# Medium-band observation of the neutrino emitting blazar, TXS 0506+056

Sungyong Hwang Astronomy Program, Department of Physics & Astronomy, Seoul
National University,
1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea<EMAIL_ADDRESS>SNU
Astronomical Research Center, Seoul National University, 1 Gwanak-ro, Gwanak-
gu, Seoul 08826, Korea  Myungshin Im Astronomy Program, Department of Physics
& Astronomy, Seoul National University,
1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea<EMAIL_ADDRESS>SNU
Astronomical Research Center, Seoul National University, 1 Gwanak-ro, Gwanak-
gu, Seoul 08826, Korea  Yoon Chan Taak Astronomy Program, Department of
Physics & Astronomy, Seoul National University,
1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea<EMAIL_ADDRESS>SNU
Astronomical Research Center, Seoul National University, 1 Gwanak-ro, Gwanak-
gu, Seoul 08826, Korea  Insu Paek  Astronomy Program, Department of Physics &
Astronomy, Seoul National University,
1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea<EMAIL_ADDRESS>SNU
Astronomical Research Center, Seoul National University, 1 Gwanak-ro, Gwanak-
gu, Seoul 08826, Korea  Changsu Choi  Astronomy Program, Department of Physics
& Astronomy, Seoul National University,
1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea<EMAIL_ADDRESS>SNU
Astronomical Research Center, Seoul National University, 1 Gwanak-ro, Gwanak-
gu, Seoul 08826, Korea  Suhyun Shin Astronomy Program, Department of Physics &
Astronomy, Seoul National University,
1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea<EMAIL_ADDRESS>SNU
Astronomical Research Center, Seoul National University, 1 Gwanak-ro, Gwanak-
gu, Seoul 08826, Korea  Sang-Yun Lee  Astronomy Program, Department of Physics
& Astronomy, Seoul National University,
1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea<EMAIL_ADDRESS>Tae-Geun
Ji  School of Space Research and Institute of Natural Sciences, Kyung Hee
University,
1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Korea
Soojong Pak School of Space Research and Institute of Natural Sciences, Kyung
Hee University,
1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Korea  Hye-
In Lee School of Space Research and Institute of Natural Sciences, Kyung Hee
University,
1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Korea  Hojae
Ahn  School of Space Research and Institute of Natural Sciences, Kyung Hee
University,
1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Korea  Jimin
Han  School of Space Research and Institute of Natural Sciences, Kyung Hee
University,
1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Korea
Changgon Kim  Department of Astronomy and Space Science, Kyung Hee University,
1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Korea
Jennifer Marshall  Mitchell Institute for Fundamental Physics and Astronomy
and Department of Physics and Astronomy, Texas A&M University, College
Station, TX 77843-4242, USA
Christopher M. Johns-Krull  Department of Physics & Astronomy, Rice
University, 6100 Main Street, Houston, TX 77005, USA  Coyne A. Gibson
McDonald Observatory, 3640 Dark Sky Dr, Fort Davis, TX 79734, USA  Luke
Schmidt Mitchell Institute for Fundamental Physics and Astronomy and
Department of Physics and Astronomy, Texas A&M University, College Station, TX
77843-4242, USA
Travis Prochaska  Mitchell Institute for Fundamental Physics and Astronomy and
Department of Physics and Astronomy, Texas A&M University, College Station, TX
77843-4242, USA

###### Abstract

TXS 0506+056 is a blazar that has been recently identified as the counterpart
of the neutrino event IceCube-170922A. Understanding blazar type of TXS
0506+056 is important to constrain the neutrino emission mechanism, but the
blazar nature of TXS 0506+056 is still uncertain. As an attempt to understand
the nature of TXS 0506+056, we report the medium-band observation results of
TXS 0506+056, covering the wavelength range of 0.575 to 1.025 $\mu$m. The use
of the medium-band filters allow us to examine if there were any significant
changes in its spectral shapes over the course of one month and give a better
constraint on the peak frequency of synchrotron radiation with quasi-
simultaneous datasets. The peak frequency is found to be $10^{14.28}$ Hz, and
our analysis shows that TXS 0506+056 is not an outlier from the blazar
sequence. As a way to determine the blazar type, we also analyzed if TXS
0506+056 is bluer-when-brighter (BL Lac type and some flat spectrum radio
quasars, FSRQs) or redder-when-brighter (found only in some FSRQs). Even
though we detect no significant variability in the spectral shape larger than
observational error during our medium-band observation period, the comparison
with a dataset taken at 2012 shows a possible redder-when-brighter behavior of
FSRQs. Our results demonstrate that medium-band observations with small to
moderate-sized telescopes can be an effective way to trace the spectral
evolution of transients such as TXS 0506+056.

††facilities: Struve (SQUEAN)††software: Astronomy.net (Lang et al. 2010),
IRAF (Tody 1986), SExtractor (Bertin & Arnouts 1996), SWarp (Bertin 2010)

##  1 Introduction

There has been a significant amount of efforts to understand the origin of
high-energy cosmic neutrinos with energies $>100$ TeV detected by the IceCube
collaboration (e.g., Aartsen et al. 2013, 2014, 2019, 2020; IceCube
Collaboration, 2013; Kankare et al. 2019), and their connection with high-
energy astrophysical phenomena such as cosmic rays. In this regard, the recent
identification of TXS 0506+056, a blazar at $z=0.3365$ (Paiano et al. 2018),
as the likely site of the high-energy neutrino emission phenomenon
IceCube-170922A marked the first time that enhanced electromagnetic (EM)
radiation from an astronomical source is found to coincide with a high-energy
cosmic neutrino event, and thus provided an exciting opportunity to understand
where and how high-energy cosmic neutrinos are produced. Studies to
convincingly associate TXS 0506+056 with the neutrino event is still ongoing
(e.g., Lipunov et al. 2020), and so as the efforts to explain the physical
mechanism for the observed neutrino and EM radiation event all together (e.g.,
Ansoldi et al. 2018; Padovani et al. 2018; Sahakyan. 2018; Banik & Bhadra.
2019; Cerruti et al. 2019; Righi et al. 2019; Xue et al. 2019; Cao et al.
2020; Li et al. 2020; Morokuma et al. 2020; Petropoulou et al. 2020; Zhang et
al. 2020). One of key ingredients for such efforts to succeed is to understand
the physical properties of the neutrino emission site, namely the blazar TXS
0506+056.

The leading candidate of high-energy neutrino site has been blazars.
Therefore, it is not very surprising that IceCube-170922A was found to be
associated with one. Blazars are active galactic nuclei (AGNs) with their
relativistic jets pointing toward us. The production of high-energy neutrinos
is thought to be a result of the interaction of protons and nuclei accelerated
in the jet with the surrounding medium that triggers chain reactions and
produce high-energy neutrinos and cosmic rays. The same environment provides
relativistically traveling leptons that give rise to the prominent two
components in the spectral energy distribution (SED) of blazars, the low-
energy peaked component (the infrared to X-ray) through synchrotron radiation
of high-speed electrons, and the high-energy peaked component (GeV to TeV
gamma-rays) through inverse Compton scattering of low-energy photons by
electrons.

However, there is a wide variety of blazars and different types of blazars
often represent very different physical environments, making it important to
pin down the dominant type of neutrino emitting blazars for constraining the
high-energy neutrino emission mechanism. Blazars can be broadly divided into
two types, BL Lac objects and Flat Spectrum Radio Quasars (FSRQs).
Phenomenologically speaking, BL Lacs are blazars with featureless optical
spectra, while FSRQs have strong, quasar-like emission lines in their optical
spectra (Padovani et al. 2017). Physically speaking, FSRQs are intrinsically
high-luminosity AGNs with their jets pointing toward us, and BL Lacs are
intrinsically low-luminosity AGNs such as radio galaxies with beamed jets very
closely aligned to us. Therefore, in terms of energetics, FSRQs are more
likely to be the site of high-energy neutrino emission (e.g., Murase et al.
2014; Dermer et al. 2014), although there are proposed mechanisms that can
produce high-energy neutrinos from BL Lacs (Tavecchio et al. 2014; Righi et
al. 2017; Ansoldi et al. 2018) or even from radio galaxies (e.g., Hooper
2016).

Interestingly, the optical spectrum of TXS 0506+056 shows no notable features
and a smooth continuum indicative of the emission being dominated by
synchrotron radiation (Halpern et al. 2003; Paiano et al. 2018). Therefore,
TXS 0506+056 is a BL Lac object from the spectral classification point of
view. However, Padovani et al. (2019, hereafter, P19) recently suggested that
TXS 0506+056 is a masquerading BL Lac with properties in line with FSRQs based
on several accounts: (i) its radio and [OIII] luminosities are consistent with
those of jetted luminous quasars rather than radio galaxies; (ii) it has the
emission line ratios of Seyfert 2 galaxies; and (iii) the Eddington ratio is
$>0.01$, which is too high for BL Lacs and low luminosity AGNs. Furthermore,
they showed that TXS 0506+056 is an outlier of the blazar sequence, a sequence
defined in the 2-D plane of the peak frequency of the synchrotron radiation of
blazar SED, $\nu^{S}_{\rm{peak}}$, versus the gamma-ray luminosity (Ghisellini
et al. 1998; Fossati et al. 1998; Giommi et al. 2012; Padovani et al. 2012).
Therefore, it is necessary to examine in detail the observational
characteristics of TXS 0506+056 to see how it fits into the blazar types.

One way to better constrain the blazar nature of TXS 0506+056 is to study its
SED in the optical-infrared (IR), and spectral variability as a function of
time. The $\nu^{S}_{\rm{peak}}$ has been found to be $\lesssim 10^{14}$ Hz for
FSRQs, and $\gtrsim 10^{14}$ Hz for masquerading BL Lacs. Previous studies
looked into this issue already, finding $\nu^{S}_{\rm{peak}}\sim 10^{14.5}$ Hz
using available archival data from various epochs and sparsely sampled
photometric information at around the IceCube-170922A event epoch (e.g., P19).
However, re-examination of the $\nu^{S}_{\rm{peak}}$ value using quasi-
simultaneous, densely sampled (in wavelength) datasets are desirable to better
constrain $\nu^{S}_{\rm{peak}}$ since TXS 0506+056 is known to show a rapid
variability and the existing data are sometimes too sparse in wavelength
coverage. Optical spectral variability of blazars have been investigated
extensively in the past, and it has been found that the optical spectra of BL
Lacs are “bluer-when-brighter” (Ikejiri et al. 2011; Bonning et al. 2012; Gaur
et al. 2012; Wierzcholska et al. 2015), while a significant portion of FSRQs
($\sim$ tens of %) have the variability of “redder-when-brighter” (Gu et al.
2006; Hu et al. 2006; Rani et al. 2010; Meng et al. 2018). Although the
fraction of “redder-when-brighter” FSRQs may be lower and some FSRQs show
“bluer-when-brighter” or no variable behavior, virtually no BL Lacs are found
to be “redder-when-brighter”. Therefore, the behavior of the optical spectral
variability can be another indicator to tell if TXS 0506+056 is a BL Lac or a
FSRQ.

In this respect, low resolution spectroscopy with narrow or medium-band (MB)
filters can be a powerful way to gain insight on the characteristics of the
blazar. Although moderate- to high-resolution spectra can give us rich
information about the object in study, a wider spectral width sampled by each
MB filter increases signal-to-noise (S/N) per spectral element in comparison
to conventional moderate- to high-resolution spectroscopy. At the same time,
MB filter widths are many times finer than broad-band filters, so that shapes
of SEDs can be better traced with MB filters than broad-band filters. With
these advantages, small ($\lesssim 1$ m) and mid-sized telescopes ($\sim 2$ m)
can be utilized for MB-based low resolution spectroscopy, even for faint
objects for which spectroscopy has been considered only possible with large
telescopes. Since observing time is more readily available for small and mid-
sized telescopes than large telescopes, time-intensive spectroscopy of many
targets, such as long-term monitoring observations, can be done with MB
filters on a small telescope. Several examples that show the power of the MB
observations are the identification of faint, high-redshift quasars (Jeon et
al. 2017; Kim et al. 2018a, 2019b; Shin et al. 2020) and the AGN reverberation
mapping (Kim et al. 2019a).

To better understand the nature of the optical variability associated with
IceCube-170922A, we conducted a series of MB filter observations using SED
camera for Quasars in Early Universe (SQUEAN, Kim et al. 2016) and the Wide-
field Integral-Field Unit (IFU) Telescope (WIT) from 2017 October 14 (UT), or
about 23 days after the IceCube-170922A event, until 2017 November 6. Here,
WIT is a 0.25 m wide-field imaging telescope, equipped with multiple MB
filters covering a field view of 2.34 $\times$ 2.34 deg2. Since it can produce
MB-based low-resolution spectra at each pixel in the entire field of view, we
name the telescope as the Wide-field “IFU” Telescope.

This paper presents results from our mostly MB-based observations of TXS
0506+056, focusing on demonstration of the effectiveness of MB-based data for
the study of the SED temporal variation and $\nu^{S}_{\rm{peak}}$ of the
blazar emission. We introduce WIT in Section 2. The observation and data
reduction are introduced in Section 3. The variability of TXS 0506+056 during
our monitoring period is given in Section 4, along with the time-series SEDs
of the blazar. In Section 5, we derive $\nu^{S}_{\rm{peak}}$ and discuss the
spectral variability of this object based on our observations. Finally, we
present our conclusion in Section 6.

##  2 Wide-field Integral-Field-Unit Telescope (WIT)

WIT is a 0.25 m, f/3.6 Takahashi CCA-250 telescope equipped with a 4k $\times$
4k CCD camera and a series of MB filters. The telescope is a modified
Cassegrain telescope with a focal reducer. The CCD camera is the Finger Lake
Instrumentation (FLI)’s ML16803 model, which has 4096 $\times$ 4096 pixels
with a pixel size of 9 $\mu$m. Therefore, the pixel scale is $2\farcs 06$, and
the camera covers a field of view of 2.34 $\times$ 2.34 deg2.

Figure 1: Wide-field IFU Telescope (middle), attached on the 0.8 m telescope
(left) of the McDonald Observatory.

The telescope was mounted on the 0.8 m telescope of the McDonald Observatory
on 2017 February 14 (Figure 1). The system includes a filter wheel that can
house eight 50 mm square filters, and a suite of softwares that allows remote
operation of the telescope. Currently, five medium-band filters,
$m575,m625,m675,m725$, and $m775$ are installed, where the alphabet $m$ in the
filter name stands for “medium-band”, and the number represents the central
wavelength of the filter in nm. The filter widths are 50 nm each. The combined
transmission curves of the filters and the quantum efficiency of the camera
are shown in Figure 2. The effective wavelength and the full-width at half-
maximum (FWHM) of the combined filter throughput of each filter of WIT are
computed following Choi & Im (2017), and they are presented in Table 1.

Table 1: Effective wavelength and FWHM of MB on WIT

Filter | $\lambda_{eff}$ | FWHM
---|---|---
$m575$ | 573.7 | 48.4
$m625$ | 623.6 | 48.7
$m675$ | 674.6 | 48.6
$m725$ | 724.8 | 48.8
$m775$ | 774.2 | 49.2

Note. — (1) - Filter name; (2) - Effective wavelength of the filter (nm); (3)
- Full-width at half-maximum (nm)

Figure 2: The throughput curves for each filter of SQUEAN (Top) and WIT
(Bottom). The black lines represent the throughput of MBs and the red, dashed
lines represent the throughput of broad-bands. The names of filters are shown
on the throughput curve.

In addition to these MB filters, WIT can be equipped with the standard Johnson
$B,V,R,I$ filters and holographic diffraction gratings for slitless
spectroscopy. WIT has been operational since 2017 February 16, and mostly
operated remotely from Korea, for monitoring observation of nearby galaxies to
discover supernovae (Im et al. 2019), and to trace the spectral variation of
AGNs in the field around each nearby galaxy.

##  3 Observations and data reduction

###  3.1 Observation

We observed TXS 0506+056 with the Otto Struve 2.1 m telescope and WIT at the
McDonald Observatory. The 2.1 m Otto Struve telescope is equipped with SQUEAN.
SQUEAN has 10 MB and 10 broad-band filters, allowing us to trace SED shapes of
targets with a moderate investment of observational resources (see, e.g. Kim
et al. 2019b).

The observations were conducted at 2017 October 14 through 2017 November 6
(UT) for a total of 10 nights. We used 9 MBs and 4 broad-bands of SQUEAN, and
5 MBs for the WIT observation. The exposure time per frame was set at 150
seconds for the WIT observations. For the case of SQUEAN, the exposure times
were set at 10 secs to 60 secs depending on the weather condition and the
passband. The observation logs are given in Tables 5 and 6 in Appendix A.

Figure 3: The $V$-band light curve of TXS 0506+056 from CRTS and ASAS-SN
surveys. Only the points that have magnitude error less than 0.1 are shown.
The $V$-band magnitude of the object increased about 0.5 mag from 50 days
before the neutrino event. The time of the neutrino detection is shown with a
red vertical line. The epoch of the Rau et al. (2012) data (inactive phase) is
indicated with the green arrow and the corresponding $V$-band magnitude is
marked with a green circle, while our observation period is indicated with the
blue arrow and the corresponding $m575$-band magnitudes are overplotted with
blue circles.

Figure 3 indicates $m575$ data from our observations, overplotted on the
Catalina Real-time Transient Survey (CRTS, Drake et al. 2009) and the All-Sky
Automated Survey of Supernovae (ASAS-SN, Shappee et al. 2014; Jayasinghe et
al. 2019) $V$-band light curve. Epochs of our observations correspond to the
time shortly after the most active phase near the IceCube-170922A event, but
still during the active phase.

###  3.2 Data reduction and calibration

We reduced the data in a standard manner, for the bias, dark, and flat-field
corrections. For flat-field correction, we used both sky flat-fields and dome
flat-fields. The IRAF (Tody 1986) and custom IDL codes were used for these
reductions.

Figure 4: Light curves of TXS 0506+056 using different filters. The blue
points are the WIT data and the black points are those from SQUEAN. It shows
the mild variabilities in the filters $m575$ through $m775$. The filter name
is shown in the upper left corner and the excess variability is shown in the
upper right corner in each plot.

Photometric calibration was done using objects near the target in the image,
in a manner similar to the method described in Jeon et al. (2016) and Choi &
Im (2017). In this procedure, we first identified point sources in the field
to be used as photometry reference stars by running the object detection
software SExtractor (Bertin & Arnouts 1996) and matched them to $B,V,g,r,i$
photometry from the AAVSO Photometric All Sky Survey (APASS) catalog (Henden
et al. 2016). For the case of WIT, we selected 10 to 40 photometry reference
stars within 36$\arcmin$ from the target, where the point sources are chosen
to be those with the probability of being point source greater than 0.98,
which is calculated by SExtractor. For the 2.1 m telescope data, we selected
about 10 stars in the SQUEAN field of view. For both broad-band and MB data,
we fitted spectral templates of 175 stars (Gunn & Stryker, 1983; Strecker et
al., 1979) to the multi-band APASS magnitudes of the reference stars, and
derived the best-fit template. After identifying the best-fit template, we
used the template to synthetically calculate the magnitude of each band,
$m_{\rm AB}$, of the reference stars using the following equation:

$m_{\rm{AB}}=-2.5\,\rm{log}(\frac{\textit{$\int$}\textit{d}\textit{$\lambda\,\lambda$}\,\textit{F}_{\lambda}\,\textit{R}_{\lambda}}{\textit{c}\,\textit{$\int$}\textit{d}\textit{$\lambda$}\,\textit{R}_{\lambda}\,\textit{/}\,\textit{$\lambda$}})-48.6,$
(1)

where $R_{\lambda}$ is the throughput function of each filter including the
detector quantum efficiency, and $F_{\lambda}$ is the specific flux of the
best-fit stellar template. Using these magnitudes of the reference stars, we
derived the zero point of each image in each filter. The error is taken as the
standard deviation of the zero points from all the reference stars. To derive
the magnitude of TXS 0506+056, we performed aperture photometry using
SExtractor. The aperture size was chosen as 1.5 times of the seeing value to
obtain the maximal signal-to-noise (S/N). The magnitudes of the reference
stars were obtained in the same way, therefore, the aperture magnitude zero
points include aperture correction. The photometric error of each magnitude is
computed as root mean square (rms) of the zero point error and the photometric
error from SExtractor. Note that the magnitudes in the tables are corrected
for the Galactic extinction, by adopting E(B-V) = 0.094 from E(B-V) map of
Schlafly $\&$ Finkbeiner (2011) with the Cardelli et al. (1989) law assuming
$\it R_{V}$ = 3.1 and the dust extinction function of Fitzpatrick (1999).

##  4 results

###  4.1 Variability of TXS 0506+056

Figure 4 shows the MB and broad-band light curves of TXS 0506+056. At a
glance, TXS 0506+056 seems to be somewhat variable during the explored period,
showing a decrease in flux, and then increase in flux again. To quantify the
significance of variability, we performed two analyses.

First, we performed a $\chi^{2}$ test to see how variable the light curves are
(de Diego 2010; Villforth et al. 2010; Kim et al. 2018b). The $\chi^{2}$ value
of each light curve is

$\chi^{2}=\sum_{i=1}^{N}\frac{(m_{i}-\mu_{mag})^{2}}{\sigma_{mag,i}^{2}},$ (2)

where $m_{i}$ is the magnitude at the epoch $i$, $\mu_{mag}$ is the mean value
of the magnitudes, and $\sigma_{mag,i}$ is the magnitude error at the epoch
$i$. The null hypothesis is that the object in the filter is not variable. We
can calculate the confidence level of rejection, $C_{reject}$ as

$C_{reject}=1-\int_{\chi^{2}}^{\infty}P(\chi^{2},dof)\,d\chi^{2},$ (3)

where $P(\chi^{2},dof)$ is the $\chi^{2}$ distribution fuction of the degree
of freedom $dof$. The results are presented in Table 2, and the $C_{reject}$
values are about 1 for all the filters except for the filters with only 3
epochs and the mean photometric error $\langle\sigma_{mag}\rangle>0.045$
Therefore, the $\chi^{2}$ result shows that the object is variable. Second, in
order to confirm the variability of the object, C-test value ($C_{test}$) was
calculated as (e.g., Kim et al. 2018b)

$C_{test}=\frac{\sigma_{\rm{LC}}}{\langle\sigma_{mag}\rangle},$ (4)

where $\langle\sigma_{mag}\rangle$ is the average photometric error over all
the epochs, and $\sigma_{\rm{LC}}$ is the standard deviation of the light
curve magnitudes. If $C_{test}$ is greater than 1.96 (2.56), the object is
variable with 95% (99%) confidence level. We find that the $C_{test}$ values
are above 1.96 and some exceed 3, except for those for the light curves of the
filters at $\geq 875$ nm with only 3 epochs and
$\langle\sigma_{mag}\rangle>0.05$ mag (Table 2). The $C_{test}$ value for the
$m975$ filter light curve is the lowest value of 1.25, and even for this case,
we can say that the confidence level is 79% in favor of variability. With the
results from the $\chi^{2}$ test and C-test, we conclude that the blazar has
been variable during our observation. To quantify how much the object was
variable, we derived the excess variability $\sigma_{ex}$ defined as (Kim et
al. 2018b, 2019a),

$\sigma_{ex}=\sqrt{\sigma_{\rm{LC}}^{2}-\langle\sigma_{mag}\rangle^{2}}\,.$
(5)

The excess variabilities are found to be at 0.04 to 0.12 mag, and they are
positive for all the MB filters. In particular, for the filters with more than
8 epochs of data ($m575$ through $m775$), suggesting inter-day variability of
TXS 0506+056 at the level of 0.1 mag level during the monitored period.

Table 2: Variability Test Results

Filter | $\chi^{2}$ | $\nu$ | $\sigma_{LC}$ | $\langle\sigma_{mag}\rangle$ | $C_{reject}$ | $C_{test}$ | $\sigma_{ex}$
---|---|---|---|---|---|---|---
$m575$ | 34.8 | 8 | 0.101 | 0.045 | 1.00 | 2.24 | 0.09
$m625$ | 394.8 | 11 | 0.114 | 0.029 | 1.00 | 3.93 | 0.11
$m675$ | 249.4 | 11 | 0.103 | 0.040 | 1.00 | 2.58 | 0.09
$m725$ | 392.9 | 11 | 0.122 | 0.032 | 1.00 | 3.81 | 0.12
$m775$ | 94.1 | 11 | 0.107 | 0.041 | 1.00 | 2.61 | 0.10
$m825$ | 23.3 | 3 | 0.083 | 0.042 | 1.00 | 1.98 | 0.07
$m875$ | 4.7 | 3 | 0.090 | 0.057 | 0.80 | 1.58 | 0.07
$m925$ | 4.2 | 3 | 0.069 | 0.045 | 0.76 | 1.53 | 0.05
$m975$ | 4.1 | 3 | 0.075 | 0.060 | 0.75 | 1.25 | 0.05
$m1025$ | 6.5 | 3 | 0.061 | 0.048 | 0.91 | 1.27 | 0.04
$r$ | 129.2 | 3 | 0.087 | 0.023 | 1.00 | 3.78 | 0.08
$i$ | 28.1 | 3 | 0.062 | 0.031 | 1.00 | 2.00 | 0.05
$z$ | 11.8 | 3 | 0.078 | 0.031 | 1.00 | 2.52 | 0.07
$Y$ | 2.9 | 3 | 0.074 | 0.058 | 0.59 | 1.28 | 0.05

Note. — (1) - Filter name; (2) - Chi-squared value; (3) - Degree of freedom;
(4) - Standard deviation of the light curve (mag); (5) - Mean photometric
error of the light curve (mag); (6) - Confidence of rejection; (7) -
C-statistic; (8) - Excess variability (mag);

Figure 5: The optical SEDs at each epoch and the fitting results. The best-fit
spectral slope $\alpha_{\lambda}$ and $\chi_{\nu}^{2}$ values are shown in the
upper right corner of each plot. UT date of the observation is shown as well.

###  4.2 SED temporal variation

Table 3: Power-law fitted parameters

Date | $\alpha_{\lambda}$ | $\sigma_{\alpha_{\lambda}}$ | $\chi_{\nu}^{2}$
---|---|---|---
2017-10-14 | -0.55 | 0.03 | 0.58
2017-10-16 | -0.60 | 0.03 | 1.02
2017-10-17 | -0.78 | 0.16 | 0.31
2017-10-18 | -0.51 | 0.07 | 0.22
2017-10-21 | -0.45 | 0.19 | 0.09
2017-10-28 | -0.71 | 0.15 | 1.05
2017-10-29 | -0.57 | 0.19 | 0.58
2017-10-30 | -0.81 | 0.12 | 0.24
2017-11-03 | -0.65 | 0.18 | 0.45
2017-11-06 | -0.62 | 0.17 | 0.60

Note. — (1) - Date of observation (UT); (2) - Power-law slope; (3) - Error of
the power-law slope; (4) - Reduced chi-squared value

The time-series MB data allow us to examine in detail how the SED of TXS
0506+056 evolves with time. To quantify the variability of the SED shape, we
modeled the specific flux of TXS 0506+056 with a power-law shape of
$F_{\lambda}\sim\lambda^{\alpha_{\lambda}}$. We did so because typical blazars
show a power-law shape of SEDs indicative of the synchrotron radiation (e.g.,
Zacharias & Schlickeiser 2010). In particular, TXS 0506+056 has been reported
as having a featureless power-law continuum in optical with the
$\alpha_{\lambda}$ value at around $\alpha_{\lambda}\sim-1$ to 0 (Rau et al.
2012; IceCube Collaboration et al. 2018; Paiano et al. 2018). We fitted the
observed SEDs at each epoch with this power law model,

$\rm{log}\,F_{\lambda}=\alpha\,\rm{log}\,\lambda+K,$ (6)

where $K$ is the logarithm of the proportional constant. Figure 5 shows SEDs
of TXS 0506+056 at different epochs. From the figure, we confirm that the
power law model can represent the SED shape of the blazar well, with the
reduced $\chi^{2}$ value of each fit showing the value around one or less.
Table 3 lists the fitted parameters of each epoch SED. The spectral slopes
$\alpha_{\lambda}$ are plotted as a function of time in Figure 6.

Figure 6: The spectral slope as a function of time. The power-law index
$\alpha_{\lambda}$ varies during our observation period.

To examine if the power-law index shows a strong variability, we applied the
$\chi^{2}$ test, similarly to what we did in Section 3.1. We find the value of
$\chi^{2}$ = 13.88 with the degree of freedom of 9 for $\alpha_{\lambda}$, and
a modest value of $C_{reject}$ = 0.87 for the null hypothesis that
$\alpha_{\lambda}$ is not variable. This result shows that $\alpha_{\lambda}$,
or the spectral shape is not strongly varying above the level that goes beyond
the measured uncertainty of $\sim 0.15$ during the monitoring period.

Combining with the result from Section 4.1, we conclude that TXS 0506+056 is
variable in short-term ($\sim$ days), but the variability is rather achromatic
during the same period.

##  5 Discussion

As we mentioned in the introduction, the blazar nature of TXS 0506+056 is an
interesting topic, to better understand the neutrino emission mechanism. We
discuss here two spectral properties of TXS 0506+056, its synchrotron peak
frequency, $\nu_{peak}^{S}$, and the spectral variability as a function of
brightness.

###  5.1 Synchrotron peak frequency, $\nu^{S}_{peak}$

The synchrotron peak frequency has been used as a rough way to sort blazars,
with FSRQs having log($\nu_{peak}^{S}$/Hz) $\lesssim$ 14, and BL Lacs having
any kind of $\nu_{peak}^{S}$. Masquerading BL Lacs are known to have
log($\nu_{peak}^{S}$/Hz) $\gtrsim$ 14 (Padovani et al. 2019). It is also a key
quantity that defines the blazar sequence. Therefore, we examine its value in
detail using our new data.

Due to the moderate but rapid variability of TXS 0506+056, we need to
construct its SED with data taken nearly simultaneously ($\lesssim$ 1 day).
The $\nu_{peak}^{S}$ value can be in near-infrared or mid-infrared, therefore
it is desirable to have IR data too. Our MB photometry data satisfy the
criteria of being taken nearly simultaneously, and we also identify the quasi-
simultaneous multi-band photometry data from UV through NIR in Rau et al.
(2012, hereafter, R12). For the R12 dataset, the $Swift$ UV and $u,b$, and $v$
and the GROND data ($g,r,i,z,J,H$, and $K$) were taken at about 2 days apart.
Therefore, the $Swift$ data and the optical/NIR data are not exactly
simultaneous. There is indeed a small offset in the $v$ and $b$ magnitudes
versus their GROND $g$ and $r$ magnitudes. Therefore, we shifted the $Swift$
magnitude to match the epoch of the GROND data by deriving the magnitude
offset with respect to a reference epoch and applying it to all the $Swift$
data. The achromatic nature of the short-term variability (Section 4.2)
justifies this approach for constructing a quasi-simultaneous dataset. The
offset was found by deriving the interpolated $v$ magnitude from the GROND
data and comparing it to the Swift $v$ mag, where quasi-simultaneous $v$
magnitude is obtained by linearly interpolating the GROND $g$ and $r$
magnitudes and taking the flux at the effective wavelength of the filter. The
offset value is found to be 0.14 mag.

Additionally, we estimated the WISE $W1$ (3.36 $\mu$m) and $W2$ (4.61 $\mu$m)
data corresponding to the two epochs. Searching for the WISE and NEOWISE data,
we find the WISE data taken about 36 and 91 days near our MB observaion and
the R12 GROND epochs not close enough in time to treat as quasi-simultaneous.
Therefore, we took an alternative approach of finding the correlation between
$V$-band data from ASAS-SN, and WISE $W1$ and $W2$ data. We matched the ASAS-
SN and WISE data in time domain, allowing a maximum time gap to be 0.2 days.
12 epochs of data are fitted within the criterion. As shown in Figure 7, there
is good correlation between $V$ and $W1$ or $W2$, and we derived the relation
between the $V$ magnitude and the WISE $W1,W2$ magnitudes as below.

$W1=1.30\,V-8.63\,\,(\pm 0.14,1\,\sigma)$ (7) $W2=1.18\,V-7.73\,\,(\pm
0.16,1\,\sigma)$ (8)

Using the above relations, we can calculate the WISE $W1,W2$ magnitude at any
epoch using a given $V$ magnitude at a given epoch. The $V$-band derived $W1$
or $W2$ magnitude errors can be obtained as the rms of the offset between the
WISE data and the best-fit correlations, which are found to be 0.14 mag and
0.16 mag respectively.

Figure 7: The $V$ \- $W1$ and $V$ \- $W2$ magnitude relation. $V$ magnitudes
are from ASAS-SN survey and $W1$, $W2$ magnitudes are from WISE survey. We
matched the data collected at 57200 and 58300 MJD with the time difference of
observation between WISE and ASAS-SN data less than 0.2 days. The correlation
between $V$ vs. $W1$ and $W2$ and the standard deviation in the $WISE$
magnitudes are shown in each plot.

To calculate the WISE $W1$, $W2$ magnitude at the epoch of our observation, we
extrapolated the MB SED at the corresponding epoch and took the flux at 550 nm
which is the effective central wavelength of the $V$-band filter. For the R12
epoch, the offset-corrected $v$ photometry was adopted to calculate the $W1$
and $W2$ data. In Figure 8, we show these quasi-simultaneous SEDs of the TXS
0506+056. The $V$, $W1$, and $W2$ photometry values are given in Table 4. For
our MB data, we used the epoch of 2017 October 14, since the optical SED shape
does not vary dramatically during our observation period (Section 4.2). We
note that our observation epoch corresponds to the active phase, while the R12
epoch corresponds to a rather inactive phase. This is evident in the long-term
$V$-band light curve (Figure 3). The $\nu_{peak}^{S}$ values were obtained by
fitting the SEDs with polynomial functions. We obtained the best-fit
$\nu_{peak}^{S}$ values for the 2nd-, 3rd-, 4th\- order polynomial functions,
since the derivation of $\nu_{peak}^{S}$ can vary significantly what fitting
function we use. Then, we calculated $\nu_{peak}^{S}$ and its error to be the
mean value and the standard deviation of the three $\nu_{peak}^{S}$ values
obtained from the three fitting methods. We find log($\nu_{peak}^{S}$)= 14.69
$\pm$ 0.11 for MJD 55540 (the R12 epoch, the darker state) and
log($\nu_{peak}^{S}$) = 14.28 $\pm$ 0.09 for MJD 58040 (our epoch, the
brighter state). If we use the magnitudes of Swift and WISE for the darker
state that are closest in time (open circles in Figure 8), instead of the
quasi-simultaneous magnitudes, we get log($\nu_{peak}^{S}$) = 14.59 $\pm$
0.10, which is slightly smaller than the log($\nu_{peak}^{S}$) from the quasi-
simultaneous magnitudes. The $\nu_{peak}^{S}$ values are consistent with the
values presented in P19, and the expected $\nu_{peak}^{S}$ of a masquerading
BL Lac, or an intermediate energy BL Lac. We also note that the
$\nu^{S}_{peak}$ determination could have imporved if there were NIR data,
since the estimated $\nu^{S}_{peak}$ occurs at NIR for MJD 58040.

Table 4: Photometry values of $V$, $W1$ and $W2$ filters

Date |  $V$ (error) |  $W1$ (error) |  $W2$ (error)
---|---|---|---
56911.555 | 14.838 (0.065) | 10.457 (0.015) | 9.569 (0.017)
56911.556 | 14.697 (0.052) | 10.457 (0.015) | 9.569 (0.017)
57276.530 | 15.065 (0.048) | 11.041 (0.019) | 10.068 (0.019)
57276.531 | 15.133 (0.046) | 11.041 (0.019) | 10.068 (0.019)
57276.532 | 15.062 (0.046) | 11.041 (0.019) | 10.068 (0.019)
57441.298 | 14.520 (0.051) | 10.478 (0.018) | 9.589 (0.013)
57441.299 | 14.600 (0.056) | 10.478 (0.018) | 9.589 (0.013)
57441.301 | 14.478 (0.048) | 10.478 (0.018) | 9.589 (0.013)
57805.117 | 14.690 (0.041) | 10.193 (0.019) | 9.282 (0.014)
57805.118 | 14.595 (0.040) | 10.193 (0.019) | 9.282 (0.014)
57805.119 | 14.640 (0.041) | 10.193 (0.019) | 9.282 (0.014)
58004.559 | 14.542 (0.048) | 10.023 (0.021) | 9.087 (0.015)

Note. — (1) - Date of observation of $V$-band (Modified Julian Date).; (2) -
$V$-band magnitude from ASAS-SN survey; (3) - $W1$-band magnitude from WISE
survey matched with $V$-band in time within 0.2 days; (4) - $W2$-band
magnitude from WISE survey matched with $V$-band in time within 0.2 days;

Figure 8: SED of the TXS 0506+056 at two different epochs, one near the
neutrino emission (brighter phase, the blue line and points), and another at
darker phase (the black dashed line and points). The Swift
$u,b,v,UVW1,UVM2,UVW2$ fluxes presented in black filled circles are corrected
for the 2-days of time gap between GROND and Swift observation. Uncorrected
Swift fluxes are also shown in gray empty circles. The WISE data that are
closest in time to the epoch of each observation are also shown as open
symbols. The best-fit of the data with $2^{nd}$-order polynomial are
overploted as solid and dashed lines.  Figure 9: The blazar sequence with
gamma-ray luminosity (left) and 5 GHz radio luminosity (right). The data of
FSRQs and BL Lacs from Fossati et al. (1998, right) and Fermi 3LAC (Ackermann
et al. (2015, left) are plotted with black squares and green triangles
respectively. The TXS 0506+056 data from P19 is presented with open blue
circles and those from our observation is plotted with red circles. The blazar
model from Ghisellini et al. (2017) is plotted with cyan circles.

###  5.2 Is TXS 0506+056 an outlier of the blazar sequence?

Figure 9 shows how this object fits in the blazar sequence, in both gamma-ray
luminosity vs. $\nu_{peak}^{S}$ and radio luminosity vs. $\nu_{peak}^{S}$
spaces. For the case of the gamma-ray luminosity vs. $\nu_{peak}^{S}$ plot,
the FSRQ and BL Lac data are taken from the Fermi 3LAC catalog (Ackermann et
al. 2015), while the Fossati et al. (1998) data are used for the radio
luminosity vs. $\nu_{peak}^{S}$ plot. The gamma-ray luminosity of TXS 0506+056
is taken from P19. The 5 GHz radio flux of TXS 0506+056 is derived from the
interpolation of 4.68 GHz and 7.02 GHz data (rest-frame) of IceCube
Collaboration et al. (2018) obtained at MJD = 58038.35, which is the closest
in time to our MB observation. The luminosity ($\rm{log}\,L_{5GHz}$) is
calculated from flux, assuming a $\Lambda$CDM concordance cosmology of
$H_{0}=70$ km s-1 Mpc-1, $\Omega_{M}=0.3$ and $\Omega_{\Lambda}=0.7$ which has
been supported by observations in the past decades (e.g., Im et al. 1997;
Planck Collaboration et al. 2018). We found $\rm{log}\,L_{5GHz}$ = 43.01. On
contrary to the suggestion by P19, Figure 9 shows that TXS 0506+056 sits in
the blazar sequence, meaning that TXS 0506+056 is not an exceptional blazar in
the blazar sequence. Compared to the blazar sequence constructed from the
blazar model of Ghisellini et al. (2017, cyan circles), TXS 0506+056 appears
as an outlier, but TXS 0506+056 sits near the Ackermann et al. (2015, black
squares and green triangles) data, which is also used to construct the model
of Ghisellini et al. (2017). We note that the blazar sequence points taken
from Ghisellini et al. (2017) deviates from the Fermi 3LAC SEDs and the
Fossati et al. (1998) points. By plotting the Fermi 3LAC SEDs of blazars, we
confirm that the values listed in the Fermi 3LAC catalogs. Therefore, it is
not clear the origin of the discrepancy, but we suspect that it is due to the
model function used to determine the $\nu_{peak}^{S}$ by Ghisellini et al.
(2017).

###  5.3 Redder-when-brighter or bluer-when-brighter?

The next interesting point in Figure 8 is the difference between the spectral
shapes during the active and inactive phases. We find that the R12 spectral
shape in the inactive phase is flatter in the optical than the active phase
(our data), suggesting a redder-when-brighter behavior found in FSRQs. The
redder-when-brighter behavior is likely to be due to the thermal component of
the SED at optical-IR range as suggested by many previous studies (Rani et al.
2010; Bonning et al. 2012). The SED of the blazars at optical-IR range can be
broken into two main components. One is the non-thermal emission by the jet
and the other is the thermal emission by the accretion disk. Intrinsically
bright quasars such as FSRQs, have a strong thermal emission component with a
blue SED, and their rapid variability is controlled by a non-thermal component
which is dominant at longer wavelength. Therefore, when the jet activity is
high, the non-thermal component brightens (SED at longer wavelength) causing
the redder-when-brighter behavior. Therefore, the redder-when-brigther
behavior of TXS 0506+056 suggests that it is an intrinsically bright AGN.
Obviously, one should not draw a definitive conclusion from the comparison of
SEDs at only the two epochs. Therefore, we examined how the optical spectral
slope changes with the apparent magnitude, assuming that the apparent
magnitude is a good proxy of the AGN activity. Figure 10 shows
$\alpha_{\lambda}$ versus $m575$ and $m775$ magnitudes which are the bluest
and reddest passbands covered by WIT. When SQUEAN MB photometry is available
we use it instead of the WIT photometry since it provides smaller errors. The
Pearson’s coefficient ($\rho$) and its p-value were 0.38 and 0.35 for the case
of $m575$-band and -0.269 and 0.45 for the case of $m775$-band. Therefore, the
significance of the correlation is only 65% and 55% each. The results shown in
Figure 10 indicate that the change in $\alpha_{\lambda}$ is consistent with
null hypothesis within the measurement error, agreeing with the achromatic
behavior of the SED shape over a short period found in Section 4.2. It appears
that a longer time and larger magnitude base line is desirable to better
characterize the spectral variability. If there was such a long-term data,
since the measurement accuracy for the $\alpha_{\lambda}$ of MB SED is
$\delta\alpha_{\lambda}$ of 0.05 - 0.2 (Table 3), the SED slope change of
$\alpha_{\lambda}\sim-0.6$ (our result) to $\alpha_{\lambda}\sim-1.0$ (R12)
would have been easily explored.

###  5.4 Implications on the neutrino emission mechanism

TXS 0506+056 being an intrinsically bright AGN is in line with the
conventional view of high energy neutrino production in AGN jets (e.g., see
discussion in Ansoldi et al. 2018). The typical production channel of high
energy neutrinos is the decay of charged pions that were produced in
collisions between high energy protons in jets and ambient matters (Halzen
2017; Meszaros 2017), such as matter ($pp$ interaction) or low energy photons
($p\gamma$ interaction). In AGN jets, with their low-density plasma, the
$p\gamma$ interaction has been a more favored channel (Mannheim 1995) where
the target photon energies are in the range of UV to soft X-ray photons. Hot,
luminous accretion disks of intrinsically bright AGNs are an excellent source
for the UV to soft X-ray photons (Ansoldi et al. 2018). Hence, FSRQ-type
blazars have been favored to be high energy neutrino sources (Mannheim et al.
1992; Atoyan & Dermer 2001; Murase et al. 2014; Dermer et al. 2014), and our
observation is consistent with such an expectation.

Figure 10: Upper pannel shows the $\alpha_{\lambda}$ versus $m575$ band
magnitude plot and lower pannel shows the $\alpha_{\lambda}$ versus $m775$
band magnitude plot. The Pearson’s coefficient ($\rho$) and its p-value are
shown as well.

###  5.5 Discussion summary

The above analyses teach us that TXS 0506+056 is not an extraordinary outlier
from the blazar sequence. From the long-term spectral variability analysis, we
find that TXS 0506+056 varies as redder-when-brighter which is consistent with
the earlier P19 report that TXS 0506+056 is not a BL Lac. However, this was
limited to only two epochs. A more definitive answer requires data at multiple
long-term epochs. Figure 10 and the spectral slope measurement accuracy
achieved from our observation gives a great promise that spectral variability
study with MB data at more extended epochs could firmly establish if the
object is redder-when-brighter or bluer-when-brighter.

##  6 Conclusion

We conducted the time-series, MB observational study of the blazar, TXS
0506+056, to understand its nature as the host galaxy of the neutrino event
IceCube-170922A. Our observation reveals the variable nature of TXS 0506+056,
and the MB data obtained using WIT and SQUEAN are found to excellently trace
the optical SED shape of the blazar allowing us to measure the spectral index,
$\alpha_{\lambda}$ to be $\sim-0.6$ to $\sim-0.8$ to the accuracy of 2 % to 10
%. Our MB data were combined with published dataset and archival data to
produce quasi-simultaneous SED at two epochs (one active, another inactive).
The multi-wavelength SEDs show that $\nu_{peak}^{S}$ is of order of
$\nu_{peak}^{S}\sim 10^{14.5}$ Hz, which is consistent with previous results.
Considering that the $\nu_{peak}^{S}$ value is in NIR, NIR monitoring
observation can help better determine $\nu_{peak}^{S}$. However, the
comparison with the blazar sequence data in both gamma-ray and radio
luminosities shows that TXS 0506+056 is not an outlier of the blazar sequence.
The comparison of the 2012 data (inactive) and the 2017 data (active) shows a
flatter SED at 2012, possibly indicating the redder-when-brighter nature of
FSRQ for TXS 0506+056, but the spectral variability analysis using our data
show no clear spectral change as a function of the brightness. A more
extensive study using a larger set of data is desirable, using a longer time-
baseline and larger phase difference.

In conclusion, our study demonstrates that the MB photometry observation with
small and medium-sized telescopes can be a powerful way to study spectral
variability of many kinds of astrophysical objects such as neutrino sources,
AGNs, gravitational-wave sources, gamma-ray bursts, and supernovae, to name a
few (e.g., Abbott et al. 2017; Im et al. 2015, 2019; Kim et al. 2019a).
Additional NIR monitoring data will be also helpful for better determining the
spectral shape, especially $\nu_{peak}^{S}$ of blazars like TXS 0506+056.
Wide-field telescopes with MB filters, such as WIT with a field of view of 5.5
deg2, can be a great force for multi-messenger astronomy to search for and
characterize optical counterparts and host galaxies of neutrino and
gravitaional-wave sources (e.g., Abbott et al. 2017; Im et al. 2017) for which
initial positional estimates can be very uncertain ($\gg 1$ deg2).

We thank John Kuehne and the staffs at the McDonald Observatory for their
support during the installation and the operation of WIT. This work was
supported by the National Research Foundation of Korea (NRF) grant
(2020R1A2C3011091) by the Korean government (MSIT) and the Korea Astronomy and
Space Science Institute under the R&D program (Project No. 2020-1-600-05)
supervised by MSIT. S.S. acknowledges support from Basic Science Research
Program through the National Research Foundation of Korea (NRF) funded by the
Ministry of Education (No.2020R1A6A3A13069198). This paper includes data taken
at the McDonald Observatory of the University of Texas at Austin.

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##  Appendix A Photometry Information Tables

Table 5: TXS 0506+056 photometry with WIT

WIT |  |  |  |  |  |  |  |
---|---|---|---|---|---|---|---|---
Date | Weather | Filter | Seeing | Exposure time | Zeropoint (error) | Detection limit | mag | $\sigma_{m}$
2017-10-14 08:31 | clear | $m575$ | 4.06 | 150s$\times$4 | 18.409 (0.045) | 17.58 | 14.38 | 0.05
2017-10-14 08:51 | clear | $m625$ | 4.39 | 150s$\times$4 | 18.212 (0.036) | 17.37 | 14.35 | 0.04
2017-10-14 09:13 | clear | $m675$ | 4.84 | 150s$\times$4 | 17.800 (0.048) | 16.90 | 14.17 | 0.05
2017-10-14 09:25 | clear | $m725$ | 4.55 | 150s$\times$4 | 17.306 (0.041) | 16.58 | 14.07 | 0.04
2017-10-14 09:40 | clear | $m775$ | 4.95 | 150s$\times$4 | 16.759 (0.046) | 16.29 | 14.01 | 0.05
2017-10-17 07:35 | clear | $m575$ | 5.18 | 150s$\times$4 | 18.412 (0.042) | 17.34 | 14.28 | 0.04
2017-10-17 07:46 | clear | $m625$ | 5.09 | 150s$\times$4 | 18.195 (0.033) | 17.34 | 14.16 | 0.03
2017-10-17 07:56 | clear | $m675$ | 4.95 | 150s$\times$4 | 17.8073 (0.046) | 17.17 | 14.04 | 0.05
2017-10-17 08:07 | clear | $m725$ | 5.86 | 150s$\times$4 | 17.386 (0.032) | 16.66 | 13.94 | 0.04
2017-10-17 08:19 | clear | $m775$ | 5.64 | 150s$\times$4 | 16.768 (0.033) | 16.18 | 13.90 | 0.04
2017-10-21 09:47 | clear | $m575$ | 3.70 | 150s$\times$4 | 18.404 (0.050) | 17.78 | 14.55 | 0.05
2017-10-21 09:58 | clear | $m625$ | 4.12 | 150s$\times$4 | 18.177 (0.034) | 17.58 | 14.40 | 0.04
2017-10-21 10:10 | clear | $m675$ | 4.46 | 150s$\times$4 | 17.786 (0.049) | 17.19 | 14.26 | 0.05
2017-10-21 10:21 | clear | $m725$ | 3.99 | 150s$\times$4 | 17.312 (0.042) | 16.95 | 14.17 | 0.05
2017-10-21 10:33 | clear | $m775$ | 4.17 | 150s$\times$4 | 16.685 (0.045) | 16.39 | 14.03 | 0.05
2017-10-28 08:27 | clear | $m575$ | 3.62 | 150s$\times$5 | 18.388 (0.044) | 14.81 | 14.49 | 0.05
2017-10-28 08:54 | clear | $m625$ | 3.94 | 150s$\times$5 | 18.099 (0.026) | 15.20 | 14.46 | 0.03
2017-10-28 09:09 | clear | $m675$ | 4.20 | 150s$\times$5 | 17.770 (0.036) | 14.83 | 14.31 | 0.04
2017-10-28 09:24 | clear | $m725$ | 3.86 | 150s$\times$5 | 17.280 (0.029) | 14.44 | 14.22 | 0.03
2017-10-28 09:40 | clear | $m775$ | 4.14 | 150s$\times$5 | 16.688 (0.027) | 13.84 | 14.12 | 0.05
2017-10-29 07:49 | clear | $m575$ | 4.01 | 150s$\times$5 | 18.426 (0.045) | 17.91 | 14.60 | 0.04
2017-10-29 08:05 | clear | $m625$ | 4.38 | 150s$\times$5 | 18.203 (0.038) | 17.60 | 14.47 | 0.04
2017-10-29 08:20 | clear | $m675$ | 4.55 | 150s$\times$5 | 17.798 (0.051) | 17.34 | 14.29 | 0.05
2017-10-29 08:36 | clear | $m725$ | 4.25 | 150s$\times$5 | 17.360 (0.044) | 17.16 | 14.26 | 0.05
2017-10-29 08:51 | clear | $m775$ | 4.68 | 150s$\times$5 | 16.768 (0.049) | 16.62 | 14.12 | 0.05
2017-10-30 06:17 | clear | $m575$ | 6.05 | 150s$\times$4 | 18.298 (0.033) | 16.81 | 14.51 | 0.04
2017-10-30 06:30 | clear | $m625$ | 5.89 | 150s$\times$4 | 18.097 (0.020) | 16.91 | 14.37 | 0.02
2017-10-30 06:42 | clear | $m675$ | 5.22 | 150s$\times$4 | 17.686 (0.036) | 16.83 | 14.26 | 0.04
2017-10-30 06:53 | clear | $m725$ | 7.63 | 150s$\times$4 | 17.440 (0.009) | 16.30 | 14.19 | 0.02
2017-10-30 07:12 | clear | $m775$ | 6.53 | 150s$\times$4 | 16.756 (0.012) | 15.97 | 14.11 | 0.03
2017-11-03 05:58 | clear | $m575$ | 4.37 | 150s$\times$4 | 18.440 (0.045) | 16.27 | 14.50 | 0.05
2017-11-03 06:10 | clear | $m625$ | 4.69 | 150s$\times$4 | 18.201 (0.030) | 16.28 | 14.36 | 0.03
2017-11-03 06:22 | clear | $m675$ | 4.51 | 150s$\times$4 | 17.829 (0.044) | 16.35 | 14.22 | 0.05
2017-11-03 06:33 | clear | $m725$ | 4.65 | 150s$\times$4 | 17.376 (0.030) | 16.23 | 14.18 | 0.04
2017-11-03 06:45 | clear | $m775$ | 4.87 | 150s$\times$4 | 16.777 (0.047) | 15.92 | 14.02 | 0.05
2017-11-06 06:01 | clear | $m575$ | 6.64 | 150s$\times$4 | 18.488 (0.035) | 15.86 | 14.42 | 0.04
2017-11-06 06:16 | clear | $m625$ | 5.37 | 150s$\times$5 | 18.195 (0.026) | 16.16 | 14.31 | 0.03
2017-11-06 06:39 | clear | $m675$ | 5.80 | 150s$\times$5 | 17.850 (0.040) | 15.97 | 14.14 | 0.04
2017-11-06 07:00 | clear | $m725$ | 4.87 | 150s$\times$4 | 17.369 (0.039) | 15.93 | 14.11 | 0.04
2017-11-06 07:22 | clear | $m775$ | 5.07 | 150s$\times$5 | 16.774 (0.047) | 15.60 | 13.96 | 0.05

Note. — (1) - Date of observation (UT); (2) - Weather; (3) - Name of filter;
(4) - Seeing value ($\arcsec$); (5) - Exposure time (sec); (6) - Zeropoint
(mag); (7) - Point source detection limit at 5-$\sigma$ with exposure time and
zeropoint in same row (mag); (8) - Magnitude; (9) - Error of the magnitude

Table 6: TXS 0506+056 photometry with 2.1m telescope

2.1m telescope |  |  |  |  |  |  |  |
---|---|---|---|---|---|---|---|---
Date | Weather | Filter | Seeing | Exposure time | Zeropoint (error) | Detection limit | mag | $\sigma_{m}$
2017-10-14 09:07 | clear | $Y$ | 1.73 | 60s$\times$3 | 22.414 (0.067) | 19.55 | 13.54 | 0.07
2017-10-14 09:22 | clear | $i$ | 2.53 | 10s$\times$36 | 24.979 (0.012) | 20.72 | 13.97 | 0.01
2017-10-14 09:02 | clear | $m1025$ | 1.86 | 60s$\times$3 | 21.046 (0.032) | 18.61 | 13.51 | 0.03
2017-10-14 08:29 | clear | $m625$ | 1.49 | 30s$\times$5 | 23.087 (0.012) | 20.52 | 14.28 | 0.01
2017-10-14 08:32 | clear | $m675$ | 1.47 | 30s$\times$5 | 23.436 (0.015) | 20.74 | 14.14 | 0.02
2017-10-14 08:36 | clear | $m725$ | 1.51 | 15s$\times$12 | 23.675 (0.012) | 20.64 | 14.05 | 0.01
2017-10-14 08:40 | clear | $m775$ | 1.66 | 15s$\times$12 | 23.670 (0.021) | 20.49 | 13.95 | 0.02
2017-10-14 08:45 | clear | $m825$ | 1.63 | 15s$\times$12 | 23.642 (0.019) | 20.43 | 13.86 | 0.02
2017-10-14 08:49 | clear | $m875$ | 1.77 | 30s$\times$6 | 23.459 (0.056) | 20.44 | 13.75 | 0.06
2017-10-14 08:54 | clear | $m925s$ | 1.48 | 30s$\times$6 | 22.375 (0.053) | 19.30 | 13.64 | 0.05
2017-10-14 08:58 | clear | $m975$ | 1.66 | 60s$\times$3 | 22.113 (0.060) | 19.65 | 13.60 | 0.06
2017-10-14 09:29 | clear | $r$ | 2.15 | 15s$\times$12 | 24.318 (0.008) | 20.80 | 14.30 | 0.01
2017-10-14 09:12 | clear | $z$ | 1.31 | 15s$\times$12 | 23.708 (0.033) | 20.38 | 13.71 | 0.03
2017-10-16 08:54 | clear | $Y$ | 1.30 | 30s$\times$10 | 21.849 (0.046) | 19.29 | 13.42 | 0.05
2017-10-16 09:08 | clear | $i$ | 2.46 | 10s$\times$30 | 24.908 (0.047) | 20.47 | 13.92 | 0.05
2017-10-16 08:45 | clear | $m1025$ | 1.52 | 60s$\times$5 | 20.600 (0.020) | 18.55 | 13.42 | 0.02
2017-10-16 07:51 | clear | $m625$ | 2.52 | 60s$\times$5 | 23.056 (0.009) | 20.51 | 14.16 | 0.01
2017-10-16 07:58 | clear | $m675$ | 1.89 | 30s$\times$10 | 23.311 (0.010) | 20.69 | 14.02 | 0.01
2017-10-16 08:06 | clear | $m725$ | 1.71 | 15s$\times$20 | 23.595 (0.009) | 20.65 | 13.95 | 0.01
2017-10-16 08:12 | clear | $m775$ | 1.61 | 15s$\times$20 | 23.538 (0.035) | 20.51 | 13.83 | 0.04
2017-10-16 08:19 | clear | $m825$ | 1.58 | 15s$\times$20 | 23.373 (0.054) | 20.40 | 13.70 | 0.05
2017-10-16 08:28 | clear | $m875$ | 1.59 | 15s$\times$20 | 23.165 (0.046) | 20.11 | 13.61 | 0.05
2017-10-16 08:33 | clear | $m925s$ | 1.86 | 60s$\times$5 | 22.494 (0.044) | 19.38 | 13.52 | 0.04
2017-10-16 08:39 | clear | $m975$ | 1.44 | 30s$\times$10 | 21.802 (0.026) | 19.44 | 13.48 | 0.03
2017-10-16 09:15 | clear | $r$ | 1.84 | 10s$\times$30 | 23.994 (0.030) | 20.55 | 14.13 | 0.03
2017-10-16 09:00 | clear | $z$ | 1.30 | 10s$\times$30 | 23.327 (0.026) | 19.92 | 13.59 | 0.03
2017-10-18 11:53 | cloudy | $Y$ | 1.13 | 60s$\times$8 | 21.475 (0.059) | 19.27 | 13.41 | 0.06
2017-10-18 12:07 | cloudy | $i$ | 1.23 | 10s$\times$18 | 23.336 (0.034) | 18.72 | 13.85 | 0.03
2017-10-18 11:43 | cloudy | $m1025$ | 1.05 | 60s$\times$10 | 18.749 (0.089) | 17.34 | 13.39 | 0.09
2017-10-18 10:24 | cloudy | $m625$ | 1.27 | 60s$\times$5 | 19.307 (0.028) | 17.81 | 14.16 | 0.03
2017-10-18 10:31 | cloudy | $m675$ | 1.18 | 60s$\times$12 | 20.156 (0.038) | 18.89 | 14.05 | 0.04
2017-10-18 10:42 | cloudy | $m725$ | 1.09 | 60s$\times$5 | 19.284 (0.018) | 17.68 | 13.91 | 0.02
2017-10-18 10:53 | cloudy | $m775$ | 1.11 | 60s$\times$8 | 19.306 (0.028) | 17.72 | 13.83 | 0.03
2017-10-18 11:02 | cloudy | $m825$ | 1.46 | 60s$\times$10 | 18.948 (0.049) | 17.01 | 13.76 | 0.05
2017-10-18 11:15 | cloudy | $m875$ | 1.04 | 60s$\times$10 | 22.212 (0.069) | 20.03 | 13.58 | 0.07
2017-10-18 11:23 | cloudy | $m925s$ | 1.84 | 60s$\times$5 | 21.462 (0.037) | 18.75 | 13.52 | 0.04
2017-10-18 11:31 | cloudy | $m975$ | 1.03 | 60s$\times$10 | 20.042 (0.094) | 18.47 | 13.46 | 0.09
2017-10-18 12:11 | cloudy | $r$ | 1.87 | 30s$\times$10 | 22.270 (0.031) | 18.33 | 14.19 | 0.03
2017-10-18 12:01 | cloudy | $z$ | 1.30 | 15s$\times$12 | 22.833 (0.032) | 19.16 | 13.57 | 0.03

Note. — (1) - Date of observation (UT); (2) - Weather; (3) - Name of filter;
(4) - Seeing value ($\arcsec$); (5) - Exposure time (sec); (6) - Zeropoint
(mag); (7) - Point source detection limit at 5-$\sigma$ with exposure time and
zeropoint in same row (mag); (8) - Magnitude; (9) - Error of the magnitude