The universe contains objects capable of accelerating particles to energies of tens or hundreds of TeV. Gamma rays are an important probe to study the extreme physics in these cosmic accelerators. High-energy gamma rays have been detected from a variety of sources such as supermassive black holes in the centers of galaxies, pulsar wind nebulae, supernova remnants, and binary star systems. Detailed measurements of high-energy gamma rays, in conjunction with observations at other wavelengths, will greatly enhance our understanding of the acceleration and emission mechanisms in these objects.
This RUI award will provide support for scientists at Grand Valley State University (GVSU) to conduct research with VERITAS, the Very Energetic Radiation Imaging Telescope Array System located at the Fred Lawrence Whipple Observatory in southern Arizona. VERITAS is an array of four imaging atmospheric Cherenkov telescopes designed to detect very-high-energy gamma rays (100 GeV - 50 TeV) by imaging the flashes of Cherenkov light produced when these photons impact the Earth's atmosphere. The research efforts at GVSU will focus on Galactic TeV sources, in particular compact binaries, pulsars, and unidentified gamma-ray sources, and on multi-wavelength studies of these objects. Service efforts will be directed towards improving the performance of the VERITAS Pointing Monitor system and improving the pointing accuracy and optical properties of the VERITAS telescopes.
Under Broader Impacts, this project will provide undergraduate students at GVSU with the opportunity to conduct focused research in high-energy astrophysics and acquire a broad range of research skills. GVSU has a diverse population of undergraduate students many of which are first-generation college students or students from traditionally underrepresented groups.
The two main objectives of the project were to conduct multi-wavelength studies of Galactic gamma-ray sources and to maintain and improve the VERITAS Pointing Monitor system. We analyzed multi-wavelength data of the millisecond gamma-ray pulsar PSR J1907+0602 discovered with Fermi. Radio data obtained with the EVLA did not reveal any point-like or extended radio emission from the pulsar, and we derived upper flux limits in multiple radio bands. The pulsar was detected in X-ray data obtained with XMM-Newton as a point-like source, and we analyzed the X-ray spectrum to determine the pulsar's spectral parameters. No extended emission which might indicate a pulsar tail was detected. However, the X-ray data showed evidence for a bow shock which supports a connection between the pulsar and a nearby extended TeV gamma-ray source. Our findings were published in Pandel & Scott (2012, AIP Conf. Proc. 1505, 329). We analyzed XMM-Newton data of the extended TeV gamma-ray source MGRO J1908+06 to search for point-like and diffuse X-ray emission. The source, whose nature is currently unknown, has so far only been detected at TeV energies. Our analysis revealed multiple X-ray point sources that might be associated with the TeV source, including the previously detected gamma-ray pulsar PSR J1907+0602. However, no extended X-ray emission coincident with MGRO J1908+06 was detected, and we derived strong upper limits on the diffuse X-ray flux. The VERITAS Pointing Monitors are an optical tracking system which improves the pointing accuracy of the VERITAS telescopes by one order of magnitude to about 10 arc seconds. As part of this project, we carried out performance studies of the system and implemented improvements to its operation. We investigated how small pointing errors of the VERITAS telescopes affect the sensitivity for the detection of gamma-ray sources. It was found that the improved pointing accuracy provided by the Pointing Monitor system can lead to a 10 per cent improvement in the significance of point-like sources near the detection threshold. We investigated several systematic effects that might impact the performance of the Pointing Monitors. Atmospheric refraction was found to have a significant impact on the reconstructed gamma-ray source localization for observations with large zenith angles. The Earth's magnetic field, while significantly distorting the gamma-ray shower images, was found to have a negligible effect on the reconstructed source positions.
