NGC 4151 is an intermediate spiral Seyfert galaxy with weak inner ring structure located 15.8 megaparsecs (52 million light-years) from Earth[4] in the constellation Canes Venatici. The galaxy was first mentioned by William Herschel on March 17, 1787; it was one of the six Seyfert galaxies described in the paper [5] which defined the term. It is one of the nearest galaxies to Earth to contain an actively growing supermassive black hole.[6] The black hole would have a mass on the order of 2.5 million to 30 million solar masses.[7] 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.[8] This is, however, still a matter of active debate.
Some astronomers nickname it the "Eye of Sauron" from its appearance.[9]
One supernova has been observed in NGC 4151: SN 2018aoq (Type II-P, mag 15.3).[10][11]
X-ray emission from NGC 4151 was apparently first detected on December 24, 1970, with the X-ray observatory satellite Uhuru,[12] 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,[13] coincident with the optical position of the nucleus and outside the error box of Uhuru.[12]
To explain the X-ray emission two different possibilities have been proposed:[14]
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