tokens: for small values of the BH spin and θ v , those effects should have a minor weight on the results. For the fitting procedure, we used the sources NGC 5548 and MRK 509 (see Appendix C for details) also studied by KD 18. We found that, as for KERRBB, the modeling with AGNSED of the optical-UV-X SED is degenerate: by changing the model parameters (i.e., BH mass, spin, Eddington ratio, corona size) appropriately, it is possible to reproduce the same set of data (for some reference values, see Fig. C.1 ). For NGC 5548, results from both models are compatible while for MRK 509, KERRBB BH masses are larger than the AGNSED ones by a factor ≲0.2 dex (Fig. C.2 , right panel). We argue that for NGC 5548, the absence of relativistic effects in AGNSED is balanced out by modeling a large corona above the disk, leading to the same BH mass. Instead, for MRK 509, the smaller X-ray corona leads to different results because it does not compensate the differences between the two models. In general, KERRBB observed disk luminosity is dimmer than the intrinsic one because of a compact corona located in the inner region of the disk, the presence of which modifies the AD emission and leads to an overestimated BH mass. By correcting the KERRBB disk lumminosity by a factor ≲0.2 dex, results from the two models related to MRK 509 show better compatibility (see Appendix C for more details about this correction). The partial compatibility between KERRBB and AGNSED shows that BH masses estimated though the SED fitting procedure clearly depend on the adopted model and their physical background. Despite these findings, KERRBB results for both sources are still compatible with SE and RM ones within uncertainties (Fig. 4 ). Moreover, results presented by KD 18 are compatible with ours for what concerns the BH spin: taking their finding for the BH masses as reference values, both their results and ours favor low values of a . 6. Discussion and conclusions We used the relativistic thin AD model KERRBB ( Li et al. 2005 ) to infer the BH masses of 28 sources from the AGN Black Hole Mass Database ( Bentz Katz 2015 ). These sources have a BH mass estimate from the RM studies and show clear evidence in the UV band of the so-called Big Blue Bump produced by the radiation coming from an AD around a SMBH. Since we did not have information about the viewing angle for the majority of them, we assumed that the Type 1 QSOs were observed with θ v ≤ 45° in order to avoid the absorption from the dusty torus (assumed to have an average opening angle of ∼45°). Our results can be summarized as follows: The majority of the sources of our sample show a good match between data and the modeling with KERRBB. The modeling led to a relatively good estimate of the spectrum peak position (i.e., peak frequency and luminosity; see Appendix B ) with a small uncertainty (∼0.05 dex on average). The total uncertainty related to the AD BH mass estimates is ∼0.45 dex, which is connected to the unknown BH spin
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