The OVRO 40-m telescope has been monitoring the 15 GHz radio flux density of
over 1200 blazars since 2008. The 15 GHz light curve of the flat spectrum radio
quasar J1359+4011 shows a strong and persistent quasi-periodic oscillation. The
time-scale of the oscillation varies between 120 and 150 days over a $\sim4$
year time span. We interpret this as the active galactic nucleus mass-scaled
analog of low-frequency quasi-periodic oscillations from Galactic microquasars,
or as evidence of modulation of the accretion flow by thermal instabilites in
the "inner" accretion disc.Comment: 4 pages, 2 figures, 1 table. Accepted for publication in MNRAS
  Letter

Hovatta, T.

King, O. G.

Max-Moerbeck, W.

Meier, D. L.

Pearson, T. J.

Readhead, A. C. S.

Reeves, R.

Richards, J. L.

Shepherd, M. C.

English

arXiv

O. G. King

T. Hovatta

W. Max-Moerbeck

D. L. Meier

T. J. Pearson

A. C. S. Readhead

R. Reeves

J. L. Richards

M. C. Shepherd

Crossref

Monthly Notices of the Royal Astronomical Society Letters

A quasi-periodic oscillation in the blazar J1359+4011

The OVRO 40 m telescope has been monitoring the 15 GHz radio flux density of over 1200 blazars since 2008. The 15 GHz light curve of the flat spectrum radio quasar J1359+4011 shows a strong and persistent quasi-periodic oscillation. The time-scale of the oscillation varies between 120 and 150 days over a ∼ 4 year time span. We interpret this as the active galactic nucleus mass-scaled analog of low-frequency quasi-periodic oscillations from Galactic microquasars, or as evidence of modulation of the accretion flow by thermal instabilites in the “inner ” accretion disc

W. Max-moerbeck

CiteSeerX

The Owens Valley Radio Observatory 40-m telescope has been monitoring the 15-GHz radio flux density of over 1200 blazars since 2008. The 15-GHz light curve of the flat spectrum radio quasar J1359+4011 shows a strong and persistent quasi-periodic oscillation. The time-scale of the oscillation varies between 120 and 150 d over an ∼4 year time span. We interpret this as the active galactic nucleus mass-scaled analogue of low-frequency quasi-periodic oscillations from Galactic microquasars or as evidence of modulation of the accretion flow by thermal instabilities in the ‘inner’ accretion disc

Caltech Authors - Main

Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 6 September 2013 (MN LATEX style file v2.2)
A quasi-periodic oscillation in the blazar J1359+4011
O.G. King1?, T. Hovatta1, W. Max-Moerbeck2, D.L. Meier3, T.J. Pearson1,
A.C.S. Readhead1, R. Reeves1, J.L. Richards4, M.C. Shepherd1
1California Institute of Technology, 1200 E California Blvd, MC 249-17, Pasadena, CA 91125, USA
2National Radio Astronomy Observatory, P.O. Box 0, Socorro, NM 87801, USA
3NASA Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, CA 91109, USA
4Department of Physics, Purdue University, West Lafayette, IN 47907, USA
6 September 2013
ABSTRACT
The OVRO 40 m telescope has been monitoring the 15 GHz radio flux density of
over 1200 blazars since 2008. The 15 GHz light curve of the flat spectrum radio
quasar J1359+4011 shows a strong and persistent quasi-periodic oscillation. The
time-scale of the oscillation varies between 120 and 150 days over a ∼ 4 year time
span. We interpret this as the active galactic nucleus mass-scaled analog of low-
frequency quasi-periodic oscillations from Galactic microquasars, or as evidence
of modulation of the accretion flow by thermal instabilites in the “inner” accretion
disc.
Key words: accretion, accretion discs – galaxies: active – galaxies: jets – galax-
ies:individual:J1359+4011.
1 INTRODUCTION
Quasi-periodic behaviour in blazars at radio wavelengths
occurs on time-scales of order years. It usually takes the
form of periodic flaring, for instance in the BL Lacertae
object OJ 287 (1.12 or 1.66 yr, Hughes et al. 1998), BL Lac
itself (∼ 2 and ∼ 8 yr, Stirling et al. 2003; Villata et al.
2004), and AO 0235+16 (∼ 5 or ∼ 8 yr, Raiteri et al.
2001, 2006). This behaviour is usually explained by pro-
cesses related to the orbital dynamics of the disc/jet sys-
tem, such as periodically varying Doppler beaming from
a precessing jet (Marscher & Gear 1985; Camenzind &
Krockenberger 1992; Abraham 2000; Caproni et al. 2013)
or processes related to the innermost stable orbit of the
accretion disc (Broderick & Loeb 2006; Pihajoki et al.
2013).
While quasi-periodic oscillations (QPOs) in the X-ray
emission of stellar mass black-hole binaries are a common
and well-studied phenomenon (Remillard & McClintock
2006), corresponding behaviour from supermassive black
holes at the heart of AGN is very rare. Only one X-ray
QPO from an AGN – the narrow line Seyfert 1 galaxy
REJ 1034+396 – has been reliably measured (Gierlin´ski
et al. 2008; Gonza´lez-Mart´ın & Vaughan 2012). It has a
period of ∼ 1 h, and is thought to be the mass-scaled
equivalent of high frequency QPOs seen in stellar black
hole binaries (Middleton & Done 2010).
In this letter we describe the discovery of a re-
markable and persistent quasi-periodic oscillation in
the 15 GHz light curve of the flat-spectrum radio
quasar CGRABS J1359+4011. The source J1359+4011
? E-mail: ogk@astro.caltech.edu
(13h59m38.1s, +40d11m38.3s J2000) is a high Galactic
latitude (b = 70.8◦) flat-spectrum radio quasar (FSRQ)
(Landt et al. 2001; Sowards-Emmerd et al. 2005) at a red-
shift of z = 0.407 ± 0.001. It has been detected at X-ray
energies using ROSAT (Landt et al. 2001), but it is not
visible at γ-ray energies and has not been included in ei-
ther of the Fermi catalogs (Abdo et al. 2010; Nolan et al.
2012). J1359+4011 is thought to contain a 108M black
hole with high Eddington ratio accretion (Table 1). The
data are described in Section 2, the QPO is explored us-
ing a wavelet decomposition in Section 3, and we interpret
the QPO in Section 4 as being either the AGN analog of
low-frequency QPOs, or as being caused by thermal in-
stabilities in the “inner” accretion disc.
2 DATA
The 40 m telescope of the Owens Valley Radio Observa-
tory (OVRO) is being used to monitor the 15 GHz flux
densities of over 1200 active galaxies twice a week. This
program has been running continuously since 2008. The
data are reduced and calibrated to form regularly updated
light curves that are made available on the internet1.
The OVRO 40 m receiver uses an alternation of two
beams on the source (Readhead et al. 1989) to remove
the varying atmospheric emission and diffuse background
emission from the source flux density measurement. The
data are calibrated and the flux density scale is set using
the primary calibrator 3C286, as described in Richards
et al. (2011).
1 http://www.astro.caltech.edu/ovroblazars/
c© 0000 RAS
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 Se
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2 O.G. King et al.
54800 55000 55200 55400 55600 55800 56000 56200
Time [MJD]
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
15
G
H
z
flu
x
de
ns
it
y
[J
y]
2008.5 2009.0 2009.5 2010.0 2010.5 2011.0 2011.5 2012.0 2012.5 2013.0
Figure 1. The OVRO 15 GHz light curve for J1359+4011. The
data cover a 4.1 yr time span and show a strong quasi-periodic
modulation (Fig. 2).
The 4.1 yr light curve of J1359+4011 is shown in
Fig. 1. The object has been observed slightly more of-
ten than once a week, on average, with occasional gaps
in the light curve due to telescope maintenance or poor
weather. A modulation of about 25% of the flux density
is visible. This is the only source in the OVRO catalogue
to show such a strong and persistent modulation.
Two instrumental effects would produce such a mod-
ulation of the light curve. The first is related to the dual-
beam measurement process. If the reference field around
the target source were to contain a patch of bright emis-
sion, this might produce elevation dependent flux-density
measurements. However, the data show no dependence
of the flux density on elevation. The second systematic
effect that might cause the modulation is linked to the
pointing procedure used by the OVRO 40 m telescope.
The sky is divided in to 134 regions and pointing correc-
tions for the 40 m telescope are calculated in each region.
Time-dependent errors in the pointing correction for the
region around J1359+4011 would result in a modulated
light curve. Sources that share the same pointing correc-
tion would share the same fractional flux density modu-
lation. However, careful study of the sources that share
the same pointing correction shows that they contain no
such common modulation. We conclude that the modula-
tion seen in the J1359+4011 light curve is intrinsic to the
source.
3 QUASI-PERIODIC BEHAVIOUR
Wavelets are an ideal tool for measuring quasi-periodic
fluctuations. The data are decomposed into localized func-
tions, which is preferable to Fourier analysis when search-
ing for short-lived fluctuations or fluctuations with a vary-
ing period. Foster (1996) introduced an alternative to
the discrete wavelet transform, the weighted wavelet Z-
transform (WWZ), that is better suited to discovering
the time-scale of fluctuations in the light curve and is
robust against missing data. It is based on the Morlet
wavelet (Grossmann & Morlet 1984) and has become a
much-used tool in fields as diverse as paleoceanography
(Incarbona et al. 2010), climate change (Johnson 2010),
pulsating variable stars (Templeton & Karovska 2009),
and AGN variability (Hovatta et al. 2008). The values
20.0
30.0
50.0
70.0
100.0
150.0
200.0
300.0
Pe
ri
od
[d
ay
s]
0 1 2 3 4 5 6 7
Normalized
Amplitude
Data
1σ
2009.0 2009.5 2010.0 2010.5 2011.0 2011.5 2012.0 2012.5
54800 55000 55200 55400 55600 55800 56000 56200
Time [mjd]
0.0
0.1
0.2
0.3
0.4
F
lu
x
de
ns
it
y
[J
y]
Figure 2. The WWZ of the light curve for the source
J1359+4011, with darker colour indicating a stronger signal.
A strong signal is seen throughout 2008 and most of 2009. It
appears to fade in strength and increase in duration before re-
turning in late 2011. The WWZ amplitude has been normalized
as described in the text; the global wavelet in the right panel
includes the 1σ significance contour determined from Monte
Carlo simulations.
Table 1. The properties of J1359+4011 from D’Elia et al.
(2003). The mass was determined using the Hβ emission line
and the luminosity calculated in the interval 7.08×1014−7.24×
1014 Hz.
logM• log M˙ log
(
L
LEdd
)
log
(
Lth
Ltot
)
logLtot(
M
) (
M yr−1
)
(erg s−1
Hz−1)
8.0 −1.7 −1.6 −1.7 30.03
that the WWZ takes can be thought of as goodness-of-fit
parameters similar to the familiar chi-square statistic.
We used Monte Carlo simulations to establish the sta-
tistical significance of the structures in the WWZ of the
light curve. The null hypothesis is that the light curve is
a Gaussian random process with power law power spec-
trum. We generated artificial light curves with the same
PSD slope of 1.5 as J1359+4011 using a modified im-
plementation of Uttley et al. (2002), as described in W.
Max-Moerbeck et al. (in preparation). The artificial light
curves had the same variance and sampling as the data.
We used 1000 artificial light curves to establish the mean
and standard deviation of the WWZ at each point in the
period/time plane for the artificial data. The WWZ of
J1359+4011 is shown in Fig. 2. The amplitude has been
normalized by the mean established through Monte Carlo
simulations. The average wavelet over time in the right
panel includes the 1σ contour.
The wavelet analysis reveals an initial ∼ 120 d oscil-
lation in 2008/2009 that then fades in amplitude and in-
creases in period, reaching a minimum amplitude in mid-
2010. It then increases in amplitude and peaks at a period
of ∼ 150 d in early 2012. The range of detectable time-
scales is from 15 to 300 d.
c© 0000 RAS, MNRAS 000, 000–000
A quasi-periodic oscillation in the blazar J1359+4011 3
4 INTERPRETATION
A variety of mechanisms has been proposed to explain
periodic behaviour in blazar radio and optical emission.
These include periodically varying Doppler beaming from
a precessing jet (Marscher & Gear 1985; Camenzind &
Krockenberger 1992; Abraham 2000; Caproni et al. 2013)
and processes related to the innermost stable orbit of the
accretion disc (Broderick & Loeb 2006; Pihajoki et al.
2013). The periods observed are generally on time-scales
of order years.
One possible explanation for the behaviour of
J1359+4011 is that the oscillation is a direct analog of the
QPOs seen in microquasars (jet-producing black hole X-
ray binaries). Since many black hole time-scales vary ap-
proximately inversely with black hole mass, scaling the os-
cillation in J1359+4011 in this manner to a 10M black
hole would result in a QPO of a few hertz – squarely in
the range of a low-frequency QPO (LFQPO) for a typical
microquasar. LFQPOs in the latter sources are usually
classified as types A, B, or C (Casella et al. 2005). The
closest analog of the oscillation in J1359+4011 may be
the type A QPO, which is associated with high Edding-
ton ratio accretion2 and low Q (ν/∆ν). Type A QPOs
also have low fractional variability (only a few percent),
but that measurement is performed in the X-ray; very
high time resolution radio studies have not yet been done
on QPO-producing microquasars so no comparable ra-
dio variability values exist yet. In principle, one might
be able to verify the expected low variability by observ-
ing J1359+4011 in the X-ray, but given past problems in
finding X-ray QPOs in AGN (not to mention the possi-
ble beamed jet contamination of the X-ray flux in this
blazar), such an observation would be very difficult. Fur-
thermore, the currently popular model for classical mi-
croquasar QPOs (particularly types A and C) is Lense-
Thirring precession of a geometrically thick, accretion
torus near the central black hole (Ingram et al. 2009; In-
gram & Done 2011; Motta et al. 2011). Such models even
predict the X-ray flux / QPO frequency correlations seen
in type C QPOs. If this torus precession would slightly
precess the pointing direction of the J1359+4011 blazar’s
jet, this model may account for the radio oscillations seen
in Fig. 1.
Another potential explanation for the observed QPO
is an instability in the disc/jet system. Magnetically
choked accretion flows (MCAFs) in general-relativistic
3D MHD simulations are seen to produce quasi-periodic
fluctuations in the energy outflow efficiency of the rela-
tivistic jet close to the black hole (Tchekhovskoy et al.
2011). The dominant mode of these fluctuations has a
period of ∼ 70 rg/c for a rapidly spinning black hole
(a = 0.9375), which is ∼ 1 d when the Schwarzschild ra-
dius rg ' 3 × 1011 m for a 108M black hole. Slower-
spinning black holes are expected to have longer periods
(McKinney et al. 2012). The different time-scale makes it
improbable that MCAFs are the mechanism behind the
QPO seen in the J1359+4011, assuming that it is even
possible for the MCAF QPOs to radiatively transfer out
to the optically thin radio region of the jet. So-called dy-
2 Here we define “high Eddington ratio” as an accretion ratio
m˙ = M˙/M˙Edd at which, in standard thin disc theory, the disc
“inner” region is expected to be radiation pressure dominated.
For M• = 10M, this is m˙ & 0.15 and for M• = 108M, a
high Eddington ratio would be anything above m˙ & 0.02.
namo cycles (Brandenburg et al. 1995) in simulations of
black hole accretion discs – oscillations in the azimuthal
magnetic field above the accretion disc – have periods of
∼ 5000 s GM•/c3 (O’Neill et al. 2011) which is ∼ 30 d
for a 108M black hole. It is unclear, however, whether
these oscillations can produce an observable signature in
the radio jet.
A third possible model that would be applicable only
to high Eddington ratio black hole systems involves an in-
stability in the accretion flow that would alternately feed
and starve the black hole with accreting material. This
is the Lightman-Eardley secular (accretion flow) insta-
bility, in which the entire radiation pressure dominated
“inner” region of the accretion disc behaves in a man-
ner opposite to that normally expected: the accretion rate
varies inversely with vertically integrated disc surface den-
sity (Lightman & Eardley 1974). The accretion flow then
breaks up into rings of high surface density (which ac-
crete slowly internally) and low surface density (which
accrete rapidly internally). The time for the entire ra-
diation pressure dominated disc region to empty (12 min
(M•/10M)4/3 (m˙/0.3)2/3) is much too long (9,000 years
for a 108M black hole accreting at m˙ ∼ 0.025; Meier
2012) to explain the J1359+4011 oscillations. However,
if the disc rings formed via thermal processes, then when
they initially form at or near the outer edge of the “inner”
disc region each would accrete toward the black hole, sep-
arated roughly by the thermal time at or near that outer
edge. The thermal time scale at the outer edge of the
“inner” disc region is ∼1s (M•/10M)8/7 (m˙/0.3)8/7, or
∼ 0.2 yr. Given the uncertainty in the coefficients and
black hole mass in these expressions (factors of 2–3), this
is in rough agreement with the observed 0.3–0.4 yr oscil-
lation period in J1359+4011.
5 CONCLUSIONS
The 15 GHz light curve for the flat spectrum radio quasar
J1359+4011 shows a strong QPO with a period that varies
between 120 and 150 days. The fluctuations in the light
curve are intrinsic to the source and are not due to in-
strumental or observing effects. It is the only source in
the OVRO catalogue of over 1200 sources to show such a
strong and persistent QPO.
The physical cause of this quasi-periodic oscillation
is unclear. The QPO period is of the right time-scale to be
the AGN analog of LFQPOs observed at X-ray energies
in stellar-mass black hole binaries. If this is the case, the
precessing thick torus (that is thought by some authors
to be the origin of the X-ray QPOs in black hole binaries)
may be precessing the direction of the blazar jet slightly,
thereby modulating the radio light curve.
Another possible explanation for the modulation of
the jet luminosity is instabilities in the accretion flow.
Inwardly-accreting rings caused by the Lightman-Eardley
secular instability would be separated by the thermal time
scale measured at the outer edge of the “inner” disc re-
gion. This time scale is of the right order to explain the
oscillation period in J1359+4011.
ACKNOWLEDGEMENTS
We thank Russ Keeney for his support of observations at
OVRO. The OVRO 40-m program is supported in part
by NASA grants NNX08AW31G and NNX11A043G and
c© 0000 RAS, MNRAS 000, 000–000
4 O.G. King et al.
NSF grants AST-0808050 and AST-1109911. T.H. was
supported by the Jenny and Antti Wihuri foundation.
Support from MPIfR for upgrading the OVRO 40-m tele-
scope receiver is acknowledged. We thank V. Pavlidou for
useful discussions. The National Radio Astronomy Ob-
servatory is a facility of the National Science Foundation
operated under cooperative agreement by Associated Uni-
versities, Inc. Part of this research was carried out at the
Jet Propulsion Laboratory, California Institute of Tech-
nology, under contract with the National Aeronautics and
Space Administration.
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A quasi-periodic oscillation in the blazar J1359+4011

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