Intra-night Optical Variability of BL Lacs, Radio-Quiet Quasars and Radio-Loud Quasars

Abstract

We report optical monitoring observations of 20 high-luminosity AGN, 12 of which are radio-quiet quasars (RQQs). Intra-night optical variability (INOV) was detected for 13 of the 20 objects, including 5 RQQs. The variations are distinctly stronger and more frequent for blazars than for the other AGN classes. By combining these data with results obtained earlier in our program, we have formed an enlarged sample consisting of 9 BL Lacs, 19 RQQs and 11 lobe-dominated radio-loud quasars. The moderate level of rapid optical variability found for both RQQs and radio lobe-dominated quasars argues against a direct link between INOV and radio-loudness. We supplemented the present observations of 3 BL Lacs with additional data from the literature. In this extended sample of 12 well observed BL Lacs, stronger INOV is found for the EGRET detected BL Lacs. Comment: 8 pages, 3 Postscript figures, Accepted for publication in MNRAS, uses mn2e.cls

Full text

Page 1

arXiv:astro-ph/0410156v1 6 Oct 2004
Mon. Not. R. Astron. Soc. 000, 1–?? (2004)Printed 2 February 2008(MN LATEX style file v2.2)
Intra-night Optical Variability of BL Lacs, Radio-Quiet
Quasars and Radio-Loud Quasars
C. S. Stalin1,2, Alok C. Gupta3, Gopal-Krishna4,5, Paul J. Wiita6, and Ram Sagar2,7
1IUCAA, Post Bag 4, Ganeshkhind, Pune 411 007, India
2State Observatory, Manora Peak, Naini Tal 263 129, India
3Harish-Chandra Research Institute, Chhatnag Road, Jhunsi, Allahabad 211019, India
4National Centre for Radio Astrophysics, TIFR, Pune University Campus, Post Bag 3, Pune 411 007, India
5Institut d’Astrophysique de Paris, 98 bis, Bd. Arago - 75014, Paris, France
6Department of Physics & Astronomy, Georgia State University, P.O. Box 4106, Atlanta, Georgia 30302-4106, USA
7Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital 263 129, India
Accepted 2004 xxxxx. Received 2004 xxxxx
ABSTRACT
We report monitoring observations of 20 high-luminosity AGN, 12 of which are radio-
quiet quasars (RQQs). Intra-night optical variability (INOV) was detected for 13 of
the 20 objects, including 5 RQQs. The variations are distinctly stronger and more
frequent for blazars than for the other AGN classes. By combining these data with
results obtained earlier in our program, we have formed an enlarged sample consisting
of 9 BL Lacs, 19 RQQs and 11 lobe-dominated radio-loud quasars. The moderate level
of rapid optical variability found for both RQQs and radio lobe-dominated quasars
argues against a direct link between INOV and radio-loudness. We supplemented the
present observations of 3 BL Lacs with additional data from the literature. In this
extended sample of 12 well observed BL Lacs, stronger INOV is found for the EGRET
detected subset.
Key words: BL Lacertae objects – galaxies: active – galaxies: jets – galaxies: pho-
tometry – quasars: general
1INTRODUCTION
In a series of papers since 2003, we have reported results
of a program to search for intra-night optical variability
(INOV; see Wagner & Witzel 1995 for a review), often
called microvariability, in a sample of 26 optically lumi-
nous active galactic nuclei (AGN), using the 104-cm Zeiss
telescope of the Aryabhatta Research Institute of Observa-
tional Sciences (ARIES), Naini Tal, India (Gopal-Krishna
et al. 2003, GK03; Stalin et al. 2004a, St04a; Sagar et al.
2004, Sa04; Stalin et al. 2004b, St04b; see also Stalin 2003).
These objects belong to the four major classes of lumi-
nous AGN, namely, radio-quiet QSOs (RQQs), radio-loud
lobe-dominated quasars (LDQs), radio-loud core-dominated
quasars (CDQs), and BL Lac objects (BLs). The sample se-
lection was such that the four classes are reasonably well
matched in the z–MB plane, with z ranging from 0.17 to
2.2 and MB ranging from −24.3 to −30.0 (taking H0 = 50
km s−1Mpc−1and q0 = 0).
The observations under this program typically achieved
convincing detectability of INOV at a level of 0.01–0.02 mag
and spanned a total of 113 nights (720 hours) between Oc-
tober 1998 and May 2002. This work provided the first pos-
itive detection of INOV for RQQs, though modest evidence
for such variations had been obtained earlier (e.g. Gopal-
Krishna et al. 1995, 2000; Sagar et al. 1996; Jang & Miller
1997). It was, moreover, found that except for BLs and high
optical polarization CDQs (HP-CDQs), the amplitude of de-
tected INOV is small (? 3 per cent) and so is the INOV
duty cycle (∼10–20 per cent), irrespective of the radio loud-
ness. Further, for the BLs and HP-CDQs, for which a strong
INOV was frequently observed, no correlation was found be-
tween the amplitude of INOV and long-term optical variabil-
ity (St04b). We argued that these results are consistent with
the hypothesis that even radio-quiet QSOs possess relativis-
tic jets emitting optical emission on sub-parsec scales, but
that we are observing them at moderately large angles to
the jet direction so that any variations are neither amplified
in magnitude nor compressed in time-scale as they are in BL
Lacs (GK03; St04a). Wills (1996) also argued that RQQs do
indeed possess jets, but that they propagate through denser
gas close to the host galaxies’ planes and are thus quickly
snuffed out. For BLs we found no correlations between ap-
parent brightness levels and INOV properties (St04b). This
is in accord with a recent study which indicates that mi-
crovariability of a blazar may be correlated with the pres-
ence of longer-term flux changes, rather than its apparent
brightness level (Howard et al. 2004).

Page 2

2
C. S. Stalin et al.
Table 1. The sample of 20 optically luminous AGN monitored
in the present work
IAU NameType
BMB
zPopt†
(%)
log R∗‡
0003+158
0025+307
0043+039
0806+315
0824+098
0832+251
0846+513
0850+440
0931+437
0935+416
0945+438
1029+329
1418+546
1422+424
1425+267
1444+407
1522+101
1553+113
1631+395
1750+507
LDQ
LDQ
RQQ
BL
RQQ
RQQ
CDQ
RQQ
RQQ
RQQ
RQQ
RQQ
BL
RQQ
LDQ
RQQ
RQQ
BL
LDQ
RQQ
16.51
15.79
16.00
15.70
15.50
16.10
16.28
16.40
16.47
16.31
16.28
16.00
16.17
16.42
15.78
15.45
16.20
15.00
16.48
15.80
−25.7
−26.7
−26.0
−25.0
−25.6
−25.5
−29.4
−26.1
−25.8
−29.6
−24.5
−26.7
−23.7
−25.1
−26.0
−25.7
−28.4
−26.8
−27.8
−25.6
0.450
0.500
0.385
0.220
0.260
0.331
1.860
0.513
0.456
1.966
0.226
0.560
0.152
0.316
0.366
0.267
1.324
0.360
1.023
0.300
0.65
—
0.27
—
—
—
—
—
—
—
—
—
7.5
—
1.9
0.4
0.3
—
1.1
—
2.6
1.8
−0.7
1.7
0.5
0.1
2.2
< −0.5
0.1
< −0.7
< −0.1
< −0.6
3.1
< −0.4
2.0
−1.1
< −0.7
2.2
1.6
0.7
†References for optical polarization: Wills et al. 1992;
Berriman et al. 1990; — for no data available
‡R∗is the K-corrected ratio of the 5 GHz radio to 2500˚ A-band
optical flux densities (Stocke et al. 1992); references for
radio fluxes: V´ eron-Cetty & V´ eron 2001; NVSS (Condon et al.
1998); FIRST (Becker et al. 1995; Bauer et al. 2000)
In this paper, we present the results of our optical mon-
itoring for another 20 AGN belonging to all of the above
mentioned four classes of luminous AGN. We then combine
the present data for 3 BLs with similar high quality light-
curves for another 9 BLs taken from literature, to arrive at
a representative sample of 51 intra-night optical light-curves
for BL Lacs. This sample allows us to make a comparative
study of the INOV properties of BL Lacs detected with the
EGRET instrument on the Compton Gamma-Ray Observa-
tory (Hartman et al. 1999), or otherwise found to emit high
energy γ-rays, and their counterparts that were not detected
by EGRET (Section 4). Our conclusions are summarized in
Section 5.
2INTRANIGHT OPTICAL MONITORING
All AGNs chosen for this additional study had to be bright
enough to allow a high temporal density for precision differ-
ential photometry using telescopes of a modest aperture.
This led to a requirement that mB < 17 mag. We also
wanted to minimize the contamination problems that arise
when the host galaxy contributes a significant portion of
the visible light (e.g. Cellone, Romero & Combi 2000), and
so restricted our sample to luminous AGNs (quasars), with
MB < −23.5 mag. For good visibility from India the sources
had to be at moderate positive declinations and within suit-
Table 2. Positions and magnitudes of the comparison stars
IAU NameStarRA(2000)DEC(2000)
BR
(mag)(mag)
0003+158 S1
S2
S3
S1
S2
S1
S2
S3
S1
S2
S3
S1
S2
S1
S3
S1
S3
S1
S2
S3
S1
S2
S1
S3
S1
S3
S4
S1
S2
S1
S2
S3
S1
S2
S3
S1
S2
S1
S2
S1
S2
S1
S3
S1
S3
S1
S2
S3
00 06 08.42
00 06 06.20
00 06 05.97
00 28 25.59
00 28 15.86
00 45 39.87
00 45 44.87
00 45 44.16
08 09 06.08
08 09 18.58
08 09 14.89
08 27 39.18
08 27 44.30
08 35 26.47
08 35 47.24
08 50 14.07
08 50 19.88
08 53 28.75
08 53 48.92
08 53 39.97
09 34 46.90
09 35 01.19
09 38 40.37
09 39 02.53
09 49 28.88
09 49 06.74
09 48 58.30
10 32 10.68
10 32 07.49
14 20 02.31
14 19 46.29
14 19 39.75
14 25 03.56
14 25 09.20
14 25 11.09
14 27 47.53
14 27 30.07
14 16 54.96
14 46 55.62
15 24 03.25
15 24 07.32
15 55 35.71
15 55 51.81
16 33 01.57
16 32 54.19
17 51 07.39
17 51 06.32
17 51 37.59
+16 09 54.4
+16 10 46.3
+16 12 15.6
+31 03 19.0
+31 03 09.8
+04 10 02.0
+04 10 57.9
+04 13 26.0
+31 22 19.3
+31 22 20.7
+31 20 18.7
+09 41 13.5
+09 45 05.6
+24 57 12.2
+24 57 19.0
+51 06 21.9
+51 09 00.0
+43 46 22.8
+43 48 28.1
+43 46 15.4
+43 32 05.9
+43 27 43.4
+41 26 11.3
+41 30 37.9
+43 37 54.4
+43 29 08.2
+43 55 11.8
+32 36 08.1
+32 37 28.2
+54 25 25.3
+54 26 43.4
+54 21 56.1
+42 14 41.8
+42 17 21.8
+42 17 51.2
+26 35 14.9
+26 36 05.5
+40 36 51.9
+40 36 16.6
+09 58 15.2
+10 01 02.9
+11 09 33.2
+11 12 28.7
+39 20 49.4
+39 21 19.8
+50 45 03.5
+50 44 33.9
+50 43 56.5
16.31
17.17
16.89
15.57
15.57
16.95
17.34
17.76
16.52
16.54
17.65
16.60
16.28
18.86
16.56
17.27
17.34
18.37
18.03
18.72
15.75
15.72
16.11
16.27
15.30
17.18
17.28
16.37
17.35
16.28
16.11
16.74
16.27
18.14
15.97
15.21
15.59
15.68
17.04
16.99
17.12
16.11
16.55
17.20
17.91
20.11
16.38
15.70
15.32
15.71
15.49
14.11
13.89
15.50
16.09
15.21
15.09
15.11
15.92
15.03
15.44
16.62
15.72
16.11
18.67
16.08
16.39
16.46
14.42
15.24
15.32
15.47
14.35
15.27
16.14
15.02
15.33
15.58
15.51
15.44
15.67
16.81
15.39
13.65
14.01
14.10
14.99
15.03
15.56
15.11
15.45
15.90
16.48
19.55
14.81
14.80
0025+307
0043+039
0806+315
0824+098
0832+251
0846+513
0850+440
0931+437
0935+416
0945+438
1029+329
1418+546
1422+424
1425+267
1444+407
1522+101
1553+113
1631+395
1750+507
able ranges of right ascensions. Basic data on our sources is
presented in Table 1. Twelve of the sources are RQQs (us-
ing the usual criterion for the K-corrected ratio of 5 GHz to
2500˚ A fluxes, R∗< 10), 4 are LDQs, 1 is a CDQ, and 3 are
BLs; their redshifts range from 0.22 to 1.97.
The majority of the data was obtained at ARIES (for-
merly UPSO), Naini Tal, India, using the 104-cm Sampur-
nanand telescope which is an RC system with a f/13 beam
(Sagar 1999). The detectors used were a cryogenically cooled

Page 3

Optical Variability of Quasars
3
1024 × 1024 CCD chip (prior to October 1999) and a 2048
× 2048 chip (after October 1999), both mounted at the
cassegrain focus. The 1k×1k chip has a readout noise of
7 electrons and a gain of 11.8 electrons/ADU, whereas the
2k×2k chip has a readout noise of 5 electrons (in the usu-
ally employed slow readout mode) and a gain of 10 elec-
trons/ADU. Each pixel of both of these CCDs correspond
to 0.38 × 0.38 arcsec2on the sky, covering a total field of
12′× 12′in the case of the larger CCD and 6′× 6′in
the case of the smaller CCD (Sagar 1999). An R Cousins
filter was used for these observations. On each night only
one AGN was monitored, as continuously as possible. The
choice of exposure times depended on the brightness of the
object, the moon’s phase, and sky transparency. The field
containing the AGN was adjusted so as to have within the
CCD frame at least 2 (usually 3 or more) comparison stars
within about a magnitude of the AGN; for nearly all ob-
jects we were able to find at least one steady comparison
star fainter, or < 0.4 mag brighter, than the AGN, so as to
obtain an equivalent S/N in the CCD frames. Seeing ranged
from about 1.5 to about 3.5 arcsec.
Four of the RQQs in the sample were monitored in V
Johnson passband using a cryogenically cooled Tektronix
CCD detector at the f/3.23 prime focus of 2.34 meter Vainu
Bappu Telescope (VBT) of the Indian Institute of Astro-
physics, at Kavalur, India (Table 3). The chip has 1024 ×
1024 pixels of approximately 24 × 24 µm2, with each pixel
dimension corresponding to about 0.63 arcsec on the sky,
so that the total area covered by a CCD frame is 10.75′×
10.75′. The readout noise was 4 electrons and the gain was
4 electrons/ADU. Typical seeing was around 2 arcsec.
One night of monitoring data for the RQQ 1422+424 re-
ported here was carried out in V Johnson passband using the
Tektronix 1k×1k CCD detector at the f/13 Cassegrain fo-
cus of the 1.2 meter Gurushikhar Telescope (GSO) at Mount
Abu, India (Table 3). Each pixel corresponds to 0.32 arcsec
in each dimension and the entire chip covers approximately
5.4′× 5.4′of sky. The readout noise was 4 electrons and gain
was 10 electrons/ADU. Typical seeing was ∼1.5 arcsec.
At all three telescopes, observations were carried out
in 2×2 binned mode, in order to increase S/N; bias frames
were taken intermittently and twilight sky flats were taken
for processing of the data. Initial processing (bias subtrac-
tion, flat-fielding and cosmic ray removal) as well as photo-
metric reductions was done in the usual manner employing
standard routines in IRAF1software.
Instrumental magnitudes of the AGN and the stars in
the images taken at Naini Tal were obtained using the rou-
tines available in the apphot package in IRAF. For these re-
ductions, a crucial parameter, the circular aperture used for
the photometry of the QSO and the comparison stars, varied
from night to night. For each night an optimum aperture for
the photometry was selected by considering a range of aper-
tures starting from a minimum corresponding to the median
seeing (FWHM) over the night; we chose the aperture that
produced the minimum variance in the star−star differen-
tial light curve (DLC) of the steadiest pair of comparison
1Image Reduction and Analysis Facility, distributed by NOAO,
operated by AURA, Inc. under agreement with the US NSF.
stars. Additional details of the observation and reduction
procedures are presented elsewhere (Stalin 2003; St04b).
Instrumental magnitudes of the AGN and stars in the
image frames acquired at VBT and GSO were determined
by using DAOPHOT II2(Stetson 1987) and employing aper-
ture photometric techniques. The best S/N was found for
data reduced with a 7.0 pixel radius and it is thus used for
our analysis.
The positions and the B and R magnitudes (taken from
the USNO-B catalog3(Monet et al. 2003) for the compari-
son stars used in our analysis are given in Table 2. Note that
the magnitudes of the comparison stars taken from this cat-
alog have uncertainties of up to 0.25 mag, though errors for
individual objects are not provided.
3RESULTS OF INTRA-NIGHT MONITORING
Figure 1 presents the DLCs for all the nights on which sig-
nificant variability was detected for any AGN in the present
sample. It can be seen that in these data, variability of the
order of 0.01 mag over the course of a few hours can be de-
tected. A log of the observations and the main results are
given in Table 3. For each night of observations of every ob-
ject, this table provides the number of data points (Npoints),
the duration, an indicator of the variability status, as well
as two quantitative measures of the variability, Ceff and ψ
(see below).
The parameter Ceff is defined, basically following Jang
& Miller (1997), for a given DLC as the ratio of the stan-
dard deviation of all its data points, σT, to the averaged
standard deviation for its individual data points, σ = ησerr.
Here η is the factor by which the average of the measure-
ment errors (σerr, as given by phot) should be multiplied; we
find η = 1.50 (Stalin 2003; St04b). We compute Ceff from
the Ci values (defined as the ratio of the standard devia-
tion of ithDLC to the mean σ of its individual data points
multiplied by the factor η) determined for the DLCs of an
AGN relative to different comparison stars, measured on a
single night (see Sa04 for details). A value of Ceff > 2.57
corresponds to a confidence level of variability in excess of
0.99 and is the criterion we use to assign variability to a
QSO. We note that for these AGN all the DLCs involving
only their comparison stars were found to show statistically
insignificant variability, using the same statistical criterion.
We quantify the actual variation of the QSO on a given
night using the error corrected amplitude of variability, ψ,
as defined by Romero, Cellone & Combi (1999),
ψ =
?
(Dmax− Dmin)2− 2σ2,
(1)
with Dmax (Dmin) the maximum (minimum) in the quasar
DLC, and σ the corrected error value described in the pre-
vious paragraph. Details are given in St04b.
The structure function (SF) is frequently used to char-
acterize variability properties such as time-scales and peri-
odicities present in the light-curves. We have also computed
the SFs for our dataset in the fashion discussed in some
detail in Sa04 and St04b. Basically, a monotonically rising
2Dominion Astrophysical Observatory Photometry Software
3http://www.nofs.navy.mil/data/fchpix

Page 4

4
C. S. Stalin et al.
Figure 1. Differential light curves (DLCs) for the quasars on nights with a positive detection of INOV. The name of the quasar, the
date, and the duration of the observation are given at the top of each night’s data. The upper panel(s) give the DLCs for the various
pairs of comparison stars available and the subsequent panels give the quasar-star DLCs, as defined in the labels on the right side.

Page 5

Optical Variability of Quasars
5
Figure 1. Continued
Table 3. Observation log and variability results
IAU NameOther
Name
TypeDate FilterTelescopeNpointsDuration
(hours)
Status∗
Ceff
ψ
(%)
0003+158PKSLDQ03.11.00
05.11.00
13.10.98
01.11.98
21.10.98
05.11.98
28.12.98
27.12.98
25.12.98
14.01.99
10.12.99
30.12.98
17.02.99
20.02.99
27.03.99
15.01.99
13.03.99
28.03.99
03.04.99
14.04.99
07.03.00
08.03.00
06.05.99
15.04.99
11.04.99
05.05.99
06.06.99
04.06.99
30.05.00
03.06.98
06.06.98
08.06.99
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
V
V
R
R
V
R
V
R
V
R
R
R
R
R
R
R
R
ARIES
ARIES
ARIES
ARIES
ARIES
ARIES
ARIES
ARIES
ARIES
ARIES
ARIES
ARIES
ARIES
ARIES
ARIES
VBT
VBT
ARIES
ARIES
VBT
ARIES
GSO
ARIES
VBT
ARIES
ARIES
ARIES
ARIES
ARIES
ARIES
ARIES
ARIES
28
32
26
24
12
28
34
58
24
63
31
37
37
24
15
10
57
31
39
40
15
28
31
28
36
20
40
28
12
44
15
34
5.8
7.0
3.6
3.4
2.4
3.2
7.3
8.2
4.7
7.3
6.7
7.1
7.7
4.5
2.7
2.2
5.4
5.6
7.2
4.1
3.9
3.0
5.8
2.9
6.6
3.6
7.1
5.7
3.5
4.7
1.6
6.1
NV
V
V
V
V
V
V
V
V
NV
NV
V
NV
NV
NV
NV
NV
V
NV
NV
NV
V
V
NV
NV
V
NV
V
NV
NV
NV
V
—
3.1
2.7
5.1
4.2
2.6
—
1.8
0.8
1.9
2.5
3.2
14.5
2.2
2.0
—
—
5.6
—
—
—
—
—
2.0
—
—
—
3.6
3.2
—
—
2.3
—
2.7
—
—
—
2.0
0025+307 RXSLDQ
0043+039 PGRQQ
0806+315
0824+098
0832+251
B2
1WGA
PG
BL
RQQ
RQQ
> 6.6
4.3
4.3
—
—
2.8
—
—
—
—
—
4.0
—
—
—
2.9
2.8
—
—
0846+513
0850+440
0931+437
0935+416
0945+438
1029+329
1418+546
1422+424
0846+51
US 1867
US 737
PG
US 995
CSO 50
OQ 530
RXS
CDQ
RQQ
RQQ
RQQ
RQQ
RQQ
BL
RQQ
1425+267
1444+407
1522+101
1553+113
B2
PG
PG
PG
LDQ
RQQ
RQQ
BL
>6.6
—
2.9
—
—
—
1631+395KUVLDQ
1750+507IRAS RQQ
>6.6
∗V = variable; NV = non-variable