October 2023
The brightest source of gamma-ray emission in any energy band is the Milky Way. It dominates the all-sky maps obtained with scanning or hemispheric detectors operating in space (e.g. the Fermi-LAT) or on the ground (e.g. the HAWC or LHAASO facilities). Imaging Cherenkov telescopes, which have a more limited field of view, still reveal the plane of the Milky Way as a particularly bright band, clustered with many individual sources. Unlike almost all extragalactic sources of very-high-energy emission, most of the gamma-ray sources on or close to the plane of the Milky Way appear to be spatially extended when studied with H.E.S.S., which champions the currently operating gamma-ray instruments in terms of their angular resolution. In many cases, H.E.S.S. images reveal Galactic sources to be spatially extended, with a wide range of shapes and often complex morphology. Interestingly, in many cases the morphology revealed in the very-high-energy domain differs from the morphology that the same sources reveal in other energy bands. This, together with the high density of galactic sources, leads to the problem that it is often difficult to distinguish between different sources. This problem of confusion (telling sources apart that appear close to each other projected on the sky, but physically disconnected and often widely separated along the line-of sight) has long become evident in other energy bands. The Milky Way, after all, which appears as a smooth and diffuse band of emission to the naked eye is resolved into billions of distinct stars when studied with sufficient angular resolution. The challenge of separating adjacent sources becomes more demanding if the objects themselves are resolved. On morphological grounds it sometimes becomes difficult to distinguish whether a distinct source of emission is an individual object, potentially projected along the line of sight on top of another object of very-high-energy gamma-ray emission or a sub-component that is part of a well-defined physical source but stands out due to a local enhancement of intensity.
Towards low Galactic latitudes (i.e. along the Galactic plane) – and in particular in the regions on either side of the Galactic Center – the line of sight from Earth passes for many tens of thousands of light-years through the inner regions of the Milky Way, where sources of very-high-energy gamma rays are numerous. It is in these regions where the distinction between ‘sources’ and ‘components’ becomes difficult. In case individual regions can likely be identified as gamma-ray counterparts of well known astrophysical sources, the separation and identification is less ambiguous. A rather large fraction of VHE gamma-ray sources, however, lacks such a clear association. While this makes studies in the gamma-ray band particularly fascinating, it leaves the identification uncertain for the time being. Fortunately, the International Astronomical Union offers a prescription on how to deal with such ambiguities that facilitates better identification if new data become available in future studies. This schemes has been applied to H.E.S.S. investigations and – among other consequences – leads to the naming convention of H.E.S.S. sources.
In this month’s rendition of our blog we introduce HESS J1645−455, which has been identified in a recent paper [1] as a distinct feature almost exactly on the plane of the Milky Way, but also projected against the rim of the nearby source HESS J1646−458, which is associated with the massive stellar cluster Westerlund 1 (Fig. 1). HESS J1646−458 is a very extended source, reveals a fairly symmetric shell-type morphology and displays a very homogeneous spectral shape across its projected surface. The shell-type morphology is thought to reflect a spherical shape of a bubble shaped by the combined stellar winds of the massive young stars in the stellar cluster. HESS J1645−455 lies on top of the shell source HESS J1646−458 and might be either an independent source projected against the rim or a local enhancement of the shell’s surface brightness. The former assumption is plausible, given the proximity to the Galactic plane, which would be consistent with an independent source, possibly at much larger distance than HESS J1646−458. The latter assumption is supported by the fact that J1645−455 broadly shares the spectral properties of HESS J1646−458 (it should be noted, though, that the spectrum of the region around HESS J1645−455 differs slightly more from the average spectrum of the shell of HESS J1646−458 than any other part of the rim. An enhanced surface brightness somewhere on top of the shell of the Westerlund 1 bubble would not be extraordinary given the density of interstellar matter in this part of the Milky Way – one might actually expect such an enhancement to be likely on that side of the shell that is closest to the plane (where the density of the ISM is expected to be highest). On the other hand, maps of the molecular gas in the velocity channel that corresponds to the distance of HESS J1646−458 do not reveal any enhancement of the ISM density at or near the position of HESS J1645−455 (Fig. 2).
Even in case the assumed distance to Westerlund 1 would be wrong, there is no density enhancement at other velocity ranges (corresponding to other distance intervals). This absence of any indication why the gamma-ray emission of Westerlund 1 should be particularly bright at the location of HESS J1645−455 supports the suggestion that it is indeed a distinct, independent source. On the other hand, there is no particularly promising counterpart for an independent source. HESS J1645−455 is spatially coincident with the X-ray source 4U 1642–45. This object is a so-called low-mass X-ray binary (LMXB), a type of source neither known nor expected to emit very-high-energy gamma rays. Furthermore HESS J1645−455 is spatially slightly extended, which would be even more unexpected if 4U 1642–45 was the counterpart.
For the time being, no clear distinction between the two options can be made. Several avenues of research promise to reveal further clues that may ultimately clarify the nature of HESS J1645−455 . Even if its distinct, enhanced brightness might ‘merely’ be caused by an enhanced density of the ISM in this part of the shell, this enhancement would still be identified as a distinct cloud of interstellar matter which, in turn, would correspond to a distinct physical entity.
References:
[1] H.E.S.S. Collaboration: H. Abdalla et al. 2022, Astronomy and Astrophysics, 666, A124
[2] McClure-Griffiths, N. M., Dickey, J. M., Gaensler, B. M., et al. 2005, ApJS, 158, 178
[3] Braiding, C., Wong, G. F., Maxted, N. I., et al. 2018, PASA, 35, e029
