MRC 0910-208: A moderate extremist

June 2022

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Most of the gamma-ray sources outside our Milky Way are active galactic nuclei. These galaxy centers outshine their entire host galaxies including all stars. One of the first ones to be studied in detail – 3C 273 – was identified in 1963 as a bright radio-source, although due to its apparent resemblance to a star, it was called a quasi-stellar radio source or "quasar" [1]. However, the large redshift of this object and thus the large distance from Earth demonstrated that this object must be located at such a large distance that its luminosity exceeds even the brightest star more than a billion times. Other quasi-stellar radio sources remained more elusive, their optical spectra lacking the spectral lines which are necessary to determine a redshift. BL Lacertae was one of the first of these objects where finally, in 1974, a host galaxy of the quasi-stellar nucleus was found and the redshift was obtained. Since then, many similar objects – knows as BL Lac-type objects – have been observed in all frequency bands of the electromagnetic spectrum and reveal different properties. When studying the amount of energy emitted from a BL Lac-type object at different electromagnetic wavelengths, it became apparent that two prominent humps dominate over a broad wavelength range (see Fig. 1).

Fig. 1: Representation of the spectral energy distribution of different types of BL Lac objects. Both, so called low-frequency-peaked BL Lac objects or 'LBL' sources and high-frequency-peaked BL Lac objects or 'HBL' objects reveal two humps in a diagram that shows the amount of energy radiated in different frequency bands. In very rare cases - so called 'extreme' BL Lac objects, the maxima of the two humps emerge at even higher frequencies.
These distributions are referred to as 'Spectral Energy Distributions (SEDs). At long wavelengths (corresponding to low photon energies) one hump peaks in the infrared to X-ray band. This is caused by the synchrotron emission of the electrons and positrons that are accelerated in the relativistic jet that is ejected from all quasi-stellar radio sources and BL Lac-type objects. At very short wavelengths (corresponding to high photon energies of 106 electron volt (or MeV) to 1012 electron volt (or TeV) gamma rays, a second hump emerges. The main models for explaining the emission in the gamma-ray band are leptonic (via inverse Compton emission of relativistic electrons), or hadronic (e.g., via protons) interactions, and both could play a role in the overall emission.

Fig. 2: Very-high energy spectrum of MRC 0910-208 measured with H.E.S.S. in 2018. (Image credit: [4])

Deriving the true physical properties of the quasi-stellar radio sources is not easy. The exact positions of the peaks of the two humps and their relative amplitudes are an important diagnostic. The positions of the maxima are also used to classify the large population of BL Lac-type objects. Interestingly, sources of distinct SEDs show rather different cosmological evolution. BL Lac-type objects that display a synchrotron peak in the optical wavelength regime (at comparatively low frequencies) are referred to as low-frequency-peaked BL Lac objects or 'LBL's. Those sources that reveal a synchrotron maximum in the X-ray domain (with frequencies about a thousand times higher than optical light) are called high-frequency-peaked or 'HBL'. The second high energy hump in the SED of these two classes peaks in the high-energy X-ray and medium energy gamma-ray band, respectively. HBLs are among the most powerful accelerators and are ideal objects for studying the physics of particle acceleration and radiation. However, objects that are even more extraordinary than HBLs also exist in the universe. In some cases, the second hump peaks at TeV energies or even higher. The search for such objects in the TeV gamma-ray band and the study of their spectral characteristics is of great interest [2,3]. These objects are referred to as 'extreme BL Lac objects'.

The HESS collaboration started a dedicated search for extreme BL Lac objects, a source class that has only a very small number of known members. Last year the H.E.S.S. collaboration announced the detection of gamma-ray emission from the object BL Lac-object MRC 0910-208 [4], which is about three billion light years away and is classified as BL Lac-object [5]. The object was observed for about 17 hours in 2018 because of its hard MeV-GeV spectrum indicating the second bump peaking in the TeV band. The observed spectrum between 0.2 and 2 TeV is affected by the absorption of the extragalactic background light, emitted by stars or dust. When gamma rays travel through the universe they interact with this light by producing electron and positron pairs. These gamma rays are therefore absorbed on the way to Earth and this effect becomes stronger the farther their origin and the higher the energy of the gamma rays is. In case of MRC 0910-208 the correction for gamma-ray absorption via the extragalactic background light result in an incredibly hard intrinsic gamma-ray spectrum of these energies (Fig. 2). Despite these characteristics, however, the high-energy maximum of the SED does not reach the very high values detected in other 'extreme BL Lac objects'.

Fig. 3: Broad-band spectral energy distribution of MRC 0910-208 including data from the low energy range in the radio up to the TeV band. The dashed line represents a model explaining the lowest energies with synchrotron emission and the highest energies with inverse Compton scattering which includes the absorption of the extragalactic background light. (Image credit: [4])

Figure 3 shows the broad-band spectral energy distribution of MRC 0910-208 with data taken from H.E.S.S. and Fermi-LAT in the gamma-ray range, Swift-XRT in X-rays and archival data from the NASA/IPAC Extragalactic Database from WISE, DENIS, and GALEX in the optical to radio band. This broad emission can be modeled with synchroton emission at low frequencies and the inverse Compton scattering of the synchroton seed photons in the high frequency range. The small bump on top of the low energy peak is the thermal component of the star light in the host galaxy resulting from the star light emission.

Objects like MRC 0910-208 do not just offer the possibility to investigate acceleration and emission processes at extreme energies, they also enable fundamental astronomical studies of the extragalactic background. Detailed studies require the combination of several such extreme BL Lac objects, an endeavor the HESS collaboration aims to complete by doubling the number of known sources of this class.


[1] M. Schmidt. “3C 273 - A star-like object with large red-shift”, Nature, 197, 487, 1963.

[2] J. Biteau et al. “Progress in unveiling extreme particle acceleration in persistent astrophysical jets”, Nature Astronomy, 4, 124–131, 2020.

[3] V. A. Acciari et al. “New hard-TeV extreme blazars detected with the MAGIC telescopes”, ApJS, 247, 16, 2020

[4] M. de Bony de Lavergne et al., “Detection of new Extreme BL Lac objects with H.E.S.S. and Swift XRT” PoS, ICRC2021, 823, 2021.

[5] E. Massaro et al. "Roma-BZCAT: A multifrequency catalogue of Blazars", A&A, 495, 691, 2009