Inner 0.8 degrees of the acceptance-corrected excess map for the IC 443 field. See Figure 3 below for more details.

Observation of extended VHE emission from the supernova remnant IC443 with VERITAS

Reference:  V. A. Acciari et al. (The VERITAS Collaboration), The Astrophysical Journal, 698: L133-L137, 2009

Full text version

ArXiv version: ArXiV:0905.3291

Contact person: Brian Humensky

FITS file of results here.

IC 443 is one of the classic examples of a supernova remnant interacting with a molecular cloud. The expanding supernova remnant appears to be surrounded by a ring of gas that cuts across the remnant in the foreground from northwest to southeast. The cloud is dense enough to attenuate the optical emission from IC 443, which is noticeably dimmer across its middle than elsewhere. IC 443 is considered middle-aged, though with large uncertainty in its age (with estimates ranging from a few thousand years to 30 thousand years). It is believed to be ~1.5 kpc away, so its 0.75-degree diameter translates to a size of ~20 pc. In the southeastern part of IC 443, a pulsar wind nebula (PWN) has been identified. This PWN may be powered by the neutron star left behind by the progenitor star whose explosion created the supernova remnant.

VERITAS observed IC 443 for 37.9 hours during 2007 and detected emission above 300 GeV with an excess of 247 events, resulting in a significance of 8.3 standard deviations (σ) before trials and 7.5σ after trials in a point-source search. The emission is centered at 6h16m51s 22°30′11′′ (J2000) ± 0.03° stat ± 0.08°sys, with an intrinsic extension of 0.16°±0.03°stat ±0.04°sys (assuming a symmetric Gaussian profile). This was the first detection of extended gamma-ray emission from IC 443, and has since been confirmed by the Fermi Gamma-ray Space Telescope. The VHE spectrum is well fit by a power law (dN/dE = N0 ×(E/TeV)^-Γ) with a photon index of Γ=2.99 ± 0.38 stat ± 0.3 sys and an integral flux above 300 GeV of (4.63 ± 0.90 stat ± 0.93 sys)×10¹² cm^-2 s^-1.

IC 443 is interesting to study in TeV gamma rays for two reasons. First, supernova remnants are the leading candidates for the accelerators of cosmic rays up to energies of 1000 TeV or so. Studying TeV gamma rays from IC 443 (and similar supernova remnants) can teach us about how cosmic rays are accelerated in supernova remnants and how efficiently they escape and diffuse into the interstellar medium. IC443 is special in that its expanding shock wave is interacting with the ring of gas that surrounds it. The gas in this cloud is much denser than the average interstellar medium and provides a rich target for cosmic rays to interact with and generate gamma rays. The fact that the TeV gamma rays seen by both VERITAS and MAGIC coincide with the densest regions of this gas supports this picture (as does the observation of GeV gamma rays by the Fermi Gamma-ray Space Telescope). In this scenario, the low flux (~3% that of the Crab Nebula above 300 GeV) and unusually steep spectrum (with a power-law index of -2.99 ± 0.38 stat ± 0.30 sys) for a supernova remnant are likely a consequence of IC 443's age and the energy-dependent escape of cosmic rays from the immediate environment.

Second, the pulsar wind nebula in IC 443 may also be responsible for at least some of the observed gamma rays. This is a very interesting pulsar wind nebula because it appears to still be embedded in the interior of the supernova remnant and because pulsed emission from the compact object in the nebula has not yet been detected.

In the future, we expect that high-statistics GeV and TeV observations will allow us to assess in more detail whether both of these mechanisms for generating gamma rays are contributing, and learn more about cosmic-ray acceleration and diffusion in this fascinating system.

Figures from paper (click to get full size image): 

Figure 1: Significance map for the IC 443 field. The black circle indicates the 1σ angular extension. The black cross-hair indicates the centroid position and its uncertainty (statistical and systematic added in quadrature), and the white cross-hair likewise indicates the position and uncertainty of MAGIC J0616+225 (Albert et al. 2007). Red contours: optical intensity (McLean et al. 2000). White star: location of the star Eta Gem. The white circle indicates the PSF of the VERITAS array.

Figure 2: Spectrum of IC 443, scaled by E^2. The red points are the VERITAS spectrum (the red tick marks along top indicate bin edges). The red line is a power-law fit (see the text), with residuals to the fit plotted in the lower box. The MAGIC spectrum is indicated by the gray band and extends down to 90 GeV. VERITAS error bars and MAGIC error band reflect statistical errors only. The Crab spectrum (V. Acciari et al. 2009, in preparation) is shown as a dashed line for comparison.

Figure 3: Inner 0.8 degrees of the acceptance-corrected excess map for the IC 443 field. Markers and contours follow Figure 1, with several additions. Thick black contours: CO survey (Huang & Thaddeus 1986); black star: PWN CXOU J061705.3+222127 (Olbert et al. 2001); open blue circle: 95% confidence radius of 0FGL J0617.4+2234 (Abdo et al. 2009); and filled black triangles: locations of OH maser emission (Claussen et al. 1997; J. W. Hewitt 2009, private communication).