September 2010
HESS J1626-490, so far only detected with the H.E.S.S. array of imaging atmospheric Cherenkov telescopes, could not be unambiguously identified with any source seen at lower energies. Recently, a new study of the multi-wavelenght environment of HESS J1626-490 was published (Eger et al. 2010), including new data from the X-ray satellite XMM-Newton, as well as data on molecular gas detected via its CO and HI line emission. The X-ray signal is vital to determine the kind of particles that are responsible for the observed gamma-rays. Energetic pulsars are thought to accelerate electrons to highly relativistic energies which then diffuse into the surrounding interstellar medium. These electrons lose their energy either via synchrotron radiation, which can be seen in X-rays, or via Inverse Compton emission which is visible in the gamma-ray regime (e.g. Aharonian et al. 1997). However, in the XMM images no significant extended X-ray emission from the direction of HESS J1626-490 is seen; the derived upper limit plus the absence of any known energetic pulsar in the vicinity make a pulsar scenario less likely.Apart from leptons, also relativistic protons in certain environments can produce gamma-ray emission. Expanding shock fronts originating from recent supernova explosions are effective accelerators for protons. Those particles that escape from the shock diffuse into the surrounding interstellar medium. In dense environments, such as Giant Molecular Clouds, these particles can effectively produce pions via hadronic interaction (e.g. Aharonian 1991). The neutral pions subsequently decay into very high energy photons which can be detected by gamma-ray telescopes. To search for gas clouds in the vicinity of HESS J1626-490, Eger et al. analyzed CO(J=1-0) molecular line data from the Nanten Galactic plane survey, HI data from the Southern Galactic Plane Survey and observations with the Spitzer infrared space telescope, from the GLIMPSE and MIPSGAL surveys.
An approximate morphological match is seen between molecular and atomic gas in specific distance (i.e. velocity) ranges and HESS J1626-490 (Fig. 1 and Fig. 2). Gas clouds are seen near HESS J1626-490 in CO (Fig. 1) and in neutral hyrogen (Fig. 2) The clouds have masses in the range of 10000 solar masses and are located roughly at a distance d = 1.8 kpc. Furthermore, a density depression in the HI gas is seen at a similar distance (Fig. 2 top), spatially consistent with the SNR G335.2+00.1. This might indicate the presence of an expanding shock front or stellar wind that has blown out the neutral gas. At 1.8 kpc the distance between the SNR and the cloud could be as small as 8 pc, which could explain gamma-ray signal which is large for a cloud of this mass (e.g. Aharonian & Atoyan 1996). Spitzer images of the region expose two HII regions in the vicinity of the gamma-ray source (Fig. 3).
Thus the current results favor a hadronic origin of the VHE gamma-ray emission more than a purely leptonic scenario. However, to better resolve the source morphology future gamma-ray telescopes, such as CTA, and further sub-mm observations tracing heavier elements will be necessary.
Reference: H.E.S.S. VHE gamma-ray sources without identified counterparts, H.E.S.S. collaboration, F. Aharonian et al., Astron. Astrophys. 477 (2008) 353-363;
A multi-wavelength study of the unidentified TeV gamma-ray source HESS J1626-490, P. Eger et al., arXiv:1009.1724
