A NICER View on the 2020 Magnetar-Like Outburst of PSR J1846-0258

Abstract: We report on our monitoring of the strong-field magnetar-like pulsar PSR J1846-0258 with the Neutron Star Interior Composition Explorer (NICER) and the timing and spectral evolution during its outburst in August 2020. Phase-coherent timing solutions were maintained from March 2017 through November 2021, including a coherent solution throughout the outburst. We detected a large spin-up glitch of magnitude \Delta\nu/\nu = 3 X 10^{-6} at the start of the outburst and observed an increase in pulsed flux that reached a factor of more than 10 times the quiescent level, a behavior similar to that of the 2006 outburst. Our monitoring observations in June and July 2020 indicate that the flux was rising prior to the SWIFT announcement of the outburst on August 1, 2020. We also observed several sharp rises in the pulsed flux following the outburst and the flux reached quiescent level by November 2020. The pulse profile was observed to change shape during the outburst, returning to the pre-outburst shape by 2021. Spectral analysis of the pulsed emission of NICER data shows that the flux increases result entirely from a new black body component that gradually fades away while the power-law remains nearly constant at its quiescent level throughout the outburst. Joint spectral analysis of NICER and simultaneous NuSTAR data confirms this picture. We discuss the interpretation of the magnetar-like outburst and origin of the transient thermal component in the context of both a pulsar-like and a magnetar-like model.

• The paper documents a notable spin-up glitch (Δν/ν ≃ 3×10⁻⁶) and a pulsed flux increase over ten times the quiescent level during the outburst.
• Data from NICER and complementary instruments reveal spectral evolution driven by the emergence of a new blackbody component while the power-law component remains constant.
• The study supports a dual interpretation of PSR J1846-0258, indicating it exhibits both rotation-powered and magnetar-like features that hint at a continuum in neutron star evolution.

Overview of the 2020 Magnetar-Like Outburst of PSR J1846-0258

The paper by Hu et al. presents an extensive examination of the magnetar-like outburst of the pulsar PSR J1846-0258 observed in 2020. This research offers significant observations derived from the monitoring of this pulsar with the Neutron Star Interior Composition Explorer (NICER) and other instruments, covering periods of quiescence, during which coherent timing solutions were maintained from March 2017 through November 2021.

The observational data revealed the occurrence of a notable spin-up glitch at the onset of the outburst, quantified by a fractional frequency increase of $\Delta\nu/\nu = 3 \times 10^{-6}$. Concurrently, the outburst period witnessed an elevation in pulsed flux exceeding ten times the base level observed during quiescence. Such an escalation in pulsed flux mirrors the behavior seen in a previous outburst of J1846-0258 in 2006. Intriguingly, it was noted that the flux had been increasing prior to the official announcement of the outburst. The pulse profile during the event altered its shape, eventually reverting to its pre-outburst state over the ensuing year. Detailed spectral analysis during and after the outburst indicated that the flux augmentations were entirely attributable to the emergence of a new blackbody component, in contrast to the power-law component which stayed relatively constant.

This paper explores the implications of these findings, proposing interpretations within the frameworks of both pulsar-like and magnetar-like models. In a pulsar-like framework, PSR J1846-0258 could be considered as a rotation-powered pulsar (RPP) whose episodic magnetar-like bursts are perhaps due to magnetic field twists altering particle acceleration and pair production dynamics. Alternatively, in a magnetar-like model, these outbursts might be underpinned by changes in crustal magnetic field configurations leading to increased magnetic activity and surface heating.

Several aspects of PSR J1846-0258 make it an enigmatic subject of study. The pulsar showcases both rotation-powered pulsar and magnetar characteristics, which blurs the demarcation between RPPs and magnetars. This duality suggests a continuum in neutron star evolutionary paths and points to the potential for such objects to demonstrate characteristics of both RPPs and magnetars, depending on their current evolutionary stage. The paper also addresses the evolution in J1846-0258's braking index—a critical factor in understanding the balance between rotation and magnetic braking and a characteristic that offers insights into the fundamental magnetic field evolution processes in neutron stars.

The quiescent and outburst states observed in PSR J1846-0258 present new opportunities to examine and model the physics of neutron stars in extreme conditions. The results give credence to the idea of further exploring spectral energy distributions in emission regions typically accessible to instruments covering a range from soft X-rays to soft $\gamma$-rays.

Looking forward, continued monitoring and multi-spectrum analysis of J1846-0258, as well as comparative studies with other similar pulsars, could provide deeper insights into the mechanisms governing neutron star outbursts, the nature of their magnetic fields, and the potential evolution from pulsar-like to magnetar-like attributes over time. These findings underscore the importance of coordinated observational campaigns to unravel the complexities of transient phenomena in neutron stars and to enhance our understanding of the diverse evolutionary pathways they may follow.