Abstract Details
| Name: Devraj Pawar Affiliation: R. J. College, Mumbai 86 Conference ID: ASI2015_545 Title : Optical Counterparts to ROSAT X-Ray Sources Authors and Co-Authors : Kaustubh Vaghmare (IUCAA, Pune), Varun Bhalerao (IUCAA, Pune), Aditya Rotti (Florida State University, USA), Eric Bellm (Caltech, USA) Abstract Type : Poster Abstract Category : Instrumentation and Techniques Abstract : The ROSAT Bright Source Catalog (BSC) consists of 18,806 objects discovered by the ROSAT satellite in its mission. Almost half of the sources are still unidentified owing to the large error circles from the ROSAT data. Past efforts have tried to associate optical counterparts to the sources by putting a cut on positional offset, and assigning the brightest object as the counterpart. More recent efforts have also investigated optical variability. We propose a method to robustly combine all information regarding positional offset, optical brightness and optical variability and assign association probabilities in order to securely identify optical counterparts. In this method, if 's' is the positional uncertainty in the X-ray source, all optical sources within 3s radius are considered as candidates. For each candidate, we assign a formal association probability based on offset, brightness and variability. For offset, a Gaussian distribution of errors is assumed while for brightness, a background brightness distribution is determined using a control field which comprises objects between 3s and 30s. To determine the association probability based on variability, we need to be able to determine the actual variability of the source over and above the measurement noise. We present a detailed theory of how this can computed and simulations to verify the same. The method is currently being used to find optical counterparts to ROSAT BSC X-ray sources using the light curves made available by the Palomar Transient Factory survey but can be employed to find counterparts to any sources discovered in any waveband with large positional uncertainties. |