Daniel E. Reichart1 and Andrew W. Stephens2
We report 1405 MHz measurements of the flux density of the approximately 320
year old supernova remnant Cassiopeia A, relative to the flux density of Cygnus
A, made between 1995 and 1999. When compared to measurements made between 1957
and 1976, we find that the rate at which Cassiopeia A has been fading at this
and nearby frequencies has changed from approximately 0.9 % yr^-1 in the 1960s
to approximately 0.6 - 0.7 % yr^-1 now. Furthermore, we have collected from
the literature measurements of this fading rate at lower (38 - 300 MHz) and
higher (7.8 - 16.5 GHz) frequencies. We show that the fading rate has dropped
by a factor of approximately 3 over the past 50 years at the lower frequencies,
while remaining relatively constant at the higher frequencies, which is in
agreement with the findings of others. Our findings at 1405 MHz, in
conjunction with a measurement of the fading rate at the nearby frequency of
927 MHz by Vinyajkin (1997), show an intermediate behavior at intermediate
frequencies. We also find that Cassiopeia A, as of approximately 1990, was
fading at about the same rate, approximately 0.6 - 0.7 % yr^-1, at all of these
frequencies. Future measurements are required to determine whether the fading
rate will continue to decrease at the lower frequencies, or whether Cassiopeia
A will now fade at a relatively constant rate at all of these frequencies.
In August of 1995, 1996, 1997, and 1999, we took drift scans of
Cassiopeia A and Cygnus A using the 40-foot telescope at the National
Radio Astronomy Observatory in Green Bank, WV.
Prior to the transit of each source, we manually adjusted the
declination of the telescope, ensuring, with the aid of a strip chart
recorder, that the pointing of the telescope contributed no more than
approximately 0.5 % of error to the flux density measurement.
We are extremely grateful to Sue Ann Heatherly for her dedication to educational and outreach activities at Green Bank, and for making the 40-foot telescope and the facilities at Green Bank available to ERIRA since 1992. We are also very grateful to Carl Chestnut for the technical assistance he has given us over the years. We also want to thank Dan Fellows, Randy Bish, and Alan Fuller for their past contributions to ERIRA, and Walter Glogowski and Jeremy Garris for their continued dedication to the program. Last, but not least, we want to thank the over 100 students and educators who have participated in ERIRA over the past eight years, many of whom helped to collect the data presented in this paper.
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