Strongly pulsed thermal X-rays from a single extended hot spot on PSR J2021+4026

The radio-quiet pulsar PSR J2021+4026 is mostly known because it is the only rotation-powered pulsar that shows variability in its gamma-ray emission. Using XMM-Newton archival data, we first confirmed that its flux is steady in the X-ray band, and then we showed that both the spectral and timing X-ray properties, that is to say the narrow pulse profile, the high pulsed fraction of 80-90%, and its dependence on the energy, can be better reproduced using a magnetized atmosphere model instead of simply a blackbody model. With a maximum likelihood analysis in the energy-phase space, we inferred that the pulsar has, in correspondence of one magnetic pole, a hot spot with a temperature of T similar to 1 MK and colatitude extension of theta similar to 20 degrees. For the pulsar distance of 1.5 kpc, this corresponds to a cap of R similar to 5-6 km, which is greater than the standard dimension of the dipolar polar caps. The large pulsed fraction further argues against emission from the entire star surface, as it would be expected in the case of secular cooling. An unpulsed (less than or similar to 40% pulsed fraction), nonthermal component, probably originating in a wind nebula, is also detected. The pulsar geometry derived with our spectral fits in the X-ray is relatively well constrained (chi=90 degrees and xi=20 degrees-25 degrees) and consistent with what is deduced from gamma-ray observations, provided that only one of the two hemispheres is active. The evidence for an extended hot spot in PSR J2021+4026, which was also found in other pulsars of a similar age but not in older objects, suggests a possible age dependence of the emitting size of thermal X-rays.

Automated summary

The radio-quiet pulsar PSR J2021+4026, known for its variability in gamma-ray emission, was studied using XMM-Newton archival data. The X-ray properties, including a hot spot, support the use of a magnetized atmosphere model rather than a blackbody model. This discovery suggests possible age dependence of emitting size of thermal X-rays in similar pulsars.