Where are you HESS J1652−462?

December 2022

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A large fraction of all astronomical objects so far discovered in the VHE gamma ray band are concentrated in the Galactic plane, i.e. they are projected against the belt referred to as the Milky Way. The high concentration of objects of different nature sometimes makes an unambiguous identification difficult.Whenever a newly discovered source is spatially coincident with an object of a class previously confirmed to be capable of emitting gamma-rays up to energies of 100 GeV and beyond, a tentative association allows to test existing models for the specific source class. Associations are sought by comparing the size and position of new objects with sources identified in other wavebands. Often enough, such an association is not obvious; in case no and the new object are referred to as 'dark accelerators' in case they are found in observations conducted in other domains of the electromagnetic spectrum. Examples of such sources have been reported, e.g. as Source of the Month 2016-12 or 2009-09, or revealed in the HESS Galactic plane survey [1], SOM 2016-01. One of the problems in identifying objects is the absence of any indicator that allows a distance measurement. In particular for objects observed towards the inner parts of the Galaxy, the distance may vary by more than an order of magnitude.

As deeper and more sensitive analyses probe deeper into the Galaxy, the surface density of extended objects becomes so large, that it is often difficult to unambiguously discriminate whether two sources placed close to each other on the plane of the sky are physically associated and simply reveal different components of a single astrophysical object or whether they are distinct and possibly separated from each other by thousands of light years but yet appear close to each other in projection.

Fig. 1: HESS J1652−462 is an extended, nearly circular source, 1.5 degrees off-set from the Galactic plane and one degree off-set from Westerlund 1, its rim touching the shell of HESS J1646–458, which is identified as the shock surrounding Westerlund 1. The left-hand figure shows the skymap at energies above 365 GeV after subtracting the diffuse emission from the Milky Way. To the west (right) the shall of HESS J1646–458 can be seen. The right-hand figure shows the same area before background-subtraction and delineates the emission identified by Ohm et al., 2013 in Fermi-LAT data along with other maxima in the LAT map (circles) and positions of known pulsars (triangles).

A new discovery of this kind was reported in a study published recently [2]. When surveying the object HESS J1646–458, an extended gamma-ray source associated with the most massive young stellar cluster in the Galaxy, Westerlund 1 (SOM 2022-08), the HESS collaboration identified HESS J 1652-462 in the vicinity of the former object (see figure 1). HESS J 1652-462 is an extended source outside the shell of HESS J1646–458, but only separated by about 1 degree on the plane of the sky. As it is further away from the Galactic plane, with a Galactic latitude of 2 degrees, the area covered by HESS J 1652-462 is not covered by many Galactic plane surveys, such as [3], which typically cover a rather more limited range in Galactic latitude. Hence no obvious counterpart to HESS J 1652-462 could be identified so far. Two possible assumptions may be considered: HESS J 1652-462 might be a source distinct and separate from HESS J1646–458, or it might be a distinct morphological component which is nonetheless physically part pf the Westerlund 1 complex which gives rise to the gamma-ray emission of HESS J1646–458, that has been associated with Westerlund 1 proper.

Fig. 2: This face-on projection of the Galaxy (taken from [1]), illustrates the positions of the sun, the Galactic Centre and the arms of the Milky Way, compared to the reach of VHE observations covered in the HESS Galactic Plane Survey HGPS. The red arrow illustrates the line-of sight towards HESS J 1652-462 which intersects the Perseus Arm over the wide range of about 8 - 12 kiloparsec , and intersects the Sagittarius-Carina-, Scutum- , the 3 kpc- and the Norma- arm of the Milky Way spiral structure at distances of 1 to 14 kpc, respectively. The wide range of possible distances implies an even wider range of possible luminosities.

In case HESS J 1652-462 is not physically connected to Westerlund 1, it would have to be considered as an unassociated 'dark accelerator' until any convincing counterpart would be identified. Its Galactic coordinates imply that it might lie in any of the five arms of the Milky Way that are intersected along the line of sight (see figure 2), allowing for a wide range of possible distances and hence implying an uncertainty in the gamma-ray luminosity of this source by a factor as large as one hundred. The Galactic latitude of 1.8 degrees suggests that the source would be rather close-by, possibly in the Sagittarius-Carina- Arm which would imply that HESS J 1652-462 is placed 28 pc off the plane of the Galaxy. Already the next, further Scutum-Arm implies such a large distance, that the offset would by 100 parsecs which is an unusually large distance compared to other Galactic sources.

The proximity to HESS J1646–458 in projection and the very similar spectral properties of these two objects in the VHE gamma-ray range may suggest that HESS J 1652-462 is an extended component of the former source. In this case, however, the emission of HESS J 1652-462 would imply that the shocks caused by the combined winds of the young massive stars in Westerlund 1 would extend out to distances as far as 100 parsecs from the center of the stellar cluster (which is only 2 parsecs in size).

On a larger scale it is apparent that HESS J 1652-462 coincides with a circular region which (in projection) displays a 'hole' in the dust distribution in a part of the Galactic plane that has a rather large amount of dust emission (see figure 3). The physical nature of this 'hole' is not well understood. While the matching size of the hole and HESS J 1652-462 is intriguing, it remains unclear whether this is a chance coincidence projected along the common line of sight, of whether a physical connection suggests that the gamma-ray emission originates from inside a bubble that has been blown into dusty ISM - possibly by young stars as those seen to the right of the centre of the bubble.

Fig. 3: The environment of HESS J 1652-462 (1.5 degree x 1.5 degree) as revealed in the infrared GLIMPSE survey, obtained with the instruments onboard the Spitzer mission (colour scale: blue: 3.6 micron, green: 4.5 micron and red: 8 micron). The image shows a portion of the full survey described in [4].


[1] H.E.S.S. Collaboration, H. Abdalla et al., Astron. Astrophys. 612, A1 (2018)

[2] H.E.S.S. Collaboration: H. Abdalla et al. ,Astron. Astrophys. 666, A124 (2022)

[3] S. Molinari et al., Astron. Astrophys. 591, A149 (2016)

[4] E. Churchwell et al., Publications of the Astronomical Society of the Pacific, 121, pp. 213 (2009)