Chinese astronomers have recently conducted a comprehensive study on the long-term variability of a blazar named PKS 0727-11, providing new insights into its behavior and nature. The study, published in The Astrophysical Journal, delves deeper into the characteristics of this particular blazar by analyzing multiwavelength data gathered from various space telescopes and astronomical surveys. The findings contribute significantly to our understanding of these intriguing and energetic cosmic phenomena.
Blazars are a subset of quasars, which are extremely compact and luminous objects associated with supermassive black holes (SMBHs) located at the centers of active galaxies. These objects are particularly noteworthy because they are the most prolific sources of gamma-ray emissions from extragalactic sources. Blazars are characterized by relativistic jets that are oriented nearly directly toward Earth, resulting in some of the most intense and variable radiation observed in the universe. These jets are powered by the SMBH at the core of the blazar, which pulls in vast amounts of surrounding material, creating an accretion disk and producing high-energy emissions.
Blazars are classified into two broad categories based on their optical emission properties: flat-spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs). FSRQs are characterized by broad and prominent optical emission lines, while BL Lacs lack such features. PKS 0727-11, which is the focus of this new study, is an FSRQ with a redshift of approximately 1.59. It is also associated with the gamma-ray source 4FGL J0730.3-1141, identified as part of the catalog of gamma-ray sources. PKS 0727-11 was first discovered in 1966 as part of the Parkes catalog of radio sources, and its behavior has intrigued astronomers ever since.
One of the notable features of PKS 0727-11 that has attracted attention is its opaque microwave spectrum at 6.63 and 10.63 GHz, as well as the presence of a compact component and rapid variability in its emissions. These characteristics make it an ideal target for studying the dynamics of blazars and their underlying processes.
In this recent study, a team of astronomers led by Yuncai Shen of Yunnan Normal University in China sought to deepen our understanding of PKS 0727-11’s variability by examining data from a range of space-based and ground-based telescopes. The researchers utilized data from NASA’s Fermi and Swift spacecraft, both of which have provided valuable observations of high-energy gamma rays and X-rays from various cosmic sources. The gamma-ray data were retrieved from the public archive of the Fermi-Large Area Telescope (Fermi-LAT), which is one of the most powerful instruments for detecting gamma rays. The X-ray data were obtained through Swift-XRT, which monitors high-energy sources like blazars. The astronomers also gathered optical R-band and near-infrared J-band data from the SMARTS program, which focuses on long-term optical and infrared observations of celestial objects. Furthermore, millimeter-wave data at 1 mm were retrieved from the Submillimeter Array (SMA) database, and radio observations at various frequencies (4.8, 8.0, and 14.5 GHz) were obtained from the University of Michigan Radio Astronomy Observatory (UMRAO).
Through this extensive dataset, the team was able to make several groundbreaking discoveries. One of the most intriguing results was the identification of a possible quasi-periodic oscillation (QPO) in the gamma-ray light curve of PKS 0727-11. QPOs are oscillations in the emission of X-rays or gamma rays that occur when gas is spiraling toward a compact object, such as a black hole or neutron star, within an accretion disk. While QPOs have been previously observed in X-ray emission from objects like neutron stars and black holes, this is the first time such oscillations have been detected in gamma-ray emissions from a blazar.
The QPO detected in PKS 0727-11 has a period of approximately 168.6 days, suggesting that there may be periodic fluctuations in the emission of high-energy radiation from this blazar. The astronomers hypothesize that these oscillations could be driven by a non-ballistic helical motion near the inner region of the accretion disk, possibly induced by the orbital motion of a close supermassive binary black hole. In this scenario, the primary black hole would have a mass estimated to be between 0.36 and 5.79 billion solar masses. The discovery of this QPO is significant because it opens up new possibilities for studying the dynamics of SMBHs in blazars and understanding how their gravitational influences can produce periodic variations in emissions.
In addition to the discovery of the QPO, the researchers also observed a strong correlation between the variations in the light curves across different wavelengths. Specifically, the gamma-ray and radio flares of PKS 0727-11 appeared to be linked, suggesting that the same underlying disturbance could be responsible for both types of emissions. This finding is consistent with previous models that propose that the emission from blazars arises from relativistic jets powered by the SMBH, where particles are accelerated to high speeds and emit radiation across a broad range of wavelengths, from radio to gamma rays.
The team also performed calculations to estimate the distance between the gamma-ray and 1-mm emission regions of PKS 0727-11. Based on their observations, they found that the two emission regions are separated by approximately 43.65 light years. This result provides valuable insight into the structure of the relativistic jets in this particular blazar, suggesting that the gamma-ray and millimeter-wave emissions originate from different regions of the jet, which are likely at different distances from the central SMBH.
Overall, the findings from this study represent a significant step forward in our understanding of the behavior and variability of blazars. By examining the multiwavelength data collected from various space telescopes and ground-based observatories, the team was able to reveal new details about the processes occurring within the relativistic jets of PKS 0727-11. The detection of the QPO and the observed correlations between different wavelength variations open up exciting avenues for further research into the complex dynamics of supermassive black holes and their jets. This research not only enhances our understanding of PKS 0727-11 specifically but also contributes to the broader field of active galactic nuclei and high-energy astrophysics.
Blazars like PKS 0727-11 are crucial to our study of the extreme environments surrounding supermassive black holes. Their intense radiation and rapid variability provide a unique laboratory for testing theories of high-energy physics, general relativity, and the behavior of matter under extreme conditions. As astronomers continue to observe and analyze these enigmatic objects, discoveries like those made in this study will undoubtedly lead to new insights into the most powerful and mysterious processes in the universe. The work done by Shen and his team marks an important contribution to this ongoing effort, and the data they have uncovered will likely influence our understanding of blazars for years to come.
More information: Yuncai Shen et al., Multiwavelength Variability Analysis of the Blazar PKS 0727-11: An ~ 168 day Quasiperiodic Oscillation in the γ-Ray, The Astrophysical Journal (2025). DOI: 10.3847/1538-4357/ada775