Discovery of TeV Gamma-ray Emission Towards Supernova Remnant SNR G Last Updated Tuesday, 30 July :01

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1 Background-subtracted gamma-ray count map of SNR G showing the VERITAS detection (VER ). For details, see Figure 1 below. Reference: E. Aliu et al. (The VERITAS Collaboration), Astrophysical Journal 770: 93, 2013 Full text version ArXiv: ArXiV: Contacts: Amanda Weinstein Motivated by preliminary indications of gamma-ray emission seen in its two-year ( ) blind survey of the Cygnus region, VERITAS observed the radio and X-ray shell-like supernova remnant SNR~G for 21.4 hours in Extended gamma-ray emission above 320 GeV overlapping the northwest part of the radio shell was detected at a post-trials significance of 7.5 standard deviations. This gamma-ray source, named VER J , has an intrinsic extent of 0.23 o ± 0.03 o stat o sys o sys; the spectrum is well-described by a differential 1 / 5

2 power law(dn/de = N index of Γ=2.37 ± 0.14 stat ± 0.20 sys and a flux normalization of No = 1.5 ± 0.02 stat ± 0.4 sys photon TeV -1 cm -2 s -1. The integral flux corresponds to 3.7% of the Crab Nebula flux above 320 GeV. o (E/TeV) -Γ ) with a photon The nature of the emission from VER J and its relationship to SNR G remains unclear. It is possible that VER J is a pulsar wind nebula (PWN), associated either with the gamma-ray pulsar PSR J or another undetected pulsar in the line of the sight. However, the most plausible explanation at the present time ascribes VER J to the acceleration of particles by shocks at the interaction of the supernova ejecta and the surrounding medium. TeV gamma-ray emission would be produced through inverse-compton scattering of accelerated electrons or through hadronic interactions of accelerated nuclei with interstellar material, possibly in the form of a dense HI shell that surrounds the remnant. The lack of non-thermal X-ray emission near the source slightly favors the hadronic scenario, since electrons energetic enough to produce very high energy photons through inverse-compton scattering would also be expected to produce X-ray synchrotron radiation. Moreover, estimates of the density of material required to produce the integral flux of VER J above 320 GeV are consistent with estimates of the density in the vicinity of VER J obtained from optical observations. VER J also poses an intriguing puzzle when placed in the context of the diffuse gamma-ray emission seen by Fermi below 100 GeV. First, given that the Fermi gamma-ray satellite sees emission between 10 and 100 GeV from the entire remnant (Lande et al. 2012), it is not yet clear why the gamma-ray emission above 320 GeV from SNR G would be localized to the remnant's northwestern rim. Assuming that the GeV and TeV gamma-ray emission both originate from the remnant, there remains the question of whether or not cosmic rays accelerated by SNR G are being fed to a nearby cocoon of freshly-accelerated 2 / 5

3 cosmic rays, visible to Fermi as a 4 square degree region of gamma-ray emission between GeV. Further observations will be required to address these questions. FITS files: Excess map (Figure 1) ; spectrum (Figure 2). Figures from paper (click to get full size image): Figure 1: Background-subtracted gamma-ray counts map of SNR G showing the VERITAS detection of VER J and its fitted extent (black dashed circle). The supernova remnant is delineated by CGPS 1420 MHz continuum radio contours at brightness temperatures of 23.6K, 33.0K, 39.6K, 50K and 100K (white) (Taylor et al. 2003); the star symbol shows the location of the central gamma-ray pulsar PSR J The inverted triangle and dot-dashed circle (yellow) show the fitted centroid and extent of the emission detected by Fermi above 10 GeV. The open and filled triangles (black) show the positions of Fermi catalog sources 3 / 5

4 1FGL J and 2FGL J which have been subsumed into the extended GeV emission from the entire remnant. The 0.16, 0.24, and 0.32 photons/bin contours of the Fermi detection of the Cygnus cocoon are shown in cyan. The white circle (bottom right corner) indicates the 68% containment size of the VERITAS gamma-ray PSF for this analysis. Figure 2: Spectrum of VER J , derived from four-telescope data only. Points are the VERITAS spectrum while the arrow indicates the upper limit on emission at 11 TeV. The solid line shows a power-law fit with a spectral index of Γ = 2.37 ± 0.14 stat ± 0.20 sys and a flux normalization of N 0 = 1.5 ± 0.2 stat ± 0.4 sys ph TeV-1 cm -2 s -1. Figure 3: ROSAT PSPC X-ray view of SNR G between 1 and 2 kev. The VER J smoothed photon excess contours (100, 150, 210 and 260 photons) are superimposed. The image is composed of a mosaic of six exposure and vignette-corrected overlapping observations, smoothed using a 5 5 pixel boxcar filter. The lower energy bound was selected to reject the background flux from the Galactic Plane. The location of the gamma-ray pulsar PSR J is marked with a white star. 4 / 5

5 150, dashed Figure 210 ellipses, 4: and ASCA 260 respectively. X-ray photons). view The of A G white region star used marks between to extract the 1 position and a spectrum 3 kev, of PSR overlaid and J the with corresponding the VER J background smoothed region are photon indicated excess by contours white solid (100, and The due Figure to solid Si. 5: line Bottom Top shows Panel Panel the ASCA fit of a Δχ X-ray Raymond-Smith residuals spectrum (residual of thermal divided region plasma of by enhanced the model statistical with X-ray parameters error) emission for the as coincident best-fit given model. the with text. VER We J , identify the as line shown at 1.9 in kev Figure as vertical Figure Lower: green 6: Time Upper: line evolution Time indicates evolution of the modeled age of the at which PWN SNR the magnetic (blue) measured and field, PWN SNR using (red) radius the radii. model is Horizontal reached. of Gelfand, (See dashed text Slane lines for & model Zhang indicate description.) (2009), the current with parameters values for CTA given 1. in The Table 5 / 5