Acta Polytechnica


doi:10.14311/AP.2013.53.0595
Acta Polytechnica 53(Supplement):595–600, 2013 © Czech Technical University in Prague, 2013

available online at http://ojs.cvut.cz/ojs/index.php/ap

NON-THERMAL EMISSION FROM CATACLYSMIC VARIABLES:
IMPLICATIONS ON ASTROPARTICLE PHYSICS

Vojtěch Šimon∗

Astronomical Institute, Academy of Sciences of the Czech Republic, 25165 Ondřejov, Czech Republic, Czech
Technical University in Prague, FEL, Prague, Czech Republic

∗ corresponding author: simon@asu.cas.cz, vojtech.simon@gmail.com

Abstract. We review the lines of evidence that some cataclysmic variables (CVs) are the sources of
non-thermal radiation. It was really observed in some dwarf novae in outburst, a novalike CV in the
high state, an intermediate polar, polars, and classical novae (CNe) during outburst. The detection of
this radiation suggests the presence of highly energetic particles in these CVs. The conditions for the
observability of this emission depend on the state of activity, and the system parameters. We review the
processes and conditions that lead to the production of this radiation in various spectral bands, from
gamma-rays including TeV emission to radio. Synchrotron and cyclotron emissions suggest the presence
of strong magnetic fields in CV. In some CVs, e.g. during some dwarf nova outbursts, the magnetic
field generated in the accretion disk leads to the synchrotron jets radiating in radio. The propeller
effect or a shock in the case of the magnetized white dwarf (WD) can lead to a strong acceleration of
the particles that produce gamma-ray emission via π0 decay; even Cherenkov radiation is possible. In
addition, a gamma-ray production via π0 decay was observed in the ejecta of an outburst of a symbiotic
CN. Nuclear reactions during thermonuclear runaway in the outer layer of the WD undergoing CN
outburst lead to the production of radioactive isotopes; their decay is the source of gamma-ray emission.
The production of accelerated particles in CVs often has episodic character with a very small duty
cycle; this makes their detection and establishing the relation of the behavior in various bands difficult.

Keywords: acceleration of particles, astroparticle physics, nuclear reactions, nucleosynthesis, abun-
dances, elementary particles, magnetic fields, radiation mechanisms: non-thermal, circumstellar matter,
accretion, accretion disks, Novae, cataclysmic variables, X-rays: binaries.

1. Introduction
In cataclysmic variable (CV), matter flows from a
companion star, the so-called donor (often a low-mass,
main-sequence star), onto the white dwarf (WD) ac-
cretor. This mass-transferring binary is a complicated
and very active system with various emission regions.
Release of the gravitational energy during accretion
of matter from the donor onto the WD is the domi-
nant energy source of the CV system. This accretion
usually occurs via accretion disk embedding the WD.
However, if the WD has a strong magnetic field, this
field largely influences the accretion flow – such sys-
tems are called polars. Thermonuclear reaction of the
accreted matter on the surface of the WD is another,
often episodic source of energy. Review can be found
in [40].
CVs have been shown to radiate in various spec-

tral regions via various emission mechanisms. The
complicated structure of the emission regions and
the operation of the individual emission mechanisms
largely vary with the state of their activity. CVs
are therefore very important laboratories for a study
of the physical processes, including a search for the
highly energetic particles.

We discuss the processes and conditions that lead to
the production of non-thermal radiation in CVs. The

detection of such a radiation suggests the presence
of highly energetic particles in these CVs. How can
this emission be detected and monitored? In which
spectral bands and with which techniques? We also
show the importance of non-thermal radiation of CVs
for the physics of these systems. We focus on the
cases where such emission was really observed – from
gamma-rays (including TeV emission) to radio.

2. Non-thermal radiation in
various types of CVs

We review the types of CVs and their states of activ-
ity in which non-thermal radiation was observed in
various spectral regions.

2.1. CVs with accretion disks and
“non-magnetized” WDs

In some CVs, the accretion disk suffers from a thermal-
viscous instability if the mass transfer rate ṁ lies
between certain limits. It gives rise to the large-
amplitude optical outbursts in the so-called dwarf
novae (e.g. [17]). The accretion disk is a source of in-
tense thermal emission during the outburst (or during
the high state in novalike CVs) (e.g. [40]). Never-
theless, we will show several CVs which are also the
sources of non-thermal radiation.

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Vojtěch Šimon Acta Polytechnica

The dwarf nova SS Cyg/3A 2140+433 is a very good
example. Its non-thermal radio emission was gener-
ated during its outburst [25]. This radio emission does
not directly follow the optical one; the radio-emitting
medium is therefore not any detached reprocessing
medium. It originates from SS Cyg itself during the
outburst. Radio emission of SS Cyg is optically thick
synchrotron radiation of a transient jet because the
size of the radio-emitting medium is much larger than
the magnetosphere of the WD. A similar behavior,
that is radio emission only during the optical out-
burst, was observed in the dwarf nova EM Cyg/1RXS
J193840.0+303035. Gyrosynchrotron emission of non-
thermal electrons is a plausible explanation. The radio
source is significantly larger than the binary separa-
tion. This is consistent with a transient jet like in
SS Cyg. This radio emission, related to the conditions
in the accretion disk [6, 7], suggests a symbiosis of
the thermal emission of the accretion disk and the
non-thermal radiation of the jet. A single process
(thermal-viscous instability of the disk) is the trigger
of both types of emission.

In addition, radio emission (frequencies of 5.5 GHz
and 9 GHz; ATCA radio telescope) was also detected
from the novalike system V3885 Sgr. The ASAS [33]
light curve shows a relation between the optical and
radio activity. This radio emission falls into a long-
lasting high state of the optical emission, with the
luminosity comparable to the peak of the outburst
in dwarf novae (e.g. SS Cyg). The analogies with
Z-sources and outbursting dwarf novae suggest syn-
chrotron emission of a jet [26].

2.2. Intermediate polars
Accretion flow of intermediate polar is controlled by
the magnetic field of the WD inside the magneto-
sphere of this accretor. Accretion of matter therefore
occurs onto the magnetic poles of this WD [16, 40].
Non-thermal radiation can be produced by electrons
and protons which are accelerated in the transition
region between the rotating WD magnetosphere and
the accreting matter. Gamma-ray emission from the
decay of neutral pions is predicted to be produced in
hadronic collisions [5].
V1223 Sgr/1H 1853−312 was observed to display

brief brightenings that cannot be explained by a
thermal-viscous instability of its accretion disk. Part
of a flare observed in far infrared (IR) (λ of 14÷21 µm)
by Spitzer satellite, with the flux declining by a factor
of 13 in 30 minutes, suggests a transient synchrotron
emission [18]. Another two brightenings were ob-
served in the optical band. One of them, with an
amplitude of more than 1 mag in the red continuum,
lasted only for several hours. It was accompanied by
an increase of the Hα line flux, which was longer than
in the continuum. This suggests that also the line
emission participated in this event [39]. Another flare
(a brightening by > 1 mag) was found on an archival
photographic plate (the Bamberg Observatory) in blue

Figure 1. (a) Position of the synchrotron flare ob-
served by [18] (arrow) in the optical (the V band) light
curve of V1223 Sgr (ASAS data [33], one CCD image
per night). The smooth line represents the HEC13
fit. (b) Statistical distribution of the brightness from
panel ‘a’. The baseline level of the synchrotron flare
is marked by the vertical line. Notice that the flare
occurred from a shallow low state of the optical bright-
ness. See Sect. 2.2 for details.

light [34]. One photographic plate of the field was
obtained per night during this monitoring.
Extrapolation of the flux of the flare with a very

flat synchrotron spectrum, observed by [18], from far
IR to the optical band can hardly explain the ob-
served bright optical flares in V1223 Sgr. It is possible
that some interaction of the inner disk region with
the synchrotron jet occurred to produce the optical
brightening; this scenario is supported by an increase
of the Hα emission during the flare.

Placing the flares in the long-term optical light curve
can give us important information about the condi-
tions suitable for generating these events in V1223 Sgr
(Fig. 1a). In the last years, this system was in the
so-called high state of its activity, but it displayed
several recurring decreases to the so-called shallow
low state. Nevertheless, even during this shallow low
state it was always much brighter that in the true low
state reported by [15].
In the statistical distribution (Fig. 1b), the upper

(brighter) limit of the brightness is significantly better
defined than its lower (fainter) limit. Also the time
fraction spent in the high state is considerably larger
than in the shallow low state. The synchrotron flare

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in Spitzer data occurred from a shallow low state
(Fig. 1b). It is close to an optical flare, but it is not
precisely coinciding with it (Fig. 1a). A cluster of
flares in a given shallow low state can explain it.
A similar situation is as regards the flare on the

Bamberg plate [34]. It therefore appears that the
episodes of shallow low state create suitable conditions
for the flares in V1223 Sgr (increase of the Alfvén
radius, inner disk region therefore being closer to the
propeller regime, but still with a high ṁ?).
AE Aqr/1RXS J204009.4−005216 is a unique sys-

tem because it is an intermediate polar in a propeller
regime. Most of the transferring matter is therefore
ejected by the rapidly spinning (33 s) magnetized
WD [41]. An interaction of these blobs with the
magnetosphere of the WD leads to the flares of the
optical (thermal) emission. A typical duration of a
single flare is several minutes but these events can
be clustered [32]. Part of matter of these blobs is
trapped in the WD magnetosphere and these particles
are accelerated according to the model by [27]. Syn-
chrotron emission from electrons in expanding clouds
then dominates in far IR and radio bands. This radio
emission is a superposition of discrete, synchrotron
emitting flares from the vicinity of the WD [10]. The
flare is an expanding plasmoid – a spherical cloud of
relativistic electrons which expands adiabatically [4].
Generation of the synchrotron emission is therefore a
consequence of the blobs [27], although the occurrence
of the individual spikes of the radio emission is not
directly correlated with the optical spikes [1].
AE Aqr is a transient TeV source detected by

ground-based Cherenkov telescopes [29, 30]. A
method of confirmation that TeV emission really
comes from the observed object (i.e. an argument
against a spurious detection) is to correlate the time
variations of its TeV intensity with some period al-
ready known to be specific for this object. Indeed,
the optical and TeV flares display the same frequency
(a 33 s spin period of the WD detected in the optical
band). TeV flares are highly transient, with quite a
small duty cycle (0.2 percent). These TeV flares were
interpreted as due to acceleration of particles by the
rotating magnetic field of the WD in intermediate
polar in the propeller regime. TeV flares occur only
during a low optical brightness; the accretion luminos-
ity must be low to allow the inner edge of the disk to
be outside the co-rotation radius. Electrons are accel-
erated to E ≈ 1013 eV and converted to gamma-rays
via π0 decay in the blobs.

All this production of non-thermal radiation of
AE Aqr depends on the state of its long-term ac-
tivity which can be measured in the optical band.
The mean level of this brightness largely varies on the
timescale of years (Fig. 2a). This activity is mainly
caused by a variable amount and brightness of the
flares. The more numerous flares lead to a higher opti-
cal brightness of the system because they emit optical
radiation. Even a season with almost absent flares was

Figure 2. (a) Long-term activity of AE Aqr in the
optical band (ASAS data [33], one CCD image per
night). The smooth lines represent the HEC13 fits.
The segment with almost absent flares is marked by
the box. (b) The light curve folded with the orbital
period according to the ephemeris of [12]. Open circles
represent all the data points from panel ‘a’. Closed
circles denote the data from the box in panel ‘a’. See
Sect. 2.2 for details.

observed (Fig. 2a). This behavior can be explained by
a transient decrease or even a cessation of the mass
inflow from the donor. Such an evolution suggests a
variable amount of the blobs on this timescale. This
implies variable conditions for the generation of the
accelerated particles.
The light curve of AE Aqr folded with the orbital

period according to the ephemeris of [12] displays a
large scatter caused by the real variations, not by a
noise (Fig. 2b). The flares can occur at any orbital
phase (see also [32]), and the detection of a higher
intensity of the flares is less frequent. The profile of the
lower envelope of the folded light curve (remaining all
the time, even when the flares are missing) is caused
by the tidal deformation of the donor [32].

2.3. Polars
Polars are CVs with a strongly magnetized WD (typ-
ically B > 107 G (e.g. [40]). No disk is formed and
the accreting matter flows directly onto the accretion
region(s) in the vicinity of the magnetic pole(s) of the
WD. This falling matter forms an accretion column
above the surface of the accretor (less than 0.1 of
the radius of the WD). This column is a source of

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Vojtěch Šimon Acta Polytechnica

radiation via several mechanisms. It radiates via cy-
clotron mechanism in the optical and near IR bands,
while the accretion shock emits bremsstrahlung in
hard X-rays (e.g. [13, 28]). Intensity of the cyclotron
emission largely varies with the orbital phase of the po-
lar, which reflects the changing aspect of the emission
region with respect to the observer [14].

AM Her/3A 1815+498 displays a significantly vari-
able ratio of the intensities of the cyclotron and
bremsstrahlung components for the individual high-
state episodes [35]. This suggests large changes of the
properties of the emission region(s) on the WD. These
properties are established already in the early phase
(several days long) of the high-state episode but they
are not reproduced for every individual episode. An
increase of ṁ from the donor that switches the polar
from the low to the high state of its long-term activity
also establishes a division of the emission released dur-
ing the accretion process into various spectral regions,
hence into the individual emission mechanisms oper-
ating in the accretion region. This division is valid
only for a given episode. Each high-state episode is
thus characterized by the specific configuration of the
accreting matter [35]. This is also supported by the
change of the profile of the optical orbital modulation,
as observed by [24]. The observed behavior can be
reconciled if the bremsstrahlung emission is confined
to a smaller region than the cyclotron emission. Also
the role of several modes of accretion, e.g. a single-
pole and two-poles accretion [19], is worth considering.
These findings show that both the positions of these
emission regions and their contributions to the to-
tal intensity varied with time. The emission region
caused by the accretion of matter and its conditions
are therefore proved to be highly unstable in time.
Several sites of non-thermalradio emission (fre-

quency of 4.9 GHz) existed in AM Her [11] in the
same time, specifically in the optical low state of its
long-term activity [20]. Quiescent radio emission was
ascribed to the energetic electrons trapped in the
magnetosphere of the WD. A radio outburst (flare)
was explained as an electron-cyclotron maser near the
surface of the late-type donor, in its corona with the
magnetic field (B ≈ 1000 G).
In the high state of the long-term optical activity,

AM Her is sometimes able to accelerate particles that
produce TeV emission [8]. Its intensity is strongly
modulated with the already known orbital period
seen in the polarized optical light of AM Her. This is
strong evidence that this gamma-ray emission really
comes from this object [8]. Protons can be accelerated
to very high energies (GeV to TeV) at the shock front
on the top of the accretion column [38], with the
subsequent gamma-ray production via π0 decay [3].

2.4. Outbursts of classical novae
Radioactive isotopes are synthesized during outburst
of classical nova (CN). Gamma-ray emission is then
produced during their decay. Detection of these

gamma-rays strongly depends on the isotope’s life-
time and on the optical depth in the ejecta of the
nova.
Only cumulative effect of the production of 26Al

(lifetime of 106 years) emitting the gamma-ray line
with E = 1.809 MeV in various types of sources, con-
centrated toward the Galactic plane, and its ejec-
tion into the space has been observed by COMP-
TEL/CGRO [9]. This emission therefore does not
come from a single source. Outbursts of CNe con-
tribute about 15 percent [23]. The situation of 22Na
with a lifetime of 3.75 years (E = 1.272 MeV) is simi-
lar. Outbursts of the Neon-type CNe contribute only
partly; excitation of 22Ne-nuclei, e.g. through the
low-energy cosmic ray interactions with the nuclei of
the interstellar matter, can dominate [21].
It is very important that in V723 Cas (Nova Cas

1995), the nucleosynthesis during its outburst was
really observed in the gamma-ray band [22] (Fig. 3).
Both the COMPTEL/CGRO field of Nova Cas 1995
(total integration time of ∼4.5 years) and a spectrum
from the position of this nova revealed a new source
emitting the 22Na line (E = 1.275 MeV). This is direct
evidence that CNe contribute to the gamma-ray flux
of 22Na in the Galaxy. This 22Na was synthesized
during thermonuclear reactions on the surface of the
WD [22]. Time evolution of the 22Na gamma-ray
flux of V723 Cas is a combination of a decreasing
absorption of the gamma-ray flux in the ejecta and the
time dependence of the undecayed 22Na nuclei. The
case of V723 Cas also enabled to identify the important
properties needed for the formation and preservation
of 22Na, hence for the detection of the 22Na gamma-
ray flux: type of CN (slow nova), massive ejected
envelope, low mass of the WD (only 0.66 M�) [22].
Strongly accelerated population of electrons with

a nonthermal energy distribution during an outburst
of the classical nova V2491 Cyg was proved by the
detection of superhard (E > 10 keV) X-ray emission
with a power-law spectral profile up to E of 70 keV,
attenuated by a heavy extinction of 1.4 × 1023 cm−2
(Suzaku satellite) [37]. According to the model by [36],
Compton degradation, i.e. repeated scattering of the
gamma-ray photons by electrons in the matter ejected
from a WD, can explain the extremely hard X-ray
spectrum and the absence of the 1.275 MeV line of
22Na. The ejecta become transparent to the gamma-
-ray photons within several tens days.

Gamma-rays from the shock in the very fast CN
in the symbiotic binary V407 Cyg (Nova Cyg 2010),
detected by LAT/Fermi, represent a unique case of
the CN activity [2]. The environment of the erupt-
ing WD was very specific. This object was deeply
embedded in the dense wind of its cool giant com-
panion [31]. According to [2], a variable gamma-ray
emission started with the rise of the optical luminosity
of the outburst and lasted for about 18 days. Con-
tinuum emission with no lines was detected in the
spectral region between 0.2 and 5 GeV. No spectral

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Figure 3. (a) Outburst of the classical nova
V723 Cas (Nova Cas 1995) in the optical band
(AAVSO data [20]). The vertical line denotes the
time of the discovery of the optical outburst. The
outburst started at most several days before the dis-
covery. (b) Time evolution of the flux of the line
(E = 1.275 MeV) of the radioactive isotope 22Na of
V723 Cas (adapted from [22]). See Sect. 2.4 for de-
tails.

variability was observed over the duration of the ac-
tive gamma-ray period. Most gamma-rays come from
the part of the nova shell approaching the red giant
donor [2]. This gamma-ray emission was interpreted
as an interaction of the material of the nova shell with
the dense medium of the red giant donor. Particles
were accelerated by this interaction of the shell to
produce π0 decay gamma-rays from proton-proton
interactions [2].

3. General conclusions
Non-thermal radiation was definitely observed in the
following types of CVs: dwarf nova in outburst, no-
valike CV in the high state, intermediate polar, CN
during outburst. This brings evidence that the pro-
cesses for the creation and/or acceleration of highly
energetic particles must operate in such CVs. The
conditions for generating the non-thermal radiation
depend on the state of the system’s activity and its pa-
rameters. These processes, states of the activity, and
the spectral bands in which non-thermal radiation can
be observed can be summarized in the following way.

Synchrotron emission provides us with evidence
of generation of the magnetic fields influencing the
transferring matter. This emission, which can be
studied in the radio band, can come from the jets
launched during the outbursts of dwarf novae and
even during the high state of some novalike systems.
However, the structure of this medium is uncertain
in some systems (e.g. the flare radiating in far IR,
observed in the intermediate polar V1223 Sgr). Also
in the case of AE Aqr, the synchrotron emission in
radio is produced by the clouds launched from the
system by the propeller effect rather than in a jet.
Also cyclotron emission carries information about

strong magnetic field existing in some CVs, namely
polars. It emerges that even a single polar can possess
several simultaneously existing cyclotron-emitting re-
gions: accretion region on the WD (emitting in the
optical and IR bands), and the donor’s magnetosphere,
radiating in radio.
Gamma-ray production via π0 decay suggests op-

eration of mechanisms that lead to acceleration of
protons in various types of CVs. π0 particles can be
created in the transition region between the rotating
WD magnetosphere and the accreting matter, by the
propeller effect, by a shock on a strongly magnetized
WD, or in ejecta of outburst of a symbiotic CN.

Production of radioactive isotopes occurs during
the nuclear reactions in the outer layer of the WD
during a CN outburst. Only some isotopes lead to
the production of the observable gamma-rays. Only
cumulative effect of many CNe can be expected from
the gamma emission of 26Al. The situation is more
optimistic as regards 22Na, which was detected from
a single CN.

We offer the following solution and further prospects
in search for the suitable states of activity of CVs
in which the highly energetic particles are produced.
The long-term activity of these objects in the optical
band appears to be a plausible indicator of suitable
conditions for the generation of these particles. The
reason is that the data coverage in other spectral bands
is fragmentary or even absent. The phenomena related
to the generation of the highly energetic particles
often have episodic character with a low duty cycle:
e.g. ultra-high energy flares in the propeller systems
(AE Aqr), radio flares in polars (AM Her). This
property makes the detection of these phenomena
and establishing a relation between various emission
processes difficult.

Acknowledgements
Support by the grant 205/08/1207 of the Grant Agency
of the Czech Republic and the project RVO:67985815 is
acknowledged. This research has made use of observa-
tions from the ASM/RXTE, ASAS, AAVSO, and AFOEV
databases. I thank Prof. P. Harmanec for providing me
with the code HEC13. The Fortran source version, com-
piled version and brief instructions how to use the program
can be obtained via http://astro.troja.mff.cuni.cz/
ftp/hec/HEC13/.

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Vojtěch Šimon Acta Polytechnica

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600


	Acta Polytechnica 53(Supplement):595–600, 2013
	1 Introduction
	2 Non-thermal radiation in various types of CVs 
	2.1 CVs with accretion disks and ``non-magnetized'' WDs
	2.2 Intermediate polars
	2.3 Polars
	2.4 Outbursts of classical novae

	3 General conclusions
	Acknowledgements
	References