Detection of the high energy cut-off from the Seyfert 1.5 galaxy NGC 5273

Abstract

We perform the NuSTAR and Swift/XRT joint energy spectral fitting of simultaneous observations from the broad-line Seyfert 1.5 galaxy NGC 5273. When fitted with the combination of an exponential cut-off power law and a reflection model, a high-energy cut-off is detected at 143 ^(+96)_(−40) keV with 2σ significance. Existence of such cut-off is also consistent with the observed Comptonizing electron temperature when fitted with a Comptonization model independently. We observe a moderate hard X-ray variability of the source over the time-scale of ∼12 yr using INTEGRAL/ISGRI observations in the energy range of 20–100 keV. When the hard-band count rate (6–20 keV) is plotted against the soft-band count rate (3–6 keV), a hard offset is observed. Our results indicate that the cut-off energy may not correlate with the coronal X-ray luminosity in a simple manner. Similarities in parameters that describe coronal properties indicate that the coronal structure of NGC 5273 may be similar to that of the broad-line radio galaxy 3C 390.3 and another galaxy MCG-5-23-16, where the coronal plasma is dominated by electrons, rather than electron–positron pairs. Therefore, the coronal cooling is equally efficient to the heating mechanism keeping the cut-off energy at low even at the low accretion rate.