NGC 4151 is an intermediate spiralSeyfert galaxy with weak inner ring structure located 19 megaparsecs (62 million light-years) from Earth in the constellation Canes Venatici. The galaxy was first mentioned by William Herschel on March 17, 1787; it was one of the two Seyfert galaxies described in the paper  which defined the term. It is one of the nearest galaxies to Earth to contain an actively growing supermassive black hole; it was speculated that the nucleus may host a binary black hole, with about 40 million and about 10 million solar masses respectively, orbiting with a 15.8-year period. This is, however, still a matter of active debate.
Astronomers using data from the European Space Agency’s XMM-Newton satellite have found a long-sought X-ray signal from NGC 4151. When the black hole’s X-ray source flares, its accretion disk reflects the emission about half an hour later.
X-ray emission from NGC 4151 was apparently first detected on December 24, 1970, with the X-ray observatory satellite Uhuru, although the observation spanned an error-box of 0.56 square degrees and there is some controversy as to whether UHURU might not have detected the BL Lac object 1E 1207.9 +3945, which is inside their error box - the later HEAO 1 detected an X-ray source of NGC 4151 at 1H 1210+393, coincident with the optical position of the nucleus and outside the error box of.
To explain the X-ray emission two different possibilities have been proposed:
radiation of material falling onto the central black hole (which was growing much more quickly about 25,000 years ago) was so bright that it stripped electrons away from the atoms in the gas in its path, and then electrons recombined with these ionized atoms
the energy released by material flowing into the black hole in an accretion disk created a vigorous outflow of gas from the surface of the disk, which directly heated gas in its path to X-ray emitting temperatures
^ abGursky H; Kellogg EM; Leong C; Tananbaum H; et al. (Apr 1971). "Detection of X-Rays from the Seyfert Galaxies NGC 1275 and NGC 4151 by the UHURU Satellite". Astrophys J. 165 (4): L43–8. Bibcode:1971ApJ...165L..43G. doi:10.1086/180713.
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