A T N F    D a i l y    A s t r o n o m y    P i c t u r e

17th of July 2015
ATNF Colloquium
Altitude Bounds in Gamma-Ray Pulsars from Pair Transparency Constraints
by Matthew Baring (Rice University)
Abstract. The Fermi gamma-ray pulsar database now exceeds 160 sources and has defined an important part of Fermi's science legacy, providing rich information for the interpretation of young energetic pulsars and old millisecond pulsars. Among the well established population characteristics is the common occurrence of exponential turnovers in the 1-10 GeV range. These turnover energies can be used to provide lower bounds to the typical altitude of GeV band emission. This diagnostic arises when one demands transparency of the magnetosphere to single-photon (magnetic) pair creation at and below the turnover energy. The altitude bounds, typically in the range of 2-5 stellar radii, provide key information on the emission altitude in radio quiet pulsars that do not possess double-peaked pulse profiles. However, the exceptional case of the Crab pulsar provides an altitude bound of around 20% of the light cylinder radius if pair transparency persists out to 350 GeV, the maximum energy detected by MAGIC. An added requirement is that magnetosphere also be transparent to two-photon non-magnetic pair creation, where surface X-rays serve as seeds for interactions with gamma-rays. Preliminary indications are that such opacity is small at moderate altitudes, and that opacity should be sensitive to field-line sweepback morphology.

Image caption & credit: Radio searches netted 17 new millisecond pulsars by examining the Fermi Gamma-ray Space Telescope's list of unidentified sources. Colored circles indicate the positions of the new pulsars on the Fermi one-year all-sky map. - NASA/DOE/Fermi LAT Collaboration

<<   |   archive   |   about   |   today   *   ATNF   |   Parkes   |   ATCA   |   Mopra   |   VLBI   |   ASKAP   |   >>