May 2019
Very-high-energy (VHE, E > 0.1 TeV) astronomy has rapidly progressed in the last years and currently about two hundred sources at TeV energies are known. The variety of TeV source classes is comparable to the variety of sources observed at lower-energy, well-established observational windows. Imaging Atmospheric Cherenkov Telescopes (IACTs) have been the main drivers of this success. IACTs record the traces of VHE gamma rays through the detection of Cherenkov light triggered by secondary particles in the atmosphere. A constant improvement of the instrument designs, the successful implementation of extensive numerical simulations of air showers and instrument responses, and the development of sophisticated data analysis techniques which can efficiently discriminate gamma rays from the much more numerous background cosmic-ray events, are behind the recent achievements of VHE astronomy with IACTs.
The good performance of IACTs make it possible to detect bright astronomical sources almost immediately: for instance with a couple of observation hours we “see” the powerful Crab Nebula, the first source discovered in the VHE domain [1], and which is still used as as a “standard candle” for TeV astronomy. Bright Galactic sources such as the Centre of our Galaxy, the shell-like supernova remnant (SNR) RX J1713.7, the extended pulsar wind nebula (PWN) HESS J1825-137, or the (variable) emission from X-ray binary systems (XRBs), are also listed amongst the first discoveries reported in the early years of operation of the third generation of IACTs [2].
While accumulating data over the years, however, IACTs improve their sensitivity “only” at a rate approximately proportional to square-root of observation time, and the discovery of new sources usually requires increasingly large amounts of observational time. In our Galaxy, the vast majority of potential new TeV sources are expected to lie along the Galactic Plane. Deep and homogeneous observations of large portions of the Galactic Plane represent an obvious (albeit expensive) strategy by which new TeV emitters can be discovered. Such an effort has recently been conducted with the H.E.S.S. array of Cherenkov telescopes. The H.E.S.S. Galactic Plane Survey (HGPS; [3]) accounted for more than 2700h of observations of a region encompassing about half of the plane’s longitudinal extension (from l=65° to l=250°) within latitudes between ± 3°. The HGPS has revealed the presence of 78 VHE sources, about half of them classified as either SNRs, PWNe and XRBs. The other half account for systems not yet firmly identified or which could not be unambiguously associated with sources in other astronomical catalogs.
HESS J1826-130 belongs to this latter class of newly discovered systems: unidentified TeV emitters in the Galactic Plane without an associated counterpart at other wavelengths. It belongs also to a growing class of sources affected by “source confusion”, for which the presence of nearby extended and bright sources makes their detection particularly challenging. HESS J1826-130 is located at R.A. 18h26m02.16s, Dec. -13d04m, or about 0.7° away from the centroid of HESS J1825-137, a bright PWN with an extended morphology occupying an angular size of about 1.5° (see Fig. 1) [4]. The presence of HESS J1825-137 not only poses problems for the detection of the much smaller (and fainter) HESS J1826-130 (with an extension of about 0.2°), but makes it also difficult to derive its spectral properties given the uncertain amount of contaminating radiation from the large PWN.
Indeed, the data set accumulated on HESS J1826-130 that triggered its initial discovery (γ-ray 2016 Symposium; [5]), and even the data used for the source analysis carried out in the HGPS, were not enough to properly characterise the spectrum and morphology of HESS J1826-130 at TeV energies. For this reason, the total observation time on the source had to be almost doubled, up to about 200 hours of H.E.S.S. time. Furthermore, distinct data analysis cuts had to be employed, as well as dedicated studies which could account and constrain the actual contamination by HESS J1825-137. When studied at increasing gamma-ray energies, the relative contribution of HESS J1826-130 with respect to its masking neighbour becomes more and more pronounced, as can bee seen in Fig. 2.
HESS J1826-130 displays a remarkably hard spectrum, with a differential flux proportional to Eγ-1.78 , where Eγ is the gamma-ray photon energy, which extends up to a cutoff energy of about 15 TeV. It is worth noting that the nearby source, HESS J1825-137, displays a much softer spectrum. Depending on the uncertain degree of contamination, HESS J1826-130 intrinsic spectrum may be even harder. HESS J1826-130 stands therefore as one of the hardest sources observed so far in the VHE domain, almost at the level of the bright PWN Vela X (see Fig. 3 for a comparison of HESS J1826-130 spectrum with that of Vela X).
The nature of the powerful accelerator in HESS J1826-130 is still unknown. A look into lower-energy emitters in the region provide however some good candidate counterparts. The first evidence for gamma-ray emission from the region came with the detection of 3EGJ1826-1302 with the EGRET satellite (Hartman et al. 1999). Follow-up X-ray observations of the FoV led to the detection of a diffuse X-ray source, AXJ1826-1300, in the 2-10 keV energy range with the ASCA satellite (Roberts et al. 2001). This diffuse emission was later resolved with the Chandra X-ray satellite (Roberts et al. 2007), which revealed the presence of a new PWN, the Eel Nebula. The detection of the radio-quiet Fermi-LAT gamma-ray pulsar PSRJ1826-1256 (Abdo et al. 2009) with a position consistent with the Eel Nebula represented a key discovery for the MWL view of the region. The surroundings of HESS J1826-130 contain also two SNRs, G18.1-0.1 and G18.6-0.2, and several dense molecular clouds with densities that can reach nH2 ~ 700 cm-3.
At least two distinct scenarios seem therefore capable to explain the nature of HESS J1826-130. In the first one, electrons/positrons accelerated at the shock interface of the Eel nebula could up-scatter low-energy photons present in the immediate environment, either infra-red photons produced locally or photons from the cosmic microwave background, up to the VHE regime. Alternatively, HESS J1826-130 could be powered by highly energetic cosmic rays accelerated in the HESS J1826-137 progenitor SNR, which upon interaction with the dense molecular clouds in the region, acting as an effective “calorimeter”, would give rise to the observed TeV emission. This latter scenario is particularly interesting, as it could shed light into the yet unresolved question on the origin of the highest-energy Galactic cosmic-rays. In this regard, the emission detected by H.E.S.S. would point to the presence of protons with energies of a few-hundreds of TeV, close to the so-called “knee” spectral feature present in the cosmic-ray differential flux observed from the Earth, making HESS J1826-130 a new pevatron candidate [6].
While the extreme properties of HESS J1826-130 were key for its discovery, together with the good performance of H.E.S.S. and an unusually large amount of observation time devoted to the source, other hard TeV emitters in crowded regions will certainly be revealed in the next years. The first telescopes of the Cherenkov Telescope Array CTA) are already in place and in commissioning phase. This observatory will bring a further step in resolving out the components of the TeV sky, and on the identification of active and powerful accelerators in our Galaxy. New systems with similar properties as HESS J1826-130 will help us understanding the processes behind their gamma-ray emission, possibly linked to the origin of the highest-energy Galactic cosmic-rays.
References:
[1] T. Weekes et al., “Observation of TeV gamma rays from the Crab nebula using the atmospheric Cerenkov imaging technique”, Astrophys. J 342 (1989) 379
[2] H.E.S.S. Collaboration (F.A. Aharonian et al.) “A new population of very high energy γ-ray sources in the Milky Way”, Science 307 (2005) 1938
[3] H.E.S.S. Collaboration (H. Abdalla et al.), “The H.E.S.S. Galactic plane survey”, Astron. Astrophys. 612 (2018) A1
[4] H.E.S.S. Collaboration (H. Abdalla et al.), “Particle transport within the pulsar wind nebula HESS J1825-137”, Astron. Astrophys. 621 (2019) A116
[5] O. Angüner et al. (H.E.S.S. Collaboration), “HESS J1826-130: A very hard γ-ray spectrum source in the galactic plane , AIP Conf Proc. 1792 (2017) 040024
[6] H.E.S.S. Collaboration (A. Abramowski et al.), “Acceleration of petaelectronvolt protons in the Galactic Centre”, Nature 531 (2016) 476
