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German and Swiss astronomers are detecting a nearby blazar Markarian 501. The observing campaign provided crucial information about the blazar's variability and spotted several flares from it.

The researchers detailed what they saw in the study titled "Long-Term Multi-Band Photometric Monitoring of Mrk 501," which they published on arXiv.org.

Markarian 501 (Mrk 501 for short) is a well-known gamma-ray blazar located 456 million light-years away, Phys.org said. It is one of the BL Lacertae objects, and the non-thermal continuum from the jet dominates its optical spectrum.

Mrk 501 produces very high-energy (VHE) gamma-rays, according to observations made with the Whipple Observatory in 1996. (over 100 GeV). It is one of the few VHE objects that can be detected with modern telescopes in a reasonable amount of time, even during low-state emission periods. Mrk 501 is a good candidate for long-term multi-wavelength monitoring because of this.

Astronomers Check Nearby Markarian 501

Markarian 501 is one of the most studied nearby bright blazars, with a redshift of 0.034. Previous observations of this source have led to the conclusion that it is an EHBL. Phys.org said a team of researchers tested this theory further. Hence, they tested a long-term multi-band photometry of Mrk 501 using a variety of ground-based and space telescopes, including the First G-APD Cherenkov Telescope (FACT).

SDSS Mrk 501
(Photo: Sloan Digital Sky Survey/via Wikimedia Commons)
Sloan Digital Sky Survey image of blazar Markarian 501

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Mrk 501's fluctuation was found in all wavebands. The radio band has the lowest fractional variability. In contrast, the TeV band has the highest, and fractional variability grows even from radio to X-rays and from GeVs to TeVs.

The TeV and X-ray fluctuations were predicted to be separated by less than 0.4 days. According to the researchers, this near-zero latency is compatible with synchrotron self-Compton (SSC) emission, in which TeV photons are generated by inverse Compton scattering.

Markarian 501 also produced several TeV and X-ray flares, according to the observations. The accretion disk around the SMBH was found to have a characteristic time interval between TeV flares that is comparable to the expectation if these flares are triggered by the so-called Lense-Thirring precession.

What Are Blazars and BL Lacertae Objects?

Astronomy.com said blazars are supermassive black holes (SMBHs) in the cores of active, large elliptical galaxies that are very compact quasars. They are the most frequent extragalactic gamma-ray sources. They are part of a broader group of active galaxies that house active galactic nuclei. Relativistic jets pointing almost precisely toward the Earth are one of its distinguishing qualities.

BL Lacertae objects (BL Lacs) are blazar with greater Doppler factors and lower-power jets than other blazars. They are classified as low (LBLs), intermediate (IBLs), or high synchrotron peak BL Lacs based on the position of the synchrotron peak (HBLs). Rare extreme HBLs (EHBLs)-known by synchrotron emission peaks at energies exceeding 1 keV-are of particular interest to astronomers. These objects are thought to be among the universe's most efficient and extreme accelerators.

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