Color indices of optical afterglows of long GRBs in the Swift era

We show the power of the method of the color indices that uses the commonly available multiband photometry of optical afterglows (OAs) of long GRBs detected by Swift. Our study shows that for most OAs, in the observer frame, these colors, corrected for the Galactic reddening, display prominent clustering in the color-color diagrams. The color indices enable us to distinguish OAs from other types of objects even without available gamma-ray emission. The color-color diagrams can be used to assess the extinction inside the host galaxy. Our approach is also generally important for investigating the numerous faint OAs for which obtaining spectra with good S/N ratio is often impossible.