Two multifrequency campaigns were carried out on OJ287 in 2005 : in April when it was in its pre-outburst state , and in November , during the main 12 yr cycle outburst .
The wavelength coverage was from radio to X-rays .
In the optical-to-UV range the differential spectrum between the observations has a bremsstrahlung spectral shape , consistent with gas at 3 \times 10 ^ { 5 } K temperature .
Our result supports the hydrogen column density of the OJ287 host galaxy of \sim 9.3 \times 10 ^ { 20 } cm ^ { -2 } , the average value found by Gosh & Soundararajaperumal .
The 3 \times 10 ^ { 5 } K bremsstrahlung radiation was predicted in the binary black hole model of OJ287 , and it arises from a hot bubble of gas which is torn off the accretion disc by the impact of the secondary .
As this radiation is not Doppler boosted , the brightness of the outburst provides an estimate for the mass of the secondary black hole , \sim 1.4 \times 10 ^ { 8 } solar mass .
In order to estimate the mass of the primary black hole , we ask what is the minimum mass ratio in a binary system which allows the stability of the accretion disc .
By using particle simulations , we find that the ratio is \sim 1.3 \times 10 ^ { 2 } .
This makes the minimum mass of the primary \sim 1.8 \times 10 ^ { 10 } solar mass , in agreement with the mass determined from the orbit solution , 1.84 \times 10 ^ { 10 } solar mass .
With this mass value and the measured K-magnitude of the bulge of the host galaxy of OJ287 , the system lies almost exactly on the previously established correlation in the black hole mass vs. K-magnitude diagramme .
It supports the extension of this correlation to brighter magnitudes and to more massive black holes than has been done previously .