In a silent way: extreme particle acceleration in HESS J1741-302 without low-energy footprints
25 years ago, when the discovery of the Crab nebula at very-high-energy (VHE, E> 0.1 TeV) gamma rays marked the beginning success of ground-based gamma-ray astronomy, it was commonly thought that most TeV gamma-ray sources would be detectable at other wavelengths, too. In recent years, however, more and more TeV sources were discovered which are not detected through observations at lower photon energies. Such VHE gamma-ray sources are tagged as ``unidentified sources'' as they cannot be associated to any astrophysical object identified through emission at longer wavelengths. HESS J1741-302 is one of these unidentified sources, being seen on the VHE gamma-ray sky up to energies of 10 TeV, but being silent at other wavelengths . As an interesting side aspect, the location of HESS J1741-302 is coincident with the position of a (now disproven) hint for a 130 GeV line-like gamma-ray signal discussed last month.
The usual approach to reveal the nature of unidentified sources is to perform multi-wavelength investigations of the source region, aiming at identifying possible objects that can be confidently associated with the observed VHE emission. Unfortunately, dedicated analyses of the region around HESS J1741-302 performed both at X-rays  and high-energy (0.1-100 GeV) gamma rays  did not result in an obvious counterpart of the VHE source. The VHE gamma-ray spectrum measured by H.E.S.S. continues up to at least 10 TeV in energy; from that information we know that the astrophysical object producing the TeV emission must be capable of accelerating protons or electrons (or both) up to energies of several hundreds of TeV. HESS J1741-302 is thus a source of ultra-relativistic particles and, at the same time, a so called ``dark accelerator'', meaning that the astrophysical object driving the particle accelerations remains unknown. >
Improved analysis techniques, leading to increased sensitivity and angular resolution, and a total of 145 hours of H.E.S.S. data recorded on this source between 2004 and 2013, enable us to update previous investigations  with studies on the VHE gamma-ray morphology and spectrum of this enigmatic source (see Fig. 1 for the gamma-ray excess map of HESS J1741-302). The angular extension of the source is compatible with the angular resolution of H.E.S.S., meaning that the emission region is point-like (we derive an upper limit of 0.077° on the intrinsic source extension at a 99% confidence level). With an integrated flux of 1% of the Crab Nebula flux above 1 TeV, HESS J1741-302 is one of the faintest sources in the VHE sky; its differential energy spectrum is compatible with a simple power-law distribution with a spectral index of Γ = 2.3 ± 0.2stat ± 0.2sys. As can be seen from Fig. 2, the emission extends at least to energies of about 10 TeV. Since HESS J1741-302 is such a faint source, the number of VHE photons in excess of 10 TeV is so small that we cannot say whether the spectrum continues as a power-law well beyond this energy or cuts off. A variability analysis suggests that the integral flux is constant within statistics for the periods during which the source was observed with H.E.S.S.
The analysis of data from surveys of atomic and molecular hydrogen around the source direction suggests the existence of dense gas material in this region (see Fig. 3 (top)), which might serve as target material for the ultrarelativistic particles to produce the observed VHE gamma-ray emission. From the energy budget point of view, each interstellar cloud feature found along the line-of-sight to HESS J1741-302 could give rise to the observed level of VHE flux if a cosmic-ray source is located in the vicinity of (or inside) one of these dense gas regions (Fig. 3 (bottom)). However, no cosmic-ray accelerator (such as a supernova remnant) is known to be present close to the source location.
In contrast to our earlier results  we are now able to provide the centroid position of HESS J1741-302. This position is, within statistics, coincident with the compact radio source 1LC 358.266+0.038 . Despite the fact that the radio source is still unidentified, and no variability has been observed in HESS J1741-302 due to low statistics, an association of the two could be a hint for a gamma-ray binary; in fact, this scenario turned out to be realised for other point-like unidentified H.E.S.S. sources in the past. On the other hand, a relic pulsar wind nebula leptonic scenario related to the pulsar PSR B1737-30, located about 0.4 kpc away from HESS J1741-302 , is not excluded and could be verified with future X-ray observations of this pulsar. Scenarios involving an extragalactic origin of the source are highly unlikely when taking into account that the source is located very close to the Galactic plane.
In fact, the field-of-view surrounding HESS J1741-302 is even more complicated: although not significantly detected in our analysis, there is evidence for large-scale emission towards North of the source (see Fig. 1). It is interesting that the extremely bright luminous blue variable (LBV) star Wray 17-96 , which is one of only ~20 high-mass-loss LBV stars known in the Galaxy, is located just 1.1 standard deviations away from the best fit position of this emission. Further studies are needed to see whether or not the VHE gamma-ray emission is related to Wray 17-96.
For the moment, despite a thorough investigation of its multi-wavelength context and consideration of a variety of plausible emission mechanisms, HESS J1741-302 stays a dark accelerator. Is it a new gamma-ray binary, yet another pulsar wind nebula, or is its emission caused by a very much different object? With its enhanced sensitivity and better angular resolution, this question may be possibly answered only by the next generation of imaging Cherenkov telescopes, the Cherenkov Telescope Array.
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