I got these answers from various collegaues. I am mentioning their names and their answers. Hope these would be helpful.
Reetanjali Moharana: It depends on the strength of the black hole, i.e., its mass etc. For typical BH of AGNs have a magnetic field of 10^4 to 10^3 Gauss.
Sibasish Laha :I too am not sure. But as far as I remember it is of the order of micro-Gauss in the disk. Please do have a look at the papers on Magneto hydrodynamical simulations ( by Fukumura et al 2010 ApJ 715.. etc...)
Harold Nations: Popular reference: https://www.sciencenews.org/...
Magnetic field of black hole measured | Science News
www.sciencenews.org
Pulsar near Milky Way’s center makes first assessment of this type possible.
But still I have not converged to a specific range. Kindly put your comments.
The typical order should be around 100 Gauss. The maximum limit could be 10^4 Gauss (1 Tesla) to 10^5 Gauss (10 Tesla). As told by Dr. Rajneev Misra, IUCAA, India.
Thank you very much Sriram for the paper link you shared. The reported value of 33 G for V404 Cygni is significantly low. The paper itself mentions that the for other sources such as Cyg X-1, the range of magnetic field is ~10^5 to 10^7 G (My theoretical paper also suggests that this range of magnetic field can be acheived in a magnetically supported disc around a black hole. Ref: Sarkar B., Das S., 2018, JApA, 39, 3)
For fully ionized hot disks the resistivity can be very low so the field can get very strong. So now we expect many, many “interesting” phenomena. First if the resistivity is low enough we may see the disks rotation profile depart from Keplerian. This might be observable remotely. Second, the disk may have Magnetic storms. Think solar flares but instead of coming from the surface of a sphere, it comes from the disk. Third, at some minimum point the orbits become unstable. This determines the inner radius of the accretion disk.
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