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Decomposition of disk and jet optical emission: Case of PKS~1510-089 and PKS~0736+017

This study utilizes polarization signatures to decompose the optical continuum of two jetted quasars, PKS~0736+017 and PKS~1510-089, into thermal disk and non-thermal jet components, revealing significant jet contamination that necessitates revised estimates of their Broad Line Region sizes and black hole masses.

Original author: Shivangi Pandey, Suvendu Rakshit, Adarsh Kumar, Anne Lähteenmäki, Merja Tornikoski, Archana Gupta

Published 2026-11-03✓ Author reviewed ⓘ

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CC BY 4.0

Original author: Shivangi Pandey, Suvendu Rakshit, Adarsh Kumar, Anne Lähteenmäki, Merja Tornikoski, Archana Gupta

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written by the author. For technical accuracy, refer to the original paper. Read full disclaimer

In the vast, dark expanse of the universe, some galaxies act as cosmic lighthouses, beaming intense radiation across the electromagnetic spectrum. Among the most powerful of these are active galactic nuclei, regions surrounding supermassive black holes that consume vast amounts of gas and dust. As this material spirals inward, it forms a swirling, superheated disk that glows with thermal light, much like a stove burner heating up. This light is the key astronomers use to measure the size of the region where gas clouds orbit the black hole, a distance known as the broad-line region. By timing how long it takes for changes in the disk's brightness to ripple out and illuminate these clouds, scientists can calculate the black hole's mass. However, in a specific class of these galaxies called flat-spectrum radio quasars, a second, much brighter light source often interferes. These objects possess jets of particles shooting out at nearly the speed of light, which emit their own intense, non-thermal radiation. This jet light can overwhelm the softer glow of the accretion disk, making it difficult to distinguish the true signal of the black hole's engine from the noise of the jet.

A team of researchers recently tackled this problem by studying two such quasars, PKS 0736+017 and PKS 1510-089, to separate the light of the disk from the light of the jet. They realized that while both sources of light travel together, they behave differently. The light from the accretion disk is thermal and relatively unpolarized, whereas the light from the jet is generated by magnetic fields and carries a distinct polarization signature, meaning its light waves vibrate in a specific direction. By analyzing decades of archival data, including optical brightness measurements, the behavior of specific emission lines, and radio observations, the team used a sophisticated statistical method to untangle the two signals. They treated the total observed light as a mixture of two independent processes: one representing the steady, thermal glow of the disk and the other representing the variable, polarized emission of the jet. This approach allowed them to isolate the disk's contribution and see how it truly behaved without the jet's interference.

The results revealed a stark difference between the two galaxies. For PKS 1510-089, the jet is the dominant player, contributing roughly 63 percent of the total optical light, leaving the disk to provide only about 37 percent. Once the researchers removed the jet's contribution from the data, the timing of the light changes shifted. The delay between the disk's brightness changes and the response of the surrounding gas clouds was found to be approximately 54 days. This is a shorter interval than previously thought, suggesting that earlier measurements had been skewed by the jet's overwhelming brightness. In contrast, the situation for PKS 0736+017 was almost the reverse. Here, the accretion disk dominates, providing about 70 percent of the optical light, while the jet contributes a smaller 30 percent. After cleaning the data of the jet's influence, the time delay for this source was calculated to be roughly 85 days, a longer duration than previous estimates.

These findings highlight a critical challenge in measuring the masses of black holes in these energetic systems. The study demonstrates that without carefully separating the jet's light from the disk's light, astronomers risk miscalculating the size of the region surrounding the black hole and, consequently, the black hole's mass itself. The researchers noted that while their method successfully isolated the disk in PKS 0736+017, the situation for PKS 1510-089 remains complex. Because the jet in that galaxy is so active and variable on short timescales, the model used to separate the light might still contain some residual jet contamination within the disk signal. This means the calculated time delay for PKS 1510-089 should be viewed with caution, as the rapid flares of the jet may have been partially misidentified as disk activity.

Ultimately, this work provides a clearer picture of how these cosmic engines operate by showing that the light we see is often a composite of different physical processes. By using the unique polarization signatures of the light, the team was able to peel back the layers of brightness to reveal the underlying structure of the accretion disk. The study confirms that for some quasars, the jet is the primary source of visible light, while for others, the disk remains the dominant voice. This distinction is vital for understanding the true nature of these distant objects and refining our measurements of the supermassive black holes that power them. The research suggests that future studies of similar objects must account for this jet contamination to avoid drawing incorrect conclusions about the scale and mass of the universe's most massive objects.

Technical Summary: Decomposition of Disk and Jet Optical Emission in PKS 1510-089 and PKS 0736+017

Problem Statement
Measuring the Broad Line Region (BLR) size and black hole mass in jetted quasars (Flat-Spectrum Radio Quasars, or FSRQs) is complicated by the significant contribution of non-thermal synchrotron radiation from the relativistic jet to the optical continuum. This jet contamination obscures the intrinsic thermal emission from the accretion disk, which serves as the ionizing source for the BLR. Consequently, Reverberation Mapping (RM) analyses that rely on the total observed optical continuum can yield biased estimates of BLR lags and black hole masses. Previous work on 3C 273 (Li et al. 2020) demonstrated that jet contamination can dilute reverberation signals, motivating a similar investigation into other FSRQs where jet dominance is pronounced.

Methodology
The authors investigate two FSRQs, PKS 1510-089 ( $z=0.361$ ) and PKS 0736+017 ( $z=0.189$ ), by decomposing their optical continuum into thermal (disk) and non-thermal (jet) components. The study utilizes a multi-epoch dataset comprising:

Optical Spectro-photometry: V-band continuum and H $\beta$ emission line light curves from the Steward Observatory (SO) monitoring program, supplemented by SMARTS and ASAS-SN data.
Radio Data: 37 GHz light curves from the 13.7 m Metsähovi radio telescope.
Polarimetry: Optical polarization measurements (Stokes parameters $Q$ and $U$ ) from the SO spectropolarimetric program.

The analysis employs DECOMP, a Bayesian decomposition framework (Li et al. 2020). The model treats the total observed optical emission ( $C_t$ ) as the sum of disk ( $C_d$ ) and jet ( $C_j$ ) contributions. Both components are modeled as independent Damped Random Walk (DRW) stochastic processes. The model incorporates transfer functions to link the disk continuum to the H $\beta$ line and the jet continuum to the 37 GHz radio emission. Crucially, the decomposition is constrained by polarization data, leveraging the distinct polarization signatures of the thermal disk (low polarization) and the synchrotron jet (variable polarization). The model simultaneously fits the optical continuum, emission line, radio, and polarization data to derive posterior distributions for physical parameters.

Key Results
The Bayesian analysis yields distinct decompositions for the two sources:

PKS 1510-089:
- Decomposition: The disk contributes approximately 37% ( $f_d = 0.37^{+0.03}_{-0.03}$ ) of the total optical flux, indicating that the jet dominates the optical continuum.
- Revised Lag: Using the decomposed disk continuum, the H $\beta$ lag is revised to 54.4 $^{+4.4}_{-5.4}$ days, which is shorter than previous estimates based on the total continuum.
- Polarization: The jet component exhibits an average polarization degree of $\sim$ 3.6% and a polarization angle of $\sim$ 78.8 $^\circ$ .
- Radio Lag: The 37 GHz radio emission lags the optical jet component by $8.3^{+2.5}_{-1.4}$ days.
PKS 0736+017:
- Decomposition: The disk contributes approximately 70% ( $f_d = 0.70^{+0.01}_{-0.01}$ ) of the total optical flux, suggesting the disk dominates the optical emission during the observed quiescent interval.
- Revised Lag: The H $\beta$ lag derived from the jet-subtracted disk continuum is 84.8 $^{+23.2}_{-18.4}$ days, slightly longer than previous measurements.
- Polarization: The jet component has an average polarization degree of $\sim$ 2.05% and a polarization angle of $\sim$ 91.8 $^\circ$ .
- Radio Lag: The 37 GHz emission lags the optical jet continuum by $41.6^{+18.5}_{-11.5}$ days.

Significance and Limitations
The paper demonstrates that careful decomposition of optical light curves using polarization constraints is essential for accurate RM in jet-dominated AGNs.

R-L Relation: After correcting for jet contamination, PKS 1510-089 shifts closer to the canonical $R_{BLR}$ – $L_{5100}$ relation, though it remains below the best-fit line, potentially due to high accretion rates and slim-disk effects. Conversely, PKS 0736+017 shifts further from the relation, exhibiting an unusually extended BLR size relative to its luminosity.
Methodological Caveats: The authors note a limitation in the DECOMP approach for highly variable blazars. Because the jet component is modeled based on the 37 GHz radio light curve, the model captures long-term jet trends but fails to reproduce rapid, short-timescale optical jet flares. In PKS 1510-089, this results in the "disk" component retaining residual short-timescale jet emission, suggesting the derived disk fraction and H $\beta$ lag for this source should be interpreted with caution.
Conclusion: The study validates the necessity of isolating thermal continuum emission to refine BLR size estimates and assess the applicability of RM-based black hole mass measurements in FSRQs, highlighting that jet contamination can artificially inflate observed luminosities and distort lag measurements.

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