BL Lacertae (BL Lac), a blazar located about 950 million light years from Earth and discovered almost a century ago, has been found to have reached its maximum brightness.
The results published in the latest study, led by postdoctoral researcher Aditi Agarwal of the Raman Research Institute, an autonomous institute funded by the Department of Science and Technology (DST), said the flare emanating from BL Lac peaked in August. 21, 2020, which is a new finding about this source.
For several decades, BL Lacertae (commonly referred to as BL Lac), the source of an active galactic nucleus (AGN), has remained a major subject of study among the global astronomical community.
AGNs, usually located in the core of a galaxy, are compact structures exhibiting anomalous luminosity from time to time. Variations in their brightness levels can vary and last for hours, days, weeks or even months. This scattering of luminosity, when seen by the human eye alone, is nothing but electromagnetic emissions that are visible at radio, microwave, infrared, optical, ultraviolet, X-ray, and gamma wavelengths.
The flare and its decline in emission from BL Lac were detected using continuous observations made by an array of eleven optical telescopes located around the world. One of them was the Himalayan Chandra Telescope located in Ladakh’s Hanle.
In early July 2020, astronomers suspected that BL Lac was starting to flare up. All eleven telescopes were immediately deployed and focused observations continued for 84 days from 13 July 2020 to – 14 September 2020.
“As time progressed, the flare was observed to gradually brighten, indicating that BL Lac became more active. For the first time on August 21, 2020, BL Lac reached its maximum brightness. This was well captured by the Modified Dall–Kirkham telescope located in Krakow, Poland ,” said Agarwal, lead author of a paper titled “Analysis of nighttime variability of BL Lacertae during its August 2020 flare,” published in the Astrophysical Journal Supplements Series.
The results showed that the magnitude (luminosity) of BL Lac increased from 14 to 11.8 (as used astronomically), while its flux increased from 13.37 milli Jansky (mJy) to a peak of 109.88 mJy, another first observation of this resources. Additionally, this international collaboration of researchers calculated the source’s magnetic field, which varied from 7.5 Gauss to 76.3 Gauss during the flare.
These significant calculations, never before possible, were made only because of the availability of terabytes of data sets obtained from a battery of telescopes. “These new parameters will form the basis for future multispectral studies of BL Lac,” Agarwal emphasized.
In addition, the scientists were also able to estimate the size of the flare-emitting region of this source, which was calculated to be 1/8 of the actual radius of the jet. Agarwal’s team is continuing further observations of this BL Lac and trying to understand its characteristics at X-ray and gamma wavelengths, which are underway.
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