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Any explanation for the origin of the BLR must provide answers to a number of fairly obvious questions:

  1. Stellar-mass black hole systems (microquasars) do not have (obvious) BLRs. Why are they different from AGNs? While Galactic binaries have accretion disks and jets, they do not have anything like the BLR. This may, of course, be related to the fact that accretion disks around stellar-mass black holes are considerably hotter than those around supermassive black holes, as the highest disk temperature is inversely proportional to the mass in simple thin-disk models.

  2. Some AGNs may lack BLRs, i.e., they are true type 2 AGNs. Whereas many type 2 AGNs have "hidden" BLRs that are detected at extremely high signal-to-noise ratios or in polarized (scattered) light, there is evidence suggesting that some Type 2 objects have no broad lines at all. Why is this? What are the physical differences between AGNs with BLRs and those without?

  3. Broad lines widths are in the range 1000 to 25,000 km s-1. What imposes these limits? And, a key question closely related to this, what does spectrum of an AGN at i = 0° look like? If not orbital motion, what causes the line broadening in this extreme case?

  4. Where does the energy to power the emission lines come from? The BLR energy budget problem is still unsolved. The observed AGN continuum is neither luminous enough nor hard enough to account for the broad lines. Does the BLR see a different continuum than we do, or is there an energy source we have not yet recognized?

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