Radio Emission Spectrum of Supernova Remnant G74.9+1.2

Abstract

Measurements of the flux densities of the supernova remnant (SNR) G74.9+1.2 (CTB 87) at frequencies of 4840 and 8450 MHz were carried out with the RT-32 radio telescope of the Svetloye observatory of the Institute of Applied Astronomy, Russian Academy of Sciences (IAA RAS), in 2018–2019. The data contain signs of the presence of a source of a variable component in the radio emission on a time scale of a month or more. The flux densities of G74.9+1.2 over the time interval 1959.7–2010 are determined from published data, which allow the intensity of G74.9+1.2 to be compared with standard sources. All the data are presented in a single system based on the exact scale of “artificial moon” (AM) fluxes. A refined spectrum of SNR G74.9+1.2 was obtained. The totality of available data is approximated by two power-law sections with different spectral indices: ${{\alpha }_{1}} = 0.31$ at frequencies $f < {{f}_{b}}$ and ${{\alpha }_{2}} = 0.71$ at $f > {{f}_{b}}$. The projections of two power-law sections intersect at a frequency ${{f}_{b}} \approx 3409$ MHz. The break in the radio spectrum of the source, considering its age (more than 4000 years), could form as a result of synchrotron losses. The increase in the steepness of the spectrum close to 0.5 above the frequency ${{f}_{b}}$ is an argument in favor of such an assumption. The totality of data obtained during measurements on the RT-32 and on the basis of published works allows us to state that the variable component in the G74.9+1.2 radio emission on all time scales is much less pronounced compared to younger PWNs. As a possible mechanism for the observed variability, a reconnection of the magnetic field lines in the pulsar magnetosphere is proposed.