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 n 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. n 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 n 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. n GRBs galaxy lensing is therefore roughly 3x less likely than previous popular estimates. n 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? n 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? n Best bet: galaxy lensing of afterglows of GRBs originally triggering Swift n Still unlikely if few high-z GRBs detected – It’s a numbers game -- just don’t have the hundreds of candidates necessary n 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? n 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. n Swift might re-point to this position n Small ground telescopes might organize a “round- the clock” campaign

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