HESS J1427-608 and – finally – its X-ray counterpart

May 2013

Discovery image of of HESS J1427-608, from Aharonian et al. 2008. The gamma-ray excess at the right of the image is the hard X-ray source known as the Kookaburra/Rabbit.

The very high energy gamma ray source HESS J1427-608 (top image) was discovered already in 2007 in the H.E.S.S. Galactic Plane Survey (Aharonian et al. 2008), as an unidentified source without X-ray or radio counterpart. HESS J1427-608 is an extended source of about 3′ Gaussian width, and has a rather hard gamma ray spectrum extending up to tens of TeV. If the gamma rays orginate from an electron population, one would normally expect accompanying synchrotron radiation in the X-ray regime. At least in the ROSAT data none was observed (Fig. 1), making this one of the puzzling ‘dark’ gamma ray sources.

Only recently, Suzaku X-ray observations revealed a plausible counterpart – a coincident extended X-ray source – Suzaku J1427–6051 (Fujinaga et al., 2013), see Fig. 2. The X-ray radial profile has an extension of about 0.9′ (Fig. 3); the X-ray energy spectrum has a spectral index of 3.1, steeper than the gamma ray spectrum with its index of about 2.2. The 2nd Fermi catalog also lists a GeV gamma-ray source consistent in position with the H.E.S.S. source and the Suzaku source. A plausible interpretation of HESS J1427-608 / Suzaku J1427–6051 is a pulsar wind nebula; gamma-ray and X-ray spectra and yields are consistent if the radiation originates from high energy electrons in a region with about 5 muG magnetic field, and are consistent with the range of values observed for other pulsar wind nebulae. However, lacking the detection of both pulsed emission from the pulsar and of a radio nebula, Fujinaga et al. also discuss a possible supernova remnant origin.

Reference: “HESS very-high-energy gamma-ray sources without identified counterparts”, H.E.S.S. Collaboration, F. Aharonian et al., Astronomy & Astrophysics 447 (2008) 353
and “An X-ray counterpart of HESS J1427-608 discovered with Suzaku”, T. Fujinaga et al., arXiv:1301.5274

Fig. 1: H.E.S.S. significance countours of HESS J1427-608 (black) overlayed on a radio image; the green contours are from a ROSAT hard-band X-ray image (from Aharonian et al. 2008)

Fig. 2: Suzaku XIS image of the HESS J1427-608 field in the 2-8 keV band. Circles indicate the position and extent of the H.E.S.S. source and the error circle for the position of the Fermi source 2FGL J1427.6–6048c. The green contours indicate the intensity of the XIS image in linear scale. From Fujinaga et al., 2013.

Fig. 3: Radial profiles of the X-ray source in the 2-8 keV band, with simulations of a point source and of an extended source on top of a background. From Fujinaga et al., 2013.