(infile, outfile, sigma, clobber=False):
+    """
+    Randomize the position (X,Y) of each X-ray event according to a
+    specified size/sigma Gaussian distribution.
+    """
+    sigma_pix = sigma / CHANDRA_ARCSEC_PER_PIXEL
+    evt_fits = fits.open(infile)
+    evt_table = evt_fits[1].data
+    # (X,Y) physical coordinate
+    evt_x = evt_table["x"]
+    evt_y = evt_table["y"]
+    rand_x = np.random.normal(scale=sigma_pix, size=evt_x.shape)\
+            .astype(evt_x.dtype)
+    rand_y = np.random.normal(scale=sigma_pix, size=evt_y.shape)\
+            .astype(evt_y.dtype)
+    evt_x += rand_x
+    evt_y += rand_y
+    # Add history to FITS header
+    evt_hdr = evt_fits[1].header
+    evt_hdr.add_history("TOOL: %s @ %s" % (
+            os.path.basename(sys.argv[0]),
+            datetime.datetime.utcnow().isoformat()))
+    evt_hdr.add_history("COMMAND: %s" % " ".join(sys.argv))
+    evt_fits.writeto(outfile, clobber=clobber, checksum=True)
+
+
+def main():
+    parser = argparse.ArgumentParser(
+            description="Randomize the (X,Y) of each X-ray event")
+    parser.add_argument("infile", help="input event file")
+    parser.add_argument("outfile", help="output randomized event file")
+    parser.add_argument("-s", "--sigma", dest="sigma",
+            required=True, type=float,
+            help="sigma/size of the Gaussian distribution used" + \
+                 "to randomize the position of events (unit: arcsec)")
+    parser.add_argument("-C", "--clobber", dest="clobber",
+            action="store_true", help="overwrite output file if exists")
+    args = parser.parse_args()
+
+    randomize_events(args.infile, args.outfile,
+            sigma=args.sigma, clobber=args.clobber)
+
+
+if __name__ == "__main__":
+    main()
+