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Acta Polytechnica CTU Proceedings 2(1): 297–302, 2015

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doi: 10.14311/APP.2015.02.0297

Study of Cataclysmic Variables with the Satellites LOFT and Gaia

R. Hudec1,2, V. Šimon 1,2

1Astronomical Institute, Academy of Sciences of the Czech Republic, CZ-25165 Ondřejov, Czech Republic
2Czech Technical University in Prague, Faculty of Electrical Engineering, Prague, Czech Republic

Corresponding author: rene.hudec@asu.cas.cz

Abstract

The goal of this paper is to discuss the capabilities of ESA satellite missions Gaia (already in space) and LOFT (considered

for the ESA M4 slot) for investigation of cataclysmic variables (CVs). Both Gaia and LOFT can contribute to study of

CVs and related objects. Spectrophotometry and low dispersion spectroscopy are the most important for CV analyses

with the observations of Gaia. We present the possible strategies for investigation of CVs in the sampled photometric

and spectroscopic data provided by Gaia. E.g. statistical properties of the long-term activity of various types of CVs can

be determined from them. LOFT can be a promising satellite to provide a sensitive X-ray monitor which will enable to

investigate the little studied long-term activity of various types of CVs (especially the magnetic ones) in the X-ray band.

Keywords: cataclysmic variables - dwarf novae - intermediate polars - X-rays - LOFT - Gaia.

1 Introduction

Although two ESA satellite missions, namely Gaia (al-
ready in space) and LOFT (considered for the ESA M4
slot), focus on different science areas, both of them can
effectively contribute to the investigation of cataclysmic
variables (CVs).

2 The ESA Gaia Mission

The ESA satellite Gaia was successfully launched on
December 19, 2013. Gaia is an ambitious mission to
chart a three-dimensional map of our Galaxy, in the
process revealing the composition, formation and evolu-
tion of the Galaxy (e.g. de Boer et al. 2000 and Eyers et
al. 2013). Gaia will provide unprecedented positional
and radial velocity measurements with the accuracies
needed to produce a stereoscopic and kinematic census
of about one billion stars in our Galaxy and throughout
the Local Group. This amounts to about 1 percent of
the Galactic stellar population.

2.1 Astrophysics with Gaia

Motivation of this paper is to outline the possibilities of
performing astrophysics of CVs with Gaia data. This
is not trivial as the main goal of the Gaia mission is to
create a catalog. The photometric sampling provided
by Gaia will not be optimal for many variable astro-
physical sources. However, the fine spectrophotometry
(in reality ultra low resolution spectroscopy) provided
by BP/RP photometers will be unique and important

for many astrophysical investigations with Gaia, includ-
ing cataclysmic variables science (Hudec & Šimon 2007a
and 2007b; Hudec et al. 2012 and 2013).

Even the sampled Gaia data can provide the follow-
ing properties of the object:

1. Determination of amplitude of the brightness vari-
ations

2. Statistical distribution of brightness

3. Absolute magnitude (from distance determined
from parallax (and interstellar extinction)). Note
that these items 1, 2, and 3 even enable to estab-
lish the physically justified sequence of types of
CVs.

4. Color indices and their time variations

In principle, it is also possible to search for the pe-
riods and cycles of the brightness variations, but one
has to very cautious in doing this procedure using the
sampled data from Gaia.

The sampling of the data provided by Gaia is not
optimal for the astrophysical work; these observations
are not dense enough and not equidistantly distributed.
Typically, one can expect 50–100 photometric points
over 5 years. However, additional data can be provided
by the ground-based experiments.

We propose the following two approaches for clas-
sification and verification of CVs in the sampled Gaia
data. The first one is to search for outbursts or high/low

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R. Hudec, V. Šimon

state transitions, i.e. analysis of parameters of the light
curve not strongly dependent on sampling.

2.2 Statistical properties of the
long-term activity of CVs,
perspectives of histograms for Gaia

We investigated the profiles of the light curves of
CVs of various types with real observations (daily
means) from the AFOEV database (http://cdsarc.u-
strasbg.fr/afoev/). We approximated the Gaia sam-
pling of these light curves by selecting the data sepa-
rated by ∼ 20 days. This procedure yielded the amount
of the sampled data and the length of the mapped time
segment which roughly corresponded to the expected
time of observing with Gaia. We investigated which
methods can yield correct identification of a CV of a
given type almost independent of the sampling. We
identified the impact of the Gaia data on the investiga-
tion of CVs which are very active objects.

Figure 1: Comparison of the observed (one-day means
of AFOEV data (crosses)) and the approximated sam-
pled data of the dwarf nova AH Her (closed circles).
Heavily sampled data are expected from the Gaia satel-
lite. Notice that it is difficult to resolve the profile of
the light curve in the sampled data.

The basic profile of the high state and its fluctu-
ations in e.g. novalike systems and supersoft X-ray
sources can be plausibly mapped by the sampled Gaia
data. Only the high state/low state transitions may
be missed or covered by only one or two data points
by Gaia. On the other hand, we expect the large out-
bursts of a dwarf nova to be covered by only a very few
data points while short outbursts may be missed in the
Gaia data set. Moreover, the individual outbursts will
be probably captured in different phases due to their
short duration. Gaia will therefore usually not provide
us with information about the profile of the outburst.
However, we found that the statistical distribution of
magnitudes of a given CV is only slightly distorted if a
long time (several years) interval is covered. The statis-
tical distribution of brightness and its parameters like

the standard deviation, skewness, excess are a repre-
sentative description of the properties of the long-term
activity of CVs. These histograms are only slightly dis-
torted by the sampling of the Gaia observations if a
long (several years) time segment is covered by the ob-
servations. Nevertheless, a distortion may appear if the
recurrence time of outbursts is close to the sampling
time of Gaia.

Figure 2: Statistical distribution of the observed data
in the dwarf nova AH Her (from Fig. 1).

Figure 3: Statistical distribution of the approximated
sampled Gaia data of the dwarf nova AH Her (from
Fig. 1). Notice that the histogram is not dramatically
influenced by the sampling and is similar to that of the
observed data.

We conclude that the sequence of the types of CVs,
justified by the physics of accretion onto the compact
objects, is reflected in the statistical distribution of the
long-term brightness variations, hence it is suitable for
analysis of the sampled Gaia data. The importance
of this sequence can be even enhanced if the apparent
magnitudes are transformed to the absolute magnitudes
by using the distances of CVs determined from the par-
allaxes.

2.3 Rare flares (different from
outbursts in dwarf novae) in CVs

We investigated the possibility to detect and identify
rare phenomena in CVs with Gaia. Deep monitoring
of a large number of objects can lead to discoveries of
more CVs with flares (phenomena different from out-
bursts) like the event discovered by van Amerongen

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Study of Cataclysmic Variables with the Satellites LOFT and Gaia

& van Paradijs (1989) and to detection of additional
such events even in the already known systems. We in-
vestigated some CVs in Bamberg photographic plates
which have a similar sampling and coverage as the ex-
pected Gaia data. We conclude that the discovery of
such events is possible with the monitoring planned for
Gaia (Šimon 2010).

2.4 Ultra-low dispersion spectra

The ultra-low dispersion spectra will be provided by
Gaia RP/BP photometers. They can play a significant
role in (1) searches for prominent spectroscopic variabil-
ity, and (2) searches for objects with prominent (maybe
variable) spectral features.

As an example, the spectra of the outburst of
V407 Cyg (Munari et al. 2011) (or of an analogous
object) could be investigated by Gaia. The spectral
appearance of V407 Cyg in outburst was a highly pe-
culiar one. The spectrum of this event is completely
different from those ever recorded for this object and
other symbiotic Mira variables. The white dwarf com-
panion to the Mira variable experienced an outburst
similar to that of classical novae, and its ejecta were
moving in the circumstellar environment already filled
by the ionized wind of the Mira.

We summarize several types of CVs and the related
systems with very bright emission lines:

• Supersoft X-ray sources in the optical high states
(Steiner & Diaz 1998). Their spectra display in-
tense and broad emission lines (especially Hα)
(e.g. V Sge (Herbig et al. 1965)). Such systems
also often have Balmer jump in emission.

• Classical novae in the nebular phase of the out-
burst (late phase, weeks to months after the maxi-
mum of the optical luminosity) – faint continuum,
very strong emission lines of various elements (e.g.
Munari et al. 2013).

• Symbiotic systems: very strong emission lines
(mainly Hα) (e.g. Davidson et al. 1978).

• Dwarf novae: highly variable strong Balmer jump
(e.g. Walker & Chincarini 1968).

2.5 The color indices from RP/BP
spectra

The color indices can be determined from the spectra
obtained by the Gaia RP/BP photometers. Significant
results are expected to be immediately available, there
is no need to wait for years as in the photometry case.

The color indices give important information on the
spectral energy distribution. These indices can play
an important role in classification and/or verification

of CVs. They are also important for a search for the
common properties of the sources of a given kind (e.g.
to help identify a source as a CV and/or to study the
evolution of the spectral profile with time as the CV un-
dergoes various states of its activity). Not only the color
indices of the object at a given time, but also the time
evolution of these indices are therefore important. They
can also play a role in resolving among the individual
radiation mechanisms (e.g. cyclotron radiation versus
thermal emission). Even variations of strong emission
lines with respect to the continuum can be resolved by
the color indices (e.g. Hα changes, changes of Balmer
jump between emission and absorption). The color in-
dices are also important for forming a representative
ensemble of events (e.g. outbursts) in a given CV or
in a given type of CVs. This is possible also for faint
objects detected by Gaia (down to mag ∼ 20).

The parallaxes from Gaia will enable to determine
the distances, and hence the absolute magnitudes of
many CVs. It will be therefore possible to determine
and study the relation between the color indices and
the absolute magnitudes of CVs of various types.

2.6 Searches for cycles or periods

Variograms allow us to search for characteristic
timescales or quasiperiods which extend just for several
epochs of the cycle. Variogram characterizes the spatial
continuity or roughness of a data set. Variograms are
important for a search for the superorbital cycles in the
long-term activity of CVs and low-mass X-ray binaries
because their long-term activity is not periodic. Stan-
dard period searches therefore often reveal nothing.

We used the light curves of CVs of various types
for testing. We applied the real observations from the
AFOEV database for feasibility study of Gaia data. We
approximated the Gaia sampling of these light curves
by the data separated by ∼20 days. We investigated
how the variograms were modified by the sampling. An
example, the supersoft X-ray source V Sge, is displayed
in Fig. 4. The amount of the sampled data and the
length of the mapped time segment roughly correspond
to the expected time of observing of Gaia. Since the
duration of the high or the low state is longer than the
time interval between the sampled data, the profile of
the light curve is still recognizable.

Our tests showed that, in the case of the Gaia data,
variograms are suitable especially for investigation of
the types of CVs which display the light curves, whose
profiles are gradual. V Sge with its alternating high
and low states and the amplitude of the brightness vari-
ations similar for the individual epochs of the cycle is
a good example (Figs. 5 and 6). Similar results can
be obtained for novalike CVs with the episodes of the
high and low states. However, it is necessary to be cau-

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R. Hudec, V. Šimon

tious in the case of dwarf novae because the outbursts,
although with a large amplitude, are usually consid-
erably shorter than the intervals of quiescence. This
implies that some outbursts may be missed in the sam-
pled data. It also emerged that binning of the data for
variograms (Fig. 6) plays a big role. Fine binning yields
a more precise determination of the cycle-length, but
artifacts and false detections can appear, especially if
the data are sampled.

Figure 4: Comparison of the observed (one-day means
of AFOEV data (crosses)) and the approximated sam-
pled data (closed circles) of the supersoft X-ray source
V Sge. Heavily sampled data are expected from the
Gaia satellite.

Figure 5: Variogram for the observed data of the su-
persoft X-ray source V Sge (from Fig. 4). N bins refers
to the number of bins used for constructing the vari-
ogram.

The orbital modulation of CVs can be searched for
and investigated in the sampled Gaia data only under
some favorable circumstances. Although the length of
the orbital period of CV can be considered to be sta-
ble (or at most slightly variable) during the proposed
lifetime of Gaia, it is reasonable to expect that the or-
bital modulation will be influenced by the long-term
variations (e.g. outbursts, episodes of the high and low
states). The amplitude of the long-term variations can
often be considerably larger than that of the orbital

modulation even in the case of a significant amplitude of
this modulation (e.g. > 0.8 mag). Only if the long-term
level of brightness remains almost stable (or if the lim-
ited segments of the long-term light curve are selected),
the orbital modulation can emerge. A preferable sit-
uation will be to fold the Gaia data with the already
known orbital period, and to investigate if and how the
profile of the modulation varies with the changes be-
tween the states of activity. The orbital period can be
determined for example by the follow-up observations
of the ground-based telescopes. This is one of the cases
of co-operation of the orbital and the ground-based ob-
serving components of the project.

Figure 6: Variogram for the approximated Gaia data
of the supersoft X-ray source V Sge (from Fig. 4). A
higher value of N bins can yield a better determination
of the possible cycle-length, but also a bigger noise.

3 ESA LOFT

LOFT (the Large Observatory For x-ray Timing) is
specifically designed to exploit the diagnostics of very
rapid X-ray flux and spectral variability in compact ob-
jects, yielding unprecedented information on strongly
curved spacetimes and matter under extreme condi-
tions of density and magnetic field strength (Feroci et
al. 2012; Brandt et al. 2013). LOFT is designed to in-
vestigate variability from submillisecond quasi-periodic
oscillations (QPO) to years long transient outbursts.

LOFT belongs to ESA Medium Class mission (M3
candidate) and was selected in Feb 2011 to complete a 3-
year Phase 0/A study. Final down selection on Jan 2014
(5 competed for single launch slot) was however nega-
tive for LOFT. There is however a chance to be selected
for M4.

LOFT is observatory-type mission: there are 2 on-
board instruments and science data center provided by
the community. The originally expected launch date
was 2022–2024 timeframe (Soyuz launcher) with 4+1
years mission lifetime and Low Earth orbit (550 km,
2 ground stations, 7 Gb science data/orbit).

The LOFT Large Area Detector (LAD) has an effec-

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Study of Cataclysmic Variables with the Satellites LOFT and Gaia

tive area ∼20 times larger than any largest predecessor,
uniquely combined with a CCD-class energy resolution.
LOFT LAT is suitable for observing CVs in a wide X-
ray band of 1–40 keV, with very fine time resolution.

The LOFT Wide Field Monitor (WFM) has a
4 steradian field of view at soft X-rays to discover and
localize X-ray transients and impulsive events and to
monitor spectral state changes, triggering follow-up ob-
servations and providing a wealth of science in its own.
LOFT WFM is suitable for monitoring of the outbursts
and high states of CVs in the energy range of 1–50 keV.

3.1 The LOFT science

The main LOFT science is the study of neutron star
structure and equation of state of ultradense matter (3+
independent methods): (1) neutron star (NS) mass and
radius measurements, and (2) neutron star crust prop-
erties. In addition to that, strong gravity and the mass
and spin of black holes will be investigated (5 inde-
pendent methods), e.g. quasi-periodic oscillation evo-
lution, and, in the time domain, nFe line tomography
and reverberation studies in bright active galactic nu-
clei (AGNs) and black hole candidates (BHCs) ((also
vs NSs), and relativistic precession. LOFT Observa-
tory Science is expected to provide important observa-
tions for virtually all classes of relatively bright sources,
including: X-ray bursters, High-mass X-ray binaries, X-
ray transients (all classes), CVs, magnetars, gamma-ray
bursts (GRBs), Nearby galaxies (SMC, LMC, M31...),
AGN etc.

In general, CVs are X-ray emitters (e.g. Warner
1995). However, the properties and time evolution of
their X-ray spectra strongly depend on the type and
the state of activity of a given CV. Although some gen-
eral relations exist for a given CV type, each CV is
really specific in this regard. Since the proposed band
of LOFT is 1–50 keV, those CVs with the hardest X-
ray spectra will be the suitable targets for observing
with this satellite. For example, the intermediate po-
lars usually display the significantly larger values of kT
in comparison with non-magnetic CVs (Šimon et al.
2006).

Magnetic CV often have the hard X-ray intensity
correlated with the mass accretion rate onto the white
dwarf because of the accretion rate onto the magnetic
poles of the WD. The dwarf nova outbursts of interme-
diate polars can be therefore accompanied by a strong
increase of the hard X-ray intensity (e.g. DO Dra
(Szkody et al. 2002), GK Per (Fig. 7)). In polars (e.g.
Warner 1995), the hard X-ray intensity strongly in-
creases in the optical high states. However, the case
of the polar AM Her shows that the relation of the op-
tical and hard X-ray emission varies for the individual
episodes of the high state (Šimon 2011). This empha-

sizes the necessity to obtain the observations of a larger
ensemble of CVs to study the processes and their pa-
rameters which influence the behavior of CVs on the
long timescales. We therefore expect the satellites like
LOFT to be important instruments for investigation of
magnetic CVs.

Figure 7: Outbursts of the intermediate polar GK Per
in the optical band (AFOEV data) (a) and the hard X-
ray band (BAT/Swift data (data of Krimm et al. 2013).

4 Conclusions

Both Gaia and LOFT can contribute to study of CVs
and related objects. Spectrophotometry and low dis-
persion spectroscopy are the most important for CV
analyses with the observations of Gaia. Both the LOFT
WFM X-ray monitoring in the energy range of 1–50 keV
and detailed LAD observations with fine time resolution
are important in this research field. Gaia was launched
on December 19, 2013. LOFT was not selected in com-
petition with other ESA M3 candidates, but it remains
a promising candidate for M4.

It is true that dense series of observations covering
the intervals of several years are necessary to investi-
gate the rising and decaying branches of outbursts and
high/low states. Nevertheless, a properly chosen strat-
egy can yield suitable results of analyses even in the
sampled photometric and spectroscopic data provided
by Gaia. We propose to obtain the statistical properties
of the long-term activity of a representative ensemble of
events (e.g. outbursts, high and low states) in a very
large ensemble of a given type of systems. This is im-
portant for our understanding of the physical processes
involved in the systems distributed in various regions
of the Galaxy which may have a different history of the
star formation and chemical evolution.

We also argue in favor of the development of more
sensitive X-ray monitors that will be able to detect the
long-term activity of CVs. Even the available short
data series obtained by the pointed X-ray observations

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R. Hudec, V. Šimon

of some known CVs revealed the very large variety of
properties of their X-ray emission (e.g. Warner 1995).
The current monitors are able to observe mainly the
luminous binary X-ray sources which contain the NS or
the BH. Since only a very few CVs are detectable by
these instruments, LOFT can be a promising satellite
to provide a sensitive X-ray monitor.

Acknowledgement

We acknowledge partial support by GA CR grants 13-
39464J and 13-33324S. We also acknowledge the use
of public data from Swift/BAT transient monitor pro-
vided by the Swift/BAT team. We also used the obser-
vations from the AAVSO International database (Mas-
sachusetts, USA) and the AFOEV database operated
in Strasbourg, France.

References

[1] Brandt, S., et al., 2013, EAS Publications Series,
Volume 61, 2013, pp.617-623
doi:10.1051/eas/1361098

[2] Davidson, K., et al., 1978, ApJ, 220, 239

[3] de Boer,K., Gilmore, G., Hog, E., Lattanzi, M.G.,
Lindegren, L., Luri, X., Mignard, F., Face, O.,
Ferryman, M., de Zeeuw, P.T. (eds) GAIA
Concept and Technology Study Report (the ’Red
Book’) ESA-SCI(2000)4 pp381 (ESA Paris) 2000

[4] Hudec, R. et al., 2013, Acta Polytechnica, Vol.
53, No. 3, p.30

[5] Eyer, L. et al., 2013, Central European
Astrophysical Bulletin, p. 115-126

[6] Feroci, M. et al., 2012, Experimental Astronomy,
Volume 34, Issue 2, pp.415-444
doi:10.1007/s10686-011-9237-2

[7] Herbig, G. H., et al., 1965, ApJ, 141, 617
doi:10.1086/148149

[8] Hudec, L., 2007, Algorithms for spectral
classification of stars, BSc. Thesis, Charles
University, Prague

[9] Hudec, R., Šimon, V., 2007a, Specific object
studies for cataclysmic variables and related
objects ESA Gaia Reference Code
GAIA-C7-TN-AIO-RH-001-1.

[10] Hudec, R., Šimon, V., 2007b, Specific object
studies for optical counterparts of high energy
sources. ESA Gaia Reference Code
GAIA-C7-TN-AIO-RH-002-1.
doi:10.14311/AP.2013.53.0799

[11] Hudec, R., Šimon, V., Hudec, L., 2013, Acta
Polytechnica, Vol 53, Supplement, p.798
doi:10.1088/0067-0049/209/1/14

[12] Krimm, H. A., et al., 2013, ApJS, 209, 14
doi:10.1111/j.1745-3933.2010.00979.x

[13] Munari, U., et al., 2011, MNRAS, 410, L52
doi:10.1093/mnras/stt1340

[14] Munari, U., et al., 2013, MNRAS, 435, 771

[15] Šimon, V., Mattei, J. A., 1999, A&AS, 139, 75

[16] Šimon, V., 2000, A&A, 360, 627

[17] Šimon, V., et al., 2006, IAUS, 230, 66

[18] Šimon, V., Hric, L., Petŕık, K., et al., 2002, A&A,
393, 921

[19] Šimon, V., 2010, Advances in Astronomy, article
id. 382936 doi:10.1016/j.newast.2011.03.001

[20] Šimon, V., 2011, NewA, 16, 405

[21] Steiner, J. E., Diaz, M. P., 1998, PASP, 110, 276

[22] Szkody. P., et al., 2002, AJ, 123, 413

[23] van Amerongen, S., van Paradijs, J., 1989, A&A,
219, 195

[24] Walker, M., & Chincarini, G., 1968, ApJ, 154, 157

302

http://dx.doi.org/10.1051/eas/1361098
http://dx.doi.org/10.1007/s10686-011-9237-2
http://dx.doi.org/10.1086/148149
http://dx.doi.org/10.14311/AP.2013.53.0799
http://dx.doi.org/10.1088/0067-0049/209/1/14
http://dx.doi.org/10.1111/j.1745-3933.2010.00979.x
http://dx.doi.org/10.1093/mnras/stt1340
http://dx.doi.org/10.1016/j.newast.2011.03.001

	Introduction
	The ESA Gaia Mission
	Astrophysics with Gaia
	Statistical properties of the long-term activity of CVs, perspectives of histograms for Gaia
	Rare flares (different from outbursts in dwarf novae) in CVs
	Ultra-low dispersion spectra
	The color indices from RP/BP spectra
	Searches for cycles or periods

	ESA LOFT
	The LOFT science

	Conclusions