Background-subtracted gamma-ray count map of SNR G78.2+2.1 showing the VERITAS detection (VER2019+407). For details, see Figure 1 below.
Reference: E. Aliu et al. (The VERITAS Collaboration), Astrophysical Journal 770: 93, 2013
Motivated by preliminary indications of gamma-ray emission seen in its two-year (2007-2009) blind survey of the Cygnus region, VERITAS observed the radio and X-ray shell-like supernova remnant SNR~G78.2+2.1 for 21.4 hours in 2009. 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 J2019+407, has an intrinsic extent of 0.23o ± 0.03ostat + 0.04osys - 0.02osys; the spectrum is well-described by a differential power law(dN/dE = No × (E/TeV)-Γ) with a photon index of Γ=2.37 ± 0.14stat ± 0.20sys and a flux normalization of No = 1.5 ± 0.02stat ± 0.4sys 10-12 photon TeV-1cm-2s-1. The integral flux corresponds to 3.7% of the Crab Nebula flux above 320 GeV.
The nature of the emission from VER J2019+407 and its relationship to SNR G78.2+21 remains unclear. It is possible that VER J2019+407 is a pulsar wind nebula (PWN), associated either with the gamma-ray pulsar PSR J2021+4026 or another undetected pulsar in the line of the sight. However, the most plausible explanation at the present time ascribes VER J2019+407 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 J2019+407 above 320 GeV are consistent with estimates of the density in the vicinity of VER J2019+407 obtained from optical observations.
VER J2019+407 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 G78.2+2.1 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 G78.2+2.1 are being fed to a nearby cocoon of freshly-accelerated cosmic rays, visible to Fermi as a 4 square degree region of gamma-ray emission between 1-100 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 G78.2+2.1 showing the VERITAS detection of VER J2019+407 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 J2021+4026. 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 1FGL J2020.0+4049 and 2FGL J2019.1+4040 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 J2019+407, 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 ± 0.14stat ± 0.20sys and a flux normalization
of N0 = 1.5 ± 0.2stat ± 0.4sys × 10-12 ph TeV-1 cm-2 s-1.
Figure 3: ROSAT PSPC X-ray view of SNR G78.2+2.1 between 1 and 2 keV. The VER J2019+407 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 J2021+4026 is marked with a white star.
Figure 4: ASCA X-ray view of G78.2+2.1 between 1 and 3 keV, overlaid with the VER J2019+407 smoothed photon excess contours (100, 150, 210 and 260 photons). The region used to extract a spectrum and the corresponding background region are indicated by white solid and dashed ellipses, respectively. A white star marks the position of PSR J2021+4026.
Figure 5: Top Panel — ASCA X-ray spectrum of the region of enhanced X-ray emission coincident with VER J2019+407, as shown in Figure 4. The solid line shows the fit of a Raymond-Smith thermal plasma model with parameters as given in the text. We identify the line at 1.9 keV as due to Si. Bottom Panel — Δχ residuals (residual divided by the statistical error) for the best-fit model.
Figure 6: Upper: Time evolution of the PWN magnetic field, using the model of Gelfand, Slane & Zhang (2009), with parameters given in Table 2. Lower: Time evolution of the modeled SNR (blue) and PWN (red) radii. Horizontal dashed lines indicate the current values for CTA 1. The vertical green line indicates the age at which the measured SNR radius is reached. (See text for model description.)