Status Report

Dirty Fireballs and Orphan Afterglows: A Tale of Two Transients

By SpaceRef Editor
January 1, 2003
Filed under , ,

Astrophysics, abstract

From: James E. Rhoads <[email protected]>
Date: Wed, 1 Jan 2003 11:19:48 GMT (52kb)

Dirty Fireballs and Orphan Afterglows: A Tale of Two Transients

James E. Rhoads

Comments: 17 pages, 4 figures, accepted for publication in The Astrophysical
Journal (April 2003)

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 gamma ray
burst collimation. However, orphans may also be produced by “dirty fireballs,”
i.e., cosmological fireballs whose ejecta carry too many baryons to produce a

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 followup
observations, the light curve can be tracked for a time exceeding 2*t_1, where
t_1 is the age of the event at first observation. Such a light curve allows
simultaneous fits to t_1 and the time decay slope alpha with sufficient
accuracy to distinguish on- and off-axis orphans. Second, radio followup 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 multi-band monitoring of the cooling,
self-absorption, and f_nu peak frequencies are also possible, although 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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