A fresh look at the Galactic Plane

February 2020

Previous | Index | Next

The H.E.S.S. Galactic Plane Survey (HGPS,H.E.S.S.-SOM 2016-01) was a 10 year long observation program. For this survey the H.E.S.S. telescopes in Nambia systematically scanned the Milky Way for very high energetic (VHE) gamma-ray emission. The analysis of the data resulted in the first homogeneous catalog of Galactic VHE gamma-ray sources and a set of sky images, both published in 2018 ([1]). In total 78 sources were discovered and 31 of these could be firmly identified and associated with already known supernova remnants, pulsar wind nebulae, binary objects and stellar clusters. Most of the remaining unidentified sources have possible associations with already known objects, but a firm identification can likely only be established when the results from the HGPS are combined and compared with data from other instruments.The HGPS was performed by taking full advantage of the angular resolution of the H.E.S.S. array. This resolving power enhances the ability to detect faint and point-like sources. To enhance the sensitivity for diffuse extended objects a new approach had to be developed. Studies of low-surface brightness emission are challenging, but allow H.E.S.S. to probe the Galactic plane in another way. Fortuitously, an independent test is available for a limited part of the sky.

On the other side of the Atlantic ocean, the HAWC gamma-ray observatory located in Mexico has been continuously monitoring the northern sky for more than 4 years and produced a skymap of the VHE gamma-ray emission ([2]). Since part of it overlaps with the Galactic plane scanned by H.E.S.S., it is a very good set of data for comparisons with the HGPS. However, to be able to compare the data obtained by the two completely different instruments, a different analysis of the HGPS data had to be use in two main aspects, allowing HESS to take a fresh view of the Galactic plane ([3]).

The complementing sensitivities achived using different approaches are displayed in in the four panels of Figure 1. All of them show the same part of the H.E.S.S. Galactic plane between longitudes 60 deg < l < 10 deg, where both instruments have reasonable sensitivity. An energy threshold of ~1 TeV was selected in both datasets as a compromise between sufficient statistics, good quality reconstruction and reasonable angular resolution. First, while the HGPS source search was performed by taking full advantage of HESS very high angular resolution, the angular resolution of HAWC is significantly lower. In the classical H.E.S.S. analysis a relatively small, so-called correlation radius is used in order to correlate the signal from pixels within this radius, leading to an enhancement of the sensitivity for the detection of sources with the size of this radius. Therefore, a classical H.E.S.S. analysis with a correlation radius of 0.1 deg favors point like sources, given that the angular resolution of H.E.S.S. at 1 TeV is ~0.07 deg. Using a correlation of ~0.4 deg, which is the angular resolution of HAWC at 1 TeV, provides higher sensitivity to extended sources. Indeed, the gamma-ray emission of these extended sources, which also have low surface brightness, can become significant when correlating the signal over a large radius. This is illustrated on the second panel of Figure 1. Secondly, in the case of extended sources the background to the gamma-ray emission will be better characterized by using the entire field of view, rather than by the restricted annulus used in the traditional "ring-background method". This "field of view background method" is very close to the HAWC background method. It uses a model of the acceptance as a function of the center of the camera, assuming radial symmetry. The map obtained with this background method is displayed in the third panel of Figure 1. For sources with size covering a significant fraction of the field of view, this method is challenging. A comparison with HAWC data is an independent test of this analysis. The bottom panel of Figure 1 shows the Galactic plane as measured using HAWC. Comparing the third and the fourth panels shows that, after the new data processing of the HGPS data the agreement between both instruments becomes obvious: most of the structures can be seen clearly in both maps. The small remaining differences can be explained by a lack of sensitivity of one instrument compared to the other to detect a specific source. Different spectral characteristics may be the main reason. Despite their intrinsic differences, both instruments have shown to be very complementary and give a very consistent image of the Galactic plane in VHE gamma-rays.

Fig. 1:

H.E.S.S.and HAWC significance maps of the galactic plane (see ([3]). All panels display the same field of the sky with photon energies E > 1 TeV and use the same color scale. From top to bottom the panels show:

1 - H.E.S.S. Galactic plane using ImPACT reconstruction ([4]) and a correlation radius of 0.1 deg ; the ring background method with an adaptive radius is applied separately on each observation position.

2 - Same H.E.S.S. data and background method as in the 1st panel but using a correlation radius of 0.4 deg, revealing extended diffuse emission.

3 - Same H.E.S.S. data and correlation radius as in 2nd panel but using the field of view background method, revealing fainter emission.

4 - HAWC galactic plane map using events in nHit bins 4 to 9 (> 1 TeV), (see [3])

In all panels sources detected in the HAWC map are marked with black dots. For clarity sources detected with H.E.S.S. are not labelled, but green circles illustrate the 68% containment of sources that were detected in the H.E.S.S. Galactic Plane Survey HGPS ([1]) using a correlation radius of 0.1 degrees and a ring background method.

The diffuse extended emission detected by H.E.S.S. and HAWC match very well, the use of different correlation radii allows studies on different anular scales..

An important consequence of this new analysis is an enhanced sensitivity of H.E.S.S. to extended sources with low-surface brightness emission. A good example is given in Figure 2: the HAWC source HWC J1928+177, likely associated with the pulsar PSR J1928+1746, was not detected in the HGPS. Using the new approach, however, significant emission is revealed. In the part of the Galactic plane shown here, four new source candidates are revealed in the H.E.S.S. data. This number is expected to increase further by applying this new analysis to the remaining part of the Galactic plane, which cannot be studied with HAWC.

Fig. 2:

Zoom into the region of Galactic longitude 51 deg < l < 56 deg. Same as Figure 1 with panels a, b, c and d corresponding to 1, 2, 3 and 4 respectively. The methods developped for the detection of diffuse extended emission reveal new objects that match the diffuse sources identified in the HAWC map ([3])



[1] The H.E.S.S. collaboration: The H.E.S.S. Galactic plane survey, 2018, Astronomy and Astrophysics, 612, p. A1.
[2] Abeysekara, A.U. et al.: The 2HWC HAWC Observatory Gamma-Ray Catalog, 2017, The Astrophysical Journal, 843, p. 40.
[3] Jardin-Blicq A., Marandon V., Brun F., the HAWC collaboration, the H.E.S.S. collaboration: A complementary view of the galactic plane in TeV gamma rays by HAWC and H.E.S.S., 2019, 36th International Cosmic Ray Conference (ICRC2019).
[4] Parsons, R.D. and Hinton, J.A.: A Monte Carlo template based analysis for air Cherenkov arrays, 2014, Astroparticle Physics, 56, p. 26.