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Lensing Prospects for Prompt GRBs and Afterglows

Lensing Prospects for Prompt GRBs and Afterglows. Robert J. Nemiroff (Michigan Tech) Jonathan Granot (IAS) & Scott Gaudi (Harvard CfA). GRB Lensing Likelihood Historically Overestimated. Previously, the multiple image fraction for any source at z=2 was estimated at 1 in 170.

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Lensing Prospects for Prompt GRBs and Afterglows

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  1. Lensing Prospects for Prompt GRBs and Afterglows Robert J. Nemiroff (Michigan Tech) Jonathan Granot (IAS) & Scott Gaudi (Harvard CfA)

  2. GRB Lensing Likelihood Historically Overestimated • Previously, the multiple image fraction for any source at z=2 was estimated at 1 in 170. • e.g. Holz, Miller & Quashnock 1999, but they just follow previous estimates in the literature. • Recent results from the CLASS lensing survey indicate the actual multiple image fraction for QSOs at z=2 source is closer to 1 in 500. • e.g. Brown et al., 2003, MNRAS

  3. GRB Lensing Likelihood Historically Overestimated • Since the GRB Log N - Log S is so shallow, GRBs likely suffer no magnification bias, which would make GRB lensing even less likely than extrapolated from the QSOs in CLASS. • GRBs galaxy lensing is therefore roughly 3x less likely than previous popular estimates. • The number of BATSE lenses therefore drops below 1/3 -- non-detection by Nemiroff et al. (1994), Marani et al. (1998) is therefore less surprising.

  4. Detectable Lensing of GRBs by Swift? • It is unlikely that Swift, but itself, will detect gravitational macrolensing (by galaxies) • Estimated rate at 100/yr for 2 years results in many fewer GRBs detected than BATSE • Were a lens-induced secondary image to come in, Swift will likely be looking in another direction (BAT sees only 1/9 of sky) • Best chance is to detect two images of a quadrapole lens, but this is a strong function of the redshift distribution of the detected GRBs.

  5. Detectable Lensing of GRBs by Swift? • Best bet: galaxy lensing of afterglows of GRBs originally triggering Swift • Still unlikely if few high-z GRBs detected • It’s a numbers game -- just don’t have the hundreds of candidates necessary • Afterglows will have a likely higher follow-up percentage than on Swift -- can maximize with a coordinated campaign

  6. Detectable Lensing of GRBs by Swift? • Afterglow lensing likelihood is not “blind” • Deep optical fields might show galaxies that could act as a lens, increasing the lensing chances for certain cases • Were any afterglow to align near a galaxy core, a “quadrapole lens alert” might be issued indicating an increased likelihood that a secondary image from a quadrapole lens might arrive within days. • Swift might re-point to this position • Small ground telescopes might organize a “round-the clock” campaign

  7. Other Lensing Prospects for Swift • Swift millilensing search will likely provide even more stringent limits on very massive black holes from composing the dark matter • Nemiroff et al. 2001, PRL & poster 3.21.6 • Highest likelihood Swift-triggered lens detection might be for microlensing of afterglows during late times • Mao & Loeb 2001, ApJ. • Could determine or limit stellar abundance at high redshift

  8. Acknowledgements • We thank Neal Dalal and Bohdan Paczynski for helpful comments. • RJN thanks the IAS for hospitality during some of this research • These results are still very preliminary. We are attempting to increase their numerical accuracy through Monte Carlo modeling.

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