The environment of the unidentified gamma-ray source HESS J1507-622
  The nature of the gamma-ray source HESS J1507-622 that is located significantly off-set from the Galactic plane is not ascertained to date. Identifying the environment of an enigmatic object may help to constrain its nature. The path of the line of sight of HESS J1507-622 through the Galaxy is compared to the characteristic length scales of stellar populations of different ages. Furthermore, for this object, the energy density in particles is contrasted to the magnetic field energy density and constraints on the distance based on equipartition between these two components are calculated. The line of sight of HESS J1507-622 reaches a minimum distance to the Galactic center at around a galactocentric distance of 5.3 kpc at about 300 pc off the Galactic disc. This location coincides with the scale length and width of stars with an age of 1.2 Gyr which could in principle be an indication that HESS J1507-622 is connected to a stellar population of similar age. For such a case the source appears to be strongly particle dominated. In a leptonic scenario, if a magnetic field in the source of 1 $\mu$G is assumed, equipartition between magnetic field and particles would be realized at a distance of >1 Mpc. This could indicate an extragalactic origin of this object. However, an extragalactic origin is challenged by the extension of the source. The environment of HESS J1507-622 still remains elusive. For the case where this source belongs to a new class of gamma-ray emitters, the distribution of related objects (if existing) may help to settle the respective environment and distance scale. 
Wilfried F. Domainko
astro-ph.HE