Abstract
Orphan afterglows are transient events that are produced by cosmological fireballs and resemble gamma-ray burst (GRB) afterglows yet are not accompanied by gamma rays. Such transients may be produced by jetlike 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 fν 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.
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