Swift Redshift Measurements
Measurements of the gamma-ray spectra obtained during the main outburst of the GRB have found little value as redshift indicators, due to the lack of well-defined features. However, optical observations of GRB afterglows have produced spectra with identifiable lines, leading to precise redshift measurements (e.g., z = 0.835, 3.418, and 0.966 for GRBs 970508, 971214, and 980703).The UVOT has a grism which can produce spectra of objects above 17th magnitude. However, most GRB afterglows will rapidly fade below this limit. For most GRBs, Swift will attempt to measure redshifts by using the UVOT to find the Lyman edge (see below), or by finding spectral features in the X-ray spectrum with the XRT. Swift will make many successful redshift measurements with an accuracy of ~ 0.1, compared to the current few ground-based measurements with accuracy ~ 0.001. At cosmological distances, the Swift measurements will be more accurate than the currently known conversion between redshift and distance, so the reduced precision will have little scientific impact. The success of these techniques for an individual GRB depends on the brightness of its afterglow, the absorbing material in front of the GRB, and the redshift of the GRB.
The amount of absorbing material is often parameterized by the column density, NH, the number of hydrogen atoms in a 1 cm square column between the observer and the source. Since hydrogen is not always the material which absorbs the photons, this is adjusted using estimates for the number of heavier atoms and dust particles per hydrogen atom, and these depend on qualities of the interstellar medium. We will be able to tell whether the region around a GRB is full of the cast-off debris of older stars, or the coalescing gas forming new stars, or even if the object that produces the GRB had a history of blowing dusty winds into the space around itself.
The optical techniques require the line-of-sight to be relatively dust-free, so that the visible and ultraviolet light is not clouded out. Ground-based optical measurements of faint sources require that strong spectral features be visible in the wavelength range observable through the atmosphere, and so have trouble with the redshift range 1.3 < z < 2.5. The XRT can detect the oxygen and iron (Fe K) absorption edges in the x-ray spectrum if z < 0.5 or z > 0.5, respectively, but only if there is enough (and not too much) of the appropriate element along the line-of-sight. For moderately bright afterglows, m ~ 20, this diagram shows the parameter ranges over which the various techniques work:
The brightness of the burst and afterglow and its approximate redshift will be available within hours of the GRB, allowing ground-based observers enough information to determine whether the GRB is worthy of follow-up studies using over-subscribed and expensive ground-based instruments. Bursts that are special in various ways, such as being extremely luminous, very distant, or excessively obscured, may be studied in different programs using different techniques to learn different things about the GRB source or its environment.
If you have a question about Swift, please contact us via the Feedback form.