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FO Aqr time-series observations requested

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發表於 2016-7-14 06:09:35 | 顯示全部樓層 |閱讀模式
AAVSO Alert Notice 545
July 13, 2016

FO Aqr time-series observations requested

Dr. Colin Littlefield (University of Notre Dame) and colleagues
Drs. Peter Garnavich (Notre Dame), Erin Aadland (Minnesota State),
and Mark Kennedy (University College Cork) have requested AAVSO
assistance in monitoring the intermediate polar cataclysmic
variable FO Aqr beginning immediately.

Dr. Littlefield, who with his colleagues recently published ATel
#9216 (http://www.astronomerstelegram.org/?read=9216) and #9225
(http://www.astronomerstelegram.org/?read=9225), writes:

"This system is in a faint state for the first time in its
observational record, implying a dropoff in the mass-transfer rate.
AAVSO observations contributed by Shawn Dvorak [the only observer
following FO Aqr at the time] were particularly helpful in detecting
this low state. Since early May, the system has recovered to V~15,
but it is still well below its normal brightness. In addition, our
time-series photometry shows a very strong 11.26-minute photometric
period. By contrast, during its bright state, FO Aqr's light curve
is dominated by a 20.9-minute period, corresponding with the spin
period of the white dwarf. We interpret our observations as evidence
that the system's accretion processes have changed dramatically as
a result of the reduced mass-transfer rate.

"We have...determined that...[the 11.26-min] periodicity is
dependent on the orbital phase of the binary. The 11.26-min period
is dominant for about half of the orbit, but for the other half, a
22.5-min period is stronger. AAVSO observers can help us study both
of these periods as well as their dependence on the orbital phase.
We are particularly interested in any changes in this behavior as
the system continues to brighten.

"...Time-series photometry of FO Aqr [is requested] in order to
better study the evolution of the 11.26-minute period as the system
rebrightens. Unfiltered photometry reduced with a V zeropoint would
be the most useful to us... A cadence of less than 60 seconds per
image is important, given the brevity of these periods (especially
the 11.26-min period). Long, uninterrupted time series would be
especially valuable, but even a short time series would be useful
in terms of monitoring the 11-minute period. Also, it's critical
that observers ensure that their computer clocks are accurate to
within a few seconds."

Littlefield reports that as of 2016 July 12-13, FO Aqr was "still
much fainter than usual, at V~14.93 (unfiltered with V zeropoint).
The data remain consistent with a gradual rebrightening at an
average rate of about -0.01 mag/day." FO Aqr's normal brightness
is V~13.4.

"We would like the time-series observations to continue until the
system returns to its normal brightness level or until it gets too
close to the Sun to be observed (whichever comes first)."

Coordinates (2000.0): R.A. 22 17 55.38  Dec. -08 21 03.8

Charts: Charts for FO Aqr with an expanded comparison star sequence
may be created using the AAVSO Variable Star Plotter (VSP) at
https://www.aavso.org/vsp. For observers who have been following
FO Aqr as part of the CBA program, the comparison stars used for
that program have been retained.

Please submit observations to the AAVSO International Database
using the name FO AQR.

This campaign is being monitored on the AAVSO Observing Campaigns
webpage at https://www.aavso.org/observing-campaigns.


This AAVSO Alert Notice was compiled by Elizabeth O. Waagen.

----------------------------------
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