First detection of a VHE gamma-ray spectral maximum from a cosmic source: HESS discovery of the Vela X nebula

Abstract

The Vela supernova remnant (SNR) is a complex region containing a number of sources of non-thermal radiation. The inner section of this SNR, within 2 degrees of the pulsar PSR B0833−45, has been observed by the HESS γ-ray atmospheric Cherenkov detector in 2004 and 2005. A strong signal is seen from an extended region to the south of the pulsar, within an integration region of radius 0.8• around the position (α = 08 h 35 m 00 s , δ = −45• 36 J2000.0). The excess coincides with a region of hard X-ray emission seen by the ROSAT and ASCA satellites. The observed energy spectrum of the source between 550 GeV and 65 TeV is well fit by a power law function with photon index Γ = 1.45 ± 0.09 stat ± 0.2 sys and an exponential cutoff at an energy of 13.8 ± 2.3 stat ± 4.1 sys TeV. The integral flux above 1 TeV is (1.28 ± 0.17 stat ± 0.38 sys ) × 10 −11 cm −2 s −1 . This result is the first clear measurement of a peak in the spectral energy distribution from a VHE γ-ray source, likely related to inverse Compton emission. A fit of an Inverse Compton model to the HESS spectral energy distribution gives a total energy in non-thermal electrons of ∼2 × 10 45 erg between 5 TeV and 100 TeV, assuming a distance of 290 parsec to the pulsar. The best fit electron power law index is 2.0, with a spectral break at 67 TeV.

Associations of very high energy gamma-ray sources discovered by H.E.S.S. with pulsar wind nebulae

“…Observations of this region with H.E.S.S. revealed a very extended source of VHE gamma-rays centered to the south of the pulsar [8], overlapping a diffuse hard X-ray emission feature first detected with ROSAT [9] and aligned with a bright radio filament within the plerion.…”

“…The VHE gamma-ray spectrum of this source significantly steepens with increasing energy, and can be described by a power law of photon index Γ = 1.45 ± 0.09 stat ± 0.2 sys with an exponential cutoff energy 13.8 ± 2.3 stat ± 4.1 sys TeV; this constitutes the first clear measurement of a peak in the spectral energy distribution at VHE energies [8]. Assuming the CMB is the main target photon component for IC scattering in the outer regions of the Galaxy, a total energy of ∼ 2 × 10 45 erg in non-thermal electrons between 5 TeV and 100 TeV could be deduced.…”

Pulsar-Wind Nebulae

“…Therefore, the different angular sizes are not surprising. We also note that in the Vela PWN the TeV emission comes from the region of brightest radio filament (Frail et al 1997;Aharonian et al 2006b) which is filled with the ejecta (based on the X-ray spectra; LaMassa et al 2008) thus providing denser target for the putative relativistic hadrons that might be present in the pulsar wind. Therefore, it may turn out that some of the prominent thermal Crab filaments are TeV bright.…”

“…The nearby Vela SNR (∼ 8 • in diameter) has a large region of non-thermal radio emission surrounding the Vela pulsar (see e.g., Bock et al 1998). One of the brightest radio filaments in Vela X, positioned at the southwest of the pulsar, was detected in X rays with ROSAT, ASCA, Suzaku, and XMM-Newton (Markwardt andÖgelman 1995(Markwardt andÖgelman , 1997Mori et al 2008;) and, more recently, in GeV γ-rays with AGILE (Pellizzoni et al 2010) and Fermi LAT (Abdo et al 2010b) and very high energies (0.5-70 TeV) with HESS (Aharonian et al 2006c) and CANGAROO (Enomoto et al 2006). The bright X-ray and VHE emission regions are positionally coincident (they sometimes referred to as a "cocoon"), and have been commonly dubbed a relic PWN displaced to the south by the unequal pressure of the reverse shock propagating within the SNR.…”

Pulsar Wind Nebulae and Cosmic Rays: A Bedtime Story

“…Motivated in part by the hard spectrum of the very high-energy gamma-ray emission seen from the Vela X PWN [28,29], some theorists have advanced hybrid PWNe models, in which the induced electric field extracts both e ± pairs and heavy nuclei (e.g. iron) from the neutron star surface [30,31,32].…”