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MNRAS 529, L108–L114 (2024) https://doi.org/10.1093/mnrasl/slae003
Advance Access publication 2024 January 10
Intra-night optical variability of peculiar narrow-line Seyfert 1 galaxies
with enigmatic jet behaviour
Vineet Ojha ,
1 ܠ Veeresh Singh ,
1 M. Berton
2 and E. J
¨
arvel
¨
a
3
1
Physical Research Laboratory (PRL), Astronomy and Astrophysics Division, Ahmedabad 380 009 , India
2
European Southern Observatory (ESO), Alonso de C
´
ordova 3107, Casilla 19, Santiago 19001 , Chile
3
Homer L. Dodge Department of Physics and Astronomy, The University of Oklahoma, 440 W. Brooks St, Norman, OK 73019, USA
Accepted 2024 January 5. Received 2023 December 22; in original form 2023 September 14
A B S T R A C T
Variability studies of active galactic nuclei are a powerful diagnostic tool in understanding the physical processes occurring in disc-
jet re gions, unresolv ed by direct imaging with currently available techniques. Here, we report the first attempt to systematically
characterize intra-night optical variability (INOV) for a sample of seven apparently radio-quiet narrow-line Seyfert 1 galaxies
(RQNLSy1s) that had shown recurring flaring at 37 GHz in the radio observations at Mets
¨
ahovi Radio Observatory, indicating
the presence of relativistic jets in them, but no evidence for relativistic jets in the recent radio observations of Karl G. Jansky Ve r y
Large Array at 1.6, 5.2, and 9.0 GHz. We have conducted a total of 28 intra-night sessions, each lasting ≥3 h for this sample,
resulting in an INOV duty cycle ( DC ∼20 per cent) similar to that reported for γ-ray-NLSy1s (DC ∼25 per cent–30 per cent),
that display blazar-like INOV. This in turn infers the presence of relativistic jet in our sample sources. Thus, it appears that
e ven lo wer mass ( M
BH
∼10
6
M
) RQNLSy1 galaxies can maintain blazar-like acti vities. Ho we ver, we note that the magnetic
reconnection in the magnetosphere of the black hole can also be a viable mechanism to give rise to the INOV from these sources.
Key words: surv e ys – galaxies: active – galaxies: jets – galaxies: photometry – galaxies: Seyfert – gamma-rays: galaxies.
1 INTRODUCTION
Accretion of gas around the central supermassive black holes
(SMBH) of masses M
SMBH ∼10
6
–10
10
M
is the main powering
mechanism of active galactic nuclei (AGNs) that makes them the
most energetic objects in the Universe with integrated luminosities
reaching up to 10
48
erg s
−1
(Koratkar & Blaes 1999 ; Bischetti et al.
2017 ). Among the dif ferent observ ational characteristics, v ariability
on different time-scales ranging from minutes to decades across
the electromagnetic spectrum is being used as one of the defining
characteristics of AGNs (Gaskell & Klimek 2003 ; P ado vani et al.
2017 ). Variability studies in AGNs play an important role in
understanding the physical processes occurring in these objects.
For instance, AGN variability has been used to probe emission
mechanisms occurring on physical scales that are unresolved by
currently available telescopes/facilities, and also used to investigate
the spin and mass of the central SMBH (Urry & P ado vani 1995 ;
Wagner & Witzel 1995 ; Ulrich, Maraschi & Urry 1997 ; Zensus
1997 ; Cackett et al. 2013 ; Emmanoulopoulos et al. 2014 ; McHardy
et al. 2014 ). It is widely believed that the optically thick accretion
disc surrounding the SMBH is primarily responsible for the optical
emission from AGNs (Shakura & Sunyaev 1973 ), but the physical
processes producing optical variability are not clearly understood. In
E-mail: vineetojhabhu@gmail.com
† Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing
100871, China
a tiny subset of AGNs called blazars, the optical variability is thought
to originate due to relativistic boosting of small fluctuations arising
through the turbulence of plasma in the jet (e.g. see Marscher &
Travis 1991 ; Goyal et al. 2012 ; Calafut & Wiita 2015 ). Short-term
optical flux variability of AGNs from minutes to hours is known as
‘Intra-Night Optical Variability’ (INOV; Gopal-Krishna et al. 2003 ).
Among the INOV study of different luminous classes co v ered
by Goyal et al. ( 2013b ), strong INOV with duty c ycle (DC) abo v e
30 per cent is exhibited by high-optical polarization core-dominated
quasars, and TeV blazars (both are radio-loud
1
).
This suggests that strong INOV can be an ef fecti ve tracer of
jet activity in AGNs. On the other hand, a low level of INOV
DC < 10 per cent observed in radio-quiet quasars (e.g. see Goyal
et al. 2013b ) is thought to originate either from its weak jet (e.g.
Kellermann et al. 2016 ) and/or transient shocks or ‘hot spots’ in
the accretion disc around the SMBH (Chakrabarti & Wiita 1993 ;
Mangalam & Wiita 1993 ).
Among the different sub-classes of lower luminosity AGNs, a
handful of the radio-loud minority of Narrow-line Seyfert 1 (NLSy1)
galaxies, marked by detection in γ-ray band, exhibit comparable
DC ( ∼30 per cent) to the jetted class of AGN (e.g. see Paliya
et al. 2013 ; Ojha, Hum & Gopal-Krishna 2021 ). NLSy1s are
characterized by smaller width of Balmer emission lines with full
1
Radio loudness is parametrized by the ratio of radio to optical flux densities
at 5 GHz and at 4400 Å, respectively, with RL ≤10 and > 10 for radio-quiet
and radio-loud AGNs, respectively (e.g. see, Kellermann et al. 1989 ).
© 2024 The Author(s).
Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative
Commons Attribution License ( https:// creativecommons.org/ licenses/ by/ 4.0/ ), which permits unrestricted reuse, distribution, and reproduction in any medium,
provided the original work is properly cited.
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INOV of RQNLSy1s L109
MNRASL 529, L108–L114 (2024)
width at half-maximum, FWHM(H β) < 2000 km s
−1
and flux ratio
of [O
III
]
λ5007
/H β< 3 (Shuder & Osterbrock 1981 ; Osterbrock &
Pogge 1985 ; Goodrich et al. 1989 ). The ample majority of NLSy1s
are radio quiet, only a tiny fraction ∼7 per cent is radio loud and
likely to harbour relativistic jets (Komossa et al. 2006 ; Singh &
Chand 2018 ). The presence of relativistic jets has been pro v en in
gamma-ray-detected RLNLSy1s ( γ-RLNLSy1s; e.g. Giroletti et al.
2011 ; Caccianiga et al. 2015 ; Jarvela et al. 2022 ). Unlike the general
population of RLNLSy1s, γ-RLNLSy1s show much stronger INOV
similar to blazars, suggesting a connection of relativistic jets with
their strong INOV. Despite many similar characteristics such as flat
radio spectrum, high brightness temperature, superluminal motion,
and γ-ray detection (Yuan et al. 2008 ; Abdo et al. 2009a , b , c ;
Foschini et al. 2010 , 2021 , 2022 ; Foschini 2011 ; D’Ammando et al.
2012 , 2015 ; Yao et al. 2015 , 2019 ; Berton et al. 2017 ; Lister 2018 ;
Paliya et al. 2018 ; Yang et al. 2018 ; Li et al. 2023 ) to blazar class
of AGN, an important difference between NLSy1s and blazars is
an order of less-massive black hole for NLSy1s (10
7
M
; Grupe &
Mathur 2004 ; Deo, Crenshaw & Kraemer 2006 ; Peterson 2011 ). This
makes them to harbour less powerful jets (Angelakis et al. 2015 ; Gu
et al. 2015 ; Fuhrmann et al. 2016 ; Paliya 2019 ).
Recently, a sample of seven radio-quiet and/or radio-silent (never
detected in radio) NLSy1s (see Table 1 ) exhibited recurring flaring
at 37 GHz in the radio observations at Mets
¨
ahovi Radio Observatory
(MRO; see L
¨
ahteenm
¨
aki et al. 2018 ). Ho we ver, when these radio-
quiet narrow-line Seyfert 1 galaxies (RQNLSy1s) were observed
with Karl G. Jansky Ver y Large Array (JVLA) in A configuration at
three different frequencies, 1.6 GHz, 5.2 GHz, and 9.0 GHz, no hints
of relativistic jet was detected from them (see Berton et al. 2020b ).
Taking advantage of INOV, here, we investigate the presence of
relativistic jets in these NLSy1s by performing intra-night optical
monitoring of these seven RQNLSy1s with 1–2.5m ground-based
optical telescopes.
The format of this paper is as follows. In Section 2 , we describe our
optical monitoring and data-reduction procedure. Section 3 provides
details of statistical analysis methods. The main results of this work
followed by discussion are given in Section 4 .
2 OPTICAL INTRA-NIGHT MONITORING AND
DATA REDUCTION
Optical telescopes from two Indian institutes namely Aryabhatta
Research Institute of Observational Sciences (ARIES) and Physical
Research Laboratory (PRL) were used for the Intra-night monitoring
of the seven RQNLSy1s in the broad-band Johnson–Cousin filter R
except for a session with 2.5m PRL telescope when it was taken in the
Sloan Digital Sky Survey (SDSS) filter r due to non-availability of
broad-band Johnson–Cousin filter R . Broad-band Johnson–Cousin
filter R and SDSS filter r were chosen for observations because
charge-coupled device (CCD) detector used has maximum sensitivity
in these bands. A total of four telescopes two from ARIES namely
1.04 metre (m) Sampurnanand telescope (ST; Sagar 1999 ), 1.30m
De v asthal Fast Optical Telescope (DFOT; Sagar et al. 2010 ) located
at Nainital, Uttarakhand, and two from PRL namely, 1.2m telescope
(Sri v astav a et al. 2021 ) and 2.5m telescope located at Mount Abu
Rajasthan, were used in this work. The details of the observational
set-ups used for each telescope in observing the sample of 7
RQNLSy1s are listed in Table 2 . At least three epochs of observation
each ≥3 h were devoted for each RQNLSy1s. Since we have used
in the present work, the optical telescopes range between 1.04 m–
2.5 m, therefore, the typical exposure time was set between 300 s–
Tab l e 1. A current sample of 7 RQNLSy1s.
SDSS name
a R -mag
b zc RL
d log( M
BH
)
e
M
J102906.69 + 555625.2 19.10 0.45 –7.33
J122844.81 + 501751.2 17.80 0.26 –6.84
J123220.11 + 495721.8 16.90 0.26 –7.30
J150916.18 + 613716.7 18.60 0.20 –6.66
J151020.06 + 554722.0 17.80 0.15 –6.67
J152205.41 + 393441.3 13.10 0.08 02 5.97
J164100.10 + 345452.7 16.00 0.16 13 7.15
a
SDSS name of RQNLSy1s.
b
R -band magnitude of NLSy1s taken from Monet ( 1998 ).
c
Redshift of the RQNLSy1s taken from L
¨
ahteenm
¨
aki et al. ( 2018 ).
d, e
Both radio-loudness or radio-silent RL ≡S
1.4 GHz
/ S
440 nm
and black hole masses of current sources are taken from
J
¨
arvel
¨
a, L
¨
ahteenm
¨
aki & Le
´
on-Tavares ( 2015 ) and L
¨
ahteenm
¨
aki et al. ( 2018 ).
In both
articles, black hole masses were estimated following the
FWHM(H β)–luminosity mass scaling relation, given
by Greene & Ho ( 2005 ).
1200 s to reach a suitable signal-to-noise ratio (SNR), depending on
the sky condition, moon phase, telescope efficiency, and magnitude
(brightness) of the RQNLSy1s.
For preliminary processing of the raw images, at least three bias
frames, and also three flat frames were taken during each observing
session. Furthermore, the standard tasks available in the IRAF
2
software package were followed for making final science images
from the raw images. Since the field of each target RQNLSy1s
was not clustered, therefore, aperture photometry (Stetson 1987 ,
1992 ) was used in the current work for extracting the instrumental
magnitudes of RQNLSy1s and the comparison stars registered in
the CCD frames, using D AOPHO T II algorithm.
3 As emphasized in
Ojha, Hum & Gopal-Krishna ( 2021 ) size of the chosen aperture is an
important parameter while estimating the instrumental magnitude
and the corresponding SNR of the individual photometric data
points registered on the CCD frames. Additionally, caution about
the point spread function (PSF) variation becomes very important
when dealing with intra-night variability of nearby ( ≤0.4) AGNs.
Because in such a situation a significant contribution to the total flux
can come from the underlying host Galaxy that can mimic the INOV
in the standard analysis of the differential light curves (DLCs) due
to the significant relative contributions of the (point-like) AGN and
the host Galaxy to the aperture photometry with the variation of PSF
during the session (Cellone, Romero & Combi 2000 ). Therefore, the
procedure of data reduction, PSF estimation for aperture photometry,
selection of aperture, and caution for PSF variations (see Section
4 ) were followed from Ojha, Hum & Gopal-Krishna ( 2021 ). Since
except for one RQNLSy1 J102906.69 + 555625.2 ( z = 0.45), all
the RQNLSy1s in the present sample are at lower redshift ( ≤0.4),
therefore proper caution has been taken about its PSF variation
during the night before commenting about its variability (see
Section 3 ).
Furthermore, DLCs of target RQNLSy1 for each intra-night
session were derived relative to a pair of non-varying (steady)
comparison stars (see online Figs A1 ), additionally, the PSF variation
2
Image Reduction and Analysis Facility ( http:// iraf.noao.edu/ ).
3
Dominion Astrophysical Observatory Photometry ( http://www.astro.wisc.
edu/ sirtf/ daophot2.pdf).
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L110 V. Ojha et al.
MNRASL 529, L108–L114 (2024)
Tab l e 2. Details of system parameters of the telescopes and detectors used in the observations of 7 RQNLSy1s.
Telescope (s) No. of sessions Detector (s) Readout speed Field of view Readout Gain Focal ratio Pixel size Plate scale
(arcmin
2
) noise (e
−of of CCD of CCD
(e
−) /ADU) telescope ( μm) (
/pixel)
1.04m ST
a 05 4k ×4k 100 kHz 15.70 ×15.70 3.0 10.0 f /13 15.0 0.23
1.30m DFOT
b 10 2k ×2k 100 kHz 18.27 ×18.27 7.5 2.0 f /4 13.5 0.53
1.20m Mt Abu
c 12 1k ×1k 50 kHz 5.21 ×5.21 5.0 5.0 f /13 13.0 0.30
2.50m Mt Abu
c 01 4k ×4k 100 kHz 10.00 ×10.00 2.1 3.0 f /8 15.0 0.15
a
Sampurnand Telescope (ST).
b
De v asthal Fast Optical Telescope (DFOT).
c
Mount Abu telescope.
during each intra-night session is plotted in the bottom panel of
DLCs.
3 METHODOLOGY FOR ANALYSIS
3.1 Statistical tests
To examine the presence of intra-night variability in the present
sample of 7 RQNLSy1s, we have applied two different fla v ours of
F -test, (i) standard F-test ( F
η-test) and (ii) the power-enhanced F-
test ( F
p–enh
-test), by following the basic requirement and procedure
of these two tests as described in (Goyal et al. 2012 ; de Diego
2014 ). Several star–star DLCs were generated for each session with
the instrumental magnitudes extracted from the aperture photometry,
and out of several pairs of star–star DLCs those two stars were chosen
as comparison stars for which no-variability resulted based upon F
η-
test. Out of the chosen two comparison stars, the one with the closest
match ( m ∼1, a requirement of F
η-test) in magnitude to the target
RQNLSy1 is chosen as a reference star, and other as comparison star.
Furthermore, two versions of F- test are applied to the DLCs of target
RQNLSy1 relative to the reference star and comparison star (basic
parameters of these two stars are tabulated in the online Table A1) .
The F
η-values for the two RQNLSy1 DLCs and star–star DLC of an
intra-night session can be written as (e.g. Goyal et al. 2012 ):
F
η
CS 1
=
σ2
( RQNLSy 1 −CS 1)
η2
σ2
RQNLSy 1 −CS 1
, F
η
CS 2
=
σ2
( RQNLSy 1 −CS 2)
η2
σ2
RQNLSy 1 −CS 2
F
η
CS 1 −CS 2
=
σ2
( CS 1 −CS 2)
η2
σ2
CS 1 −CS 2
, (1)
where σ2
( RQNLSy 1 −CS 1)
, σ2
( RQNLSy 1 −CS 2)
, and σ2
( CS 1 −CS 2)
are the
variances with σ2
RQNLSy 1 −CS 1
=
N
i= 1
σ2
i, err
( RQNLSy 1 −CS 1) /N ,
σ2
RQNLSy 1 −CS 2
, and σ2
CS 1 −CS 2
being the mean square (formal) rms
errors of the i
th
data points in the DLCs of target RQNLSy1, and N
is the number of observations. A computed value of η= 1.54 ±0.05
based upon the data of 262 intra-night monitoring sessions of AGNs
by Goyal et al. ( 2013a ) is used here for the correct use of rms errors
on the photometric data points.
In column 6 of online Tab le A2 , we compare the computed F -
values, resulting from equation ( 1 ) for a session, with its estimated
critical value ( = F
( β)
cri
) for the same session, here βis the level
of significance for the test. The βvalues in the current work
are set by us to be 0.05 and 0.01, corresponding to 95 per cent
and 99 per cent confidence levels for INOV detection. The null
hypothesis (i.e. non-detection of INOV) is discarded at the βlevel
of significance if the computed value of F
ηexceeds its F
( β)
cri
at the
corresponding confidence level. Thus a RQNLSy1 is assigned as
a variable ( V ) if the computed value of F
ηis found to be greater
than its F
cri
(0.99); probable variable (PV) if the same is found to
be greater than F
cri
(0.95) but less or equal to F
cri
(0.99), and non-
variable if F
ηis found to be less than or equal to F
cri
(0.95). Summary
of computed values of F
ηand correspondingly inferred status of
INOV detection for all the 28 sessions are tabulated in columns 6
and 7 of online Table A2 .
The second fla v our of F -test (the F
p–enh
-test) can be written
following de Diego ( 2014 ) as below
F
p –enh
=
σ2
RQNLSy1
σ2
comb
, σ2
comb
=
1
(
q
j= 1
R
j
) −p
q
j= 1
R
j
i= 1
D
2
j,i
, (2)
here σ2
RQNLSy1
is the variance of the ‘target RQNLSy1-reference
star’ DLC, while σ2
comb
is the combined variance of ‘comparison star-
reference star’ DLC having R
j
data points (number of observations)
and p comparison stars, computed using scaled square deviation D
2
j , i
as
D
2
j,i
= ω
j
( m
j,i
−¯
m
j
)
2
, (3)
where, m
j , i
’s is the ‘ j
th comparison star-reference star’ differential
instrumental magnitudes value and ¯
m
j represent the corresponding
average value of the DLC for its R
j
data points. The scaling factor ω
j
is taken here as described in Ojha, Hum & Gopal-Krishna ( 2021 ).
Columns 10 and 11 of online Table A2 represent the F
p–enh
-test
values and corresponding inferred INOV status for the entire session,
following the criteria as set for F
η-test (see abo v e).
In addition to variability resulting from two versions of the F -test,
proper caution has also been taken about its PSF variation during the
night before finalizing its variability status because except for one
RQNLSy1 J102906.69 + 555625.2 ( z = 0.45), all the RQNLSy1s in
the present sample are at lower redshift ( ≤0.4). Thus for a genuine
INOV detection from the current sample, we have first carefully
inspected the seeing variations of all the variable (including probable
variable cases) intra-night sessions, resulting from F
p–enh
-test. An
RQNLSy1 is designated as V if either the FWHM of the session
w as f airly steady during the time of RQNLSy1’s flux variations or
gradients in the FWHM of the session are anti-correlating with the
systematic variations of differential magnitude of target RQNLSy1
and chosen comparison stars (see Cellone, Romero & Combi 2000 ).
3.2 INOV duty cycle estimation
Adopting the definition given by Romero, Cellone & Combi ( 1999 ;
see, also Stalin et al. 2004 ) for the DC of intra-night variability,
we have computed it for the current sample of RQNLSy1s with the
following expression
DC = 100
n
p = 1
C
p
(1 /T
p
)
n
p = 1
(1 /T
p
)
per cent , (4)
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MNRASL 529, L108–L114 (2024)
where T
p
= T
p , observed
(1 + z)
−1
is the observed duration of p
th
monitoring session obtained after redshift correction for the target.
For p
th
session, C
p
is considered to be 1 in equation ( 4 ) only when
INOV is detected, otherwise C
p = 0. The computed INOV duty
cycles for the current sample of RQNLSy1s are listed in Table 3 ,
based on two statistical tests.
To compute the peak-to-peak amplitude of INOV ( ψ) detected in
a given DLC, we followed the definition given by Heidt & Wagner
( 1996 )
ψ =
( H
max
−H
min
)
2
−2 σ2 (5)
with H
min,max
= minimum (maximum) values in the DLC of target
NLSy1 relative to steady comparison stars and σ2
= η2
σ2
NLSy1 −CS
,
where, σ2
NLSy1 −CS
is the mean square (formal) rms errors of
individual data points. The mean value of ( ψ ) for different sets
(e.g. see Table 3 ) of RQNLSy1 galaxies is computed by taking the
average of the computed ψ values for the DLCs belonging to the ‘ V ’
category. In Tabl e 3 , we have also summarized the DC and ψ values
based on the two statistical tests for the different sets of RLNLSy1s,
accessed from Ojha, Hum & Gopal-Krishna ( 2021 ) and Ojha et al.
( 2022 ).
4 RESULTS AND DISCUSSION
The current study presents the first attempt to systematically charac-
terize the INOV for a sample of seven RQNLSy1s that had shown
recurring flaring at 37 GHz when observed at MRO (L
¨
ahteenm
¨
aki
et al. 2018 ). These seven NLSy1s are either radio-quiet or radio-
silent (never detected in radio at any frequency). We monitored the
current sample (see online Table A2 ) of seven RQNLSy1s in a total
of 28 intra-night sessions, each ≥3 h (see online Figs A1 ). It may be
emphasized that an AGN may not sho w v ariability on every night it
was observed, therefore to improve INOV statistics, we have devoted
at least three intra-night sessions, each ≥3 h for each RQNLSy1s.
We applied two versions of the F -test i.e. F
η-test and F
p–enh
-test on
the derived 28 intra-night DLCs to confirm the presence/absence of
INOV in an intra-night session. Out of 28 intra-night sessions signifi-
cant INOV with ψ 10 per cent was detected from the DLCs of four
RQNLSy1s namely J102906.69 + 555625.2, J122844.81 + 501751.3,
J123220.11 + 495721.8, and J152205.41 + 393441.3 (see also first
three online figures of Figs A1 ) with F
η-test. The DLCs of another
session for an RQNLSy1 J102906.69 + 555625.2 showed PV case
with the same test even though with ψ 21 per cent. The PV
case would have stemmed even though ψ ∼21 per cent, in this
case, may be due to comparatively more noise in its DLCs (see
column 12 of online Table A2 ). Furthermore, eight variable cases
with ψ 10 per cent from the DLCs of six RQNLSy1s resulted with
F
p–enh
-test. Another two sessions with ψ > 4 per cent were placed
under PV category using the same test. Thus, using statistical tests
(see online Tab le A2 ) we find that all RQNLSy1s exhibit INOV
except one RQNLSy1, J150916.17 + 613716.6 that did not show
INOV with any of the statistical tests for all intra-night sessions.
It may be recalled here that out of seven RQNLSy1s observed
with JVLA, RQNLSy1s J150916.17 + 613716.6 is the only one with
absolutely no detection in the JVLA, while the others are typically
showing at least one data point at some frequencies (see Berton
et al. 2020b ; J
¨
arvel
¨
a et al. 2023 ). Therefore, the non-detection of
INOV in this source may be due to its quiescent phase in the optical
band too.
From Ta ble 3 , a DC of ∼28 per cent ( ∼35 per cent when two ‘PV’
cases are considered to be ‘ V ’) with ψ (the mean value of ψ for all
the DLCs belonging to the type ‘ V ’) of ∼19 per cent resulted from
F
p–enh
-test for the current sample. Ho we ver, DC of ∼14 per cent
( ∼18 per cent when a PV case is also considered to be ‘ V ’) with
ψ of 19 per cent are estimated based on the more conserv ati ve
F
η-test. Considering the average DC ( DC ) of conserv ati ve F
η-
test and F
p–enh
-test which is ∼21 per cent ( ∼26 per cent when
‘PV’ cases are considered to be ‘ V ’), found to be comparable
to DC of 25 per cent–30 per cent, exhibited by γ-RLNLSy1s,
that display blazar-like INOV (Ojha, Hum & Gopal-Krishna
2021 ).
The strong INOV level of ψ 10 per cent in any variable cases
resulting from the current sample appear striking because even
powerful radio-quiet quasars empowered by SMBH never displayed
ψ > 4 per cent (e.g. see Gopal-Krishna & Wiita 2018 , and references
therein). From Ta ble 3 , it also appears that duty cycles of individual
variable sources are al w ays 20 per cent with ψ 10 per cent,
consistent with the DC estimated for the whole sample. Here, it may
be recalled that in the e xtensiv e INOV study of six prominent classes
of luminous AGNs co v ered in 262 monitoring sessions by Goyal et al.
( 2013b ) where a very similar telescopes and analysis procedure were
used, only blazars class of AGN displayed a DC abo v e ∼10 per cent
for ψ > 3 per cent. Thus, we conclude that the DC resulting from
the present sample appears to be blazar-like.
Considering the median black hole mass log( M
BH
/M
) = 6.84
that implied to harbour inevitably less powerful jets for the current
sample (Heinz & Sunyaev 2003 ; Foschini 2014 ). The resulting DC of
21 per cent from the current sample appears striking and it suggests
the origin of their INOV from the relativistic plasma jets. However, it
may be recalled here that the median black hole mass of the current
sample is an order of less massive than the median black hole mass
of log( M
BH
/M
) = 7.72 of jetted-RLNLSy1s (see last column of
table 5 of Ojha et al. 2022 ). Thus, despite having low SMBH our
sample of NLSy1s is capable of launching relativistic jets. This is
in agreement with the recent results of blazar-like INOV displayed
by a sample of 12 low-mass (median M
BH = 10
6
M
) radio-quiet
AGNs (Gopal-Krishna et al. 2023 ), which strengthened the case
for the ability to launch relativistic jets from apparently radio-quiet
AGNs.
The detection of recurring flaring at 37 GHz at MRO (L
¨
ahteenm
¨
aki
et al. 2018 ) and strong INOV levels found here from the current
sample of seven RQNLSy1s hints at the presence of relativistic jets
in the current sample of RQNLSy1s, ho we ver, non-detection of jet
activity in JVLA observations at 1.6, 5.2, and 9.0 GHz frequencies
appears contrasting. Here, it may be recalled that powerful relativistic
jets that are capable of propagating outside the host Galaxy are found
in approximately 10 per cent of AGNs (P ado vani 2017 ). Therefore,
non-detection of jet activity in JVLA observations from the current
sample may be due to their low-integrated luminosity which is indeed
low ≤1.0 ×10
39
erg s
−1
(see table 2 of Berton et al. 2020b ). Such
scenario straightens with integrated luminosity ≥1.5 ×10
39
erg s
−1
found in jetted sources (see Berton et al. 2018 ). In addition to the
lo w po wer of jets, non-detection of jet activity in JVLA observations
might be due to absorbed jets because of a more tied connection of
optical emission to nuclear jet emission at millimetre wavelengths as
compared to its emission at lower radio frequencies (see Gopal-
Krishna et al. 2023 ), which is largely attenuated due to a high
opacity around the nuclear jet, as interpreted from very long baseline
interferometry studies (Gopal-Krishna & Steppe 1991 ; Boccardi
et al. 2017 ).
The detection of multiple flaring at 37 GHz and INOV in the
current work while non-detection of jet activity in JVLA observations
at 1.6, 5.2, and 9.0 GHz frequencies may be due to the flaring and
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L112 V. Ojha et al.
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Tab l e 3. Duty cycle and amplitude of INOV ( ψ ) for the current sample of 7 RQNLSy1s based on the two versions of F -test.
Duty cycle and amplitude of INOV ( ψ ) for the individual RQNLSy1
F
η-test F
p–enh
-test
RQNLSy1s DC ψ
a DC ψ
a
(per cent) (per cent) (per cent) (per cent)
J102906.69 + 555625.2 [3] 31.3 (64.3)
b 17.6 (19.5)
b 64.3 (64.3)
b 19.5 (19.5)
b
J122844.81 + 501751.3 [5] 22.6 (22.6)
b 32.7 (32.7)
b 38.8 (38.8)
b 26.3 (26.3)
b
J123220.11 + 495721.8 [5] 21.0 (21.0)
b 29.0 (29.0)
b 21.0 (42.2)
b 29.0 (21.7)
b
J150916.17 + 613716.6 [3] 00.0 (00.0)
b 00.0 (00.0)
b 00.0 (00.0)
b 00.0 (00.0)
b
J151020.05 + 554722.0 [4] 00.0 (00.0)
b 00.0 (00.0)
b 25.2 (25.2)
b 10.1 (10.1)
b
J152205.41 + 393441.3 [4] 20.1 (20.1)
b 09.6 (09.6)
b 20.1 (40.7)
b 09.6 (07.1)
b
J164100.10 + 345452.7 [4] 00.0 (00.0)
b 00.0 (00.0)
b 29.6 (00.0)
b 11.4 (11.4)
b
Duty cycle and amplitude of INOV ( ψ ) for the current and control sample of NLSy1s
F
η-test F
p–enh
-test
No. of RQNLSy1s DC ψ
a DC ψ
a
(per cent) (per cent) (per cent) (per cent)
7 RQNLSy1s [28] 14.3 (17.5)
b 19.2 (22.0)
b 27.7 (35.0)
b 18.9 (17.1)
b
c
15 γ-RLNLSy1s [36] 30.4 (30.4)
b 14.1 (14.1)
b 40.5 (47.5)
b 13.9 (13.1)
b
d
8 J- γ-RLNLSy1s [23] 25.9 (25.9)
b 08.6 (08.6)
b 37.5 (54.7)
b 08.1 (07.7)
b
a
The mean value for all the DLCs belonging to the type ‘ V ’. The number of sessions used is tabulated inside the bracket ‘[]’.
b
Values inside parentheses have resulted
when ‘PV’ cases are considered to ‘ V ’.
c
DC and ψ of 15 radio-loud γ-ray detected NLSy1s ( γ-RLNLSy1s) are estimated
using their 36 intra-night sessions from Ojha, Hum & Gopal-Krishna ( 2021 ).
d
DC and ψ of eight radio-loud jetted with γ-ray detected NLSy1s (J- γ-RLNLSy1s)
are estimated using their 23 intra-night sessions from Ojha et al. ( 2022 ).
quiescent states of RQNLSy1s when 37 GHZ, our observations, and
JVLA observations were taken place, respectively.
Another possibility of non-detection of jet activity in JVLA
observations as also emphasized in Berton et al. ( 2020b ) from
the current sample could be due to their high-frequency peakers
nature which usually happens with extremely young objects like
NLSy1s which are expected to be in an early phase of their evolution
(see Komossa 2018 ; Paliya 2019 ). It may be emphasized here that
NLSy1s are young and typically characterized by high Eddington
ratios (Boroson & Green 1992 ; Ojha et al. 2020 ), therefore expected
to be associated with a dense circumnuclear environment around
non-flattened broad-line region (Heckman & Best 2014 ; Vietri et al.
2018 ; Berton et al. 2020a ) that might also hinder relativistic jets
propagation through its interaction with the clouds (vanBreugel,
Miley & Heckman 1984 ).
In addition to non-detection of jet activity in the JVLA obser-
vations at 1.6, 5.2, and 9.0 GHz, recent observations with JVLA at
higher frequencies 10, 15, 22, 33, and 45 GHz showed no signs of
jets from the current sample rather than resulting in either steep
spectrum or no detection at all from most of the sources except for a
source J122844.81 + 501751.2 that showed the flat spectrum (J
¨
arvel
¨
a
et al. 2023 ). Ho we ver, blazar-like INOV found here from the current
sample of RQNLSy1s could be due to the peculiar geometry of jet
in these sources that causes changes in viewing angle toward the
observer’s line of sight thus changes in Doppler factor (Raiteri et al.
2017 ).
Finally, one last possibility of non-detection of jet activity in JVLA
observations might be magnetic reconnection in the magnetosphere
of black hole (Lazarian & Vishniac 1999 ; deGouveia Dal Pino,
Pio v ezan & Kadowaki 2010 ; Kadowaki, de Gouveia Dal Pino &
Singh 2015 ; Kimura et al. 2022 ; Ripperda et al. 2022 ) which does
not require the presence of a permanent relativistic jet and can
account time-scales of variability ranging from minutes to days.
In brief, this model invokes the interactions of magnetic field lines
emerging from the accretion disc with the magnetosphere anchored
into the central black hole horizon (Blandford & Znajek 1977 ).
With the enhancement of the accretion rate, magnetic fluxes from
the accretion disc and those anchored into the black hole horizon
are pushed together in the inner disc region and reconnected under
finite magnetic resistivity (see fig. 1 of Kadowaki, de Gouveia Dal
Pino & Singh 2015 ). This reconnection becomes very efficient and
fast under turbulence instability and releases a huge amount of
magnetic power. A part of released magnetic power accelerates
particles to relativistic velocities and thus is attributed to radio
emissions and flares. Therefore, recurring flaring at 37 GHz observed
at MRO might be due to fast magnetic reconnection driven by
turbulence. Thus, it implies that jets may not be present in our
sample as evidenced from their recent radio studies by JVLA
and Very Long Baseline Array (VLBA, see Berton et al. 2020a ;
J
¨
arvel
¨
a et al. 2023 ), but the INOV seen in the current study may
be due to magnetic reconnection in the black hole magnetosphere.
Furthermore, for the low-luminosity AGNs, deGouveia Dal Pino,
Pio v ezan & Kadowaki ( 2010 ) and Kadowaki, de Gouveia Dal Pino &
Singh ( 2015 ) showed that the turbulence-induced fast magnetic
reconnection events in an efficiently accreting black hole with a mass
of M
BH ∼10
6
M
can release ∼10
39
–10
43
erg s
−1 power which is
sufficient to explain the detection of INOV and flares. The jetted AGN
such as gamma-ray NLSy1s and blazars can also possess magnetic
reconnection and instabilities in their accretion discs, ho we ver,
jet-dominated non-thermal emission completely o v erwhelms the
thermal emission from the accretion disc in them (see Mangalam &
Wiita 1993 ; Ulrich, Maraschi & Urry 1997 ; Blandford, Meier &
Readhead 2019 ).
Additionally, magnetic reconnection events causing acceleration
of plasma to relativistic velocities can also potentially heat the corona
of the accretion disc resulting in the enhancement of thermal X-
ray emission. Furthermore, emissions in high energy gamma-rays
might also be possible through interactions of these accelerated
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INOV of RQNLSy1s L113
MNRASL 529, L108–L114 (2024)
relativistic electrons with photon density in the surrounds of the
black hole via synchrotron self-Compton and/or inverse Compton
interactions (Kadowaki, de Gouveia Dal Pino & Singh 2015 ).
Therefore, coordinated radio, X-ray, and gamma-ray observations
can be useful in confirming or ruling out the possibility of a
magnetic reconnection mechanism (deGouveia Dal Pino, Pio v ezan &
Kadowaki 2010 ; Kimura et al. 2022 ).
5 CONCLUSIONS
In the current study, we present the first attempt to systematically
characterize the INOV for a sample of seven RQNLSy1s that
had shown recurring flaring at 37 GHz when observed with MRO
(L
¨
ahteenm
¨
aki et al. 2017 , 2018 ), ho we ver, no signs of jet were
detected from them when these RQNLSy1s were observed in radio
bands with JVLA at low frequencies. Recurring flaring at 37 GHz
from them strongly suggests the presence of jets in them. Ho we ver,
the non-detection of jet activity in JVLA and VLBA observations
appears puzzling. We have addressed this issue by taking advantage
of INOV which is being used to infer the presence of relativistic jets
in AGNs based on their blazar-like duty cycle and the amplitude of
INOV. Therefore, we monitored the current sample in a total of 28
intra-night sessions, each ≥3 h. The resulting level of INOV from this
sample found to be similar to the INOV level of γ-RLNLSy1s, which
displays blazar-like INOV. Thus detection of recurring flaring at
37 GHz with MRO and strong INOV level found here from the current
sample of seven RQNLSy1s hints at the presence of relativistic jets
in the present set of RQNLSy1s. Furthermore, the resulting strong
level of INOV from the current sample of low-mass RQNLSy1s and
almost a similar INOV level observed recently by Gopal-Krishna
et al. ( 2023 ) from 12 low-mass AGNs, suggest that even low-mass
radio-quiet and/or radio-silent AGNs can launch relativistic jets.
Furthermore, inferred jet activity from the current sample along
with the presence of jet activity in 12 low-mass AGNs (Gopal-
Krishna et al. 2023 ) would be useful in understanding relativistic jet
mechanism in lower black hole mass ( M
BH
∼10
6
–10
7
M
) AGNs.
The resulting variability nature of these sources in radio and
optical wavebands is difficult to explain with the usual variability
mechanisms of AGNs. Therefore, studying such types of sources
with bigger sample sizes is very important to unveil the nature of
these sources as they might represent a new type of AGN variability.
ACKNOWLEDGEMENTS
We acknowledge the Director of Physical Research Laboratory
(PRL) and the Department of Space (DOS), Go v ernment of India,
for supporting this research work and VO’s postdoctoral fellowship.
The PRL runs and supports the Observational activities with the
1.2m and 2.5m telescopes at Mt. Abu. We acknowledge the PRL
Mt. Abu Observatory staff for our observations with the PRL’s 1.2m
and 2.5m telescopes. This work has used observations from the Mt.
Abu Faint Object Spectrograph and Camera-Pathfinder (MFOSC-
P) and we acknowledge the help from Dr. Vipin Kumar and the
entire MFOSC-P team led by Dr. Mudit K. Sri v astav a. This work has
also used observations with the Faint Object Camera (FOC) on the
2.5m telescope and we acknowledge the efforts of the FOC devel-
opment team led by Prof. Abhijit Chakraborty and Dr. Vishal Joshi.
We also thank the Aryabhatta Research Institute of Observational
Sciences (ARIES) authorities and staff for their assistance during
the observations taken from the 1.04m Sampurnanand Telescope
(ST) at Nainital, 1.30m De v asthal Fast Optical Telescope (DFOT)
at De v asthal, both ST and DFOT are run by ARIES. We thank the
anonymous referee for providing useful comments and suggestions,
which helped us to impro v e the manuscript.
DATA AVAILABILITY
The data from the
ARIES telescopes and the Mount Abu observatory
Mt. Abu telescopes of PRL used in this paper will be shared on
reasonable request to the corresponding author.
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SUPPORTING INFORMATION
Supplementary data are available at MNRASL online.
Figure A1 . Differential light curves (DLCs) of three RQNLSy1s
from the sample of seven RQNLSy1s.
Tabl e A1 . Basic observational parameters and log of the target
RQNLSy1s and comparison stars used in the current study.
Tabl e A2 . Observation dates, duration of monitoring along with
DLCs details, and the status of the statistical tests for the sample of
7 RQNLSy1 galaxies studied in the present work.
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