A Relook at the Black Hole Binary Candidate J1328+2752 with VLBI

We present multiband follow-up observations of the supermassive binary black hole (BBH) candidate and misaligned double–double radio galaxy, J1328+2752. To investigate its parsec-scale structure, we have carried out observations with the Very Long Baseline Array (VLBA) and the European Very Long Baseline Interferometry (VLBI) Network. Additionally, we have obtained optical spectroscopic observations with the 3.6 m Devasthal Optical Telescope. Within 3.5 yr of our previous VLBI observations, the central parsec-scale radio structure of J1328+2752 has changed from a double component to a single central component and its flux density has increased by a factor of 3 in the 5 GHz VLBA observations. The new radio component is largely unresolved at 3, 5, and 8 GHz. The multifrequency radio data shows a convex-shaped spectrum for this unresolved component. The turnover frequency is at ∼3 GHz. While the total intensity image does not reveal any details, a super-resolved VLBA 5 and 8 GHz spectral index image does indicate the presence of two components at a separation of ∼4.2 pc with spectral indices −0.5 ± 0.3 and −0.9 ± 0.3. We have not observed a simultaneous change in the optical line profiles or intensities over the past few years. The radio structure, the variation of the flux density, and spectral shape can potentially be consistent both with the signature of a young radio source or a BBH at the center of J1328+2752.


Introduction
In recent decades, several systematic searches for supermassive binary black holes (BBHs) have been carried out in the literature (Komossa 2006;Deane et al. 2014;Comerford et al. 2015).Temporal variability, double-peaked emission lines, and helical jet structures have all been used to identify BBH candidates.Direct imaging of parsec-or subparsec-scale separated BBHs has been carried out by Very Long Baseline Interferometry (VLBI) techniques (e.g., Rodriguez et al. 2006;Deane et al. 2014;Kharb et al. 2017;Yang et al. 2017;Kharb et al. 2020).Multiple direct and indirect observational techniques throughout the whole electromagnetic spectrum have been used to confirm two supermassive black holes (SMBHs) in any given binary system (see De Rosa et al. 2019 for a review).To date, few definite cases of supermassive BBHs are known.One spatially resolved source, 4C+37.11,shows two compact radio cores (i.e., two accreting black holes) at a projected separation of 7.3 pc and S-shaped jets on kiloparsec scales (Rodriguez et al. 2006).The optical spectroscopy of this object shows Hα line splitting with a velocity separation ∼400 km s −1 .Similarly, the Seyfert galaxy NGC7674 is a candidate BBH source with a projected black hole separation of 0.35 pc and Z-shaped jets on kiloparsec scales (Kharb et al. 2017).OJ287 is the strongest supermassive BBH candidate with a parsec-scale BBH separation (∼0.1 pc).The optical light curve of OJ 287 shows strong periodic magnitude variations, which can be explained with the help of a BBH system (Sillanpaa et al. 1988;Dey et al. 2021).In a recent search, Charisi et al. (2016) and Graham et al. (2015a) reported a population of close SMBH binary quasar candidates with strong periodic variability in optical light curves.In this sample, quasar PG 1302-102 is a possible supermassive BBH system with a subparsecscale separation (Graham et al. 2015b).Blazar PKS 2131−021 is also reported as a candidate supermassive BBH with subparsecscale separation, and it exhibits periodic variations in radio flux density (O'Neill et al. 2022).Some blazars also exhibit quasiperiodic changes in the apparent velocities and position angles of their superluminal jet components that may be interpreted as jet precession induced, for example by a supermassive BBH (Caproni & Abraham 2004a, 2004b;Caproni et al. 2013;Roland et al. 2013;Caproni et al. 2017;Britzen et al. 2018).
We have recently identified a supermassive BBH candidate in the radio galaxy J1328+2752 (Nandi et al. 2021a).All its optical narrow emission lines of the host are double-peaked, with the average velocity separation of the two components being ∼235 km s −1 (Nandi et al. 2017).The observations from the Giant Metrewave Radio Telescope revealed that J1328 +2752 has a ∼400 kpc scale, and a large radio structure with two epochs of misaligned jet activity (Nandi et al. 2017).Previously observed (the observation was taken in the year 2018), the 5 GHz Very Long Baseline Array (VLBA) image reveals a core-jet structure and another single component The estimated binary separation obtained from the optical analysis is 6.3 2.1 3.1 -+ pc.Within the errors, this value was consistent with the 2018 VLBA observations (Nandi et al. 2021a).The separation between the two peak positions in the 2018 August 11 VLBA data was 2.75 milliarcseconds at a position angle of 97°.8; this separation corresponded to 4.7 pc.A kinematic jet precession model (Caproni et al. 2017) was applied to this source.This model indicates that the jet precession may have been induced by torques in the primary accretion disk due to the secondary black hole in a noncoplanar orbit around the primary (compatible with the projected separation of ∼4.7 pc).However, we could not neglect an alternative scenario where the jet precession is driven by the Bardeen-Petterson effect (Bardeen & Petterson 1975).The Sloan Digital Sky Survey (SDSS) photometric data of the host galaxy suggested that the extended 2.5 kpc disklike substructure of the galaxy may represent a gas-rich, unequal-mass merger (Nandi et al. 2021a).
The optical emission lines, the helical kiloparsec-scale jets, the VLBI image, disturbed host morphology, and the jet precession model strongly suggested the presence of a BBH in this source.Motivated by this compelling evidence, we observed this source with the VLBA and European VLBI Network (EVN) in order to investigate the parsec-scale structure of the central host in detail and search for possible changes since 2018.We also carried out observations from the 3.6 m Devasthal Optical Telescope (DOT) to understand any systematic changes in the optical emission lines.In this paper, we present these new VLBA, EVN, and DOT observations in Section 2. The observational results are discussed in Section 3. The discussion and conclusions follow in Section 4.

Observations and Data Reduction
The radio galaxy J1328+2751 was observed in the 2021 February-March EVN session at 8 and 22 GHz under proposal codes EN008A and EN008B, respectively.They were phasereferenced continuum observations in which 13 telescopes were available for EN008A, while 14 antennas were employed during the EN008B session.For both bands, "J1327+2210" and "J1333+2725" were used as a fringe-finder and phasereference calibrator, respectively.The EVN data were correlated at JIVE and calibrated using the standard EVN pipeline.We used the Astronomical Image Processing System (AIPS) to image these calibrated data.We also observed this target from VLBA under proposal codes BN060A (C band) and BN060B (S/X band).For both observations, 10 telescopes were available.Fringe-finder J1642+39 was observed at the beginning and the end of the observations.The same EVN phase calibrator was adopted for these observations.The details of the observations are given in Table 1.We used VLBARUN to calibrate VLBA data and then used IMAGER in AIPS.
We performed long-slit low-resolution spectroscopic observation of J1328+2752 using the ARIES-Devasthal Faint Object Spectrograph and Camera, mounted on the 3.6 m DOT under the proposal code DOT-2023-C1-P58.A total of 2.5 hr was allocated for this observation.The host spectra in two preferred slit orientations were taken.The first slit position was along the inner components of the radio jet to probe the associations of the emission lines with outflows.Whereas the second slit orientation was perpendicular to the first position, where outflow is not expected.Spectroscopic data were reduced under the standard IRAF5 environment.Bias and flat-fielding were performed on all the frames.Cosmic-ray rejection was done using the Laplacian kernel detection method (van Dokkum 2001).Flux calibration was carried out using standard spectrophotometric fluxes from Hamuy et al. (1994).

Results
The source has been detected at 3, 5, and 8 GHz with the VLBA and at 8 GHz with the EVN (see Figure 1).However, we do not detect any emission in the 22 GHz EVN observations.The final image rms noise, peak intensity, and total flux density are provided in Table 2.We used AIPS task JMFIT to obtain peak intensity and the total flux density measured using AIPS verb TVSTAT.We also used historical Very Large Array (VLA) data to obtain intermediate resolution (∼300 mas) data.This archival data set was observed at 8.4 GHz A-array configuration under project code AL0187.To reduce the VLA archival data, we used both AIPS and the Common Astronomy Software Applications package.The final rms noise in the historical VLA image (see Figure 2) made using a UVTAPER of 0-300 kλ in  −11.3, 11.3, 22.5, 45, and 90) mJy beam −1 .The beam is of size 0 486 × 0 305 at PA = 2.6°.IMAGR is ∼80 μ Jy beam −1 .The total flux density in the western extended emission is 576 μJy using the AIPS verb TVSTAT.The VLBA and EVN data mostly show the presence of a single component at all of the frequencies (Figure 1).In the new epoch of 5 GHz observation, the flux density of the component has increased by a factor of 3 compared to the 2018 VLBA observation.The VLBA 5 and 8 GHz spectral index image (Figure 3) shows two features with slightly different spectral indices (α is −0.4 and −0.9 with errors of ± 0.3).These two features are not completely resolved in the total intensity images.The separation between these two spectral index features is 2.452 milliarcseconds at a position angle of 125°.34.For the cosmology scale of 1.709 kpc arcsec −1 , this separation corresponds to 4.2 pc.We note that the total extent of the radio emission estimated from the 2018 August 11 VLBA image was ∼6.8 pc, the peak separation between components A and B was 4.7 pc, and the optical line separation was 6.3 pc.Considering all these, we reported an average BBH separation of ∼6 pc in our previous work (Nandi et al. 2021a).The new 3 and 5 GHz spectral index image of J1328+2752 obtained with the VLBA data shows a single inverted spectrum component with α = 0.51 ± 0.35.We do not see two inverted spectrum components in this spectral index image (Figure 3).The optical spectra are shown in Figure 4. Here, the upper two spectra are from DOT with a two-slit orientation.The lower spectrum is from the SDSS.All emission lines detected in SDSS are also present in the DOT spectra.

Discussion
The previous VLBA image of J1328+2752 by Nandi et al. (2021a) detected two distinct radio components.The west component (A) had a core-jet structure, whereas the east component (B) was a compact source (see bottom panel of Figure 1).While the core is not resolved in archival VLA X-band (8.4 GHz) observations, the low-resolution image does show a ∼1.5 kpc weak outflow roughly in the east-west direction, consistent with having been ejected from Source A (Figure 2).
The new data present us with different possibilities.It is possible that the previous A component has disappeared within the 3.5 yr timescale or has come closer.It is also possible that the old component B as well as component A2 were jet components that have moved away and expanded (and therefore become invisible to the VLBA) and the two spectral index components are the new core-jet components.Finally, it is also possible that A1 and A2 were jet components, and B is the true jet inlet region.It is noteworthy that the location of the B component does not match exactly with the new epoch VLBA component.This is one of the different possibilities that has been assumed arbitrarily (see the Appendix).We discuss the possible scenarios ahead after a brief spectral aging analysis of the VLBA/EVN-detected component.

A Spectral Ageing Analysis
Two spectral index images indicate that the component does not follow classical power-law distribution and there is a possibility of low-frequency deviation.Low-frequency turnover due to the synchrotron self-absorption process can be seen in the case of very optically thick emission regions of young radio sources.An estimation of radiative age helps us to quantify the central source nature.Figure 5 displays the integrated radio spectrum of the core.The measurements for the new epoch are indicated as solid black circles in the figure, while the 5 GHz VLBA observation in 2018 is indicated by the magenta solid diamond.The rms noise level in the 22 GHz image is marked with a down-facing triangle.We used the SYNAGE software (Pacholczyk 1970;Murgia et al. 1999) to fit the spectrum.For this fitting, the flux density errors for each frequency have been set to 10%.We consider the JP (Jaffe & Perola 1973) model modified by synchrotron self-absorption.Keeping α inj (injection spectral index) as a free parameter, we get the synchrotron selfabsorption frequency to be ∼3 GHz (see Figure 5) and a highfrequency break (ν br ) is at 11 GHz.The high-frequency break, magnetic field, and radiative synchrotron age are related through the following equation (Konar et al. 2012): where τ syn is the spectral age in the unit Myr.B iC = 0.318(1+z) 2 is the magnetic field strength equivalent to the inverse-Compton microwave background radiation.The units of B is in nT.The break frequency is ν br in GHz.We followed the classical minimum energy approach by Miley (1980) and Konar et al. (2008) to calculate the magnetic field.We integrated the spectrum from 10 MHz to 100 GHz and considered a cylindrical geometry, filling factor 1.

A Gigahertz-peaked Spectrum Source
Gigahertz-peaked spectrum (GPS) sources with slightly resolved radio morphology and turnover frequency ∼1 GHz are widely believed to be the young precursors of large radio galaxies (O'Dea et al. 1991;Nandi et al. 2021b).Therefore, a very young GPS-like component at the center of J1328+2752 implies that the core is going through an event that may have retriggered new jet activity with increased flux density.Previous studies indicate that large-size radio galaxies can show some GPS-like characteristics at their centers (Brienza et al. 2018;Bruni et al. 2019).J1328+2752 is unique among these radio galaxies because the finding of a GPS source at the center suggests three episodes of jet activity in this source.
The radiation pressure instabilities in the accretion disk of the short-lived radio sources can produce changes in radio luminosity.It has been suggested that the slow transient phenomenon in the case of the young radio source is likely associated with renewed active galactic nucleus (AGN) activity (Kunert-Bajraszewska et al. 2020).Wołowska et al. (2021) reported transient phenomena in a sample of 12 young radio sources, which includes both quasars and galaxies.All these sources have convex radio spectra with a range of peak frequency from 2 to 12 GHz.Most of them also show radio flux density variability in the optically thin part of the radio spectrum.A few of these sources have resolved jets in the early epochs of observations, while these are no longer visible in the later epochs.This may be due to a complete dissipation of jet energy in the latter epochs.This study concludes that GPS galaxies can maintain the shape of the convex spectrum.However, GPS quasars can change into spectrum sources quickly.We note similar features in J1328+2752, in which the GPS-like central component shows a turnover at ∼3 GHz, and the increase of radio flux density is in the optically thin region.The disappearance of the second component may be due to the complete jet energy dissipation.

A Changing-look AGN
A changing-look AGN shows an enhancement and cessation of the radio activity that can be related directly to the black hole efficient/inefficient accretion mode.Koay et al. (2016) reported a changing-look behavior in Mrk 590 in the radio domain by studying its parsec-scale radio structure, variability, and radio spectral shape.This is the first changing-look AGN that shows coincident radio variability.This finding may connect changing-look AGNs with short-term radio activity.However, the changing look behavior in AGN is rare, and their optical broad emission lines show dramatic changes in their appearance.Either they disappear or emerge due to variable accretion rates.In 2019, the changing look AGN IRAS 23226−3843 went through an X-ray and optical outburst.The optical spectrum of this AGN has an asymmetric broad H α emission line as well as a broad double-peaked H β emission line.In addition, the source also has narrow emission lines like Kollatschny et al. (2023) reported that the optical continuum and the Balmer line intensities showed drastic differences when the spectra were compared between the years 1999 and 2017.The double-peaked emission line is from the underlying accretion disk in this source.
To examine possible changes in the optical emission lines, like their amplitude and disappearance or reappearance, we carried out spectroscopic observations of J1328+2752.The DOT low-resolution spectra could not identify the double peak features.After fitting Gaussian components, we checked the line ratios for the emission lines H β λ4862 Å, [O III] λ5007 Å, H α λ6563 Å, and [S II] λ6781 Å (the lines are marked in Figure 4).All these ratios are consistent for DOT and SDSS spectra.We did not notice any underlying broad component for the slit position along the radio jets.Therefore, outflow signatures are absent in the optical lines.The emission line strengths are not diminished or magnified in the new spectra.We did not notice any disappearance or reappearance of emission lines either.Therefore, the scenario of a changinglook AGN is very unlikely for J1328+2752.However, it is possible that the radio variability is not coincident with the optical line variation timescale.

A Supermassive BBH System
When two SMBHs are in the actual process of merging, they go through different merger stages.In the final merger stage, the two nuclei come inside a common envelope or show only a single unresolved nucleus (Ricci et al. 2017).At this stage, they spiral more closely, start to produce gravitational radiation, and finally coalesce (Begelman et al. 1980).Numerical simulations have shown that during mergers, material channels from the kiloparsec to the parsec scales via tidal torques (Di Matteo et al. 2005), and the AGN luminosity strongly increases.Supermassive BBHs often show modulation in the flux density due to their orbital motion.In the case of J1328+2752, Nandi et al. (2021a) showed that the putative BBH in this source is gravitationally bound for any value of q BH = M s /M p , where M p and M s are, respectively, the masses of the primary and secondary black holes.They also showed that the implied orbital motions of these black holes are Keplerian, in an evolutionary phase prior to the hard phase of a BBH system (e.g., Begelman et al. 1980).As discussed in the Appendix, the supermassive BBH scenario in J1328+2752 still holds in light of the new interferometric observations presented in this work.
The VLBA 5 and 8 GHz spectral index image actually shows a closer separation in the new epoch.The increase in flux density in the recent epoch could be an indication of an enhanced accretion rate.Similar behavior has been observed in OJ 287.For this binary system, Agudo et al. (2012) reported a sharp rise in the core emission at 43 GHz during the year 2004 along with a change in the jet position angle.It is important to investigate if there exist any periodic variations of flux density as well as any systematic change in the optical emission lines of J1328+2752.Future long-term monitoring of high-resolution radio continuum emission and high-resolution optical spectroscopy is mandatory to confirm the supermassive BBH hypothesis.

Summary and Conclusions
We present multiband follow-up observations of the supermassive BBH candidate and double-double radio galaxy, J1328+2752.We have presented here the results from VLBA and EVN observations, along with optical spectroscopic observations with the DOT.We summarize the main results below.
1. Within 3.5 yr of our previous VLBI observations, the central parsec scale radio structure of J1328+2752 has changed from a double component to a single central component, and its flux density has increased by a factor of 3 in the 5 GHz VLBA observations.2. The new radio component is largely unresolved at 3, 5, and 8 GHz.The multifrequency radio data shows a convex-shaped spectrum for this unresolved component.3.While the total intensity image does not reveal any details, a super-resolved VLBA 5 and 8 GHz spectral index image does indicate the presence of two components at a separation of ∼4.2 pc with spectral indices −0.5 ± 0.3 and −0.9 ± 0.3.4. We have not observed a simultaneous change in the optical line profiles or intensities over the past few years.The radio structure, the variation of the flux density, and spectral shape can potentially be consistent with the signature of a young radio source or a BBH at the center of J1328+2752. 5. Galaxy mergers are common phenomena in the Universe but there are few instances of galaxies that harbor two supermassive BBH at subparsec separation.At this close separation, they expect to produce gravitational waves at nHz frequencies.J1328+2752 is one such strong candidate, which shows several direct and indirect evidence of a binary system along with other possibilities.
In the current epoch, radio flux density of J1328+2752 has increased by 3 times.This study shows that this source may exhibit significant radio periodicity, which is another promising signature to detect two supermassive BBH.Future regular VLBI monitoring will help to understand its periodic modulation of radio flux density and confirm its supermassive BBH interpretation.
as well as their respective angular separations from B in the bottom panels.As can be seen, A2 is the component closer to B in the two epochs of observation in scenario 1 (bottom left panel), while it is only true in 2018 for scenario 2 (bottom right panel).Linear least-squared regressions were applied to the data, aiming only to obtain a crude estimate of the apparent proper motions of the individual components (since no statistical meaning can be attributed to those fits because of the small number of observational epochs).For scenario 1 (bottom left panel in Figure 6), proper motions for A1 and A2 are approximately 0.34 and 0.40 mas yr −1 , respectively, which means 2.0c and 2.4c for their apparent velocities considering the redshift of J1328+2752.On the other hand, A2 has a higher proper motion of 0.78 mas yr −1 (or ∼4.4c) in scenario 2 (bottom right panel in Figure 6), while A1 presents a very low proper motion (absolute value of about 0.006 mas yr −1 ), still compatible with being stationary between 2018 and 2022 given the uncertainties in the structural parameters of the Gaussian model fittings and possible core-shift opacity effects between 5 and 8 GHz data used in such analysis (e.g., Blandford & Königl 1979;Lobanov 1998).
The mildly superluminal jet components found in scenario 1 are in agreement with a jet launched recently by a single black hole in a different direction when compared with that seen at kiloparsec scales.In the case of scenario 2, the lack of a substantial motion argues in favor of a second core region where a secondary black hole could be residing.Indeed, the separation of about 2.9 mas between A1 and B is similar to that found for the two features seen in the spectral index map (∼2.5 mas), as well as that derived from the double peak lines in the spectrum of J1328+2752 (Nandi et al. 2021a).The lack of signatures of outflow motions in the DOT spectra, as well as the constancy of the intensity of the emission lines also argue in favor of the presence of a supermassive BBH system in the nucleus of J1328+2752.
Although the above results agree with the presence of a GPS source or a supermassive BBH system at the parsec-scale core of J1328+2752, the small number of epochs and the very faint western 8 GHz components used in the kinematic studies, as well as possible core-shift opacity effects due to the usage of data from two frequencies, suggest caution in those interpretations.New interferometric monitoring of the parsec-scale activity of J1328+2752 is essential to confirm such findings.

Figure 4 .
Figure 4.The uppermost spectrum is from DOT with a slit position along the jet direction (DOT position 1; observed on 2023 February 21).The middle spectrum is also from DOT but with a slit position perpendicular to the jet direction (DOT position 2; observed on 2023 March 17).The lower spectrum is from SDSS (observation date 2005 April 13).

Figure 5 .
Figure 5.The radio spectrum of J1328+2752 and its best-fitting model.The black filled points are new VLBA and EVN observations.The magenta point represents the 5 GHz VLBA observation in 2018.The filled down-pointing triangle represents the rms level of 22 GHz EVN observation.