The NANOGrav Nine-year Data Set: Measurement and Analysis of Variations in Dispersion Measures
- Department of Physics and Astronomy, West Virginia University, Morgantown, WV 26506 (United States)
- Department of Astronomy and Cornell Center for Astrophysics and Planetary Science, Cornell University, Ithaca, NY 14853 (United States)
- Jodrell Bank Centre for Astrophysics, Alan Turing Building, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL (United Kingdom)
- Center for Research and Exploration in Space Science and Technology and X-Ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20771 (United States)
- Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1 (Canada)
- National Radio Astronomy Observatory, P.O. Box 0, Socorro, NM 87801 (United States)
- Department of Physics, Hillsdale College, 33 E. College Street, Hillsdale, MI 49242 (United States)
- Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr. Pasadena, CA 91109 (United States)
- Department of Physics, McGill University, 3600 rue Universite, Montreal, QC H3A 2T8 (Canada)
- Department of Physics, Columbia University, 550 W. 120th St. New York, NY 10027 (United States)
- Department of Physics, Lafayette College, Easton, PA 18042 (United States)
- National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903 (United States)
- Department of Physics and Astronomy, Oberlin College, Oberlin, OH 44074 (United States)
We analyze dispersion measure (DM) variations of 37 millisecond pulsars in the nine-year North American Nanohertz Observatory for Gravitational Waves (NANOGrav) data release and constrain the sources of these variations. DM variations can result from a changing distance between Earth and the pulsar, inhomogeneities in the interstellar medium, and solar effects. Variations are significant for nearly all pulsars, with characteristic timescales comparable to or even shorter than the average spacing between observations. Five pulsars have periodic annual variations, 14 pulsars have monotonically increasing or decreasing trends, and 14 pulsars show both effects. Of the four pulsars with linear trends that have line-of-sight velocity measurements, three are consistent with a changing distance and require an overdensity of free electrons local to the pulsar. Several pulsars show correlations between DM excesses and lines of sight that pass close to the Sun. Mapping of the DM variations as a function of the pulsar trajectory can identify localized interstellar medium features and, in one case, an upper limit to the size of the dispersing region of 4 au. Four pulsars show roughly Kolmogorov structure functions (SFs), and another four show SFs less steep than Kolmogorov. One pulsar has too large an uncertainty to allow comparisons. We discuss explanations for apparent departures from a Kolmogorov-like spectrum, and we show that the presence of other trends and localized features or gradients in the interstellar medium is the most likely cause.
- OSTI ID:
- 22663539
- Journal Information:
- Astrophysical Journal, Vol. 841, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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The NANOGrav nine-year data set: Astrometric measurements of 37 millisecond pulsars
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