MNRAS 469, 2457–2463 (2017) doi:10.1093/mnras/stx1062
Advance Access publication 2017 May 4
Radio and optical intra-day variability observations of five blazars
X. Liu,1,2‹ P. P. Yang,1,3 J. Liu,1,2‹ B. R. Liu,4 S. M. Hu,5 O. M. Kurtanidze,6,7,8
S. Zola,9,10 A. Kraus,11 T. P. Krichbaum,11 R. Z. Su,1,3 K. Gazeas,12 K. Sadakane,13
K. Nilson,14 D. E. Reichart,15 M. Kidger,16 K. Matsumoto,13 S. Okano,17 M. Siwak,10
J. R. Webb,18 T. Pursimo,19 F. Garcia,20 R. Naves Nogues,21 A. Erdem,22,23
F. Alicavus,22,23 T. Balonek24 and S. G. Jorstad25
Affiliations are listed at the end of the paper
Accepted 2017 April 28. Received 2017 April 18; in original form 2017 March 14
ABSTRACT
We carried out a pilot campaign of radio and optical band intra-day variability (IDV) obser-
vations of five blazars (3C66A, S5 0716+714, OJ287, B0925+504 and BL Lacertae) on 2015
December 18–21 by using the radio telescope in Effelsberg (Germany) and several optical
telescopes in Asia, Europe and America. After calibration, the light curves from both 5 GHz
radio band and the optical R band were obtained, although the data were not smoothly sam-
pled over the sampling period of about four days. We tentatively analyse the amplitudes and
time-scales of the variabilities, and any possible periodicity. The blazars vary significantly in
the radio (except 3C66A and BL Lacertae with only marginal variations) and optical bands on
intra- and inter-day time-scales, and the source B0925+504 exhibits a strong quasi-periodic
radio variability. No significant correlation between the radio- and optical-band variability ap-
pears in the five sources, which we attribute to the radio IDV being dominated by interstellar
scintillation whereas the optical variability comes from the source itself. However, the radio-
and optical-band variations appear to be weakly correlated in some sources and should be
investigated based on well-sampled data from future observations.
Key words: scattering – BL Lacertae objects: general – galaxies: jets – quasars: general.
1 IN T RO D U C T I O N
Among active galactic nuclei (AGNs), blazars (a combination of
flat-spectrum radio quasars and BL Lac objects) exhibit extreme
variations in flux over almost the entire electromagnetic spectrum
on time-scales ranging from less than hours to many years. This
violent behaviour of blazars is generally explained in terms of the
shock-in-jet model (Marscher & Gear 1985) and the beaming effect,
in which a relativistic jet is oriented very close to our line of sight
(Urry & Padovani 1995).
Discovered in the 1980s (Witzel et al. 1986; Heeschen et al. 1987),
the intra-day variability (IDV) of AGNs at centimetre wavelengths
occurs in 56 per cent of the flat-spectrum radio AGNs (Lovell et al.
2008) and in ∼60 per cent of the γ -ray blazars (Liu et al. 2012a).
The cm-radio IDV has been largely attributed to the interstellar
scintillation (ISS) for the dependence of variability on the Galactic
latitude and the H α emission [the Wisconsin H Alpha Mapper
(WHAM) Northern sky survey, Haffner et al. 2003], see Lovell et al.
(2008), and especially for the annual modulation (owing to the Earth
revolution around the Sun) of variability time-scales discovered in
 E-mail: liux@xao.ac.cn (XL); liuj@xao.ac.cn (JL)
a couple of IDV sources (e.g. Jauncey & Macquart 2001; Rickett
et al. 2001; Dennett-Thorpe & de Bruyn 2003; Bignall et al. 2003,
2006; Gaba´nyi et al. 2007; Liu et al. 2012b, 2013) and the time
delays of IDV found in trans-continent observations (Jauncey et al.
2000; Dennett-Thorpe & de Bruyn 2002; Bignall et al. 2006). For
more details, see the review by Jauncey et al. (2016).
In the optical band, however, IDV should be intrinsic to the
source because ISS does not affect extragalactic emission in optical
wavelengths.
It is generally believed that the brightness temperature Tb of the
radio-jet component of AGNs cannot exceed the inverse Comp-
ton limit TIC = 1011.5 K (Kellermann & Pauliny-Toth 1969) or
the equipartition temperature Teq = 5 × 1010  1010.7 K (Readhead
1994). If radio IDV at cm wavelengths is source-intrinsic, the bright-
ness temperature of emitting components will exceed the limit TIC
or Teq by several orders of magnitudes, which would require a very
large Doppler factor δ (e.g. >100). However, for blazars, Doppler
factors less than 30 are usually estimated from Very Long Base-
line Interferometry (VLBI) observations or flux monitoring data
(Liodakis et al. 2017).
Although cm-radio IDV is generally attributed to ISS, source-
intrinsic radio IDV might still be possible to detect in the follow-
ing situations: (1) In some blazar cores, the intrinsic brightness
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2458 X. Liu et al.
temperature Tint = Tobs/δ is greater than 1012 K (where Tobs is
measured from the RadioAstron space VLBI observations and δ is
estimated from ground VLBI arrays; see Kovalev et al. 2016), im-
plying that TIC could be violated for these blazars (Marscher 2016),
but such apparent detections of high temperatures on very long
baselines as seen by RadioAstron may also be caused by refractive
substructure introduced by scattering in the interstellar medium,
which is more evident at 1.6 GHz (Johnson et al. 2016). (2) A large
fraction of IDV sources are intermittent (Kedziora-Chudczer 2006;
Lovell et al. 2008), which should allow source-intrinsic IDV to be
observed when ISS disappears. (3) Sometimes the radio and optical
IDVs seem to be correlated, e.g. in blazar S5 0716 + 714, Quirren-
bach et al. (1991) found, during a four-week monitoring campaign
in 1990, simultaneous transitions from fast to slow variability modes
among 5 GHz radio-band and optical-band wavelengths; they inter-
pret the similar transitions as source intrinsic variability.
However, the existence of source-intrinsic IDV, if present, is
difficult to disentangle from ISS-induced variability, and it may
require multiwavelength measurements with, for example, radio,
optical, X-ray and γ -ray IDV observations to search for correlations
between the bands. Note that the optical and high-energy-band IDVs
are thought to be intrinsic to the source. Very few multiband IDV
observations have been made over the past decades. To address this
shortcoming, we are planning more radio-optical IDV observations.
As a first step in this plan, we carried out a pilot campaign on 2015
December 18–21, using two radio telescopes and several optical
telescopes.
2 O B S E RVAT I O N S A N D DATA R E D U C T I O N
We carried out quasi-simultaneous radio-optical observations of five
blazars on 2015 December 18–21 using the following telescopes:
Effelsberg and Urumqi (5 GHz radio), Weihai (1 m), Urumqi (1 m),
Abastumani (70 cm), the Blazar Optical Sky Survey (BOSS Project)
at the University of Athens Observatory (40 cm) and OJ287-15/16
Collaboration telescopes (optical R band). The target sources were
3C66A, S5 0716 + 714, OJ287, B0925 + 504 and BL Lac, which
were selected from the RadioAstron 2015 December observing sam-
ple, except B0925 + 504 that we added in this campaign for its sig-
nificant radio IDV observed before. These sources are also bright
in optical, so we want to compare their radio and optical variations,
and it will be interesting to see if there is a radio-optical correlation
for S5 0716 + 714 as suggested before (Quirrenbach et al. 1991).
Unfortunately, the Urumqi radio telescope was being reconstructed
and was still in testing at that time. It suffered from serious prob-
lems involving pointing, etc. that prevented the data from being used
in our analysis. In addition, the Urumqi optical observation had a
problem with its camera gate, which led to serious light pollution
that prevented that data from being used as well.
The radio observations at Effelsberg, however, were successful.
These observations were performed by cross-scans in azimuth and
elevation (all sources were point-like for the 100-m telescope). For
these observations, the source was actually scanned two times in
both azimuth and elevation. After initial calibration of the raw data,
the intensity profile of each scan was fitted with a Gaussian function
after subtracting the baseline. The fitted scans were then averaged
over both azimuth and elevation. Next, after pointing correction, the
azimuth and elevation scans were averaged together, and then cor-
rections for the gain-elevation effect of the antenna and the (rather
small) atmospheric absorption were applied, with the secondary cal-
ibrators monitored during the observation. These calibrators were
Figure 1. Radio-band (5 GHz) light curve (Effelsberg, on right-hand axis)
and optical R-band light curve (Weihai, Abastumani, on left-hand axis)
versus JD−2457300, for 3C66A.
also used to correct the data for systematic time-dependent effects,
with an accuracy of ∼0.4 per cent (1σ ) of total flux density. Fi-
nally, the amplitudes were converted to absolute flux density by
using the average scale of the primary calibrators (3C48, 3C286
and NGC7027). The observation lasted nearly four days, with a
typical sampling rate of 1–2 h for each calibrated data point. How-
ever, because sources were not always in the sky for 24 h, some
gaps appear in the light curves of the sources.
The raw optical data were processed by using standard methods
e.g. by using the standard routines of the Image Reduction and
Analysis Facility or DAOPHOT II calibrated with comparison stars in
the observation field; see Valtonen et al. 2016 for the data-reduction
method of the OJ287-15/16 Collaboration). We do not describe in
detail here the telescopes and cameras involved in these observa-
tions. However, note that, because the sources are compact, the
different apertures used at different telescopes (≤1 m) had a negli-
gible effect on the measured flux. Photometric error was estimated
from the root-mean-square (rms) deviations of the reference stars
and found to be generally less than 0.02 mag, although some larger
errors appeared in some time slots of individual telescopes (e.g. in
the last day of the Weihai observations), which were mostly due to
bad weather.
3 R E S U LT S A N D VA R I A B I L I T Y
C H A R AC T E R I S T I C S
3.1 Light curves and variability analysis
Figs 1–4 and 6 show the radio and optical light curves, for 3C66A,
S5 0716 + 714, OJ287, B0925 + 504 and BL Lac, respectively. We
used the following methods to analyse the variability:
(1) The relative amplitude of variability is defined as the maxi-
mum Smax minus the minimum Smin divided by the maximum plus
the minimum of a light curve (Kovalev et al. 2005):
V = (Smax − Smin)/(Smax + Smin). (1)
The results are shown in Table 1 for both the radio and optical
variability for the five sources. We also used the rms flux variation
(in percent), which is the ratio of the rms variability σ var divided
by the mean flux density S: m = σvar/S. Note that, the units of the
optical magnitude are not those of the power intensity of the source,
such as the radio-flux density; here, we are concerned only with the
relative variations and do not intend to transform these variations
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Radio and optical IDV observation of blazars 2459
Figure 2. Radio-band (5 GHz) light curve (Effelsberg, on right-hand axis)
and optical R-band light curve (Weihai, Abastumani, on left-hand axis)
versus JD−2457300, for S5 0716+714.
Figure 3. Radio (5 GHz) light curve (Effelsberg, on right-hand axis) and
optical R-band light curve (Weihai, Abastumani, OJ287−15/16 Collabora-
tion, on left-hand axis) versus JD−2457300, for OJ287.
Figure 4. Radio (5 GHz) light curve (Effelsberg, on right-hand axis) and
optical R-band (V–C) light curve (Abastumani, on left-hand axis) versus
JD−2457300, for B0925 + 504.
into flux density. Note that, in Table 1, m < V by definition for both
radio and optical variations.
(2) We analyse the variability time-scale t and possible variability
period P in the light curves. For this, we used the first-order standard
structure function (SF); see Simonetti, Cordes & Heeschen (1985).
The SF usually has a power-law form that reaches its maximum at
a saturation level. Here, we define the intersection of the power-law
fit with the plateau corresponding to this saturation level as the time-
scale of the characteristic variability. The error of the time-scale can
be estimated by accounting for the formal errors of the power-law
fit and the fitting to the SF plateau. Sometimes, the SF has more
than one plateau, in which case multiple variability time-scales are
estimated.
To determine the possible periodicity of the variations in light
curves, we applied the power spectrum density (PSD) analysis, in
which strong narrow peaks in the PSD are assumed to be suggestive
of variability periods, and the error is estimated from the full width
at half power of the peak. Only three sources (S5 0716 + 714,
OJ287 and B0925 + 504) show possible periodic variations (see
Table 1).
Note that, the resulting variability time-scales and periods are
tentative because of the gaps in the light curves of the sources
(e.g. 3C66A, OJ287 and BL Lac in radio band, and 3C66A, S5
0716 + 714, B0925 + 504 and BL Lac in optical band).
(3) The correlation between radio and optical light curves was
analysed by using the Pearson correlation coefficient and a null
probability or confidence level (e.g. Olkin & Pratt 1958; Fan et al.
2017); the result is listed in Table 1. The results reveal no significant
correlations between the radio and optical variations for the five
blazars, although weak correlation seems to appear in some cases
(S5 0716 + 714, B0925 + 504), but with low significance.
4 C O M M E N T S O N I N D I V I D UA L S O U R C E S
All five blazars are γ -ray loud BL Lac objects and three of them
are also TeV emitters (3C66A, S5 0716 + 714 and BL Lac), and
their radio spectra are flat at centimetre wavelengths. Four of them
show kpc-scale core-halo features but not more than 100 kpc, and
one (B0925 + 504) is compact and unresolved with the Very Large
Array (VLA). The BL Lac objects are low-accreting AGNs and
could be in the early evolutionary stage of becoming larger scale
Fanaroff–Riley Class I radio sources. In the following, we briefly
discuss each of the five sources based on the literature and on the
present results.
3C66A, 2FGL J0222.6 + 4302 (ISP BL)
This is an intermediate synchrotron peaked BL Lac object (ISP
BL, Abdo et al. 2010; Fan et al. 2016) and a TeV γ -ray source.
Optical variability at short time-scales has been reported for this
source (Xie et al. 1994). In the radio band, it appears as a core
halo on the <100 kpc scale and has a one-sided VLBI core jet to
the south, in which fast apparent superluminal motions of up to
29c were reported by Jorstad et al. (2001). No evident correlation
between long-term γ -ray by the Fermi-Large Area Telescope and
multiband radio emission was found in 3C66A(Fuhrmann et al.
2014).
The present results show a marginal inter-day variability at 5 GHz
(with reduced Chi-squared of 1.2) and rapid optical variations with
time-scales of 0.6 ± 0.1 and 1.4 ± 0.2 d (see Fig. 1 and Table 1),
and no correlation between the radio and optical variations is found.
S5 0716+714, 2FGL J0721.9 + 7120 (ISP BL)
This object is a TeV γ -ray blazar and displays optical short-
term variability and micro variability (Hu et al. 2014; Man et al.
2016). In the radio band, it shows a two-sided halo on the kpc
scale, and a one-sided VLBI core jet to the north with highly rel-
ativistic speeds (Rani et al. 2015; Lister et al. 2016). A significant
positive correlation exists between the position angle and the peak
flux density of the inner jet, which is suggestive of a helical jet
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Table 1. Variability amplitudes and possible variability time-scales and periods of blazars, and the correlation parameter between radio and optical
variations. The columns are (1) source name, (2) redshift, (3) observation band (5 GHz radio in first line, optical R band in second line), (4) rms
amplitude variation divided by average value in percent, (5) relative amplitude variations in percent as defined in the text, (6) variability time-scales (in
days) estimated from the SF, (7) variability periods (in days) estimated from the PSD, (8) Pearson correlation coefficient r and (9) null probability pnull.
The calibrator’s modulation index m0 is 0.35 per cent at radio band.
(1) (2) (3) (4) (5) (6) (7) (8) (9)
Source z Band m (per cent) V (per cent) t(d) P(d) r pnull
3C66A 0.444 Radio 1.5 4.7 >3.0 0.07 0.81
Optical 0.3 1.2 0.6(0.1), 1.4(0.2)
S5 0716 + 714 ∼0.3 Radio 3.6 7.4 2.3(0.2) 0.35 0.13
Optical 0.6 1.5 0.5(0.1) 0.8(0.2)
OJ287 0.306 Radio 3.0 7.3 0.4(0.1), >1.8 − 0.47 0.04
Optical 1.2 3.4 0.3(0.1), 1.0(0.1) 0.5(0.1), 1.3(0.2)
B0925 + 504 0.37 Radio 5.2 10.0 0.3(0.1) 0.5(0.1) 0.34 0.26
Optical 1.1 2.5 0.3(0.1)
BL Lac 0.0686 Radio 1.1 2.4 1.9(0.2) 0.32 0.21
Optical 0.6 1.6 0.4(0.1), 1.1(0.1)
(Liu et al. 2012c). The VLBI jet-position angle and the γ rays are
also correlated, which suggests that radio waves and γ rays are
emitted on the same helical filament, which rotates with the jet
(Rani et al. 2014), Rani et al. (2015) further state that ‘our analysis
favours the major optical/γ -ray flares in S5 0716 + 714 being pro-
duced upstream of the 7-mm VLBI core’. This statement makes it
clear that the radio and optical/γ -ray flares in S5 0716 + 714 come
from separate regions. During a four-week monitoring campaign
in 1990, simultaneous transitions were observed from fast to slow
variability modes among 5-GHz radio and optical bands, suggest-
ing a source intrinsic origin of the inter-day variability (Quirrenbach
et al. 1991). Fuhrmann et al. (2008) found that the increase of the
variability amplitudes from cm to mm bands contradicts expecta-
tions from standard ISS and suggests a source-intrinsic origin for
the inter-day variability. Gupta et al. (2012) observed multiwave-
band IDV and found no significant correlation between cm-radio
and optical-band IDV. At 5 GHz, Liu et al. (2012b) found strong
evidence in the form of an annual cycle of IDV time-scales of S5
0716 + 714, which suggests that the 6-cm IDV is dominated by
ISS.
The present results show strong IDVs in both the radio and optical
bands (Fig. 2 and Table 1). The radio flux initially increases and
then decreases, and contains small IDV wiggles. In the optical
band, it undergoes pronounced variations. A weak radio-optical
correlation exists in the ‘common radio-optical data time’, but this
is not significant (Table 1).
OJ287, 2FGL J0854.8 + 2005 (LSP BL)
The BL Lac object is known for its ∼12-yr periodicity in the
optical light curve (Sillanpa¨a¨ et al. 1988). The optical outbursts
appear as double peaked and their periodic occurrence is interpreted
primarily in terms of a binary black hole system (Valtonen 2007).
The model predicted a major optical outburst in 2015 December,
which occurred within the expected time range, peaking on 2015
December 5 at a magnitude of 12.9 in the optical R band (Valtonen
et al. 2016). Various time-scales and periods (or quasi-periodic
oscillations) have been reported in the optical band (Sandrinelli
et al. 2016; Bhatta et al. 2016; Zola et al. 2016). In the radio band, it
shows a one-sided core jet on the kpc scale to the west, similar to its
VLBI jet direction (Perlman & Stocke 1994). A prominent erratic
wobbling of the inner-jet-position angle changes by up to 40◦ over
2 yr (Agudo et al. 2012), and it has been suggested that OJ287 is a
rotating helix (Cohen 2017).
The present results show prominent intra- and inter-day variations
in the radio and optical bands, with the shortest time-scale being a
few hours in the optical band (Fig. 3 and Table 1). The radio-band
light curve has three gaps, so the radio-band time-scale assignment
is only tentative. Finally, a weak (non-significant) anti-correlation
appears between the radio and optical variations in the common
radio-optical data time (see Table 1).
B0925 + 504, 2FGL J0929.5 + 5009 (ISP BL)
The optical R-band image of this ISP BL Lac object at redshift
0.37 shows a very compact host galaxy that can be fitted with the
de Vaucouleurs model (Nilsson et al. 2003). In the radio band, it
is not resolved at 5 GHz with the VLA (Taylor et al. 1996) but is
resolved with the VLBI, showing a core jet to the southeast. Rapid
radio IDVs have previously been detected towards this source at
5 GHz (Koay et al. 2011; Liu & Liu 2015).
Our results show prominent intra- and inter-day variations with
similar time-scales of 0.3 ± 0.1 d in the radio and optical bands
(but the optical time-scale is tentative because the data are from
only a single site), and exhibit a remarkable quasi-periodic radio
variability with P ∼ 0.5 ± 0.1 d estimated from the light curve (see
Fig. 4 and Table 1). Liu & Liu (2015) find that, from our previous
observations at Urumqi and Effelsberg, the IDV time-scales of this
source can be fitted with an ISS model, with the time-scales mostly
<0.5 d and the longer time-scales becoming relevant only during
days 250–300 of the year. The IDV time-scale of 0.3 ± 0.1 d
observed on 2015 December 18–21 about day 355 of the year is
shorter than that predicted by the model. We combined all previous
and current observations of this source and fit the time-scales again
with the model of the anisotropic scattering ‘screen’ (see Fig. 5).
The parameters of the ISM model are updated with the projected
velocity of the screen, VRA = −4.2 ± 1.1 km s−1 and VDec = −2.5 ±
1.8 km s−1, the ratio of the two-dimensional screen axis, R = 1.5 ±
0.3, and the position angle of the major axis of the screen (measured
from the north to east), ψ = 111.◦1 ± 8.4◦. Note that the data are
fitted with D = 110 pc, with the assumption that the distance D
from the ISM to Earth did not vary too much over the past years.
See Marchili et al. (2012) and Liu & Liu (2015) for more details of
fitting to the ISS model.
Furthermore, a weak but non-significant radio-optical corre-
lation exists (see Table 1) for the common radio-optical data
time (note, however, that only two nights of optical data are
available). Further study of this correlation requires a longer
optical-data train. However, the fitted annual cycle in Fig. 5 is
clear evidence of ISS as a major cause of the radio IDV on
B0925 + 504.
BL Lacertae, 2FGL J2202.8 + 4216 (LSP BL)
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Radio and optical IDV observation of blazars 2461
Figure 5. Radio (5 GHz) IDV time-scales from Liu & Liu (2015) and
new observations (green) that we made in 2015 July and December (the
campaign reported herein) against day of year for B0925 + 504. The red
solid line represents the updated model-expected time-scales. See text for
the values of the ISS model parameters.
Figure 6. Radio-band (5 GHz) light curve (Effelsberg, on right-hand axis)
and optical R-band light curve (Weihai, Abastumani, the University of
Athens Observatory, on left-hand axis) versus JD−2457300, for BL Lac.
BL Lacertae (the prototype of BL Lac objects) lies at the centre
of a giant elliptical galaxy with weak emission lines and is a TeV
γ -ray source. The source exhibits optical variabilities with multiple
time-scales (Fan, Xie & Bacon 1999). In the radio band, it is a
kpc-scale core halo and has a VLBI core jet to the south. The radio
emission varies at higher frequencies, leading the lower-frequency
emissions by several days to a few months (Villata et al. 2004), and
the optical variations lead the 15-GHz radio emissions by ∼100 d
(Bach et al. 2006).
The present results (Fig. 6) show some intra- and inter-day optical
variations with relatively weak variations in the radio band (with
reduced Chi-squared of 1.5 and m = 1.1 per cent in Table 1, about
3-σ detection). A weak but non-significant correlation exists be-
tween radio- and optical-band variations in the common radio-
optical data time (Table 1).
5 D ISCUSSION AND S UMMARY
Striking highly periodic radio-band variations of B0925 + 504 are
detected in this campaign, which share with the optical band a sim-
ilar variability time-scale of ∼0.3 d to the optical one. Furthermore,
a weak positive correlation between the radio- and optical-band
variability seems to appear in the common data time range. Liu &
Liu (2015) summarized the 5-GHz IDV data of B0925 + 504 ob-
served prior to 2015, which can be fitted with an ISS model. With the
two data points observed in 2015 added, the combined data can also
be fitted to an updated ISS model. The new model, which may be
more reliable because it has been tested against more data, suggests
the time-scale of 0.3 ± 0.1 d for B0925 + 504 is roughly consistent
with the annual modulation model of ISS. As for the highly periodic
radio-band variations of B0925 + 504, a tentative interpretation is
that the ISS can induce periodic variations when the scintillation
pattern forms a periodic distribution of the focus and defocus scin-
tillation patches. The quasi-periodic variations have also been seen
at 4.8 and 8.6 GHz in PKS1257 − 326, an IDV source about half
as strong as B0925 + 504, by Bignall et al. (2003).
For a source to produce intrinsic radio IDV at 5 GHz with a time-
scale of 0.5 d, for example, we can estimate the variability brightness
temperature Tb, var ≈ 1.3 × 1018 K (Wagner & Witzel 1995) for a
flux-density change of 0.1 Jy at 5 GHz during the variability period
of 0.5 d for the B0925 + 504. Based on this Tb, var and the Doppler
effect in total flux variability, Tb, var = δ3Tint (Hovatta et al. 2009),
we estimate a Doppler factor δ ≥ 160 if we assume an intrinsic
brightness temperature Tint ≤ TIC = 1011.5 K. This δ ∼ 160 is
much greater than that estimated from VLBI data or from total flux
variability on longer time-scales for blazars (δ < 30, La¨hteenma¨ki &
Valtaoja 1999; Hovatta et al. 2009; Savolainen et al. 2010; Liodakis
et al. 2017). The Doppler factor for a test with a viewing angle
θ = 1◦ and an intrinsic jet speed v = 0.98c is δ ∼ 10. The intrinsic
variations at 5 GHz, with such a short time-scale of 0.5 d, could
only be possible for a source in the most extreme conditions, for
example, for an extreme case of θ = 0.◦1 and v = 0.999 92c, the
Doppler factor is δ ∼ 155, which is close to the value of 160 obtained
above. However, for the B0925 + 504, as discussed previously, an
annual cycle surely demonstrates that these radio variations seen in
B0925 + 504 are predominantly caused by ISS.
To summarize, we conducted a pilot campaign of radio- and
optical-band IDV observations of five blazars around 2015 Decem-
ber 18–21 with the radio telescope at Effelsberg, and several optical
telescopes. After calibration, the light curves from both the 5-GHz
radio band and the optical R band were obtained, although the data
are not well sampled in time. We tentatively analyse the ampli-
tudes and time-scales of the variabilities and any possible periodic-
ity. The blazars vary significantly in the radio (except 3C66A and
BL Lac with only marginal variations) and optical bands on intra-
and inter-day time-scales. No significant correlation between radio-
and optical-band variability is found in the five sources, which is
most likely due to the radio IDV being dominated by ISS, whereas
the optical variability comes from the source itself. However, the
weak correlation between radio- and optical-band variation in some
sources, whether real or not, should be investigated again based on
observations that yield well-sampled data.
Some gaps appear in the light curves obtained in this pilot cam-
paign, so future radio-optical IDV observations over continuous
days with no gaps are required to well identify any correlation
between radio- and optical-band variations and the time delay be-
tween them. In addition, the optical band is better suited for observ-
ing radio-band IDV at both cm and mm wavelengths because the
ISS-induced variability is not present at mm wavelengths.
AC K N OW L E D G E M E N T S
We gratefully acknowledge the referee for careful reading and
valuable comments. This work is supported from the following
MNRAS 469, 2457–2463 (2017)Downloaded from https://academic.oup.com/mnras/article-abstract/469/2/2457/3798208
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2462 X. Liu et al.
funds: the 973 Program 2015CB857100; the Key Laboratory of
Radio Astronomy, the Chinese Academy of Sciences; the Na-
tional Natural Science Foundation of China (Grants No. 11273050,
No. 11463001 and No. 11503071); and the Guangxi Science
Foundation (2014GXNSFAA118024). OMK acknowledges fi-
nancial support from the Shota Rustaveli NSF under contract
FR/217554/16; and SMH acknowledges support from the National
Natural Science Foundation of China under Grant No. 11203016
and support from the Young Scholars Program of Shandong Uni-
versity, Weihai. SZ acknowledges partial support from National
Science Centre Poland (NCN) Grant No. 2013/09/B/ST9/00599.
Optical monitoring observations of OJ287 and BL Lac collected
within the framework of the Blazar Optical Sky Survey (BOSS
Project), is conducted at the University of Athens, Greece. We
thank The Scientific and Technological Research Council of Turkey
(TUBITAK) for partial support for using the T60 telescope through
Project No. 10CT60-76. The radio data are based on observations
with the 100-m telescope at the MPIfR (Max-Planck-Institut fu¨r
Radioastronomie) at Effelsberg.
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1Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 150
Science 1-Street, Urumqi 830011, China
2Key Laboratory of Radio Astronomy, Chinese Academy of Sciences,
Nanjing 210008, China
3Graduate University of Chinese Academy of Sciences, Beijing 100049,
China
4Guangxi Key Laboratory for Relativistic Astrophysics, Guangxi University,
Nanning, Guangxi 530004, China
5Shandong Provincial Key Laboratory of Optical Astronomy and Solar-
Terrestrial Environment, Institute of Space Sciences, Shandong University,
Weihai, 264209 Weihai, China
6Abastumani Observatory, Mt. Kanobili, 0301 Abastumani, Georgia
7Engelhardt Astronomical Observatory, Kazan Federal University, 420008
Tatarstan, Russia
8Landessternwarte Heidelberg, Ko¨nigstuhl 12, D-69117 Heidelberg,
Germany
9Astronomical Observatory, Jagiellonian University, ul. Orla 171, PL-30-
244 Krakow, Poland
10Mt. Suhora Observatory, Pedagogical University, ul. Podchorazych 2,
PL-30-084 Krakow, Poland
11Max-Plank-Institut fu¨r Radioastronomie, Auf dem Hu¨gel 69, D-53121
Bonn, Germany
12Section of Astrophysics, Astronomy and Mechanics, National and
Kapodistrian University of Athens, Zografos, GR-15784 Athens, Greece
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Radio and optical IDV observation of blazars 2463
13Astronomical Institute, Osaka Kyoiku University, 4-698 Asahi-
gaoka,Kashiwara, Osaka 582-8582, Japan
14Tuorla Observatory, Department of Physics and Astronomy, University of
Turku, F-21500 Turku, Finland
15Department of Physics and Astronomy, University of North Carolina,
Chapel Hill, NC 27599, USA
16Herschel Science Centre, ESAC, European Space Agency, C/Bajo el
Castillo, s/n, Villanueva de la Cana˜da, E-28692 Madrid, Spain
17Planetary Plasma and Atmospheric Research Center, Tohoku University,
Sendai 980-8578, Japan
18Florida International University and SARA Observatory, University Park
Campus, Miami, FL 33199, USA
19Nordic Optical Telescope, Apartado 474, E-38700 Santa Cruz de La
Palma, Spain
20Muna˜s de Arriba La Vara, E-33780 Valde´s, Spain
21C/Jaume Balmes No 24, Cabrils, E-08348 Barcelona, Spain
22Department of Physics, Faculty of Arts and Sciences, Canakkale Onsekiz
Mart University, Canakkale TR-17100, Turkey
23Astrophysics Research Center and Ulupinar Observatory, Canakkale
Onsekiz Mart University, Canakkale TR-17100, Turkey
24Foggy Bottom Observatory, Colgate University, 13 Oak Drive, Hamilton,
NY 13346, USA
25Institute for Astrophysical Research, Boston University, 725 Common-
wealth Ave., Boston, MA 02215, USA
This paper has been typeset from a TEX/LATEX file prepared by the author.
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