The bright extreme BL Lac object HESS J1958-301
December 2020
Many active galactic nuclei (AGN) harbour powerful cosmic particle accelerators. Charged particles, such as protons and electrons, acquire very high energies in these natural accelerators, whose efficiency vastly outperforms the most powerful accelerators on Earth. The objects in which particles are accelerated to the highest energies are of particular interest to high-energy astrophysics.
Charged particles do not move on straight lines in the presence of cosmic magnetic fields, and it has not yet been possible to directly associate one of the ultra-high-energy charged particles (called 'Cosmic Rays') with a specific active galactic nucleus. On the flip side, observations of the highest energy gamma-rays (which are neutral particles and hence travel straight from the source to the observer) serve as the best pointer to the powerful accelerators. During the last decades X-ray observations revealed a population of AGN displaying synchrotron emission with photon energies up to 100 kilo-electronvolt (keV). Astrophysicists refer to these objects as high energy BL Lac objects or extreme BL Lac objects. Their peculiar spectral properties may help in understanding the properties of cosmic accelerators. It has long been a goal of Gamma-ray astrophysics to study these extreme sources and to measure the maximum energies that are provided by the powerful accelerators residing in their centres. The accelerated particles do not only emit the energetic synchrotron emission observed in the X-ray band, they also scatter these synchrotron photons to still higher energies - sometimes exceeding 1 Tera-Electronvolt (10.000.000 more energetic than the already very energetic synchrotron photons of up to 100 keV) by so-called inverse Compton scattering. The nature of these processes can be revealed by measuring the spectrum at the highest gamma-ray energies. Extreme BL Lac objects are also important probes for many cosmological studies. An important prerequisite for such investigations is the discovery of extreme BL Lac objects. They have unique spectral properties but are rare objects.
During the last decades the sky has been surveyed in very many energy bands with very sensitive instruments, detecting up to millions of objects in the various surveys each. Particularly well suited candidate 'extreme BL Lac objects' are aAGN whose flux increases with higher photon energies. A number of such sources have been identified at lower gamma-ray energies (below 300 GeV) with the Large Area Telescope (LAT) on board the Fermi satellite. Combining the data for each and every object allows appropriate classification. Even rare objects with peculiar spectra can be identified like needles in a haystack. Figure 1 presents broadband properties of a sample of sources, combining data from many telescopes to derive the frequency of maximum energy flux of the synchrotron spectra and the spectral shapes of these objects as derived from Fermi-LAT measurements. Promising candidates have been observed with the H.E.S.S. array, and resulted in the discovery of new sources.
HESS J 1958-301 is the gamma-ray source detected at the position of a BL Lac object which was identified in radio-, X-ray-, and low-energy gamma-ray surveys conducted with the VLA, the ROSAT satellite, and the Fermi-LAT instrument, respectively. Following astronomical naming convention, the object is hence referred to as NVSS J195814-301112, 1RXS J195815.6-301119, and 3FHL J1958.3-3011 by the New VLA Sky Survey catalogue, the ROSAT X-ray catalogue, and the 3rd Fermi-LAT catalogue of hard spectrum sources. Observations at the location of this candidate revealed the new, spatially unresolved VHE gamma-ray source very clearly (Fig. 2). Already after the first few hours, the source was recorded with more than 5 standard deviations (5 'sigma'), which is conventionally considered a requirement for a reliable detection.
Spectroscopic observations have shown that 1RXS J195815.6-301119 is receding from Earth with a velocity of 33650 km/s, which corresponds to a redshift of z=0.1193. HESS J 1958-301, coincident with 1RXS J195815.6-301119 is hence assumed to be 560 Mpc away from the Milky Way. At such large distances, the VHE gamma-ray spectra emitted by the object are significantly absorbed by intervening radiation fields (see source of the month, January 2013). The spectra observed on Earth of sources lying at such distances have to be corrected for this absorption in order to reconstruct the original spectral shape. The techniques developed for such a correction and the knowledge about the true amount of absorption have been improved for many years and different estimates are now fairly consistent. Applying the correction to the observed spectral energy distribution of HESS J 1958-301/3FHL J1958.3-3011 reveals an intrinsic spectrum that follows a power-law with a spectral index Gamma of approximately Gamma ~2 (Fig. 3), This suggests that HESS J 1958-301 is indeed an extreme BL Lac object, whose intrinsic energy spectrum still rises up to photon energies of 1 TeV. Only very few sources are known so far which have such extreme spectra.
On the one hand, scenarios involving only electrons and positrons accelerated in the jet of the AGN (so-called leptonic scenarios) for such extreme BL Lac objects require that these leptons are accelerated up to very high energies of up to 100,000 times their rest mass and very low magnetic fields in order to limit their energy loss due to synchrotron emission. On the other hand, scenarios also including the acceleration of protons, require high magnetic fields and very high jet powers. In these scenarios, the TeV emission could be due to a combination of proton-synchrotron radiation and the decay of neutral pions ([7], [8]). Interestingly, such scenarios would also predict neutrino emission from such objects.
As other extreme BL Lac objects, HESS J 1958-301 will also be an ideal candidate to search for emission of an electromagnetic cascade that could unfold in the intergalactic medium. The gamma rays emitted by the source will collide with photons from radiation fields and generate electron-positron pairs. These pairs can in-turn Compton scatter photons of the Cosmic microwave background and the scattered photons can end up having gamma-ray energies and again produce pairs. As the electron-positron pairs are deflected in the intergalactic magnetic field (IGMF), the cascade emission could be observable as a faint halo around the BL Lac (Aharonian et al. 1994). Together with the observations of other extreme BL Lacs, the detection of HESS J 1958-301 could ultimately help to indirectly determine the strength of the IGMF (Biteau et al. 2020).
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