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PSR J2021+4026 in the gamma cygni region: the first variable γ-ray pulsar seen by the Fermi LAT

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Allafort, A.
Venter, C.
Baldini, L.
Ballet, J.
Barbiellini, G.
Fermi LAT

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IOP Publishing

Abstract

Long-term monitoring of PSR J2021+4026 in the heart of the Cygnus region with the Fermi Large Area Telescope unveiled a sudden decrease in flux above 100 MeV over a timescale shorter than a week. The "jump" was near MJD 55850 (2011 October 16), with the flux decreasing from (8.33 ± 0.08) × 10−10 erg cm−2 s−1 to (6.86 ± 0.13) × 10−10 erg cm−2 s−1. Simultaneously, the frequency spindown rate increased from (7.8 ± 0.1) × 10−13 Hz s−1 to (8.1 ± 0.1) × 10−13 Hz s−1. Significant (>5σ) changes in the pulse profile and marginal (<3σ) changes in the emission spectrum occurred at the same time. There is also evidence for a small, steady flux increase over the 3 yr preceding MJD 55850. This is the first observation at γ-ray energies of mode changes and intermittent behavior, observed at radio wavelengths for other pulsars. We argue that the change in pulsed γ-ray emission is due to a change in emission beaming and we speculate that it is precipitated by a shift in the magnetic field structure, leading to a change of either effective magnetic inclination or effective current.

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Allafort, A. et al. 2013. PSR J2021+4026 in the gamma cygni region: the first variable γ-ray pulsar seen by the Fermi LAT. Astrophysical journal letters, 777(1): Article no L2. [https://doi.org/10.1088/2041-8205/777/1/L2]

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