Composite supernova remnants ( SNRs ) constitute a small subclass of the remnants of massive stellar explosions where non-thermal radiation is observed from both the expanding shell-like shock front and from a pulsar wind nebula ( PWN ) located inside of the SNR .
These systems represent a unique evolutionary phase of SNRs where observations in the radio , X-ray , and \gamma -ray regimes allow the study of the co-evolution of both these energetic phenomena .
In this article , we report results from observations of the shell-type SNR G 15.4 + 0.1 performed with the High Energy Stereoscopic System ( H.E.S.S . )
and XMM-Newton .
A compact TeV \gamma -ray source , HESS J1818 - 154 , located in the center and contained within the shell of G 15.4 + 0.1 is detected by H.E.S.S .
and featurs a spectrum best represented by a power-law model with a spectral index of -2.3 \pm 0.3 _ { stat } \pm 0.2 _ { sys } and an integral flux of F ( > 0.42 \mathrm { TeV } ) = ( 0.9 \pm 0.3 _ { \mathrm { stat } } \pm 0.2 _ { \mathrm { sys } } ) \times 10 ^ { -12 } cm ^ { -2 } s ^ { -1 } .
Furthermore , a recent observation with XMM-Newton reveals extended X-ray emission strongly peaked in the center of G 15.4 + 0.1 .
The X-ray source shows indications of an energy-dependent morphology featuring a compact core at energies above 4 keV and more extended emission that fills the entire region within the SNR at lower energies .
Together , the X-ray and VHE \gamma -ray emission provide strong evidence of a PWN located inside the shell of G 15.4 + 0.1 and this SNR can therefore be classified as a composite based on these observations .
The radio , X-ray , and \gamma -ray emission from the PWN is compatible with a one-zone leptonic model that requires a low average magnetic field inside the emission region .
An unambiguous counterpart to the putative pulsar , which is thought to power the PWN , has been detected neither in radio nor in X-ray observations of G 15.4 + 0.1 .