FERMI LARGE AREA TELESCOPE OBSERVATIONS OF THE SUPERNOVA REMNANT W28 (G6.4−0.1)

By A. Abdo, M. Ackermann, M. Ajello, A. Allafort, L. Baldini, J. Ballet, G. Barbiellini, D. Bastieri, K. Bechtol, R. Bellazzini and 144 more. 2010

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We present detailed analysis of two gamma-ray sources, 1FGL J1801.3−2322c and 1FGL J1800.5−2359c, that have been found toward the supernova remnant (SNR) W28 with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. 1FGL J1801.3−2322c is found to be an extended source within the boundary of SNR W28, and to extensively overlap with the TeV gamma-ray source HESS J1801−233, which is associated with a dense molecular cloud interacting with the SNR. The gamma-ray spectrum measured with the LAT from 0.2 to 100 GeV can be described by a broken power-law function with a break at ∼1 GeV and photon indices of 2.09 ± 0.08 (stat) ± 0.28 (sys) below the break and 2.74 ± 0.06 (stat) ± 0.09 (sys) above the break. Given the clear association between HESS J1801−233 and the shocked molecular cloud and a smoothly connected spectrum in the GeV–TeV band, we consider the origin of the gamma-ray emission in both GeV and TeV ranges to be the interaction between particles accelerated in the SNR and the molecular cloud. The decay of neutral pions produced in interactions between accelerated hadrons and dense molecular gas provides a reasonable explanation for the broadband gamma-ray spectrum. 1FGL J1800.5−2359c, located outside the southern boundary of SNR W28, cannot be resolved. An upper limit on the size of the gamma-ray emission was estimated to be ∼16′ using events above ∼2 GeV under the assumption of a circular shape with uniform surface brightness. It appears to coincide with the TeV source HESS J1800−240B, which is considered to be associated with a dense molecular cloud that contains the ultra compact H ii region W28A2 (G5.89−0.39). We found no significant gamma-ray emission in the LAT energy band at the positions of TeV sources HESS J1800−230A and HESS J1800−230C. The LAT data for HESS J1800−230A combined with the TeV data points indicate a spectral break between 10 GeV and 100 GeV.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/798e68e4faab9351e96bf194333e3f300cdd6df3

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Gamma-rays from molecular clouds illuminated by cosmic rays escaping from interacting supernova remnants (2010, background): “Comparison of the model results (solid line) with Fermi (red) (Abdo et al. 2010b) and HESS (blue) (Aharonian et al. 2008) observations for the source N of SNR W28.”
NEUTRAL PION EMISSION FROM ACCELERATED PROTONS IN THE SUPERNOVA REMNANT W44 (2011, background): “Moreover, Abdo et al. (2010a) excluded the presence of a pulsation in the γ-ray signal.”
The origin of galactic cosmic rays (2013, unknown): “7-3946 (Aharonian et al. 2004, 2006, 2007), later followed by the detection of the same remnant in the GeV energy range with the Fermi-LAT telescope (Abdo et al. 2011).”
TOWARD UNDERSTANDING THE COSMIC-RAY ACCELERATION AT YOUNG SUPERNOVA REMNANTS INTERACTING WITH INTERSTELLAR CLOUDS: POSSIBLE APPLICATIONS TO RX J1713.7−3946 (2011, background): “The Fermi Gamma-ray Space Telescope revealed gamma-ray emissions from middle-aged SNRs interacting with molecular clouds (Abdo et al. 2009, 2010a, 2010b, 2010c).”
THE SECOND FERMI LARGE AREA TELESCOPE CATALOG OF GAMMA-RAY PULSARS (2013, background): “The Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope did not just confirm the expectation: by discovering dozens of radio-quiet gamma-ray pulsars and millisecond pulsars (MSPs, thought to be old pulsars spun up to rapid periods via accretion from a companion, Alpar et al. 1982), the”
FERMI LARGE AREA TELESCOPE FIRST SOURCE CATALOG (2010, methodology): “The association between GeV γ-ray PSRs and the PWNs visible in TeV γ-rays seems well established, as has already been discussed in Abdo et al. (2010m).”
THE FIRST FERMI LAT SUPERNOVA REMNANT CATALOG (2015, background): “…so we compared our results to the fluxes, indices, localizations, and extensions for extended sources that were reported in these publications (Abdo et al. 2010d; Lande et al. 2012; Castro & Slane 2010; Abdo et al. 2010c,e,f; Giordano et al. 2012; Katsuta et al. 2012; Brandt & Fermi-LAT…”