I mentioned the “Black Hole Astrophysics” workshop I attended this past weekend. It was a very interesting discussion between gravitational wave (GW) astronomers and traditional “electromagnetic” (EM) astronomers. The primary focus was on the overlap between the two methods in attempts to detect the merger of massive black holes ( 3-4 < log M_bh/M_sun < 7). So, there was a lot of discussion about the expected merger rates as well as electromagnetic counterparts.

One interesting note from the workshop was the complementary nature of GW and EM observations. Typically, the Laser Interferometer Space Antenna will observe GW from BH mergers at high redshift (z > 7; large distances). In contrast, radio astronomy observations are generally more sensitive to local black holes in the mass range mentioned above. The reason for this is that AGN emission loosely scales with mass of the black hole.

For an object of a given mass, the “maximum” luminosity can be determined by setting the gravitational force equal to the radiation pressure from the photons. When the gravitational force is less than the radiation pressure, the outer levels of the object will be blown off in a wind, and the object will lose mass. Of course, black holes can’t lose mass, but if the radiation pressure exceeds their gravitational pull, the matter falling onto the black hole will be blown away. So, with increasing mass, an object can be more luminous. This “maximum” luminosity for a given mass is called the Eddington Luminosity. Note that I put “maximum” in quotes. This calculation assumes spherical symmetry, so if you have mass flowing onto a black hole from a disk, the luminosity can exceed this maximum luminosity. While not an absolute limit, it provides a metric to compare the emission from black holes.

Consistent with the Eddington Luminosity, more massive (active) black holes can be more luminous, so we can see them farther away. However, less massive active black holes won’t be visible as far. It turns out active black holes in the mass range LISA is sensitive to are only bright enough to be seen in the relatively local universe. So EM astronomy may not be able to see the same sources LISA will see. However, as the physics is the same, we can still learn about these objects.

Electromagnetic counterparts to the BH mergers were also discussed. These occur on a wide range of timescales. These include AGN activity prior to the merger and interaction of a kicked BH with the surrounding gas. (BH mergers often result in a non-isotropic emission of GW which imparts a net velocity ["kick"] on the resulting BH of up to ~4000 km/s [depending on the spins of the merging black holes]).

It was a very interesting meeting with a lot of good insight. There is likely to be a white paper on the subject, which will be posted to astro-ph (preprint server).