Wikipedia:SDSS J1408+0257

SDSS J1408+0257 (full designation SDSS J140821.67+025733.2) is a high-luminosity quasar, a special type of a class of objects known as active galactic nuclei, located within the constellation Virgo. An article publised in 2017 indicated that it possesses one of the most massive black holes known, if not the most massive, with a rough estimate of 196 billion. However closer examination of the data showed that the 196 billion estimate was the erroneous, and a better estimate of the mass is 8 billion.

Properties
Similar to other quasars, SDSS J1408+0257 is believed to be an active nucleus of a galaxy; the primary engine of which being a supermassive black hole feeding on an extensive accretion disc, allowing the generation of heat and light energy that makes the quasar visible across astronomical distances. The quasar itself has an absolute bolometric luminosity of -29.1, about $1.69$ (16.9 trillion) times more luminous than the Sun. The central host galaxy is estimated to be 60 kiloparsecs (200,000 light years) across.

The quasar has been one of the 280,000 AGNs of the comprehensive Sloan Digital Sky Survey's DR12Q catalogue which had been examined by Szymon Kozlowski. Despite being ten times less luminous as the other quasars of the survey, by a measure using CIV emission line spectra, the quasar hosts the biggest black hole mass, reported to be 196 billion. This is considered the highest mass ever attributed to a supermassive black hole; 45,500 times the mass of Sagittarius A* - the Milky Way's central black hole, and even higher than the mass of all stars in the Andromeda Galaxy, the Local Group's largest galaxy, with estimates at 150 billion. Such a high mass classifies it in a new class of ultramassive black holes, or it may even be in the class of stupendously large black holes (SLABs); those with masses above 100 billion. The diameter of this black hole's event horizon is estimated to be 1.16 trillion kilometers (7000 AU), about 1/9 of a light-year or 40.5 light-days, and 98 times the diameter of Pluto's orbit.

However, caution should be taken when considering this estimate. Trakhtenbrot and Netzer (2012) notes that the method of obtaining masses via the CIV emission method is generally considered to be less reliable compared to other methods (such as MgII or Hβ). Denney et al. also noted factors that may lead to unreliable redshift estimates in using the CIV method,, which in turn as Kozlowski points out in his paper can lead to biases in black hole mass estimations. Nevertheless, Kozlowski also has pointed out that there had been corrections made in these biases in order to fit it with confidence levels comparable to MgII methods (within 0.4 orders of magnitude); such method has been utilized earlier by Y. Shen et al to estimate quasar masses in the DR7Q catalogue (a precursor to the DR12Q), which Kozlowski himself considers to be a reliable method. The correction proceeded minimizing their offset differences and offset dispersion by some 0.07 and 0.39 orders of magnitude, respectively. Kozlowski reiterates that the black hole mass estimations based on CIV emissions, like the one used in SDSS J1408+0257, should be taken with caution due to the unreliability of the method, and the use of CIV emission should only be treated as simply the best estimates from the DR12Q catalogue.

Measurements of the light boson
SDSS J1408+0257 was among the quasars selected by Zu et al to be used to model out the destabilizing effects of scalar and vector bosons among rotating black holes; the others being OJ 287 and TON 618, also two objects with very large black hole masses.