Multiwavelength observations of PSR J2021+4026 across a mode change reveal a phase shift in its X-ray emission

¹ Università di Pisa and Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, 56127 Pisa, Italy
e-mail: massimiliano.razzano@unipi.it
² Santa Cruz Institute for Particle Physics, Department of Physics and Department of Astronomy and Astrophysics, University of California at Santa Cruz, Santa Cruz, CA, 95064 USA
³ INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E. Bassini 15, 20133 Milano, Italy
⁴ Kepler Institute of Astronomy, University of Zielona Góra, Lubuska 2, 65-265 Zielona Góra, Poland
⁵ Istituto Universitario di Studi Superiori (IUSS), 27100 Pavia, Italy
⁶ Los Alamos National Laboratory, Los Alamos, NM, 87545 USA
⁷ Space Science Division, Naval Research Laboratory, Washington, DC, 20375-5352 USA
⁸ INAF Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Roma, Italy

Received: 9 January 2023
Accepted: 31 May 2023

Abstract

Context. We have investigated the multiwavelength emission of PSR J2021+4026, the only isolated γ-ray pulsar known to be variable, which in October 2011 underwent a simultaneous change in γ-ray flux and spin-down rate, followed by a second mode change in February 2018. Multiwavelength monitoring is crucial to understand the physics behind these events and how they may have affected the structure of the magnetosphere.

Aims. The monitoring of pulse profile alignment is a powerful diagnostic tool for constraining magnetospheric reconfiguration. We aim to investigate timing or flux changes related to the variability of PSR J2021+4026 via multiwavelength observations, including γ-ray observations from Fermi-LAT, X-ray observations from XMM-Newton, and a deep optical observation with the Gran Telescopio Canarias.

Methods. We performed a detailed comparison of the timing features of the pulsar in γ and X-rays and searched for any change in phase lag between the phaseogram peaks in these two energy bands. Although previous observations did not detect a counterpart in visible light, we also searched for optical emission that might have increased due to the mode change, making this pulsar detectable in the optical.

Results. We have found a change in the γ-to X-ray pulse profile alignment by 0.21 ± 0.02 in phase, which indicates that the first mode change affected different regions of the pulsar magnetosphere. No optical counterpart was detected down to g′ = 26.1 and r′ = 25.3.

Conclusions. We suggest that the observed phase shift could be related to a reconfiguration of the connection between the quadrupole magnetic field near the stellar surface and the dipole field that dominates at larger distances. This is consistent with the picture of X-ray emission coming from the heated polar cap and with the simultaneous flux and frequency derivative change observed during the mode changes.