HESS J1507-622: an unique unidentified source off the Galactic Plane

TL;DR

HESS J1507-622 is a unique very high energy gamma-ray source located off the Galactic plane, with no identified counterparts in other wavelengths, challenging existing models of Galactic gamma-ray sources.

Contribution

This paper reports the discovery and detailed analysis of HESS J1507-622, a unique gamma-ray source with unusual position and properties, and explores possible origins and models.

Findings

Located at ~3.5° latitude, off the typical Galactic plane clustering.

No counterparts found in radio, infrared, or X-ray wavelengths.

Possible distances range from within 1 kpc to several kpc, depending on the model.

Abstract

Galactic very high energy (VHE, > 100 GeV) gamma ray sources in the inner Galaxy H.E.S.S. survey tend to cluster within 1 degree in latitude around the Galactic plane. HESS J1507-622 instead is unique, since it is located at latitude of ~3.5 degrees. HESS J1507-622 is slightly extended over the PSF of the instrument and hence its Galactic origin is clear. The search for counterparts in other wavelength regimes (radio, infrared and X-rays) failed to show any plausible counterparts; and given its position off the Galactic plane and hence the absorption almost one order of magnitude lower, it is very surprising to not see any counterparts especially at X-rays wavelengths (by ROSAT, XMM Newton and Chandra). Its latitude implies that it is either rather close, within about 1 kpc, or is located well off the Galactic plane. And also the models reflect the uniqueness of this object: a leptonic PWN scenario would place this source due to its quite small extension to multi-kpc distance whereas a hadronic scenario would preferentially locate this object at distances of < 1 kpc where the density of target material is higher.