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Astronomical Observatory of the Jagiellonian University


Astronomy Object of the Month: 2019, August

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Rhythmic oscillations of the blazar Markarian 501

Dr. Gopal Bhatta from the Jagiellonian University in Kraków has detected transient rhythmic oscillations in the gamma-ray emission from the blazar Markarian 501. The discovery could be helpful in improving our understanding of the most energetic processes taking place in the universe.

Figure 1: Sloan Digital Sky Survey image of blazar Markarian 501. Credit: SDSS.

Blazars, classified as members of a larger group of active galaxies that host active galactic nuclei (AGN), are the most luminous sources in the Universe. They are believed to harbor monstrously giant black holes fed by the matter falling from the parsec-scale accretion disks. These sources launch the most powerful relativistic jets beamed upon us and ejecting matter with high speeds (their characteristic features are jets pointed almost exactly toward the Earth).

From the astronomical point of view, blazars are high-energy engines serving as natural laboratories to study particle acceleration, relativistic plasma processes, magnetic field dynamics and black hole physics.

Located some 456 million light-years away, Markarian 501 (Mrk 501) is a blazar or a type known as BL Lacertae source, with a spectrum extending to the highest energy gamma rays. It is also one of the nearest blazars that shines bright in the X-ray and one of the earliest extragalactic sources detected in the TeV ( teraelectronovolt) energy range.

Within the last decade, Mrk 501 was observed by the Large Area Telescope (LAT) of NASA's Fermi Gamma-ray Space Telescope. A group of astronomers led by Dr. Bhatta from the Astronomical Observatory of the Jagiellonian University has analyzed the observational data of Mrk 501 collected by LAT between August 2008 and June 2018. The study resulted in the detection of rhythmic oscillations in the blazar's gamma-ray emission.

Blazar shows a strong signal of quasi-periodic oscillation (QPO) with a periodicity of around 332 days. Moreover, the gamma-ray flux modulations in Mrk 501 gradually decayed in strength during the recent years. Researchers emphasized that the detected gamma-ray oscillation in this blazar persisted nearly seven cycles until it finally weakened. Given that not many gamma-ray QPOs have been found to show more than five cycles, the interesting case of Mrk 501 might be one of the few examples in which astronomers witness a relatively high frequency with a period of less than a year, and the first event where we observe a transient gamma-ray periodic modulations in the high energy emission of the blazar that persist up to 7 cycles before fading away.

The study also presents several hypotheses about what could be the driving force behind such rhythmic oscillations in Markarian 501. Dr. Bhatta suggest various scenarios, including supermassive binary black holes, jet precession and accretion disk precessing under gravitational torque. However, he also concludes that further analysis of Mrk 501 and discussion on the topic are needed in order to definitely determine the most plausible theory explaining the origin of the oscillations in this blazar.

Original publication: Gopal Bhatta: Blazar Mrk 501 shows rhythmic oscillations in its γ-ray emission, Monthly Notices of the Royal Astronomical Society, 18 August 2018.

The above description was based partly on the article previously published in Phys.org

The research was conducted at the Department of High Energy Astrophysics of the Jagiellonian University’s Astronomical Observatory (OAUJ). The work was supported by the Polish National Science Centre through the grant UMO-2017/26/D/ST9/0117.

Gopal Bhatta
Astronomical Observatory
Jagiellonian University
Gopal.Bhatta [at] uj.edu.pl