Independent Estimation of Black Hole Mass for the γ-ray-detected Archetypal Narrow-line Seyfert 1 Galaxy 1H 0323+342 from X-Ray Variability

γ-ray-detected narrow-line Seyfert 1 galaxies (NLS1s) are a newly discovered class of radio-loud active galactic nuclei (AGNs) that launch powerful jets that are generally found only in blazars and radio galaxies. However, their black hole (BH) masses as estimated from their broad emission lines are an order of magnitude or more lower than those in blazars. This poses new challenges in explaining the triggering of radio loudness in AGNs. It is still under debate whether their BH masses are underestimated by the commonly used virial method. Here we present an estimate of the BH mass for the γ-ray-detected NLS1 1H 0323+342, an archetype of this class, from its X-ray variability, which is independent of inclination. Our results independently confirm that this NLS1 harbors a (2.8{--}7.9)× {10}⁶ {M}_⊙ BH similar to those in normal NLS1s rather than those in blazars.