The new source HESS J1641-463 emerging near HESS J1640-465

October 2012

Radio image of the vicinity of the gamma ray source HESS J1640-456, with the two remnants G338.3-0.0 and G338.5+0.1. 843 MHz image from the Sydney University Molonglo Sky Survey.

With increasing exposure and statistics in the Galactic plane, complex gamma-ray sources are better resolved and frequently new structures appear. This is the case for a new source in the vicinity of the strong gamma ray source HESS J1640-465: in about 80 hours of data, a second nearby object emerges, see Fig. 1. The new source is particularly prominent at higher gamma ray energies: Fig. 2 illustrates how a tail in the distribution of gamma rays around HESS J1640-465 develops into a separate source, when the energy of gamma rays is gradually increased. The new source, HESS J1641-463, has a size consistent with the angular resolution of the instrument (shown as inset in Fig. 1) and is hence consistent with a point source of gamma rays. In contrast, HESS J1640-465 is clearly an extended object. HESS J1641-463 has a flux of about 2% of the Crab flux (above 1 TeV) and its gamma ray spectrum exhibits a spectral index of about 2.0. This new discovery nicely demonstrates the power and resolution of modern Cherenkov telescopes.

The newly discovered source is coincident with the radio supernova remnant G338.5+0.1, adjacent to the remnant G338.3-0.0 which coincides with HESS J1640-465 Fig. 3; in radio the two remnants are connected by the bright HII region G338.4+0.0. The pointlike nature of HESS J1641-463 suggests that the emission may not be connected with the extended supernova remnant, but rather with a pulsar wind nebula at the center of the remnant, driven by a – yet undetected – pulsar. Another option could be a binary system, similar to HESS J0632+057 However, no variability is detected in the gamma ray flux. Deeper X-ray observations covering the field will help to reveal the nature of this new source, for example by exposing an X-ray pulsar wind nebula as seen in other H.E.S.S. sources, e.g. in SNR G327.1-1.1.

Reference: “Discovery of the hard VHE γ-ray source HESS J1641−463”, H.E.S.S. Collaboration, A. Abramoswki et al., in preparation.

Fig. 1: Very high energy gamma ray map of the field around HESS J1640-465 / HESS J1641-463, for gamma rays about 4 TeV. Black solid contours indicate the significance of the emission at the 5, 6, 7 and 8 sigma level. The inset illustrates the point spread function, i.e. the angular resolution of the instrument. The red box indicates the slices through the gamma ray images, shown in Fig. 2.

Fig. 2: Distribution of gamma ray flux along the slice indicated in Fig. 1, for different energy bands: all energies, and above 1, 2, 3, 4 and 5 TeV. The full lines model the distribution as a superposition of two sources with a Gaussian intensity distribution. Dashed vertical lines indicate the centers of the two radio supernova remnants. A single Gaussian source (dashed Gaussians) is not compatible with the data.

Fig. 3: Radio view (MOST 843 MHz) of the field of G338.5+0.1 and G338.3-0.0. The 6, 7 and 8 sigma significance contours of the gamma ray emission > 4 TeV are shown as black contours. The catalog position and extension of G338.5+0.1 is indicated as a dashed black circle, the best fit position of HESS J1641−463 is shown as a black cross. The positions of nearby HII regions, of SNR G338.3-0.0 and of HESS J1640-465 are also indicated.