The Inner Light Year of the Nearest Seyfert 1 Nucleus in NGC 4395

Abstract

The NRAO VLBA was used at 1.4 GHz to image the inner 550 mas (23 lt-years) of the nearest known Seyfert 1 nucleus, in the Sm galaxy NGC 4395. One continuum source was detected, with flux density $530\pm130 μ$Jy, diameter d<11 mas (0.46 lt-year), and brightness temperature $T_{\rm b} > 2.0\times10^6$ K. The spectral power $P$ of the VLBA source is intermediate between those of Sagittarius A and Sagittarius A$^*$ in the Galactic Center. For the VLBA source in NGC 4395, the constraints on $T_b$, $d$, and $P$ are consistent with an origin from a black hole but exclude an origin from a compact starburst or a supernova remnant like Cassiopeia A. Moreover, the spectral powers of NGC 4395 at 1.4 and 4.9 GHz appear to be too low and too constant to allow analogy with SN 1988Z, a suggested prototype for models of compact supernova remnants. The variable and warm X-ray absorber in NGC 4395 has a free-free optical depth much larger than unity at 1.4 GHz and, therefore, cannot fully cover the VLBA source.