The Swift Soft X-ray Serendipitous Survey

Since the launch in November 2004, Swift has observed over 500 gamma-ray bursts (GRBs). These GRB fields are uniformly distributed on the sky and not correlated with other known X-ray sources. Therefore, these XRT observations form an excellent soft X- ray serendipitous survey with a sky area of 75 square degrees and a median flux limit of 4e-15 erg/cm^2/s. This survey is about an order of magnitude deeper than previous surveys of similar area, and an order of magnitude wider than previous surveys of similar depth. The unique combination of the survey area and depth enables it to fill in the gap between the deep, pencil beam surveys (such as the Chandra Deep Fields) and the shallow, wide area surveys measured with ROSAT. With it, we will place independent and complementary measurements on the number counts and luminosity functions of X- ray sources, with a focus on galaxy clusters. The highest priority goal is to produce the largest X-ray selected cluster catalog and increase the sample of high redshift clusters (z > 0.9) by an order of magnitude, from which we will study the evolution of cluster number counts, luminosity function, scaling relations, and eventually the mass function. This is a crucial step in mapping cluster evolution and constraining cosmological models. First, we propose to extract the complete serendipitous point source and extended source lists for the existing 500 GRB fields. We plan to analyze 800 fields ( 120 square degrees) at the late stage of the project. Second, we will use optical observations to further identify galaxy clusters. These optical observations include existing data from SDSS, UKIDSS, and deep optical follow-up observations of GRB fields, and new data obtained with MDM, Magellan, LBT, or NOAO telescopes. We expect to detect 460 and 60 clusters in the redshift bin of 0.5<z<0.9 and 0.9 < z < 1.3, respectively, 12 and 10 times larger than the current sample. Deep X-ray follow-up observations of the 60 high redshift clusters will constrain the equation of state of dark energy with Delta w < 0.07 by this method alone, reducing the uncertainty of the current measurement using X-ray selected clusters by a factor of 2.5.