May 2017
The pioneering Cherenkov telescope generation (Whipple, HEGRA, CAT, CANGAROO) has already been used very successfuly to detect TeV emission from known objects, i.e. from objects that have been discovered and characterised in other wavebands before. But the sensitivity of dedicated large-scale scans, e.g., of the Galactic plane (which indeed had been attempted, e.g., with the HEGRA telescope system) has not been sufficient to detect new TeV sources. The only TeV-first discovery of a new source (TeV J2032+4130 with the HEGRA telescopes) was a serendipitous discovery in the field of view of another target. This situation has changed dramatically with the current generation of Cherenkov telescopes (H.E.S.S., VERITAS, MAGIC). For the first time, the available sensitivity has enabled us to successfully perform blind searches for new TeV sources (see for example SOM 01/2016).
On the order of 100 new TeV sources have been discovered in the Galactic plane scans over the past years, a large fraction with H.E.S.S. To identify the astrophysical nature of the individual sources, the usual approach is to search for counterparts in other wavebands. If the counterpart association is secured and the astrophysical nature of the counterpart is known, then the nature of the TeV source is also identified (though not necessarily the nature of the TeV-radiating particles and the details of the particle acceleration mechanism). Identified Galactic TeV sources belong to one of the following categories: pulsar wind nebulae, supernova remnants, stellar binary systems hosting a compact object, and molecular clouds (the last case is special because it does not identify the particle acceleration site, only the radiating site). Several sources are classified to have candidate identifications (for example TeV pulsar wind nebula candidates, or TeV binary candidates). Those classifications are motivated by possible associations with objects in other wavebands, but without a firm confirmation of the suggested identifications.
What has been done now for the first time with the available H.E.S.S. Galactic plane data is to classify individual unidentified TeV sources as strong candidates for (young to middle-aged) supernova remnants, purely from their morphological appearance in the TeV maps. Supernova remnants exhibit in general a shell-type appearance from the spherical expansion of the shock waves following the supernova explosion. Most of the well-identified TeV supernova remnants show this shell-type morphology. Therefore, it has been natural to search for shell-type appearances also amongst the H.E.S.S. sources in the Galactic plane survey data set. A homogeneous treatment of the entire H.E.S.S. data set following a pre-defined procedure has been applied, that was shaped based on the appearance of the known TeV supernova remnants. The procedure has shown that three H.E.S.S. sources have a significant shell-type appearance, HESS J1534-571, HESS J1912+101, and HESS J1614-518. These sources have consequently been classified as supernova remnant candidates. HESS J1534-571 has meanwhile been confirmed to be a supernova remnant, from the clear morphological identification with a supernova remnant candidate detected in the radio band (cf. SOM 02/2017).
HESS J1912+101 (Fig. 1) stands out of the three sources. The updated analysis of the full H.E.S.S. data set [1] yields a nearly perfect shell appearance (an earlier analysis of a limited data set has been superseded). Yet, no convincing counterpart has been found in any other waveband, despite very intensive searches. It is however difficult to judge whether the lack of counterparts is due to intrinsic source properties. If so, this would make this a unique object. It could however also be that the existing data in other wavebands are just not sufficiently sensitive yet. In fact, it is for example very difficult to perform X-ray satellite observations in the sky field in which HESS J1912+101 is located. Strong stray-light contamination from a very bright X-ray source (GRS 1915+105) prevents very sensitive X-ray observations in the field with instruments such as XMM-Newton. Similar effects might have prevented the detection of a radio counterpart so far. To really judge whether HESS J1912+101 is physically fundamentally different from the other identified TeV-emitting supernova remnants, very constraining flux limits in the radio and X-ray bands have to be established. This is going to be very challenging.
Still, why don’t we simply classify HESS J1912+101 as a new supernova remnant, given the nearly perfect morphological shell-type appearance? There are a few reasons why we are a bit conservative. In principle, we may be fooled by a superposition of several unidentified TeV sources which by chance mimic a shell appearance. For HESS J1912+101, the probability for this seems rather low. However, and perhaps more importantly, the discovered shell of TeV-emitting particles (HESS J1912+101) might in principle also have been formed by other processes than a single supernova remnant. For example, a collective phenomenon called superbubble might also appear like a shell. In this case, an ensemble of supernova remnants and stellar wind bubbles forms a shell at which particles might be accelerated to TeV energies. One such case is the superbubble 30 Dor C in the Large Magellanic Cloud, which is known to emit TeV gamma-rays [2], see also here). For all these reasons, HESS J1912+101 remains for the moment just classified as supernova remnant candidate.
References:
[1] H.E.S.S. Collaboration, H. Abdalla et al., “A search for new supernova remnant shells in the Galactic plane with H.E.S.S.”, submitted.
[2] H.E.S.S. Collaboration, A. Abramowski et al., ” The exceptionally powerful TeV gamma-ray emitters in the Large Magellanic Cloud”, Science 347 (2015) 406-412
