FRESIP Project observations of cataclysmic variables would provide unique data sets. In the study of known cataclysmic variables they would provide extended, well sampled temporal photometric information and in addition, they would provide a large area deep survey; obtaining a complete magnitude limited sample of the galaxy in the volume cone defined by the FRESIP field of view

Howell, Steve B.

English

NASA Technical Reports Server

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N95. 14507
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FRESIP Project Observations of Cataclysmic Variables -
A Unique Opportunity
Steve B. Howell
Planetary Science Institute, 620 N. 6th Avenue, Tucson, AZ 85705
Abstract. FRESIP Project observations of cataclysmic variables would
provide unique data sets. In the study of known cataclysmic variables
they would provide extended, well sampled temporal photometric infor-
mation and in addition, they would provide a large area deep survey;
obtaining a complete magnitude limited sample of the galaxy in the vol-
ume cone defined by the FRESIP field of view.
1. Introduction
If one reviews the many leaps of understanding that have been made in observa-
tional astronomy, it becomes clear that most of these have come about through
many year efforts and dedicated observational programs. The classification of
stellar spectra, the National Geographic/Palomar Observatory Sky Survey, long
term records of certain variable stars collected by the AAVSO, objective prism
surveys for QSO's, and the Hubble guide star catalogue are but a few examples.
The FRESIP Project can provide astronomers with another leap into the
understanding of planetary systems and could potentially provide humankind
with much understanding about our existence, as well as simultaneously giving
us a better view and understanding of our own galaxy and the universe beyond.
One of the unique opportunities available from the FRESIP Project is in
the observational study of cataclysmic variables. These binary systems consist
of a low mass red main sequence star in orbit about a white dwarf primary. The
red star fills its Roche lobe and material flows through the inner Lagrangian
point to usually form an accretion disk about the white dwarf. Some of these
systems have highly magnetic white dwarfs in which the associated magnetic
fields are so strong that the flow of material is interrupted and only an inner
disk or even no disk at all is permitted to form.
Three basic types of CV exist. These are the dwarf novae (DN) which have
fairly regular outbursts due to disk instabilities. These outbursts have ampli-
tudes of usually 2-5 magnitudes but a larger amplitude group (with outburst
amplitudes up to 10 magnitudes) exists. Novalike systems (NL) are similar to
DN but are essentially always in outburst as they have a high rate of mass
transfer. Finally there are the Novae (N) which undergo thermonuclear out-
bursts with long temporal spacing. Mass transfer rates from the secondary to
the primary and the orbital period of the system are the basic causes for the
different types of CV.
Low mass X-ray binaries (LMXRBs) are related systems in which the sec-
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ondary can be a subgiant or giant and the primary is a neutron star or black
hole.
In both cataclysmic variables and LMRXBs, there is a poor understanding
of any temporal process. These systems are usually faint enough that they can
not be monitored by amateur groups, are not easy to perform detailed studies
on as they require large telescope time, and they spend most of their time in
quiescence. Except for a few bright, somewhat well studied, probably nontypical
systems, we are at a loss at understanding their behavior and evolution. This
is unfortunate for astronomy, as CVs are very good laboratories for accretion
studies. They undergo changes on minute to hour to daily to longer timescales,
accretion and accretion disks are important for our understanding not only of
CVs, but also our understanding of accretion processes in other astrophysical
objects such as AGNs and newly forming solar systems.
In the following paper, I emphasize the need and value of essentially com-
plete temporal monitoring of CVs that the FRESIP telescope could provide.
The types of data sets which would be available divide up into two major areas;
1) A well sampled temporal study of known or newly discovered CVs within
the FRESIP field of view (FOV) and 2) A deep survey conducted in some test
areas within the FRESIP FOV. This second dataset would be of use to persons
wishing to obtain deep surveys of extragalactic objects, studies of other possibly
yet unknown faint or transient phenomena, and studies that would help our un-
derstanding of CCDs as instruments capable of observing low S/N objects and
long-term operation in a space environment.
2. Temporal Studies of Cataclysmic Variables
The best long term time series data sets that are available for CVs to date, are
visual and photoelectric measurements of CVs. These lightcurves are mostly
obtained by amateurs belonging to the AAVSO or other equivalent organiza-
tions. These, of course, have a number of flaws such as 24-hour alaising, large
uncertainties, and the limiting magnitude is usually brighter than about 12th
magnitude.
Cannizzo and Mattei (1992) for example, have presented the entire almost
100 year lightcurve of SS Cygni. Even thought this represents a milestone in the
study of CVs, it still is incomplete in two ways. It represents only one CV, and
possibly a non-typical one at that, there are gaps in the data of days (> 10 days
at times), and at particularly important places, such as the fast rise to outburst.
These problems are in addition to those already stated above.
Richter (1992) shows outburst lightcurves for a few large amplitude DN.
These lightcurves are from Sonneburg Observatory plates but are typical of
data of this sort from any of the observatory plate collections; they are missing
data for various reasons, have uncalibrated plate materials, and as above, the
limiting magnitude is usually not very faint (about 16-17th magnitude). In his
work, Richter discusses some interesting features seen in the outburst decline
lightcurves. Interpretation of these gaps is very important to our understanding
of the outburst physics, yet it is very difficult due to lack of contiguous data
points.
We do not have complete coverage of the entire outburst of many kinds of
55
CVs, and Howell(1993)hasshownthat therearemany faint CVswith large
amplitudeoutburstsfor which wehavejust a singledatum. Thesesamestars
haveessentiallyNO informationduringminimumlight dueto their faintness
and thereis evidence(Howellet. al. 1990)that manysuchsystemshavelarge
variationsduringquiesenceastheirmasstransferrate changeswith time. Only
a long-term,well sampleddataset cananswerall the abovequestions.
Photometricstudiesof CVs canbe roughlydividedup into four regimes:
1) quiescence,2) the outburstcycle(rise-maximum-decline),3) the longerterm
cycleto cyclevariations,and 4) quasi-periodicoscillations(QPOs). Wehavea
fairly goodunderstandingfor somesystemsof regime2 but only sketchyinfor-
mation on varioussystemsin the others. No completepicture existsand it is
unlikelyit will without datafrom theFRESIPproject.
To demonstratewhat canbe accomplishedwith a long-termphotometric
study, I will usean examplebasedon a 4-yearongoingproject to study an
LMXRB. The star V404 Cyguiunderwentan outburst in May 1989. It had
beenlisted asa classicalnovaesinceits last outburstin 1938. Wagneret al.
(1992)showedthe systemto havean orbital periodnear 6.5daysand a black
holeprimary of 8-12solarmasses.Photometricinformationwascollectedover
manynightsby myselfandmy colleagues.
Usingnearbystarswithin the sameFOV, andpoolingall the data,nearly
3000data frames,wewereableto detect the 0.2 mag full amplitudein the
lightcurveorbitalmodulationandarefinally beginningto understandthemany
otherdetailedphotometricvariationswithin this system.Quasi-periodicoscilla-
torieshavebeena sourceof confusionandfrustrationfor CV workersfor many
years.Data from a singlenight or evenmanynightsobservationcanproduce
spuriousfrequencieswithin a powerspectrumto fool the observer.Thepapers
of PattersonandThomas(1993)beartestimonyto theobservationaldifficulties
of QPOstudies.
Theoriginof thesequasi-periodicoscillationsis of greatimportanceto the-
oreticalstudies.The originandnatureof whycertainfrequenciesarepreferred
in certainsystemsis a completemystery.Thelargestumblingblockto datehas
beenthe needfor findingandpositivelyidentifyingQPOs.
Figure1showssamplefrequencyspectrafor constantcomparisonstarsand
ourcurrentfrequencyspectrumonV404Cygin whichwecanclearlyidentify the
manyQPOsthat existin the system.Rememberthat this frequencyspectrum
is basedonmorethan4 yearsof ground-basedphotometricdataandismarred
by daytimedatalossesandweatherinterruptions.Wearenowableto provide,
for this onesystem,the dataneededfor better modelsof accretionmechanisms
andbinary structuresin blackholesystems.
ThecurrenttargetedFOVof theFRESIPtelescopecontains6 knownCVs
(Downesand Shara1993).Photometricinformationof theseobjectsin 'white
light' will revealmuchneedednew informationof the nature of CVs. The
study of thesestars alonewith data takeneveryhour for manyyearswould
dramaticallyimprovethedata-basefor CV research.
57
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Frequency (days -1)
Figure 1A. Frequency Spectrum of assumed constant comparison stars used in the
study of V404 Cygni. The two comparison stars C and K used in the bottom panel are
about 3 magnitudes fainter than those in the top panel. Note that the number of
observations used and time coverage are not the same. Adapted from Kreidl (1992).
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10
Figure lB. Frequency Spectrum for the Black Hole LMRXB V404 Cygni. The top
panel shows the spectral window, the middle panel is the raw spectrum and the bottom
panel in the cleaned spectrum. Note the presence of many significant periods.
58
3. The FRESIP Deep Cataclysmic Variable Survey
The study of CVs, since their "discovery" as a class of star in the mid-fifties, has
lead to a list of _500 known systems. Of these, only about 1/4 have a known
orbital period and any other associated detailed information. It has become
apparent in recent years (Howell and Szkody 1990, Howell 1993) that the well
known and well studied systems, the brightest and closest ones, are likely to be
atypical of the entire population of CVs. Howell (1993) has provided evidence
which suggests that there may be an entire population of low luminosity CVs
which have very faint minimum magnitudes (18-22+), and are rarely observed
due to their infrequent, yet large, outbursts.
The collected information on the current well studied systems may be lead-
ing us to a false conclusion about the entire population of CVs in general. Sur-
veys to find these fainter systems have been limited to very small areas of the
sky and single epochs. They usually employ 2-color techniques (looking for blue
objects) in an attempt to isolate candidates. Other methods have been literature
searches for randomly seen outbursting systems and then attempted follow-up
studies to try to identify the candidate stars.
Long-term, deep survey data is badly needed. Many CVs are NOT blue
most of the time. Most CVs spend the majority of their time at (faint) mini-
mum, and single epoch snapshots will miss any indication of orbital variation or
other photometric signatures. The long-term luminosity of these stars is also of
interest as they are likely to have their mass transfer start and stop stochastically.
Studies of the nature of mass transfer will provide information on how binaries
evolve, the thermal evolution of the secondary star, and binary dynamics.
If these fainter systems exist in abundance as appears likely from recent
clues (Shara et al. 1993), then the current beliefs on the space density of CVs
must be revised. This large increase in numbers of binary systems would have
important implications to CV evolution, binary star formation rates, and the
evolutionary status of stellar systems in our galaxy.
In order to determine how the FRESIP Project could be used for such
a deep survey, I have run simulations for the FRESIP telescope based on a
lm aperture and use of a broadband filter. A larger telescope using unfiltered
light will give higher S/N values for a given integration time. These are shown
in Figure 2. Figure 2A shows the predicted S/N obtainable for four sample
magnitudes in a 1 hour exposure. We see that stars of even 20th magnitude
are easily detectable and their variability deduced. Figure 2B shows the same
information but for a single 24-hour period. We are now able to begin to detect
24th magnitude sources. Over only a small time span compared to the mission
lifetime, variable objects at quite faint magnitudes will be easily detectable.
Detection of serendipitous CVs and supernovae in background galaxies should
also occur. Over many years, FRESIP will provide us with a complete sample
of variable sources such as CVs, to a limiting magnitude which should detect
all unobscured CVs in the volume cone, defined by the FOV, within our entire
galaxy.
The FRESIP Project will provide photometric information showing orbital
modulations and other variations for many CVs. These would be the first such
data sets and be of a unique nature.
59
ld
'l'''ll''e'l''l'l''''i''''ll'''l'''ll' 1
10000 _ IOTH MAG
1000 15TH MAG
•01 FRESIP TELESCOPE SIMULATION
.ooi I,,,,I,,,,I,,,, I,,,,!,,,,I,,,=1,,,=!,
0 500 1000 .1500 2000 2500 ,3000 3500
TIME (SECS)
Figure 2A. Simulations of the S/N obtainable with the FRESIP telescope during one
hour of observation.
lO'
1OOOO
1000
100
f
15TH MAG
20TH MAG
°'°I1
f 25TH MAG
.1
.01
.001
10000 20000 30000 40000 5(XX)O 60(X)0 70000 80000 90000
TIME (SECS)
Figure 2B. Simulations of the S/N obtainable with the FRESIP telescope during one
day of observation.
60
References
Cannizzo, J.K., and Mattei, J.A. 1992, ApJ 401, 642.
Downes, R., and Shara, M. 1993, PASP, 105, 127.
Howell, S.B., and Szkody, P. 1990, ApJ, 356, 623.
Howell, S.B., Szkody, P., Kreidl, T.J., Mason, K.O., and
Puchnarewicz,E.M. 1990, PASP, 102, 758.
Howell, S.B. 1993, Annals of the Israel Physical Society, eds. O. Regev
and G. Shaviv, Vol. 10, 67.
Kreidl, T.J. 1992, Proceedings of the IAU Colloquium No. 136, eds.
C.J. Butler and I. Elliott, p. 311.
Patterson, J., and Thomas, G_ 1993, PASP, 105, 59.
Richter, G.A. 1992, ASP Conference Series, ed. N. Vogt, Vol. 29, 12.
Shara, M., Moffat, A., Potter, M., Bode, M., and Stephenson, F.R.
1993, Annals of the Israel Physical Society, eds. O. Regev and G.
Shaviv, Vol. 10, 84.
Wagner, R.M., Kreidl, T.J., Howell, S.B., and Starrfield, S.G. 1992,
ApJ Let., 401, L97.
61



FRESIP project observations of cataclysmic variables: A unique opportunity

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