Dirty fireballs and orphan afterglows: A tale of two transients

Orphan afterglows are transient events that are produced by cosmological fireballs and resemble gammaray burst (GRB) afterglows yet are not accompanied by gamma rays. Such transients may be produced by jet-like GRBs observed off-axis and therefore hold great promise as a test of GRB collimation. However, orphans may also be produced by "dirty fireballs," i.e., cosmological fireballs whose ejecta carry too many baryons to produce a GRB. A well-designed orphan afterglow search can distinguish between on-axis dirty fireballs and off-axis orphans in at least two ways. First, by combining real-time triggers from a wide-area, multicolor search with deeper follow-up observations, the light curve can be tracked for a time ≳2t₁, where t₁ is the age of the event at first observation. Such a light curve allows simultaneous fits to t₁ and the time decay slope α with sufficient accuracy to distinguish on- and off-axis orphans. Second, radio follow-up of orphan afterglows will show whether the radio flux is falling in time (as expected for an off-axis orphan) or not (as expected for on-axis events). Additional tests involving multiband monitoring of the cooling, self-absorption, and f_v peak frequencies are also possible, although they are much more observationally demanding. A further complication in orphan searches is that dirty fireballs are likely to also be collimated and that collimated dirty fireballs viewed off-axis will individually be practically indistinguishable from off-axis GRB afterglows. To recognize their presence, orphan afterglow surveys must be sufficiently extensive to catch at least some dirty fireballs on-axis.