Since GRBs are the most luminous objects in the Universe, they provide a unique opportunity to probe the intergalactic medium (IGM) and the insterstellar medium (ISM) of the host galaxies via measurement of absorption along the line of sight. Depending on evolution, GRBs might originate from redshifts up to ∼20. They have a median redshift of ∼2 and the most distant GRB to date is at a redshift of 8.3.
Log N - Log P
Log N - Log P distribution for three models: no evolution (NE), star-formation rate (SFR), and SFR plus GRB luminosity function (SLF). SLR does not fit BATSE/PVO data. NE and SLF give starkly different redshift-intensity relations. Swift can disentangle distance, luminosity, and evolution effects with redshift measurements of large numbers of GRBs.
GRB Intensities & Redshifts
Swift bursts vs peak flux: All BAT detections (blue) and complete XRT and UVOT redshift observations (green) compared to normalized BATSE and BeppoSAX/HETE-II distributions. Top scales show corresponding redshift for the three models in the Log N - Log P plot. UVOT and XRT redshift measurements of faint bursts measured by BAT will distinguish between models.
Swift had obtained ∼500 GRBs over a wide range of fluences and redshifts. This data has shown that their evolution follows that of star formation in the Universe. Because the X-ray flux does not depend greatly on the line of sight column, this result is independent of absorption.
By enabling the measurement of a large number of redshifts, Swift-discovered bursts directly measures the luminosity function of GRBs and their evolution with cosmic time. Such analysis typically requires a large sample (hundreds). This is only the third population (after quasars and galaxies) for which such a direct measurement is possible. This measurement is of fundamental cosmological importance as it provides the deepest measurement of the star-formation rate of the Universe.
By rapidly providing both accurate positions and optical magnitudes Swift enables the rapid follow-up of those GRBs bright enough for high resolution optical absorption line spectroscopy at redshifts large enough to study the re-ionization of the IGM. This information on the high-z Ly-α forest is unique because there are no known bright (m < 17) galaxies or quasars at z > 8.3.
Swift notifies observers of high-redshift GRBs with bright afterglows, which provide the best back lights to illuminate z > 4 Ly-α forest.