Matthew Baring
(Rice University)
Matthew Baring Colloquium: Altitude Bounds in Gamma-Ray Pulsars from Pair Transparency Constraints
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.
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