1 Physical Research Laboratory (PRL), Astronomy and Astrophysics Division,
Ahmedabad, India - 380 009
2Aryabhatta Research Institute of observational sciencES (ARIES), Nainital,
India - 263001

# Intra-night optical variability study of a non-jetted narrow-line Seyfert 1
galaxy: SDSS J163401.94$+$480940.1

Vineet Ojha1, 2*

###### Abstract

SDSS J163401.94$+$480940.2 is a non-jetted radio-loud narrow-line Seyfert 1
(NLSy1) galaxy. Optical monitoring of this object was carried out in two
intra-night sessions each $\geq$ 3 hrs with 3.6m DOT. Intra-night optical
variability (INOV) characterization is presented for the first time for this
source. We have detected an unexpected remarkable flare in one of two
monitoring sessions of SDSS J163401.94$+$480940.2, whose rapid brightening
phase implied a minute like doubling time of $\sim$ 22 minutes, thereby
approaching to the extremely fast minute like variability, observed from FSRQ
PKS 1222$+$21 at 400 GeV. The detection of a minute-like variability suggests
the existence of relativistic jets with a small viewing angle. We briefly
discuss the possible mechanisms for the non-detection of relativistic jets in
its Very Long Baseline Array (VLBA) observations.

###### keywords:

surveys – galaxies: active – galaxies: jets – radio-loud galaxies: photometry
– galaxies: Seyfert – gamma-rays: galaxies.

<EMAIL_ADDRESS><EMAIL_ADDRESS>———- ———

12.3456/s78910-011-012-3 #### 1– 1

##  1 Introduction

Narrow-line Seyfert 1 (NLSy1) galaxies are a distinctive subclass of Seyfert
galaxies with intense multi-wavelength properties. While in optical wave-band
both permitted and forbidden emission lines are present in their spectra but
the width of their broad component of Balmer emission lines are narrower than
those of normal broad-line Seyfert 1 galaxies with the full width at half
maximum of the broad component of Balmer emission line (FWHM(H${\beta}$)) less
than 2000 km s-1 (Osterbrock & Pogge, 1985; Goodrich $et~{}al$., 1989). In
addition to the criterion of FWHM(H${\beta}$), two extreme optical
characteristics such as relatively weak [OIII] and strong permitted Fe ii
emission lines with [O${}_{III}]_{\lambda 5007}/H\beta$ $<$ 3 are used to
define the NLSy1 galaxy (Shuder & Osterbrock, 1981). Moreover, NLSy1 galaxies
(NLSy1s) also display other utmost observational characteristics such as steep
soft X-ray spectra, rapid X-ray (in optical sometimes) flux variability,
strong soft X-ray excess below 2 keV, and blue-shifted line profile (e.g.,
Brandt $et~{}al$., 1997; Leighly, 1999; Vaughan $et~{}al$., 1999; Komossa &
Meerschweinchen, 2000; Miller $et~{}al$., 2000; Zamanov $et~{}al$., 2002;
Klimek $et~{}al$., 2004; Leighly & Moore, 2004; Boroson, 2005; Liu $et~{}al$.,
2010; Paliya $et~{}al$., 2013a; Kshama $et~{}al$., 2017; Ojha $et~{}al$.,
2019, 2020b, 2020a). It is believed that NLSy1s are young active galactic
nuclei (AGNs) and represents an earlier stage in their evolution (e.g.,
Mathur, 2000; Sulentic $et~{}al$., 2000; Mathur $et~{}al$., 2001; Fraix-Burnet
$et~{}al$., 2017; Komossa, 2018; Paliya, 2019). Observational evidence
suggests that the average estimated black hole mass of NLSy1s based upon
various methods such as reverberation mapping, luminosity-radius relationship,
and single-epoch virial method, has been found to be relatively lower $\sim
10^{6}-10^{7}M_{\odot}$ (Grupe & Mathur, 2004; Deo $et~{}al$., 2006; Zhou
$et~{}al$., 2006; Peterson, 2011; Wang $et~{}al$., 2014; Rakshit $et~{}al$.,
2017), and they accrete with a high fraction of the Eddington rate, in
contrast to quasars (Boroson & Green, 1992; Peterson $et~{}al$., 2000).
However, a systematic underestimation of their black hole masses is suggested
(Decarli $et~{}al$., 2008; Marconi $et~{}al$., 2008; Calderone $et~{}al$.,
2013; Viswanath $et~{}al$., 2019; Ojha $et~{}al$., 2020a). NLSy1s are mainly
hosted by spiral/disc galaxies (Deo $et~{}al$., 2006; Ohta $et~{}al$., 2007;
Olguín-Iglesias $et~{}al$., 2020). However, elliptical hosts are also
suggested for a few $\gamma$-ray detected NLSy1s (e.g., see D’Ammando
$et~{}al$., 2017, 2018).

Conventionally, it was thought that NLSy1s are often being radio-quiet,
usually defined with the ratio (R) of rest-frame flux densities at 5 GHz and
4400Å to be $\leq$ 10 (see Kellermann $et~{}al$., 1994, 1989, 2016, and
references therein). However, with the recent statistically large sample of
NLSy1s, about $\sim$ 7% of NLSy1s are found to be radio-loud having R $>$ 10
(hereafter RLNLSy1s, Komossa $et~{}al$., 2006; Zhou $et~{}al$., 2006; Rakshit
$et~{}al$., 2017; Singh & Chand, 2018), which suggests that in a few of them,
jets are present (Zhou $et~{}al$., 2003; Yuan $et~{}al$., 2008). In fact,
parsec-scale blazar-like radio jets were revealed in the Very Long Baseline
Array (VLBA) observations of several NLSy1s (Lister $et~{}al$., 2013; Gu
$et~{}al$., 2015; Lister $et~{}al$., 2016). The launching of relativistic jets
in a subclass of AGN having smaller black hole masses and higher accretion
rates counters the historical trend of the launching of relativistic jets with
larger black hole masses and lower accretion rates (Urry $et~{}al$., 2000;
Boroson, 2002; Böttcher & Dermer, 2002; Urry, 2003; Marscher, 2009; Chiaberge
& Marconi, 2011), and also challenges the theoretical scenarios of jet
formation (e.g., Böttcher & Dermer, 2002). Furthermore, in the case of
stellar-mass black holes with high accretion rates generally accord to
quenched states of jets (Boroson, 2002; Maccarone $et~{}al$., 2003).
Therefore, studying jet related aspects of the NLSy1 is important to
understand physical processes that are capable to launch relativistic jets in
this unique class of AGN.

On the other hand, in addition to a similar double-humped spectral energy
distribution (SED) of some RLNLSy1s with blazars (e.g., Abdo $et~{}al$.,
2009c; Paliya $et~{}al$., 2013b; Paliya, 2019), a significant fraction of
RLNLSy1s especially very radio-loud (R $>$ 100) display blazar-like
characteristics such as rapid infrared and X-ray flux variability (Boller
$et~{}al$., 1996; Grupe $et~{}al$., 1998; Leighly, 1999; Hayashida, 2000;
Komossa & Meerschweinchen, 2000; Jiang $et~{}al$., 2012; Itoh $et~{}al$.,
2013; Yao $et~{}al$., 2015; Gabanyi $et~{}al$., 2018), compact radio cores,
high brightness temperature, superluminal motion, flat radio and X-ray spectra
(Yuan $et~{}al$., 2008; Orienti $et~{}al$., 2012; Berton $et~{}al$., 2018;
Lister, 2018). All these characteristics give indirect evidence of the
presence of jets in them and detections of $\gamma$-ray emissions by Fermi-
Large Area Telescope (Fermi-
LAT)111https://heasarc.gsfc.nasa.gov/docs/heasarc/missions/fermi.html from a
handful of RLNLSy1s support the scenario that these jets are relativistic
(Abdo $et~{}al$., 2009a, b, c; Foschini $et~{}al$., 2010; Foschini, 2011;
D’Ammando $et~{}al$., 2012, 2015; Yao $et~{}al$., 2015; Paliya $et~{}al$.,
2018; Yang $et~{}al$., 2018; Yao $et~{}al$., 2019).

Flux variability of AGNs on minutes to hour time scales in the optical
waveband is termed as intra-night optical variability (INOV, Gopal-Krishna
$et~{}al$., 1993), and this alternative tool is also used to indirectly verify
the presence of jets, as it has been observationally well established that
radio-loud jet dominated sources such as blazars exhibit a distinctive
stronger INOV, both in INOV amplitude ($\psi$) and duty cycle (DC, fractional
time for which an AGN is found to be variable) as compared to their radio-
quiet counterparts i.e. QSOs. In fact, this tool has been used for a decade as
a diagnostic to search for the Doppler boosted optical jets in X-ray detected
NLSy1s, $\gamma$-ray detected NLSy1s, and weak emission line QSOs (e.g., see
Liu $et~{}al$., 2010; Paliya $et~{}al$., 2013a; Kumar & Gopal-Krishnaand
Chand, 2015; Kumar $et~{}al$., 2016, 2017; Ojha $et~{}al$., 2018, 2019, 2021).
Therefore, to continue these variability studies, we present here the intra-
night variability study of a RLNLSy1 galaxy ($R=204$) SDSS
J163401.94$+$480940.2, which we observed with 3.6m Devasthal Optical Telescope
(DOT) of the Aryabhatta Research Institute of observational SciencES (ARIES),
India. The RLNLSy1 SDSS J163401.94$+$480940.2 discussed in this paper is from
the member of eight non-jetted NLSy1s (Ojha et al. 2021 under preparation).
Out of these eight NLSy1s, six were already reported (see sample section of
Ojha et al. 2021 under preparation for more detail) in Gu $et~{}al$. (2015)
where they have confirmed no-jet in them based upon their VLBA observations.

The layout of this paper is as follows. A brief introduction about the source
is presented in Sect. 2. Sect. 3 provides details of our photometric
monitoring and data reduction procedure. The statistical method is presented
in Sect. 4. Our main results followed by discussion are given in Sect. 5.

##  2 SDSS J163401.94 + 480940.2

The rather narrower FWHM of H${\beta}$ about 1609$\pm$79 km s-1 of SDSS
J163401.94$+$480940.2 resulted in a smaller black hole mass of $\sim$
$2.5\times 10^{7}$ M⊙ based upon single-epoch optical spectroscopy virial
method (see Yuan $et~{}al$., 2008). In addition to narrower FWHM(H${\beta}$),
the small flux ratio [O${}_{III}]_{\lambda 5007}/H\beta$ of 0.3 and strong
permitted Fe ii emission line make it conventional NLSy1 (Yuan $et~{}al$.,
2008). It is a radio-loud NLSy1 at $z$ = 0.49 with R${}_{1.4~{}GHz}$ = 204 (Gu
$et~{}al$., 2015). Its radio spectrum between the Westerbork Northern Sky
Survey (WENSS) 325 MHz and the NRAO VLA Sky Survey (NVSS) 1.4 GHz data of
similar resolution has been found to be flat with $\alpha=-0.47$ and its
brightness temperature is estimated to be $10^{10.1}$K from its high-
resolution VLBA image (Gu $et~{}al$., 2015). Only a compact component was
detected in its high-resolution VLBA image (see figure 14 of Gu $et~{}al$.,
2015), therefore, Gu $et~{}al$. (2015) have classified this source as a non-
jetted source.

##  3 Photometric monitoring and data reduction

Intranight monitoring of target source SDSS J163401.94$+$480940.1 was carried
out in the Bessel broad-band filter R in two epochs each $\geq$ 3.0 hrs with
3.6m DOT of the ARIES, located at Devasthal, India (Sagar $et~{}al$., 2012).
DOT has a Ritchey–Chretien design, with an f/9 beam at the Cassegrain focus,
and an alt-azimuth mounting. Our observations were performed with a
4k$\times$4k CCD imager mounted on the main port of the telescope (see Pandey
$et~{}al$., 2018). The 4k$\times$4k CCD is cooled with liquid nitrogen (LN2)
to $-$120∘C having a CCD pixel size of 15 $\mu$m and a plate scale of 0.095
arcsec/pixel, covering a field-of-view (FOV) of $\sim$ 6.52$\times$6.52
arcmin2 on the sky. Our observations were taken in 2$\times$2 binning mode
with a readout speed of 1 MHz which corresponds to the system rms noise and
gain of 8.0 $e^{-}$ and 2.0 $e^{-}$ ADU-1, respectively.

For each night, sky flat-field images were taken during dusk and dawn and at
least three bias frames were taken in each night. The dark frames were not
taken during our observations due to a relatively low temperature (of about
$-120^{\circ}$C) of the CCD detector used. Preliminary processing of the
observed frames was done following the standard routines within the
IRAF222Image Reduction and Analysis Facility (http://iraf.noao.edu/) software
package. Since the optical field of monitored NLSy1 was not crowded and target
object is a point-like without extended emission, therefore, aperture
photometry (Stetson, 1987, 1992) was done for extracting the instrumental
magnitudes of the target and the comparison stars recorded in the CCD frames,
using DAOPHOT II algorithm333Dominion Astrophysical Observatory Photometry
(http://www.astro.wisc.edu/sirtf/daophot2.pdf). Aperture size is a key
parameter in the photometry for measuring the instrumental magnitude and the
corresponding signal-to-noise ratio (S/N) of the individual photometric data
points. In addition to aperture size, caution about seeing disc (FWHM)
variation during an intra-night session becomes very important for the nearby
AGNs because a significant contribution to the total flux can come from the
underlying host galaxy. Thus, the relative contributions of the (point-like)
AGN and the host galaxy to the aperture photometry can vary significantly as
the point spread function (PSF) changes during the session. As a result, in
the standard analysis of the differential light curves (DLCs), it might lead
to statistically significant, yet spurious claims of INOV for small apertures
comparable to the PSF (see Cellone $et~{}al$., 2000). Therefore, data
reduction, aperture selection, and caution for seeing disc (FWHM) variations
were done following the procedure adopted in Ojha $et~{}al$. (2021).

Figure 1: Intranight differential light curves (DLCs) of SDSS
J163401.94$+$480940.2. In each panel, the upper DLC is derived using the
chosen two (non-varying) comparison stars, while the lower two DLCs are the
‘RLNLSy1-star’ DLCs, as defined in the labels on the right side. The bottom
panel displays the variations of the seeing disc (FWHM) during the monitoring
session. The redshift, date of observation, telescope, CCD used, and
monitoring time are given at the top of panel.

(a) A 300s sampled DLCs.

(b) A 100s sampled DLCs from flaring point to end point.

Figure 2: Intranight differential light curves (DLCs) of SDSS
J163401.94$+$480940.2 monitored on 2018.03.26, (a): 300 seconds sampled DLCs
for whole monitoring session, (b): 100s sampled DLCs of same epoch from
flaring point (22.37 UT) to end point (23.88 UT). The panels detail are same
as Fig. 1.

##  4 Statistical method

Since the prime interest of this work is to search INOV in the observed
target, therefore, the differential photometric technique is used to produce
differential light curves of each observed night following the procedure
described in Ojha $et~{}al$. (2021). Furthermore, for the confirmation of INOV
in DLCs of each observed night statistically, we have applied two different
versions of the _F $-$test_ proposed by de Diego (2010). These two tests are
known as standard _F $-$test_ (hereafter $F^{\eta}$-test, e.g., see Goyal
$et~{}al$., 2012) and the power-enhanced _$F-$ test_ (hereafter
$F_{enhced}$-test, e.g., see de Diego, 2014). A comprehensive explanation
about these two tests is demonstrated in our old papers (Ojha $et~{}al$.,
2020b, 2021, and references therein).

In short, following Goyal $et~{}al$. (2012), $F^{\eta}$-test can be written as

$F_{1}^{\eta}=\frac{Var_{(NLSy1-cs1)}}{\eta^{2}\langle\sigma_{NLSy1-cs1}^{2}\rangle},\hskip
8.5359ptF_{2}^{\eta}=\frac{Var_{(NLSy1-cs2)}}{\eta^{2}\langle\sigma_{NLSy1-cs2}^{2}\rangle}$
(1)

where $Var_{(NLSy1-cs1)}$ and $Var_{(NLSy1-cs2)}$ are the variances with
$\langle\sigma_{NLSy1-cs1}^{2}\rangle=\sum_{i=1}^{N}\sigma^{2}_{i,~{}err}(NLSy1-cs1)/N$
and $\langle\sigma_{NLSy1-cs2}^{2}\rangle$ being the mean square (formal) rms
errors of the individual data points in the ‘target NLSy1 - comparison star1’
and ‘target NLSy1 - comparison star2’ DLCs, respectively. Here, “$\eta$” an
error scaling factor is taken to be $1.5$ (see Goyal $et~{}al$., 2012; Ojha
$et~{}al$., 2021). Two critical significance levels, $\alpha=0.01$ and
$\alpha=0.05$ that correspond to the confidence levels of 99% and 95%,
respectively are set by us in the present work. The estimated values of
$F^{\eta}$ employing Eq. 1 were compared with its adopted critical $F$-values
($F_{c}$), and the SDSS J163401.94$+$4809 is considered to be variable only
when the $F^{\eta}$-values computed for both its DLCs are found to be greater
than its critical value at 99% confidence level (hereafter, $F_{c}(0.99)$). In
columns 6 and 7 of Table 1, we have tabulated the estimated $F^{\eta}$-values
and the correspondingly inferred variability status of our target source SDSS
J163401.94$+$4809 for its two intra-night sessions.

Table 1: Observational details and the inferred INOV status for the target SDSS J163401.94$+$480940.2 (photometric aperture radius used for analysis = 2$\times$FWHM). RLNLSy1s |  Date(s)a |  Tb |  Nc |  Mediand |  $F^{\eta}$-test | INOV |  $F_{enh}$-test | INOV | $\sqrt{\langle\sigma^{2}_{i,err}\rangle}$ | $\overline{\psi}^{g}_{s1,s2}$
---|---|---|---|---|---|---|---|---|---|---
(SDSS name) | yyyy.mm.dd | (hrs) |  | FWHM |  $F_{1}^{\eta}$,$F_{2}^{\eta}$ |  statuse | $F_{enh}$ |  statusf |  (AGN-s)g | (%)
|  |  |  | (arcsec) |  | 99% |  | 99% |  |
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9) | (10) | (11)
J163401.94$+$480940.2 | 2018.03.23 | 3.04 | 34 | 0.98 | 00.62, 00.71 | NV, NV | 01.61 | NV | 0.012 | –
| 2018.03.26 | 3.00 | 29 | 1.56 | 03.49, 03.83 | V , V | 03.50 | V | 0.023 | 17.50
aDate(s) of the monitoring session(s). bDuration of the monitoring session in
the observed frame. cNumber of data points in the DLCs
of the monitoring session. dMedian seeing (FWHM in arcsec) for the session.
e,fINOV status inferred from Fη and Fenh tests, with V =
variable, i.e. confidence level $\geq$ 99%; and NV = non-variable, i.e.
confidence level $<$ 95%. gMean amplitude of variability in the two
DLCs of the target NLSy1 (i.e., relative to the two comparison stars).

The second version of $F$-test employed in this study is $F_{enhced}$-test. As
described in Ojha $et~{}al$. (2020b) $F_{enhced}$-test can be written as

$F_{{\rm enhance}}=\frac{s_{{\rm NLSy1}}^{2}}{s_{\rm stc}^{2}},\hskip
2.84544pts_{\rm
stc}^{2}=\frac{1}{(\sum_{p=1}^{k}T_{p})-k}\sum_{p=1}^{k}\sum_{i=1}^{T_{p}}s_{p,i}^{2}$
(2)

where $s_{{\rm NLSy1}}^{2}$ is the variance of the DLC of the target NLSy1 and
the reference star (the one proximity in magnitude to the target NLSy1 out of
the two selected non-varying comparison stars), while $s_{\rm stc}^{2}$ is the
stacked variance of the DLCs of the comparison stars and the reference star
(de Diego, 2014). $T_{p}$ is the number of observed frames taken for the
$p^{th}$ star, and $k$ is the total number of non-varying comparison stars.

The scaled square deviation $s_{{\rm p,i}}^{2}$ defined as

$\hskip 76.82234pts_{p,i}^{2}=\omega_{p}(m_{p,i}-\bar{m}_{p})^{2}$ (3)

where $m_{p,i}$’s are the differential instrumental magnitudes, and
$\bar{m_{p}}$ is the mean differential magnitude of the reference star and the
pth comparison star. The scaling factor $\omega_{p}$ (see also Joshi
$et~{}al$., 2011) is taken as

$\hskip
76.82234pt\omega_{p}=\frac{\langle\sigma^{2}_{i,err}(NLSy1-ref)\rangle}{\langle\sigma^{2}_{i,err}(s_{p}-ref)\rangle}.$
(4)

The $F_{{\rm enhced}}$ value estimated using Eq. 2 is compared with its
$F_{c}(0.99)$ and $F_{c}(0.95)$, and SDSS J163401.94$+$4809 is considered to
be variable only when the $F_{{\rm enhced}}$ computed for both its DLCs are
found to be greater than its $F_{c}(0.99)$. In columns 8 and 9 of Table 1, we
have tabulated the estimated values of $F_{{\rm enhced}}$ and the
correspondingly inferred variability status of our target source SDSS
J163401.94$+$4809 for its two intra-night sessions.

Furthermore, the peak-to-peak amplitude of INOV ($\psi$) for quantifying the
actual variation featured by SDSS J163401.94$+$4809 in its variable session is
computed by following the definition given by Heidt & Wagner (1996)

$\hskip 71.13188pt\psi=\sqrt{({P_{max}}-{P_{min}})^{2}-2\sigma^{2}}$ (5)

with $P_{min,~{}max}$ = minimum (maximum) values in the DLC of target NLSy1
relative to comparison stars and
$\sigma^{2}=\eta^{2}\langle\sigma^{2}_{NLSy1-CS}\rangle$, where
$\langle\sigma^{2}_{NLSy1-CS}\rangle$ is the mean square (formal) rms errors
of individual data points and $\eta$ = 1.5 (see Goyal $et~{}al$., 2012).

##  5 Results and discussion

The $\gamma$-ray emissions along with the presence of stronger INOV both in
amplitude ($\psi$) and duty cycle (DC) suggest the presence of Doppler boosted
relativistic jets in the AGNs because of well known beaming effect (e.g., see
Sect. 1, and also Urry & Padovani, 1995). Recently Ojha $et~{}al$. (2021) have
reported the INOV characterization of an unbiased sample of 15
$\gamma$-RLNLSy1s, but INOV characteristics of jetted and non-jetted RLNLSy1s
are still lacking and poorly known for non-jetted RLNLSy1s. Therefore, in the
present work, we report the INOV characterization of a non-jetted RLNLSy1s
SDSS J163401.94$+$480940.2 for first the time which was monitored in two
observing sessions each $\geq$ 3 hrs with 3.6m DOT.

For the unambiguous variability detection of SDSS J163401.94$+$480940.2 in its
both observing sessions, we have first selected two non-variable comparison
stars in each observing session and then generated DLCs of SDSS
J163401.94$+$480940.2 with respect to them. On 2018.03.23, both the selected
comparison stars (S1 and S2) were steady (non-variable) during 3.04 hrs of the
monitoring, and seeing disc variation was also steady except for a slight
variation of about 0.5 arcsec at the end (see Fig. 1). In this monitoring
session, the sampling time was about 300s, and we did not find the source to
be variable based upon both tests (see Table 1). Three days later, on
2018.03.26, the source was again monitored for a total duration of 3.0 hrs
with a sampling time of 100s. In this monitoring session, the two chosen non-
variable comparison stars S1 and S2 of the earlier session become non-steady,
therefore, we have chosen two other non-variable comparison stars S3 and S4
for generating DLCs of SDSS J163401.94$+$480940.2 with respect to them.
Furthermore, for achieving comparable S/N of DLCs to earlier monitoring
session (i.e., 2018.03.23), we have first stacked three 100s frames and
thereafter generated the DLCs for the monitoring session of 2018.03.26. On
this night, both the comparison stars (S3 and S4) were steady during 3 hrs of
the monitoring session, and seeing disc was also steady throughout the session
(see Fig. 2 (a)). Unambiguous evidence of blazar like INOV has been detected
on this night based on both tests (see Table 1).

Although, the blazar types optical variability is rarely expected in non-
jetted AGNs or misaligned AGNs (Paliya $et~{}al$., 2013a; Bhattacharya
$et~{}al$., 2019). But, in the present study, we have been found the
unexpected remarkable sharp feature in the differential light curves of the
non-jetted-RLNLSy1 SDSS J163401.94$+$480940.2 on dated 2018.03.26 with 3.6m
DOT. Therefore, we focus here on this sharp feature only. During the 3.0 hrs
of continuous monitoring with high sensitivity and about 5 minutes of sampling
time, it can be seen in the DLCs of SDSS J163401.94$+$480940.2 that at around
_22.37_ UT there was a sharp rise (between two consecutive points) of $\sim$
_14_ % within _6.01_ minutes and then, after remaining quiescent for _12_
minutes, faded back to almost its initial level (see Fig. 2 (a)). Caution
about seeing disc (FWHM) variation during a monitoring session becomes
important when AGNs are at small redshift (Ojha $et~{}al$., 2021) because a
significant contribution to the total flux can come from the underlying host
galaxy and hence the relative contributions of the (point-like) AGN and the
host galaxy to the aperture photometry can vary significantly as the PSF
changes during the session. This might lead to statistically significant, yet
spurious claims of INOV in the standard analysis of DLCs (Cellone $et~{}al$.,
2000). However, in the present situation, the high redshift $z=0.49$ of the
source and a very small around _0.25_ arcsec seeing disc variation during the
monitoring session (see bottom panel of Fig. 2 (a)), suggest that this sharp
variation (flare) is unlikely to be affected by the host galaxy contribution
of this source and seems to be genuine. This is in accord with a recent deep
near-infrared imaging study of RLNLSy1s by Olguín-Iglesias $et~{}al$. (2020)
using the ESO Very Large Telescope (VLT) from which It can be inferred that
any variable contamination arising from the host galaxy can be safely
discounted in the case of AGNs at z$\gtrsim$0.5. Nonetheless, the sharp
variation in the DLCs of an AGN like we caught in SDSS J163401.94$+$480940.2
is sometimes suspectable if it is accommodated by just two points. Therefore,
we have regenerated DLCs of SDSS J163401.94$+$480940.2 from the flaring point
(i.e., at _22.37_ UT) to endpoint (i.e., at _23.88_ UT) using its 100s
sampling time which we had fortunately taken with 3.6m DOT on 2018.03.26 (see
above). This extra caution has been taken by us to ensure that whether a
flaring event occurred in the SDSS J163401.94$+$480940.2 at _22.37_ UT is
accommodated by more than two points or not. The regenerated 100s sampled DLCs
of SDSS J163401.94$+$480940.2 (see Fig. 2 (b)) has ensured that the flaring
event is accommodated by _four_ points. This has again ensured that the
flaring event discovered in the SDSS J163401.94$+$480940.2 with 3.6m DOT is
genuine.

Such sharp variations are quite uncommon even in case of $\gamma$-ray detected
NLSy1s (e.g., see Eggen $et~{}al$., 2013; Maune $et~{}al$., 2014; Ojha
$et~{}al$., 2019) and extremely rare for blazars (e.g., see Gopal-Krishna &
Wiita, 2018, and reference theirin). The high redshift nature of this source
($z=0.49$) implying high intrinsic luminosity capable to swamp the host galaxy
allowed us to assert that there is almost no contribution from its host galaxy
to our chosen aperture for photometry and most of its optical emissions are
due to synchrotron and accretion disc (e.g., see Ojha $et~{}al$., 2019). Now,
if we estimate the flux doubling time on our conservative assumption, that
equal contributions i.e. 50% each is coming from AGN’s accretion disc and
synchrotron (jet) then in order to account for the observed brightening of
$\sim$ _14_ % (in _6.01_ minutes), occurred at around _22.37_ UT (see Fig. 2
(a)), the optical synchrotron component of AGN is required to have brightened
up by a factor of _1.27_. This corresponds to a flux doubling time of $\sim$
_0.37_ hrs ($\sim$ _22_ minutes).

The spectacular variation observed in the DLCs of SDSS J163401.94$+$480940.2
is remarkable, as such a variation is genuinely unexpected for the non-jetted-
RLNLSy1s. However, exceptional variation, and deduced minutes like flux
doubling time support the jet based origin and thereby approaching to the
extremely fast minute like variability (with a flux doubling time of $\sim$ 10
minutes), observed from flat spectrum radio quasar (FSRQ) PKS 1222$+$21 at 400
GeV (e.g., see Aleksić $et~{}al$., 2011). On the other hand, the detection of
a compact core component only in the VLBA observation at 5 GHz for this
source, classifies it into the non-jetted-RLNLSy1 category (see Gu $et~{}al$.,
2015), which is contrary to the present findings from this source. This might
be either due to the quiescent state of this source during its VLBA
observation, performed in 2013 (see Gu $et~{}al$., 2015) or due to limited
sensitivity of VLBA. Another reason for non-detection of jet component, in the
VLBA observation of this source by Gu $et~{}al$. (2015) could be its lower
black hole mass of $\sim 2.5\times 10^{7}$ M⊙ (see Sect. 2) and lower radio
luminosity of $1.02\times 10^{41}$ erg s-1 at 1.4 GHz (see table 1 of Gu
$et~{}al$., 2015), indirectly implies to harbor less powerful jet (Heinz &
Sunyaev, 2003; Foschini, 2014), hence not capable to escape from the confines
of its host galaxy (Berton $et~{}al$., 2020). Finally, one last possibility of
non-detection of radio jet in the VLBA observation of SDSS
J163401.94$+$480940.2, could be either through synchrotron self-absorption as
it occurs in gigahertz peaked sources or, more probably, via free-free
absorption that can be understood as follows. Since NLSy1s are typically
characterized by a high Eddington ratios (Boroson & Green, 1992; Peterson
$et~{}al$., 2000; Ojha $et~{}al$., 2020a), and they are generally associated
with a dense circumnuclear enviroment (Heckman & Best, 2014) with a high star
formation activity with respect to regular Seyfert galaxies (Chen $et~{}al$.,
2009; Sani $et~{}al$., 2010). Therefore, the high star formation along with
the nuclear activity of NLSy1s can ionize the circumnuclear gas around it, and
thus, the large quantities of ionized gas produced via this process could be
responsible for screening the jet emission at low frequency, and hence
resulting in non-detection of the jet component at low frequencies
observations. Even formation of a cocoon of ionized gas (Wagner $et~{}al$.,
2012; Morganti, 2017) can also be possible when the jet passes through the
interstellar medium, which could also be responsible for the free-free
absorption (Bicknell $et~{}al$., 1997).

## Acknowledgements

I thank an anonymous referee for his/her very important comments that have
improved this paper considerably. I am very grateful to Prof. Hum Chand for
his helpful discussion and useful suggestions. I also thank the concerned
ARIES staff for assistance during the observations obtained at the 3.6m
Devasthal Optical Telescope (DOT), which is a National Facility run and
managed by ARIES, an autonomous Institute under the Department of Science and
Technology, Government of India.

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