Detection of a second jet within the nuclear core of Mrk 501

Abstract The blazar Mrk 501 still poses challenging questions for astrophysics, primarily because of the different orientations observed at pc- and kpc-scales. A much debated issue is whether its core harbors a supermassive binary black hole (SMBBH). We re-model and re-analyze 83 datasets observed with the Very Long Baseline Array (VLBA) (2011/09/24 – 2023/07/24) at 43 GHz and compare them with earlier results for 15 GHz and 8 GHz. In addition to the previously known parsec-scale jet in the core of Mrk 501, we detect a second nuclear jet (Jet 2) which appears to start off on the counter-jet side and loop anti-clockwise around the primary core. The image on 2022/06/24 indicates Jet 2 being lensed into a partial Einstein ring. The entire jet-system (Jet 1 + Jet 2) at 43 GHz appears significantly displaced in clockwise direction, relative to the 15 GHz jet and periodic transverse motion on a time scale of about 7 years is revealed. The total flux-density light-curve at 43 GHz also shows modulations with a similar period (7.4 yrs). A shorter period of about 121 days is found in the 43 GHz flux-density light-curve of the core, albeit with less certainty. This shorter period is supported by the quasi-periodic appearance of Jet 2. We explore several physical scenarios and find that an SMBBH with a 7-year orbital plane precessional period and a 121-day orbital period can reasonably explain our observational results. Assuming equal masses, we estimate the SMBBH to have its two components separated by 27–128 Schwarzschild radii and each having a mass of 108 - 109 M⊙.