# Optical Spectropolarimetric Variability Properties in Blazars PKS 0637–75
and PKS 1510–089

Stephanie A. Podjed Department of Physics and Astronomy, Dartmouth College,
6127 Wilder Laboratory, Hanover, NH 03755-3528  Ryan C. Hickox Department of
Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH
03755-3528  Jedidah C. Isler The SeRCH Foundation, Inc, PO Box 442335, Fort
Washington, MD 20749-2335  Markus Böttcher Centre for Space Research, North-
West University, Potchefstroom 2520, South Africa  Hester M. Schutte Centre
for Space Research, North-West University, Potchefstroom 2520, South Africa
Stephanie Podjed<EMAIL_ADDRESS>
(Received 2024 February 8; Revised 2024 April 16; Accepted 2024 April 18)

###### Abstract

Spectropolarimetry is a powerful tool to investigate the central regions of
active galactic nuclei (AGNs), as polarization signatures are key to probing
magnetic field structure, evolution, and the physics of particle acceleration
in jets. Optical linear polarization of blazars is typically greater than a
few percent, indicating the emission is dominated by non-thermal synchrotron
radiation, while polarization less than a few percent is common for other type
1 AGNs. We present a spectropolarimetric study of PKS 0637–75 and PKS 1510–089
to determine how the head-on orientation of a jet and dominant emission
processes influence polarimetric variations in the broad lines and continuum.
Observations were obtained bi-weekly from the Robert Stobie Spectrograph on
the Southern African Large Telescope. Variability in the continuum
polarization is detected for both PKS 0637–75 and PKS 1510–089, with a total
average level of 2.5% $\pm$ 0.1% and 7.5% $\pm$ 0.1%, respectively. There is
no clear polarization in the broad Balmer emission lines and weak polarization
in Mg II as the average level across all observations is 0.2% $\pm$ 0.1% for
H$\beta$, 0.2% $\pm$ 0.3% for H$\gamma$, and 0.6% $\pm$ 0.2% for Mg II. We
find that polarization measurements confirm the conclusions drawn from
spectral energy distribution modeling of the disk-jet contributions to the
emission, as optical polarization and time variability for PKS 0637–75 is
shown to be dominated by accretion disk emission while that of PKS 1510–089 is
due to both disk and jet emission, with greater jet contribution during
flaring states.

active galaxies: blazars — FSRQs: individual: PKS 0637–75, PKS 1510–089 —
galaxies: jets — polarization: optical; spectropolarimetry

††journal: The Astrophysical Journal††software: polsalt (Crawford et al.,
2016), lmfit (Newville et al., 2020), Astropy (Astropy Collaboration et al.,
2013, 2018, 2022), IRAF (Tody, 1986), PHEW (Núñez et al., 2022)

2024 June 20

##  1 Introduction

Blazars are a class of radio-loud (jet-dominated) active galactic nuclei
(AGNs) whose relativistic jet is oriented at small angles with respect to our
line of sight (Antonucci, 1993; Urry & Padovani, 1995; Padovani et al., 2017).
The bulk outflow of these charged particles, in addition to the small viewing
angles, induces Doppler beaming along the direction of the outflow which makes
the jet appear brighter and the variability timescale shorter in the observed
frame. Blazars can be divided into two subclasses based on optical spectral
features: Flat Spectrum Radio Quasars (FSRQs) and BL Lacertae objects (BL
Lacs). The spectrum of an FSRQ generally has prominent broad and narrow
emission lines, whereas that of a BL Lac object has absent or weak (EW $<$ 5
Å) emission lines (Urry & Padovani, 1995).

Blazar broadband emission experiences rapid variability and is generally jet-
dominated. During quiescent periods the contribution of thermal emission from
an optically-thick accretion disk can be detected, making blazars an ideal
laboratory to study connections between the nonthermal jet and thermal disk
components. Radio through optical, and more recently through X-ray with the
Imaging X-ray Polarimetry Explorer (Weisskopf et al., 2016; Di Gesu et al.,
2022; Liodakis et al., 2022; Peirson et al., 2023; Middei et al., 2023a, b;
Ehlert et al., 2023), emission of blazars is also characterized by a high
degree of polarization, with polarization percentages ranging from a few to
tens of percent, while their non-beamed AGN counterparts usually show
polarization levels of less than a few percent. The optical emission of
blazars is often dominated by synchrotron radiation from the relativistic jet
and is well-known to be polarized, with a theoretical maximum synchrotron
polarization percentage attainable ranging from 69% to 75% (Rybicki &
Lightman, 1986). The polarization percentage (P) and electric vector position
angle are both often highly variable, even on intranight timescales
(D’Arcangelo et al., 2007; Zhang et al., 2014).

Polarimetry as a complement to spectroscopy is a useful tool to investigate
the central regions of AGNs that are unresolved by direct observations since
polarization is sensitive to the geometry and magnetic fields of the
scattering region. Optical spectropolarimetric studies of type 2 AGNs
(Antonucci & Miller, 1985; Miller et al., 1991) provided the basis for the
phenomenological geometric unification scheme of active galactic nuclei
(Antonucci, 1993; Urry & Padovani, 1995; Netzer, 2015). Though the optical
light coming from the central engine and broad-line region (BLR) in these
objects is thought to be geometrically blocked by an obscuring torus, it can
escape perpendicularly along the unobstructed direction by scattering off of
dust or free electrons located above or below the central region (Antonucci,
1983; Lira et al., 2020). This scattering induces linear polarization in the
emission, making the once-hidden broad emission lines observable. Similar
studies of type 1 AGNs, objects where the BLR is directly observable, found
that the optical polarization properties are not consistent with this polar
scattering; instead, the PA is typically aligned with the radio axis/axis of
symmetry, suggesting that the scattering material is equatorially located
(Antonucci, 1983; Smith et al., 2002, 2004, 2005).

The mechanisms responsible for polarization in the continuum and broad lines
of AGNs can be different, and include the intrinsic geometry of the emitting
region and the geometry of the scattering region (Afanasiev et al., 2014), as
polarization due to radiative transfer alone cannot account for any
polarization levels in the BLR (Afanasiev et al., 2019). Additionally in
blazars, the thermal components, i.e., the AD, BLR, and dusty torus, are
expected to be unpolarized and in the optical spectrum will be characterized
by a decrease in the degree of polarization in spectropolarimetric
observations (Böttcher et al., 2017).

For blazars, which generally have a face-on geometry and unobscured view of
the accretion flow, spectropolarimetry is a diagnostic tool that can be used
to better understand the interplay between the nonthermal jet and thermal disk
components. These findings can be used to comment on the presence and location
of scattering regions, how blazar spectra respond to accretion disk dominated
emission or synchrotron dominated emission, and to better constrain emission
mechanism models when included in spectral energy distribution (SED) studies
(Böttcher et al., 2017; Schutte et al., 2022).

FSRQs are extremely well studied in time series observations across a range of
wavelengths, but a relative paucity of spectropolarimetric observations of
blazars exist (e.g., Böttcher et al., 2017; Patiño-Álvarez et al., 2018;
Schutte et al., 2022; Aharonian et al., 2023), with such studies becoming
prominent only recently. Since we aim to increase the characterization of
blazar spectropolarimetric properties, we focus on FSRQs as they generally
show prominent quasar-like emission lines in contrast to BL Lacs. Particularly
in this work, from our sample of seven Southern Hemisphere gamma-ray active
and quiescent blazars, we present the optical linear polarization variability
properties for the two objects displaying emission lines: PKS 0637–75 (z =
0.653, J2000 RA = 06h35m46s.5079, Dec = -75d16m16s.814 as given in the
NASA/IPAC Extragalactic Database, NED111https://ned.ipac.caltech.edu/) and PKS
1510–089 (z = 0.36, J2000 RA = 15h12m50s.53, Dec = -09d05m59s.82 as given in
NED). We focus on the Mg II $\lambda$2798 emission line, a typically strong
low ionization line seen in the optical-UV band of AGNs, in PKS 0637–75 and on
the broad H$\gamma$ and H$\beta$ emission lines in PKS 1510–089 .

In this paper, the observations and data analysis of PKS 0637–75 and PKS
1510–089 are described in Section 2. Results of our polarization variability
study are presented in Section 3 and discussed in Section 4. Our main
conclusions are summarized in Section 5.

##  2 Observations and Data Reduction

###  2.1 Optical Spectropolarimetry

Optical spectropolarimetrc observations have been performed with the Robert
Stobie Spectrograph (RSS; Burgh et al., 2003; Kobulnicky et al., 2003) on the
Southern African Large Telescope (SALT; Buckley et al., 2006), a 10 m class
telescope located at the South African Astronomical Observatory near
Sutherland, South Africa. The effective area of the telescope is constantly
changing as a part of the SALT design, so accurate absolute flux calibration
is not available. The data were collected using the RSS in the
spectropolarimetry LINEAR mode with slit-width of 1$\arcsec$.25 between 2019
February and 2020 March for PKS 0637–75 and between 2019 March and 2021 August
for PKS 1510–089.

Figure 1: Optical SALT nonpolarized (left) and polarized (right) nonnormalized
spectra of PKS 0637–75 (a) and PKS 1510–089 (b) obtained with the medium-
resolution RSS. Identified emission and masked telluric ($\oplus$) lines are
labeled. The continuum wavelength range used for analysis is highlighted. Gaps
around 5500 and 6550 Å correspond to the gaps in the CCD as it is a mosaic of
three individual chips. Vertical gray dotted–dashed lines in the polarized
spectra indicate the location of the main emission lines studied here: Mg II,
H$\gamma$, and H$\beta$. Emission features in the nonpolarized emission are
not detected in polarized light.

The RSS CCD detector consists of a mosaic of three CCDs with a total size of
6362 $\times$ 4102 pixels, with a single pixel size of $\sim$15 $\mu$m,
corresponding to a spatial resolution of 0$\arcsec$.13 per pixel. We used the
2 $\times$ 4 binning, faint gain, slow readout mode. The mean gain of the
mosaic is 1.7 ADU/electron and the read-out noise is typically around 2.48
electrons. We used the volume phase holographic (VPH) PG0900 grating with a
grating angle of 15∘.88, which gives the observed wavelength coverage of 4500
Å – 7500 Å. The average spectral resolving power is R = 1300. Order blocking
was done with the UV PC03850 filter. Each spectrum (flux, polarization degree,
and polarization angle as a function of wavelength) was obtained through four
total exposures per spectrum, one for each of the half-wave plate orientations
(0∘, 45∘, 22.5∘, and 67.5∘) necessary to obtain linear polarization data. We
began with an exposure time of 325 seconds that we reduced to 250 seconds for
each subsequent observation after the first three. Each observing block
consisted of flat-field images, slit acquisition images, the four science
exposures, and an exposure of the calibration Xe lamp. See Figures 1, 2, and 3
for the flux, average polarization, and average PA variability respectively.

Figure 2: Average polarization levels for PKS 0637–75 (a) and PKS 1510–089
(b); dotted–dashed gray lines mark the emission lines Mg II, H$\gamma$, and
H$\beta$. Depolarization associated with the H$\gamma$ and H$\beta$ emission
lines are noticeable in all observations for PKS 1510–809. Figure 3: Average
polarization angle variability across wavelength and time of PKS 0637–75 (a)
and PKS 1510–089 (b); dotted–dashed gray lines mark the emission lines Mg II,
H$\gamma$, and H$\beta$. No PA swing across emission lines is seen for either
object.

###  2.2 Optical and Gamma-Ray Photometry

We supplement our spectropolarimetric observations with g-band photometry from
the Ohio State All-Sky Automated Survey for Supernovae (ASAS-
SN222http://www.astronomy.ohio-state.edu/~assassin/index.shtml) project
(Shappee et al., 2014; Kochanek et al., 2017), in order to monitor any
continuum changes. Thermal continuum emission in the ultraviolet and optical
is thought to originate within the AD (Perlman et al., 2008) and it is
possible to have nonthermal continuum emission originating from synchrotron
emission from the jet (see panel (a) of Figures 4 and 5). Additionally, we use
the weekly binned 100–300,000 MeV Fermi Large Area Telescope (Fermi-LAT) light
curve data for continually monitored
sources333https://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/ to monitor
the jet behavior. After converting from mission elapsed time to MJD, we
trimmed the data to cover a date range similar to that of our SALT data for
PKS 1510–089 (panel (b) of Figure 5).

Figure 4: PKS 0637–75 ASAS-SN photometric g-band flux corresponding to the
period of our SALT spectropolarimetric study. Upper limits were removed for
plotting purposes. Vertical gray dashed lines represent the MJD of specific
nights of SALT observations. Panels (b) and (c) are the average continuum
polarization and PA measurements respectively. The lack of strong optical
flares (almost constant optical flux state) as well as low polarization levels
during the epoch of observation may be indicating disk dominance of the
emission and generally low to no disk activity in PKS 0637–75. Figure 5: PKS
1510–089 ASAS-SN photometric g-band flux (a) and Fermi gamma-ray flux (b)
corresponding to the period of obtained SALT spectropolarimetric data. Upper
limits were removed for plotting purposes. Vertical gray dashed lines
represent the MJD of specific nights of SALT observations. Panels (c) and (d)
are the average continuum polarization and PA SALT measurements from this work
and from Aharonian et al. (2023). The noticeable optical flare around MJD
59400 with high levels of polarization are indicating the jet dominance of the
emission even when we don’t have a gamma-ray flare. Between 59260 to 59900
MJD, we can see that our SALT polarization and PA observations follow the
trends found in Aharonian et al. (2023) and provide more detail to the drop in
polarization of PKS 1510–089 after 2021 July 18 (MJD 59413; vertical gray
loosely dashed line) to an almost constant value of 2%–3%.

###  2.3 Data Reduction

Data analysis was performed using the graphical user interface (GUI) of the
polsalt444https://github.com/saltastro/polsalt reduction pipeline (Crawford et
al., 2016) for Python 2.7. reducepoldataGUI.py is a step-wise execution for
reducing RSS spectropolarimetric data. It begins with raw image reductions,
where basic CCD reduction techniques are undertaken including overscan
subtraction, gain and cross talk correction, and amplifier mosaicing. Once
this is finished, wavelength calibration is done for the two beams of the
beamsplitter using the IRAF specidentify interface to identify the emission
lines from the calibration arc images; cosmic ray rejection happens here as
well. Next, in each image the beams are corrected for beamsplitter distortion
and tilt, and the sky and target spectrum is extracted versus wavelength. Raw
and final Stokes calculations are completed for the polarimetry reduction. In
the raw Stokes calculation, wave-plate position pairs are identified and
together result in linear polarization signal swapping between the O and E
beams. The “raw Stokes” files contain unnormalized I and S plane data, with
the degree of polarization being S/I. In the final Stokes calculation, the
full polarization pattern is evaluated to determine Q and U. Finally,
polarimetric zero point, wave-plate efficiency, and axis calibrations are
applied to give final Stokes parameters. Once all of these reduction steps are
accomplished, the GUI provides an interactive results visualization window
showing plots of the intensity, linear polarization percentage, equatorial
position angle, Stokes Q or Stokes U behavior.

In analyzing the SALT spectropolarimetric data we take a weighted average of
the polarization (P) PA data in 50 Å wide bins to investigate any polarization
variability associated with each object in its continuum and broad emission
lines. Error associated with these is the standard error on the weighted mean;
see equations directly below, where x is the parameter of which we find the
weighted average:

$\bar{x}=\frac{\Sigma\>x_{i}/\sigma_{x_{i}}^{2}}{\Sigma\>1/\sigma_{x_{i}}^{2}}\>,\>\>\>\sigma_{\bar{x}}=\sqrt{\frac{1}{\Sigma\>1/\sigma_{x_{i}}^{2}}}$
(1)

Continuum polarization measurements were made between 5650 Å and 6260 Å, with
the telluric Na I absorption feature around 5900 Å masked, for PKS 0637–75,
and between 4480 Å and 5090 Å for PKS 1510–089 due to the lack of any other
prominent features within these regions. An example of the total flux spectra,
Q and U normalized Stokes parameters, polarized flux, polarization percentage
and PA measurements are shown in Figure 6 for PKS 0637–75 (a) and PKS 1510–089
(b).

After the standard CCD reduction procedures were carried out, we continuum-
normalized the spectra by dividing out the modeled continuum from the overall
spectra using a polynomial of degree 2, as it provided the best overall fit to
our data. In doing so, the chip gaps, emission lines, and any noticeable
cosmic rays missed from the reduction pipeline were masked so as to not
negatively influence the fit.

Figure 6: Example of the total flux spectra, Q and U normalized Stokes
parameters, polarized flux, polarization percentage and PA measurements for an
arbitrary observation of PKS 0637–75 (a) and PKS 1510–089 (b). Vertical gray
dotted–dashed lines mark Mg II, H$\gamma$, and H$\beta$.

In PKS 0637–75, we concentrate on modeling a relatively narrow band between
4500 Å and 5000 Å in the observed frame. The spectrum consists of the
continuum, Fe II pseudocontinuum (Bruhweiler & Verner, 2008; Kovačević et al.,
2010), and the Mg II emission line. In PKS 1510–089, we concentrate on
modeling the two main broad emission lines, H$\gamma$ and H$\beta$, between
5800 Å and 6800 Å in the observed frame. The Mg II line is treated as a
singlet and for all three broad emission lines the kinematic shape is modeled
as a single Gaussian using the lmfit package to minimize the $\chi^{2}$
statistic (Newville et al., 2020) and obtain the Gaussian parameters plus
error used for further analysis.

As a first step to determine if the broad emission lines of our two blazars
show variability in polarization levels, we checked if the lines can be seen
in polarized light. To do so, we took the nonnormalized spectral intensities,
I, and multiplied them by the polarization percentage, P, to obtain polarized
spectra, P $\times$ I. To increase the signal-to-noise ratio, we bin the full
spectra by 50 Å wide bins (Figure 1, right). After this visual inspection, to
further constrain an upper limit of the polarization percentage for the
emission lines in this study, $P_{L}$, we create a simple model where the line
polarization is some fraction of the continuum polarization, $P_{C}$, it is
sitting on (Equation (2)):

$P_{L}=P_{C}\>\frac{A}{R}$ (2)

We assume the emission line retains its same shape in polarized light as was
displayed in the nonpolarized spectra. Under this assumption the fraction A/R
compares the amplitude of the Gaussian used to fit where an emission line
would be in normalized polarized spectra (A) to the amplitude of a Gaussian
used to fit the emission line in normalized nonpolarized spectra (R), keeping
the centroid and $\sigma$ of the Gaussian fixed to what their values were in
the non-polarized emission line fit. Standard error propagation was followed
to obtain the 1$\sigma$ error values. Negative (nonphysical) polarization
values may be returned in this model when a trough is encountered instead of a
peak near the emission line position in the polarized spectra.

We measure the equivalent width (EW) of Mg II, H$\gamma$, and H$\beta$ by
using the code PHEW (PytHon Equivalent Widths; Núñez et al. 2022). Within
PHEW, we use EW.py to calculate the EW of an emission or absorption line for a
given spectrum using PySpecKit, which takes 13 input parameters to find a
best-fit EW value and its associated error via Monte Carlo (MC) iterations.
Parameters updated for each emission line in the equivalent_width function
include: spec, a list of the spectrum that includes the wavelength, flux, and
flux error of the spectrum; bandloc, the value specifying the central location
of the spectral line to be measured in Å; xmin,xmax, which specify the
wavelength space region of interest around the emission line;
exclude_min,exclude_max values that specify the start and stop wavelength
range of the emission line; n, the number of MC iterations to go through; and
blorder, the order of the polynomial used to fit the pseudocontinuum. For all
three emission lines studied here, n and blorder used were 500 and 2
respectively. For Mg II we used 4632, 4489, 4840, 4590, and 4720 Å for
bandloc, xmin,xmax, and exclude_min,exclude_max respectively. We do not use
the center value output from the lmfit single-Gaussian fitting for the bandloc
values here as they do not create an accurate fit to the emission line when
using PHEW to get the EW values; the Gaussian has some additional structure,
so the fit is off from the actual centroid location by tens of angstroms in
each instance. For H$\gamma$ we used the center value output from the lmfit
Gaussian fit for each observation as the bandloc value, with 5800, 6100, 5850,
5970 Å for xmin,xmax, and exclude_min,exclude_max respectively. For H$\beta$,
we also used the center value output from the lmfit Gaussian fit for each
observation as the bandloc value, with 6400, 6800, 6550, 6670 Å for xmin,xmax,
and exclude_min,exclude_max respectively. Tables 1 and 2 give the calculated
EW and errors for Mg II and H$\gamma$ and H$\beta$. Generally, the EW of the
Balmer emission lines follow an inverse relationship to the ASAS-SN g-flux,
showing a decrease as the flux increases.

Table 1: PKS 0637–75 Mg II EW Measurements and Associated Error

|  | Equivalent Width
---|---|---
|  | (Å)
Date | MJD | Mg II
20190223 | 58537 | 21.4 $\pm$ 0.6
20190301 | 58543 | 21.4 $\pm$ 0.6
20190304 | 58546 | 21.5 $\pm$ 0.6
20191105 | 58792 | 20.3 $\pm$ 0.8
20191130 | 58817 | 20.2 $\pm$ 0.8
20191221 | 58838 | 18.8 $\pm$ 0.8
20191230 | 58847 | 20.2 $\pm$ 1.0
20200114 | 58862 | 19.0 $\pm$ 1.0
20200125 | 58873 | 19.4 $\pm$ 0.9
20201213 | 59196 | 21.7 $\pm$ 0.7
20210107 | 59221 | 4.7 $\pm$ 0.1
20210131 | 59245 | 20.9 $\pm$ 0.7
20210209 | 59254 | 21.6 $\pm$ 0.6
20210304 | 59277 | 23.1 $\pm$ 0.9

Note. — Absolute value of EW measurements are shown.

Table 2: PKS 1510–089 H$\gamma$ and H$\beta$ EW Measurements and associated
Error

|  | Equivalent Width
---|---|---
|  | (Å)
Date | MJD | H$\gamma$ | H$\beta$
20190306 | 58548 | 17.9 $\pm$ 0.6 | …
20190413 | 58586 | 9.1 $\pm$ 0.3 | …
20200319 | 58927 | 15.4 $\pm$ 0.5 | …
20210208 | 59253 | 22.2 $\pm$ 0.8 | 67.2 $\pm$ 0.9
20210407 | 59311 | 17.0 $\pm$ 0.6 | 47.3 $\pm$ 0.5
20210416 | 59320 | 22.8 $\pm$ 0.7 | 60.7 $\pm$ 0.7
20210506 | 59340 | 15.2 $\pm$ 0.6 | 40.8 $\pm$ 0.5
20210510 | 59344 | 8.3 $\pm$ 0.3 | 23.3 $\pm$ 0.3
20210512 | 59346 | 11.4 $\pm$ 0.5 | 29.5 $\pm$ 0.4
20210517 | 59351 | 19.6 $\pm$ 0.6 | 50.7 $\pm$ 0.6
20210530 | 59364 | 11.4 $\pm$ 0.6 | 29.8 $\pm$ 0.5
20210606 | 59371 | 8.2 $\pm$ 0.4 | 22.5 $\pm$ 0.4
20210612 | 59377 | 3.0 $\pm$ 0.7 | 13.9 $\pm$ 0.5
20210707 | 59402 | 10.2 $\pm$ 0.5 | 23.2 $\pm$ 0.4
20210711 | 59406 | 5.2 $\pm$ 0.4 | 12.1 $\pm$ 0.3
20210714 | 59409 | 5.1 $\pm$ 0.3 | 13.4 $\pm$ 0.2
20210731 | 59426 | 22.0 $\pm$ 0.7 | 55.5 $\pm$ 0.7
20210804 | 59430 | 18.6 $\pm$ 0.7 | 54.5 $\pm$ 0.6
20210824 | 59450 | 27.1 $\pm$ 0.8 | 66.8 $\pm$ 0.7

Note. — Absolute value of EW measurements are shown. The ellipses (…) for the
first three observations are due to the H$\beta$ emission line being on a CCD
chip gap.

##  3 Results

###  3.1 Level of Continuum Polarization

Bright and transient sources that have shown flares during the LAT mission and
reach the minimum gamma-ray flux threshold of 1$\times$10-6 cm-2 s-1 are added
to the Fermi monitored source list which provides daily and weekly flux values
of such objects of interest. PKS 1510–089 has crossed the abovementioned flux
threshold for continual monitoring, representing one of our gamma-ray loud
blazars. PKS 0637–75 on the other hand, while a Fermi blazar in that it has
been gamma-ray detected by Fermi and is included in all of the annual Fermi
catalogues, has not reached the minimum threshold, thus representing one of
our gamma-ray quiescent blazars.

For PKS 0637–75, the continuum region was defined between 5650 Å and 6260 Å in
the observer’s frame to avoid the underlying Fe II at the wavelength of Mg II,
as well as any other spectral features. The level of continuum polarization
ranges from a minimum of 1.4% $\pm$ 0.1% to a maximum of 4.0% $\pm$ 0.2%, with
an average value of 2.5% $\pm$ 0.1%. For PKS 1510–089, the continuum region
was defined between 4200 Å and 4900 Å for the first three observations and
between 4480 Å and 5090 Å for the remaining observations, as H$\beta$ fell on
a chip gap for the first few observing windows. The level of continuum
polarization ranges from a minimum of 1.8% $\pm$ 0.1% to a maximum of 21.4%
$\pm$ 0.1%, with an average value of 7.5% $\pm$ 0.1%.

Continuum polarization for PKS 1510–089 shows stronger variability than for
PKS 0637–75. In particular, the highest levels of polarization ($\sim$17%-21%)
reached by PKS 1510–089 happens between MJDs 59406 and 59409, which
corresponds to an optical flare as seen with ASAS-SN (see (a) and (c) of
Figure 5). We are able to clearly detect a change in the dominant emission
processes before (MJD 59253, purple) and near (MJD 59409, blue) the optical
flaring period (Figure 7); the emission mechanism changes from thermally
dominated, low polarization to nonthermal synchrotron dominance with high
levels of polarization detected and smaller EWs of both emission lines.

Figure 7: Intensity and polarization variability during optical quiescent
(59253) and flaring (59409) periods of PKS 1510–089. We are able to observe
the change in thermal to nonthermal emission dominance in the transition from
the quiescent to flaring state, with a noticeable decrease in H$\gamma$ and
H$\beta$ line strength compared to the continuum and an approximately
4$\times$ increase in overall polarization.

In both blazars, there is modest evidence for a wavelength dependence of the
polarization, with the polarization fraction appearing to rise at redder or
bluer wavelengths depending on the date of observation. For those with an
increased polarization fraction at redder wavelengths, the continuum is
contaminated by bluer unpolarized disk and BLR emission, causing the dilution
of the polarization signal at the shorter wavelengths. The dominance of jet or
disk emission is reflected in this wavelength dependence, particularly in the
quiescent and flaring states of PKS 1510–089 (Figure 7). When the synchrotron
jet emission is enhanced compared to the thermal AD, polarization levels are
high and the wavelength dependence is more noticeable. When the thermal
emission is more dominant, as is seen in PKS 0637–75 and the quiescent state
of PKS 1510–089, the polarization fraction is low and the wavelength
dependence is marginally detected.

Table 3: PKS 0637–75 Continuum and Mg II Polarization Percentage and
Associated 1-$\sigma$ Error

|  | Polarization
---|---|---
|  | (%)
Date | MJD | Continuum | Mg II
20190223 | 58537 | 1.5 $\pm$ 0.1 | 0.3 $\pm$ 0.7
20190301 | 58543 | 1.5 $\pm$ 0.1 | 0.4 $\pm$ 0.5
20190304 | 58546 | 1.4 $\pm$ 0.1 | 0.6 $\pm$ 0.4
20191105 | 58792 | 2.8 $\pm$ 0.1 | 1.4 $\pm$ 1.3
20191130 | 58817 | 2.8 $\pm$ 0.1 | 0.7 $\pm$ 1.2
20191221 | 58838 | 2.9 $\pm$ 0.1 | 2.5 $\pm$ 0.9
20191230 | 58847 | 3.1 $\pm$ 0.1 | < 1.0∗
20200114 | 58862 | 4.0 $\pm$ 0.2 | 2.1 $\pm$ 1.0
20200125 | 58873 | 3.2 $\pm$ 0.1 | 1.3 $\pm$ 1.3
20201213 | 59196 | 2.3 $\pm$ 0.1 | 2.3 $\pm$ 0.9
20210107 | 59221 | 2.3 $\pm$ 0.1 | 0.6 $\pm$ 0.7
20210131 | 59245 | 2.3 $\pm$ 0.1 | 1.4 $\pm$ 1.2
20210209 | 59254 | 2.2 $\pm$ 0.1 | < 0.2∗
20210304 | 59277 | 2.9 $\pm$ 0.1 | < 0.5∗

Note. — Polarization values with an * denote upper limit values.

Table 4: PKS 1510–089 Continuum and Emission Line Polarization Percentage and
1-$\sigma$ Error

|  | Polarization
---|---|---
|  | (%)
Date | MJD | Continuum | H$\gamma$ | H$\beta$
20190306 | 58548 | 2.1 $\pm$ 0.1 | 0.1 $\pm$ 1.0 | < 0.1∗
20190413 | 58586 | 4.5 $\pm$ 0.1 | < 1.2∗ | 0.4 $\pm$ 0.5
20200319 | 58927 | 7.3 $\pm$ 0.1 | < 0.7∗ | 0.7 $\pm$ 0.5
20210208 | 59253 | 4.5 $\pm$ 0.1 | < 1.4∗ | 0.3 $\pm$ 0.9
20210407 | 59311 | 6.0 $\pm$ 0.1 | < 0.4∗ | < 0.1∗
20210416 | 59320 | 3.0 $\pm$ 0.1 | 0.2 $\pm$ 1.8 | 0.9 $\pm$ 0.6
20210506 | 59340 | 5.0 $\pm$ 0.1 | < 0.2∗ | 0.6 $\pm$ 0.6
20210510 | 59344 | 13.2 $\pm$ 0.1 | 0∗∗ | 0.9 $\pm$ 0.8
20210512 | 59346 | 14.8 $\pm$ 0.1 | 0∗∗ | 0.8 $\pm$ 0.8
20210517 | 59351 | 3.2 $\pm$ 0.1 | 2.1 $\pm$ 1.3 | 0.4 $\pm$ 0.4
20210530 | 59364 | 6.5 $\pm$ 0.1 | 2.2 $\pm$ 1.3 | 0.1 $\pm$ 0.9
20210606 | 59371 | 1.8 $\pm$ 0.1 | 0.3 $\pm$ 1.9 | 1.4 $\pm$ 1.0
20210612 | 59377 | 12.2 $\pm$ 0.1 | 0∗∗ | 3.1 $\pm$ 4.0
20210707 | 59402 | 9.1 $\pm$ 0.1 | 0.3 $\pm$ 1.1 | 0.2 $\pm$ 0.7
20210711 | 59406 | 17.0 $\pm$ 0.1 | 0∗∗ | < 0.5∗
20210714 | 59409 | 21.4 $\pm$ 0.1 | 0∗∗ | 1.8 $\pm$ 1.5
20210731 | 59426 | 3.2 $\pm$ 0.1 | 4.3 $\pm$ 1.1 | 0.6 $\pm$ 0.5
20210804 | 59430 | 3.2 $\pm$ 0.1 | 5.8 $\pm$ 6.8 | < 0.7∗
20210824 | 59450 | 3.4 $\pm$ 0.1 | 0.2 $\pm$ 1.3 | 0.5 $\pm$ 0.7

Note. — Polarization values with an * denote upper limit values, and those
with ** denote the $1\sigma$ upper limit was a (nonphysical) negative value,
so we mark the line as not polarized. The measured values with uncertainties
are shown in Figure 8.

###  3.2 Level of Emission Line Polarization

Figure 8: (a) PKS 0637–75 continuum and Mg II average polarization percentage
as a function of time. Variability in continuum polarization is detected
across the full observation period, with slight polarization of the emission
line potentially detected. (b) PKS 1510–089 continuum, H$\gamma$ and H$\beta$
average polarization percentage as a function of time. This shows a more
polarized continuum as compared to PKS 0637–75, and while the continuum
polarization increases and varies over the full observation period, the
emission line polarization values are nonpolarized in all observations.

From our initial visual inspection of P$\times$I in Figure 1 (right), neither
PKS 0637–75 nor PKS 1510–089 display noticeable features at the wavelengths of
the emission lines Mg II, H$\gamma$, or H$\beta$. In Figure 2, there is an
observable depolarization dip in the average polarization levels most notably
at the wavelengths corresponding to the centroid of the H$\gamma$ and H$\beta$
lines, marked with the vertical dotted-dashed gray lines. Moving to our simple
model used to find $P_{L}$ for each emission line, we find the following
polarization level limits: For Mg II, the minimum and maximum $P_{L}$ values
are <0.2% and 2.5% $\pm$ 0.9%. H$\gamma$ is consistent with zero polarization
throughout, with a maximum $P_{L}$ value of 5.8% $\pm$ 6.8%. Similarly, the
minimum and maximum $P_{L}$ values for H$\beta$ are <0.5% and 3.1% $\pm$ 4.0%.
Mg II shows a weak level of polarization while the calculated line
polarization percentages are almost all identically similar to zero within
their 1$\sigma$ error bars for the Balmer lines, i.e. these broad emission
lines are not polarized. Tables 3 and 4 give the calculated continuum and
emission line polarization for PKS 0637–75 and PKS 1510–089 respectively; see
Figure 8 for visual representation.

###  3.3 Polarization Angle

Table 5 and Figure 3 show the variation with time in the PA for both blazars.
PKS 1510–089 shows a range of values between 0∘ and 175∘ and PKS 0637–75 has a
few distinct clumps of values at around 0∘–30∘, 80∘ and 150∘–200∘. The
continuum polarization angle of PKS 0637–75 ranges from -3∘.3 $\pm$ 1∘.2 to
190∘.4 $\pm$ 1∘.0, with an average value of 127∘.8 $\pm$ 1∘.0. For PKS
1510–089, the continuum polarization angle ranges from -1∘.2 $\pm$ 0∘.7 to
180∘.5 $\pm$ 0∘.8, with an average value of 95∘.9 $\pm$ 0∘.5.

Near the time of enhanced optical flux between MJDs 59375 and 59415 for PKS
1510–089, the total average polarization percentage shows elevated levels and
the PA changes by about 120∘. During this time, the H$\beta$ and H$\gamma$
line polarization is nominally 0% within their errors.

Table 5: Average Continuum Polarization Angle and Associated 1-$\sigma$ Error for Both PKS 0637–75 and PKS 1510–089 |  | Polarization Angle
---|---|---
Date | MJD | (deg)
_PKS 0637–75_
20190223 | 58537 | -3.3 $\pm$ 1.2
20190301 | 58543 | 16.9 $\pm$ 0.9
20190304 | 58546 | 168.0 $\pm$ 1.1
20191105 | 58792 | 170.8 $\pm$ 1.0
20191130 | 58817 | 164.8 $\pm$ 0.9
20191221 | 58838 | 174.4 $\pm$ 0.9
20191230 | 58847 | 167.1 $\pm$ 1.0
20200114 | 58862 | 66.1 $\pm$ 1.0
20200125 | 58873 | 190.4 $\pm$ 1.0
20201213 | 59196 | 174.2 $\pm$ 1.0
20210107 | 59221 | 7.0 $\pm$ 1.1
20210131 | 59245 | 158.7 $\pm$ 1.0
20210209 | 59254 | 157.4 $\pm$ 1.0
20210304 | 59277 | 177.2 $\pm$ 1.0
_PKS 1510–089_
20190306 | 58548 | 180.5 $\pm$ 0.8
20190413 | 58586 | 91.5 $\pm$ 0.2
20200319 | 58927 | 120.4 $\pm$ 0.3
20210208 | 59253 | -1.2 $\pm$ 0.7
20210407 | 59311 | 14.4 $\pm$ 0.3
20210416 | 59320 | 70.4 $\pm$ 0.9
20210506 | 59340 | 135.4 $\pm$ 0.5
20210510 | 59344 | 164.6 $\pm$ 0.1
20210512 | 59346 | 22.0 $\pm$ 0.1
20210517 | 59351 | 61.3 $\pm$ 0.7
20210530 | 59364 | 98.5 $\pm$ 0.4
20210606 | 59371 | 44.3 $\pm$ 0.9
20210612 | 59377 | 63.3 $\pm$ 0.3
20210707 | 59402 | 43.1 $\pm$ 0.2
20210711 | 59406 | 171.3 $\pm$ 0.1
20210714 | 59409 | 166.3 $\pm$ 0.1
20210731 | 59426 | 125.1 $\pm$ 0.7
20210804 | 59430 | 142.3 $\pm$ 0.8
20210824 | 59450 | 51.1 $\pm$ 0.7

###  3.4 Broadband Spectral Energy Distribution Modeling

PKS 0637–75, unlike PKS 1510–089, is a source that does not exhibit much
variability generally. To better understand what is contributing to the
observed flux and polarization of PKS 0637–75, we compiled the SED for
semicontemporaneous observations between ASAS-SN g-band and Swify-UVOT
photometry corresponding to our SALT 20190303 epoch of observation, with NED
(NASA/IPAC Extragalactic Database (NED) (NASA/IPAC Extragalactic Database
(2019), NED) archival data from 2005 onward. A similar analysis of PKS
1510–089 has been done by Aharonian et al. (2023), so we refer to that work
for a detailed study of its SED.

The leptonic single-zone blazar model of Böttcher et al. (2013) has been used
to produce a fit by eye to the broadband SED of PKS 0637–75. A summary of this
model is given here; see Böttcher et al. (2013) for a more detailed and
thorough description. This model follows a homogeneous one-zone framework
where a power-law distribution of ultrarelativistic electrons is injected into
a spherical emission region of radius $R$. The emission region moves with a
constant speed $\beta_{\Gamma}$c along the jet, which corresponds to the bulk
Lorentz factor $\Gamma$.

The cooling of the electron distribution is influenced by synchrotron and
Compton emission processes. Synchrotron emission is determined by a tangled
magnetic field of strength B. The model accounts for Compton scattering
involving the synchrotron radiation field (synchrotron self Compton, SSC) and
external radiation fields (external Compton, EC), including direct AD emission
— EC (disk) — and AD emission reprocessed by the BLR, represented numerically
by an isotropic external radiation field with a blackbody spectrum of
temperature $kT_{\rm BLR}=10^{4}$ K — EC (BLR).

The code self-consistently computes an equilibrium electron distribution
considering particle injection/acceleration, escape on an energy-independent
escape time scale $t_{\rm esc}=\eta_{\rm esc}\,R/c$ with $\eta_{\rm esc}\geq
1$, and radiative cooling; evaluates the kinetic jet power $L_{e}$
corresponding to the final electron population in the emission region and the
magnetic field (Poynting flux) power $L_{B}$; and calculates the energy
partition ratio $\epsilon_{Be}$ $\equiv$ $L_{B}/L_{e}$.

Observational parameters used to constrain the model include the redshift (z =
0.653), the black hole mass (MBH $\sim$ 109 M⊙; Xiong & Zhang, 2014; Ito et
al., 2021), the apparent superluminal speed ($\beta_{\perp,app}$ $\sim$ 13;
Godfrey et al., 2012), and the AD and BLR luminosities ($L_{AD}$ $\sim$ 1047
erg s-1, $L_{BLR}$ $\sim$ 1045 erg s-1; Xiong & Zhang, 2014). The fit
parameters adjusted during the fit-by-eye procedure include the low-energy and
high-energy cutoffs of the injected electron spectrum ($\gamma_{min}$,
$\gamma_{max}$), the electron injection spectral index (qe), the emission
region radius ($R$), the magnetic field strength (B), the distance of the
emission region from the black hole (z0), the bulk Lorentz factor ($\Gamma$),
the observing angle ($\theta_{obs}$) in the observer’s frame, the BLR
radiation field black-body temperature (TBB), and the BLR radiation field
energy density (uBB). The minimum variability time scale corresponding to the
light-crossing time scale (tvar,min), the kinetic power in relativistic
electrons in the AGN frame ($L_{e}$), the power carried in the magnetic field
($L_{B}$), and the energy partition parameter ($\epsilon_{Be}$) are computed
from the other model parameters and the resulting equilibrium electron
distribution.

Figure 9: Single-zone leptonic model fit to the broadband SED of PKS 0637–75
using the code of Böttcher et al. (2013). Photometry data (points with error
bars) are as follows: archival multiwavelength data points from NED (black),
quasi-simultaneous ASAS-SN g-band (green) and Swift-UVOT u (orchid) and
uvw2bands (blue violet). The plotted radiation components include: ADflux
(blue double dotted–dashed), synchrotron emission (coral dotted), synchrotron
self-Compton emission (gray dashed), external Compton emission from the AD (EC
(Disk); orange dotted–dashed) and external Compton emission from the broad-
line region (EC (BLR); light purple loosely double dotted–dashed), and the
best fit of the total components (teal solid). The light shaded box around
1015 Hz indicates the frequency coverage of our spectropolarimetry data from
the RSS on SALT. While the fit-by-eye is rough and nonunique, it is apparent
that we are detecting mainly thermal emission in our SALT observations.

A representative broadband SED fit is plotted in Figure 9 with the leptonic
model fit parameters listed in Table 6. However, due to the substantial number
of free parameters and the fact that some of the SED photometric measurements
were taken at different times, the fit is rough and nonunique. In order to
reduce the degeneracies in the fit procedure, we aimed for a fit with exact
equipartition between relativistic electrons and magnetic fields, which we
were able to achieve. On the basis of the SED fit, we determined that the
optical emission is dominated by the thermal disk, though the listed
parameters are only indicative and no strict conclusions on the physical
conditions in the emission region should be drawn from it.

Table 6: Model Parameters for the SED Fit of PKS 0637–75 Parameter | Value
---|---
MBH (M⊙) | 2.5 $\times$ 109
$L_{disk}$ (erg s-1) | 8 $\times$ 1046
$L_{BLR}$ (erg s-1) |  $\sim$ 1045
$\gamma_{min}$ | 700
$\gamma_{max}$ | 2 $\times$ 105
qe | 2.5
R (cm) | 1.5 $\times$ 1016
B (G) | 2
z0 (pc) | 0.06
$\Gamma$ | 15
$\theta_{obs}$ (deg) | 3.82
TBB (K) | 1 $\times$ 104
uBB (erg cm-3) | 2 $\times$ 10-2
$L_{e}$ (erg s-1) | 7.60 $\times$ 1044
$L_{B}$ (erg s-1) | 7.59 $\times$ 1044
$\epsilon_{Be}$ | 1.0
tvar,min (hr) | 15.3

##  4 Discussion

The optical emission from blazars is composed of a variety of components
including the accretion disk, the broad-line region, and synchrotron emission
from the jet. Each of the emission mechanisms associated with these regions
contributes differently to the polarization characteristics of the emitted
radiation (Kovalev et al., 2020). The mechanisms inducing polarization in AGNs
can be divided into internal (central parsec and smaller scales) and external
(greater-than-parsec scales). Polarization due to the influence of magnetic
fields close to the SMBH, radiation transfer from the AD, the synchrotron jet
radiation, and electron scattering in the hot corona all contribute to the
internal polarization mechanisms. Equatorial scattering at the torus and polar
scattering at the ionization cone make up the external polarization mechanisms
(Shablovinskaya & Afanasiev, 2019).

Polarized emission lines are typically due to scattering by material through
the above mentioned external mechanisms, where the number and location of the
scattering regions determine the observed polarization properties. If
equatorial scattering is the dominant mechanism, a couple of characteristic
polarization signatures would be observed – mainly a dip in the polarization
degree and an S-shaped PA swing along the emission line profiles
(Shablovinskaya et al., 2023). Of the two, we do see a dip in the percentage
of polarization at each emission line core region, but there is no evidence
for any swing of the PA across the line (Figure 3).

The linear polarization that is associated with optically thin synchrotron
radiation depends on the structure of the magnetic fields in the emitting
region and on the emitting electrons’ energy distribution (Itoh et al., 2016).
Additionally, polarization from nonthermal electrons in an anisotropic
magnetic field should vary with the total flux of an object as magnetic field
configurations evolve, giving rise to synchrotron polarization that is
strongly variable and different from the polarization of emission lines
(Marin, 2019). Especially evident for PKS 1510–089 , the polarization fraction
spanning the 2–3 yr of our study displays variability with flux and is
different from that of the emission lines, which are consistently unpolarized.

The orientation of an AGN with respect to the observer’s line of sight can
have a strong influence on the polarization detection as well. Single SMBHs
surrounded by coplanar, axisymmetric, or spherically-shaped scattering regions
produce low amounts of polarization when viewed close to face-on inclinations
(Savić et al., 2019). For objects with inclination angles close to 0∘ (i.e.
blazars), we have a mostly direct, unobstructed view of the AD, BLR, and
relativistic jet. As such, the jet will continue to have a polarization signal
due to the intrinsically polarized synchrotron radiation, while the
polarization vectors of the disk undergo geometric cancellations, resulting in
no net disk polarization in total light (Oudmaijer & Harries, 2008) nor in the
broad lines.

Various spectropolarimetry studies of quasars have been undertaken with
similar results to what we have found for PKS 0637–75 and PKS 1510–089.
Kishimoto et al. (2003) found an absence of broad lines in polarized light for
two quasars in their study, suggesting the continuum scattering region
potentially is located interior to or cospatially with the BLR (from Smith et
al., 2005). Additionally, Kishimoto et al. (2004) found that polarization was
confined to the continua for five quasars in their study with depolarized BLR
emission and wavelength-independent position angles, suggesting a single
source of the observed polarization. For radio-loud quasars studied by Popović
et al. (2021), polarization was not detected in or across the broad emission
lines which the authors discuss could have been due to a lack of equatorial
scattering, a region of depolarization above the BLR, or an inner equatorial
scattering region comparable in size to the BLR. As was discussed for higher
inclination quasars in Capetti et al. (2021), if a single scattering medium
physically close to the BLR is present, geometric cancellation of polarization
vectors is possible.

Lower levels of synchrotron polarization during the nonflaring epochs of PKS
1510–089 indicate a less ordered magnetic field during such quiescent states,
suggesting more tangled magnetic field lines with different field-line
directions that can cancel out (Schutte et al., 2022). The generally low
degree of synchrotron polarization in PKS 0637–75 is likely due to the
dominant AD emission.

As we see in Figure 9, the upturn at the optical-UV regime in the SED of PKS
0637–75 is well modeled by a Shakura-Sunyaev AD (Shakura & Sunyaev, 1973). The
SED is modelled with an emission region located 0.06 pc down the jet, i.e.,
within the BLR, hence the dominant external Compton contributions from the
disk and BLR (EC (disk), EC (BLR)). The lightly shaded box around 1015 Hz
illustrates the frequency range covered by the RSS on SALT used for our
spectropolarimetry data collection. We see that in our model, SSC is sub-
dominant and the optical emission is dominated by thermal processes (direct AD
emission) over the nonthermal synchrotron jet. Thus the low degree of
polarization of the optical-UV emission in PKS 0637–75 is consistent with
dilution by the AD and BLR which are expected to be intrinsically
nonpolarized.

From the beginning of 2021 April to mid-June, there exists an overlap of SALT
spectropolarimetry observations of PKS 1510–089 between this observing
campaign and that of Aharonian et al. (2023), which used H.E.S.S., ATOM, and
SALT observations to better understand the primary emission region of PKS
1510–089 (see panels (c) and (d) of Figure 5). In this study, we observed a
drop in the optical continuum polarization and an increase in the EW of broad
emission lines H$\gamma$ and H$\beta$ during the quiescent stages of PKS
1510–089, most notably after the optical flaring event around 2021 14 July.
After this observation in 2021, our SALT data display significantly lower
levels of polarization (less than 4%). In Aharonian et al. (2023), the
polarization level reached a maximum of 12.5% and minimum of 2.2% in 2021,
similar to the values we obtained from our SALT observations at similar times
(within $\pm$ 1–2 days).

Using the SED + spectropolarimetry model of Schutte et al. (2022), Aharonian
et al. (2023) were able to show that contributions from the synchrotron jet,
AD, and BLR can explain the observed emission and polarization levels in 2021,
whereas for 2022, the drop in polarization level to being consistently below
2% is consistent with no polarization in the blazar, such that the AD and BLR
flux is sufficient to fully explain the optical spectrum. The decrease in
polarization levels after 2021 mid-July we observe gives support to the above
suggestion that the optical-UV spectrum became dominated by the thermal AD and
BLR, while low levels of polarization (4%) between 2021 mid-July and August
may be consistent with only interstellar polarization and no intrinsic
polarization in the source, in agreement with what was found by Aharonian et
al. (2023) in their later observations; see the bottom panels (c) and (d) in
Figure 5 where we see in more detail the decline in polarization levels of
PKS–1510-089.

##  5 Conclusions

We obtained spectropolarimetric observations of FSRQs PKS 0637–75 and PKS
1510–089 using the Southern African Large Telescope during the 2019–2021
period. Blazar optical emission is composed of the thermal accretion disk and
broad-line region and nonthermal synchrotron jet. The connection between these
thermal and nonthermal components are explored through the polarization
characteristics of the emitted radiation.

Variability in continuum polarization is on the order of approximately half to
a few percent on various time scales for PKS 0637–75 and approximately a few
to tens of percent on day–week timescales for PKS 1510–089. While we detect
variability in the continuum polarization levels, the same cannot be said for
the broad emission lines these blazars exhibit. The broad H$\gamma$ and
H$\beta$ lines of PKS 1510–089 are not detected in polarized emission and
within their errors are consistent with zero polarization. In PKS 0637–75, the
low-ionization Mg II line is not detected in polarized light as well, though
it does occasionally demonstrate weak levels of polarization.

It has been shown for 4C+01.02 that the AD emission can dilute the synchrotron
emission toward higher optical frequencies, causing a decrease in the total
degree of polarization, as well as a detected decrease in polarization at the
frequency of unpolarized emission lines (Schutte et al., 2022). Both of these
phenomena are observed in this work – during the nonflaring period of time
when the thermal emission components are dominant, PKS 1510–089 was in a lower
polarization state compared to the epoch of nonthermal dominance during the
flare. Likewise, there is a noticeable drop in polarization at the wavelengths
of the unpolarized emission lines, especially for the Balmer lines.

We conclude that the broad emission lines of PKS 0637–75 and PKS 1510–089 are
intrinsically nonpolarized, though geometric cancellations due to the pole-on
orientation potentially exist as well (Oudmaijer & Harries, 2008; Capetti et
al., 2021) and are causing our nondetection of polarized emission from the
lines analyzed here. Additionally, changes in the dominant emission process
can lead to continuum polarization variability. The gamma-ray quiet FSRQ PKS
0637–75 is not as variable a source as compared to the gamma-ray loud FSRQ PKS
1510–089 and seems to be consistently dominated by thermal emission in the
optical-UV regime, resulting in very low levels of observed polarization.
Emission from PKS 1510–089 prior to 2021 mid July was consistent with being
associated with a nonthermal synchrotron jet, thermal AD and BLR which then
underwent a change to being dominated by the thermal components, as evidenced
by the drastic decrease in polarization levels observed in our SALT
spectropolarimetric observations and supports the photometric and
spectropolarimetric observations from Aharonian et al. (2023).

We thank the anonymous reviewer for comments that improved this work. S.A.P.
acknowledges support from the Dartmouth Fellowship and Sigma Xi grant
G201903158443203 and would like to thank Keighley E. Rockcliffe, Aylin García
Soto, John R. Thorstensen, Elisabeth R. Newton, Rujuta A. Purohit, and Emily
M. Boudreaux for various mentoring, advice, and conversations that improved
the manuscript. R.C.H. acknowledges support from NASA through Astrophysics
Data Analysis grant No. 80NSSC23K0485. All of the spectropolarimetric
observations reported in this paper were obtained with the Southern African
Large Telescope (SALT), under proposals 2018-2-SCI-039 (PI: J. Isler),
2019-2-SCI-040 (PI: J. Isler), 2020-2-SCI-017 (PI: S. Podjed), and
2021-1-SCI-027 (PI: S. Podjed) with SALT astronomers Danièl Groenewald & Lee
Townsend–special thanks to the SALT observation team for their diligent
communications, data collection, and initial reduction of data for our program
study. This research has made use of the NASA/IPAC Extragalactic Database
(NED) which is operated by the Jet Propulsion Laboratory, California Institute
of Technology, under contract with the National Aeronautics and Space
Administration. This work includes data collected by the ASAS-SN mission and
Fermi-LAT mission. SALT (RSS)

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