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Title: Nanolensing of gamma-ray bursts
Authors: Walker, Mark A.
Lewis, Geraint F.
Keywords: Cosmology: Dark Matter
gamma-ray bursts
Cosmology: Gravitational Lensing
Issue Date: Jun-2003
Citation: Astrophysical Journal, 2003, Vol.589, p844-860
Abstract: All quasars vary in their optical flux on a timescale of years, and it has been proposed that these variations are principally due to gravitational lensing by a cosmologically distributed population of planetary-mass objects. This interpretation has implications for the observable properties of gamma-ray bursts (GRBs)-as a source expands across the nanoarcsecond caustic network, variability is expected-and data on GRBs can be used to test the proposed model of quasar variability. Here we employ an ultrarelativistic blast wave model of the source, with no intrinsic variations, to study the effects of nanolensing on GRBs. Taken in isolation, the light curves of the caustic crossings are predictable, and we find that a subset of the predicted light curves (the image-annihilating fold crossings) resembles the ``pulses'' that are commonly seen in long GRBs. Furthermore, for sources at high redshift, the expected time between caustic crossings is of the order of seconds, comparable to the observed time between pulses. These points suggest that it might be possible to model some of the observed variations of GRBs in terms of nanolensing; however, our simulated light curves exhibit a small depth of modulation compared to what is observed. This means that the GRB data do not significantly constrain the quasar nanolensing model; it also means that the simplest nanolensing model cannot explain the observed GRB pulses. Viable nanolensing models for pulses probably require a large external beam shear. If a viable model can be constructed, it would effect a considerable simplification in source modeling and, ironically, it would explain why no macrolensed GRBs have been identified to date. Independent of the particular theoretical model, we can test for the presence of nanolensing in GRB data because any variability due to nanolensing should manifest parallax: the timing of caustic crossings, and hence the temporal substructure of bursts, should be different as seen by separated observers. Parallax therefore shifts triangulated burst locations away from their true positions; this displacement is typically expected to be at the few-arcminute level, and existing astrometry is not good enough to reveal the predicted effects. Useful constraints can, however, be obtained by comparing the relative timing of individual peaks in the light curves recorded by spacecraft in the Interplanetary Network; published data show hints of the predicted temporal shifts, but the photon counting statistics are not good enough to categorically decide the matter. There is no plausible alternative interpretation for this phenomenon, and if it is confirmed as a real effect then it compels acceptance of a cosmology that is very different from the currently popular model.
Description: Open Access.
ISSN: 0004-637X
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Copyright: 2003 American Astronomical Society
Appears in Collections:Research Papers (A&A)

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