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Using NASA’s Neil Gherels Swift Observatory, Indian astronomers observed a blazar known as OJ 287. The results of the observing campaign, published May 25 in the arXiv pre-print server, has shed more light on the spectral changes of OJ 287, improving our understanding of the behavior of this source.
Blazars are very compact quasars associated with supermassive black holes (SMBHs) at the centers of active giant elliptical galaxies. They belong to a larger group of active galaxies that host active galactic nuclei (AGNs) and are the most numerous extragalactic gamma-ray sources. Their features are relativistic jets aimed almost exactly at the Earth.
Based on their optical emission properties, astronomers divide blazars into two classes: flat-spectrum radio quasars (FSRQs) that have prominent and broad optical emission lines, and BL Lacertae objects (BL Lacs), which do not.
OJ 287 is an optically bright BL Lac located approximately 3.5 billion light-years from Earth. Its supermassive black hole is estimated to be 100 million times more massive than the sun. The blazar exhibits a 1112-year periodic variation with a narrow double peak at maximum brightness, suggesting that it hosts a binary SMBH.
OJ 287 is one of the best monitored blazars by the Swift spacecraft. Previous Swift observations of OJ 287 found that it exhibits distinctive spectral phases and that its spectra are representative of the entire blazar class. Recently, a team of astronomers led by Pankaj Kushwaha of the Indian Institute of Science Education and Research Mohali in India decided to delve further into this topic by conducting another Swift study of OJ 287.
‘In the current work, we focus on a detailed spectral investigation of the simultaneous optical to X-ray spectrum of the lowest observed measured X-ray flux state of OJ 287 from the Swift Observatory,’ the researchers wrote.
The new observations found that OJ 287 has an extended optical-ultraviolet synchrotron spectrum, with a power-law photon spectral index of 2.71, that continues down to X-ray energies during the plus X-ray flux state. low (and intermediate). Taking this into account in the spectral fit, the astronomers inferred a hard intrinsic X-ray spectrum with a power-law photon spectral index at a level of 1.22, thus harder than the hardest spectrum reported for this blazar considering X-ray data only.
According to the study, OJ 287’s extended optical-ultraviolet spectrum indicates a very broad spectrum of high-energy particles. Additionally, the reported hard X-ray spectrum naturally reproduces most of the source’s observed flat X-ray spectra when combined with the corresponding optical-ultraviolet spectrum.
The paper’s authors noted that these findings could have implications for our overall understanding of the X-ray flat spectra of OJ 287.
“The finding further strengthens our previous claim that most of the flat X-ray source spectra in the LBL/LSP [low-frequency peaked/low-synchrotron peaked] The spectral phase of the source is due to the optical-UV synchrotron spectrum extending to X-ray energies,” concluded the scientists.
Pankaj Kushwaha, On the Spectral Changes of OJ 287I: Particle Spectrum during the Lowest X-ray state extended at Optical-UV and Hard at X-rays, arXiv (2023). DOI: 10.48550/arxiv.2305.16144
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